Th

TNa

b

c

d

A

a

ARRAA

KEITER

1

aartcthhscctr

0h

Electrochimica Acta 113 (2013) 87– 93

Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

owards ionic liquid-based thermoelectrochemical cells for thearvesting of thermal energy

heodore J. Abrahama, Douglas R. MacFarlanea, Ray H. Baughmanb, Liyu Jina,a Lib,c, Jennifer M. Pringled,∗

ARC Centre of Excellence for Electromaterials Science, School of Chemistry, Monash University, Clayton, Victoria 3800, AustraliaUniversity of Texas Dallas, 800W Campbell Road, Richardson, TX 75080, United StatesInstitute of Polymer Chemistry, Nankai University, Tianjin 300071, ChinaARC Centre of Excellence for Electromaterials Science, Institute for Frontier Materials, Deakin University, 221 Burwood Highway, Burwood, VIC 3125,ustralia

r t i c l e i n f o

rticle history:eceived 16 May 2013eceived in revised form 13 August 2013ccepted 14 August 2013vailable online 5 September 2013

eywords:nergy conversiononic liquidshermal harvesting

a b s t r a c t

Liquid-state thermoelectrochemical cells offer an alternative design to traditional semiconductor-basedthermoelectric devices for the harvesting of thermal energy. They are capable of continuous and cheapoperation at moderate temperatures, are low maintenance and produce zero carbon emissions. The goodthermal and electrochemical stability, non-volatility and non-flammability of ionic liquids (ILs) makesthem promising electrolytes for these devices, especially for applications involving low grade thermalenergy available at temperatures in the 100–200 ◦C range. Power generation characteristics have beendetermined for a number of such ionic liquid-based devices using the iodide/triiodide (I−/I3

−) redox cou-ple. Power densities as high as 29 mW/m2 were measured in unoptimised devices operating with a hotside at 130 ◦C. The performance of these devices is a complex function of thermodynamic and transport

lectrolyteedox couple

properties of the electrolyte. An adjusted thermoelectric figure of merit (ZT*) was designed to comparephysical properties of these liquid electrolytes that determine their viability as electrolytes in thermo-electric energy conversion. This comparison requires evaluation of the diffusivity of the redox couple, thethermal conductivity of the electrolyte, and the Seebeck coefficient (Se) of the redox electrolyte. Theseparameters were determined independently and then combined in the figure of merit relationship toanalyse device performance using a range of different electrolytes.

. Introduction

The discovery and use of renewable resources for energypplications is essential given the continuing decline in fossil fuels,nd environmental effects from global warming [1]. Intermittentesources such as solar, wind and thermal energy provide oppor-unities for energy storage and conversion that are sustainable andan be utilised in zero or low emission processes [1–6]. Electricityoday is predominately generated by the indirect conversion ofeat into electricity. The use of thermal energy in this process isighly inefficient, producing waste heat that is released into theurroundings. Thermoelectric energy is produced by the directonversion of thermal energy into electrical energy. The design of a

ost effective and efficient thermoelectric device to harness wastehermal energy would save money, fossil fuels, and benefit the envi-onment. Thermoelectric cells rely on the Seebeck effect, where a

∗ Corresponding author. Tel.: +61 392446391.E-mail address: j.pringle@deakin.edu.au (J.M. Pringle).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.08.087

© 2013 Elsevier Ltd. All rights reserved.

temperature difference induces a potential difference across amaterial. The performance of thermoelectric materials is oftencompared by reference to a dimensionless figure of merit, definedas:

ZT = S2e T�

�(1)

where T is the absolute temperature, Se is the Seebeck coefficient, �is the electrical (or ionic) conductivity and � is the thermal conduc-tivity [7–9]. Se is the material constant describing the magnitude ofpotential difference created when a temperature difference exists:

Se = ∂E(T)∂T

(2)

where E(T) is the equilibrium electrode potential, which is afunction of the temperature, T. Se can be measured using a

8 ochim

nd

ccsmstrptmeuisd[[tachpt

toud∂hcttme

widpimont

tcoecda

fmS

8 T.J. Abraham et al. / Electr

on-isothermal electrochemical cell where a temperatureifference is created and the potential is measured1 [10].

