Final Report Summary - NEST (Nanowires for Energy STorage)
Executive Summary:
The NEST project propose to design and develop a new kind of micro-supercapacitor assembly based on ultra-high surface area electrodes fabricated thanks to material structuration at the nanometer scale level. NEST is based on the growth of silicon nanostructures in combination with high thermal stability of ionic liquids, ILs, used as electrolyte and/or various coatings either to enlarge the electrochemical window (diamond coating) or to induce additional pseudocapacitance properties (MnO2 or ElectroConducting Polymers, ECPs, coating). One of the primary targets of the NEST project were to validate the integrated electrode and electrolyte designs to produce a micro-supercapacitor via a process compatible with microelectronics processes that can withstand solder reflow (280°C for few seconds).
During the first 18 months of the project, the main achieved results were the following:
- Silicon substrates were successfully covered with various nanostructures according to WP3 : SiNWs, SiNTrs, D-NWs, SiNWs/covered with diamond, D-SiNWs and supernanostructures by electropolymerization according to WP4: SiNWs/PEDOT, PPy or PAni,.
- Various ionic liquid were selected through successive rounds of screening: compatibility with the electrode, resistance to thermal treatment i.e. ability to sustain the reflow treatment, which was consequently demonstrated.
- Preliminary electrochemical characterisations of the performances were carried out in 3-electrode cell set-ups and Swagelok® symmetrical assembly.
Furthermore, the activities were focused on the assembly and evaluation of the set-up and prototype and finally the achievement of a demonstrator.
Project Context and Objectives:
Among today challenges that of energy needs is one of the most important. An obvious question is its production but the need of energy storage systems is almost as large. Renewable energies will not have an impact unless we find an efficient way to store the electricity that they produce. Energy should be available everywhere and at any time, this translates in a strong need for energy containers in the form of electrochemical storage. In this context, the NEST project aims to demonstrate and develop a new kind of integrated supercapacitors, electrochemical capacitors (ECs), as well as novel pseudocapacitors devices able to drastically enhance the energy storage capacity. The primary target of the project was to produce a micro-supercapacitor with integrated electrodes compatible with microelectronics process that can withstand solder reflow (280°C for few minutes). We will associate the high surface area of a new kind of silicon nanostructures, to the high thermal stability of ionic liquids used as the electrolyte. We propose to integrate Si nanowires with sub-nanostructures such as silicon branches and nano-diamond coatings. Diamond coating will bring the additional advantage to allow using protonic electrolyte while keeping a wide 2-3 V electrochemical window. In addition to the gained surface area provided by the nanotree design, even higher capacitance will be achieved by using redox-active coating such as metal oxides and electro-conducting polymers (ECPs). As a result, this combination will lead to highly reversible surface redox reaction with electrochemical double layer capacitance. These new devices well adapted to peak power demand and storage while improving energy capacity will enhance the energy efficiency and consequently will increase the competitiveness of Europe’s industries.
Project Results:
WP2 Technical specifications
The main objective of this task was to provide initial specifications for the energy storage systems developed during the course of the NEST project, and then precise or re-adapt them by taking into account the first results obtained experimentally.
Initial specifications: energy microsources for electronics applications
A first market study has been performed focused on energy microsources for electronics applications (smartcards, MEMS, RFID ...) which has fixed constraints on the energy of the systems (above 1 mWh/cm2) as well as on the ionic liquids used as electrolytes. Following these specifications, a set of 15 ionic liquids were chosen for initial characterizations. Focus was put on their ability to withstand very high temperature (280°C) but also their electrochemical properties (conductivity and electrochemical stability).
Targeted market: from aerospace to aeronautic applications
Considering the unique advantage of ionic liquids to be non-volatile as well as their very high chemical stability, aerospace segment has been first targeted. One major constraint for such application is the very long lifetime required for the devices. For an energy storage system to be used for satellite application, a cycle life of more than 3 million charge/discharge cycles is mandatory. Some of the NEST products have shown to possess this capability, though with reduced energy density. On the other side, more energetically dense products had lower cycle lifetime. Following these first results it has thus been decided to switch from aerospace to aeronautics applications, which require lower cycle lifetime (200 000 cycles).
Wireless autonomous sensors as potential applications
During the NEST workshop, a survey has been proposed to several potential customers for the final devices. Results have shown a very high interest for energy micro-sources for wireless autonomous sensors applications.
These applications have particular requirements in terms of cyclability and operating temperature range, but also safety, which are fitting the systems developed within the project. Indeed, supercapacitors have proven to offer unrivaled cyclability and ionic liquids are non-flammable and non-volatile. Supercapacitors are also less sensitive to cycling conditions (highly variable charging-discharging depths and rates) as compared to other battery technologies. Furthermore, the device will have to sustain vibrations and shocks.
WP3 Nanowires design & fabrication
Since the achievable capacitance of Supercapacitors is strongly dependent on the active surface area of the electrodes, nanostructured materials are attractive candidates for such devices. Heavily boron-doped diamond surfaces are able to further enhance the performance of supercapacitors due to chemical hardening, the widest potential window among all electrode materials in aqueous solution and an additional increase of surface area.
We used Silicon Nanowires (Si-NWs) and Nanotrees (SiNTs) provided by CEA grown by a VLS-process as substrates for the overgrowth with conducting boron doped diamond (BDD) coatings. The SiNWs had typical dimensions of 5 to 20 µm length at diameters of 150 nm.
Alternatively, we prepared electrodes with all-diamond nanowires by ICP etching of 10 µm thick boron-doped polycrystalline CVD diamond with Ti- or Ni nanoparticles as an etching mask. The all-diamond nanowires have a much lower aspect ratio compared to overgrown Si-NWs; however, the surface enhancement is up to 25 times larger compared to unstructured polycrystalline films.
GROWTH OF SINWs or SINTs
Silicon nanowires (SiNWs) and Silicon nanotrees (SiNTs) samples were realized by Vapor Liquid Solid mechanism (VLS) in Chemical Vapor Deposition (CVD) reactor by CEA and distributed to other partners. For the last half of the project dewetted gold thin film was used as catalyst (high density). For the entire project, 78 batches of samples was distributed to all partners. After the mid-term meeting it was decided to produce electrodes for coin-cell and pouch-cell devices. In order to realize coin-cell and pouch-cell devices, we have developed a growth process on metal substrates. Three metals has been tested as substrates: Al, Cu and Stainless Steel. The best candidate is the Stainless Steel. Then we have realized SiNWs and SiNTs (length of the SiNWs: 50µm SiNTs trunk length: 50µm branches length: 10 to 20µm) on circles and custom shape in stainless steel foil and silicon, specially cut by FHG. Furthermore, we have realized double face growth on stainless steel substrates to produce staking electrodes. We have sent 25 batches of these growths to all partners.
OVERGROWTH OF SINWs or SINTs
Since a spontaneous nucleation of diamond on Si does not occur, the seeding with diamond particles is essential. A colloidal diamond suspension in water with typical particle size of 5nm is used.
The higher the density of nuclei and the smaller their size, the higher the quality of the achievable continuous diamond film coating of the SiNWs.
Two principal seeding methods have been established:
- A ten minute seeding dip, preceded by a cleaning step
- Ultrasonic treatment of the sample in seeding solution for one minute at low power (26 W, i.e. 20% of maximum power capacity of the ultrasound bath).
The prerequisites in both cases were the most homogenous distribution of seeds and least possible damage to the samples.
