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ScienceDirect Materials Today: Proceedings 8 (2019) 672–679
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ECT 2017
Novel Ionic Liquids for Thermoelectric Generator Devices Edith Lauxa*, Laure Jeandupeuxa, Stefanie Uhla, Herbert Keppnera, Pilar Pérez Lópezb, Pauline Sanglardb, Ennio Vanolib and Roger Martib a
Haute Ecole Arc Ingénierie (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
In recent years, ionic liquids (ILs) have been investigated as thermoelectric materials in thermoelectric generators (TEG). Reduced toxicity, high boiling point, low thermal conductivity at good conductivity are striking properties. With thermal conductivities, typically 1/10th of solid state materials, a small thermal flow can generate a high temperature difference and, with that, a high voltage (or Seebeck coefficient, SE) can be achieved. However, carrier extraction (current) from the liquid to an external load is still an issue and needs further improvement. The creation of the electrode potential could be described by the Nernst equation taking into account the involvement of charged species in the liquid, such as the anions and cations of the IL and the species for the redox couples. In the past, more than 25 ILs were screened, focusing in particular on non-hazardous and environmental friendly ILs, based on amino acids. Most of them show large positive (p-SE). There is, however, a lack of ILs with similar large negative Seebeck coefficients (n-SE) allowing to be joined combined with p-SE cells to better internal resistance matching for output power improvement. In this paper, we show further progress only focusing on finding large n-SE liquids; carrier extraction for TEGcurrent will not be considered. We argue that the ion attachment at the electrode interface plays a crucial role for high generator voltages. For current extraction, the IL must be “blended” with redox couples, allowing carrier extraction to an external load in a reversible cyclic process and the role of attached ions from ILs may be considered as internally created “bias” . Blending, however, was experienced in the past by our work and others, leads unavoidably to both p- and n-SE reduction. Based on new experiments a first exception was found using Co2+/3+(bpy)3(TFSI2)2/3 redox couples in combination with choline lactate; it was observed that the n-SE could be increased up to -3630 µV/K, making it a promising candidate for a high performance TEG.
* Corresponding author. Tel.: +41 32 930 14 63; fax: +41 32 930 29 30. E-mail address:
[email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 15th European Conference on Thermoelectrics.
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© 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 15th European Conference on Thermoelectrics. Keywords: Ionic liquid; Thermoelectric generators, Thermoelectricity; negative Seebeck coefficient; redox;
1. Introduction The thermoelectric generators (TEGs) considered in this paper consist of heated and cooled serial connected electrodes, with an ionic liquid (IL) trapped in between. In order to extract a current via the electrodes, redox couples are added to the IL. In contrast to solid state TEG materials, there is a large variety of potentially usable liquids and more work must be carried out in future to find those materials that have appropriated thermoelectric properties and that are compatible to the environment. An overview of a large variety of ILs is given by [1]. Among the pioneering works using IOls as active materials the work of [2, 3] was contributed. Detailed theoretic studies about thermogalvanic cells were contributed by [4]. It is assumed that the redox couples are dissociated by the IL, and there is no other solvent involved. Four particles make up the liquid: IL-anions, IL-cations, oxidized and reduced forms of a redox couple. The active electrode area (4.3 cm2) in contact with the liquid has a larger diameter as compared to the distance between the electrodes (0.5 cm). It was shown in previous papers [5] that such configuration lead to turbulent- and convectivedominated heat transport through the generator; note, in liquids and gases heat-transport is in general coupled to material transport. Under pronounced heat/mass flow it is assumed that all four particles touch all surfaces with a probability depending on their concentration. There is a large variety of liquids with a positive Seebeck coefficient [6]. However, to avoid important internal electric losses through series connection of the TEG-cells to reach the maximum power output, the internal resistance of all liquid cells (positive Seebeck coefficient and negative Seebeck coefficient (p-SE and n-SE respectively) must be the same at a given temperature difference. The lack of suitable liquids with high n-SE was faced in previous publications [7], whereby theoretical considerations were carried out in order to predict favourable molecular structures [8]. Due the complexity of the task there is need of more experimental support. In previous studies [5-8, 9, 10] a large variety of combinations between IL-anions, IL-cations and redox couples where