Final Report Summary - GRENADA (GRaphenE for NAnoscaleD Applications)
Executive Summary:
The discovery of new structures in carbon materials is giving way to vast opportunities for new scientific knowledge and emerging industrial applications. As we celebrate the ten year anniversary of the discovery of graphene, the number of scientific publications and related press articles exposing the potential of graphene continues to grow exponentially. In the history of our collective knowledge on materials science, no other material has received such intense scientific attention in so little time.
The GRENADA project set out to explore the inputs and conditions that influence the final properties of graphene with the objective to make it fit for purpose. The two-dimensional structure of graphene is responsible for many of its unique properties, in particular the excellent thermal and electrical properties. Paradoxically, it is also this same two-dimensional structure which makes graphene highly sensitive to minor changes in its surroundings. By gaining a thorough understanding of the synthesis conditions that produce exceptional graphene, as well as the external factors that can subsequently alter or degrade such properties, the path to industrial implementation will be traced.
Results obtained during the GRENADA project demonstrate the potential of graphene to compete with and exceed the performance of many existing materials in industrially relevant applications. For example, for graphene to replace indium tin oxide (ITO) in display applications, both excellent electrical conductivity and high optical transparency are required. Progress has been made in this direction with films of graphene exhibiting sheet resistivity as low as 135 ohms per square and optical transparency exceeding 95%, in line with application requirements. More importantly, display test vehicles have validated the functionality of graphene as an electrode in operating OLED devices with light emission at voltages as low as 4 volts.
In parallel, significant results have been achieved for the future use of graphene in energy storage devices such as supercapacitors and lithium batteries. Functional battery test cells were fabricated with reduced graphene oxide proving the feasibility of battery fabrication based on chemically synthesized graphene. High capacity supercapacitor test vehicles were also validated with graphene exhibiting energy density performance equivalent to commercial supercapacitors in side-by-side evaluations.
Nevertheless, integrating today’s graphene into industrial applications is not as easy as a “drop-in” replacement. There are still significant challenges to overcome before the materials properties of graphene obtained in a laboratory setting can be regularly repeated in a large scale industrial setting. The loss of fidelity of the material can be explained on the one hand by the difficulties to synthesize on a large scale, as well as by the susceptibility of the material to modification upon interaction with its surrounding environment. We have made significant progress on these issues and now see that graphene is slowly but surely making its way into the marketplace.
Project Context and Objectives:
Project Context
Graphene is a two dimensional, one-atom thick sheet of carbon arranged in a hexagonal network. Owing to its exceptional properties, graphene has led to the emergence of an interesting bridge between condensed-matter physics and quantum field theory, since it allows the study of relativistic charge carrier effects in relatively simple, lab experiments. The measured performance of graphene in the laboratory is exceptional, including remarkable values of electron mobility, transparency, Young’s modulus, thermal conductivity and a high specific surface to mass ratio. The combination of such unique properties in graphene offer exciting prospects for graphene to replace existing materials in many industrial applications.
Industry analysts predict the market for graphene materials will be in the range of 100-200M€ in the 2020-2024 time period. While this base materials market remains somewhat modest, the products that graphene will enable are of far greater importance. Flexible displays will provide an entirely new generation of wearable devices. Conductive coatings and printable inks containing graphene will help propel the emergence of 3D printing to conquer new markets. Energy storage devices will likewise see the replacement of traditional carbon materials by graphene to achieve longer lifetime and higher charge capacity.
Despite the exceptional promise offered by graphene as a “game changer”, there are still many challenges to overcome to allow graphene to find widespread market introduction. For example, minor variations in synthesis conditions can drastically modify the materials properties of the resulting graphene, quickly shifting from a regime of excellent material to one of useless rubbish. In a similar fashion, graphene properties are largely influenced by their surroundings, including contact with other solid state materials, as well as with gaseous or liquid environments. Consequently, the main challenge of industrial implementation of graphene is to find the appropriate combination of synthesis conditions, coupled with scalable production processes and a stable environment for the long term viability of graphene.
General Objectives
The work program of the GRENADA project was designed to address the major stumbling blocks currently preventing large-scale implementation of graphene in industry. In order to fully understand and exploit the potential properties of graphene, a comprehensive study of the various synthesis methods and the resulting material properties was needed. The expectation was to provide a correlation between synthesis conditions and final materials properties for several parameters, including number of graphene layers, crystalline structure of the graphene, electrical, optical and mechanical properties.
A careful study of defects in graphene and the effect of defects on materials properties forms an integral part of the Grenada project. While many defects are a result of the synthesis conditions of graphene, other defects can develop over time as a result of external factors. Defects can also be intentionally induced in defect-free graphene to provide specific, localized defects that can be studied individually to measure the effects on performance. The final output expected is a comprehensive overview of the effects of defects in graphene on performance.
Modifying graphene synthesis conditions to optimize for a particular material property can have a negative influence on other material properties. By probing the existing parameter space, a knowledge base could be developed which would effectively allow one to choose the best synthesis techniques and “engineer” the desired graphene material properties to fit specific application requirements.
While optimum materials properties are important, the focus is to provide processes and methods which are amenable to mass manufacturing. For instance, while ultra-high vacuum CVD is a synthesis method which can provide graphene with exceptional materials properties, the technique is comparatively expensive and difficult to control in high throughput production environments. Alternatively, atmospheric pressure CVD is better suited to mass production and has been a focus of study within Grenada. The balance between achieving optimal materials properties and at the same time a scalable manufacturing process is core to the objectives of the project.
Reliable characterization methods are vital for fast and effective feedback and control of process development in graphene growth. They are equally essential for elucidating the mechanisms linking growth conditions and resulting graphene morphology to materials performance. Highly advanced measurement capabilities and methods can provide a better understanding of the local properties of graphene, particularly as a function of the surrounding environment. New experimental set-ups coupled with advanced test methodologies are needed to understand the fundamental mechanisms of graphene interaction at the atomic scale. Nanoscale probing of the materials can surely bring new information and knowledge to eventually quantify and predict materials behavior at a system level.
Local environment has a significant effect on the mobility of carriers, adhesion to the substrate and chemical stability of graphene. The nature of the local environment (e.g. polarity of media in immediate contact with graphene) can enhance or reduce effects of the defects on material properties. Such knowledge is absolutely essential for understanding and predicting graphene behavior in real life applications. This objective addresses exploration and modeling of fundamental properties of graphene, and its performance in applications including graphene in contact with polymers and OLED materials, as well as graphene in ionic, polar and non-polar liquids in super-capacitors and batteries.
The final objective will be to bring together all of the knowledge that has been learned from the development of high quality graphene (with minimized defects), the investigation of graphene / environment interaction and the tailoring of graphene properties to fit the specifications. The combined know-how will be applied in test vehicles which are designed to validate the final performance of graphene in applications. The results provide a direct indication of the potential impact of graphene on the performance of actual end-products. The target applications for graphene as a key material are for energy storage in supercapacitors and batteries (demanding materials with superior electrical properties in a hostile, lithium ion environment) and for display applications (requiring optical transparency and high conductivity).
Energy Storage (Accumulators, including supercapacitors and batteries)
Today’s modern society is mobile, connected and “constantly looking for a recharge”. As the mobile energy needs of our society continue to grow, energy storage devices will require higher energy density, faster charge/discharge capability and high reliability in order to bring sufficient value to consumers. In parallel to the straightforward need for increased primary energy storage capacity of storage devices, there is also a need to efficiently manage power demand fluctuations. An acknowledged approach to serve such fluctuations is to couple large capacity accumulators, such as Li-ion batteries, with high power capacitors which provide high energy levels over short periods of time. By using supercapacitors to provide high surges of power, research has already shown that the talk time of a cell phone can be increased by a factor of four and the total battery life can be doubled. An excellent example of such a hybrid system, which today combines high power with high energy and tie a supercapacitor in parallel with a battery, are implantable defibrillators. These systems use low-power batteries to charge conventional capacitors that provide the required power pulse.
Supercapacitors coupled with batteries can be used to manage peak power in electrical vehicles, thereby decreasing the overall demand on the batteries and improving battery lifetime. Stationary power systems providing emergency power back-up will also benefit from the coupling of batteries with supercapacitors, especially during start-up conditions. Although super- and ultra-capacitors are a potential long-term competitor to batteries, they offer the short term possibility of enhancing battery performance by extending battery life and reducing battery weight.
The capacitors (supercapacitors and ultracapacitors), rely on polarization effects at the electrodes/electrolyte interface while batteries exploit red-ox reactions which take place at their electrodes/electrolyte interface. From this statement, it follows that to improve the energy accumulation capacity of batteries as well as to increase the supercapacitor power, the electrode/electrolyte interfaces are of crucial importance. Graphene being an extremely good conductor exhibiting a large surface to volume ratio is one of the most promising nanostructured materials to be investigated as an electrode to improve the energy density and the power of these devices.
A capacitor, like a battery, is an electrical energy storage device. Whereas a battery stores energy in chemical form, a capacitor stores electrostatic energy as charge separation. The term supercapacitor usually describes a device that uses the charge separation at an electrode-electrolyte interface—that is, an electrochemical capacitor. An electrochemical device requires at least two electrodes, so a supercapacitor is actually two capacitors in series. Whereas supercapacitors store significantly less energy than batteries store—one normally expresses their capacity in ampere-seconds rather than ampere-hours—they benefit from extremely fast charge and discharge capabilities and longer life cycles. In Japan supercapacitors are already a $100 million dollar-a-year business, finding widespread application in short-term memory retention for electronic devices. Growing applications for supercapacitors include defibrillators, pulse power units for vehicle propulsion, power stabilizers for computers, handheld tools, memories, camcorders, and backup devices on computers and plant production lines.
OLED Displays
Nearly all display technologies have one thing in common, they require at least one conductive transparent electrode in order to allow light to pass in the direction of the user. Nearly all displays manufactured today use Indium Tin Oxide (ITO) as the transparent conductive material. Graphene is a prime candidate to replace ITO in displays, especially for flexible displays.
Today, liquid crystal displays (LCD) dominate the market, while OLED technology is expected to gradually replace LCD in applications such as televisions, laptop computer screens, tablets and mobile phones. According to market forecasts, in the short run active matrix screens (AMOLED) of mobile phones and tablets should become the main applications of the OLED market for displays. The high color saturation, high resolution, low power consumption and the very high contrast inherent to the emitting aspect of the OLED make it a technology of choice for future applications. For strategic reasons, the testing of graphene compatibility with OLED displays was preferred over LCD based on potential impact for Europe.
Light-emitting organics (OLED) provide a low cost, high reliability process over large areas on many kinds of substrates including flexible plastic substrates. Their self-emissive and wide viewing angle properties offer advantages over the current widely used liquid crystals in many display applications. Particularly in the automotive display applications, polymer OLEDs could offer price and performance advantages over vacuum florescence display (VFD) and liquid crystal displays (LCD).
Beyond these first applications, many similar industrial applications (solar cells, touch screens, discrete electronic components, etc.) could benefit from the experience gained with energy storage and displays. Determining the best conditions for production and performance of graphene opens many new perspectives and provides a step change in these technological areas.
The Consortium
In order to achieve the objectives of GRENADA, the consortium required a broad range of competencies, from pure academic theory all the way to product-oriented industrial testing. The final configuration of the consortium, led by the CEA (France), includes the Institut Néel CNRS (France), LGC-CNRS (France), University of Cologne (Germany), Lancaster University (UK), Polytechnic University of Valencia (Spain), CRF (Italy), Varta Microbattery (Germany) and Varta Micro Innovation (Austria). The capabilities of the partners are highly complementary while avoiding duplication of efforts. The inclusion of some of Europe’s most active industrial developers in energy storage provides an excellent outlet for exploitation of the results of the project.
Project Results:
Graphene synthesis has been the subject of numerous scientific studies in recent years, and Grenada in this respect is no exception. The most widely studied methods in literature are exfoliation and chemical vapor deposition (CVD), both of which are capable of yielding high quality graphene which can potentially be transferred to any type of substrate. The high potential of CVD has been extensively investigated during the GRENADA project. Several CVD techniques, employing a wide variety of metallic substrates, have provided positive results in terms of graphene growth with high quality. For example, under UHV, large-area, ultra-high quality graphene has been achieved. On Ir(111), Co(0001) and Re(0001) thin films, purely single-layer graphene films having single crystallographic orientation have been prepared (Institut Néel CNRS). This was carefully validated in a number of in situ experiments using various techniques: X-ray diffraction at synchrotron radiation facilities, electron diffraction, low-energy electron microscopy, and scanning tunneling microscopy. The growth mechanisms on Co and Re were unknown so far, and were addressed with care. The interplay between surface carbide, carbon-in-the-bulk, and graphene phases was studied. On Co(0001) it was found that growth temperatures as low as 600°C yielded extremely high quality graphene. Low- and ambient-pressure CVD produced graphene islands, single-layer graphene, multi-layer graphene, and nanostructured interconnected few-layer graphene (LGC CNRS). In parallel, very large-area growth (fully covered 200 mm wafers) of few layer graphene, using carbon segregation, has been demonstrated (CEA). The growth process is performed inside industrial production equipment compatible with CMOS technologies at relatively low temperatures (600°C-800°C) with high throughput (2-15 min annealing). The metal catalysts used are polycrystalline metal thin films that have the advantage of being low-cost as compared to monocrystalline metal films.
