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Fabrication of Highly-integrated Thermoelectric Generators Based on Ionic Liquids
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 2 (2015) 669 – 674
12th European Conference on Thermoelectrics
Fabrication of highly-integrated thermoelectric generators based on Ionic Liquids Stefanie Uhl*, Michaël Pellet, Jessica Tschanz, Edith Laux, Tony Journot, Laure Jeandupeux, Herbert Keppner Haute Ecole Arc Ingénierie (HESSO), Eplatures-Grise 17, 2300 La Chaux-de-Fonds, Switzerland
Abstract
Recent research in Ionic Liquids as new thermoelectric materials [1, 2] demands a suitable implementation in an industrially producible generator. Such a flexible micro generator can harvest electrical energy in order to extent battery lifetime or serve as single energy source for medical and wearable applications. The paper presents the fabrication steps of a new, highly integrated thermoelectric generator (TEG) based on ionic liquids. Furthermore, the applicability of ionic liquids with copper electrodes in thermoelectric devices is shown due to a passivation process at the electrode surface, which allows the usage of commercially available copper clad laminates. © 2014 Elsevier Ltd. All rights reserved. © 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of Conference Committee members of the 12th European Conference on Selection and peer-review under responsibility of Conference Committee members of the 12th European Conference on Thermoelectrics. Thermoelectrics. Keywords: thermoelectric generator, thermo-electricity, Ionic Liquids, Solid-on-Liquid, flexible devices, medical power supply, energy harvesting, packaging
1. Introduction In recent years, Ionic Liquids as thermoelectric materials have been demonstrated by several researches [1, 2]. Ionic Liquids (IL) with n- and p-type equivalent Seebeck coefficient [3] could be found, allowing serial connection of many individual single p/n junction-equivalent liquid cells. Handling of liquids at this level is not at all a standard procedure and a new technology must be developed at feasibility level, allowing large-scaling and manufacturing at industrial level. The conventional fabrication techniques of flexible thermoelectric modules including solid thermoelectric material are hence not applicable using Ionic Liquids. In this paper first attempts for serial connection * Corresponding author. Tel.: +41 32 930 1464; fax: +41 32 930 29 30. E-mail address: [email protected]
2214-7853 © 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of Conference Committee members of the 12th European Conference on Thermoelectrics. doi:10.1016/j.matpr.2015.05.084
670
Stefanie Uhl et al. / Materials Today: Proceedings 2 (2015) 669 – 674
combining well known MEMS technologies such as isotropic plasma etching, Laser micromachining with a novel technology called Solid-on-Liquid deposition [4] are presented. Increasing needs associated with portable electronic devices (medical applications, entertainment, communication etc.) and the decreasing power consumption of those, envisage self-sufficiency from local supply sources. Thermal energy recovered in the human environment could be sufficient to drive low-power electronics. Looking at an available spot size of 20x20mm2 (591 integrated IL junctions) and a temperature difference of 5K the following estimation using a combination of the ionic liquids 1-Butyl-3-methylimidazolium-tetrafluoroborate [BMIM]+[BF4]- and 1-Hexyl-3-methylimidazolium iodide [HMIM]+[I]- for obtaining an output voltage which is sufficient to drive low-voltage electronics. The data for Dp and Dn are taken from Fig. 1, also presented in [3]. D TEG = N · (Dp - Dn ) = 591· (421 μV – (-83 μV) = 297.86 mV/K VTEG
(1)
= D TEG · 'T = 297.8 mV/K · 5K = 1.49 V (2)
αTEG: Total Seebeck coefficient of TEG N: Number of p/n-junction of TEG αp: Seebeck coefficient of p-type equivalent IL αn: Seebeck coefficient of n-type equivalent IL VTEG: Open circuit potential of TEG ΔT: Temperature difference between hot and cold electrodes
Fig. 1. Seebeck coefficient measurements of [BMIM]+[BF4]- and [HMIM]+[I]- at a redox concentration of 0.01M of I2/I- and with different electrode spacing.
The layout of the TEG is briefly sketched in Fig. 2, a view of a first technological prototype is shown in Fig. 3.
Fig. 2. Principle of monolithic serial connected thermoelectric module [5].
Fig. 3. Prototype with 1182 serial connected IL-cells.
