FIELD OF INVENTION

[0001] The present invention relates to the field of energy generating devices.

[0002] In one form, the invention relates to use of thermo-electrochemical devices suitable for generating electricity from thermal energy.

[0003] In one particular aspect the present invention is suitable for use as a thermo- electrochemical cell, or device incorporating the thermo-electrochemical cell.

BACKGROUND ART

[0004] It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.

[0005] Waste heat, such as from geothermal sources or in industrial power plants, is a major energy source that is extremely underutilised. For example, currently about 90 % of the world's power is generated by heat engines that use fossil fuel combustion as a heat source and operate at only ~30-40 % efficiency. The development of efficient thermo-electrochemical devices could enable harvesting of some of this waste heat for use in low voltage applications such as lighting and communications. Even capturing some of the waste heat generated from internal combustion engines could provide electricity for functions such as on-board controls. Many sources of waste heat are continuous, which is an advantage over other renewable energy sources. Thermo- electrochemical devices allow the direct conversion of thermal energy to electrical energy.

Thermo-electrochemical Cells Generally

[0006] Thermo-electrochemical cells, (also sometimes termed thermogalvanic cells, thermocells, thermal cells, galvanic thermocells, electrochemical thermocouples or galvanic thermocouples) produce electrical energy by utilizing the temperature dependence of electrochemical redox potentials. Within the cells are two electrodes, held at different temperatures, and this temperature difference generates a potential difference, thereby allowing the heat to be converted to electrical energy (FIG. 1 ). The driving force for the cell is the transport of entropy from the high temperature reservoir to the low temperature sink, while chemical and thermal gradients drive the movement of redox species within the cell and prevent build up of oxidized/reduced species at the electrodes.

[0007] The magnitude of the voltage that can be induced in response to a temperature difference across a material is given by the Seebeck coefficient, S _e . (Bouty, E. Journal de Physique Theorique et Appliquee 1879, 8, 289; Rockwood, A. L. Physical Review A 1984, 30, 2843)

[0008] If the cell contains a redox system A + ne ^" -» B, then the temperature dependence of the electrochemical redox potentials is given by the Seebeck coefficient, S, such that: e _ W_ ₌ ^BA

where V is the electrode potential, T is the temperature, n is the number of electrons involved in the redox reaction, F is Faraday's constant and ASB A is the reaction entropy for the redox couple.

[0009] The efficiency of a thermoelectric material or device can be described by the Figure of Merit, ZT, where: ZT = S ² a/K where σ is the electrical conductivity and κ is the total thermal conductivity.

[0010] One of the earliest thermocell - the Ag|AgNO ₃ |Ag thermocell - was first reported in the 1890s and there has since been a range of different metal electrodes, solid electrolytes and liquid electrolytes investigated (Chum, H. L; Osteryoung, R. A. Review of thermally regenerative electrochemical systems, Solar Energy Res. Inst., Golden, CO, USA., 1981 ).

[001 1] In this archetypal system, the reaction at the anode (hot) is Ag→ Ag ⁺ + e ^" . The Ag ⁺ then moves through the electrolyte from the hot to cold side, where the reaction at the cathode is Ag ⁺ + e ^" → Ag (Weininger, J. L. J. Electrochem. Soc. 1964, 111, 769). In this system the metal of the anode is being consumed, and therefore the hot and cold side of the cell must, at some point, be reversed to allow further operation of the cell.

[0012] A thermocell using solid Agl and gaseous l ₂ was patented in 1957, (Weininger, J. L; (General Electric Co.). Application: US 1957-651758, 2890259, 1959) and a range of other gases and electrolytes have also been trialled, (Chum, H. L; Osteryoung, R. A. Review of thermally regenerative electrochemical systems, Solar Energy Res. Inst., Golden, CO, USA., 1981 ). These devices can theoretically be run continuously, however they use gas electrodes and often require high temperatures.

[0013] There is also literature on the use of concentration cells (where the cathode and anode are placed in electrolyte solutions of different concentrations, such as strong and weak acid). As these systems can be thermally regenerated, they are sometimes referred to as thermo-electrochemical systems, (Gordon, J. H.; Joshi, A. V.; Balagopal, S. H.; (Ceramatec, Inc., USA). Application: US 2005-286637, 20060141346, 2006, p 14 pp) but this is a different concept to the redox couple based systems described here.

[0014] In the 1990s Opallo reported studies on the electrode reactions of various redox couples in "frozen" electrolytes i.e. salt hydrates taken below their melting point, (Opallo, M. J. Solid State Electrochem. 1998, 2, 347; Opallo, M. J. Electroanal. Chem. 1995, 399, 169; Opallo, M. J. Electroanal. Chem. 1996, 418, 91 ; Opallo, M.J Electroanal. Chem. 1996, 411, 145; Opallo, M. J. Electroanal. Chem. 1998, 446, 39;

[0015] Opallo, M. J. Electroanal. Chem. 1998, 444, 187) and the change in the electrochemical potentials of these redox couples with temperature was described. However, these materials never progressed to use in thermo-electrochemical cells.