The deposition of boron-doped diamond (BDD) was carried out in Microwave plasma reactors with 6 kW power at 2.45 GHz (accommodating samples up to 75 mm in diameter) and 30 kW power at 915 MHz (for large samples up to 150 mm in diameter) respectively.
FHG/IAF performed numerous experiments to optimize the seeding technique leading to homogenous thin closed diamond layers on SiNWs.
Typical growth parameters were the following:
Pressure 40 mbar, microwave power 2.75 kW, temperature 660 °C, [CH4]/[H2] = 4%, [B]/[CH4] = 3000 ppm, growth time 30 min. As a boron source we used trimethyboron (TMB); the doping level was up to 5x1020 cm-3.
We realized uniform diamond coatings with a thickness in the range of 200 nm. Even densely packed Si-NWs with high aspect ratio could be readily coated.
FHG prepared a total of 133 samples with SiNWs or SiNTs overgrown with BDD.
ALL DIAMOND NANOWIRES
For the preparation of all diamond nanowires FHG produced highly boron-doped nanocrystalline diamond films without nanowires with a thickness of 10 µm on 3 inch Si substrates. All diamond nanowires, D-NWs, were obtained by ICP etching of these substrates (see Fig. 6) using Ni and Ti masks. Results showed that for both of the metal masks one thick wire will split into several smaller wires during the latter stage of the ICP etching, and the surface enhancement due to this effect is very large.
In order to optimize the diamond wires in terms of length and density, a series of samples was prepared using different ICP etching parameters.
Typical parameters of preparation are the following:
Mask: Ti; Thickness: 3 nm; Dewetting: 1000 °C, 1min; ICP power: 1200 W; HF power: 300 W; 50 sccm O2; Pressure: ~ 3 Pa; Duration: 5 min.
FHG prepared a total of 51 samples with all diamond nanowires.
SURFACE WETTING OF DIAMOND WITH IONIC LIQUIDS (IL)
For the investigation of the wetting behavior of ionic liquids to silicon- and diamond surfaces an experimental setup for the measurement of the contact angle was used. All tests have been carried out on unstructured surfaces, since it became apparent, that surfaces with dense nanowires tend to soak the liquids making measurements impossible. A standard droplet volume of 1 µL was used for the measurements. It turned out that the wetting angle for frequently used ILs (EMIMBF4, N222 TFSI, PMPiperTFSI) was always around 0° indicating complete wetting of diamond as well as silicon surfaces. Measurements with commonly used solvents revealed the same result.
LASER CUTTING OF SI-SUBSTRATES FOR THE PREPARATION OF SUPERCAP PROTOTYPES
The preparation of the collectors used so far required the cutting of silicon wafers into suitable parts for pouch cells as well as coin cells. For this purpose we received standard 4inch silicon wafers from CEA. They were cut into 30 x 30 mm2 electrodes with a contact tab for the prototype pouch cell and into circular parts with 15 mm and 10 mm diameter respectively for coin cell prototypes. For the cutting we used a pulsed NdYAG-Laser system. After cutting, the samples were sent back to CEA for the preparation of SiNWs or SiNTs.
WP4 Electrolyte & pseudo-capacitance: Design & fabrication
Functionalization
The WP4 carried out the deposition of different electroactive conducting polymers (ECPs) such as PEDOT, PPy and PANi on SiNWs by means of electrochemical methods using ionic liquid electrolytes in a 3- electrode cell configuration.
Uniform, adherent and homogeneous polymer coatings on SiNWs were succesfully achieved. The electrochemical performance and morphological characterization of such functionalized electrodes were evaluated in symmetric supercapacitor devices. A complete study of these systems was reported in recent publications in collaboration with CEA, CSIC and IL (RSC Advances 2014, 4, 26462 – J. Mater. Chem. A 2015, 3, 13978). In this direction, the functionalization concept was also addressed to the deposition of MnO2 and diamond coatings respectively. The deposition of MnO2 onto SiNWs was carried out using a chemical bath deposition (CBD). Furthermore, studies on supercapacitor applications were described in a recent study (Scientific Reports 2015, 5, 9971). Finally, in the last phase of the NEST project, intensive studies were focused on deposition of diamond on SiNWs. More specifically, during the second stage of this project, the functionalization concept was addressed to other nanostructured systems described in the NEST project. Thus, important research activities were addressed mainly to the deposition of ECPs on silicon nanotrees (SiNTrs), diamond-coated silicon nanowires and diamond nanowires. In conclusion, the coating of SiNWs using ECPs and diamond showed an important improvement on the capacitive properties compared to SiNWs due to its pseudocapacitive behaviour associated with faradaic reactions. Thereby, high specific capacitance and energy density values were found higher than bare SiNWs. However, bare SiNWs exhibited higher power density and extraordinary cycling stability owing to
its EDL behaviour.
Electrolyte
For the final phase of NEST, a selection of different electrolytes was carried out taking into account the different technical specifications described in the project memory. Thus, the criteria were chosen according to the next objectives:
a) Functionalization: The electrochemical deposition of ECPs onto the corresponding nanostructures was carried out using PYR13TFSI (PEDOT and PPy) and Dema OTF (PANi).
b) Electrolyte: In this section, PYR13 TFSI, N1114 TFSI and Et3NH TFSI were employed as electrolytes in supercapacitor devices at room temperature. The criteria was chosen due to their moderate viscosity, high conductivity and good thermal properties (e.g. solder reflow conditions, 240 C during 40s in average). For the final application of the NEST project, an important criteria concerning the use of electrolyte for a wide temperature working range (-40°C up to 120°C) was examined. Thus, conventional pure ionic liquids exhibit melting points at around 0°C, which limits its potential for specific applications. In this direction, the approach of ionic liquid mixtures has emerged as a promising strategy to overcome the working temperature range at very low temperatures. Consequently, Iolitec synthesized several mixtures based on EMIM TFSI and ammonium-based ionic liquids. Additionally, various homemade pure ILs were also synthesized in this direction. Among them, MMEIM BTA and the AMIM TFSI – EMIM TFSI mixture exhibited the best electrochemical performances at a wide range temperature from -40°C up to 30°C. This electrolyte was employed in several commercial cell technologies, such as coin and Swagelok cells, based on SiNWs, SiNTrs and ECP-coated SiNTrs. In all the cases, excellent electrochemical responses were obtained through CV and galvanostatic charge discharge cycles.