Purely single layer graphene was demonstrated for the first time and fully characterized with a comprehensive set of techniques (Institut Néel CNRS), and the partial pressure, nature of the carbon precursor gas, sample temperature and chemical composition of the metallic substrate, were found crucial in the view of controlling the number and of graphene layers and their quality (LGC CNRS, CEA).
Graphene synthesis using APCVD as an approach to industrial processes
Lessons learned from high vacuum CVD of graphene have been applied to atmospheric pressure CVD (APCVD), a production technique which is more conducive to large-scale production. At LGC CNRS, experiments have been performed on Cu foil (2 cm2) at 700 Torr, using very highly diluted methane in argon and hydrogen. The influences of numerous synthesis parameters (pre-treatment and synthesis durations, methane and hydrogen partial pressures, position of the substrate into the reactor,) have been studied and optimized.
The use of very low methane partial pressure allows obtaining graphene without D band on the Raman spectra, i.e. of high crystalline quality. A decrease of the methane pressure (30 to 15 mTorr) allows decreasing the number of layers down to the monolayer, probably thanks to a lower amount of reactive carbon species near the copper surface. However, for these conditions, graphene is not continuous, even if the surface coverage of the copper surface is very high. First results about an increase of the synthesis duration at 15 mTorr of methane have shown that graphene is still not fully continuous and is heterogeneous (monolayers and multilayers areas), showing that even at these low methane pressures, the formation of graphene is not self-limiting. This result could indicate that even when the catalytic copper surface cannot be active anymore because it is practically fully covered by graphene, reactive carbon species are still created to form additional graphene layers. This could be explained by the fact that graphene is formed from two kinds of unsaturated reactive species, the first one born near the copper surface from catalytic phenomena, and the second one in the gas phase from methane pyrolysis, as deduced from modeling analysis in LGC-CNRS. Experiments have also shown that a decrease of the H2/CH4 ratio for the conditions tested leads to an increase of the number of layers and to heterogeneous zones of multi-layers graphene. This confirms the positive role of hydrogen as an etching agent of the graphene layer in formation.
Further work has been performed in order to understand the reason for the only partial coverage of the Cu foils after growth, observed even for such large growth times as 90 min. The role of hydrogen, a well-known etching agent for graphene, was ruled out by progressively decreasing its concentration in the gas mixture used during growth. A two-step process was then developed, in which the methane concentration is changed from low to high at the first and second steps respectively. This process yields very high quality purely single layer graphene which is however not fully covering the surface of the Cu foil. A three-step process eventually was developed, which allows reaching full coverage to the expense of a lesser control over the number of layers, mostly one and two.
LGC CNRS also has worked on lower temperature processes for growing graphene on Cu foils, by using a more reactive carbon precursor than methane, ethylene. Growth at various temperatures (750, 800, 900°C) and H2 pressures yields graphene whose defect density and number of layers are ill-defined.
Graphene synthesis on large surface areas
An alternative to graphene growth from carbon in the gaseous phase is to utilize a solid carbon source. A thin film of amorphous silicon carbide combined with a thin film of metal catalyst can produce well-controlled graphene films, as demonstrated by work at CEA. At temperatures between 600-800°C, carbon will diffuse into the metal catalyst film and subsequently precipitate at the surface of the metal upon cooling. The thickness (number of layers) of graphene depends on the thickness of the carbon source, the solubility of carbon in the metal and the time/temperature of the thermal anneal step. The entire surface of 200mm silicon wafers have been coated with high quality graphene using this method.
For applications in energy storage, large quantities of graphene will be required. Most planar substrates will not provide sufficient quantities of graphene at a reasonable cost for large-scale production. An alternative is to synthesize graphene on substrates with large surface area, such as metallic foams. Foams made of nickel have been introduced into the APCVD process machine at CNRS LGC. After some work to improve graphene growth on the surface of Ni foams, LGC CNRS achieved final graphene weight percent of 15.5%, better than published in the literature for thick graphene. It was found that the mechanisms proposed in the literature cannot provide an explanation to these values. Further investigations are in progress.
Defects in Graphene
At University of Cologne, in order to study the influence of structural imperfections on the properties of graphene, defects were purposefully created in epitaxial graphene on Ir (111) using ion bombardment in the keV-range. It was found that grazing incident Xe ions penetrate graphene and are channeled in between the carbon sheet and its substrate. Chains of vacancy clusters in graphene result, with their edges bend down to the substrate to saturate their dangling bonds. These chains arise as the ions are repeatedly reflected at the metal surface and the bottom side of the graphene layer. The impacts on the graphene result in carbon sputtering, causing a track of holes in the graphene layer. The guided motion of the ion in the interface region is analogous to subsurface channeling observed earlier for the case of pristine crystal surfaces.
An irradiated sample was gradually heated in steps of 150 K (30 s at each temperature). From the resulting topographies, the number of defect patterns as well as the fraction of defect patterns accompanied by protrusions were quantitatively estimated as a function of annealing temperature.
The holes in the graphene sheet remain present at 850 K. Their edges appear not very sharp in the STM, the holes rather have the shape of funnels reaching down towards the substrate. As in previous publications the presence of unsaturated carbon bonds was verified by the detection of a peak in the density of states at the Fermi energy, an effect which was sought for in scanning tunneling spectroscopy experiment on the funnel-shaped holes. No peak close to 0 V could however be detected, proving the absence of dangling bonds in the system, in contrast to previous experiments for graphene/Pt(111). It can thus be assumed that the carbon atoms at the edge of the hole are bound to the metal substrate, as it is also shown in accompanying density functional theory calculations. The holes start to disappear just around 970 K, which resembles the temperature of the transition from amorphous carbon to graphene. This is due to the onset of the diffusion vacancies and small vacancy clusters. At the highest investigated temperatures the graphene layer appears to be entirely rebuilt on a large scale. However, some well-defined point defects persist.
Graphene synthesis by wet processes
Nanostructured graphene was also prepared by a new wet process (UPVLC), from graphite powders, following a two-step procedure consisting in oxidation according to a modified Hummers method, and then a fast reduction step using an environmentally friendly reducing agent (ascorbic acid). Functionalization with PSS allowed efficient dispersion of reduced graphene oxide in water solution (Fig. 3). The formation of graphene oxide and reduced graphene oxide was confirmed by UV-visible spectroscopy, transmission electron microscopy, and Raman spectroscopy among other techniques. Reduced graphene oxide is found to have a low density of defects, as compared to that reported in the literature for graphene prepared by wet methods, most oxygen-containing groups being removed, but still comprising a certain density of vacancies and showing the presence of few-layer stacked fractions. Unfortunately, modification to the fabrication process has contrary effects in both issues (defects and stacked layers), as increasing the degree of exfoliation increases also the quantity of defects.
Given the strong tendency to aggregation of the graphene oxide (GO) sheets, covalent functionalization on the surface of GO can be pursued in order to achieve either i) homogeneous dispersion or ii) efficient interfacial interactions between the GO and the substrate of choice. Two approaches were followed depending on the targeted functional group developed on the GO after oxidation, i.e. hydroxyl (-OH) and epoxy (C-O-C) groups on the basal plane or carboxylic groups (–COOH) present on the edge of GO sheets. In relation to the former, silanes are used, as these compounds are well known coupling agents usually applied to the surface modification of nanofillers in the presence of catalyst. Chemically functionalized GO sheets by 3-aminopropyltriethoxysilane (APTS) are reported. Secondly, the presence of abundant carboxylic groups on graphene oxide (GO) edges was exploited to covalently bound EDC–NHS (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride–N-hydroxysuccinimide) resulting in the formation of an active ester intermediate.
The successful attachment of the functionalization groups by the two approaches was confirmed by nuclear magnetic resonance (NMR), UV-Vis and Raman spectroscopies and visualized by transmission electron microscopy (NMR). Further on, after functionalisation, the successful reduction of these new materials was confirmed by UV-Vis spectroscopy.
The fabrication of graphene by the wet chemical route was continuously improved based on feedback from partners developing energy storage applications. After several iterations, graphene synthesis conditions settled on a three step method: first oxidation of graphite to obtain graphene oxide (GO); second, an initial reduction step to obtain reduced graphene oxide (rGO); third, further reduction step by heat treatment in reducing atmosphere
While most “traditional” graphene synthesis methods require very high temperatures, often coupled with ultra-high vacuum environments, we now can describe novel techniques which provide graphene at lower temperatures and which are considered to be in line with standard industrial processing. To summarize the progress, major accomplishments of the project with respect to graphene synthesis include:
- Validation of growth on platinum, nickel, copper, iridium, cobalt and rhenium metal films.
- High quality graphene synthesis with electrical mobility as high as 5000 cm2/(Vs).
- Validation of growth on very large sized substrates (up to 200mm diameter).
- Growth on large sized substrates at temperatures as low as 600°C.
- Growth on nanostructured substrates with control over the number of defects (including the possibility to intentionally induce defects with a pre-defined periodicity).
- Wet chemical synthesis of dispersed graphene oxide in water using an optimized three-step process capable of producing large batch sizes.
Atomic scale characterisation of as-grown graphene
Furthermore, defects have been studied in epitaxial graphene on a metal, Ir, using X-ray diffraction at a synchrotron source (ESRF, Grenoble). It has been shown that tiny imperfections are still present in the highest quality graphene samples. These imperfections were not evidenced before; they include small shears, strains, and misorientations, in the form of domains (see Figure for schematic models for the imperfections).
We have analysed the formation of strains in epitaxial graphene, in real time, during growth, ion bombardment and oxygen etching, with the help of a standard widespread in situ technique, reflection high-energy electron diffraction. We found substantial strains developing in graphene which are ascribed to defects, including carbon vacancies and ions trapped between graphene and its substrate, and epitaxial stress. Strains are found to be adjustable from a tenth to as much as 2.2%. Such strains can be canceled by removing the defects. The weak interaction of graphene with metals such as Ir, Cu or Pt also has important consequences regarding the structure of graphene. Indeed, graphene domains with different stacking (twins) with respect to the substrate tend to form on these metals, presumably because the formation of each of the domain involves similar energetic costs. At the boundary between these domains, dislocations (heptagon pentagon pairs) are found. Such structural defects hinder the improvement of graphene performances, e.g. for electronic transport, heat conduction, or mechanical resistance, towards state-of-the-art ones obtained for exfoliated, suspended graphene. Recently, strategies have been developed in order to avoid the formation of grain boundaries, in graphene having single crystalline orientation at the scale of a centimeter. We investigated with high resolution the structure of graphene on iridium, which is a model system for graphene weakly interacting with a transition metal substrate. Even the highest quality epitaxial graphene displays tiny imperfections, i.e. small strains, ~0.3%, rotations, ~0.5°, and shears over distances of ~100 nm, and is found incommensurate, as revealed by X-ray diffraction and scanning tunneling microscopy. These structural variations are mostly induced by the increase of thermal mismatch between the preparation temperature and the actual sample temperature. Although graphene weakly interacts with iridium, its thermal expansion is found positive, contrary to free standing graphene. The structure of graphene and its variations are very sensitive to the preparation conditions. All these effects are consistent with initial growth and subsequent pinning of graphene at steps. These structural imperfections may influence the graphene properties.
Free-suspended Graphene
Free-suspended graphene films were synthesized via carbon diffusion through Pt based compounds and imaged using SEM confirming that the graphene of the full stack Si/SiO2/Pt+Pt3Si/graphene sample is suspended. Sample growth occurs in a number of stages, determined by substrate temperature during annealing. Of particular interest are: i) secondary stage, mirror-like regions corresponding to graphene on almost flat metal/metal silicide and; ii) tertiary stage frosted-like green region where SEM images appear to show large areas of graphene suspended between islands of Pt+Pt3Si.