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Stefanie Uhl et al. / Materials Today: Proceedings 2 (2015) 669 – 674
2. Fabrication of thermoelectric generator 2.1. Cavity structuring The base material of the device consists of a commercially available metal-clad laminated polyimide sheet (DuPont, Pyralux AP8565). In a first step, blind holes with a diameter of 200μm are laser-machined through the copper and deep into the polyimide (Laser Nanosecondes, Type Nd:YAG Fresco-Ultra, Fab. Coherent, spot size 20μm); the laser parameters had to be adapted to the present material. Best results could be achieved using a laser- pulse frequency of 25kHz and a scanning speed of beam 192mm/s. The deep hole machining ends close to the adjacent copper electrode. This is important to exclude creating a hole that would let escape the IL form the cavity after filling, therefore laser machining was stopped before; the cross section of the sample in this state is shown in Fig. 4b. In order to perfectly control the machining of the 1182 holes the finishing was carried out using isotropic plasma etching using an O2/SF6 plasma at high pressure and low power. (Fig.5b). Plasma etching (PE) is preferred due to the possibility to remove all polymer traces and to stop the machining at the copper level due to the high selectivity of the process. (Plasma etching system used: SENTEC SI500). The increase of temperature due to plasma exposure must be well controlled in order to avoid delamination of the copper sheets from the polyimide. In contrast to Deep Reactive Ion Etching (DRIE) that is used for obtaining high aspect ratios in etching, plasma etching is preferred because undercutting of the masking hole of the top-copper is desired, allowing the liquid to create the top contact (Fig. 5b).
a 4a
Copper Polyimide
4b Fig. 4. (a) Schematic profile of the cavity after micro-machining by laser. (b) Image with optical microscope: The ablation is stopped before damaging the opposite copper layer. Laser parameter: For Copper: pulse intensity 2W, 5 cycles; For Polyimide: pulse intensity 0.3.W, 15 cycles.
a 5a
Copper Polyimide
5b Fig. 5. (a) Schematic profile of cavity after isotropic plasma etching. (b) Image with optical microscope: The copper is cleared on the bottom and topside of the cavity to enable a conductive contact with the liquid. Etching parameter: Power ICP: 1200W; pressure 40Pa, temperature 10°C, O2 flow 7.8sccm, SF6 flow 5.2sccm, time 30min.
Special care must be taken for stabilizing the laser and the PE process because the loss of one the 1182 contact will let fail the generator. In Fig. 6 a cross section of six wells is represented, the top-and bottom well will be reserved for n and p –equivalent ILs.
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Stefanie Uhl et al. / Materials Today: Proceedings 2 (2015) 669 – 674
Fig. 6. Larger view of a partly completed (underneath side Laser + PE) and party uncompleted top-side (laser) of a sequence of volumes prepared prior to IL filling; note the upper ones will be filled with e.g. [BMIM]+[BF4]- (p-equivalent IL) and the bottom wells will be filled with [HMIM]+[I]- (n-equivalent IL).
2.2. Cavity filling, packaging and electrode structuring Before filling, the cavity surface is activated with argon plasma and subsequently filled with the corresponding liquids in a specially developed filling chamber Fig. 7. It allows in vacuum plasma activation of the inside of the wells, top and bottom-well filling with two different ILs. The vacuum is needed in order to evacuate the air in the well prior to filling. After filling the samples must be bubble-free sealed. 1
+
3 4
6 7 8 10
5
9
Fig. 7. Schema of filling chamber: 1.gas inlet valve, 2. Argon source, 3. Rotating sample holder, 4. Ventilation valve, 5. Vacuum chamber, 6. IL reservoirs, 7. DC-Electrode with feed through 8. DC source, 9. Vacuum valve, 10. Vacuum pump
Fig. 8. Electrode structuring by laser micro machining. Laser parameter for machining: For Parylene: pulse intensity 0.3.W, 5 cycles. For Copper: pulse intensity 1.7W, 9 cycle;
The liquids are encapsulated with Parylene using Solid-on-Liquid Technology [4]. The polymer poly-di-chloropara-xylylene, known as Parylene C, is grown on low pressure ILs using conventional low-pressure chemical vapor deposition process. Parylene encapsulates the liquid. After the deposition of the first Parylene layer, the serial connection of the electrodes are structured, again, by laser micromachining (Fig. 8).
Fig. 9. Schematic profile of thermoelectric module to demonstrate the different encapsulation steps after filling the cavities with IL and laser engraving of serial connected electrodes.