Electrochemical performances
As aforementioned in our previous section, PYR13TFSI, N1114 TFSI aprotic ionic liquids and Et3NH TFSI protic ionic liquid were selected as potential electrolytes for supercapacitor devices at room temperature. PYR13TFSI and N1114 TFSI aprotic ionic liquids were employed for supercapacitor devices based on SiNWs, SiNTrs and ECPs-SiNWs, whereas Et3NH TFSI was used for supercapacitors based on diamond electrodes.
a) Diamond-coated SiNWs. The electrochemical performance of diamond-coated SiNWs was evaluated in a 3-electrode standard cell configuration using a PYR13TFSI/PC mixture (40:60 v/v) as electrolyte. The single electrode showed a specific capacitance of 105 μF cm-2 and an energy density value of 84 μJ cm-2 with a remarkable stability over 10 000 cyclic voltammetry curves at a scan rate of 5 Vs-1 (e.g. 7% of the initial capacitance was found to be lost). These first preliminary results reflected the promising application of diamond-coated SiNWs for high performance supercapacitors (D5.4). Consequently, over the past months, diamond-coated SiNWs were evaluated in a symmetric micro-supercapacitor using Et3NH TFSI as electrolyte. The electrochemical performance of such device was able to deliver a specific capacitance of 1.5 mF/cm2 and energy and power densities as high as 11 mJ/cm2 and 25 mW/cm2 using a cell voltage of 4V. Furthermore, a remarkable cycling stability was evaluated after 1 x 106 galvanostatic cycles, exhibiting a quasi-ideal capacitive behaviour at a high current density of 10 mA/cm2. Accordingly, the device retained 65% of its initial capacitance with a coulombic efficiency of ~99 % during the cycling test.(D5.4). A complete and exhaustive electrochemical performance of this device was reported in our recently published study. (Electrochem. Commun. 2016, 63, 34). The improvement of capacitive properties of SiNWs by using the elaboration of hyperbranched SiNWs (known as silicon nanotrees) was exhibited in our previous work. On the basis of the obtained results, an important research activity was addressed to the deposition of diamond on SiNTrs to enhance specific capacitance, energy and power densities of diamond-coated SiNWs.
However, for technical reasons it was not possible to obtain uniform and homogeneous diamond coatings on SiNTrs due to its high density including trunks and branches.
b) ECP- coated SiNTrs. As it was mentioned in the previous section, the electrochemical performance of ECPs-coated SiNWs was described in our past deliverables and articles. However, important studies should be addressed to the functionalization of SiNTrs. Within this context, various studies were carried out using PEDOT and PPy films on SiNTrs by electrochemical methods. Our previous knowledge acquired for the electropolymerization of PEDOT, PPy and PANi on SiNWs allowed us to achieve uniform and homogeneous coating on SiNTrs. A representative example is shown below concerning PPy-coated SiNTrs.
The electrochemical performance of this hybrid supercapacitor was compared to our previous results, demonstrating the enormous potential to enhance capacitive properties at specific capacitance and energy density level. (J. Mater. Chem. A 2015, 3, 13978)
c) ECP-coated DiNWs. During the elaboration of the D4.1 the conformal deposition of ECPs on diamond nanowires was an important hurdle because of the formation of polymer thin films on the top of diamond nanowires, without diffusion of the polymer along the nanowires. This serious drawback was solved by changing some electrochemical conditions and electropolymerization procedures. Thus, conformal coatings of PEDOT on DiNWs were succesfully obtained applying low polymerization charges controlled by pulse techniques. The electrochemical deposition of PEDOT was carried out by using a PC solution containing 0.01 M EDOT and 0.1 M NBu4 PF6.
The electrochemical response of PEDOT-coated DiNWs was evaluated in a 3-electrode cell configuration using Et3NH TFSI as electrolyte. The specific capacitance of the electrode showed a value of 3.31 mF cm-2 at a scan rate of 1Vs-1, whereas DiNWs exhibited a value of 0.084 mF cm-2. This enhancement was ascribed to the faradaic reactions involved in the polymer coating. The electrochemical stability of the electrode was examined by applying 10 000 CV curves at a scan rate of 1Vs-1. As can be seen, a loss of approximately 40% was found after cycling without any degradation of the structure.
d) ECP-coated Diamond/SiNWs. Recently, we have completed the study concerning the electrochemical performance of supercapacitors based on PEDOT/DiNWs. The morphological and structural characterization of the electrode was analyzed by SEM, HRTEM equipped with EDX and XPS techniques. Later, the electrodes were assembled in a symmetric supercapacitor device using N1114 BTA electrolyte.
This work represents a novel approach focused on the multi-hierarchical functionalization of diverse hetero-nanostructures (PEDOT, Diamond and SiNWs). The results show the synergistic effect produced by the combination of such nanostructures in terms of energy and power densities as well as its excellent cycling stability. As a consequence, this hybrid device reflected one of the best performances reported in the literature concerning SiNWs-based supercapacitors.
Demonstrator
Concerning the demonstrator elaboration, coin cell prototype was adapted as one of the best configurations (D5.4). Thus, round shape substrates (15 mm) based on silicon and stainless steels were employed for the growth of SiNTrs and functionalized SiNWs and SiNTrs, which show the best electrochemical performances. Particularly, CEA carried out coin cells (CR2032) based on SiNTrs, PEDOT-coated SiNW and PEDOT-coated SiNTrs electrodes, which were tested by HUT. In this direction, over the past months, important efforts were done for the machine set up, preparation of substrates, and delivery of samples.
WP5 Material testing & electro - chemical performance
Work Package 5 deals with the characterization of electrodes and electrolytes with special emphasis on their electrochemical properties. This is a very important intermediate stage for determining how well a given electrode will perform, but also to determine the compatibility of electrode-electrolyte couples. Since the interface between electrode and electrolyte is the core structure responsible for the final performance of a supercapacitor these studies provided most useful information.
We began determining the thermal stability of a long series of Ionic Liquids provided by Iolitec under the soldering reflow conditions all materials should stand (280ºC for 40 seconds), conditions mimicking the heat produced during soldering microelectronic components. We also studied the electrochemical stability window (i.e. the range of potential within which each electrolyte was stable) for each and all the many ionic liquid electrolytes. All of these studies provided a preliminary selection of the best IL electrolytes which was the used to test the selected electrolytes with a variety of electrodes: Si nanowires, diamond-coated silicon nanowires prepared by CEA and Fraunhofer partners respectively and also a new line of electrodes incorporating materials with pseudocapacitive properties able to boost the energy density of the final devices. These pseudocapacitive materials were of two types: oxides, such as MnO2 (prepared by CSIC partners) and conducting polymers such as Polyaniline (PAni) (prepared by CEA and CSIC partners). Each of the electrode materials was thoroughly characterized by means of a series of analytical techniques including Electron Microscopy, X-Ray diffraction, Thermogravimetric analyses, XPS etc. And of course the electrolytes also suffered a standard battery of test including NMR, spectroscopies, density and viscosity measurements and analyses of water contents before being used in electrochemical cells.
Each of these electrodes was studied by means of cyclic voltammetry (CV) using the Ionic Liquid electrolytes previously selected as well as charge-discharge cycles. The CVs measured current as voltage is scanned for the working electrode under study with respect to a reference electrode. These basic electrochemical characterization gives a very accurate idea of the electrochemical activity of a given electrode immersed in a given electrolyte and is the basis for later setting up and testing the final device, in which two electrodes are combined and polarized in a real supercapacitor cell.
WP6 Assembling: Setup & prototype
The objective of the prototype developed in this work package was to provide an efficient demonstration of the use of NEST energy storage system in a wireless autonomous sensor which could be representative of the many possible applications of this very broad market.
Specific constraints of wireless autonomous sensors
A wireless autonomous sensor is composed of an energy harvesting unit, which should be able to gather energy from the environment of the device or during its working. It can be mechanical energy (wind, vibrations), thermal energy (thermoelectrics) or solar energy (photoelectrics) for example. This energy should be stored in an energy storing device, and delivered to a communication system which can send the information gathered by the sensor.
An energy storage used in a wireless autonomous sensor will have to be able to sustain very random charging profile (intermittent power peaks with non-predictable length), while having a huge cycle life. It should be able to operate in a very wide temperature range as well as sustaining vibrations and shocks without compromising the safety of the device.
