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Final Report Summary - NANOCATE (Nano-carbons for versatile power supply modules)

Executive Summary:
The NanoCaTe project aims on the development of nanoscale carbon based materials for energy applications, namely energy harvesting for body wearables and satellites.
During the project we produced, modified and investigated different nanocarbon species, intrinsically conducting polymers and composite material for thermoelectric and energy storage applications.
The thermoelectric properties of printable material could be improved; however, performance is still not enough to address commercialization of the technology. Therefore a second material development route was established in which bulk material on the basis of Bismuth Telluride was improved due to incorporation of modified nanocarbon material. The thermoelectric performance of this new developed composite material was increased from zT(300K) 1.2 to 1.4.
With this new material an innovative TEG design combining the best of both worlds is realized. This TEG utilizes a high performance composite nano carbon bismuth telluride bulk material assembled on a flexible and durable substrate. It features a significant weight reduction by more than 40 % and flexibility for easy integration on non-flat surfaces while maintaining superior thermoelectric performance.
Two energy storages system based on nanocarbons such as CNT and rGo (reduced graphene oxdide) have been developed in the framework of NanoCaTe. Secondary batteries as well as supercapacitors have been developed with modified carbon materials. Both electrical conductivity and areal capacity have been increase respectively using CNT and rGo. Additionally the synthesis and functionality of robust ionogel electrolyte suitable for both energy storage systems with enhanced performances respecting to commercial electrolyte such as EC, DEC or ACN.
Throughout the NanoCaTe project, developments on integrated electronics towards an “all-silicon” TEG-harvester operated wireless and contactless sensor node, particularly for on-body use case, reaching energy autonomy by exploiting the heat flow between skin surface and environment.
The developed key innovations addressed highest integration, optimization of thermal to electric energy conversion and optimization of power and energy consumption, in detail:
- All-silicon Sub-GHz wireless transmitter, based on an internal LC resonator, exhibiting crystal-oscillator equivalent stability properties (frequency stability < 100 ppm), which could be met by the integration of proper temperature compensation, combined with mechanical stress sensors. Due to the very fast power up settling time and the omission of an external quartz device this exhibits great benefits for low-power, seamless sensor integration.
- Highly compact, multi modal “sensor grain” with advanced power management, integrated temperature monitoring and NFC readout interface including on-chip coil, operating at very low power level.
- Fully integrated TEG-interface based on sub-threshold CMOS circuitry, which allows thermal energy conversion at very low temperature gradients of dT~1 °C across the TEG element to charge a battery or capacitor up to 3 V at 1 μA. A second chip version includes an innovative approach for MPPT (Maximum Powerpoint Tracking), which dynamically maximizes the output power by matching to the source and inevitable chip-internal losses at the same time.
Several full functional sensor node prototypes, operating in different modes (background monitoring and ad-hoc reporting, use of NFC-interface and wireless interface, with and without energy storage) could be successfully demonstrated.
Project Context and Objectives:
The NanoCaTe project aims on the development of nanoscale carbon based materials for energy applications, namely energy harvesting for body wearables and satellites.

In developed countries, ca. 40% of the total fuel consumption is used for heating. Of this, about one-third is wasted due to insufficient transformation technologies into electricity.
Waste heat is one of the biggest industrial losses which can be used by exploiting the Seebeck effect enabling special materials to generate electrical energy if exposed to a temperature gradient, e.g. from waste heat. Devices built up from these materials, thermoelectric generators (TEGs), have the advantages of being a motionless and durable way for energy supply.
Since in every technical process waste heat is produced, there is a lot of potential in using thermoelectricity for the energy conversion. However, in this project the energy sources aimed for are solar and body heat. The applications addresses potential energy supply for satellites and wearables.

The materials developed within the project are thermoelectric and energy storage materials, based on nanocarbon material. Nanocarbon species, like carbon nanotubes, are modified and introduced to these material systems in order to improve their overall efficiency. In the project 1D and 2D structured nanocarbons will be studied and developed due to the predicted improvements in thermoelectric properties, like electrical conductance.

The developed materials are used for the production of thermoelectric generators and secondary batteries and should contain properties like environmentally friendly, abundant, flexible and printable.