Thermoelectric devices can be divided into (i) solid state, semi-onductor types [11], and (ii) liquid types involving various redoxouples in aqueous [10,12], organic [12,13] or ionic liquid [14–16]olutions. Solid-state thermoelectric cells using semiconductoretal alloys (e.g. Bi–Pb–Te) are commercially available and have

hown ZT values in the 1.5–1.8 range [17,18]. The drawbacks tohese devices are they have low Se (of the order of �V/K), theyely on expensive and in some case poisonous metal alloy com-ositions and they are difficult to make [11,17,18]. Liquid statehermoelectrochemical cells have a simpler design and have shown

uch higher Se values (of the order of mV/K) [4,15,19,20]. How-ver, the high volatility of solvent-based electrolytes limits thepper temperature range of most liquid based devices, rendering

mportant sources of thermal energy out of reach and thereforeeverely limiting the efficiencies currently possible from theseevices. The use of ILs can overcome this temperature limitation15,21,22], as many exhibit excellent thermal stability (>200 ◦C)23,24]. ILs are molten salts with a melting point below 100 ◦C [25],hat have been investigated for a wide range of electrochemicalpplications because of their electrochemical, thermal and chemi-al stability [1,26]. The ability to harvest higher temperature wasteeat sources, in the 100–200 ◦C range, enhances both the range ofossible applications for these devices and also the power outputshat can be achieved (by increasing �T).

The high Se in redox couple-based thermoelectrochemical sys-ems can be attributed to the large influence that temperature hasn the free energy difference between the reactants and prod-cts of an electrochemical redox reaction [20]. As discussed inetail by DeBethune et al. [27], the thermal temperature coefficient,Etherm/∂T, of the electromotive force of an electrochemical cellaving electrodes at different temperatures is related to entropyhanges in the cell, in particular the entropy changes arising fromhe redox reactions at the electrodes and the entropy of migrationransport. The latter term is usually quite small; therefore, the ther-

al temperature coefficient can be approximated to the reactionntropy, �Src, for the redox reaction:

∂Etherm

∂T≈ �Src

nF(3)

here n is the number of electrons involved in the reaction and Fs Faraday’s constant [20,27–29]. DeBethune et al. [27] discuss theistinction in this context between �Src (= S∗

e in the DeBethuneaper) and the entropy change, �S, of the electrode reaction when

t is referenced to a standard hydrogen electrode. Since measure-ents of ∂Etherm/∂T in this field usually involve measurements

f the whole cell potential under a temperature gradient and doot require a reference electrode, the standard hydrogen electrodeerm does not arise.

In contrast to the electronic and thermal conductivity limita-ions of semiconductor-based thermoelectric devices, in a redoxouple-based thermoelectrochemical system the conductivityf the electrolyte is not necessarily a determining property,specially in the case of an ionic liquid (because of the additionalonductivity of the inert ions). In this case the operation of theevice will be limited by diffusion and concentration of the redox

ctive species. Below we redevelop this figure of merit for the

1 The term Seebeck coefficient was first used to describe the thermoelectric effector a junction of two different metals. In the electrochemical context the externally

easured quantity ∂Etherm/∂T is the same and therefore it is often referred as theeebeck coefficient, even though the mechanistic origins of the effect are different.

ica Acta 113 (2013) 87– 93

redox thermoelectrochemical cell taking this into account. TheNernst–Einstein equation states:

� = �

c=

(z2F2

RT

)[Dox + Dred] (4)

where � is molar conductivity, z is the charge on the ion, F is theFaraday constant, R is the gas constant, T is the temperature, andDox and Dred are the diffusion coefficients of the individual ions, inthis case the oxidised and reduced forms of the redox couple [30];c is the concentration of the redox couple. If one of the diffusiveredox species is limiting, then its limiting diffusion coefficient,Dlim, dictates the rate of the electrochemical reaction that drivesthe current in the device, therefore it alone is a better measurefor the merit of the electrolyte. For a thermoelectrochemical cellutilising a redox couple in an electrolyte the figure of merit in Eq.(1) (now termed ZT*) is more appropriately written as:

ZT∗ =(

z2F2

R

)S2

e Dlimc

�(5)

This revised relationship thereby demonstrates the importantparameters for optimisation of redox couple-based thermoelectro-chemical cells, in particular the relative importance of Se, Dlim and c.This formulation of the figure of merit, of course, does not take intoaccount the effect of electrode structure or properties. In the case ofhighly porous electrodes where redox couple diffusion within thedepth of the electrode is an important factor, a more sophisticatedmodel is required.