studied and the overall observations where reduced to the so-called attachment model [5]. In this model, the origin of the Seebeck potential that is created between the hot and the cold electrode is based on the affinity of one ion type to be selectively attached to the (rhodium) electrode. This attachment is temperature-dependent leading to an asymmetry in ion population between the hot and cold electrode. One of the most striking findings is the fact that adding LiI/I2 redox couple, iodide ions are also (together with e.g. ethylammonium nitrate EAN+) attached to the hot rhodium electrode, leading to a compensation of the positive charge, and consequently to a reduction of the Seebeck coefficient. This effect is generally witnessed in all observed systems. In extreme cases, (e.g. using Butyl-3-methylimidazolium iodide BMIM+ I-), the addition of iodine (acting as redox couple) even leads to an inversion of the Seebeck coefficient to negative values [5]. In this paper, we summarize the possible mechanism we found so far for obtaining the highest possible negative Seebeck coefficients. 2. Experimental All experiments were carried out in a set-up as sketched in [5, 6], whereby the redox-couple containing IL was sandwiched between two rhodium-coated sapphire plates. Both electrodes were temperature-controlled via thermocouples in a solid aluminum block situated directly behind sputter-coated rhodium on sapphire. Sapphire was chosen as Rh electrode substrate because it is electrically insulating and exhibits a thermal conductivity (24 W/m·K) which is high as compared to ILs (typically BMIM 0.18 W/m·K). Furthermore, it has a very flat surface that allows the creation of a IL container in form of a both sides polished glass ring that is clamped between the Rh-coated
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sapphire electrodes. Rhodium stands for high electrical conductivity and high chemical inertness. Such Rhodium sputter-coated thin-film electrodes were preferred rather to Rhodium electroplated massive metal electrodes. In this latter case, pin-hole appearance across the Rh-layer let the overall electrode property be dominated by the copper substrate, already after few heating/ cooling cycles. The heated electrode was equipped with a heater and the cooled electrode with a liquid containing heat exchanger. The equipment allowed temperature differences between 300°C and 0°C. For the electrical measurements, the heated and the cooled rhodium-coated sapphire disc was contacted for measuring the potential across the TEG. The setup was computer controlled and during the temperature scans the potential-value was recorded as soon as it fulfilled a stability criterion (< 1% variation after 10s). For the results of this paper, only the electrode potential at open circuit was used. For SE-measurements the open circuit voltagevariation was evaluated in function of the temperature variation; a two-point probe was sufficient because the contact was not charged by a current. Aiming to specifically find Ils with n-SE, the focus was put on the role of the anions and cations. First, we measured the SE for imidazolium-based cations with different side-chain lengths and anions, always using LiI/I2 redox couples. The same strategy was applied for ammonium-based cations. We then looked at choline-based cations and the impact on n-SE with respect to carboxylate anions. For this series, the role of different redox couples was studied and LiI/I2 replaced by Co2+/3+(bpy)3(TFSI2)2/3, later abbreviated by Co2+/3+. 3. Results and discussion 3.1. Imidazolium-based cations So as to identify ILs with a negative SE, ILs with imidazolium-based cations were chosen with varying lengths of alkyl side chains. Looking at Tab. 1, comparable ILs with increasing side chains are highlighted in italic, not showing a drastic difference in SE (between 240 and 310 µV/K). However as soon as the anion is varied, more significant differences can be observed, the highest values measured with hydrogensulfate as anion (541 µV/K), and the lowest values for bis(trifluoromethane)sulfonimide (TSFI, 75 µV/K). For (future) current extraction, redox couple LiI/I2 was used for most of the ILs, though for those with iodide as anion, iodine was preferred. For imidazolium-based cations with iodide as anion, only iodine was dissolved in the IL under brown colour. It is assumed that the formation of I3- (triiodide) causes the coloration. For 1-proypl-3-methylimidazoilium iodide, it is shown that the higher the concentration of triiodide the lower the n-SE-value. It is assumed that the attachment of the anion iodide/triiodide is responsible for the n-SE, independently of the content of the redox partner lithium iodide. We measured for 1-hexyl-3-methylimidazolium iodide the same tendency: the higher the concentration of iodine, the less the n-SE value. This outlines a more efficient adsorption of iodide than triiodide. Out of 12 ILs, only four, all with iodine anions, showed n-SE but with quite unsatisfactory values (-55 µV/K). Such values are not enough to be efficiently combined with very high p-SE. In all cases, iodide is the critical anion and I/I2 the redox couple enabling n-SE.
