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Development of Thermoelectric generator based on Ionic Liquids for high temperature applications
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 10195–10202
www.materialstoday.com/proceedings
ECT 2016
Development of Thermoelectric generator based on Ionic Liquids for high temperature applications Edith Lauxa,*, Stefanie Uhla, Nicolas Gauthiera, Laure Jeandupeuxa, Herbert Keppnera, Pilar Pérez Lópezb, Sanglard Paulineb, Ennio Vanolib and Roger Martib a
University of Applied Sciences Western Switzerland (HES-SO), Eplatures-Grise 17, 2300 La Chaux-de-Fonds, Switzerland b
Haute école d’ingénierie et d’architecture (HES-SO), Perolles 80, 1705 Fribourg, Switzerland
Abstract Current, Seebeck-coefficients (SE) and maximal power output (Pmax) of thermoelectric generators using Ionic Liquids (ILs) were measured to determine the optimal temperature window for their use as thermoelectric generators (TEGs) in the range between room temperature (RT) and 300°C. The IL was sandwiched in a thermoelectric cell between a heated and a cooled electrode, allowing temperature-dependent current, voltage, and power-output characterization. Dissolving redox-couples (e.g. I2/I ) enables charge transfer from the IL to the electrode. It was found, that protic ILs degraded irreversibly at 140°C. Aprotic ILs, however, in combination with LiI/I2 redox couples exhibit higher temperature stability being finally limited either by redox-couple-electrode reactions, by temperature-induced redox couple degradation or by reaching at the boiling point at about 280°C. ILs being solid at room temperature could successfully be activated, as soon as at least the hot electrode of the set-up was heated above the melting point (m.p.) of the IL. As a striking observation, the thermo-voltage (linked to the SE) changes the sign as soon as the cold electrode was kept below the m.p. when Ethylammonium tetrafluoroborate (EA BF4) was used. Another IL, Tetrabutylammonium tetrafluoroborate (TBA BF4) exhibited in the full temperature range negative SE as high as 7 mV/K. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS.
* Corresponding author. Tel.: +41 32 930 14 63; fax: +41 32 930 29 30. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS.
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Keywords: Thermoelectric generators, Thermoelectricity, Ionic liquid, high temperature range, Seebeck-coefficient, Power output, redox-couple,
1. Introduction It was shown in previous work [1, 2, 3, 4] that Ionic Liquids (ILs) in combination with redox-couples can be used as thermoelectrically active substances. The application of a temperature gradient across the liquid creates a potential difference and gives rise to charged electrodes. The heat-flow, however, creates a strong convection in a way that the thermal conductivity of a laminar system can not necessarily be used for calculating the ZT-values in the classic form according to equation (1). 2
dV ZT T / dT
(1)
Whereby dV/dT = SE, and is the electric conductivity. In the present case is a complex value of ionic conductivity of the IL (mass transfer) and the electron transfer through the liquid/electrode interface. Looking at the turbulent nature of the liquid flow in the cell, it was concluded in previous work [5] that the origin of charging the electrodes of the cell is not due to the difference in mobility of the thermal drift between anions and cations. It is rather assumed that temperature-dependent selective attachment of ions creates the potential difference. The resulting Seebeck coefficients are found to be two to three times higher compared to solid-state-based TEGs (Bi2Te3: SE = 287μV/K [6], BMIM BF4: SE = 851 μV/K [1]). Adding redox couples (RCs) allows current extraction in a cyclic reversible process of the accumulated charge from the electrodes into an external circuit. However, as soon as RCs are added, a concentration-dependent decay of the thermal induced voltage can be observed, exclusively for ILs systems that were studied so far [7] and may be considered as a general effect. The temperature-dependent selective attachment of ions from the ILs competes with the ions of the RCs; adding a high concentration of RC (in the considered case LiI/I2 was used) can even turn SE to negative values [5]. When considering that the thermal conductivity of ILs is one tenth less as compared to solid-state materials (BMIM BF4: λ=0.184W/mK [8], Bi2Te3: λ=1.81W/mK [9])a thermal short circuit can be avoided allowing enhanced electrode temperature differences for a given heat flow. The latter point gives rise to higher generator voltages. Nevertheless, the application of IL-based TEGs at increased temperature makes sense since the Carnot factor in the overall expression of the maximum obtainable efficiency of a TEG (eq. 2), is higher for higher temperature differences between hot and cold electrodes [10]:
max
T TH
1 Z T 1 1 Z T TC TH
(2)
Whereby T/TH is the Carnot factor, T temperature difference between the hot (TH) and the cold (TC) electrode, and Z·T (eq. 1) is the figure of merit, respectively. It is well known that ILs exhibit high thermal stability e.g. Tetraethylammonium tetrafluoroborate (TEA BF4) that can be heated up to 745°C [9]. Most of the thermally stable ILs are solid at room temperature and it must be assured that they work as TEG as soon as the melting point is exceeded by the heat-flow throughout the generator. If this would be possible, an overwhelming variety of ILs could potentially be available for thermoelectric generation. 2. Experimental For the experiments a set-up was chosen allowing the application of a large temperature difference across the ILfilled cell. The key element of the set-up is a container made of a glass ring that is closed on both sides by Rhodium (Rh) coated sapphire electrodes (Fig. 1). The glass container is equipped with an inlet and an outlet tube for IL
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loading. Furthermore, its faces are polished. The 1 mm thick sapphire disks are pressed via the force of a spring against aluminum housings, meaning that first the electrode-container interfaces are first sealed against IL-loss and second, the thermal conductance throughout all interfaces is ensured. The IL was filled into a glass-ring before the experiment start. It is important to note that the IL is always exposed to air (open inlets and outlets) during the heating experiments. The Rh-coated sapphire disks are embedded in the aluminum housings containing both a heater and a thermocouple for precise TH and TC control. The high temperature housing has a thermal insulation jacket (PEEK), and the low-temperature housing is equipped with ribs for air cooling. Sapphire for the electrode carrier disk was chosen for its high thermal conductivity between the housing and the IL, Rh as contacting thin-film was chosen for its high chemical stability. The total set-up is shown in Fig.1a. For the read-out the Rh-layer is contacted using fingers that are spring assisted. Fig.1b is a close look side-view between the electrodes and Fig.1c looks at the TH housing from the (removed) TC-housing side.