We observe a clear progression in the degree of heterogeneity in the surface morphology with height of surface features increasing significantly from primary to secondary and tertiary stages corresponding, initially, to graphene “nano-domes” formation followed by the Pt3Si island growth. Progression of surface heterogeneity corresponds to increased graphene film decoupling that is revealed in AFM images. It is reasonable to expect that the topographical features are strain induced and arise during the cooling phase of the growth due to the radically different coefficients of thermal expansion (CTE) of the substrate material (CTEPt = 9 x 10-6 K-1) and graphene film (CTEGr = -8 x 10-6 K-1) [29, 30] The primary growth stage, shows conformal graphene film growth with good mechanical contact with the substrate signified by dominating high brightness (i.e. high stiffness) areas in the UFM analysis. AFM contact mode friction contrast also showed little variation suggesting continuous graphene coverage. A mechanical decoupling in the primary stage is mainly observed along the underlying substrate grain boundaries of the polycrystalline platinum substrate and revealed as darker lines in UFM images. Secondary growth stage, occurring at higher substrate temperature during the annealing phase, displays the formation of clearly decoupled bulged regions (“nano-domes”) in the AFM topography and drastically reduced stiffness (dark contrast) zones in UFM images. The location of nano-domes points out to their formation at the intersections of the substrate domain boundaries. These secondary growth stage nano-domes have a typical height of 10 - 35 nm and a width of 100 – 600 nm
In order to investigate the morphology of the graphene layer, in particular, the reason for separation of the graphene layer from the substrate and the properties of the substrate under the nano-domes, we performed UFM scans with increasing normal force over a typical 2 µm x 2 µm nano-dome containing region of the secondary growth stage. Due to practically complete reduction of lateral forces in UFM mode, a known extra bonus of UFM, application of moderate normal force of up to 100-200 nN does not modify the AFM tip or the sample, even on the stiff substrates. We observed that secondary stage 150-200 nm wide nano-domes were fully collapsing at the application of moderate forces around 51±5 nN by the AFM tip. Surprisingly, such collapse rather than resulting in a flat surface, revealed the cavity below each dome that shows the same area post-high force UFM scanning imaged again at standard low normal force. Dome heights were in the range of 10 - 35 nm and post-compression cavity depths are found to be also comparable in the range of 14 - 38 nm. This value of inversion force was also comparable to that seen for the domes in tertiary growth stages, suggesting that the areas on top of the Pt/Pt3Si islands are comparable to those in the secondary phase. Additionally, we observe that the post-inversion UFM response of the nano-domes now shows a high stiffness response suggesting a high degree of mechanical support – evidently, the previously unsupported graphene is now sat in a good mechanical contact with the bottom of the cavity.
Underliquid nanoscale thermal mapping - immersion SThM
Nanoscale heat transport is of increasing importance as it often defines performance of modern processors and thermoelectric nanomaterials, and affects functioning of chemical and biosensors. Scanning Thermal Microscopy (SThM) is the leading tool for nanoscale mapping of thermal properties, but it is often negatively affected by unstable tip-surface thermal contacts. While operating SThM in liquid environment may allow unimpeded thermal contact and open new application areas, it was so far regarded impossible due to increased heat dissipation into liquid, and perceived non-local thermal interaction between the probe and the sample. Nevertheless, we show for the first time that such liquid immersion SThM (iSThM) is fully feasible and has sufficient thermal contrast to detect thermal conductivity variations in high thermal conductivity materials such as graphene.
After testing the system on metal-polymer nanostructured interconnects, we have measured heat transport in the few tens of nm thick graphite nanoflake and iSThM’s spatial resolution was on the order of 50 nm, very similar to the resolution of the same probe in the standard air environment. These results confirm localised thermal sensing in iSThM and, coupled with absence of tip snap-in due to elimination of capillary forces, suggest the possibility for true non-contact nanoscale thermal mapping in liquids, including thermal phenomena in energy storage devices, catalysts and biosystems.
Characterization of Graphene for OLED displays
Graphene has been proposed as a transparent conductor for display applications, potentially replacing Indium Tin Oxide. In such applications, in particular for OLED displays, requirements dictate that transparent conductive films have high transmittance, low resistivity and a work function similar to ITO.
The optical transmittance of graphene transferred on glass substrates was measured from 280 to 1400 nm with a Cary 500 UV–VIS–NIR spectrophotometer. For resistivity measurements, graphene was transferred to a silicon dioxide substrate, followed by evaporation of metallic contact pads (800 um size, 200 um interdistance) of nickel (20 nm thickness) and gold (100 nm) on top of the graphene. Sheet resistance was then measured with a 4-point probe. During the course of the project, the resistivity obtained for transferred graphene has been improved to 135 Ohms/square (single transfer of graphene) and transparencies from 70% to 97% depending on number of layers.
In parallel, the work function (WF) of graphene grown at CEA on 8 inch wafers and transferred onto glass has been measured with Kelvin Force Microscopy.
In order to calibrate the quality of the tip, measurements were completed on Ru and Al samples. For measurements on graphene, the difference of potential measured between the tip and sample is about 0.13 eV when the tip is close to the sample (local WF) and 0.19eV when the tip is far from the sample (corresponding to a more average value of WF since the tip senses a larger area of the sample). The tip is made of Pt-Ir with a WF in the range of 5.0 to 5.3 eV. As there is no technique to measure exactly the WF of the tip, the range of 5.0 to 5.3 eV is used as the baseline reference. Thus the obtained WF values of our graphene on glass are in the range of 4.87 to 5. These values are similar to those of ITO that graphene is intended to replace. The WF on the sample is quite uniform with a variation of. The spatial variation of WF on the sample (± 30meV) is minimal. Thus the graphene work function is well suited to replace ITO without modification of the WF of the OLED stack materials.
Characterization of Graphene for Lithium Batteries
All basic electrochemical studies of graphene materials in lithium ion batteries were performed in half-cell measurements. These measurements allow the electrochemical investigation of processes occurring at the graphene based electrodes against a metallic lithium counter electrode. Although the half-cell measurement setup allows studying effects occurring at the graphene based electrode it actually does not represent the behavior of these electrodes in full cells. This is mainly caused by the almost endless lithium reservoir at the metallic lithium counter electrode, while in commercial lithium ion cells the lithium contributing to the cells capacity is limited by the amount of lithium incorporated in the positive electrode material. In other words, the crucial effect on the cell´s performance by irreversible losses of lithium due to SEI formation cannot correctly be observed in half cells against a metallic lithium electrode. For this reason, full cells containing graphene based negative electrodes and commercial positive electrodes were fabricated and electrochemically investigated. This demonstrator like cell setup allowed studying graphene materials in a configuration close to the actual commercial application.
Cyclic voltammetry is an effective method to characterize electrochemical phenomena at an electrode. A potential is applied between the reference and working electrode and this potential is changed linearly over the time, described as scan rate (V/s). Due to the applied potential change, a potential-depended current flow is generated between the working and the counter-electrode. This method is especially interesting to study electrolyte/electrode phenomena and to study the formation of passivation films on graphene based negative electrodes. Furthermore the method is capable of describing electrode kinetics and the reversibility of the electrode reactions. The usual measurement parameters for characterization are a scan rate of 30µV/s with potential limits of 0v and 1.5V.
Charge/discharge experiments are performed to estimate the capacity of the electrode (for half cells of graphene vs. lithium metallic electrode) or the cell stack (for full cells of graphene vs. Lithiumcobaltoxide positive electrode). This method uses a defined galvanostatic step combined with a constant voltage step to lithiate the graphene negative electrode, which is followed by a galvanostatic delithiation process to delithiate the material. For the galvanostatic step the charger varies the voltage applied to the battery to maintain a constant current flow (C-rate), switching off when the voltage reaches a defined cut off potential. During the additional constant voltage step the cell is kept at the cut off potential and further lithiation of the graphene is enabled independent from overpotentials. The C-rate can be described as the current needed to fully charge/discharge a cell in a certain time. If a C-rate of 1 is applied, the cell is totally charged or discharged within 60 minutes, if a C-rate of 0.5 is applied the charge/discharge time is 120 minutes.
This method allows measuring the End-of-charge voltage, the End-of-discharge voltage and the capacity of the electrode (for half cells) or cell (for full cells) respectively. These values are strongly dependent on the applied rate during the charge and discharge process. In order to study this influence the C-rate was varied in the above described procedures.
Characterisation of lithiated graphene layers
Lithiated graphene samples where the measurements are sensitive to changes corresponding to outer most graphene layers. Here we studied the surface and immediate sub-surface morphology of the sample to understand the effects of lithiation – a vital process for the development of rechargeable batteries – on the sample.
It is possible to lithiate the samples to a certain potential vs. Li/Li+ reference electrode for example. We observe a clear change in morphology (both AFM and UFM) post-lithiation of the samples. The most striking changes occur between the non-lithiated and lithiated samples and between the 1st lithiation step and 5th full lithiation steps. Globular surface features would appear to correspond to subsurface morphology changes arising from lithium ion intercalation, although the rough nature of the surface topography means that quantitative analysis was not fully indicative.
Characterization of Graphene for Supercapacitors
The proliferation of electronics and networked systems creates increasing demand for energy availability and storage. Batteries, with their large energy densities are the most common applied solution to this challenge, albeit their low power densities. In recent times, supercapacitors are thought to be among the list of promising candidates to replace batteries for these applications, attributing to their high power densities. Supercapacitors can store energy through 1) electrochemical double layer capacitance, which is the accumulation of electrostatic charges on the electrodes, and 2) pseudocapacitance, which involves faradic reactions. In comparison to batteries, the supercapacitor modus operandi offers relatively higher power densities, fast charging rates and theoretically unlimited cyclability. However, the main drawback of the commercially available supercapacitors is their limited energy densities in comparison to batteries. Therefore, increasing the energy density of a supercapacitor is one of the main goals of research today.
With the discovery of novel advanced materials, the energy density of a supercapacitor can be increased to a level comparable to batteries. The two main components of a supercapacitor cell consist of two electrodes and the electrolyte. The cathode and anode electrodes are current collectors coated with materials that possess high specific surface area. The purpose of using materials with high surface area is to provide the electrode with more active sites for the storage of charge compounds such as the ions of the electrolyte, thereby enhancing the capacitance of the supercapacitor cell. To date, the electrodes of supercapacitors available commercially are mainly carbon based, with activated carbon as the most widely used due to its high specific surface area (larger than 2000 m²/g), low cost and long cycling life. However the complex pore structures, with irregular pore sizes, bring about an increase in the electrode resistance and reduced capacitance due to the inaccessibility of the electrolyte ions into its micro-pores. A promising candidate to replace activated carbon as an electrode material is graphene. The target properties of graphene are low electrical resistivity and large degree of exfoliation in order to make the material present a large specific surface to a liquid electrolyte. The same features are addressed when embedding the graphene flakes in a binding polymer typically used to prepare a solid electrode. Graphene is expected to provide a larger specific surface, 2630 m²/g when all its carbon atoms are exposed to the electrolyte, as well as high electrical conductivity attributed to the π-conjugated network of sp² hybridized carbons. Apart from electronic transport properties, graphene can be structured on the current collectors to provide paths with higher accessibility for the electrolyte ions. A suitable way to probe, indirectly, the mentioned graphene-based-electrode properties is cyclic voltammetry. By means of this technique, both the specific capacitance and the equivalent series resistance give an indication of the performance of storing and delivering energy to the system.
Cyclic Voltametry applied to supercapacitors
The electrodes were evaluated with both cyclic voltammetry (CV) and constant-current charge/discharge (CC) measurements using an AMEL 500 potentiostat. The CV and CC curves resulting from the measurements were then used to calculate the electrode specific capacitance. In the case of CV measurements, the specific capacitance was obtained by integrating the curves over a full cycle; while for CC measurements, the specific capacitance was calculated from the slopes of its resulting curves using the following equations:
C=I/(dV/dt) (1)
C_sp=4×C/m (2)
With C as the capacitance as an absolute value and m the total mass of the cell.
In equation (2), a factor of 4 was included in the equation to adjust the cell’s capacitance to the electrodes specific one.
The table below summarizes the parameters of major importance when a supercapacitor has to be employed in a specific circuitry to serve an end-use application. Most of the automotive applications: moving a mirror, heating a seat, starting and stopping a car’s engine need large power and energy density, high charge and discharge rates, and the smallest possible equivalent series resistance (ESR). In this table the properties of 3 exemplary specimens are reported to show the differences between the two testing set ups used for the purpose: 3-electrode configuration and a 2-planar electrochemical cell. As one can note sample 1 exhibits a specific capacitance larger than the commercial activated carbon electrode, up to almost 250% larger by the 3-electrode setup measurement, and up to the 75% by the 2-planar-electrode testing cell. As acknowledged, the 3-electrode methodology presents significant limits, with respect the 2-planar-electrode one, to predict how a material would perform if employed in the real device. By a 3-electrode cell only one electrode, called working electrode, containing the material under analysis is used, meanwhile, the 2-planar-electrode cell employs two of them, thus mimicking the real capacitor conditions. Nevertheless, the applied voltage and charge transfer are substantially different between the two setups. In a symmetrical 2-planar-electrode cell the potentials applied to each electrode is the same, and equal to the half of the value at the working electrode in a 3-electrode setup. Therefore, for a given potential window, the working electrode of a 3-electrode cell has twice of the potential as it applied to an identical electrode placed in a 2-planar-electrode cell, resulting in doubling the calculated capacitance. Other significant differences between the two methods are due to different quantities of electrolyte contained in the two setups and different cells’ geometries. For all these reasons, to probe the quality of the innovative electrode fabricated in the present work, a 2-planar-electrode cell was designed and fabricated with the same geometrical constraints and materials of a commercial supercapacitors.
Electrochemical test of significant specimen – Cyclic voltammetry, charge and discharge cycling, ESR, power and energy densities.