Stefanie Uhl et al. / Materials Today: Proceedings 2 (2015) 669 – 674
673
3. Self-passivation of copper electrode Up to now, only little scientific work was carried out using ILs for thermoelectric generators. Most of the work used inert electrodes such as Rh and Pt, carbon nanotubes, etc. in order to avoid chemical reactions with the electrode and to study pure effects in the IL. Looking at commercial applications, the concept that is presented in this paper has advantages because the copper/polyimide/copper substrates allow carrying out all process steps for an IL based generator. It is needed to study the compatibility (stability) between promising ILs and copper. In our study 1-Butyl-3-methyl-imidazolium-tetrafluoroborate [BMIM]+[BF4]- (Iolitec, Germany) has been tested. The tests were carried out in a single-junction chamber as shown in Fig. 10. The first warm-up run (12°C temperature difference between hot and cold electrode, a pronounced hysteresis was observed between heating up (black curve in Fig. 11) and cooling down (red curve in Fig 11). Obviously a reaction occurred leading to an accumulation of reaction products such as CuI2 was formed. Cooling down delivered a different electrochemical potential between the electrodes; the behavior is similar to a battery effect. After 15 day the experiment was repeated, showing an improvement of the effect in terms of hysteresis (comparison between blue and green curve in Fig. 11); at the end of the cool down-process the potential came back to its initial value. The experiment using copper and [BMIM]+[BF4]showed that there is a self-stabilizing effect of the electrode that is in contact with the IL. Iodide can be specifically adsorbed at copper surfaces. This spontaneously adsorption creates highly ordered Iodide layers on the Cu surface. Depending on the adsorption conditions, stable Cu-I layers are created whose dissolution is kinetically inhibited in a potential window of (+250 mV to -400 mV) versus normal hydrogen electrode NHE [6]. The XRF- analyses (Fischerscope X-Ray XDV-SDD) of the copper electrode surface after measurement showed Iodine.
Fig. 10. Schematic of thermo-electrochemical cell as Cu/Cu-I/IL/Cu-I/Cu structure.
Fig. 11. Self-stabiliuzing of the copper electrode after 15 days in contact with [BMIM]+[BF4]-.
4. Conclusion All fabrication steps of the thermoelectric module have been implemented and the serial connection of the individual cells was demonstrated. The 1182 wells containing generator could not be fully tested, because some imperfections in the well manufacturing might have interrupted the continuous structure of the generator, or some “battery” type electrode coatings on the copper still exists. The self-passivation of the copper surface happens after sufficiently long storage at IL exposure. Acknowledgements The authors acknowledge the financial support of the ‘Polish-Swiss-Research Programme’ for the project ‘‘Enerliq’’ in
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Stefanie Uhl et al. / Materials Today: Proceedings 2 (2015) 669 – 674
collaboration with Gdansk University of Technology in Poland. Grant number: PSPB-051/2010.
References [1] T. J. Abraham, High Seebeck coefficient redox ionic liquid electrolytes for thermal energy harvesting, Energy & Environmental Science, Issue 9, 2013. [2] Theodore J. Abraham, Douglas R. MacFarlane and Jennifer M. Pringle, Seebeck coefficients in ionic liquids – prospects for thermo-electrochemical cells, Chem. Commun., 2011, 47, 6260–6262. [3] S. Uhl, E. Laux, T. Journot, L. Jeandupeux, J. Charmet, H. Keppner, Development of flexible micro-thermoelectrochemical generators based on ionic liquids, Journal of electronic Materials, 2014, DOI: 10.1007/s11664-0143126-1. [4] J. Charmêt, O. Banakh, E. Laux, B. Graf, F. Dias, A. Dunand, H. Keppner, G. Gorodyska, M. Textor, W. Noel, N.F. de Rooij, A. Neels, M. Dadras, A. Dommann, Ch. Borter, M. Benkhaira, Solid on liquid deposition, Thin Solid Films, 518 (18) pp. 5061-5064, 2010 [5] H. Keppner WO/2003/00632257 PCT/CH2003/000016: Highly-integrated miniature thermoelectric converter, July 2003 [6] M.S. Röefzaad, Ordnungsphänomene redox-aktiver Moleküle auf Elektrodenoberflächen unter reaktiven Bedingungen, 2011. http://hss.ulb.uni-bonn.de/2011/2436/2436.pdf