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Table 1. SE of IL with imidazolium-based cation, sorted by increasing side chains lengths (case 1). Seebeck coefficient [µV/K] Ionic liquid
Redox couple
1-Ethyl-3-methylimidazolium tetrafluoroborate 1-Ethyl-3-methylimidazolium triflate 1-Ethyl-3-methylimidazolium dicyanamide 1-Ethyl-3-methylimidazolium bromide 1-Propyl-3- methylimidazolium iodide
LiI/I2 LiI/I2 LiI/I2 LiI/I2 I2 I2
0.2 mol/L 0.2 mol/L 0.2 mol/L 0.2 mol/L 0.01 mol/L 0.2 mol/L
297 301 132 123
1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-Butyl-3-methylimidazolium tetrafluoroborate 1-Butyl-3-methylimidazolium iodide 1-Butyl-3-methylimidazolium tetrafluoroborate
LiI/I2 LiI/I2 I2 LiI/I2
0.2 mol/L 0.2 mol/L 0.2 mol/L 3 mol/L
75 310
1-Hexyl-3-methylimidazolium tetrafluoroborate 1-Hexyl-3-methylimidazolium iodide 1-Hexyl-3-methylimidazolium iodide 1-Hexyl-3-methylimidazolium iodide 1-Hexyl-3-methylimidazolium iodide
LiI/I2 neat I2 I2 I2
0.2 mol/L
240
1-Octyl -3- methylimidazolium tetrafluoroborate 1-Octyl -3- methylimidazolium trifluoromethanesulfonate
LiI/I2 0.2 mol/L LiI/I2 0.2 mol/L
Positiv e
Negative
-190 -55
-55 -90 -163 -132 -85 -42
0.01 mol/L 0.1 mol/L 0.2 mol/L 249 226
3.2. Ammonium-based cations For most anions, the study of ammonium-based cations resulted in significantly higher p-SE than for imidazolium-based cations. Moreover, as shown in Tab. 2, ethylammonium formate in combination with 0.2 mol/L LiI/I2 shows a large n-SE. It can then be assumed that a change of charge attachment at the electrodes occurs. The temperature-dependent ion attachment at the electrodes, leading in previous cases to a positive net charge at the hot electrode, is now inverted (Fig. 2). It appears in this case that the formate (carboxylate anion) dominates the polarity of the TEG over the cation influence. Table 2. SE of IL with ammonium-based cations, sorted by increasing side chains lengths (case 2). Seebeck coefficient [µV/K]
Ionic liquid
Redox couple
Methylammonium trifluoroacetate
LiI/I2 0.2 mol/L
726
Ethylammonium trifluoroacetate Ethylammonium nitrate Ethylammonium propanoate Ethylammonium acetate Ethylammonium formate
LiI/I2 LiI/I2 LiI/I2 LiI/I2 LiI/I2
580 614 513 369
Butylammonium nitrate
LiI/I2 0.2 mol/L
0.2 mol/L 0.2 mol/L 0.2 mol/L 0.01 mol/L 0.2 mol/L
Positive
Negative
-1451 573
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3.3. Choline-based cations Motivated by the previous results and the need to use more environmental-friendly ILs, a series of choline-based ILs with carboxylate anions was measured. A visual aid of the chosen anions and cations is depicted in Tab. 4. For a given redox couple, SE can reach high p-SE or n-SE values, depending on the anion. Moreover, it is observed that even a small amount of Co2+/3+ redox couple reinforces p-SE and n-SE as compared to LiI/I2 redox couple. Looking at Tab. 3, for choline trifluoroacetate p-SE increases from 187µV/K using LiI/I2 to 1545 µV/K using Co2+/3+. This is in agreement with previous results [11]. For choline lactate, n-SE increases from -1502 µV/K using LiI/I2 to -3630 µV/K using 0.01 mol/L of Co2+/3+. Table 3. SE of IL with choline-based cations (case 3). Redox couple
Ionic liquid
2+/3+
Seebeck coefficient [µV/K] Positive
Choline glycine Choline alanine Choline proline Choline acetate
Co Co2+/3+ Co2+/3+ LiI/I2
0.01mol/L 0.01mol/L 0.01mol/L 0.2 mol/L
545
Choline trifluoroacetate Choline trifluoroacetate
LiI/I2 0.2 mol/L Co2+/3+ 0.01 mol/L
187 1545
Choline lactate Choline lactate
LiI/I2 0.2 mol/L Co2+/3+ 0.01 mol/L
Negative -1670 -53 -638
-1502 -3630
Table 4. Overview of choline cation and chosen anions. Glycine
Alanine
Trifluoroacetate
Lactate
Proline
Acetate
Choline
3.4. Attachment model To try and explain the effects observed on p-SE and n-SE in sections 3.1 – 3.3, a simple model – called the attachment model – is proposed. Attachment means that a charged particle will be absorbed in proximity of an electrode and contributes to charging this electrode (electrical double layer). It is also considered that the attached ion can be released and carry its charge away.