A connected shoe as an efficient representation of the wireless autonomous sensors
A connected shoe has been chosen as a simple yet effective representation of the wireless autonomous sensors. Piezoelectric patches are generating energy through flexion of the tip of the sole. This energy is stored into NEST supercapacitors coin-cells and delivered to a bluetooth system. Information are gathered from an acceleterometer implemented in the shoe and send to a tablet application which is showing the amount of steps as well as the walking speed. Furthermore, in order to facilitate the testing of NEST current products and future implementations, an automated test-bench has been produced. Automated walking profile can thus be created in order to compare the cells on the exact same basis.
Testing of NEST coin-cells and perspective for future implementations
The testing have shown that NEST products were adapted to the very random charging discharging profile. The device is both working under automated testing and real walking. Both the device and the coin-cells were able to sustain the vibrations during a real walking.
The testing have however also stressed out the very high requirement for low self-discharge of the energy source. Thus, in order for NEST products to be able to reach the market of autonomous wireless sensors, future work will need to be oriented to the limitation of the self-discharge.
WP7 Performance evaluation; Setup & prototype
Upscaling of the capacitor systems to industrial assembly, i.e. pouch and coin cell, was successfully realized with silicon nanostructure-based electrodes. EMIM TFSI, MPPyrr TFSI, AMIM TFSI + EMIM TFSI ionic liquids were selected to obtain high energy/power device. The research was focused on the comparison of electrode materials deposited on two different substrates. The results show that better electrochemical behavior is obtained when the silicon substrate is used. It is caused by the reproducible deposition process of stable silicon nanostructures on a silicon wafer. The stainless steel (316L) as a substrate faced the problems concerning adhesion and peeling of the nanotrees from the surface. On the other hand, the stainless steel substrate is more suitable for pouch cell assembly. It is the comprise between closing easiness and Si nanostructured electrode damage risk.
Moreover, silicon-based supercapacitors with ionic liquids are able to operate at different temperatures. It was proved that supercapacitor with MPPyrr TFSI can effectively operate up to +60oC. At low temperatures, a mixture of AMIM TFSI + EMIM TSFI played a role of a suitable electrolyte. This IL mixture can sufficiently work even at -40oC.
An additional coating made on silicon nanotrees can significantly enhance overall electrochemical performance. The presence of electrically conductive polymer (PEDOT) or boron-doped diamond improved the energy output and total power (ca. 10 times). Diamond coated electrodes exhibit higher temperature stability – IL electrolyte does not decompose so rapidly (research made with MPPyrr TFSI). Moreover, diamond coated electrodes demonstrate much higher specific capacitance values than pure SiNTs. Despite the fact that PEDOT presence increases specific capacitance at room temperature, it should be taken into account that PEDOT pseudocapacitance does not contribute at -40oC and the system behave similarly as with pure SiNTs.
The cycle-life performance was studied using galvanostatic charging/discharging (GCPL) at a relatively low current density (0.1 mA cm-2). All the systems (independently on assembly type) are able to operate effectively for thousand cycles (> 50 000). Even in harsh conditions (-40oC or +60oC), silicon-based supercapacitors work sufficiently. Maximum relative specific capacitance decay does not exceed 20%, what is in accordance with International Standard IEC 62391-1. The supercapacitors with silicon nanostructures and ionic liquids meet the future application requirements, due to their long cycle-life.
In order to select the most appropriate system, comparison of coin cell assembly with pouch cell one was done. It shows that coin cell construction is the most appropriate one – the highest capacitance values are obtained, and the highest reproducibility is achieved. Additionally, the behaviour of the system at OCV conditions (self-discharge) for coin cell construction reveals the smallest decay of voltage after 20 h. Furthermore, it can be observed that specific resistance at 100 kHz for coin cell oscillates between 3-4 Ohm cm2 while for pouch cell construction is ca. 2 times higher – it equals to 7.5 Ohm cm2.
Time constant was evaluated for capacitor devices. This value ranges from a few ms to 5s. Obviously, silicon nanostructured electrodes decorated with PEDOT have the highest time constant.
A new type of bipolar pouch cell was constructed in order to improve the overall performance of the system. Connected in parallel three microcapacitors were implemented with the use of bipolar electrodes. In this assembly, two substrates (with three electrodes) were connected to the positive pole, while the other two substrates (with three electrodes) placed on the opposite side of coffee bag cell were connected to the negative one. The most important achievement obtained in this system is total power value which is ca. 3 times higher than for the other assemblies.
WP8 Exploitation & dissemination
Work package 8 is about Dissemination. That means that the main goal is to inform the community, both scientific and social about the results of the project, and the benefits from it, including scientific, economical and ecological aspects. As part of the project a total of 16 scientific papers were published in 6 different scientific journals. In order to discuss the results with more industrial partners, who might like to use the supercapacitor technology we have developed within the project, a workshop was organized in October 2014, which included a total of 14 talks in front of 50 representatives of industry and academia from the EU. Another workshop, together with some partners from EU projects on the same call was organized in Aril 2016 in Grenoble. A further way to inform the scientific community of our results is to participate in scientific conferences and fairs. All partners of the project have participated in such events have made oral or poster presentations (a total of 34 oral and 7 posters) about the innovation potential of our technology. As with all other social groups, one of the best ways to communicate a message is through personal contacts, and this is what is used the most in such events.
Another important part of this work package was to estimate if the technology developed within the project is competitive with other commercial and soon to be commercialized technologies. This was done by creating and constantly updating a patent database. It showed, that although the amounts of patents on supercapacitors is constantly growing, showing a high interest in this technology, only a very small part of them (around 30 ) to some extent use some of the materials as we do. This indicates that we are using very competitive technology that has a high value in future markets. This allows for the results of the project to be commercially exploited, either in the form of patents (1 EU patent filed) or as share growth and expansion on the available markets for the different industrial partners. The market study, which was also made as part of this work package shows more than 30 industrial producers of such electrochemical devices, who might be interested in our technology and be willing to license and implement it in their deices. The market size for such devices I expected to grow up to $2 billion by 2020.
The project website (www.project-nest.eu) is the window of the project to the world. The website has two domains: a public area, which can be accessed without restrictions and includes general information about the project, project results, information on the partners and a list of published results. In the private part of the website, which is intended only for the members of the project consortium, all documents, generated by the partners (reports, presentations, publications and deliverables) are available for download and upload.
Potential Impact:
The NEST project was remarkably productive in terms of innovation and dissemination of knowledge: 13 publications (among them 7 involved 3 partners and 2 involved 4 partners), 2 dissemination reports, one patent, 34 oral communications and 7 posters.
In addition to the mandatory midterm meeting, CEA organized a meeting “Highlights on electrochemical energy storage at CEA-Grenoble” the day before the final NEST meeting on April 28th 2016. The participants were from technological Institute LITEN and fundamental institute INAC with the attendance of EU representatives (PO and reviewers) and of a part of the NEST Partners.
The exploitation of the project results could be illustrated by the creation of the start-up EnWires for the use of Si Nanowires in electrochemical energy storage. The company was founded in April 2016.