One key aspect in the material and device development is the aim for low-cost and high throughput production techniques, e.g. printing. Especially printed thin film solutions for harvester and storage devices can be easily miniaturized and so embedded into a device with an area of some cm² with integrated energy management circuitry and a communication unit. Targeted low-power applications are for example sensors working in a pulsed mode. This can be a wireless sensor e.g. structural health monitoring or wearable health care purposes. The use of secondary batteries or supercapacitors will make replacement obsolete and improve the lifetime of the device. Especially easily applicable wireless sensor networks provide a large application area and market.

Together with the development of ultra-low power electronics, a complete system consisting of energy harvester, energy management, energy storage and interfaces for sensors is set up for self-sustaining applications. This sensor platform will be used as self-sustaining, universally usable, and maintenance-free power supply.

The project will substantially strengthen the position of Europe in the field of thermoelectric and storage materials by developing devices with increased lifetime produced by cost-efficient technologies and therefore contributing to a further promotion of cleaner energy technologies.

The main objectives are understanding and improvement of nanocarbon material and composite material for thermoelectric applications, the development of high temperature stable secondary batteries, the investigation and process development of alternative production techniques for thermoelectric generators and batteries like printing and the development of miniaturized ultra-low power energy management systems. These technological investigations are accompanied with theoretical simulations on the materials and system scale as well as demonstrators for the aimed applications.
Project Results:
Material

Carbon nanostructure material

An extensive screening of different kinds of nanocarbons like graphene nanoplettelets, commercial and in-house single walled carbon nanotubes, multiwall carbon nanotubes were performed. Additionally purification methods like based on wet chemical treatment as well by physical means were successfully done. Due to proper purification the powerfactor of the in-house SCWNT material could be increased by a factor of 10 to ~15 µW/mK². However, Seebeck coefficient is highly sensitive to purification, especially chemical, and heat treatment, due to adsorbents.
Also purified semiconducting SWCNT were produced which showed higher Seebeck-coefficient compared to as produced mixtures of semiconducting and metallic tubes. With our method an enrichment of 98% of semi conducting SWCNT was achieved. It is concluded that purely semiconducting tubes would be more suitable for subsequent modification. Due to the low yield of the recent manufacturing process this material was not further utilized.
The main outcome can be drawn that pure material has to be chosen for its specific applications like electrodes or thermoelectric. By separating of semiconducting nanotubes, the Seebeck coefficient of SWCNTs has been dramatically increased up to 100 μV/K. Treatment of these nanotubes with oxygen plasma led to further increase of Seebeck coefficient up to 160 μV/K. SWCNT tend to be the most suitable for both applications (in respect to each unique properties among the group of SWCNT). MWCNT were not suitable at all due to a small Seebeck-coefficient, low electrical conductivity and its lack of efficient doping.
Modification with oxidizing agents appears to be beneficial for the improvement of the electrical conductivity, however a drastic decrease in Seebeck-coefficient was observed. In the end the powerfactor remained almost unaltered.
Further direct covalent modification of SWCNT was investigated by Bor and Nitrogen doping. The incorporation of up to 8 at% of Bor into the carbon nanostructure, the powerfactor was increased up to ~ 120 µW/mK² (mainly due to improvements in electrical conductivity).
Further, doping of SWCNTs with nitrogen atoms using hydrothermal reaction in order to achieve negative Seebeck coefficient for carbon nanotubes was systematically studied. A maximum n-doping was achieved with 3.2 at.% of Nitrogen. Under vacuum conditions, n-doped SWCNTs showed Seebeck coefficient of –20 μV/K.