b
a)
b)
c)
Fig. 1. Experimental set-up for the high temperature measurements. (a) overview; (b) close look between the heated and cooled housings. The IL is in a glass ring between the Rh-coated sapphire electrodes; (c) looking from the removed cooled electrode onto the glass ring with the Al-body, the electrodes, and the inlets and outlets for the IL.
One pair of contact finger in Fig.1a measures the voltage drop between the adjacent Rh electrodes exposed to TH and TC, the other pair is connected to a computer-controlled switch system allowing to apply different load resistors between 2 and 100 M for current determination. Ethylammonium tetrafluoroborate and Methylammonium trifluoroacetate were synthetized at HES-SO (Fribourg, Switzerland), Butylmethylimidazolium tetrafluoroborate and Ethylammonium nitrate were purchased from Iolitec (Germany), TBA PF6 and all redox-chemicals were purchased from Sigma-Aldrich (Switzerland) and used as received without further purifications. In the set-up, three classes (types) of ILs were investigated: protic and aprotic ILs, liquid at room-temperature and ILs with m.p. above room-temperature. 3. Results and discussion 3.1. Stability of protic and aprotic ILs + RC In the first step the thermal stability of an IL+RC system is observed. It is important that the stability of the IL is characterized together with its RC, because chemical reactions can occur within the mixture and between the liquid and the electrodes. The ILs that were chosen are Ethylammonium nitrate (EAN) and Methylammonium trifluoroacetate (MA TFA) both are protic and contain 0.4 mol/L LiI/I2 as redox couple. The voltage-power (V-P) characteristics for EAN being heated up to irreversible degradation are represented in Fig. 2.
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a)
b)
Fig. 2. V-P characteristic of EAN; a) ∆T was kept constant at 20°C and TH was varied b) TC was kept constant at 26°C and TH was varied
Fig. 2a shows a surprising result indicating that for the same temperature difference the output power of an EANTEG is strongly increasing as soon as the cold electrode is heated. The power increase for ∆T =20°C, starting at TC = 26°C up to TC =146°C is about 35 times. This behaviour is not observed looking at solid state TEGs. Fig. 2b shows the stability behaviour: heating EAN higher as 146°C, an irreversible degradation of the generator occurs. See the black curve (P_46-26_after) in Fig. 2b for the V-P after 164°C exposure of the EAN and subsequent cooling down to ∆T =46°C-26°C. Looking at a higher temperature stability, aprotic ILs are known [11] to be more stable. In a next step 1-Butyl-3methylimidazoliumtetrafluoroborate BMIM BF4 + 0.4 mol/L LiI/I2 was exposed to temperatures of about 246°C. The continuously increasing power curve indicates that the IL+RC systems is stable at these temperatures (Fig. 3a).
a)
b)
Fig. 3. V-P characteristic of EAN a) Continuous increase of the power for heating BMIM BF4+RC up to 246°C. b) black deposition (Rh-I reactions) on the electrode after V-P characteristic at T ≥ 280°C.