List of Websites:
Project website: http://www.project-nest.eu/
Project coordinator contact details:
CEA-Grenoble, 17 rue des Martyrs
38054 Grenoble cedex 09
FRANCE
Tel: + 33 4 38 78 48 41
Fax: + 33 4 38 78 56 91
Mob: + 33 6 87 98 45 93
E-mail: gerard.bidan@cea.fr
The NEST project propose to design and develop a new kind of micro-supercapacitor assembly based on ultra-high surface area electrodes fabricated thanks to material structuration at the nanometer scale level. NEST is based on the growth of silicon nanostructures in combination with high thermal stability of ionic liquids, ILs, used as electrolyte and/or various coatings either to enlarge the electrochemical window (diamond coating) or to induce additional pseudocapacitance properties (MnO2 or ElectroConducting Polymers, ECPs, coating). One of the primary targets of the NEST project were to validate the integrated electrode and electrolyte designs to produce a micro-supercapacitor via a process compatible with microelectronics processes that can withstand solder reflow (280°C for few seconds).
During the first 18 months of the project, the main achieved results were the following:
- Silicon substrates were successfully covered with various nanostructures according to WP3 : SiNWs, SiNTrs, D-NWs, SiNWs/covered with diamond, D-SiNWs and supernanostructures by electropolymerization according to WP4: SiNWs/PEDOT, PPy or PAni,.
- Various ionic liquid were selected through successive rounds of screening: compatibility with the electrode, resistance to thermal treatment i.e. ability to sustain the reflow treatment, which was consequently demonstrated.
- Preliminary electrochemical characterisations of the performances were carried out in 3-electrode cell set-ups and Swagelok® symmetrical assembly.
Furthermore, the activities were focused on the assembly and evaluation of the set-up and prototype and finally the achievement of a demonstrator.
Project Context and Objectives:
Among today challenges that of energy needs is one of the most important. An obvious question is its production but the need of energy storage systems is almost as large. Renewable energies will not have an impact unless we find an efficient way to store the electricity that they produce. Energy should be available everywhere and at any time, this translates in a strong need for energy containers in the form of electrochemical storage. In this context, the NEST project aims to demonstrate and develop a new kind of integrated supercapacitors, electrochemical capacitors (ECs), as well as novel pseudocapacitors devices able to drastically enhance the energy storage capacity. The primary target of the project was to produce a micro-supercapacitor with integrated electrodes compatible with microelectronics process that can withstand solder reflow (280°C for few minutes). We will associate the high surface area of a new kind of silicon nanostructures, to the high thermal stability of ionic liquids used as the electrolyte. We propose to integrate Si nanowires with sub-nanostructures such as silicon branches and nano-diamond coatings. Diamond coating will bring the additional advantage to allow using protonic electrolyte while keeping a wide 2-3 V electrochemical window. In addition to the gained surface area provided by the nanotree design, even higher capacitance will be achieved by using redox-active coating such as metal oxides and electro-conducting polymers (ECPs). As a result, this combination will lead to highly reversible surface redox reaction with electrochemical double layer capacitance. These new devices well adapted to peak power demand and storage while improving energy capacity will enhance the energy efficiency and consequently will increase the competitiveness of Europe’s industries.
Project Results:
WP2 Technical specifications
The main objective of this task was to provide initial specifications for the energy storage systems developed during the course of the NEST project, and then precise or re-adapt them by taking into account the first results obtained experimentally.
Initial specifications: energy microsources for electronics applications
A first market study has been performed focused on energy microsources for electronics applications (smartcards, MEMS, RFID ...) which has fixed constraints on the energy of the systems (above 1 mWh/cm2) as well as on the ionic liquids used as electrolytes. Following these specifications, a set of 15 ionic liquids were chosen for initial characterizations. Focus was put on their ability to withstand very high temperature (280°C) but also their electrochemical properties (conductivity and electrochemical stability).
Targeted market: from aerospace to aeronautic applications
Considering the unique advantage of ionic liquids to be non-volatile as well as their very high chemical stability, aerospace segment has been first targeted. One major constraint for such application is the very long lifetime required for the devices. For an energy storage system to be used for satellite application, a cycle life of more than 3 million charge/discharge cycles is mandatory. Some of the NEST products have shown to possess this capability, though with reduced energy density. On the other side, more energetically dense products had lower cycle lifetime. Following these first results it has thus been decided to switch from aerospace to aeronautics applications, which require lower cycle lifetime (200 000 cycles).
Wireless autonomous sensors as potential applications
During the NEST workshop, a survey has been proposed to several potential customers for the final devices. Results have shown a very high interest for energy micro-sources for wireless autonomous sensors applications.
These applications have particular requirements in terms of cyclability and operating temperature range, but also safety, which are fitting the systems developed within the project. Indeed, supercapacitors have proven to offer unrivaled cyclability and ionic liquids are non-flammable and non-volatile. Supercapacitors are also less sensitive to cycling conditions (highly variable charging-discharging depths and rates) as compared to other battery technologies. Furthermore, the device will have to sustain vibrations and shocks.
WP3 Nanowires design & fabrication
Since the achievable capacitance of Supercapacitors is strongly dependent on the active surface area of the electrodes, nanostructured materials are attractive candidates for such devices. Heavily boron-doped diamond surfaces are able to further enhance the performance of supercapacitors due to chemical hardening, the widest potential window among all electrode materials in aqueous solution and an additional increase of surface area.
We used Silicon Nanowires (Si-NWs) and Nanotrees (SiNTs) provided by CEA grown by a VLS-process as substrates for the overgrowth with conducting boron doped diamond (BDD) coatings. The SiNWs had typical dimensions of 5 to 20 µm length at diameters of 150 nm.
Alternatively, we prepared electrodes with all-diamond nanowires by ICP etching of 10 µm thick boron-doped polycrystalline CVD diamond with Ti- or Ni nanoparticles as an etching mask. The all-diamond nanowires have a much lower aspect ratio compared to overgrown Si-NWs; however, the surface enhancement is up to 25 times larger compared to unstructured polycrystalline films.
GROWTH OF SINWs or SINTs
Silicon nanowires (SiNWs) and Silicon nanotrees (SiNTs) samples were realized by Vapor Liquid Solid mechanism (VLS) in Chemical Vapor Deposition (CVD) reactor by CEA and distributed to other partners. For the last half of the project dewetted gold thin film was used as catalyst (high density). For the entire project, 78 batches of samples was distributed to all partners. After the mid-term meeting it was decided to produce electrodes for coin-cell and pouch-cell devices. In order to realize coin-cell and pouch-cell devices, we have developed a growth process on metal substrates. Three metals has been tested as substrates: Al, Cu and Stainless Steel. The best candidate is the Stainless Steel. Then we have realized SiNWs and SiNTs (length of the SiNWs: 50µm SiNTs trunk length: 50µm branches length: 10 to 20µm) on circles and custom shape in stainless steel foil and silicon, specially cut by FHG. Furthermore, we have realized double face growth on stainless steel substrates to produce staking electrodes. We have sent 25 batches of these growths to all partners.
OVERGROWTH OF SINWs or SINTs
Since a spontaneous nucleation of diamond on Si does not occur, the seeding with diamond particles is essential. A colloidal diamond suspension in water with typical particle size of 5nm is used.
The higher the density of nuclei and the smaller their size, the higher the quality of the achievable continuous diamond film coating of the SiNWs.
Two principal seeding methods have been established:
- A ten minute seeding dip, preceded by a cleaning step
- Ultrasonic treatment of the sample in seeding solution for one minute at low power (26 W, i.e. 20% of maximum power capacity of the ultrasound bath).
The prerequisites in both cases were the most homogenous distribution of seeds and least possible damage to the samples.