ScienceDirect Materials Today: Proceedings 2 (2015) 669 – 674
12th European Conference on Thermoelectrics
Fabrication of highly-integrated thermoelectric generators based on Ionic Liquids Stefanie Uhl*, Michaël Pellet, Jessica Tschanz, Edith Laux, Tony Journot, Laure Jeandupeux, Herbert Keppner Haute Ecole Arc Ingénierie (HESSO), Eplatures-Grise 17, 2300 La Chaux-de-Fonds, Switzerland
Abstract
Recent research in Ionic Liquids as new thermoelectric materials [1, 2] demands a suitable implementation in an industrially producible generator. Such a flexible micro generator can harvest electrical energy in order to extent battery lifetime or serve as single energy source for medical and wearable applications. The paper presents the fabrication steps of a new, highly integrated thermoelectric generator (TEG) based on ionic liquids. Furthermore, the applicability of ionic liquids with copper electrodes in thermoelectric devices is shown due to a passivation process at the electrode surface, which allows the usage of commercially available copper clad laminates. © 2014 Elsevier Ltd. All rights reserved. © 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of Conference Committee members of the 12th European Conference on Selection and peer-review under responsibility of Conference Committee members of the 12th European Conference on Thermoelectrics. Thermoelectrics. Keywords: thermoelectric generator, thermo-electricity, Ionic Liquids, Solid-on-Liquid, flexible devices, medical power supply, energy harvesting, packaging
1. Introduction In recent years, Ionic Liquids as thermoelectric materials have been demonstrated by several researches [1, 2]. Ionic Liquids (IL) with n- and p-type equivalent Seebeck coefficient [3] could be found, allowing serial connection of many individual single p/n junction-equivalent liquid cells. Handling of liquids at this level is not at all a standard procedure and a new technology must be developed at feasibility level, allowing large-scaling and manufacturing at industrial level. The conventional fabrication techniques of flexible thermoelectric modules including solid thermoelectric material are hence not applicable using Ionic Liquids. In this paper first attempts for serial connection * Corresponding author. Tel.: +41 32 930 1464; fax: +41 32 930 29 30. E-mail address: [email protected]
2214-7853 © 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of Conference Committee members of the 12th European Conference on Thermoelectrics. doi:10.1016/j.matpr.2015.05.084
670
Stefanie Uhl et al. / Materials Today: Proceedings 2 (2015) 669 – 674
combining well known MEMS technologies such as isotropic plasma etching, Laser micromachining with a novel technology called Solid-on-Liquid deposition [4] are presented. Increasing needs associated with portable electronic devices (medical applications, entertainment, communication etc.) and the decreasing power consumption of those, envisage self-sufficiency from local supply sources. Thermal energy recovered in the human environment could be sufficient to drive low-power electronics. Looking at an available spot size of 20x20mm2 (591 integrated IL junctions) and a temperature difference of 5K the following estimation using a combination of the ionic liquids 1-Butyl-3-methylimidazolium-tetrafluoroborate [BMIM]+[BF4]- and 1-Hexyl-3-methylimidazolium iodide [HMIM]+[I]- for obtaining an output voltage which is sufficient to drive low-voltage electronics. The data for Dp and Dn are taken from Fig. 1, also presented in [3]. D TEG = N · (Dp - Dn ) = 591· (421 μV – (-83 μV) = 297.86 mV/K VTEG
(1)
= D TEG · 'T = 297.8 mV/K · 5K = 1.49 V (2)
αTEG: Total Seebeck coefficient of TEG N: Number of p/n-junction of TEG αp: Seebeck coefficient of p-type equivalent IL αn: Seebeck coefficient of n-type equivalent IL VTEG: Open circuit potential of TEG ΔT: Temperature difference between hot and cold electrodes
Fig. 1. Seebeck coefficient measurements of [BMIM]+[BF4]- and [HMIM]+[I]- at a redox concentration of 0.01M of I2/I- and with different electrode spacing.
The layout of the TEG is briefly sketched in Fig. 2, a view of a first technological prototype is shown in Fig. 3.
Fig. 2. Principle of monolithic serial connected thermoelectric module [5].
Fig. 3. Prototype with 1182 serial connected IL-cells.