Product and technological benchmarking, definition of test systems
Lithium Ion Batteries
Analysis of the technological advances and relative market analysis for the graphene applications in lithium ion batteries were performed at Varta Micro innovation. According to project objectives the graphene materials prepared by UPVLC and LGC CNRS were tested in Swagelok test devices (half-cells).
Half-cell test devices enable the study of graphene properties in electrodes. By charge/discharge cycling of the half cells, indications of electrode material lifetime can be deduced. In the case of this test system a lithium host material electrode (graphene) is combined with a lithium counter electrode. This metallic lithium counter electrode provides the system with an almost endless lithium source. Consequently, the test system lacks the capability to accurately predict decreases in cell capacity due to losses of lithium.
Although this test system allowed the study of electrode properties, the major influence on the performance of graphene based electrodes cannot be fully described in this half-cell system. The research work in half cells revealed high irreversible capacity of graphene due to the high surface area exposed to the electrolyte as the main drawback. To study the origins of this effect and its influence on the achievable capacity of lithium ion cells, demonstrator full cells were prepared based on the graphene material prepared by UPVLC.
In order to construct these demonstrator cells, so-called “coffee bag” full cells were prepared. The preparation of electrode pastes for the negative and positive electrodes, the preparation of electrodes and the cell setup are standard procedure for battery development. The preparation of a single full test cell required large amounts of graphene; at least 500 grams are typically necessary.
Cycling studies as well as CV measurements displayed that the irreversible capacity in the first cycle (=passivation film formation) could be lowered significantly. Furthermore the additional reducing step and the annealing allowed higher reversible capacity and a significant improvement of the electrode performance. The performance of the graphene electrodes could be improved by about 100% by this additional step.
Based on these encouraging results, different reducing materials and annealing parameters were tested. The achieved results demonstrate that performance is highly dependent on the choice of reducing agent.
The investigated cells have a nominal capacity of 16 mAh based on the positive electrodes capacity.
In the case of the graphite/ LCO cell system, which is comparable to commercially used industrial cell systems, a reversible capacity of 13 mAh could be achieved. This represents around 80% of the initial nominal capacity.
In the case of the graphene/ LCO cell system a reversible capacity of 5 to 6 mAh could be achieved. This represents around 35% of the initial nominal capacity.
The measured cells reveal clearly the main difficulty graphene has to face when entering markets.
Currently no electrolytes providing high charge ionic conductivity are stable at the working potential of graphene based electrodes. That´s why the high surface area exposed to the electrolyte and the connected formation of passivation films on the surface result in a high loss of “mobile lithium”. As the amount of lithium is limited by the cathode material this results in lower achievable capacities.
The results achieved to date with the lithium battery test vehicles may look discouraging at first glance, but in fact they are in perspective quite promising. One must keep in mind that today’s commercial graphite based electrodes required decades of intensive research work to achieve the current performance levels. Graphene, while still in its industrial infancy, has already proven its ability to achieve minimal capacity loss after up to 500 charge/discharge steps. Graphene based batteries are thus expected to have very long lifetimes with high charge/discharge capacity. The remaining challenge is to improve the chemical preparation steps to achieve graphene material that does not consume lithium on contact with the electrolyte.
OLED test vehicles
Having validated the electrical and optical properties of graphene for display devices, the final step was to produce actual OLED test vehicles to demonstrate the functionality of graphene for display applications. Graphene was produced by the segregation method on 200mm silicon wafers and transferred to glass substrates. The graphene was patterned to form a first electrode, then the OLED stack was evaporated on top (consisting of p-doped layer / AlQ3 + yellow dopant / n-doped layer / Ag cathode). Initial trials showed high leakage currents (200mA) which prevented light emission. The addition of a PEDOT:PSS layer on top of graphene (the stack becomes: Graphene / PEDOT:PSS / p-doped layer / AlQ3 + yellow dopant / n-doped layer / Ag cathode) reduces the leakage current to less than 20mA and results in light emission. A total of 11 of the 16 test vehicles with this configuration showed at least partial illumination of the active display area at drive voltages between 4 and 9 volts.
More recently, graphene sheets have been proposed as a good alternative to ITO for flexible applications because their high Young modulus makes them suitable for applications where repeatable bending occurs. Their other main advantage is obviously the high conductivity even compared to nanotubes. Thus graphene sheets can theoretically fulfill the requirements for OLED applications when going flexible and/or large area illumination.
The GRENADA project has gone beyond the initial challenge of producing graphene materials with high quality by also taking into account the specific materials requirements identified by some of the first industrial applications for graphene. For example, if graphene is to replace indium tin oxide (ITO) in display applications, it will require not only excellent electrical conductivity but also high optical transparency. Progress has been made in this direction with films of graphene exhibiting sheet resistivity as low as 135 ohms per square and optical transparency exceeding 95%. Test vehicles have validated the functionality of graphene as an electrode in operating OLED devices with light emission at voltages as low as 4 volts.
Likewise, significant results have been achieved for the future use of graphene in energy storage devices such as supercapacitors and lithium batteries. Functional battery test cells were fabricated with reduced graphene oxide proving the feasibility of battery fabrication based on chemically synthesized graphene produced within the consortium. Supercapacitor test vehicles were also validated with graphene exhibiting equivalent performance to commercial supercapacitors in terms of energy density.
Potential Impact:
Today’s modern society is faced with a number of increasingly urgent societal challenges. Our fossil-fuel based transportation systems are consuming existing resources at a feverish pace. In turn, the environment is pushed to extremes as global warming continues its upward trend. Finding an alternative to carbon-based fuels continues to escalate on the priority list of actions to be engaged by public administrations. While alternatives such as electric vehicles have existed for over a century, improvements in cost, autonomy and reliability are needed before consumers are willing to make the switch away from petrol. Making the electric vehicle competitive will depend on developing new materials which provide higher battery performance at a lower cost.
Energy in all forms is a major concern of our society, from basic energy resource management to downstream environmental impact. Actions which have a positive impact on energy efficiency and longer useful lifetime of devices will always be preferred by society. In parallel, there is increased awareness that new technologies and products should avoid dependency on scarce mineral resources. Our expectation is that graphene will exactly match these expectations. Graphene will play an important role as a transparent and flexible conductive film in the future realization of handheld mobile devices (displays and touchscreens), general lighting applications based on OLED materials and eventually flexible solar cell applications. In the energy storage domain, graphene and its derivatives will outperform and replace current carbon based materials in lithium batteries and supercapacitors.
The importance of graphene implementation is expected to influence many fields. The results of the project GRENADA specifically have an impact on: Research and Development, Society and Regulation, Economy, Education and Training and Environment.
Impact on Research and Development innovation
GRENADA has contributed to the general advancement of scientific knowledge with respect to graphene. In addition to the scientific publications generated, a significant amount of know-how related to techniques, treatments and handling of graphene have been developed and disseminated within the project partners and their larger scientific communities. Some of the most significant contributions to the scientific community include:
Defect free graphene growth by CVD on copper – conditions for achieving defect free graphene on single crystal metal substrates has been demonstrated and documented for future duplication.
Large area graphene growth by the segregation method – by making use of the solubility of carbon in thin metal films, a segregation method has been developed which yields high quality films with controlled number of graphene layers. The method has been demonstrated using standard semiconductor processing equipment on 20mm wafers. The potential of this method could easily be extended to large area sheet or roll-to-roll processing for future industrial application.
Intercalation of elements between graphene and a supporting substrate – intercalation provides a means of injecting a layer between the graphene and its supporting substrate, thereby effectively dissociating the interaction between the two. This method allows producing graphene films which act as if they are suspended and exhibit intrinsic properties.
Nanomechanical characterization under liquid – techniques for characterizing graphene (or other materials) under polar and non-polar liquid environments has been demonstrated. This technique promises to provide greater understanding of the mechanical, structural or tribological modifications of materials properties brought about by immersion of graphene in liquid environments.
Socio-economic impact
The GRENADA project contributes to the realization of a robust European advanced materials community which has the potential to provide breakthroughs in new graphene based products and systems. In particular, a novel graphene based industry will open up new possibility to enhance employment with a positive resulting impact on society. The GRENADA project will contribute to the important impact graphene will have on different economic sectors such as: automotive (car assembly and components), electronics, biomedical, composite materials, and many others. The wide adoption of innovative materials and processes as well as the wide-spectrum of different disciplines converging in GRENADA will allow the improvement of technical skills of existing workers and the creation of new job careers. Therefore, the project fully complies with the European job Policy aimed at the safeguard of existing jobs and creation of new ones. In particular, the project addresses key issues affecting the competitive position and growth of the European renewable industry. This project unites many of the leading experts available in Europe to achieve the S&T objectives and the ambition of major progress in research and development, deployment and use of emerging innovations, joint mutual learning (skills and training requirements) and the foundations of the future graphene based economy. In particular, the contribution of GRENADA to the realization of energy management and storage systems will provide an outstanding social impact on the use of renewable energy and the future limitation of the use of fossil based energy.
Impact on Education
GRENADA contributes to efficient training of young scientists through many channels. In the frame of the academics (CNRS, Univ. Lancaster, Univ. Cologne and UPVLC) and organizations for R&D (CEA, CRF, VMI) this consortium gathers highly recognized senior and junior scientists, with extensive experience in training of research students in both public and industrial contexts. Several of the group members teach in renowned universities. Members from all groups take part regularly in summer schools on nanotechnology and other workshops to stimulate interest in cutting edge research. GRENADA has promoted innovation through PhD training, from basic science towards applications in the fields of nanotechnology and renewable energy.
The educational and training activities of GRENADA are focused on making communication simpler and to facilitate the diffusion of results within the various communities ranging from nanotechnology to energy. The education of young scientists on converging sciences at the intersection of various disciplines will promote the development of a new common language to allow practitioners from different science and technology fields to understand one another. The training of students on cutting edge science is of strategic importance to shape the minds of the engineers and managers of our future industries, which represent an essential element for European socio-economic growth in the coming years.
Impact on Transportation
Mobility is considered to be one of the most challenging, while at the same time one of the most promising societal challenges facing Europe and the rest of the world. Our current transportation systems rely heavily on petroleum based fuels which contribute to global warming. According to the International Energy Agency, over 50% of petroleum consumption is for transportation and three-quarters of that is used by road vehicles. At the current growth rates of consumption, the world’s generation of CO2 is expected to double by 2050. Clearly, drastic measures are needed to alter our use of fossil fuels and provide cleaner, greener transportation.
During the past decade of quite intense R&D, micro-nanotechnology contributed to enhanced transportation safety and comfort, but their main impact has been on the reduction of fuel consumption and exhaust gas emissions. The developments of new propulsion systems and the applications of nano catalysts for gasoline, diesel and natural gas systems can be considered the most relevant on energy saving and public health. All automotive manufacturers have spent considerable resources so that in spite that most car segments increased in size, weight and power, the overall fuel consumption and emissions have decreased. The use of micro nanotechnologies to enhance current mobility based on the Internal Combustion Engine will continue for several years, but we are now focused on using micro-nano technologies as the enablers of the cleanest and the most energy efficient form of mobility: the electrical vehicle.
In alignment with the European Green Cars initiative, the GRENADA consortium, oriented by the end users (CRF and Varta), contributes to the innovative scenario of graphene based energy management and storage systems serving electrical mobility. The major drawback of electrical vehicles today is the limited autonomy coupled with the high cost of the energy storage devices (generally Li-ion batteries). The challenge is therefore to improve the capacity of the storage devices while simultaneously decreasing the cost for a given energy capacity. The results of Grenada have proven that graphene is an excellent candidate for replacing many of the carbon-based materials that are used in today’s energy storage devices. Graphene has been produced in large quantities from low cost starting material using bulk chemical processing that is compatible with industrial scale. The energy storage devices produced with this graphene material have shown performance equivalent to commercial supercapacitors, with expectations for sizeable improvements still ahead. In parallel, the batteries produced show excellent cycling performance during endurance testing, although there is still room for improvement needed to achieve batteries with high initial capacity. Overall, the potential impact of graphene as a vector to achieve low cost, high capacity energy storage devices will help make electrical vehicle transportation an economically viable alternative to the combustion engine.
Impact on the Environment
The integration of graphene based materials in technologies for power management and energy storage will enable the performance improvements needed to achieve widespread consumer acceptance of the transition to electric vehicles. It is our confident expectation that the achieved results of GRENADA will have an impact on electric vehicle transportation, thereby preserving our environment by reducing greenhouse gas emissions. The shift from diesel vehicles to electrical vehicles will also have a strong positive impact on the levels of fine particulate pollution known to be a health hazard. In other industry sector applications, such as displays and touchscreens, graphene and its compounds are expected to replace the ITO as a transparent electrode. Some analysts predict a worldwide shortage of indium in coming years because of the ever-increasing need for transparent conductors. The continuation of work based on the results of Grenada will help graphene make a lasting impact on our environment.
List of Websites:
www.fp7grenada.eu
All contact information for the partner organizations and participants can be found on the website.