The deposition of boron-doped diamond (BDD) was carried out in Microwave plasma reactors with 6 kW power at 2.45 GHz (accommodating samples up to 75 mm in diameter) and 30 kW power at 915 MHz (for large samples up to 150 mm in diameter) respectively.
FHG/IAF performed numerous experiments to optimize the seeding technique leading to homogenous thin closed diamond layers on SiNWs.
Typical growth parameters were the following:
Pressure 40 mbar, microwave power 2.75 kW, temperature 660 °C, [CH4]/[H2] = 4%, [B]/[CH4] = 3000 ppm, growth time 30 min. As a boron source we used trimethyboron (TMB); the doping level was up to 5x1020 cm-3.
We realized uniform diamond coatings with a thickness in the range of 200 nm. Even densely packed Si-NWs with high aspect ratio could be readily coated.
FHG prepared a total of 133 samples with SiNWs or SiNTs overgrown with BDD.
ALL DIAMOND NANOWIRES
For the preparation of all diamond nanowires FHG produced highly boron-doped nanocrystalline diamond films without nanowires with a thickness of 10 µm on 3 inch Si substrates. All diamond nanowires, D-NWs, were obtained by ICP etching of these substrates (see Fig. 6) using Ni and Ti masks. Results showed that for both of the metal masks one thick wire will split into several smaller wires during the latter stage of the ICP etching, and the surface enhancement due to this effect is very large.
In order to optimize the diamond wires in terms of length and density, a series of samples was prepared using different ICP etching parameters.
Typical parameters of preparation are the following:
Mask: Ti; Thickness: 3 nm; Dewetting: 1000 °C, 1min; ICP power: 1200 W; HF power: 300 W; 50 sccm O2; Pressure: ~ 3 Pa; Duration: 5 min.
FHG prepared a total of 51 samples with all diamond nanowires.
SURFACE WETTING OF DIAMOND WITH IONIC LIQUIDS (IL)
For the investigation of the wetting behavior of ionic liquids to silicon- and diamond surfaces an experimental setup for the measurement of the contact angle was used. All tests have been carried out on unstructured surfaces, since it became apparent, that surfaces with dense nanowires tend to soak the liquids making measurements impossible. A standard droplet volume of 1 µL was used for the measurements. It turned out that the wetting angle for frequently used ILs (EMIMBF4, N222 TFSI, PMPiperTFSI) was always around 0° indicating complete wetting of diamond as well as silicon surfaces. Measurements with commonly used solvents revealed the same result.
LASER CUTTING OF SI-SUBSTRATES FOR THE PREPARATION OF SUPERCAP PROTOTYPES
The preparation of the collectors used so far required the cutting of silicon wafers into suitable parts for pouch cells as well as coin cells. For this purpose we received standard 4inch silicon wafers from CEA. They were cut into 30 x 30 mm2 electrodes with a contact tab for the prototype pouch cell and into circular parts with 15 mm and 10 mm diameter respectively for coin cell prototypes. For the cutting we used a pulsed NdYAG-Laser system. After cutting, the samples were sent back to CEA for the preparation of SiNWs or SiNTs.
WP4 Electrolyte & pseudo-capacitance: Design & fabrication
Functionalization
The WP4 carried out the deposition of different electroactive conducting polymers (ECPs) such as PEDOT, PPy and PANi on SiNWs by means of electrochemical methods using ionic liquid electrolytes in a 3- electrode cell configuration.
Uniform, adherent and homogeneous polymer coatings on SiNWs were succesfully achieved. The electrochemical performance and morphological characterization of such functionalized electrodes were evaluated in symmetric supercapacitor devices. A complete study of these systems was reported in recent publications in collaboration with CEA, CSIC and IL (RSC Advances 2014, 4, 26462 – J. Mater. Chem. A 2015, 3, 13978). In this direction, the functionalization concept was also addressed to the deposition of MnO2 and diamond coatings respectively. The deposition of MnO2 onto SiNWs was carried out using a chemical bath deposition (CBD). Furthermore, studies on supercapacitor applications were described in a recent study (Scientific Reports 2015, 5, 9971). Finally, in the last phase of the NEST project, intensive studies were focused on deposition of diamond on SiNWs. More specifically, during the second stage of this project, the functionalization concept was addressed to other nanostructured systems described in the NEST project. Thus, important research activities were addressed mainly to the deposition of ECPs on silicon nanotrees (SiNTrs), diamond-coated silicon nanowires and diamond nanowires. In conclusion, the coating of SiNWs using ECPs and diamond showed an important improvement on the capacitive properties compared to SiNWs due to its pseudocapacitive behaviour associated with faradaic reactions. Thereby, high specific capacitance and energy density values were found higher than bare SiNWs. However, bare SiNWs exhibited higher power density and extraordinary cycling stability owing to
its EDL behaviour.
Electrolyte
For the final phase of NEST, a selection of different electrolytes was carried out taking into account the different technical specifications described in the project memory. Thus, the criteria were chosen according to the next objectives:
a) Functionalization: The electrochemical deposition of ECPs onto the corresponding nanostructures was carried out using PYR13TFSI (PEDOT and PPy) and Dema OTF (PANi).
b) Electrolyte: In this section, PYR13 TFSI, N1114 TFSI and Et3NH TFSI were employed as electrolytes in supercapacitor devices at room temperature. The criteria was chosen due to their moderate viscosity, high conductivity and good thermal properties (e.g. solder reflow conditions, 240 C during 40s in average). For the final application of the NEST project, an important criteria concerning the use of electrolyte for a wide temperature working range (-40°C up to 120°C) was examined. Thus, conventional pure ionic liquids exhibit melting points at around 0°C, which limits its potential for specific applications. In this direction, the approach of ionic liquid mixtures has emerged as a promising strategy to overcome the working temperature range at very low temperatures. Consequently, Iolitec synthesized several mixtures based on EMIM TFSI and ammonium-based ionic liquids. Additionally, various homemade pure ILs were also synthesized in this direction. Among them, MMEIM BTA and the AMIM TFSI – EMIM TFSI mixture exhibited the best electrochemical performances at a wide range temperature from -40°C up to 30°C. This electrolyte was employed in several commercial cell technologies, such as coin and Swagelok cells, based on SiNWs, SiNTrs and ECP-coated SiNTrs. In all the cases, excellent electrochemical responses were obtained through CV and galvanostatic charge discharge cycles.