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Stefanie Uhl et al. / Materials Today: Proceedings 2 (2015) 669 – 674
2. Fabrication of thermoelectric generator 2.1. Cavity structuring The base material of the device consists of a commercially available metal-clad laminated polyimide sheet (DuPont, Pyralux AP8565). In a first step, blind holes with a diameter of 200μm are laser-machined through the copper and deep into the polyimide (Laser Nanosecondes, Type Nd:YAG Fresco-Ultra, Fab. Coherent, spot size 20μm); the laser parameters had to be adapted to the present material. Best results could be achieved using a laser- pulse frequency of 25kHz and a scanning speed of beam 192mm/s. The deep hole machining ends close to the adjacent copper electrode. This is important to exclude creating a hole that would let escape the IL form the cavity after filling, therefore laser machining was stopped before; the cross section of the sample in this state is shown in Fig. 4b. In order to perfectly control the machining of the 1182 holes the finishing was carried out using isotropic plasma etching using an O2/SF6 plasma at high pressure and low power. (Fig.5b). Plasma etching (PE) is preferred due to the possibility to remove all polymer traces and to stop the machining at the copper level due to the high selectivity of the process. (Plasma etching system used: SENTEC SI500). The increase of temperature due to plasma exposure must be well controlled in order to avoid delamination of the copper sheets from the polyimide. In contrast to Deep Reactive Ion Etching (DRIE) that is used for obtaining high aspect ratios in etching, plasma etching is preferred because undercutting of the masking hole of the top-copper is desired, allowing the liquid to create the top contact (Fig. 5b).
a 4a
Copper Polyimide
4b Fig. 4. (a) Schematic profile of the cavity after micro-machining by laser. (b) Image with optical microscope: The ablation is stopped before damaging the opposite copper layer. Laser parameter: For Copper: pulse intensity 2W, 5 cycles; For Polyimide: pulse intensity 0.3.W, 15 cycles.
a 5a
Copper Polyimide
5b Fig. 5. (a) Schematic profile of cavity after isotropic plasma etching. (b) Image with optical microscope: The copper is cleared on the bottom and topside of the cavity to enable a conductive contact with the liquid. Etching parameter: Power ICP: 1200W; pressure 40Pa, temperature 10°C, O2 flow 7.8sccm, SF6 flow 5.2sccm, time 30min.
Special care must be taken for stabilizing the laser and the PE process because the loss of one the 1182 contact will let fail the generator. In Fig. 6 a cross section of six wells is represented, the top-and bottom well will be reserved for n and p –equivalent ILs.
672
Stefanie Uhl et al. / Materials Today: Proceedings 2 (2015) 669 – 674
Fig. 6. Larger view of a partly completed (underneath side Laser + PE) and party uncompleted top-side (laser) of a sequence of volumes prepared prior to IL filling; note the upper ones will be filled with e.g. [BMIM]+[BF4]- (p-equivalent IL) and the bottom wells will be filled with [HMIM]+[I]- (n-equivalent IL).
2.2. Cavity filling, packaging and electrode structuring Before filling, the cavity surface is activated with argon plasma and subsequently filled with the corresponding liquids in a specially developed filling chamber Fig. 7. It allows in vacuum plasma activation of the inside of the wells, top and bottom-well filling with two different ILs. The vacuum is needed in order to evacuate the air in the well prior to filling. After filling the samples must be bubble-free sealed. 1
+
3 4
6 7 8 10
5
9
Fig. 7. Schema of filling chamber: 1.gas inlet valve, 2. Argon source, 3. Rotating sample holder, 4. Ventilation valve, 5. Vacuum chamber, 6. IL reservoirs, 7. DC-Electrode with feed through 8. DC source, 9. Vacuum valve, 10. Vacuum pump
Fig. 8. Electrode structuring by laser micro machining. Laser parameter for machining: For Parylene: pulse intensity 0.3.W, 5 cycles. For Copper: pulse intensity 1.7W, 9 cycle;
The liquids are encapsulated with Parylene using Solid-on-Liquid Technology [4]. The polymer poly-di-chloropara-xylylene, known as Parylene C, is grown on low pressure ILs using conventional low-pressure chemical vapor deposition process. Parylene encapsulates the liquid. After the deposition of the first Parylene layer, the serial connection of the electrodes are structured, again, by laser micromachining (Fig. 8).
Fig. 9. Schematic profile of thermoelectric module to demonstrate the different encapsulation steps after filling the cavities with IL and laser engraving of serial connected electrodes.