Coordinator contact information:
David Holden
CEA Leti
17 rue des Martyrs
38054 Grenoble Cedex 9
France
david.holden@cea.fr
The discovery of new structures in carbon materials is giving way to vast opportunities for new scientific knowledge and emerging industrial applications. As we celebrate the ten year anniversary of the discovery of graphene, the number of scientific publications and related press articles exposing the potential of graphene continues to grow exponentially. In the history of our collective knowledge on materials science, no other material has received such intense scientific attention in so little time.
The GRENADA project set out to explore the inputs and conditions that influence the final properties of graphene with the objective to make it fit for purpose. The two-dimensional structure of graphene is responsible for many of its unique properties, in particular the excellent thermal and electrical properties. Paradoxically, it is also this same two-dimensional structure which makes graphene highly sensitive to minor changes in its surroundings. By gaining a thorough understanding of the synthesis conditions that produce exceptional graphene, as well as the external factors that can subsequently alter or degrade such properties, the path to industrial implementation will be traced.
Results obtained during the GRENADA project demonstrate the potential of graphene to compete with and exceed the performance of many existing materials in industrially relevant applications. For example, for graphene to replace indium tin oxide (ITO) in display applications, both excellent electrical conductivity and high optical transparency are required. Progress has been made in this direction with films of graphene exhibiting sheet resistivity as low as 135 ohms per square and optical transparency exceeding 95%, in line with application requirements. More importantly, display test vehicles have validated the functionality of graphene as an electrode in operating OLED devices with light emission at voltages as low as 4 volts.
In parallel, significant results have been achieved for the future use of graphene in energy storage devices such as supercapacitors and lithium batteries. Functional battery test cells were fabricated with reduced graphene oxide proving the feasibility of battery fabrication based on chemically synthesized graphene. High capacity supercapacitor test vehicles were also validated with graphene exhibiting energy density performance equivalent to commercial supercapacitors in side-by-side evaluations.
Nevertheless, integrating today’s graphene into industrial applications is not as easy as a “drop-in” replacement. There are still significant challenges to overcome before the materials properties of graphene obtained in a laboratory setting can be regularly repeated in a large scale industrial setting. The loss of fidelity of the material can be explained on the one hand by the difficulties to synthesize on a large scale, as well as by the susceptibility of the material to modification upon interaction with its surrounding environment. We have made significant progress on these issues and now see that graphene is slowly but surely making its way into the marketplace.
Project Context and Objectives:
Project Context
Graphene is a two dimensional, one-atom thick sheet of carbon arranged in a hexagonal network. Owing to its exceptional properties, graphene has led to the emergence of an interesting bridge between condensed-matter physics and quantum field theory, since it allows the study of relativistic charge carrier effects in relatively simple, lab experiments. The measured performance of graphene in the laboratory is exceptional, including remarkable values of electron mobility, transparency, Young’s modulus, thermal conductivity and a high specific surface to mass ratio. The combination of such unique properties in graphene offer exciting prospects for graphene to replace existing materials in many industrial applications.
Industry analysts predict the market for graphene materials will be in the range of 100-200M€ in the 2020-2024 time period. While this base materials market remains somewhat modest, the products that graphene will enable are of far greater importance. Flexible displays will provide an entirely new generation of wearable devices. Conductive coatings and printable inks containing graphene will help propel the emergence of 3D printing to conquer new markets. Energy storage devices will likewise see the replacement of traditional carbon materials by graphene to achieve longer lifetime and higher charge capacity.
Despite the exceptional promise offered by graphene as a “game changer”, there are still many challenges to overcome to allow graphene to find widespread market introduction. For example, minor variations in synthesis conditions can drastically modify the materials properties of the resulting graphene, quickly shifting from a regime of excellent material to one of useless rubbish. In a similar fashion, graphene properties are largely influenced by their surroundings, including contact with other solid state materials, as well as with gaseous or liquid environments. Consequently, the main challenge of industrial implementation of graphene is to find the appropriate combination of synthesis conditions, coupled with scalable production processes and a stable environment for the long term viability of graphene.
General Objectives
The work program of the GRENADA project was designed to address the major stumbling blocks currently preventing large-scale implementation of graphene in industry. In order to fully understand and exploit the potential properties of graphene, a comprehensive study of the various synthesis methods and the resulting material properties was needed. The expectation was to provide a correlation between synthesis conditions and final materials properties for several parameters, including number of graphene layers, crystalline structure of the graphene, electrical, optical and mechanical properties.
A careful study of defects in graphene and the effect of defects on materials properties forms an integral part of the Grenada project. While many defects are a result of the synthesis conditions of graphene, other defects can develop over time as a result of external factors. Defects can also be intentionally induced in defect-free graphene to provide specific, localized defects that can be studied individually to measure the effects on performance. The final output expected is a comprehensive overview of the effects of defects in graphene on performance.
Modifying graphene synthesis conditions to optimize for a particular material property can have a negative influence on other material properties. By probing the existing parameter space, a knowledge base could be developed which would effectively allow one to choose the best synthesis techniques and “engineer” the desired graphene material properties to fit specific application requirements.
While optimum materials properties are important, the focus is to provide processes and methods which are amenable to mass manufacturing. For instance, while ultra-high vacuum CVD is a synthesis method which can provide graphene with exceptional materials properties, the technique is comparatively expensive and difficult to control in high throughput production environments. Alternatively, atmospheric pressure CVD is better suited to mass production and has been a focus of study within Grenada. The balance between achieving optimal materials properties and at the same time a scalable manufacturing process is core to the objectives of the project.
Reliable characterization methods are vital for fast and effective feedback and control of process development in graphene growth. They are equally essential for elucidating the mechanisms linking growth conditions and resulting graphene morphology to materials performance. Highly advanced measurement capabilities and methods can provide a better understanding of the local properties of graphene, particularly as a function of the surrounding environment. New experimental set-ups coupled with advanced test methodologies are needed to understand the fundamental mechanisms of graphene interaction at the atomic scale. Nanoscale probing of the materials can surely bring new information and knowledge to eventually quantify and predict materials behavior at a system level.
Local environment has a significant effect on the mobility of carriers, adhesion to the substrate and chemical stability of graphene. The nature of the local environment (e.g. polarity of media in immediate contact with graphene) can enhance or reduce effects of the defects on material properties. Such knowledge is absolutely essential for understanding and predicting graphene behavior in real life applications. This objective addresses exploration and modeling of fundamental properties of graphene, and its performance in applications including graphene in contact with polymers and OLED materials, as well as graphene in ionic, polar and non-polar liquids in super-capacitors and batteries.
The final objective will be to bring together all of the knowledge that has been learned from the development of high quality graphene (with minimized defects), the investigation of graphene / environment interaction and the tailoring of graphene properties to fit the specifications. The combined know-how will be applied in test vehicles which are designed to validate the final performance of graphene in applications. The results provide a direct indication of the potential impact of graphene on the performance of actual end-products. The target applications for graphene as a key material are for energy storage in supercapacitors and batteries (demanding materials with superior electrical properties in a hostile, lithium ion environment) and for display applications (requiring optical transparency and high conductivity).
Energy Storage (Accumulators, including supercapacitors and batteries)
Today’s modern society is mobile, connected and “constantly looking for a recharge”. As the mobile energy needs of our society continue to grow, energy storage devices will require higher energy density, faster charge/discharge capability and high reliability in order to bring sufficient value to consumers. In parallel to the straightforward need for increased primary energy storage capacity of storage devices, there is also a need to efficiently manage power demand fluctuations. An acknowledged approach to serve such fluctuations is to couple large capacity accumulators, such as Li-ion batteries, with high power capacitors which provide high energy levels over short periods of time. By using supercapacitors to provide high surges of power, research has already shown that the talk time of a cell phone can be increased by a factor of four and the total battery life can be doubled. An excellent example of such a hybrid system, which today combines high power with high energy and tie a supercapacitor in parallel with a battery, are implantable defibrillators. These systems use low-power batteries to charge conventional capacitors that provide the required power pulse.
Supercapacitors coupled with batteries can be used to manage peak power in electrical vehicles, thereby decreasing the overall demand on the batteries and improving battery lifetime. Stationary power systems providing emergency power back-up will also benefit from the coupling of batteries with supercapacitors, especially during start-up conditions. Although super- and ultra-capacitors are a potential long-term competitor to batteries, they offer the short term possibility of enhancing battery performance by extending battery life and reducing battery weight.
The capacitors (supercapacitors and ultracapacitors), rely on polarization effects at the electrodes/electrolyte interface while batteries exploit red-ox reactions which take place at their electrodes/electrolyte interface. From this statement, it follows that to improve the energy accumulation capacity of batteries as well as to increase the supercapacitor power, the electrode/electrolyte interfaces are of crucial importance. Graphene being an extremely good conductor exhibiting a large surface to volume ratio is one of the most promising nanostructured materials to be investigated as an electrode to improve the energy density and the power of these devices.
A capacitor, like a battery, is an electrical energy storage device. Whereas a battery stores energy in chemical form, a capacitor stores electrostatic energy as charge separation. The term supercapacitor usually describes a device that uses the charge separation at an electrode-electrolyte interface—that is, an electrochemical capacitor. An electrochemical device requires at least two electrodes, so a supercapacitor is actually two capacitors in series. Whereas supercapacitors store significantly less energy than batteries store—one normally expresses their capacity in ampere-seconds rather than ampere-hours—they benefit from extremely fast charge and discharge capabilities and longer life cycles. In Japan supercapacitors are already a $100 million dollar-a-year business, finding widespread application in short-term memory retention for electronic devices. Growing applications for supercapacitors include defibrillators, pulse power units for vehicle propulsion, power stabilizers for computers, handheld tools, memories, camcorders, and backup devices on computers and plant production lines.
OLED Displays
Nearly all display technologies have one thing in common, they require at least one conductive transparent electrode in order to allow light to pass in the direction of the user. Nearly all displays manufactured today use Indium Tin Oxide (ITO) as the transparent conductive material. Graphene is a prime candidate to replace ITO in displays, especially for flexible displays.
Today, liquid crystal displays (LCD) dominate the market, while OLED technology is expected to gradually replace LCD in applications such as televisions, laptop computer screens, tablets and mobile phones. According to market forecasts, in the short run active matrix screens (AMOLED) of mobile phones and tablets should become the main applications of the OLED market for displays. The high color saturation, high resolution, low power consumption and the very high contrast inherent to the emitting aspect of the OLED make it a technology of choice for future applications. For strategic reasons, the testing of graphene compatibility with OLED displays was preferred over LCD based on potential impact for Europe.
Light-emitting organics (OLED) provide a low cost, high reliability process over large areas on many kinds of substrates including flexible plastic substrates. Their self-emissive and wide viewing angle properties offer advantages over the current widely used liquid crystals in many display applications. Particularly in the automotive display applications, polymer OLEDs could offer price and performance advantages over vacuum florescence display (VFD) and liquid crystal displays (LCD).
Beyond these first applications, many similar industrial applications (solar cells, touch screens, discrete electronic components, etc.) could benefit from the experience gained with energy storage and displays. Determining the best conditions for production and performance of graphene opens many new perspectives and provides a step change in these technological areas.
The Consortium
In order to achieve the objectives of GRENADA, the consortium required a broad range of competencies, from pure academic theory all the way to product-oriented industrial testing. The final configuration of the consortium, led by the CEA (France), includes the Institut Néel CNRS (France), LGC-CNRS (France), University of Cologne (Germany), Lancaster University (UK), Polytechnic University of Valencia (Spain), CRF (Italy), Varta Microbattery (Germany) and Varta Micro Innovation (Austria). The capabilities of the partners are highly complementary while avoiding duplication of efforts. The inclusion of some of Europe’s most active industrial developers in energy storage provides an excellent outlet for exploitation of the results of the project.
Project Results:
Graphene synthesis has been the subject of numerous scientific studies in recent years, and Grenada in this respect is no exception. The most widely studied methods in literature are exfoliation and chemical vapor deposition (CVD), both of which are capable of yielding high quality graphene which can potentially be transferred to any type of substrate. The high potential of CVD has been extensively investigated during the GRENADA project. Several CVD techniques, employing a wide variety of metallic substrates, have provided positive results in terms of graphene growth with high quality. For example, under UHV, large-area, ultra-high quality graphene has been achieved. On Ir(111), Co(0001) and Re(0001) thin films, purely single-layer graphene films having single crystallographic orientation have been prepared (Institut Néel CNRS). This was carefully validated in a number of in situ experiments using various techniques: X-ray diffraction at synchrotron radiation facilities, electron diffraction, low-energy electron microscopy, and scanning tunneling microscopy. The growth mechanisms on Co and Re were unknown so far, and were addressed with care. The interplay between surface carbide, carbon-in-the-bulk, and graphene phases was studied. On Co(0001) it was found that growth temperatures as low as 600°C yielded extremely high quality graphene. Low- and ambient-pressure CVD produced graphene islands, single-layer graphene, multi-layer graphene, and nanostructured interconnected few-layer graphene (LGC CNRS). In parallel, very large-area growth (fully covered 200 mm wafers) of few layer graphene, using carbon segregation, has been demonstrated (CEA). The growth process is performed inside industrial production equipment compatible with CMOS technologies at relatively low temperatures (600°C-800°C) with high throughput (2-15 min annealing). The metal catalysts used are polycrystalline metal thin films that have the advantage of being low-cost as compared to monocrystalline metal films.