Polymer material

Different intrinsically conductive materials like Polynaniliin, Polycarbazole, PEDOT:PSS and PEDOT:tos, and the n-type poly(Kx[Ni-ett]) were synthesized and investigated for the purpose of preparing an printable thermoelectric active composite material based on carbon nanostructures and intrinsically conducting polymers. During the screening commercial and self-synthesized polymers were investigated. It was concluded that commercial PEDOT:PSS was most suitable for further investigations due to constant quality and availability.
Extensive effort was put in the understanding of favorable film formation of PEDOT:PSS. In a nutshell, optimum annealing temperature and annealing time was identified as well as Co-Solvent addition (stirring time, temperature), like ethylene glycol or dimethyl sulfoxide, in order to improve the electrical conductivity by 3 orders of magnitude. The electrical conductivity of coated PEDOT:PSS films varies between 600 – 800 S/cm with an Seebeck coefficient between 12 – 17 μV/K. Several modifications were tested in order to increase the relatively low Seebeck coefficient. In order to modify this material, PEDOT:PSS was mixed with dimethyl sulfoxide and films were spin coated and annealed. Subsequently samples were immersed in sulfuric acid and annealed until the film was dry. It was seen that the electrical conductivity increases significantly from ~ 600 S/cm up to 2200 S/cm while the Seebeck coefficient decreases. Effects like increased brittleness of the previously very flexible films prevent further utilization, though. The powerfactor for the used PEDOT:PSS blend was ~ 25 µW/mK².
Composites of PEDOT:PSS and nanocarbons were investigated. It showed that no beneficial combinations could be found compared to the bulk values of the polymer or the carbon species itself. This is mainly attributed due to necessary additives such as surfactants or the change of microstructure of the polymer matrix.
Even though to the extensive research effort regarding the polymeric material development, it was not able to reproduce the alleged state of the art from the scientific literature. Undoubtedly this disappointing situation however helped to understand the problem of the matter. The main factors for the non-reproducibility are the characterization and the sample preparation itself.
The characterization of the material plays an important role. Since the literature used spin coated samples with thicknesses of around 100 nm the contacting and mounting of the samples is crucial. After detailed investigation in the literature on the used characterization methods shown (especially Seebeck-coefficient) it turned out that it was not appropriate for conducting this task (due to used evaporated contacts and to small electrode distances, later a scientific paper was published which addressed exactly this case and concluded that the Seebeck-coefficient can be overestimate by a factor of 3 when using improper contact geometry). This statement is supported by other groups in this field.
The second reason is the poor controllable morphology of the PEDOT:PSS film. It is known that the properties of polymer thin films can be highly anisotropic (especially in terms of electrical conductivity and thermal conductivity). The microstructure of the polymer chain alignment and crystallinity plays therefore an important role. The final film morphology strongly depends on the preparation procedure and thermal history of the material. These two factors may originate in the dramatic overestimation of thermoelectric performance of this p-type material.
A novel printable n-type polymer which is rather stable under ambient conditions was developed. This polymer based on poly(Kx[Ni-ett]) show adjustable rheological parameters and relatively good thermoelectric properties. Initially this polymer can be obtained as powder because it precipitates during the synthesis. Due to adjustments of solvents and synthesis procedure we obtained this polymer over a wide viscosity range like water like liquid to gel. The power factor is in the range of ~2 µW/mK².

Bismuth Telluride based material

Different BiTe mateirals were investigated. It was concluded that SPS or hot press cause clear anisotropic properties within the ingots produced. Based on this the thermoelectric properties where investigated according to the dimensional arrangement of the ingot. It could be shown that the spatial arrangement can cause a difference in the zT value of more than 0.3. Further the incorporation of carbon nanotubes into the BiTe material resulted in an overall increase in the electrical conductivity and thus increased the zT value of up to 1.5 (p-type) and 0.8 (n-type), to date one of the best values for BiTe based bulk material.

Thermoelectric generators
In combination with the material development two innovative thermoelectric generator (TEG) designs, one for the utilization of body heat and one for satellite applications, were developed. Both designs are made in order to feature flexibility and robustness.
For the satellite application a new TEG design combining the best of both worlds is realized. This TEG utilizes a high performance composite nano carbon bismuth telluride bulk material assembled on a flexible and durable substrate. It features a significant weight reduction by more than 40 % and flexibility for easy integration on non-flat surfaces while maintaining superior thermoelectric performance.
A belt like generator for the utilization of body heat is proposed. Printed TEG films can be assembled to reach the voltage required for running and maintaining charging of secondary batteries. Simulations for both use cases with respect to optimized geometry and material properties are made and evaluated.

Printed TEG
Different TEG designs based on the printing approach has been investigated (e.g. coiled-up, stacked and conventional). Printing techniques like dispenser printing and screen printing and spray deposition techniques like AerosolJet® were investigated. Materials used for the printed TEG were carbon nanotubes, intrinsically conducting polymers, composites thereof, as well as BiTe based pastes and combinations of composites. It was concluded that the performance of the printed materials is still several orders of magnitude lower compared to the best state of the art bulk material prepared in this project. However it was possible to print lab demonstrators with all material combinations and produce TEG. The performance of theses printed devices followed the known physical rules for bulk TEG. By simulation it was shown that due to simple geometrical variations like number of legs and leg length the power output can be improved by a factor of 50.
The printed TEG devices showed an power output in the nW to low µW region. By connecting several TEG the power output can be further increased.