The irreversible degradation of BMIM BF4+ LiI/I2 was observed for temperatures higher than 280°C.After the experiment, it was observed that the Rhodium electrode showed a significant colour change (Fig. 3b) and the X-ray
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fluorescence analysis indicated traces of iodine. In a next step the BMIM BF4 from the previous experiment was inserted into a cell with new electrodes and the same behaviour as in Fig. 3a could be repeated. This observation indicates that the BMIM BF4 was stable up to 280°C, however the electrode I2 reaction limited the high temperature utilisation of this TEG. 3.2. High temperature behavior of RT-solid ILs Most of the high temperature stable ILs are solid at room temperature (RT). In a next step is was verified if such ILs, once they have been heated up above their melting point, together with mixed-in RCs, exhibit TEG behavior. First, as a promising IL with a melting point at 167°C, Ethylammonium tetrafluoroborate [EA BF4 + 0.2 mol/L LiI/I2 as RC was studied. The surprising result is represented in Fig. 4.
a) Fig. 4. V-I-P characteristics of EA BF4 + RC: a) V-I characteristics at TH = 206°C and TC = variable: the sign of voltage changes as soon as TC < m.p. and b) P-V characteristics: SE is changing its sign at TC < 167°C.
If TH is kept at 206 °C and TC is continuously reduced, the transition over the m.p. induces a change in the sign of the thermo-voltage. Further increase of the hot electrode was not carried out, due to electrode degradation, as shown in Fig. 3b. In a next step Tetrabutylammonium tetrafluoroborate (TBA BF4) was studied. This IL is solid at room temperature and has a melting point of 167°C. The IL is aprotic and Ferrocene was used as RC. In contrast to EA BF4 + LiI/I2 shown in Figs 4 and 5 the TBA BF4 did not exhibit a change in the sign of the Seebeck coefficient as soon as TC drops under the m. p.; one could just observe a strong power reduction. TBA BF4+Ferrocene with high mol fraction of Ferrocene (1:4 w/w) was assumed to be more thermal stable than the pure components (m.p. 240 - 249 °C). Looking at the V-P diagram one can observe that keeping TH at 306°C, the power output is not linked to the temperature difference TH-TC. Highest power-output values can be observed for TH - TC = 306°C-166°C and TH - TC = 306°C-186°C. For all other temperatures of TC, either hotter or colder than the plateau between 166°C and 186°C the output power is found to be reduced. It can be assumed that this observation is due to the melting region of the Ferrocene at 176°C. Fig. 5a shows the V-P diagram for ascending TC until 166°C and Fig. 5b shows the descending power curves as soon as TC becomes higher than 186°C.
b)
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a)
b)
Fig. 5. V-P diagram of TBA BF4 +Ferrocene: a) showing increasing power values for ascending TC up to 166°C. b) showing descending power values for further increase of TC from 166°C up to 286°C.
3.3. Seebeck effect considerations for TBA BF4 For TBA BF4 it is interesting to observe the development of the SE across the phase transition of the IL from solid to liquid. Fig. 6 shows this behavior:
a)
b)
Fig. 6. SE development of TBA BF4 with and without Ferrocene as RC. a) 25 % w/w Ferrocene was added to the IL. TH was kept constant whereas TC was varied over a wide range of temperature. b) SE development of TBA BF4 without RC, whereby TH-TC was kept constant at 20°C and TC was continuously increased.
Looking at Fig. 6a. the Seebeck coefficient was taken by measuring the voltage at a given temperature difference. The difference TH - TC for the point at TC=286°C was only 20°C, whereby for the point at TC = 25°C the difference TH-TC is 281°C. The negative SE values increase strongly (despite the temperature difference decrease) up to values close to -3500 µV/K. Values show pronounced maximum between 206°C < TC < 226°C as well as for 100° < TH-TC < 80°C. Fig. 6b. was taken from TBA BF4 without RC. In this case TH-TC =20°C was scanned for 86°C < TC < 286°C and a continuous increase of the SE with values up to -7600 µV/K could be measured.
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3.4. Discussion Ionic liquids that are solid at room temperature behave, if both electrodes are above the melting point, similar to ILs which are still liquid at RT. Power output increases with increasing temperature of both electrodes and increases as soon as both electrodes are heated (maintaining the same temperature difference) as well as only one electrode is heated and the second remain at the same level 26 °C. They show also high SE, which are affected if RCs are added for current extraction. The case of EA BF4 + LiI/I2 appears interesting because keeping TH at 206°C the voltage across the TEG undergoes a change of polarity; positive when TC > 186°C, and negative if TC < 146°C (Fig. 4). It is assumed that this change occurs at TC = 167°C which is the melting point of EA BF4. A simple model (Fig. 7) is developed that could explain the phenomenon.
a)
b)
c) Fig. 7. a) The bulk liquid is quasi-neutral, containing free carriers symbolized by cations (+) and anions (-) of IL+RC. Both electrodes at the same temperature with IL in the liquid phase show no voltage. b) Voltage can be measured if electrodes are asymmetrically heated up above phase transition due to a temperature-dependent attachment of ions (formation of asymmetric double-layer). c) With one electrode at temperature below the phase transition temperature, a condensed layer can be formed with different attachment of ions and therefore changed voltage.