Purely single layer graphene was demonstrated for the first time and fully characterized with a comprehensive set of techniques (Institut Néel CNRS), and the partial pressure, nature of the carbon precursor gas, sample temperature and chemical composition of the metallic substrate, were found crucial in the view of controlling the number and of graphene layers and their quality (LGC CNRS, CEA).
Graphene synthesis using APCVD as an approach to industrial processes
Lessons learned from high vacuum CVD of graphene have been applied to atmospheric pressure CVD (APCVD), a production technique which is more conducive to large-scale production. At LGC CNRS, experiments have been performed on Cu foil (2 cm2) at 700 Torr, using very highly diluted methane in argon and hydrogen. The influences of numerous synthesis parameters (pre-treatment and synthesis durations, methane and hydrogen partial pressures, position of the substrate into the reactor,) have been studied and optimized.
The use of very low methane partial pressure allows obtaining graphene without D band on the Raman spectra, i.e. of high crystalline quality. A decrease of the methane pressure (30 to 15 mTorr) allows decreasing the number of layers down to the monolayer, probably thanks to a lower amount of reactive carbon species near the copper surface. However, for these conditions, graphene is not continuous, even if the surface coverage of the copper surface is very high. First results about an increase of the synthesis duration at 15 mTorr of methane have shown that graphene is still not fully continuous and is heterogeneous (monolayers and multilayers areas), showing that even at these low methane pressures, the formation of graphene is not self-limiting. This result could indicate that even when the catalytic copper surface cannot be active anymore because it is practically fully covered by graphene, reactive carbon species are still created to form additional graphene layers. This could be explained by the fact that graphene is formed from two kinds of unsaturated reactive species, the first one born near the copper surface from catalytic phenomena, and the second one in the gas phase from methane pyrolysis, as deduced from modeling analysis in LGC-CNRS. Experiments have also shown that a decrease of the H2/CH4 ratio for the conditions tested leads to an increase of the number of layers and to heterogeneous zones of multi-layers graphene. This confirms the positive role of hydrogen as an etching agent of the graphene layer in formation.
Further work has been performed in order to understand the reason for the only partial coverage of the Cu foils after growth, observed even for such large growth times as 90 min. The role of hydrogen, a well-known etching agent for graphene, was ruled out by progressively decreasing its concentration in the gas mixture used during growth. A two-step process was then developed, in which the methane concentration is changed from low to high at the first and second steps respectively. This process yields very high quality purely single layer graphene which is however not fully covering the surface of the Cu foil. A three-step process eventually was developed, which allows reaching full coverage to the expense of a lesser control over the number of layers, mostly one and two.
LGC CNRS also has worked on lower temperature processes for growing graphene on Cu foils, by using a more reactive carbon precursor than methane, ethylene. Growth at various temperatures (750, 800, 900°C) and H2 pressures yields graphene whose defect density and number of layers are ill-defined.
Graphene synthesis on large surface areas
An alternative to graphene growth from carbon in the gaseous phase is to utilize a solid carbon source. A thin film of amorphous silicon carbide combined with a thin film of metal catalyst can produce well-controlled graphene films, as demonstrated by work at CEA. At temperatures between 600-800°C, carbon will diffuse into the metal catalyst film and subsequently precipitate at the surface of the metal upon cooling. The thickness (number of layers) of graphene depends on the thickness of the carbon source, the solubility of carbon in the metal and the time/temperature of the thermal anneal step. The entire surface of 200mm silicon wafers have been coated with high quality graphene using this method.
For applications in energy storage, large quantities of graphene will be required. Most planar substrates will not provide sufficient quantities of graphene at a reasonable cost for large-scale production. An alternative is to synthesize graphene on substrates with large surface area, such as metallic foams. Foams made of nickel have been introduced into the APCVD process machine at CNRS LGC. After some work to improve graphene growth on the surface of Ni foams, LGC CNRS achieved final graphene weight percent of 15.5%, better than published in the literature for thick graphene. It was found that the mechanisms proposed in the literature cannot provide an explanation to these values. Further investigations are in progress.
Defects in Graphene
At University of Cologne, in order to study the influence of structural imperfections on the properties of graphene, defects were purposefully created in epitaxial graphene on Ir (111) using ion bombardment in the keV-range. It was found that grazing incident Xe ions penetrate graphene and are channeled in between the carbon sheet and its substrate. Chains of vacancy clusters in graphene result, with their edges bend down to the substrate to saturate their dangling bonds. These chains arise as the ions are repeatedly reflected at the metal surface and the bottom side of the graphene layer. The impacts on the graphene result in carbon sputtering, causing a track of holes in the graphene layer. The guided motion of the ion in the interface region is analogous to subsurface channeling observed earlier for the case of pristine crystal surfaces.
An irradiated sample was gradually heated in steps of 150 K (30 s at each temperature). From the resulting topographies, the number of defect patterns as well as the fraction of defect patterns accompanied by protrusions were quantitatively estimated as a function of annealing temperature.
The holes in the graphene sheet remain present at 850 K. Their edges appear not very sharp in the STM, the holes rather have the shape of funnels reaching down towards the substrate. As in previous publications the presence of unsaturated carbon bonds was verified by the detection of a peak in the density of states at the Fermi energy, an effect which was sought for in scanning tunneling spectroscopy experiment on the funnel-shaped holes. No peak close to 0 V could however be detected, proving the absence of dangling bonds in the system, in contrast to previous experiments for graphene/Pt(111). It can thus be assumed that the carbon atoms at the edge of the hole are bound to the metal substrate, as it is also shown in accompanying density functional theory calculations. The holes start to disappear just around 970 K, which resembles the temperature of the transition from amorphous carbon to graphene. This is due to the onset of the diffusion vacancies and small vacancy clusters. At the highest investigated temperatures the graphene layer appears to be entirely rebuilt on a large scale. However, some well-defined point defects persist.
Graphene synthesis by wet processes
Nanostructured graphene was also prepared by a new wet process (UPVLC), from graphite powders, following a two-step procedure consisting in oxidation according to a modified Hummers method, and then a fast reduction step using an environmentally friendly reducing agent (ascorbic acid). Functionalization with PSS allowed efficient dispersion of reduced graphene oxide in water solution (Fig. 3). The formation of graphene oxide and reduced graphene oxide was confirmed by UV-visible spectroscopy, transmission electron microscopy, and Raman spectroscopy among other techniques. Reduced graphene oxide is found to have a low density of defects, as compared to that reported in the literature for graphene prepared by wet methods, most oxygen-containing groups being removed, but still comprising a certain density of vacancies and showing the presence of few-layer stacked fractions. Unfortunately, modification to the fabrication process has contrary effects in both issues (defects and stacked layers), as increasing the degree of exfoliation increases also the quantity of defects.
Given the strong tendency to aggregation of the graphene oxide (GO) sheets, covalent functionalization on the surface of GO can be pursued in order to achieve either i) homogeneous dispersion or ii) efficient interfacial interactions between the GO and the substrate of choice. Two approaches were followed depending on the targeted functional group developed on the GO after oxidation, i.e. hydroxyl (-OH) and epoxy (C-O-C) groups on the basal plane or carboxylic groups (–COOH) present on the edge of GO sheets. In relation to the former, silanes are used, as these compounds are well known coupling agents usually applied to the surface modification of nanofillers in the presence of catalyst. Chemically functionalized GO sheets by 3-aminopropyltriethoxysilane (APTS) are reported. Secondly, the presence of abundant carboxylic groups on graphene oxide (GO) edges was exploited to covalently bound EDC–NHS (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride–N-hydroxysuccinimide) resulting in the formation of an active ester intermediate.
The successful attachment of the functionalization groups by the two approaches was confirmed by nuclear magnetic resonance (NMR), UV-Vis and Raman spectroscopies and visualized by transmission electron microscopy (NMR). Further on, after functionalisation, the successful reduction of these new materials was confirmed by UV-Vis spectroscopy.
The fabrication of graphene by the wet chemical route was continuously improved based on feedback from partners developing energy storage applications. After several iterations, graphene synthesis conditions settled on a three step method: first oxidation of graphite to obtain graphene oxide (GO); second, an initial reduction step to obtain reduced graphene oxide (rGO); third, further reduction step by heat treatment in reducing atmosphere
While most “traditional” graphene synthesis methods require very high temperatures, often coupled with ultra-high vacuum environments, we now can describe novel techniques which provide graphene at lower temperatures and which are considered to be in line with standard industrial processing. To summarize the progress, major accomplishments of the project with respect to graphene synthesis include:
- Validation of growth on platinum, nickel, copper, iridium, cobalt and rhenium metal films.
- High quality graphene synthesis with electrical mobility as high as 5000 cm2/(Vs).
- Validation of growth on very large sized substrates (up to 200mm diameter).
- Growth on large sized substrates at temperatures as low as 600°C.
- Growth on nanostructured substrates with control over the number of defects (including the possibility to intentionally induce defects with a pre-defined periodicity).
- Wet chemical synthesis of dispersed graphene oxide in water using an optimized three-step process capable of producing large batch sizes.
Atomic scale characterisation of as-grown graphene
Furthermore, defects have been studied in epitaxial graphene on a metal, Ir, using X-ray diffraction at a synchrotron source (ESRF, Grenoble). It has been shown that tiny imperfections are still present in the highest quality graphene samples. These imperfections were not evidenced before; they include small shears, strains, and misorientations, in the form of domains (see Figure for schematic models for the imperfections).
We have analysed the formation of strains in epitaxial graphene, in real time, during growth, ion bombardment and oxygen etching, with the help of a standard widespread in situ technique, reflection high-energy electron diffraction. We found substantial strains developing in graphene which are ascribed to defects, including carbon vacancies and ions trapped between graphene and its substrate, and epitaxial stress. Strains are found to be adjustable from a tenth to as much as 2.2%. Such strains can be canceled by removing the defects. The weak interaction of graphene with metals such as Ir, Cu or Pt also has important consequences regarding the structure of graphene. Indeed, graphene domains with different stacking (twins) with respect to the substrate tend to form on these metals, presumably because the formation of each of the domain involves similar energetic costs. At the boundary between these domains, dislocations (heptagon pentagon pairs) are found. Such structural defects hinder the improvement of graphene performances, e.g. for electronic transport, heat conduction, or mechanical resistance, towards state-of-the-art ones obtained for exfoliated, suspended graphene. Recently, strategies have been developed in order to avoid the formation of grain boundaries, in graphene having single crystalline orientation at the scale of a centimeter. We investigated with high resolution the structure of graphene on iridium, which is a model system for graphene weakly interacting with a transition metal substrate. Even the highest quality epitaxial graphene displays tiny imperfections, i.e. small strains, ~0.3%, rotations, ~0.5°, and shears over distances of ~100 nm, and is found incommensurate, as revealed by X-ray diffraction and scanning tunneling microscopy. These structural variations are mostly induced by the increase of thermal mismatch between the preparation temperature and the actual sample temperature. Although graphene weakly interacts with iridium, its thermal expansion is found positive, contrary to free standing graphene. The structure of graphene and its variations are very sensitive to the preparation conditions. All these effects are consistent with initial growth and subsequent pinning of graphene at steps. These structural imperfections may influence the graphene properties.
Free-suspended Graphene
Free-suspended graphene films were synthesized via carbon diffusion through Pt based compounds and imaged using SEM confirming that the graphene of the full stack Si/SiO2/Pt+Pt3Si/graphene sample is suspended. Sample growth occurs in a number of stages, determined by substrate temperature during annealing. Of particular interest are: i) secondary stage, mirror-like regions corresponding to graphene on almost flat metal/metal silicide and; ii) tertiary stage frosted-like green region where SEM images appear to show large areas of graphene suspended between islands of Pt+Pt3Si.