Electrochemical performances
As aforementioned in our previous section, PYR13TFSI, N1114 TFSI aprotic ionic liquids and Et3NH TFSI protic ionic liquid were selected as potential electrolytes for supercapacitor devices at room temperature. PYR13TFSI and N1114 TFSI aprotic ionic liquids were employed for supercapacitor devices based on SiNWs, SiNTrs and ECPs-SiNWs, whereas Et3NH TFSI was used for supercapacitors based on diamond electrodes.
a) Diamond-coated SiNWs. The electrochemical performance of diamond-coated SiNWs was evaluated in a 3-electrode standard cell configuration using a PYR13TFSI/PC mixture (40:60 v/v) as electrolyte. The single electrode showed a specific capacitance of 105 μF cm-2 and an energy density value of 84 μJ cm-2 with a remarkable stability over 10 000 cyclic voltammetry curves at a scan rate of 5 Vs-1 (e.g. 7% of the initial capacitance was found to be lost). These first preliminary results reflected the promising application of diamond-coated SiNWs for high performance supercapacitors (D5.4). Consequently, over the past months, diamond-coated SiNWs were evaluated in a symmetric micro-supercapacitor using Et3NH TFSI as electrolyte. The electrochemical performance of such device was able to deliver a specific capacitance of 1.5 mF/cm2 and energy and power densities as high as 11 mJ/cm2 and 25 mW/cm2 using a cell voltage of 4V. Furthermore, a remarkable cycling stability was evaluated after 1 x 106 galvanostatic cycles, exhibiting a quasi-ideal capacitive behaviour at a high current density of 10 mA/cm2. Accordingly, the device retained 65% of its initial capacitance with a coulombic efficiency of ~99 % during the cycling test.(D5.4). A complete and exhaustive electrochemical performance of this device was reported in our recently published study. (Electrochem. Commun. 2016, 63, 34). The improvement of capacitive properties of SiNWs by using the elaboration of hyperbranched SiNWs (known as silicon nanotrees) was exhibited in our previous work. On the basis of the obtained results, an important research activity was addressed to the deposition of diamond on SiNTrs to enhance specific capacitance, energy and power densities of diamond-coated SiNWs.
However, for technical reasons it was not possible to obtain uniform and homogeneous diamond coatings on SiNTrs due to its high density including trunks and branches.
b) ECP- coated SiNTrs. As it was mentioned in the previous section, the electrochemical performance of ECPs-coated SiNWs was described in our past deliverables and articles. However, important studies should be addressed to the functionalization of SiNTrs. Within this context, various studies were carried out using PEDOT and PPy films on SiNTrs by electrochemical methods. Our previous knowledge acquired for the electropolymerization of PEDOT, PPy and PANi on SiNWs allowed us to achieve uniform and homogeneous coating on SiNTrs. A representative example is shown below concerning PPy-coated SiNTrs.
The electrochemical performance of this hybrid supercapacitor was compared to our previous results, demonstrating the enormous potential to enhance capacitive properties at specific capacitance and energy density level. (J. Mater. Chem. A 2015, 3, 13978)
c) ECP-coated DiNWs. During the elaboration of the D4.1 the conformal deposition of ECPs on diamond nanowires was an important hurdle because of the formation of polymer thin films on the top of diamond nanowires, without diffusion of the polymer along the nanowires. This serious drawback was solved by changing some electrochemical conditions and electropolymerization procedures. Thus, conformal coatings of PEDOT on DiNWs were succesfully obtained applying low polymerization charges controlled by pulse techniques. The electrochemical deposition of PEDOT was carried out by using a PC solution containing 0.01 M EDOT and 0.1 M NBu4 PF6.
The electrochemical response of PEDOT-coated DiNWs was evaluated in a 3-electrode cell configuration using Et3NH TFSI as electrolyte. The specific capacitance of the electrode showed a value of 3.31 mF cm-2 at a scan rate of 1Vs-1, whereas DiNWs exhibited a value of 0.084 mF cm-2. This enhancement was ascribed to the faradaic reactions involved in the polymer coating. The electrochemical stability of the electrode was examined by applying 10 000 CV curves at a scan rate of 1Vs-1. As can be seen, a loss of approximately 40% was found after cycling without any degradation of the structure.
d) ECP-coated Diamond/SiNWs. Recently, we have completed the study concerning the electrochemical performance of supercapacitors based on PEDOT/DiNWs. The morphological and structural characterization of the electrode was analyzed by SEM, HRTEM equipped with EDX and XPS techniques. Later, the electrodes were assembled in a symmetric supercapacitor device using N1114 BTA electrolyte.
This work represents a novel approach focused on the multi-hierarchical functionalization of diverse hetero-nanostructures (PEDOT, Diamond and SiNWs). The results show the synergistic effect produced by the combination of such nanostructures in terms of energy and power densities as well as its excellent cycling stability. As a consequence, this hybrid device reflected one of the best performances reported in the literature concerning SiNWs-based supercapacitors.
Demonstrator
Concerning the demonstrator elaboration, coin cell prototype was adapted as one of the best configurations (D5.4). Thus, round shape substrates (15 mm) based on silicon and stainless steels were employed for the growth of SiNTrs and functionalized SiNWs and SiNTrs, which show the best electrochemical performances. Particularly, CEA carried out coin cells (CR2032) based on SiNTrs, PEDOT-coated SiNW and PEDOT-coated SiNTrs electrodes, which were tested by HUT. In this direction, over the past months, important efforts were done for the machine set up, preparation of substrates, and delivery of samples.
WP5 Material testing & electro - chemical performance
Work Package 5 deals with the characterization of electrodes and electrolytes with special emphasis on their electrochemical properties. This is a very important intermediate stage for determining how well a given electrode will perform, but also to determine the compatibility of electrode-electrolyte couples. Since the interface between electrode and electrolyte is the core structure responsible for the final performance of a supercapacitor these studies provided most useful information.
We began determining the thermal stability of a long series of Ionic Liquids provided by Iolitec under the soldering reflow conditions all materials should stand (280ºC for 40 seconds), conditions mimicking the heat produced during soldering microelectronic components. We also studied the electrochemical stability window (i.e. the range of potential within which each electrolyte was stable) for each and all the many ionic liquid electrolytes. All of these studies provided a preliminary selection of the best IL electrolytes which was the used to test the selected electrolytes with a variety of electrodes: Si nanowires, diamond-coated silicon nanowires prepared by CEA and Fraunhofer partners respectively and also a new line of electrodes incorporating materials with pseudocapacitive properties able to boost the energy density of the final devices. These pseudocapacitive materials were of two types: oxides, such as MnO2 (prepared by CSIC partners) and conducting polymers such as Polyaniline (PAni) (prepared by CEA and CSIC partners). Each of the electrode materials was thoroughly characterized by means of a series of analytical techniques including Electron Microscopy, X-Ray diffraction, Thermogravimetric analyses, XPS etc. And of course the electrolytes also suffered a standard battery of test including NMR, spectroscopies, density and viscosity measurements and analyses of water contents before being used in electrochemical cells.
Each of these electrodes was studied by means of cyclic voltammetry (CV) using the Ionic Liquid electrolytes previously selected as well as charge-discharge cycles. The CVs measured current as voltage is scanned for the working electrode under study with respect to a reference electrode. These basic electrochemical characterization gives a very accurate idea of the electrochemical activity of a given electrode immersed in a given electrolyte and is the basis for later setting up and testing the final device, in which two electrodes are combined and polarized in a real supercapacitor cell.
WP6 Assembling: Setup & prototype
The objective of the prototype developed in this work package was to provide an efficient demonstration of the use of NEST energy storage system in a wireless autonomous sensor which could be representative of the many possible applications of this very broad market.
Specific constraints of wireless autonomous sensors
A wireless autonomous sensor is composed of an energy harvesting unit, which should be able to gather energy from the environment of the device or during its working. It can be mechanical energy (wind, vibrations), thermal energy (thermoelectrics) or solar energy (photoelectrics) for example. This energy should be stored in an energy storing device, and delivered to a communication system which can send the information gathered by the sensor.
An energy storage used in a wireless autonomous sensor will have to be able to sustain very random charging profile (intermittent power peaks with non-predictable length), while having a huge cycle life. It should be able to operate in a very wide temperature range as well as sustaining vibrations and shocks without compromising the safety of the device.