In Fig. 7 it is assumed that below the melting point, a “pseudo-solid” layer is grown in the vicinity of the cold electrode surface that takes over the role of the Rh-electrode. This means that this layer favours rather cationselective surface attachment at lower temperatures more as the Rh-electrode at TH. In addition to that, this layer must have a good ion-conductivity, because the power that is observed in Fig. 4 is enhanced in the negative part of the V-P diagram, as compared to the positive one. Looking at TBA BF4 the conditions are different: as soon as TC drops below the melting point there is no change of polarity of the TEG; hence a “pseudo-solid” layer might (and will) exist as soon as TC is sufficiently low. However it must be assumed that neither the charge attachment properties of this layer nor the ion-conducting properties are different. The power output reduction at low TC can rather be explained by the diffusion-limited ion transport through the “pseudo-solid” area and not by limited carrier extraction from the liquid to solid Rh electrode. The surprisingly high SE /thermo-voltages of RT-solid ILs may, again, be attributed to the convection model that could explain the high values in terms of selective temperature-dependent carrier attachment at the electrodes.
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4. Summary and Conclusions ILs which can be used for TEG purposes in a temperature range of 150°C to 300°C were presented. Looking at the almost unlimited structural variations of ILs, desired properties of IL can be easily adjusted to all kinds of use. The stability for the already studied ILs in higher temperature was found to be limited due to the following reasons: The class of protic Ils with LiI/I2 as RC exhibit less stability (decomposition at 150°C, shown for the cases of EAN and MA TFA), than the aprotic ILs (decomposition at 300°C shown for BMIM BF4). To achieve current extraction RCs must be added; they must guarantee that they can be oxidized and reduced in a reversible cyclic process. These RCs can be thermally degraded (case of LiI/I2 in BMIM BF4). ILs as well as RC and the mixture of them must be thermally stable and must not react with the electrode material (case of LiI/I2 in BMIM BF4). The reaction product may prevent current extraction or can create a new stable layer (case of TBA BF4 + Ferrocene). Special attention must be drawn to ILs that are solid at room temperature and have m. p. > RT. In general such ILs exhibit also a higher decomposition temperature, such as TEA BF4 that can be heated up to 745°C (m. p. 300°C). In this paper two ILs, solid at RT, were studied. They have a melting point between 100°C and 200°C. In the case of EA BF4 (m.p. 167°C) the SE as well as the thermo-voltage of the TEG changes its sign as soon as the cold electrode was heated above the m. p.. However, no current limits below TC < m.p. could be observed. It is assumed that there is a solidification in proximity of the cold electrode. Surprisingly the highest power output is observed for the case TH-TC =206°C- 46°C= 160°C. For the explanation of the phenomenon, a simple model is proposed that is based on a selective ion attachment to the electrode leading to the creation of a thermo-voltage. Condensation of TBA BF4 apparently inverses the initial Rhodium electrode polarisation. It was further found that TBA BF4 exhibits Seebeck coefficients as high as -7.6 mV/K without RC and –3.27 mV/K with RC. These values are very high as compared to typical solid state TEGs. It must be assumed, that the role of the SE in equation 1 must be corrected in terms of its contribution to the Z·T value in IL-TEGs. The authors assume further that in the latter case the current extraction will remain the bottleneck for obtaining higher conversion efficiencies. Acknowledgements The project was funded by HES-SO Interdisciplinary project no. 011-2015-PI ; The Swiss office of Energy BFE under contract SI/501212-01; The Swiss Space Center, contract 0009_CC_001_SSC01 HE-ARC Cfl2015; The Polish-Swiss research Program, project PSPB-051/2010, ENERLIQ; The European project H2020, FETPOACT2016 RIA / 731976 MAGENTA. References [1]
S. Uhl, E. Laux, T. Journot, L. Jeandupeux, J. Charmet, H. Keppner, Development of flexible micro-thermo-electrochemical generators based on Ionic Liquids, Journal of Electronic Materials 43 (2014) 3759-3766. [2] T. Abraham, D. MacFarlane, J. Pringle, High Seebeck coefficient redox ionic liquid electrolytes for thermal energy harvesting, The Royal Society of Chemistry, Energy Environ. Sci. 6 (2013) 2639-2645. [3] J. He, D. Al-Masri, D. R. MacFarlane, J. M. Pringle, Temperature dependence of the electrode potential of a cobalt-based redox couple in ionic liquid electrolytes for thermal energy harvesting, Faraday Discussions 190 (2016) 205-218. [4] N. Jiao, T. J. Abraham, D. R. MacFarlane, J. M. Pringle, Ionic Liquid Electrolytes for Thermal Energy Harvesting Using a Cobalt Redox Couple, ECS (2014) D3061-D3065 [5] H. Keppner, S. Uhl, E. Laux, L. Jeandupeux, J. Tschanz, T. Journot; Ionic Liquid-based Thermoelectric Generator: Links between Liquid Data and Generator Characteristics, Mater. Today: Proceedings 2 (2015) 680-689. [6] J. Tan, K. Kalantar-Zadeh, W. Wlodarski, S. Bhargava, D. Akolekar, A. Holland, G. Rosengarten, Thermoelectric properties of bismuth telluride thin films deposited by radio frequency magnetron sputtering, Proceedings of SPIE 5836 (2005) 711-718. [7] E. Laux, S. Uhl, T. Journot, J. Brossard, L. Jeandupeux, H. Keppner, Aspects of protonic Ionic Liquids as electrolyte in thermoelectric generators, Journal of Electron Materials 45 no.7 (2016) 3383-3389. [8] M. Valkenburg, R. Vaughn, M. Williams, J. Wilkes, Thermochemistry of ionic liquid heat-transfer fluids, Thermochimica Acta 425 (2005) 181-188. [9] D. Park, S. Park, K. Jeong, H.S. Jeong, J. Y. Song, M. H. Cho, Thermal and electrical conduction of single-crystal Bi2Te3 nanostructures grown using a one step process, Nature Scientific Reports 6 : 19132 (2016). [10] Albert Van der Ziel, Solid State Physical Electronics, Prentice-Hall, inc Englewook Cliffs, New Jersey 1976. [11] Subbiah Sowmiah, Venkatesan Srinivdasadesikan, Ming-Chung Tseng and Yen-Ho Chu, On the Chemical Stabilities of Ionic Liquids, Molecules 14 (2009) 3780-3813.