We observe a clear progression in the degree of heterogeneity in the surface morphology with height of surface features increasing significantly from primary to secondary and tertiary stages corresponding, initially, to graphene “nano-domes” formation followed by the Pt3Si island growth. Progression of surface heterogeneity corresponds to increased graphene film decoupling that is revealed in AFM images. It is reasonable to expect that the topographical features are strain induced and arise during the cooling phase of the growth due to the radically different coefficients of thermal expansion (CTE) of the substrate material (CTEPt = 9 x 10-6 K-1) and graphene film (CTEGr = -8 x 10-6 K-1) [29, 30] The primary growth stage, shows conformal graphene film growth with good mechanical contact with the substrate signified by dominating high brightness (i.e. high stiffness) areas in the UFM analysis. AFM contact mode friction contrast also showed little variation suggesting continuous graphene coverage. A mechanical decoupling in the primary stage is mainly observed along the underlying substrate grain boundaries of the polycrystalline platinum substrate and revealed as darker lines in UFM images. Secondary growth stage, occurring at higher substrate temperature during the annealing phase, displays the formation of clearly decoupled bulged regions (“nano-domes”) in the AFM topography and drastically reduced stiffness (dark contrast) zones in UFM images. The location of nano-domes points out to their formation at the intersections of the substrate domain boundaries. These secondary growth stage nano-domes have a typical height of 10 - 35 nm and a width of 100 – 600 nm
In order to investigate the morphology of the graphene layer, in particular, the reason for separation of the graphene layer from the substrate and the properties of the substrate under the nano-domes, we performed UFM scans with increasing normal force over a typical 2 µm x 2 µm nano-dome containing region of the secondary growth stage. Due to practically complete reduction of lateral forces in UFM mode, a known extra bonus of UFM, application of moderate normal force of up to 100-200 nN does not modify the AFM tip or the sample, even on the stiff substrates. We observed that secondary stage 150-200 nm wide nano-domes were fully collapsing at the application of moderate forces around 51±5 nN by the AFM tip. Surprisingly, such collapse rather than resulting in a flat surface, revealed the cavity below each dome that shows the same area post-high force UFM scanning imaged again at standard low normal force. Dome heights were in the range of 10 - 35 nm and post-compression cavity depths are found to be also comparable in the range of 14 - 38 nm. This value of inversion force was also comparable to that seen for the domes in tertiary growth stages, suggesting that the areas on top of the Pt/Pt3Si islands are comparable to those in the secondary phase. Additionally, we observe that the post-inversion UFM response of the nano-domes now shows a high stiffness response suggesting a high degree of mechanical support – evidently, the previously unsupported graphene is now sat in a good mechanical contact with the bottom of the cavity.
Underliquid nanoscale thermal mapping - immersion SThM
Nanoscale heat transport is of increasing importance as it often defines performance of modern processors and thermoelectric nanomaterials, and affects functioning of chemical and biosensors. Scanning Thermal Microscopy (SThM) is the leading tool for nanoscale mapping of thermal properties, but it is often negatively affected by unstable tip-surface thermal contacts. While operating SThM in liquid environment may allow unimpeded thermal contact and open new application areas, it was so far regarded impossible due to increased heat dissipation into liquid, and perceived non-local thermal interaction between the probe and the sample. Nevertheless, we show for the first time that such liquid immersion SThM (iSThM) is fully feasible and has sufficient thermal contrast to detect thermal conductivity variations in high thermal conductivity materials such as graphene.
After testing the system on metal-polymer nanostructured interconnects, we have measured heat transport in the few tens of nm thick graphite nanoflake and iSThM’s spatial resolution was on the order of 50 nm, very similar to the resolution of the same probe in the standard air environment. These results confirm localised thermal sensing in iSThM and, coupled with absence of tip snap-in due to elimination of capillary forces, suggest the possibility for true non-contact nanoscale thermal mapping in liquids, including thermal phenomena in energy storage devices, catalysts and biosystems.
Characterization of Graphene for OLED displays
Graphene has been proposed as a transparent conductor for display applications, potentially replacing Indium Tin Oxide. In such applications, in particular for OLED displays, requirements dictate that transparent conductive films have high transmittance, low resistivity and a work function similar to ITO.
The optical transmittance of graphene transferred on glass substrates was measured from 280 to 1400 nm with a Cary 500 UV–VIS–NIR spectrophotometer. For resistivity measurements, graphene was transferred to a silicon dioxide substrate, followed by evaporation of metallic contact pads (800 um size, 200 um interdistance) of nickel (20 nm thickness) and gold (100 nm) on top of the graphene. Sheet resistance was then measured with a 4-point probe. During the course of the project, the resistivity obtained for transferred graphene has been improved to 135 Ohms/square (single transfer of graphene) and transparencies from 70% to 97% depending on number of layers.
In parallel, the work function (WF) of graphene grown at CEA on 8 inch wafers and transferred onto glass has been measured with Kelvin Force Microscopy.
In order to calibrate the quality of the tip, measurements were completed on Ru and Al samples. For measurements on graphene, the difference of potential measured between the tip and sample is about 0.13 eV when the tip is close to the sample (local WF) and 0.19eV when the tip is far from the sample (corresponding to a more average value of WF since the tip senses a larger area of the sample). The tip is made of Pt-Ir with a WF in the range of 5.0 to 5.3 eV. As there is no technique to measure exactly the WF of the tip, the range of 5.0 to 5.3 eV is used as the baseline reference. Thus the obtained WF values of our graphene on glass are in the range of 4.87 to 5. These values are similar to those of ITO that graphene is intended to replace. The WF on the sample is quite uniform with a variation of. The spatial variation of WF on the sample (± 30meV) is minimal. Thus the graphene work function is well suited to replace ITO without modification of the WF of the OLED stack materials.
Characterization of Graphene for Lithium Batteries
All basic electrochemical studies of graphene materials in lithium ion batteries were performed in half-cell measurements. These measurements allow the electrochemical investigation of processes occurring at the graphene based electrodes against a metallic lithium counter electrode. Although the half-cell measurement setup allows studying effects occurring at the graphene based electrode it actually does not represent the behavior of these electrodes in full cells. This is mainly caused by the almost endless lithium reservoir at the metallic lithium counter electrode, while in commercial lithium ion cells the lithium contributing to the cells capacity is limited by the amount of lithium incorporated in the positive electrode material. In other words, the crucial effect on the cell´s performance by irreversible losses of lithium due to SEI formation cannot correctly be observed in half cells against a metallic lithium electrode. For this reason, full cells containing graphene based negative electrodes and commercial positive electrodes were fabricated and electrochemically investigated. This demonstrator like cell setup allowed studying graphene materials in a configuration close to the actual commercial application.
Cyclic voltammetry is an effective method to characterize electrochemical phenomena at an electrode. A potential is applied between the reference and working electrode and this potential is changed linearly over the time, described as scan rate (V/s). Due to the applied potential change, a potential-depended current flow is generated between the working and the counter-electrode. This method is especially interesting to study electrolyte/electrode phenomena and to study the formation of passivation films on graphene based negative electrodes. Furthermore the method is capable of describing electrode kinetics and the reversibility of the electrode reactions. The usual measurement parameters for characterization are a scan rate of 30µV/s with potential limits of 0v and 1.5V.
Charge/discharge experiments are performed to estimate the capacity of the electrode (for half cells of graphene vs. lithium metallic electrode) or the cell stack (for full cells of graphene vs. Lithiumcobaltoxide positive electrode). This method uses a defined galvanostatic step combined with a constant voltage step to lithiate the graphene negative electrode, which is followed by a galvanostatic delithiation process to delithiate the material. For the galvanostatic step the charger varies the voltage applied to the battery to maintain a constant current flow (C-rate), switching off when the voltage reaches a defined cut off potential. During the additional constant voltage step the cell is kept at the cut off potential and further lithiation of the graphene is enabled independent from overpotentials. The C-rate can be described as the current needed to fully charge/discharge a cell in a certain time. If a C-rate of 1 is applied, the cell is totally charged or discharged within 60 minutes, if a C-rate of 0.5 is applied the charge/discharge time is 120 minutes.
This method allows measuring the End-of-charge voltage, the End-of-discharge voltage and the capacity of the electrode (for half cells) or cell (for full cells) respectively. These values are strongly dependent on the applied rate during the charge and discharge process. In order to study this influence the C-rate was varied in the above described procedures.
Characterisation of lithiated graphene layers
Lithiated graphene samples where the measurements are sensitive to changes corresponding to outer most graphene layers. Here we studied the surface and immediate sub-surface morphology of the sample to understand the effects of lithiation – a vital process for the development of rechargeable batteries – on the sample.
It is possible to lithiate the samples to a certain potential vs. Li/Li+ reference electrode for example. We observe a clear change in morphology (both AFM and UFM) post-lithiation of the samples. The most striking changes occur between the non-lithiated and lithiated samples and between the 1st lithiation step and 5th full lithiation steps. Globular surface features would appear to correspond to subsurface morphology changes arising from lithium ion intercalation, although the rough nature of the surface topography means that quantitative analysis was not fully indicative.
Characterization of Graphene for Supercapacitors
The proliferation of electronics and networked systems creates increasing demand for energy availability and storage. Batteries, with their large energy densities are the most common applied solution to this challenge, albeit their low power densities. In recent times, supercapacitors are thought to be among the list of promising candidates to replace batteries for these applications, attributing to their high power densities. Supercapacitors can store energy through 1) electrochemical double layer capacitance, which is the accumulation of electrostatic charges on the electrodes, and 2) pseudocapacitance, which involves faradic reactions. In comparison to batteries, the supercapacitor modus operandi offers relatively higher power densities, fast charging rates and theoretically unlimited cyclability. However, the main drawback of the commercially available supercapacitors is their limited energy densities in comparison to batteries. Therefore, increasing the energy density of a supercapacitor is one of the main goals of research today.
With the discovery of novel advanced materials, the energy density of a supercapacitor can be increased to a level comparable to batteries. The two main components of a supercapacitor cell consist of two electrodes and the electrolyte. The cathode and anode electrodes are current collectors coated with materials that possess high specific surface area. The purpose of using materials with high surface area is to provide the electrode with more active sites for the storage of charge compounds such as the ions of the electrolyte, thereby enhancing the capacitance of the supercapacitor cell. To date, the electrodes of supercapacitors available commercially are mainly carbon based, with activated carbon as the most widely used due to its high specific surface area (larger than 2000 m²/g), low cost and long cycling life. However the complex pore structures, with irregular pore sizes, bring about an increase in the electrode resistance and reduced capacitance due to the inaccessibility of the electrolyte ions into its micro-pores. A promising candidate to replace activated carbon as an electrode material is graphene. The target properties of graphene are low electrical resistivity and large degree of exfoliation in order to make the material present a large specific surface to a liquid electrolyte. The same features are addressed when embedding the graphene flakes in a binding polymer typically used to prepare a solid electrode. Graphene is expected to provide a larger specific surface, 2630 m²/g when all its carbon atoms are exposed to the electrolyte, as well as high electrical conductivity attributed to the π-conjugated network of sp² hybridized carbons. Apart from electronic transport properties, graphene can be structured on the current collectors to provide paths with higher accessibility for the electrolyte ions. A suitable way to probe, indirectly, the mentioned graphene-based-electrode properties is cyclic voltammetry. By means of this technique, both the specific capacitance and the equivalent series resistance give an indication of the performance of storing and delivering energy to the system.
Cyclic Voltametry applied to supercapacitors
The electrodes were evaluated with both cyclic voltammetry (CV) and constant-current charge/discharge (CC) measurements using an AMEL 500 potentiostat. The CV and CC curves resulting from the measurements were then used to calculate the electrode specific capacitance. In the case of CV measurements, the specific capacitance was obtained by integrating the curves over a full cycle; while for CC measurements, the specific capacitance was calculated from the slopes of its resulting curves using the following equations:
C=I/(dV/dt) (1)
C_sp=4×C/m (2)
With C as the capacitance as an absolute value and m the total mass of the cell.
In equation (2), a factor of 4 was included in the equation to adjust the cell’s capacitance to the electrodes specific one.
The table below summarizes the parameters of major importance when a supercapacitor has to be employed in a specific circuitry to serve an end-use application. Most of the automotive applications: moving a mirror, heating a seat, starting and stopping a car’s engine need large power and energy density, high charge and discharge rates, and the smallest possible equivalent series resistance (ESR). In this table the properties of 3 exemplary specimens are reported to show the differences between the two testing set ups used for the purpose: 3-electrode configuration and a 2-planar electrochemical cell. As one can note sample 1 exhibits a specific capacitance larger than the commercial activated carbon electrode, up to almost 250% larger by the 3-electrode setup measurement, and up to the 75% by the 2-planar-electrode testing cell. As acknowledged, the 3-electrode methodology presents significant limits, with respect the 2-planar-electrode one, to predict how a material would perform if employed in the real device. By a 3-electrode cell only one electrode, called working electrode, containing the material under analysis is used, meanwhile, the 2-planar-electrode cell employs two of them, thus mimicking the real capacitor conditions. Nevertheless, the applied voltage and charge transfer are substantially different between the two setups. In a symmetrical 2-planar-electrode cell the potentials applied to each electrode is the same, and equal to the half of the value at the working electrode in a 3-electrode setup. Therefore, for a given potential window, the working electrode of a 3-electrode cell has twice of the potential as it applied to an identical electrode placed in a 2-planar-electrode cell, resulting in doubling the calculated capacitance. Other significant differences between the two methods are due to different quantities of electrolyte contained in the two setups and different cells’ geometries. For all these reasons, to probe the quality of the innovative electrode fabricated in the present work, a 2-planar-electrode cell was designed and fabricated with the same geometrical constraints and materials of a commercial supercapacitors.
Electrochemical test of significant specimen – Cyclic voltammetry, charge and discharge cycling, ESR, power and energy densities.