The thermodynamics and kinetics of electron transfer reactionsof redox couples can be significantly influenced by the solvent envi-ronment [10,13,15,21,22]. Yamato et al. [16] recently reported on∂E/∂T and �Src of a series of iron, nickel and ruthenium redoxcomplexes in ionic liquids. The change in oxidation state thatencompasses an electrochemical redox reaction can affect the ioncoordination shell to different degrees in different solvents [13].Thus, as Se is related to the redox reaction entropy, the extentof the change in solvent ordering around the redox couple uponoxidation or reduction is influential to its performance in a thermo-electrochemical cell. In our initial communication [15], we reportedthe first measurements of Se of the I−/I3− couple in a series of ILs.Here we expand this range of ILs and report full device character-istics using these new I−/I3− electrolyte systems, with a detailedinvestigation of the parameters relating to the performance of thethermoelectrochemical cells. The figures of merit for IL electrolyte-based thermoelectrochemical cells were determined, to providea basis for understanding the variation with solvent type [3].Operating temperatures above 100 ◦C were targeted in this study,highlighting one of the benefits of IL-based electrolytes. Powergeneration from the thermoelectrochemical devices was measuredusing the best electrolytes, at different I−/I3− concentrations, usingplatinum electrodes. This demonstrates the first I−/I3− IL-basedthermoelectrochemical device and its use for power generationusing thermal energy at temperatures >100 ◦C.

2. Experimental

2.1. Analysis

Diffusion coefficients of the iodide and triiodide in the differ-ent ILs were measured by cyclic voltammetry using its relationshipto the limiting current under conditions of near steady-statebehaviour.

Ilim = 4nFDrc (6)

where n is the number of electrons transferred in the redox reac-tion, F is the Faraday constant, D is the diffusion coefficient, r is

T.J. Abraham et al. / Electrochimica Acta 113 (2013) 87– 93 89

Fig. 1. Disassembled thermoelectrochemical device set up: disc electrodes fittedito

tc(uTsp

mfdic

fc1dtTattatwtStsveTdrdahdt[t

2

adb

Table 1The effect of I−/I3

− concentration on ∂Etherm/∂T in [C2mim][BF4].

[I−/I3−] ∂Etherm/∂T (mV/K)

(±0.02)�Src (J/(K mol))(±4)

nto Teflon caps (left and right squares) with polyimide heaters on the backside ofhe electrodes. Teflon insert (middle square) with electrolyte reservoir in centre,-ring designating exposed electrode area, and platinum temperature sensors.

he radius of the electrode and c is the concentration of the redoxouple in mol/L [31]. A three-electrode cell with platinum working10 �m diameter), counter and pseudo-reference electrodes wassed, scanning between −0.8 V and +0.4 V at a scan rate of 20 mV/s.he platinum wire counter electrode was coiled to provide a largeurface area, while the pseudo-reference electrode was a straightlatinum wire.

The thermal conductivities were either found in the literature oreasured using a C-Therm TCi thermal conductivity analyzer at dif-

erent temperatures, confirming a linear relationship. The effect ofissolved redox couple on the thermal conductivity was not stud-

ed as the I−/I3− based electrolytes are corrosive to the thermalonductivity analyzer.

Thermoelectrochemical cell power measurements were per-ormed using a custom designed cylindrical cell (Fig. 1). Twoircular platinum electrodes (18 mm diameter) were separated by a0 mm cylinder insert with 9 mm internal diameter. The insert wasesigned with a groove to hold a Pt RTD probe with ceramic body,o make direct contact with the front surface of each Pt electrode.his assures accurate measurements of the temperature differencecross the cell. Polyimide ThermofoilTM heaters were attached tohe back of the electrodes and controlled using a custom-madeemperature controller connected to the temperature sensors and

power supply. The electrolyte was injected into the cylinderhrough a hole in the side, which was then sealed. The whole cellas placed onto dry ice, to allow full control of the temperature of