ScienceDirect Materials Today: Proceedings 5 (2018) 10195–10202
www.materialstoday.com/proceedings
ECT 2016
Development of Thermoelectric generator based on Ionic Liquids for high temperature applications Edith Lauxa,*, Stefanie Uhla, Nicolas Gauthiera, Laure Jeandupeuxa, Herbert Keppnera, Pilar Pérez Lópezb, Sanglard Paulineb, Ennio Vanolib and Roger Martib a
University of Applied Sciences Western Switzerland (HES-SO), Eplatures-Grise 17, 2300 La Chaux-de-Fonds, Switzerland b
Haute école d’ingénierie et d’architecture (HES-SO), Perolles 80, 1705 Fribourg, Switzerland
Abstract Current, Seebeck-coefficients (SE) and maximal power output (Pmax) of thermoelectric generators using Ionic Liquids (ILs) were measured to determine the optimal temperature window for their use as thermoelectric generators (TEGs) in the range between room temperature (RT) and 300°C. The IL was sandwiched in a thermoelectric cell between a heated and a cooled electrode, allowing temperature-dependent current, voltage, and power-output characterization. Dissolving redox-couples (e.g. I2/I ) enables charge transfer from the IL to the electrode. It was found, that protic ILs degraded irreversibly at 140°C. Aprotic ILs, however, in combination with LiI/I2 redox couples exhibit higher temperature stability being finally limited either by redox-couple-electrode reactions, by temperature-induced redox couple degradation or by reaching at the boiling point at about 280°C. ILs being solid at room temperature could successfully be activated, as soon as at least the hot electrode of the set-up was heated above the melting point (m.p.) of the IL. As a striking observation, the thermo-voltage (linked to the SE) changes the sign as soon as the cold electrode was kept below the m.p. when Ethylammonium tetrafluoroborate (EA BF4) was used. Another IL, Tetrabutylammonium tetrafluoroborate (TBA BF4) exhibited in the full temperature range negative SE as high as 7 mV/K. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS.
* Corresponding author. Tel.: +41 32 930 14 63; fax: +41 32 930 29 30. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS.