[0016] Many thermoelectric devices of the prior art utilise alternating p- and n-type semiconductor materials, which commonly have S _e of a few 100 μν/Κ, can be very expensive and are suitable only for small scale, niche applications or those utilising very high temperatures. There has been considerable research into thermoelectric materials (e.g. Bi ₂ Te ₃ ) over the last four decades, but these have achieved a maximum ZT of only ca. 1 , which is generally not sufficient to make up for the high materials costs, particularly for lower temperature applications. This apparent limit to ZT for bulk materials is a result of the fact that materials with a high conductivity also have a high thermal conductivity, and the interdependence of these factors makes improvement of ZT challenging.

[0017] Research into further optimising the performance of these devices relies on manipulating the materials at the nano-scale (Szczech, J. R.; Higgins, J. M.; Jin, S. J. Mater. Chem. 201 1 , 21 , 4037). In recent years there have been significant advances in ZT using nanomaterials, which have allowed ZT of up to 3.2, at a 300°C temperature difference. However, the high ZT values achieved recently using nanoscale materials, e.g. thin films or nanowires, are not yet practical for industrial-scale commercial use because they are fabricated by techniques which are expensive and unsuitable for scale- up, such as atomic layer deposition (Bell Lon, E. Science 2008, 321, 1457).

[0018] US patent 6,838,208 (De Crosta et al) teaches the use of a modified thermal concentration galvanic cell for conversion of heat to electrical energy, has half-cells with metallic and inert electrodes, and electrolyte having metal or metallic salt electrode solution. One of the disadvantages of this type of cell is that one half of the cell contains a chemically active electrode (metal electrode) which undergoes oxidation while the other half is a concentration cell comprising a non-active electrode. Serious degradation of materials or cell components can occur due to oxidation of the metal electrode and metal salt electrolyte solution. [0019] US patent 4,376,155 (Peck) describes a thermal galvanic cell which works on the principal of establishing a heat gradient along the electrode as well as across it. However, this type of cell has the disadvantage that it includes electrode materials that are at least in part comprised of a conductive metal and there is an inherent risk of leakage because the electrolyte is a liquid.

[0020] US patent 3,441 ,441 (Iverson) teaches the use of sodium salt mixtures as the electrolyte for galvanic cells. Specifically it discloses the chemical reaction of the cathode and anode materials, Na ⁺ and Hg ⁺ , and migration of Na ⁺ ions through the electrolyte. Thus, the electrolyte is likely to be efficient in transporting the ions across, stable in high temperature and not prone to leakage.

[0021] US patent 4,396,690 (Gordon et al) relates to a device for the simultaneous conversion of light energy (but not solar energy) into electrical energy and thermal energy using a liquid-junction semiconductor photocell (PEC).

[0022] US patent 5,310,608 (Ishizawa et al) discloses the use of two half cells separated by an impermeable membrane that stops a redox couple passing between the two halves of the cell. One half of the device is heated and the other is not, thus forming a concentration gradient. When the heat is subsequently removed, they act as concentration cells where the energy is stored via a concentration gradient, until needed. However this type of device has the disadvantage that it does not continuously produce electrical energy but instead stores energy for later use.

[0023] US patent 5,487,790 (Yasuda et al) relates to an electric power generating element for converting low temperature heat to electricity, the element including a positive electrode and a negative electrode composition based on polyethylene glycol. More specifically, in an electric power generating element, either the positive or negative electrode includes a composition containing an organic compound as a main agent. The positive electrode has an electrically conductive substance so that relatively low- temperature thermal energy is efficiently converted to electric energy. Polyethylene glycol is employed as the organic compound and graphite or a graphite composition is employed as the conductive substance. Salt providing ionic conductivity may be added to the organic compound or polyethylene glycol, and the negative electrode may be formed of a metal having an ionization tendency as large as or larger than copper or a composition of the metal. One of the disadvantages of this type of element is that it uses a metal based electrode that is oxidised during use.

SUMMARY OF INVENTION

[0024] An object of the present invention is to provide a thermo-electrochemical cell having improved voltage/power outputs as compared with thermo-electrochemical cells of the prior art.

[0025] A further object of the present invention is to provide an improved device incorporating a thermo-electrochemical cell.

[0026] A further object of the present invention is to provide a thermo-electrochemical device for continuous generation of electricity.

[0027] A further object of the present invention is to alleviate at least one disadvantage associated with the related art.

[0028] It is an object of the embodiments described herein to overcome or alleviate at least one of the above noted drawbacks of related art systems or to at least provide a useful alternative to related art systems.

[0029] In a first aspect of embodiments described herein there is provided a thermo- electrochemical cell comprising inert electrodes and a solid or multiphase electrolyte wherein the electrolyte comprises a redox couple that generates a potential difference across the cell in response to application of heat at one electrode.

[0030] In thermo-electrochemical cells of the prior art, the metal anodes were consumed and as a result, the hot and cold sides of the cell had to be reversed to allow ongoing operation of the cell. In contrast, the thermo-electrochemical cells of the present invention can operate continuously without having to be reversed or recharged. Electrolyte

[0031] The electrolyte of the present invention is preferably solid or multiphase that is, solid/solid, solid/liquid or liquid/liquid. The use of solid state electrolytes, such as molecular or organic ionic plastic crystals or polymer-based systems, provides advantages over liquid-based devices in terms of reduced leakage, thermal stability, non-volatility and thus longer lifetimes, in addition to further improvements in the Seebeck coefficients of different redox couples. Solid or multiphase electrolytes also enable the devices to be used at temperatures above 100°C, which is a significant advantage over thermo-electrochemical cells of the prior art that utilise water or other molecular solvent electrolytes and thus are limited to lower temperature applications. The ability to use the devices at higher temperatures not only increases the variety of their potential applications but also allows higher voltages to be achieved as the temperature difference between the hot and cold electrodes is increased. Furthermore, the use of a solid state electrolyte means the temperature difference is more easily maintained as there is not convection in the solid. The use of a multiphase electrolyte, or more preferably a biphasic electrolyte system is also predicted to yield significant increases in power output of the cell, as a result of increased entropy difference between the electrodes.