he electrodes by heating them using the temperature controller.e values were calculated from the open circuit potential of thehermoelectrochemical cell at each �T, using Eq. (2). Voltage mea-urements were taken after 30 min equilibration at open circuitoltage at the required Thot/Tcold. At each resistance, the system wasquilibrated for 15 min and then 30 data points collected over 300 s.his allows the temperature, concentration profile and potentialifference to reach steady-state [20,28,32]. Any equilibrium Soretedistribution effect of redox species due to the temperature gra-ient is implicitly part of the measurement of ∂Etherm/∂T, however,s the Soret effect is known to be small and takes a long time toave an effect [29] it is common to consider initial temperatureependences without Soret contributions under these experimen-al conditions. The small differences in Se from the previous report15] result from a more accurate control and measurement of theemperatures of the two electrodes in the full device.

.2. Materials

Ionic liquids 1-ethyl-3-methylimidazolium bis(trifluorometh-nesulfonyl) amide [C2mim][NTf2], 1-ethyl-3-methylimidazoliumicyanamide [C2mim][DCA] and 1-butyl-1-methylpyrrolidiniumis(trifluoro-methanesulfonyl) amide [C4mpyr][NTf2] were

0.02 M 0.45 870.1 M 0.39 750.4 M 0.23 44

synthesised following literature procedures [23] (see Sup-porting Information), while 1-ethyl-3-methylimidazoliumtetrafluoroborate (electronic grade, [C2mim][BF4]) and 1-ethyl-3-methylimidazolium tetracyanoborate (high purity,[C2mim][B(CN)4]), were purchased from Merck and used withoutfurther purification. The I−/I3− couple was prepared from lithiumiodide (Sigma Aldrich, 99.9%) and iodine (Sigma Aldrich, 99.999%).

3. Results and discussion

A series of ILs with 0.4 M I−/I3− were used as electrolytes inthis work due to their high thermal stability and low to negligiblevapour pressure. Table 1 presents the ∂Etherm/∂T results determinedin this work using the thermo-electrochemical cell. The concentra-tion dependence can be analysed at a first level of approximationby a simple application of the Nernst equation. The I−/I3− redoxreaction is shown in Eq. (7),

I3− + 2e− � 3I− (7)

and the corresponding Nernst equation can be written as:

Etherm = E◦therm − RT

2Fln

�3I− [I−]3

�I3− [I3

−](8)

In the situation where [I−] = [I3−], and the activity coefficients areapproximately independent of temperature:

∂Etherm

∂T= ∂E◦

therm

∂T− R

Fln

[�3

I−

�I3−

[I−]

](9)

if the concentration scale used in Eq. (9) is mol/kg (use of the molar-ity scale introduces terms involving the temperature dependenceof the volume of the solution). From this equation it is clear thata concentration dependence of ∂Etherm/∂T is expected in this case,as is observed in Table 1. However, the activity coefficients (andtheir temperature dependence) of the redox couple species arisingin the ionic liquid media are unknown at this stage and thereforethe exact form of the concentration dependence is not quantifiableand the origins of the concentration dependence cannot be furtheranalysed.

Table 1 also shows calculated values of �Src for I−/I3− in[C2mim][BF4] at each concentration, a pronounced decrease in �Src

occurring at 0.4 M.We take this concentration study further with measurements

of the diffusion coefficients, ZT* calculations and output powermeasurements from a full thermoelectrochemical device, to deter-mine the optimal concentration of I−/I3−. As the concentration ofdissolved redox couple is increased, the diffusion coefficient, ZT*and consequently the output power from the device also increases(Table 2), indicating that an electrolyte saturated with redox couplemay be optimal. Therefore, the 0.4 M electrolytes were used for thesubsequent study.

The measured ∂Etherm/∂T and calculated �Src for 0.4 M I−/I3− ina range of ILs (Table 3) shows the significant influence that solvent

environment has on the reaction entropy in these electrolytes.

The diffusion rates of the iodide and triiodide in the different0.4 M I−/I3− electrolytes at 80 ◦C, 95 ◦C and 110 ◦C (Fig. 2) were cal-culated by measuring the limiting current obtained by steady state

90 T.J. Abraham et al. / Electrochim

Table 2Concentration study of I−/I3

− in [C2mim][BF4]: diffusion coefficients of iodide, D(I−),triiodide, D(I3

−), figure of merit, ZT*, and maximum output power, Pmax.