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Keywords: Thermoelectric generators, Thermoelectricity, Ionic liquid, high temperature range, Seebeck-coefficient, Power output, redox-couple,
1. Introduction It was shown in previous work [1, 2, 3, 4] that Ionic Liquids (ILs) in combination with redox-couples can be used as thermoelectrically active substances. The application of a temperature gradient across the liquid creates a potential difference and gives rise to charged electrodes. The heat-flow, however, creates a strong convection in a way that the thermal conductivity of a laminar system can not necessarily be used for calculating the ZT-values in the classic form according to equation (1). 2
dV ZT T / dT
(1)
Whereby dV/dT = SE, and is the electric conductivity. In the present case is a complex value of ionic conductivity of the IL (mass transfer) and the electron transfer through the liquid/electrode interface. Looking at the turbulent nature of the liquid flow in the cell, it was concluded in previous work [5] that the origin of charging the electrodes of the cell is not due to the difference in mobility of the thermal drift between anions and cations. It is rather assumed that temperature-dependent selective attachment of ions creates the potential difference. The resulting Seebeck coefficients are found to be two to three times higher compared to solid-state-based TEGs (Bi2Te3: SE = 287μV/K [6], BMIM BF4: SE = 851 μV/K [1]). Adding redox couples (RCs) allows current extraction in a cyclic reversible process of the accumulated charge from the electrodes into an external circuit. However, as soon as RCs are added, a concentration-dependent decay of the thermal induced voltage can be observed, exclusively for ILs systems that were studied so far [7] and may be considered as a general effect. The temperature-dependent selective attachment of ions from the ILs competes with the ions of the RCs; adding a high concentration of RC (in the considered case LiI/I2 was used) can even turn SE to negative values [5]. When considering that the thermal conductivity of ILs is one tenth less as compared to solid-state materials (BMIM BF4: λ=0.184W/mK [8], Bi2Te3: λ=1.81W/mK [9])a thermal short circuit can be avoided allowing enhanced electrode temperature differences for a given heat flow. The latter point gives rise to higher generator voltages. Nevertheless, the application of IL-based TEGs at increased temperature makes sense since the Carnot factor in the overall expression of the maximum obtainable efficiency of a TEG (eq. 2), is higher for higher temperature differences between hot and cold electrodes [10]:
max
T TH
1 Z T 1 1 Z T TC TH
(2)
Whereby T/TH is the Carnot factor, T temperature difference between the hot (TH) and the cold (TC) electrode, and Z·T (eq. 1) is the figure of merit, respectively. It is well known that ILs exhibit high thermal stability e.g. Tetraethylammonium tetrafluoroborate (TEA BF4) that can be heated up to 745°C [9]. Most of the thermally stable ILs are solid at room temperature and it must be assured that they work as TEG as soon as the melting point is exceeded by the heat-flow throughout the generator. If this would be possible, an overwhelming variety of ILs could potentially be available for thermoelectric generation. 2. Experimental For the experiments a set-up was chosen allowing the application of a large temperature difference across the ILfilled cell. The key element of the set-up is a container made of a glass ring that is closed on both sides by Rhodium (Rh) coated sapphire electrodes (Fig. 1). The glass container is equipped with an inlet and an outlet tube for IL
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loading. Furthermore, its faces are polished. The 1 mm thick sapphire disks are pressed via the force of a spring against aluminum housings, meaning that first the electrode-container interfaces are first sealed against IL-loss and second, the thermal conductance throughout all interfaces is ensured. The IL was filled into a glass-ring before the experiment start. It is important to note that the IL is always exposed to air (open inlets and outlets) during the heating experiments. The Rh-coated sapphire disks are embedded in the aluminum housings containing both a heater and a thermocouple for precise TH and TC control. The high temperature housing has a thermal insulation jacket (PEEK), and the low-temperature housing is equipped with ribs for air cooling. Sapphire for the electrode carrier disk was chosen for its high thermal conductivity between the housing and the IL, Rh as contacting thin-film was chosen for its high chemical stability. The total set-up is shown in Fig.1a. For the read-out the Rh-layer is contacted using fingers that are spring assisted. Fig.1b is a close look side-view between the electrodes and Fig.1c looks at the TH housing from the (removed) TC-housing side.
b
a)
b)
c)
Fig. 1. Experimental set-up for the high temperature measurements. (a) overview; (b) close look between the heated and cooled housings. The IL is in a glass ring between the Rh-coated sapphire electrodes; (c) looking from the removed cooled electrode onto the glass ring with the Al-body, the electrodes, and the inlets and outlets for the IL.
One pair of contact finger in Fig.1a measures the voltage drop between the adjacent Rh electrodes exposed to TH and TC, the other pair is connected to a computer-controlled switch system allowing to apply different load resistors between 2 and 100 M for current determination. Ethylammonium tetrafluoroborate and Methylammonium trifluoroacetate were synthetized at HES-SO (Fribourg, Switzerland), Butylmethylimidazolium tetrafluoroborate and Ethylammonium nitrate were purchased from Iolitec (Germany), TBA PF6 and all redox-chemicals were purchased from Sigma-Aldrich (Switzerland) and used as received without further purifications. In the set-up, three classes (types) of ILs were investigated: protic and aprotic ILs, liquid at room-temperature and ILs with m.p. above room-temperature. 3. Results and discussion 3.1. Stability of protic and aprotic ILs + RC In the first step the thermal stability of an IL+RC system is observed. It is important that the stability of the IL is characterized together with its RC, because chemical reactions can occur within the mixture and between the liquid and the electrodes. The ILs that were chosen are Ethylammonium nitrate (EAN) and Methylammonium trifluoroacetate (MA TFA) both are protic and contain 0.4 mol/L LiI/I2 as redox couple. The voltage-power (V-P) characteristics for EAN being heated up to irreversible degradation are represented in Fig. 2.