[0032] In a particularly preferred embodiment, the electrolyte is biphasic that is, consisting of two electrolytes of different compositions, and/or two electrolytes of different phases.

[0033] Embodiments and examples of suitable biphasic electrolytes include the following:

[0034] In one embodiment, the electrolyte is a biphasic solid/liquid system where the solid and the liquid have the same composition but different form (because T(cold) < T(phase change) < T(hot)) thereby establishing an additional entropy difference of the redox couple between the phases that adds to the overall AS between the cathode and anode (and hence increases the voltage output of the device).

[0035] For example, the electrolyte may be a molten molecular plastic crystal such as succinonitrile. Succinonitrile can provide two phases of identical composition but due to the temperature difference between the cathode/anode the electrolyte is present as both a solid and liquid within the device (Thot>Tm plastic crystal>Tcold). Another example of a suitable electrolyte is an organic ionic plastic crystals such as N,N- methylethylpyrrolidinium tetrafluoroborate.

[0036] In another embodiment, the biphasic electrolyte is a solid/solid electrolyte, composed of two different solid state materials, thereby contributing to an increased additional entropy difference of the redox couple between the phases that adds to the overall AS between the cathode and anode (and hence increasing the voltage output of the device).

[0037] In another embodiment the biphasic electrolyte is a solid/solid electrolyte wherein the two phases are of the same composition but as a result of the temperature difference between the cathode/anode the electrolyte is present in two different plastic crystal (solid) phases (Thot>Tsolid-solid of plastic crystal>Tcold).

[0038] In another embodiment the biphasic electrolyte is a solid/solid electrolyte comprising two immiscible solids, for example a molecular plastic crystal such as succinonitrile in combination with an organic ionic plastic crystal such as N,N- methylethylpyrrolidinium tetrafluoroborate.

[0039] In another embodiment the biphasic electrolyte is a liquid/liquid electrolyte wherein the two phases are of different compositions. For example the biphasic electrolyte may be comprised of an ionic liquid such as 1 -ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide and a molecular solvent such as water.

[0040] In another embodiment the biphasic electrolyte is a liquid/liquid electrolyte comprising two immiscible molecular solvents.

[0041] In another embodiment the biphasic electrolyte is a liquid/liquid electrolyte comprising two immiscible ionic liquids such as 1-ethyl-3-methylimidazolium chloride and trihexyltetradecylphosphonium chloride. [0042] Particularly preferred electrolytes for use in the present invention include solid state electrolytes such as plastic crystals, either molecular or ionic, such as succinonitrile, pentaglycerine, tetrabromomethane, tetramethylammonium dicyanamide, tetraethylammonium dicyanamide, Λ/,/V-methylethylpyrrolidinium bis(trifluoromethanesulfonyl)amide, /V,/V-methylethylpyrrolidinium tetrafluoroborate. Preferred solid state electrolytes also include materials such as polymer electrolytes, polyelectrolytes, gels and liquid crystals.

[0043] The electrolytes may incorporate added nanoparticles such as inorganic nanoparticles (such as SiO ₂ , Ti0 ₂ etc.) or organic nanoparticles (such as carbon nanotubes and so forth). They may also include added polymer fibres/mats. In one embodiment the electrolyte is a quasi-solid state material wherein a liquid (such as an ionic liquid) has been formed into a physical gel such as by addition of nanoparticles.

[0044] In a particularly preferred embodiment the electrolyte is a gel, more particularly a hydrogel. For example, one such particularly preferred hydrogel comprises a polymeric structure formed by the interaction of gelatine or cellulose with water.

[0045] High boiling point solvents suitable for use in the present invention include, for example, 2-Methoxy ethyl acetate, dimethylformamide, dimethylsulfoxide, propylenecarbonate, ethylene carbonate, o-xylene, triglyme, tetraglyme, 2- ethoxyethanol, phenol, naphthalene, nitrobenzene, dimethyl acetamide, chlorobenzene, ionic liquids and molten salts.

Redox couples

[0046] Redox couples suitable for use in the present invention have a temperature dependant voltage. Preferred redox couples for use in the present invention include the /I3 ^" couple in the form of Lil /l _2i Fe(bpy)3 ³⁺ /Fe(bpy)3 ²⁺ (bpy = 2,2-bipyridine), Fe(Cp) ₂ ⁺ /FeCp ₂ (Cp = cyclopentadienyl), FeCI ₄ 7FeCI ₄ ^2~ , Fe(CN) ₆ ³ 7Fe(CN) ₆ ^4" , FeCI ₄ 7FeCI ₄ ^2" , H H ₂ , Fe ²⁺ /Fe ³⁺ , Cu7Cu ²⁺ , Te ² 7Te ⁴⁺ , Hg7Hg ²⁺ , Sn ² 7Sn ⁴⁺ , Co ² 7 ³⁺ , [Co(bpy) ₃ ] ^{2+ 3+} . Furthermore, organic couples such as TEMPO/TEMPO ⁺ (TEMPO = 2,2,6,6-tetramethyl-1 -piperidinyloxide), TEMPO derivatives, sulphur-based redox couples, and mixed valence compounds such as Prussian blue, Turnbull's blue and soforth may be suitable for use in the present invention.