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The only assumption made in the attachment model is that in a fully convection-dominated thin-layer TEG cell, the probability of a particle touching a surface element of the wall depends only on its concentration in the bulk liquid. In the considered case, the liquid consists of two (IL-anion / IL-cation) or four different particles (anions, cations, redox couples in two different oxidation states).
hot
cold Figure 1. Assumption of the attachment model: if a liquid (consisting of e.g. four particles) is heated in a thin-film TEG cell, the surface collision rate of the particles corresponds to the concentration of each particle. It is assumed that a perfect isotropic mixture occurs (ideal thermally induced stirring). Green: cations, yellow: anions, blue/red Redox oxidized/ reduced state.
Convection is assumed to give rise to ideal “stirring”. The trajectory of all particles is somewhat cyclic. In a previous publication computer analysis was carried out and indicated the appearance of so-called convection cells [12]. Fig. 1 sketches briefly this assumption. In the concrete case, looking at ILs, the green particles represent cations of the IL, the yellow ones the anions and the red and blue ones the redox couples in oxidized and reduced state, respectively. An external observer measures a steady-state potential difference between the hot and the cold electrode. This asymmetry can be attributed to a temperature-dependent steady-state attachment/detachment of charged particles at the electrodes. The appearance of asymmetry is only induced as soon as a temperature difference is applied. Fig. 2 sketches how at different temperatures the steady-state attachment of charged particles can give rise to a potential difference ∆V. For simplicity, only two kind of IL particles (e.g. IL-anions yellow and IL-cations green) are considered.
Figure 2. Different scenarios of voltage generation in thin-film TEG cells.
We assume that the electrode attachment is a temperature-dependent affinity between the ion type and the electrode material. This affinity plays a crucial role in the creation of the potential difference. This attachment is
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temperature-induced in the vicinity of the electrode surface and can be expressed, like the Seebeck coefficient, as ∆V/∆T. According to Maxwell equations one has to note that the value of the resulting potential difference does not depend on the total number of attached charged particles on the electrodes but only on their difference between hot and cold electrode. Chemically, one can note that the attachment happening at the hot electrode occurs also at the cold one, but at reduced scale. The attachment model was applied to our cases: In case 1 (Tab. 1), a p-SE is observed because the imidazolium-based cations are strongly attached rather to the hot electrode. Therefore, a positive voltage results of the difference in population between the hot and cold electrode. With the addition of LiI/I2 redox couple, negative iodide attach to the electrodes as well. This increase of redox concentration at the electrode leads to reduction of the p-SE, and is dependent on the redox couple concentration. This reduction of voltage is due to partial compensation of the positive charge from the imidazolium-based cations. Thecompensation effect is enhanced as soon as iodide is the anion. Adding more iodide as redox couple can even lead to a switch from p-SE to n-SE, as result of overcompensation of the cations by negative iodide [7]. In case 2 (Tab. 2), ammonium-based cations are used, and enhanced p-SEs can be achieved. A striking exception is observed as soon as formate is the anion of the IL, leading to a surprisingly high n-SE. The attachment model explains this effect by the fact that the formate anion-attachment at the hot electrode is preferred rather to positive ammonium-based cation, leading to an n-SE of -1451 µV/K. As already shown in different publications [13], formate anion has specific adsorption behaviour much more pronounced as chemically similar anions, such as acetate, which exhibits exclusively (moderate) p-SE-values. In case 3 (Tab. 3), using choline-based cations with different anions, in combination with redox couples, have an impact on p-SE and n-SE. The n-SE-value without redox could not be measured, but we assume that a strong attachment of lactate anions at the hot electrode leads to a high n-SE-value. The high value of n-SE of -3630 µV/K using 0.01 mol/L of Co2+/3 redox couple let us assume that the affinity of this kind of this redox couple is small, compared to lactate anions. A similar high n-SE-value can be achieved using glycine as anion and the same concentration of Co2+/3 redox couple. In the case that acetate and trifluoroacetate are used as anions, it can be considered like a case 1. 