[I−/I3−]a D(I−) (m2/s)

(50 ◦C)D(I3

−) (m2/s)(50 ◦C)

ZT* (×10−6)(50 ◦C)

Pmax (mW/m2)(�T = 100 ◦C)

0.02 M 9.1e−12 1.6e−11 0.91 9.00.1 M 3.1e−11 2.3e−11 10 170.4 M 6.3e−11 7.1e−11 30 29

Thot = 130 ◦C; Tcold = 30 ◦C.a Room temperature concentration given. To allow for thermal expansion, the

density at 50 ◦C was measured and used to calculate the actual concentration ofeach solution at this temperature. The concentration of the electrolyte solutions at50 ◦C decreased by <5% compared to the room temperature value (see ESI).

Table 3∂Etherm/∂T and the calculated �Src values for various 0.4 M I−/I3

− in IL electrolytes.

Electrolyte ∂Etherm/∂T (mV/K)(±0.02)

�Src (J/(K mol))(±4)

[C2mim][BF4] 0.23 44[C2mim][NTf2] 0.13 25[C4mpyr][NTf2] 0.13 25

cdapTa

F[(

[C2mim][B(CN)4] 0.10 19[C2mim][DCA] −0.070 −14

yclic voltammetry using a microelectrode [33]. The redox coupleiffusion rate of the standard thermoelectrochemical electrolyte,queous 0.4 M ferri/ferrocyanide, was measured at 55 ◦C for com-

arison: I−/I3− diffuses slower in ILs even at increased temperature.he relatively high viscosities of ILs limit the diffusion of redoxctive species. However, the diffusion of the I−/I3− redox couple is

ig. 2. Diffusion coefficients of I− (a) and I3− (b) in ionic liquid electrolytes. ( )

C2mim][BF4], ( ) [C4mpyr][NTf2], ( ) [C2mim][B(CN)4], ( ) [C2mim][NTf2] and ) [C2mim][DCA]. (�) Ferrocyanide (a) and (�) ferricyanide (b).

ica Acta 113 (2013) 87– 93

likely accelerated by a Grotthuss-type mechanism [33,34], and thediffusion rates increase with temperature (Fig. 2a and b), whichis particularly advantageous for IL-based thermoelectrochemicalcells that can be operated at relatively high temperatures. The dif-fusion rates are fastest in the low viscosity (66 cP at 20 ◦C) [35][C2mim][BF4] and slowest in the most viscous IL (89 cP at 20 ◦C)[36] [C4mpyr][NTf2]. On average, the iodide and triiodide diffusedat similar rates, ±25%. The iodide ion in [C2mim][DCA] has thefastest diffusion rate: 80% faster than triiodide in the same elec-trolyte. The dicyanamide anion appears to impede the diffusion oftriiodide, but supports the diffusion of iodide. Dicyanamide exhibitspseudo-halide behaviour such that it can react with iodine to formspecies such as [I-I-DCA]− [37,38], potentially impeding its diffu-sion. Similarly, measurement of the Se of I−/I3− in [C2mim][SCN]gave very unstable Se results and no reproducible data were col-lected. Thiocyanate can also behave as a pseudo-halide and mayinteract strongly with the I−/I3− redox equilibrium creating suchinstability.

The diffusion rates of both iodide and triiodide were similarin the tetracyanoborate and tetrafluoroborate ILs, ca. 4e−10 m2/s.However, the large differences in Se (Table 3) suggest that the sol-vation environments provided by the two ILs are very different.

The temperature dependence of the thermal conductivity ofthe neat ILs (without I−/I3−) has been reported in the literature[39–42], with the exception of [C2mim][B(CN)4], ranging from0.12 to 0.19 W/(K m), which is significantly lower than water [42](0.6 W/(K m)). The lower thermal conductivity of ILs helps maintaina greater �T and increases ZT* in this context. The thermal con-ductivities of the various neat ILs was calculated at three elevatedtemperatures based on the reported temperature dependences(see Supporting Information). The I−/I3− redox couple’s corrosivenature prevents any study of its effect on the thermal conductivityusing the C-Therm device, as the sensor is prone to corrode in thismedium.