[0047] In another example of suitable redox couples, the redox couple may be present as the cation or anion of an ionic liquid, e.g. 1 -butyl-3-methylimidazolium FeCI ₃ /FeCI ₄ .

[0048] Transition metal redox couples used in the present invention may include those utilising mono-, bi-, tri-, tetra-, penta- or hexa-dentate ligands. For example, the Co redox couple may include 2,2-bipyridyl ligands and substituted analogues wherein the bipyridyl species is substituted with alkyl groups, aryl groups, electron withdrawing or donating functional groups and soforth. The ligand may be any previously reported for transition metal complexes, including acetylacetonate, alkenes, benzene, 1 ,2- bis(diphenylphosphino)ethane, 1 ,1 -bis(diphenylphosphino)methane (dppm), crown ethers, cryptates, cyclopentadienyl (Cp), diethylenetriamine (dien), dimethylglyoximate (dmgH-), ethylenediaminetetraacetate (EDTA), ethylenediaminetriacetate glycinate (Glycinato), nitrosyl, oxo, pyrazine, sulfite, 2,2',5',2-terpyridine (terpy), triazacyclononane (tacn), tricyclohexylphosphine, triethylenetetramine, trimethylphosphine, tri(o- tolyl)phosphine, tris(2-aminoethyl)amine (tren), tris(2-diphenylphosphineethyl)amine and terpyridine.

[0049] In a preferred embodiment the redox couple comprises [Co(bpy)3] ^2+/3+ [NTf2]2/3 dissolved in one or more low or negligible volatility liquids ([C2mim][NTf2], [C2imim][eFAP], methoxypropionitrile, benzonitrile, valeronitrile etc.) at different concentrations as electrolytes in thermoelectrochemical cells (NTf ₂ = bis(trifluoromethanesulfonyl)amide), eFAP = tris(pentafluoroethyl) trifluorophosphate. Cobalt complexes may be particularly useful for use in the present invention because they are known to have large internal reorganization energy that accompanies the transition from d ⁷ (high spin) to d ⁶ (low spin) upon oxidation (Feldt, S. M.; Gibson, E. A.; Gabrielsson, E.; Sun, L; Boschloo, G.; Hagfeldt, A., Journal of the American Chemical Society 2010, 132 (46), 16714-16724). This is an advantage for thermoelectrochemical cells and a potential explanation for their relative high S _e and good performance in thermoelectric devices. Adding different substitution to the polypyridine ligands surrounding the cobalt can affect performance, by influencing the electron transfer rates by affecting the diffusion of the reactants, the reorganization energy associated with the change in redox state and the solubility in different electrolyte media.

[0050] Polypyridine based ligands surrounding cobalt such as 2,2'-bipyridine, 1 ,10- phenanthroline and 2,2';6',2"-terpyridine have been shown previously as viable redox couples (Sapp, S. A.; Elliott, C. M.; Contado, C; Caramori, S.; Bignozzi, C. A., Journal of the American Chemical Society 2002, 124 (37), 11215-1 1222; and Feldt et al as listed above).

[0051] Varying the supporting anion of the cobalt salt may also influence the Seebeck coefficient of the redox couple. Examples of counter ions include [BF ₄ ], [dicyanamide], [SCN], [PF ₆ ], P0 ₄ ³ \ H ₂ PO ₄ \ HP0 ₄ ^2" , AsF ₆ , SbF ₆ , SbCI ₆ , S0 ₄ ² \ HS0 ₄ , alkyl-S0 ₃ , perfluoroalkylSOa, aryl-S0 ₃ , F ^" , CI ^" , l ₃ ^" , FeCI ₄ ^" , Fe(CN) ₆ ^3" , CI0 ₄ ^" , N0 ₃ ^" , CF ₃ S0 ₃ ^" , N(S0 ₂ CF ₃ ) _2, p-toluenesulfonate, methanesulfonate, cyanate, B(CN) , C(CN) ₃ , dicyano(nitroso)methanide, carbamoylcyano-nitrosomethanide. Hence by varying the supporting anion and/or ligands, the redox couple may be optimised to provide the best performance in a thermoelectrochemical cell.

Electrodes

[0052] The electrodes suitable for use in the present invention are inert, typically non- metals or unreactive metals and includes coated electrodes. In contradistinction to electrodes of the prior art, they are not consumed by the cell reaction.

[0053] Certain types of coated electrodes may comprise a biphasic electrolyte system. For example, polymer coated electrodes may comprise a biphasic electrolyte system. Tsou et al have shown that redox reactants can exhibit formal potentials that are shifted from their values in solution when incorporated into polyelectrolyte electrode coatings. This occurs when the two oxidation states of the reactant have different equilibrium constants for incorporation into the polyelectrolyte (Tsou, Y. M.; Anson, F. C. J. Electrochem. Soc. 1984, 131 (3), 595-601 . Thus, coating an electrode with a suitable polymer potentially increases the voltage difference across the thermoelectrochemical cell. Suitable polymer coatings will be recognised polymer or polyelectrolyte known to the person skilled in the art, such as Nafion, poly(phenylene oxide) and poly(methyl methacralate). Electrocatalyst

[0054] The thermo-electrochemical cell of the present invention may additionally include other features known to those skilled in the art such as an electrocatalyst. An electrocatalysts may be a separate entity incorporated onto the cathode/anode (e.g. a Pt coated carbon), or the cathode/anode may function as both the current collector and the electrocatalyst (eg conducting polymer 3,4-ethylenedioxythiophene). The electrocatalyst may be heterogeneous or homogeneous.