4. Conclusion A large variety of ILs are available, and the study of three different families of IL (imidazolium-, ammoniumand choline-based), in combination with different redox (LiI/I2, I2, Co2+/3+), showed the possibility of obtaining enhanced negative Seebeck coefficients. Having both high positive and negative Seebeck coefficients will allow the matching of internal resistances of TEGs, therefore optimizing the overall maximum power output given by serial connections in a highly integrated TEG, for a specific heat-flow. In this study, it was shown that for three classes of cations, SE depends sensitively on the choice of the anion, even changing the sign of the coefficient for a same cation and redox couple (ethylammonium formate: -1451µV/K, ethylammonium nitrate: 679µV/K). Moreover, it was also shown that the choice of the redox couple has a strong influence also going as far as changing the SE sign (ethylammonium nitrate + LiI/I2 0.2M: 614µV/K, ethylammonium nitrate + I2 0.2M: -55µV/K). Finally, a special role of Co2+/3+ redox couple was revealed, by surprisingly enhancing the SE values, both in the negative (choline lactate + LiI/I2 0.2M: -1502µV/K, choline lactate + Co2+/3+ : -3630µV/K) and positive (choline trifluoroacetate + LiI/I2 0.2M 187µV/K, choline trifluoroacetate + Co2+/3+ : 1545µV/K values. This is in strong contradiction looking at all previous experiments where adding redox couples (LiI/I2) always lead to a reduction of the Seebeck coefficient. This finding could be explained by the attachment model, indicating that the potential between the electrodes is due to specific temperature-dependent attachment of ions to the electrodes. Using simple attachment model that is only based on observation of the difference of the electrode potentials, all observed phenomena can be explained.
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Acknowledgements The authors gratefully acknowledge the financial support of the University of Applied Sciences Western Switzerland (HES-SO), interdisciplinary project No 011-2015-PI and the European Union’s Horizon 2020 research and innovation programme under grant agreement No H2020-FETPROACT 731976 Magenta. References [1] S. Zhang, N. Sun, X. He, X. Lu, and X. Zhang; Physical Properties of Ionic Liquids: Database and Evaluation; J. Phys. Chem. Ref. Data, Vol. 35, No. 4, (2006) 1475-1517; [2] T. J. Abraham, D. R. MacFarlane, and J. M. Pringle. High Seebeck coefficient redox ionic liquid electrolytes for thermal energy harvesting. Energy Environ. Sci., 2013,6, 2639-2645 (2013). [3] T. J. Abraham, D. R. MacFarlane, and J. M. Pringle. Seebeck coefficients in ionic liquids –prospects for thermoelectrochemical cells; Chem. Commun. 47 (2011), 6260–6262. [4] A. V. SOKIRKO; Theoretical Study of Thermogalvanic Cells in steady-state. Electrochimica Acta. Vol. 39. No. 4. (1994) 597-609, [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] 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 [7] S. Uhl, E. Laux, T. Journot, L. Piervittori, L. Jeandupeux, H. Keppner, Potential of thermoelectric generators based on ionic liquids, Journal of Energy Challenges and Mechanics 2, issue 2 (2016) 42-50. [8] A. Sosnowska, M. Barycki, A. Gajewicz, M. Bobrowski, S. Freza, P. Skurski, S. Uhl, E. Laux, T. Journot, L. Jeandupeux, H. Keppner, T. Puzyn. Towards the application of structure-property relationship modeling in material science: Predicting the Seebeck coefficient for ionic liquids ChemPhysChem (2016), 17, 1591. [9] 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 43 (2014) 3759-3766. [10] E. Laux, S.Uhl, N. Gauthier, L. Jeandupeux, H. Keppner, P. Pérez López, S. Pauline, E. Vanoli, R. Marti. Development of Thermoelectric generator based on Ionic Liquids for high temperature appliactions. Mater. Today: Proceedings 2 (2016), submitted. [11] N. Jiao, T.J. Abraham, D. R. MacFarlane, and J. M. Pringle; Ionic Liquid Electrolytes for Thermal Energy Harvesting Using a Cobalt Redox Couple; Journal of The Electrochemical Society, 161 (7) D3061-D3065 (2014) D3061. DOI: 10.1149/2.009407jes [12] T. Ikeshoji, F.N. Bravo de Nahui, S. Kimura, M. Yoneya. Computer analysis on natural convection in thin-layer thermocells with a soluble redox couple. J. Electroanal. Chem. 312(1991), 43-56. [13] A. Ferre-Vilaplana, J. V. Perales-Rondón c, C. Buso-Rogero c, J. M. Feliu c and E. Herrero, Formic acid oxidation on platinum electrodes: a detailed mechanism supported by experiments and calculations on welldefined surfaces, J. Mater. Chem. A. 5 (2017) 21773-21784.