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a)
b)
Fig. 2. V-P characteristic of EAN; a) ∆T was kept constant at 20°C and TH was varied b) TC was kept constant at 26°C and TH was varied
Fig. 2a shows a surprising result indicating that for the same temperature difference the output power of an EANTEG is strongly increasing as soon as the cold electrode is heated. The power increase for ∆T =20°C, starting at TC = 26°C up to TC =146°C is about 35 times. This behaviour is not observed looking at solid state TEGs. Fig. 2b shows the stability behaviour: heating EAN higher as 146°C, an irreversible degradation of the generator occurs. See the black curve (P_46-26_after) in Fig. 2b for the V-P after 164°C exposure of the EAN and subsequent cooling down to ∆T =46°C-26°C. Looking at a higher temperature stability, aprotic ILs are known [11] to be more stable. In a next step 1-Butyl-3methylimidazoliumtetrafluoroborate BMIM BF4 + 0.4 mol/L LiI/I2 was exposed to temperatures of about 246°C. The continuously increasing power curve indicates that the IL+RC systems is stable at these temperatures (Fig. 3a).
a)
b)
Fig. 3. V-P characteristic of EAN a) Continuous increase of the power for heating BMIM BF4+RC up to 246°C. b) black deposition (Rh-I reactions) on the electrode after V-P characteristic at T ≥ 280°C.
The irreversible degradation of BMIM BF4+ LiI/I2 was observed for temperatures higher than 280°C.After the experiment, it was observed that the Rhodium electrode showed a significant colour change (Fig. 3b) and the X-ray
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fluorescence analysis indicated traces of iodine. In a next step the BMIM BF4 from the previous experiment was inserted into a cell with new electrodes and the same behaviour as in Fig. 3a could be repeated. This observation indicates that the BMIM BF4 was stable up to 280°C, however the electrode I2 reaction limited the high temperature utilisation of this TEG. 3.2. High temperature behavior of RT-solid ILs Most of the high temperature stable ILs are solid at room temperature (RT). In a next step is was verified if such ILs, once they have been heated up above their melting point, together with mixed-in RCs, exhibit TEG behavior. First, as a promising IL with a melting point at 167°C, Ethylammonium tetrafluoroborate [EA BF4 + 0.2 mol/L LiI/I2 as RC was studied. The surprising result is represented in Fig. 4.
a) Fig. 4. V-I-P characteristics of EA BF4 + RC: a) V-I characteristics at TH = 206°C and TC = variable: the sign of voltage changes as soon as TC < m.p. and b) P-V characteristics: SE is changing its sign at TC < 167°C.
If TH is kept at 206 °C and TC is continuously reduced, the transition over the m.p. induces a change in the sign of the thermo-voltage. Further increase of the hot electrode was not carried out, due to electrode degradation, as shown in Fig. 3b. In a next step Tetrabutylammonium tetrafluoroborate (TBA BF4) was studied. This IL is solid at room temperature and has a melting point of 167°C. The IL is aprotic and Ferrocene was used as RC. In contrast to EA BF4 + LiI/I2 shown in Figs 4 and 5 the TBA BF4 did not exhibit a change in the sign of the Seebeck coefficient as soon as TC drops under the m. p.; one could just observe a strong power reduction. TBA BF4+Ferrocene with high mol fraction of Ferrocene (1:4 w/w) was assumed to be more thermal stable than the pure components (m.p. 240 - 249 °C). Looking at the V-P diagram one can observe that keeping TH at 306°C, the power output is not linked to the temperature difference TH-TC. Highest power-output values can be observed for TH - TC = 306°C-166°C and TH - TC = 306°C-186°C. For all other temperatures of TC, either hotter or colder than the plateau between 166°C and 186°C the output power is found to be reduced. It can be assumed that this observation is due to the melting region of the Ferrocene at 176°C. Fig. 5a shows the V-P diagram for ascending TC until 166°C and Fig. 5b shows the descending power curves as soon as TC becomes higher than 186°C.
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b)
Fig. 5. V-P diagram of TBA BF4 +Ferrocene: a) showing increasing power values for ascending TC up to 166°C. b) showing descending power values for further increase of TC from 166°C up to 286°C.
3.3. Seebeck effect considerations for TBA BF4 For TBA BF4 it is interesting to observe the development of the SE across the phase transition of the IL from solid to liquid. Fig. 6 shows this behavior:
a)
b)
Fig. 6. SE development of TBA BF4 with and without Ferrocene as RC. a) 25 % w/w Ferrocene was added to the IL. TH was kept constant whereas TC was varied over a wide range of temperature. b) SE development of TBA BF4 without RC, whereby TH-TC was kept constant at 20°C and TC was continuously increased.
Looking at Fig. 6a. the Seebeck coefficient was taken by measuring the voltage at a given temperature difference. The difference TH - TC for the point at TC=286°C was only 20°C, whereby for the point at TC = 25°C the difference TH-TC is 281°C. The negative SE values increase strongly (despite the temperature difference decrease) up to values close to -3500 µV/K. Values show pronounced maximum between 206°C < TC < 226°C as well as for 100° < TH-TC < 80°C. Fig. 6b. was taken from TBA BF4 without RC. In this case TH-TC =20°C was scanned for 86°C < TC < 286°C and a continuous increase of the SE with values up to -7600 µV/K could be measured.