[0055] Suitable electrode and electrocatalytic material may include, for example metals and metal oxides, including Pt, gold, zinc, stainless steel, steel etc., conducting polymers, including but not limited to: polyacetylene; polyphenylene vinylene; polypyrrole, polythiophenes, polyaniline, polyphenylene sulfide, poly3,4-dioxythiophene (PEDOT), bisPEDOT, polycarbazoles, polyindoles, polyazepines, poly(fluorene)s, polyphenylenes, polypyrenes, polyazulenes, polynaphthalenes not limited by nature of the counter ions, which can include BF , PF ₆ , P0 ₄ , AsF ₆ , SbF ₆ , SbCl ₆ , S0 ₄ ^2" , HS0 ₄ , alkyl-S0 ₃ , perfluoroalkylSOs, aryl-S0 ₃ , F ^" , CI ^" , l ₃ ^" , FeCI ₄ \ Fe(CN) ₆ ^3" , CI0 ₄ \ I0 ₃ \ N0 ₃ ^" , CF ₃ S0 ₃ ^" , N(S0 ₂ CF ₃ ) ₂ and p-toluenesulfonate. Suitable electrode and electrocatalytic material may include carbon-based materials, including but not limited to, activated carbon, carbon fibres, templated carbons, graphite, graphene, graphene oxide, chemically-substituted graphene, single and multi-walled carbon nanotubes, carbon paper, carbon cloth, carbon composites (with Si etc), glassy carbon, platinised carbon. These materials also include the above mentioned materials with substituted heteroatoms in their structure e.g. substituted graphene. Ceramics including TiN, TiC, may also be used in conjunction with the above-mentioned electrode materials. These can be conducting and can also act as a template for the increased surface area of, for example, conducting polymer electrodes.

Device

[0056] The thermo-electrochemical cell of the present invention may be incorporated into a device for direct, continuous conversion of thermal energy to electrical energy. The device of the present invention generally give Seebeck coefficients of 1 mV/K or higher (depending on factors such as the nature of the redox couple and the electrolyte used) and thus has the potential to provide significantly higher voltage/power outputs than the semiconductor-based devices of the prior art.

[0057] In yet a further aspect of embodiments described herein there is provided a device for direct, continuous conversion of thermal energy to electrical energy, the device comprising a thermo-electrochemical cell having inert electrodes and a solid or multi-phase electrolyte comprising a redox couple, wherein the redox couple has a redox potential that generates a potential difference across the cell in response to application of heat.

[0058] In a further aspect of embodiments described herein there is provided a thermo-electrochemical cell for generating an electrical current comprising:

(a) a cathode and an anode being connectable for generation of said electrical current;

(b) a single or multiphase electrolyte, comprising a redox couple in contact with the cathode and anode wherein the redox couple has a redox potential that generates a potential difference across the cell in response to application of heat at one electrode.

[0059] The device may, but does not necessarily include, compartments. Hence, in a further aspect of embodiments described herein there is provided a thermo- electrochemical cell for generating an electrical current comprising:

(a) a cathode compartment and an anode compartment, where said compartments may have common-ion-permeable separation wall;

(b) a cathode and an anode located within their respective compartments, said cathode and anode being connectable externally of said compartments for generation of said electrical current;

(c) a single or multiphase electrolyte comprising a redox couple located in said cathode compartment and in said anode compartment and in contact with the cathode and anode respectively.

[0060] The device may optionally include an electrocatalysts, current collector or other features well known in the art.

[0061] In a further aspect of embodiments described herein there is provided a thermo-electrochemical system for generating an electrical current comprising:

(i) a source of thermal energy,

(ii) a thermo-electrochemical cell according to the present invention, and

(iii) a cell cooling means.

[0062] In a preferred embodiment, multiple thermo-electrochemical cells according to the present invention are combined to form the thermo-electrochemical system.

[0063] Other aspects and preferred forms are disclosed in the specification and/or defined in the appended claims, forming a part of the description of the invention.

[0064] In essence, embodiments of the present invention stem from the realization that thermo-electrochemical devices can be developed using a redox couple in an electrolyte, wherein the temperature dependence of the electrochemical redox potential generates the potential difference across the device. Furthermore, the realisation extends to how the use of solid or multiphase electrolyte systems can yield significant increases in power output of a thermo-electrochemical cell, as a result of increased entropy difference between the cathode and anode.

[0065] Advantages provided by the present invention comprise one or more of the following:

• novel use of a redox couple in thermo-electrochemical devices; • environmentally acceptable direct conversion of thermal energy to electrical energy;

• continuous generation of electricity while heat is supplied - the continuous generation of electrical energy being made available through continuous and free movement of the redox couple;

• no degradation of the components - neither electrode is oxidised or reduced;

• absence of metal based electrode (which is prone to oxidation) thus prolonging life of the cell;

• use of a solid electrolyte thus eliminating risk of leakage;

• can contribute to harvesting of heat that would otherwise be dissipated into the environment without producing useful energy;

• does not need any moving parts;

• low maintenance;

• no carbon emissions

• simple design and easy to assemble; and

• comparatively inexpensive to operate.