Using Ohm’s Law (I = V/R) and Joule’s Law (P = I2R = V2/R), byapplying known resistances and measuring the potential acrossa cell, one can calculate the current and the output powerof a cell. The maximum output power can be deduced fromthe second order relationship of potential to power in Joule’sLaw, by fitting a quadratic function (R2 > 0.95) to the dataand calculating the local maximum. The output powers of thethermoelectrochemical cells, using platinum electrodes, were per-formed to test the predictions made by the figure of meritcalculations. Temperature differences created by hot/cold combi-nations of 130 ◦C/30 ◦C, 130 ◦C/60 ◦C and 130 ◦C/90 ◦C were testedfor each IL system. Based on our initial results (Table 2) aconcentration of 0.4 M I−/I3− was used for these power mea-surements. Fig. 3 illustrates the power per unit area distributionas different loads are applied to the thermoelectrochemical cell,with the different electrolytes, at the three temperature differ-ences.

The [C2mim][BF4] electrolyte was found to be the best perform-ing at all temperature differences, producing 5.4, 16 and 29 mW/m2

respectively. The other electrolytes performed similarly to oneanother, giving substantially lower power outputs (Table 4). The[C2mim][DCA] electrolyte behaved abnormally in the thermoelec-trochemical device, giving a potential with the opposite sign to allthe other electrolytes, as a result of the negative Se. A possible expla-nation for this behaviour is that the pseudohalide nature of the DCAanion results in different interactions with the I−/I3− redox cou-ple, changing the redox chemistry and its associated temperaturedependence.

Calculation of the modified figure of merit for the differentelectrolytes allows comparison of their performance in the experi-mental devices. Combining the data obtained thus far allows ZT*calculations for these materials, as listed in Table 5, at average

T.J. Abraham et al. / Electrochimica Acta 113 (2013) 87– 93 91

Fig. 3. Thermoelectrochemical device measurements of 0.4 M I−/I3− electrolytes at three different operating temperatures: (a) 130 ◦C/30 ◦C, (b) 130 ◦C/60 ◦C and (c)

130 ◦C/90 ◦C. [C2mim][DCA] potentials are negative, as a result of the negative Se, but are

( ) [C2mim][B(CN)4], ( ) [C2mim][NTf2] and ( ) [C2mim][DCA].

Table 4Maximum output powers of the thermoelectrochemical devices using 0.4 M I3

−/I−

electrolytes at three operating temperatures.

Electrolyte (0.4 M I3−/I−) Pmax (mW/m2) (±0.5)

Thot 130 ◦CTcold 30 ◦C

Thot 130 ◦CTcold 60 ◦C

Thot 130 ◦CTcold 90 ◦C

[C2mim][BF4] 29 16 5.4[C2mim][NTf2] 3.0 3.5 1.4

oemat8[ti

TFt

[C4mpyr][NTf2] 2.7 2.6 0.8[C2mim][B(CN)4] 5.9 3.4 1.4[C2mim][DCA] 1.3 0.6 0.2

perating temperature (Thot + Tcold)/2 = 80 ◦C for the top performinglectrolytes studied. To allow for potentially different rates of ther-al expansion of the electrolytes the density at 80 ◦C was measured

nd used to calculate the actual concentration of each solution athis temperature. The concentration of the electrolyte solutions at

0 ◦C decreased by <5% compared to the room temperature value.C2mim][BF4] illustrates the best thermoelectrochemical proper-ies while [C2mim][DCA] is expected to have poor performancen a thermoelectrochemical cell. The ZT* values are smaller than

able 5igure of merit, ZT*, calculations of 0.4 M I3

−/I− electrolytes at the average operatingemperature (Thot + Tcold)/2 = 80 ◦C.

Electrolyte ZT*

[C2mim][BF4] 1.3e−04[C2mim][NTf2] 3.9e−05[C4mpyr][NTf2] 3.3e−05[C2mim][B(CN)4] 2.8e−05[C2mim][DCA] 2.1e−05

shown as positive for ease of comparison. ( ) [C2mim][BF4], ( ) [C4mpyr][NTf2],

those calculated for semiconductor devices because of the lowerconductivity (diffusivity), however the ability to work at lowertemperatures is key advantage of these devices. We propose a tar-get for the modified ZT* for the liquid state thermoelectrochemicalcells of >0.01, to better represent the required parameters for liquidthermoelectrochemical electrolyte materials.