Flexible bulk TEG
For the satellite application a new TEG design combining the best of both worlds is realized. This TEG utilizes a high performance composite nano carbon bismuth telluride bulk material assembled on a flexible and durable substrate. It features a significant weight reduction by more than 90 % and flexibility for easy integration on non-flat surfaces while maintaining superior thermoelectric performance.
This FlexTEG (15x50 mm²) consists of 50 pn-pairs using the materials developed in NanoCaTe. Test result shows promising power generating characteristics. At a hot side temperature of ~200 °C, this FlexTEG produces a maximum power of over 160 mW with an open voltage of about 1000 mV and a maximum current of 800 mA. An upscale of this prototype is planned in the future.

Storage material and devices
Two energy storages system based on nanocarbons such as CNT and rGo (reduced graphene oxdide) have been developed in the framework of NanoCaTe. Lithium ion battery has been developed based on LiFePO4 cathode active material using nanocarbons as conductive additive fully satisfying the applications targeted (2.25 cm², 2.5 mWh, 2.5mW, leakage current <1 μA). Carbon based supercapacitor have been optimized using functionalized hydrophilic nanocarbon for the aqueous ink formulation of the printed electrodes. Both electrical conductivity and areal capacity have been increase respectively using CNT and rGO. Contrary to the lithium ion battery, the supercapacitor show an important leakage current similar to any commercial devices and not suitable to be coupled with the TEG (1.5x1.5 cm², 1,8 V, 10 Wh/kg, 100 mJ/cm², 20 KW/Kg, 5 mW/cm², flexible with radius of 20 mm, leakage current between 0.3 to 1.0 mA).
Further it was shown that the synthesis and functionality of robust ionogel electrolyte suitable for both energy storage systems with enhanced performances respecting to commercial electrolyte such as EC, DEC or ACN (4.5 V, 350 °C) was successful.

Sensor node and demonstrators

Several modular system evaluation platforms were built and evaluated. All intended functions could be successfully demonstrated, e.g. contactless NFC-communication, background temperature monitoring and reporting, RF-transmission @868MHz, all in combination with a bulk-TEG and energy storage.
Work on integrated electronics towards an “all-silicon” TEG-harvester operated wireless and contactless sensor node, particularly for on-body use case, reaching energy autonomy by exploiting the heat flow between skin surface and environment, was carried out. For the semiconductor process an in-house 130 nm standard CMOS technology has been used.
The developed key innovations addressed highest integration, optimization of thermal to electric energy conversion and optimization of power and energy consumption, in detail:
- All-silicon Sub-GHz wireless transmitter, based on an internal LC resonator, exhibiting crystal-oscillator equivalent stability properties (frequency stability < 100 ppm), which could be met by the integration of proper temperature compensation, combined with mechanical stress sensors. Due to the very fast power up settling time and the omission of an external quartz device this exhibits great benefits for low-power, seamless sensor integration.
- Highly compact, multi modal “sensor grain” with advanced power management, integrated temperature monitoring and NFC readout interface including on-chip coil, operating at very low power level.
- Fully integrated TEG-interface based on sub-threshold CMOS circuitry, which allows thermal energy conversion at very low temperature gradients of dT~1 °C across the TEG element to charge a battery or capacitor up to 3 V at 1 μA. A second chip version includes an innovative approach for MPPT (Maximum Powerpoint Tracking), which dynamically maximizes the output power by matching to the source and inevitable chip-internal losses at the same time.
Several full functional sensor node prototypes, operating in different modes (background monitoring and ad-hoc reporting, use of NFC-interface and wireless interface, with and without energy storage) could be successfully demonstrated.
An integration demonstrator consisting of a satellite structure panel and TEG was build. The main focus was to develop integration strategies and techniques. This all was carried out with careful consideration of space certification aspects.
A belt like wearable demonstrator is suggested for utilizing body heat. This approach has to be realized and further elaborated for possible future applications concerning body heat.

For wearables use cases with thermoelectric energy sources tend to be one of the most challenging fields, since just small temperature gradients are available and high requirements regarding material performance are demanded.