[0066] Further scope of applicability of embodiments of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the disclosure herein will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Further disclosure, objects, advantages and aspects of preferred and other embodiments of the present application may be better understood by those skilled in the relevant art by reference to the following description of embodiments taken in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the disclosure herein, and in which:

FIG. 1 illustrates the operating principle of a thermo-electrochemical cell, that is, the cycling of redox active species A and B between hot (4) and cold (2) electrodes separated by an electrolyte (6) within which A + ne- <→ B, generating a potential difference depending on the Seebeck coefficient. FIG. 1 shows the process that takes place in the case of a negative Seebeck coefficient;

FIG. 2 illustrates the configuration of the H-cell used in Example 1 , the dimensions of the cell, being height 1 = 30mm, width 3=17.5mm, height 5=45mm, diameter 7=10mm, diameter 9=12.5mm, and thickness = 1 mm;

FIG. 3 is a plot of measured potential difference (mV) versus the temperature difference (K) between the hot and cold sides of the H tubes used in Example 1. Plot 10 uses 0.001 M I7I ₃ ^" in succinonitrile (Pt wire) to give 1.13 mV/K. Plot 12 uses 0.01 M I7I ₃ ^~ in succinonitrile (Pt wire) to give 0.59 mV/K;

FIG. 4 illustrates the configuration of the Pt electrode used in Example 2. The thickness of the disc is 1 .0mm, diameter 14=18.0mm, length 16=24mm, diameter 18=0.5mm;

FIG. 5 illustrates the 'disc cell' configuration used in Example 2, comprising a disc heater 20 on the hot side 22 opposite the cold side 28, a thermal sensor 24, an electrolyte pellet 26, and Pt electrodes 30;

FIG. 6 illustrates the circuit used for the disc cell testing used in Example 2, incorporating the disc cell 32 shown in FIG. 5 and a variable resistor 34; FIG. 7 is a plot of measured potential difference versus the temperature difference in the disc cell of Example 2. Plot 36 is 0.01 M /Ι3 ^" in succinonitrile with the Pt disc. Plot 38 is a 2nd batch of the same.

FIG. 8 is a plot of the power density and current density created by the disc cell versus voltage at 10°C/65°C. Plots 40 and 44 are 0.01 M I7I ₃ ^" in succinonitrile with Pt electrode. Plots 42 and 46 are the 2nd batch of the same.

FIG. 9 is a plot of the thermo-electrochemical cell performance (Example 3) with the gel electrolyte consisting of 10wt% gelatin and 0.4M Fe(CN)6 ^{3~ 4~} (aq) redox couple.

FIG. 10 is a plot of the thermo-electrochemical cell results (Example 4) with the gel electrolyte consisting of 10wt% gelatin and 0.05M /Ι3 ^" redox couple.

FIG. 1 1 is a plot of the thermo-electrochemical cell results (Example 5) with 5wt% cellulose and 0.4M Fe(CN)6 ^3~/4~ redox couple gel electrolyte.

FIG. 12 is a plot of the thermo-electrochemical cell results (Example 6) with the solidified 0.4M Fe(CN) ₆ ^{3~ 4~} (aq) and alumina nanoparticles.

DETAILED DESCRIPTION

[0068] The present invention will be further described with reference to the following non-limiting examples. The examples illustrate how the use of biphasic electrolyte systems can yield significant increases in power output of the cell, as a result of increased entropy difference between the cathode and anode.

[0069] For the most efficient thermo-electrochemical cell, it is desirable to have as high as Seebeck coefficient as possible. For the redox systems of the present invention, the Seebeck coefficient can first be measured using a simple H-tube cell design as described in Example 1 with different ionic liquid-electrolytes (Abraham, T.J. MacFarlane, D. R. Pringle, J. M. Chemical Communications (Cambridge, United Kingdom) 2011 , 47, 6260). This permitted assessment of this parameter in different electrolytes prior to the assembly of a full thermo-electrochemical cell as described in Example 2.

Example 1

[0070] The Seebeck coefficient was measured for a redox couple according to the present invention (in this case I7I ₃ ^" ) in a multi-phasic electrolyte (in this case involving the liquid and plastic crystal phases of succinonitrile).

Procedure:

[0071] The Seebeck coefficient was measured using a method having the following steps:

Step 1 : Succinonitrile was melted (ca. 80°C) and a chosen concentration of redox couple (e.g. Lil/I _2, to give I7I ₃ ^" ) was dissolved to prepare the electrolyte.

Step 2: An H-tube (FIG. 2) was filled with the molten succinonitrile and air bubbles removed from the melt.

Step 3: Clean Pt wires were inserted through the tube caps until they protruded halfway into the tubes, one on each side.

Step 4: The succinonitrile was allowed to fully solidify (about 5 minutes at room temperature).

Step 5: One tube was wrapped with a thin film heater and a thermal sensor positioned between the film heater and the glassware - this side of H tube being designated as the "hot" side.

Step 6: The heater was connected to a power supply. The heater, thermal sensor and power supply were all connected to a temperature control module which allowed a PID control of the temperature of the hot side. The cold side was kept at room temperature (about 20°C).