Stefanie Uhl et al. / Materials Today: Proceedings 2 (2015) 669 – 674
673
3. Self-passivation of copper electrode Up to now, only little scientific work was carried out using ILs for thermoelectric generators. Most of the work used inert electrodes such as Rh and Pt, carbon nanotubes, etc. in order to avoid chemical reactions with the electrode and to study pure effects in the IL. Looking at commercial applications, the concept that is presented in this paper has advantages because the copper/polyimide/copper substrates allow carrying out all process steps for an IL based generator. It is needed to study the compatibility (stability) between promising ILs and copper. In our study 1-Butyl-3-methyl-imidazolium-tetrafluoroborate [BMIM]+[BF4]- (Iolitec, Germany) has been tested. The tests were carried out in a single-junction chamber as shown in Fig. 10. The first warm-up run (12°C temperature difference between hot and cold electrode, a pronounced hysteresis was observed between heating up (black curve in Fig. 11) and cooling down (red curve in Fig 11). Obviously a reaction occurred leading to an accumulation of reaction products such as CuI2 was formed. Cooling down delivered a different electrochemical potential between the electrodes; the behavior is similar to a battery effect. After 15 day the experiment was repeated, showing an improvement of the effect in terms of hysteresis (comparison between blue and green curve in Fig. 11); at the end of the cool down-process the potential came back to its initial value. The experiment using copper and [BMIM]+[BF4]showed that there is a self-stabilizing effect of the electrode that is in contact with the IL. Iodide can be specifically adsorbed at copper surfaces. This spontaneously adsorption creates highly ordered Iodide layers on the Cu surface. Depending on the adsorption conditions, stable Cu-I layers are created whose dissolution is kinetically inhibited in a potential window of (+250 mV to -400 mV) versus normal hydrogen electrode NHE [6]. The XRF- analyses (Fischerscope X-Ray XDV-SDD) of the copper electrode surface after measurement showed Iodine.
Fig. 10. Schematic of thermo-electrochemical cell as Cu/Cu-I/IL/Cu-I/Cu structure.
Fig. 11. Self-stabiliuzing of the copper electrode after 15 days in contact with [BMIM]+[BF4]-.
4. Conclusion All fabrication steps of the thermoelectric module have been implemented and the serial connection of the individual cells was demonstrated. The 1182 wells containing generator could not be fully tested, because some imperfections in the well manufacturing might have interrupted the continuous structure of the generator, or some “battery” type electrode coatings on the copper still exists. The self-passivation of the copper surface happens after sufficiently long storage at IL exposure. Acknowledgements The authors acknowledge the financial support of the ‘Polish-Swiss-Research Programme’ for the project ‘‘Enerliq’’ in
674
Stefanie Uhl et al. / Materials Today: Proceedings 2 (2015) 669 – 674
collaboration with Gdansk University of Technology in Poland. Grant number: PSPB-051/2010.
References [1] T. J. Abraham, High Seebeck coefficient redox ionic liquid electrolytes for thermal energy harvesting, Energy & Environmental Science, Issue 9, 2013. [2] Theodore J. Abraham, Douglas R. MacFarlane and Jennifer M. Pringle, Seebeck coefficients in ionic liquids – prospects for thermo-electrochemical cells, Chem. Commun., 2011, 47, 6260–6262. [3] S. Uhl, E. Laux, T. Journot, L. Jeandupeux, J. Charmet, H. Keppner, Development of flexible micro-thermoelectrochemical generators based on ionic liquids, Journal of electronic Materials, 2014, DOI: 10.1007/s11664-0143126-1. [4] J. Charmêt, O. Banakh, E. Laux, B. Graf, F. Dias, A. Dunand, H. Keppner, G. Gorodyska, M. Textor, W. Noel, N.F. de Rooij, A. Neels, M. Dadras, A. Dommann, Ch. Borter, M. Benkhaira, Solid on liquid deposition, Thin Solid Films, 518 (18) pp. 5061-5064, 2010 [5] H. Keppner WO/2003/00632257 PCT/CH2003/000016: Highly-integrated miniature thermoelectric converter, July 2003 [6] M.S. Röefzaad, Ordnungsphänomene redox-aktiver Moleküle auf Elektrodenoberflächen unter reaktiven Bedingungen, 2011. http://hss.ulb.uni-bonn.de/2011/2436/2436.pdf