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3.4. Discussion Ionic liquids that are solid at room temperature behave, if both electrodes are above the melting point, similar to ILs which are still liquid at RT. Power output increases with increasing temperature of both electrodes and increases as soon as both electrodes are heated (maintaining the same temperature difference) as well as only one electrode is heated and the second remain at the same level 26 °C. They show also high SE, which are affected if RCs are added for current extraction. The case of EA BF4 + LiI/I2 appears interesting because keeping TH at 206°C the voltage across the TEG undergoes a change of polarity; positive when TC > 186°C, and negative if TC < 146°C (Fig. 4). It is assumed that this change occurs at TC = 167°C which is the melting point of EA BF4. A simple model (Fig. 7) is developed that could explain the phenomenon.
a)
b)
c) Fig. 7. a) The bulk liquid is quasi-neutral, containing free carriers symbolized by cations (+) and anions (-) of IL+RC. Both electrodes at the same temperature with IL in the liquid phase show no voltage. b) Voltage can be measured if electrodes are asymmetrically heated up above phase transition due to a temperature-dependent attachment of ions (formation of asymmetric double-layer). c) With one electrode at temperature below the phase transition temperature, a condensed layer can be formed with different attachment of ions and therefore changed voltage.
In Fig. 7 it is assumed that below the melting point, a “pseudo-solid” layer is grown in the vicinity of the cold electrode surface that takes over the role of the Rh-electrode. This means that this layer favours rather cationselective surface attachment at lower temperatures more as the Rh-electrode at TH. In addition to that, this layer must have a good ion-conductivity, because the power that is observed in Fig. 4 is enhanced in the negative part of the V-P diagram, as compared to the positive one. Looking at TBA BF4 the conditions are different: as soon as TC drops below the melting point there is no change of polarity of the TEG; hence a “pseudo-solid” layer might (and will) exist as soon as TC is sufficiently low. However it must be assumed that neither the charge attachment properties of this layer nor the ion-conducting properties are different. The power output reduction at low TC can rather be explained by the diffusion-limited ion transport through the “pseudo-solid” area and not by limited carrier extraction from the liquid to solid Rh electrode. The surprisingly high SE /thermo-voltages of RT-solid ILs may, again, be attributed to the convection model that could explain the high values in terms of selective temperature-dependent carrier attachment at the electrodes.
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4. Summary and Conclusions ILs which can be used for TEG purposes in a temperature range of 150°C to 300°C were presented. Looking at the almost unlimited structural variations of ILs, desired properties of IL can be easily adjusted to all kinds of use. The stability for the already studied ILs in higher temperature was found to be limited due to the following reasons: The class of protic Ils with LiI/I2 as RC exhibit less stability (decomposition at 150°C, shown for the cases of EAN and MA TFA), than the aprotic ILs (decomposition at 300°C shown for BMIM BF4). To achieve current extraction RCs must be added; they must guarantee that they can be oxidized and reduced in a reversible cyclic process. These RCs can be thermally degraded (case of LiI/I2 in BMIM BF4). ILs as well as RC and the mixture of them must be thermally stable and must not react with the electrode material (case of LiI/I2 in BMIM BF4). The reaction product may prevent current extraction or can create a new stable layer (case of TBA BF4 + Ferrocene). Special attention must be drawn to ILs that are solid at room temperature and have m. p. > RT. In general such ILs exhibit also a higher decomposition temperature, such as TEA BF4 that can be heated up to 745°C (m. p. 300°C). In this paper two ILs, solid at RT, were studied. They have a melting point between 100°C and 200°C. In the case of EA BF4 (m.p. 167°C) the SE as well as the thermo-voltage of the TEG changes its sign as soon as the cold electrode was heated above the m. p.. However, no current limits below TC < m.p. could be observed. It is assumed that there is a solidification in proximity of the cold electrode. Surprisingly the highest power output is observed for the case TH-TC =206°C- 46°C= 160°C. For the explanation of the phenomenon, a simple model is proposed that is based on a selective ion attachment to the electrode leading to the creation of a thermo-voltage. Condensation of TBA BF4 apparently inverses the initial Rhodium electrode polarisation. It was further found that TBA BF4 exhibits Seebeck coefficients as high as -7.6 mV/K without RC and –3.27 mV/K with RC. These values are very high as compared to typical solid state TEGs. It must be assumed, that the role of the SE in equation 1 must be corrected in terms of its contribution to the Z·T value in IL-TEGs. The authors assume further that in the latter case the current extraction will remain the bottleneck for obtaining higher conversion efficiencies. Acknowledgements The project was funded by HES-SO Interdisciplinary project no. 011-2015-PI ; The Swiss office of Energy BFE under contract SI/501212-01; The Swiss Space Center, contract 0009_CC_001_SSC01 HE-ARC Cfl2015; The Polish-Swiss research Program, project PSPB-051/2010, ENERLIQ; The European project H2020, FETPOACT2016 RIA / 731976 MAGENTA. References [1]
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