Step 7: The Pt wires (electrodes) were connected to a multimeter in order to read the potential difference created by the temperature difference between two sides.

[0072] These steps were repeated so that three concentrations of I7I ₃ ^" in succinonitrile were used, specifically 0.1 M, 0.01 M and 0.001 M.

[0073] The results are shown in FIG. 3 in the form of a plot of the measured potential difference as a function of the temperature difference between the hot and cold sides of the H tube. The resistance between the two electrodes is so large that it can be viewed as an open circuit test.

[0074] Initially, the potential differences are roughly proportional to the temperature differences and the slopes of the plots indicate the Seebeck Coefficient of the redox couple in this solid state electrolyte, that is, 1 .13 mV/K for 0.001 M and 0.59 mV/K for 0.01 M /Ι3 ^" . However, when the electrolyte in the hot side of the H-tube starts to melt, both the systems show a significant increase in the potential difference, which then plateaus at higher temperatures (due to variations in room temperature, which govern the temperature of the 'cold side' of the H-tube, the melting occurs at different values of temperature differences for two Examples). This phenomenon is attributed to the extra entropy difference created by the different phases and hence reference is made herein to the use of 'biphasic electrolyte materials'.

Example 2

[0075] A full thermo-electrochemical cell was assembled and used for measurement of power output. The configuration of the Pt electrode (FIG. 4), the 'disc cell' (FIG. 5) and circuit (FIG. 6) for disc cell testing are as illustrated. The Resistance in the circuit was varied from 100 to 200,000 Ω. Procedure:

Step 1 : Succinonitrile was melted (ca. 80°C) and chosen concentrations of redox couple were dissolved (for example, Lil/I ₂ to give I7I ₃ ^" couple).

Step 2: The succinonitrile was fully solidified, then 700mg succinonitrile/ /Ι3 ^" electrolyte was weighed out (for about. 2.5mm height of electrolyte pellet)

Step 3: A pestle and mortar were used to grind solid electrolyte into uniform fine powder which was then loaded into a KBr die (13mm in diameter).

Step 4: A uniaxial presser was used to press the die and kept at 2 tonnes for

5 minutes.

Step 5: The pressed succinonitrile pellet was retrieved and inserted into the spacer then thermal sensors and Pt electrodes were assembled for both sides (as shown in FIG. 5).

Step 6: The circuit was completed as shown in FIG. 6.

Step 7: The resistances was varied to obtain corresponding current/voltage data.

[0076] FIG. 7 shows the Seebeck coefficient of 0.01 M /Ι3 ^" doped succinonitrile in disc cell setup. Once the hot side becomes molten, the potential difference increases dramatically, as observed in H tube measurements (FIG. 4). The Example is reproducible.

[0077] FIG. 8 illustrates the power created by the disc cell plotted as function of voltage. The power is calculated by P=UI, where U is the voltage and I is the current. The cold and hot sides are controlled at 10°C and 65°C, respectively. The Example is reproducible (the same two batches of electrolyte are used here as used in Example 1 ). [0078] A range of potential applications exist for devices according to the present invention, depending on the characteristics of the electrolyte used such as the melting point and the design of the device (such as the use of flexible substrates). A plethora of natural and man made sources of waste heat exist and the design of thermo- electrochemical devices can be optimised each application. In particular, devices according to the present invention will be best suited for targeting low grade heat, in the temperature range of about. 100 to about 200°C. This is above the temperature range suitable for utilisation by water-based thermo-electrochemical devices of the prior art. At these lower temperatures, bulk semi-conductor thermo-electrics are not efficient enough to be commercially viable as they require a large temperature difference between the hot and cold sides to achieve the efficiencies required to compensate for their high materials and manufacturing costs. Thermo-electrochemical cells of the present invention may be used to the best advantage in relatively large scale applications, taking advantage of their lower materials costs and allowing many cells to be connected in series to capture the maximum amount of waste heat.

[0079] Two potential applications of devices of the present invention are as follows:

(i) Utilisation of waste heat in power stations, for example by wrapping the device around hot water pipes. Power stations also commonly have a source of cooling water, which may be utilised to maintain the temperature of the cold side of the thermo-electrochemical cell. This setup could consist of many modules, all connected in series, completely enclosing long lengths of pipe.

(ii) Utilisation of geothermal heat. Around the world there are many sources of geothermal heat sources, such as hot springs, and many of these sources are in remote locations. Insertion of one side of a device according to the present invention into the hot ground or water, and exposure of the cold side to air for cooling will enable the production of electricity either for use in-situ or for transportation to other points of demand. These applications would not require the deep drilling needed for many other geothermal power production technologies. Furthermore, it allows the direct production of electricity rather than the use of fluids to turn turbines and the subsequent production of electricity using generators. Example 3 - Gelatin hydrogel with Fe(CN) ₆ ^{3 4} - redox couple

(a) Preparation of the gelatin hydrogel:

[0080] Gelatin powder (from Sigma Aldrich, sourced from porcine skin) was dissolved in distilled water to make a 10wt% solution, which was stirred at 90 degree Celsius for 1 hour and cooled to room temperature (about 20°C) in air. Potassium ferricyanide and potassium ferrocyanide were added to the solution, which was kept stirring at room temperature until the added potassium salts were fully dissolved, to form a solution with a redox couple concentration of 0.4M. The solution was then kept still for about 20 min to allow solidification.