A connected shoe as an efficient representation of the wireless autonomous sensors
A connected shoe has been chosen as a simple yet effective representation of the wireless autonomous sensors. Piezoelectric patches are generating energy through flexion of the tip of the sole. This energy is stored into NEST supercapacitors coin-cells and delivered to a bluetooth system. Information are gathered from an acceleterometer implemented in the shoe and send to a tablet application which is showing the amount of steps as well as the walking speed. Furthermore, in order to facilitate the testing of NEST current products and future implementations, an automated test-bench has been produced. Automated walking profile can thus be created in order to compare the cells on the exact same basis.
Testing of NEST coin-cells and perspective for future implementations
The testing have shown that NEST products were adapted to the very random charging discharging profile. The device is both working under automated testing and real walking. Both the device and the coin-cells were able to sustain the vibrations during a real walking.
The testing have however also stressed out the very high requirement for low self-discharge of the energy source. Thus, in order for NEST products to be able to reach the market of autonomous wireless sensors, future work will need to be oriented to the limitation of the self-discharge.
WP7 Performance evaluation; Setup & prototype
Upscaling of the capacitor systems to industrial assembly, i.e. pouch and coin cell, was successfully realized with silicon nanostructure-based electrodes. EMIM TFSI, MPPyrr TFSI, AMIM TFSI + EMIM TFSI ionic liquids were selected to obtain high energy/power device. The research was focused on the comparison of electrode materials deposited on two different substrates. The results show that better electrochemical behavior is obtained when the silicon substrate is used. It is caused by the reproducible deposition process of stable silicon nanostructures on a silicon wafer. The stainless steel (316L) as a substrate faced the problems concerning adhesion and peeling of the nanotrees from the surface. On the other hand, the stainless steel substrate is more suitable for pouch cell assembly. It is the comprise between closing easiness and Si nanostructured electrode damage risk.
Moreover, silicon-based supercapacitors with ionic liquids are able to operate at different temperatures. It was proved that supercapacitor with MPPyrr TFSI can effectively operate up to +60oC. At low temperatures, a mixture of AMIM TFSI + EMIM TSFI played a role of a suitable electrolyte. This IL mixture can sufficiently work even at -40oC.
An additional coating made on silicon nanotrees can significantly enhance overall electrochemical performance. The presence of electrically conductive polymer (PEDOT) or boron-doped diamond improved the energy output and total power (ca. 10 times). Diamond coated electrodes exhibit higher temperature stability – IL electrolyte does not decompose so rapidly (research made with MPPyrr TFSI). Moreover, diamond coated electrodes demonstrate much higher specific capacitance values than pure SiNTs. Despite the fact that PEDOT presence increases specific capacitance at room temperature, it should be taken into account that PEDOT pseudocapacitance does not contribute at -40oC and the system behave similarly as with pure SiNTs.
The cycle-life performance was studied using galvanostatic charging/discharging (GCPL) at a relatively low current density (0.1 mA cm-2). All the systems (independently on assembly type) are able to operate effectively for thousand cycles (> 50 000). Even in harsh conditions (-40oC or +60oC), silicon-based supercapacitors work sufficiently. Maximum relative specific capacitance decay does not exceed 20%, what is in accordance with International Standard IEC 62391-1. The supercapacitors with silicon nanostructures and ionic liquids meet the future application requirements, due to their long cycle-life.
In order to select the most appropriate system, comparison of coin cell assembly with pouch cell one was done. It shows that coin cell construction is the most appropriate one – the highest capacitance values are obtained, and the highest reproducibility is achieved. Additionally, the behaviour of the system at OCV conditions (self-discharge) for coin cell construction reveals the smallest decay of voltage after 20 h. Furthermore, it can be observed that specific resistance at 100 kHz for coin cell oscillates between 3-4 Ohm cm2 while for pouch cell construction is ca. 2 times higher – it equals to 7.5 Ohm cm2.
Time constant was evaluated for capacitor devices. This value ranges from a few ms to 5s. Obviously, silicon nanostructured electrodes decorated with PEDOT have the highest time constant.
A new type of bipolar pouch cell was constructed in order to improve the overall performance of the system. Connected in parallel three microcapacitors were implemented with the use of bipolar electrodes. In this assembly, two substrates (with three electrodes) were connected to the positive pole, while the other two substrates (with three electrodes) placed on the opposite side of coffee bag cell were connected to the negative one. The most important achievement obtained in this system is total power value which is ca. 3 times higher than for the other assemblies.
WP8 Exploitation & dissemination
Work package 8 is about Dissemination. That means that the main goal is to inform the community, both scientific and social about the results of the project, and the benefits from it, including scientific, economical and ecological aspects. As part of the project a total of 16 scientific papers were published in 6 different scientific journals. In order to discuss the results with more industrial partners, who might like to use the supercapacitor technology we have developed within the project, a workshop was organized in October 2014, which included a total of 14 talks in front of 50 representatives of industry and academia from the EU. Another workshop, together with some partners from EU projects on the same call was organized in Aril 2016 in Grenoble. A further way to inform the scientific community of our results is to participate in scientific conferences and fairs. All partners of the project have participated in such events have made oral or poster presentations (a total of 34 oral and 7 posters) about the innovation potential of our technology. As with all other social groups, one of the best ways to communicate a message is through personal contacts, and this is what is used the most in such events.
Another important part of this work package was to estimate if the technology developed within the project is competitive with other commercial and soon to be commercialized technologies. This was done by creating and constantly updating a patent database. It showed, that although the amounts of patents on supercapacitors is constantly growing, showing a high interest in this technology, only a very small part of them (around 30 ) to some extent use some of the materials as we do. This indicates that we are using very competitive technology that has a high value in future markets. This allows for the results of the project to be commercially exploited, either in the form of patents (1 EU patent filed) or as share growth and expansion on the available markets for the different industrial partners. The market study, which was also made as part of this work package shows more than 30 industrial producers of such electrochemical devices, who might be interested in our technology and be willing to license and implement it in their deices. The market size for such devices I expected to grow up to $2 billion by 2020.
The project website (www.project-nest.eu) is the window of the project to the world. The website has two domains: a public area, which can be accessed without restrictions and includes general information about the project, project results, information on the partners and a list of published results. In the private part of the website, which is intended only for the members of the project consortium, all documents, generated by the partners (reports, presentations, publications and deliverables) are available for download and upload.
Potential Impact:
The NEST project was remarkably productive in terms of innovation and dissemination of knowledge: 13 publications (among them 7 involved 3 partners and 2 involved 4 partners), 2 dissemination reports, one patent, 34 oral communications and 7 posters.
In addition to the mandatory midterm meeting, CEA organized a meeting “Highlights on electrochemical energy storage at CEA-Grenoble” the day before the final NEST meeting on April 28th 2016. The participants were from technological Institute LITEN and fundamental institute INAC with the attendance of EU representatives (PO and reviewers) and of a part of the NEST Partners.
The exploitation of the project results could be illustrated by the creation of the start-up EnWires for the use of Si Nanowires in electrochemical energy storage. The company was founded in April 2016.
List of Websites:
Project website: http://www.project-nest.eu/
Project coordinator contact details:
CEA-Grenoble, 17 rue des Martyrs
38054 Grenoble cedex 09
FRANCE
Tel: + 33 4 38 78 48 41
Fax: + 33 4 38 78 56 91
Mob: + 33 6 87 98 45 93
E-mail: gerard.bidan@cea.fr