Product and technological benchmarking, definition of test systems
Lithium Ion Batteries
Analysis of the technological advances and relative market analysis for the graphene applications in lithium ion batteries were performed at Varta Micro innovation. According to project objectives the graphene materials prepared by UPVLC and LGC CNRS were tested in Swagelok test devices (half-cells).
Half-cell test devices enable the study of graphene properties in electrodes. By charge/discharge cycling of the half cells, indications of electrode material lifetime can be deduced. In the case of this test system a lithium host material electrode (graphene) is combined with a lithium counter electrode. This metallic lithium counter electrode provides the system with an almost endless lithium source. Consequently, the test system lacks the capability to accurately predict decreases in cell capacity due to losses of lithium.
Although this test system allowed the study of electrode properties, the major influence on the performance of graphene based electrodes cannot be fully described in this half-cell system. The research work in half cells revealed high irreversible capacity of graphene due to the high surface area exposed to the electrolyte as the main drawback. To study the origins of this effect and its influence on the achievable capacity of lithium ion cells, demonstrator full cells were prepared based on the graphene material prepared by UPVLC.
In order to construct these demonstrator cells, so-called “coffee bag” full cells were prepared. The preparation of electrode pastes for the negative and positive electrodes, the preparation of electrodes and the cell setup are standard procedure for battery development. The preparation of a single full test cell required large amounts of graphene; at least 500 grams are typically necessary.
Cycling studies as well as CV measurements displayed that the irreversible capacity in the first cycle (=passivation film formation) could be lowered significantly. Furthermore the additional reducing step and the annealing allowed higher reversible capacity and a significant improvement of the electrode performance. The performance of the graphene electrodes could be improved by about 100% by this additional step.
Based on these encouraging results, different reducing materials and annealing parameters were tested. The achieved results demonstrate that performance is highly dependent on the choice of reducing agent.
The investigated cells have a nominal capacity of 16 mAh based on the positive electrodes capacity.
In the case of the graphite/ LCO cell system, which is comparable to commercially used industrial cell systems, a reversible capacity of 13 mAh could be achieved. This represents around 80% of the initial nominal capacity.
In the case of the graphene/ LCO cell system a reversible capacity of 5 to 6 mAh could be achieved. This represents around 35% of the initial nominal capacity.
The measured cells reveal clearly the main difficulty graphene has to face when entering markets.
Currently no electrolytes providing high charge ionic conductivity are stable at the working potential of graphene based electrodes. That´s why the high surface area exposed to the electrolyte and the connected formation of passivation films on the surface result in a high loss of “mobile lithium”. As the amount of lithium is limited by the cathode material this results in lower achievable capacities.
The results achieved to date with the lithium battery test vehicles may look discouraging at first glance, but in fact they are in perspective quite promising. One must keep in mind that today’s commercial graphite based electrodes required decades of intensive research work to achieve the current performance levels. Graphene, while still in its industrial infancy, has already proven its ability to achieve minimal capacity loss after up to 500 charge/discharge steps. Graphene based batteries are thus expected to have very long lifetimes with high charge/discharge capacity. The remaining challenge is to improve the chemical preparation steps to achieve graphene material that does not consume lithium on contact with the electrolyte.
OLED test vehicles
Having validated the electrical and optical properties of graphene for display devices, the final step was to produce actual OLED test vehicles to demonstrate the functionality of graphene for display applications. Graphene was produced by the segregation method on 200mm silicon wafers and transferred to glass substrates. The graphene was patterned to form a first electrode, then the OLED stack was evaporated on top (consisting of p-doped layer / AlQ3 + yellow dopant / n-doped layer / Ag cathode). Initial trials showed high leakage currents (200mA) which prevented light emission. The addition of a PEDOT:PSS layer on top of graphene (the stack becomes: Graphene / PEDOT:PSS / p-doped layer / AlQ3 + yellow dopant / n-doped layer / Ag cathode) reduces the leakage current to less than 20mA and results in light emission. A total of 11 of the 16 test vehicles with this configuration showed at least partial illumination of the active display area at drive voltages between 4 and 9 volts.
More recently, graphene sheets have been proposed as a good alternative to ITO for flexible applications because their high Young modulus makes them suitable for applications where repeatable bending occurs. Their other main advantage is obviously the high conductivity even compared to nanotubes. Thus graphene sheets can theoretically fulfill the requirements for OLED applications when going flexible and/or large area illumination.
The GRENADA project has gone beyond the initial challenge of producing graphene materials with high quality by also taking into account the specific materials requirements identified by some of the first industrial applications for graphene. For example, if graphene is to replace indium tin oxide (ITO) in display applications, it will require not only excellent electrical conductivity but also high optical transparency. Progress has been made in this direction with films of graphene exhibiting sheet resistivity as low as 135 ohms per square and optical transparency exceeding 95%. Test vehicles have validated the functionality of graphene as an electrode in operating OLED devices with light emission at voltages as low as 4 volts.
Likewise, significant results have been achieved for the future use of graphene in energy storage devices such as supercapacitors and lithium batteries. Functional battery test cells were fabricated with reduced graphene oxide proving the feasibility of battery fabrication based on chemically synthesized graphene produced within the consortium. Supercapacitor test vehicles were also validated with graphene exhibiting equivalent performance to commercial supercapacitors in terms of energy density.
Potential Impact:
Today’s modern society is faced with a number of increasingly urgent societal challenges. Our fossil-fuel based transportation systems are consuming existing resources at a feverish pace. In turn, the environment is pushed to extremes as global warming continues its upward trend. Finding an alternative to carbon-based fuels continues to escalate on the priority list of actions to be engaged by public administrations. While alternatives such as electric vehicles have existed for over a century, improvements in cost, autonomy and reliability are needed before consumers are willing to make the switch away from petrol. Making the electric vehicle competitive will depend on developing new materials which provide higher battery performance at a lower cost.
Energy in all forms is a major concern of our society, from basic energy resource management to downstream environmental impact. Actions which have a positive impact on energy efficiency and longer useful lifetime of devices will always be preferred by society. In parallel, there is increased awareness that new technologies and products should avoid dependency on scarce mineral resources. Our expectation is that graphene will exactly match these expectations. Graphene will play an important role as a transparent and flexible conductive film in the future realization of handheld mobile devices (displays and touchscreens), general lighting applications based on OLED materials and eventually flexible solar cell applications. In the energy storage domain, graphene and its derivatives will outperform and replace current carbon based materials in lithium batteries and supercapacitors.
The importance of graphene implementation is expected to influence many fields. The results of the project GRENADA specifically have an impact on: Research and Development, Society and Regulation, Economy, Education and Training and Environment.
Impact on Research and Development innovation
GRENADA has contributed to the general advancement of scientific knowledge with respect to graphene. In addition to the scientific publications generated, a significant amount of know-how related to techniques, treatments and handling of graphene have been developed and disseminated within the project partners and their larger scientific communities. Some of the most significant contributions to the scientific community include:
Defect free graphene growth by CVD on copper – conditions for achieving defect free graphene on single crystal metal substrates has been demonstrated and documented for future duplication.
Large area graphene growth by the segregation method – by making use of the solubility of carbon in thin metal films, a segregation method has been developed which yields high quality films with controlled number of graphene layers. The method has been demonstrated using standard semiconductor processing equipment on 20mm wafers. The potential of this method could easily be extended to large area sheet or roll-to-roll processing for future industrial application.
Intercalation of elements between graphene and a supporting substrate – intercalation provides a means of injecting a layer between the graphene and its supporting substrate, thereby effectively dissociating the interaction between the two. This method allows producing graphene films which act as if they are suspended and exhibit intrinsic properties.
Nanomechanical characterization under liquid – techniques for characterizing graphene (or other materials) under polar and non-polar liquid environments has been demonstrated. This technique promises to provide greater understanding of the mechanical, structural or tribological modifications of materials properties brought about by immersion of graphene in liquid environments.
Socio-economic impact
The GRENADA project contributes to the realization of a robust European advanced materials community which has the potential to provide breakthroughs in new graphene based products and systems. In particular, a novel graphene based industry will open up new possibility to enhance employment with a positive resulting impact on society. The GRENADA project will contribute to the important impact graphene will have on different economic sectors such as: automotive (car assembly and components), electronics, biomedical, composite materials, and many others. The wide adoption of innovative materials and processes as well as the wide-spectrum of different disciplines converging in GRENADA will allow the improvement of technical skills of existing workers and the creation of new job careers. Therefore, the project fully complies with the European job Policy aimed at the safeguard of existing jobs and creation of new ones. In particular, the project addresses key issues affecting the competitive position and growth of the European renewable industry. This project unites many of the leading experts available in Europe to achieve the S&T objectives and the ambition of major progress in research and development, deployment and use of emerging innovations, joint mutual learning (skills and training requirements) and the foundations of the future graphene based economy. In particular, the contribution of GRENADA to the realization of energy management and storage systems will provide an outstanding social impact on the use of renewable energy and the future limitation of the use of fossil based energy.
Impact on Education
GRENADA contributes to efficient training of young scientists through many channels. In the frame of the academics (CNRS, Univ. Lancaster, Univ. Cologne and UPVLC) and organizations for R&D (CEA, CRF, VMI) this consortium gathers highly recognized senior and junior scientists, with extensive experience in training of research students in both public and industrial contexts. Several of the group members teach in renowned universities. Members from all groups take part regularly in summer schools on nanotechnology and other workshops to stimulate interest in cutting edge research. GRENADA has promoted innovation through PhD training, from basic science towards applications in the fields of nanotechnology and renewable energy.
The educational and training activities of GRENADA are focused on making communication simpler and to facilitate the diffusion of results within the various communities ranging from nanotechnology to energy. The education of young scientists on converging sciences at the intersection of various disciplines will promote the development of a new common language to allow practitioners from different science and technology fields to understand one another. The training of students on cutting edge science is of strategic importance to shape the minds of the engineers and managers of our future industries, which represent an essential element for European socio-economic growth in the coming years.
Impact on Transportation
Mobility is considered to be one of the most challenging, while at the same time one of the most promising societal challenges facing Europe and the rest of the world. Our current transportation systems rely heavily on petroleum based fuels which contribute to global warming. According to the International Energy Agency, over 50% of petroleum consumption is for transportation and three-quarters of that is used by road vehicles. At the current growth rates of consumption, the world’s generation of CO2 is expected to double by 2050. Clearly, drastic measures are needed to alter our use of fossil fuels and provide cleaner, greener transportation.
During the past decade of quite intense R&D, micro-nanotechnology contributed to enhanced transportation safety and comfort, but their main impact has been on the reduction of fuel consumption and exhaust gas emissions. The developments of new propulsion systems and the applications of nano catalysts for gasoline, diesel and natural gas systems can be considered the most relevant on energy saving and public health. All automotive manufacturers have spent considerable resources so that in spite that most car segments increased in size, weight and power, the overall fuel consumption and emissions have decreased. The use of micro nanotechnologies to enhance current mobility based on the Internal Combustion Engine will continue for several years, but we are now focused on using micro-nano technologies as the enablers of the cleanest and the most energy efficient form of mobility: the electrical vehicle.
In alignment with the European Green Cars initiative, the GRENADA consortium, oriented by the end users (CRF and Varta), contributes to the innovative scenario of graphene based energy management and storage systems serving electrical mobility. The major drawback of electrical vehicles today is the limited autonomy coupled with the high cost of the energy storage devices (generally Li-ion batteries). The challenge is therefore to improve the capacity of the storage devices while simultaneously decreasing the cost for a given energy capacity. The results of Grenada have proven that graphene is an excellent candidate for replacing many of the carbon-based materials that are used in today’s energy storage devices. Graphene has been produced in large quantities from low cost starting material using bulk chemical processing that is compatible with industrial scale. The energy storage devices produced with this graphene material have shown performance equivalent to commercial supercapacitors, with expectations for sizeable improvements still ahead. In parallel, the batteries produced show excellent cycling performance during endurance testing, although there is still room for improvement needed to achieve batteries with high initial capacity. Overall, the potential impact of graphene as a vector to achieve low cost, high capacity energy storage devices will help make electrical vehicle transportation an economically viable alternative to the combustion engine.
Impact on the Environment
The integration of graphene based materials in technologies for power management and energy storage will enable the performance improvements needed to achieve widespread consumer acceptance of the transition to electric vehicles. It is our confident expectation that the achieved results of GRENADA will have an impact on electric vehicle transportation, thereby preserving our environment by reducing greenhouse gas emissions. The shift from diesel vehicles to electrical vehicles will also have a strong positive impact on the levels of fine particulate pollution known to be a health hazard. In other industry sector applications, such as displays and touchscreens, graphene and its compounds are expected to replace the ITO as a transparent electrode. Some analysts predict a worldwide shortage of indium in coming years because of the ever-increasing need for transparent conductors. The continuation of work based on the results of Grenada will help graphene make a lasting impact on our environment.
List of Websites:
www.fp7grenada.eu
All contact information for the partner organizations and participants can be found on the website.
Coordinator contact information:
David Holden
CEA Leti
17 rue des Martyrs
38054 Grenoble Cedex 9
France
david.holden@cea.fr