(b) Insertion of the gel electrolyte into the thermo-electrochemical cell:

[0081] The gel was heated to 70°C to achieve water-like fluidity. The structural parts of the thermo-electrochemical cell and a glass pipette were heated to 70°C to keep sufficient fluidity of the electrolyte solution during transferal. The thermo-electrochemical cell setup was then kept still for an hour at room temperature to make sure the solidification was complete before any tests.

[0082] Thermo-electrochemical cell results, with the cold electrode at 25°C and the hot electrode at 30°C are depicted graphically at FIG. 9, the gel electrolyte consisting of 10wt% gelatin and 0.4M Fe(CN)6 ^3~/4~ (aq) redox couple. The squares indicate power density and the circles depict current density.

Example 4 - Gelatin hydrogel with iodide/triiodide redox couple

(a) Preparation of gelatin hydrogel:

[0083] Gelatin powder (from Sigma Aldrich, sourced from porcine skin) was dissolved in distilled water to make a 20wt% solution, which was stirred at 90°C for 1 hour and cooled to room temperature (about 20°C) in air. A pre-prepared 0.1 M I3VI ^" aqueous solution (2 equivalent of lithium iodide reacts with one equivalent iodine to form a 1 :1 ratio of iodide/triiodide redox couple in water) was added into the same volume of gelatin solution to form a 0.05M ₃ / solution containing 10wt% gelatin. The solution was then kept still at room temperature for about 20 minutes to achieve solidification.

(b) Insertion of gel electrolyte into the thermo-electrochemical cell:

[0084] The gel was heated to 70°C to achieve water-like fluidity. The structural parts of the thermo-electrochemical cell and a glass pipette were heated to 70°C to keep sufficient fluidity of the gel solution during the transferring processes. The thermo- electrochemical cell setup was kept still at room temperature for an hour to make sure the gelling was complete before any tests.

[0085] Thermo-electrochemical cell results, with the cold electrode at 25°C and the hot electrode at 30°C are depicted graphically at FIG. 10, the gel electrolyte consisting of 10wt% gelatin and 0.05M I7I ₃ ^" redox couple. Due to insufficiently high resistance in the external circuit, the maximum power output was not reached. The squares indicate power density and the circles depict current density.

Example 5 - Cellulose hydrogel with Fe(CN) ₆ ^{3 /4"} redox couple

(a) Preparation of cellulose hydrogel:

[0086] 5wt% cellulose powder was dissolved in 1 -ethyl-3-methylimidazolium diethyl phosphate (an ionic liquid) at 100°C and stirred for 2 hours. The viscous liquid was poured into a mold to set for 2 hours at room temperature (20°C). The gel was then soaked in methanol for 48 hours to remove the ionic liquid, followed by soaking in distilled water for 24 hours to replace the methanol with water. Finally, the hydrogel was soaked in 0.4M Fe(CN)6 ^3~/4~ aqueous solution for 96 hours.

(b) Preparation of cellulose gel electrolyte:

[0087] The cellulose gel containing 0.4M Fe(CN)6 ^{3~ 4~} aqueous solution could be easily cut into the shape appropriate for the thermo-electrochemical cell. [0088] Thermo-electrochemical cell results, with the cold electrode at 25°C and the hot electrode at 60°C (Example 5) are depicted graphically at FIG. 1 1 with 5wt% cellulose and 0.4M Fe(CN) ₆ ^{3~ 4~} redox couple gel electrolyte. The squares indicate power density and the circles depict current density.

Example 6 - Gel of alumina nanoparticles and Fe(CN)6 ^{3 4"} redox couple.

[0089] Previous examples illustrate the use of hydrogels in which the solidification is a result of the polymeric structure of gelatine and cellulose. By contrast, the present example illustrates a 'physical gel' in which the solidification is by virtue of the addition of nanoparticles. There is no polymer in this system.

(a) Preparation of nanoparticle gel:

[0090] Alumina powder (50 nm average diameter) and 0.4M Fe(CN) ₆ ^3~/4~ aqueous solution were mixed at a mass ratio of 1 : 2.6 to form a thick paste.

(b) Insertion of the nanoparticle gel electrolyte into the thermocell:

[0091] The paste-like gel was easily deformed and was therefore easily pressed into the thermo-electrochemical cell spacer.

[0092] Thermo-electrochemical cell results, with the cold electrode at 25°C and the hot electrode at 50°C (Example 6) are depicted graphically at FIG. 12, with the solidified 0.4M Fe(CN)6 ^{3" 4"} (aq) and alumina nanoparticles. The squares indicate power density and the circles depict current density.

[0093] While this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification(s). This application is intended to cover any variations uses or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth. [0094] As the present invention may be embodied in several forms without departing from the spirit of the essential characteristics of the invention, it should be understood that the above described embodiments are not to limit the present invention unless otherwise specified, but rather should be construed broadly within the spirit and scope of the invention as defined in the appended claims. The described embodiments are to be considered in all respects as illustrative only and not restrictive.

[0095] Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention and appended claims. Therefore, the specific embodiments are to be understood to be illustrative of the many ways in which the principles of the present invention may be practiced. In the following claims, means-plus-function clauses are intended to cover structures as performing the defined function and not only structural equivalents, but also equivalent structures.

[0096] "Comprises/comprising" and "includes/including" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. Thus, unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', 'includes', 'including' and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".