The trend in ZT* values for the different IL electrolytes is thesame as the trend in power outputs, with the exception of thetetracyanoborate ionic liquid, [C2mim][B(CN)4]. The current volt-age characteristics of the thermoelectrochemical cells (Fig. 4) wereinvestigated for additional insights into the factors limiting cellperformance [29]. Ideally the potential would not decrease fromthe open circuit potential as current increases, i.e., if activation,concentration and mass transport overpotentials, or ohmic lossesdid not exist [29]. However, a typical electrochemical cell exhibitsall of these sources of potential loss [29]. The [C2mim][B(CN)4]based electrolyte has one of the lowest Se values but produceshigher currents than the [NTf2] ionic liquids. The higher diffusioncoefficients of I−/I3− in the [C2mim][BF4] and [C2mim][B(CN)4]over the other ILs enable higher currents in the cell, and results in asteeper slope of the I–E curve (Fig. 4). The steeper slope representsmore ideal electrochemical behaviour and results in higher outputpowers.

The improvement in Pmax and Isc that [C2mim][NTf2] displayswhen the cold electrode is heated from 30 ◦C to 60 ◦C indicatesthat at Tcold = 30 ◦C the device must be diffusion limited. If no diffu-sion limitation existed, reducing the �T would decrease the Pmax.

However, Pmax decreases with a further increase in Tcold to 90 ◦C(Fig. 3), indicating that the voltage decrease associated with thelower �T outweighs the benefits to diffusion rates associated withan increased average temperature.

92 T.J. Abraham et al. / Electrochimica Acta 113 (2013) 87– 93

F e diffe[ parat[

4

shspiTpdtfmaepirmtmtFetp

A

R

ig. 4. Currents in thermoelectrochemical device of 0.4 M I3−/I− electrolytes at thre

C2mim][DCA] potentials and currents are negative but shown as positive for comC2mim][NTf2] and ( ) [C2mim][DCA].

. Conclusions

In conclusion, thermoelectrochemical cells using IL electrolyteshow promise for higher temperature (>100 ◦C) thermal energyarvesting applications. The good thermal and electrochemicaltability, non-volatility and non-flammability of ILs makes themromising electrolytes for these devices, especially for applications

nvolving thermal energy at temperatures in the 100–200 ◦C range.he top performing I−/I3− dissolved in [C2mim][BF4] electrolytesroduced power densities as high as 29 mW/m2 in unoptimisedevices operating with a hot side at 130 ◦C. This demonstrateshe first I−/I3− IL-based thermoelectrochemical device and its useor power generation using >100 ◦C thermal energy. The perfor-

ance of these devices is a complex function of thermodynamicnd transport properties of the electrolyte. An adjusted thermo-lectric figure of merit, ZT*, was designed to compare the physicalroperties of these liquid electrolytes that determine their viabil-

ty for thermoelectrochemical energy conversion. This comparisonequires evaluation of the diffusivity of the redox couple, the ther-al conductivity of the electrolyte, and the Seebeck coefficient of

he redox electrolyte. However, a more detailed electrochemicalodel of the operation of these devices is clearly required in order

o fully understand the interplay of the various material properties.uture experiments involving modifications of the redox couple,lectrode material, and various IL and high boiling organic elec-rolytes are in progress and will be reported on in a subsequentublication.

cknowledgements

The authors gratefully acknowledge funding from the Australianesearch Council through its Centre of Excellence programme, the

rent operating temperatures: (a) 130 ◦C/30 ◦C, (b) 130 ◦C/60 ◦C and (c) 130 ◦C/90 ◦C.ive reasons. ( ) [C2mim][BF4], ( ) [C4mpyr][NTf2], ( ) [C2mim][B(CN)4], ( )

Natural Sciences and Engineering Research Council of Canada andalso for fellowship support for D.R. MacFarlane (Federation Fellow).

Appendix A. Supplementary data

Supplementary material related to this article can be found,in the online version, at http://dx.doi.org/10.1016/j.electacta.2013.08.087.

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