However, the NanoCaTe project improved the state of the art in every aspect of the investigated field ranging from thermoelectric and energy storage material, the processing of those as well as the electrical energy management and the identification of use cases for space applications and wearables.
Potential Impact:
Socio economic impact
The results of NanoCaTe include the development of printable thermoelectric materials and generators, battery materials and devices as well as energy management devices for practical utilization. The aimed applications were energy harvesting on satellite radiator panels and the utilization of body heat for sensing.
At this stage of the project several conclusions can be drawn.
First, requirements for the TEG and accompanied storage devices for body heat utilization by thermoelectric conversion seem to be the most challenging scenario. The reason is that only a very small temperature gradient (~1 K) is present. This implies a high number of thermolegs in order to reach the start-up voltage for the energy management device. Currently, printed thermoelectric devices still lack in efficiency. However, during the project a novel flexible bulk TEG with outstanding properties was developed. This device show high efficiency of 6 - 7 %. The next step in this development will be the integration into a wearable device. By this the utilization of body heat to power electronic devices has made a big step forward. The market for wearables is expected to grow by 17 % in 2017. In combination with the worldwide digitalization further markets grow and relevance is expected.
The application in satellites shows significantly different challenges, e.g. light weight, mechanical resistance. The temperature gradient is expected to be also below 10 K. The utilization of energy harvesting in radiator panels of satellites can have several benefits: available electrical energy for structure health monitoring, the operation of an additional electrical circuit decoupled from the main power supply and working as an emergency power supply.

The utilization of body heat to power wearable electronics will have also a big share on social implications. The ability to power, charge or extending battery runtime will influence the acceptance of wearable devices due to less power grid dependency. It is expected that in near future people (in work space environments and also noncorporate) will be equipped and outfitted by an increasing number of wearable gadgets. These devices will be more powerful, hence the demand for portable electrical energy in total will be increasing.
Satellite

Main dissemination activities
The Consortium aims to disseminate information, knowledge and results of the project within the partners and outside the consortium regarding the development of a more efficient thermoelectric- and storage material based on nanocarbon (e.g. graphene and CNT) to reclaim waste heat by thermoelectric generators and to storage the energy in super capacitors or secondary batteries for manifold applications like pulsed sensors or mobile electronic devices.

Workshops
A workshop with industrial stakeholders was held at Barcelona, Spain at KIM with the title “KIMconference 2014”. The aim was to introduce the NANOCATE.
Between 25th and 26th of June 2015, the first international NanoCaTe workshop/conference was held in Dresden. This event organized by Fraunhofer IWS with the support of KIM was entitled: “Energy harvesting systems – FlexTEG 2015”.
This event was conceived as a dissemination workshop for European Projects regarding Energy Harvesting (NanoCaTe, SiNERGY, MATFLEXEND, MANpower). Several partners from each project presented their project objectives and results accomplished. The workshop consisted of over 50 international attendees and 15 talks.
Between 26th and 27th of September, the second international NanoCaTe workshop was held in Dresden. This event organized by Fraunhofer IWS was entitled “Energy harvesting systems – FlexTEG 2016”. This event was also conceived as a dissemination workshop for european Projects regarding Energy Harvesting (NanoCaTe, SiNERGY, MATFLEXEND, MANpower), however more focus was given to researchers outside of the European projects. The workshop consisted of over 35 participants and 19 talks from international researchers.

Over 27 conference attendances on national and international conferences with oral and or poster presentations were carried out by the whole consortium of NanoCaTe.
Additionally several contributions at accompanied fairs like “International Conference on Theromelectrics” and LOPEC, Munich with booth presence were realized to spread NanoCaTe results.

Over 17 peer reviewed publications have been accepted during the project phase. In addition to the project phase further publications based on the findings within NanoCaTe are expected.

Five technologies have been identified during the NanoCaTe project for further dissemination and impact.
- Printable thermoelectric polymers (n-type) and composite material
- Self-Supported Printed 3D energy storage micro-device
- Analog Chip Design for body sensor applications
- Printed thermoelectric generators
- Flexible bulk thermoelectric generators

List of Websites:
www.nanocate.eu

Related information

Reported by

FRAUNHOFER-GESELLSCHAFT ZUR FOERDERUNG DER ANGEWANDTEN FORSCHUNG E.V
Germany
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