Thermoelectric device

Abstract

The present invention provides thermoelectric device comprising a first electrode, a second electrode, a first electrolyte composition capable of transporting cations, a second electrolyte composition capable of transporting anions and a connector comprising mobile cations and mobile anions, wherein the first electrolyte composition is connected to said first electrode by being in ionic contact and the second electrolyte composition is connected to said second electrode by being in ionic contact and said connector is in ionic contact with said first and said second electrolyte composition, such that an applied temperature difference over said electrolyte compositions or an applied voltage over said electrodes facilitate transport of ions to and/or from said electrodes via said electrolyte compositions. There is also provided a method for generating electric current and a method for generating a temperature difference.

Claims

1. A thermoelectric device comprising: a first electrode; a second electrode; a first electrolyte composition capable of transporting cations; a second electrolyte composition capable of transporting anions; and a connector comprising a cation and anion transporting composition, the connector comprising mobile cations and mobile anions, wherein the first electrolyte composition is connected to said first electrode by being in ionic contact and the second electrolyte composition is connected to said second electrode by being in ionic contact and said connector is in ionic contact with said first and said second electrolyte composition, such that an applied temperature difference over said first and second electrolyte compositions or an applied voltage over said first and second electrodes facilitate transport of ions to and/or from said first and second electrodes via said first and second electrolyte compositions in accordance with a Soret effect, the ions themselves not being electrochemically active.

2. The thermoelectric device according claim 1, wherein said first electrolyte composition comprises an anionic polymer and/or said second electrolyte composition comprises a cationic polymer.

3. The thermoelectric device according to claim 1, wherein the first and second electrodes comprise a material having a specific capacitance in a range of 10 F/g to 1000 F/g, a material having a specific surface area in a range of 50 m.sup.2/g to 5000 m.sup.2/g, or an electrically conductive polymer composition capable of being reduced and/or oxidized.

4. The thermoelectric device according to claim 1, further comprising at least one ion reservoir at a junction between said first electrolyte composition and said first electrode and/or at a junction between said second electrolyte composition and said second electrode.

5. The thermoelectric device according to claim 1, wherein said first electrode, said second electrode, said first electrolyte composition, said second electrolyte composition and/or said connector can be applied by liquid deposition techniques.

6. The thermoelectric device according to claim 1, arranged on a flexible solid substrate.

7. The thermoelectric device according to claim 1 for generating electric current comprising a first leg connected to said first electrode and a second leg connected to said second electrode, wherein said first and second legs are coupled via the connector, wherein said first leg is connected to said first electrode by being in ionic contact, said second leg is connected to said second electrode by being in ionic contact, and said connector is in ionic contact with said first and said second legs; wherein said connector comprises a composition comprising mobile cations and mobile anions said device further comprises a first ion reservoir being in ionic contact with said first leg, and said first electrode and a second ion reservoir being in ionic contact with said second leg and said second electrode, wherein said first and second ion reservoirs and said connector are spatially isolated from each other; wherein said first leg comprises the first electrolyte composition being capable of transporting the cations from said connector to said first ion reservoir, said second leg comprises the second electrolyte composition being capable of transporting the anions from said connector to said second ion reservoir; and wherein said first electrode comprises a layer of a first electrically conductive polymer composition capable of being reduced which is in ionic contact with said first ion reservoir, and said second electrode comprises a layer of a second electrically conductive polymer composition capable of being oxidized which is in ionic contact with said second ion reservoir.

8. The thermoelectric device according to claim 1 for generating a temperature difference comprising a first leg connected to said first electrode and a second leg connected to said second electrode, wherein said first and second legs are coupled via the connector, wherein said first leg is connected to said first electrode by being in ionic contact, and said second leg is connected to said second electrode by being in ionic contact; said device further comprises a first ion reservoir comprising mobile cations, being in ionic contact with said first leg, and said first electrode and a second ion reservoir comprising mobile anions, being in ionic contact with said second leg and said second electrode, wherein said first and second ion reservoirs and said connector are spatially isolated from each other; wherein said first leg comprises the first electrolyte composition being capable of transporting the cations from said first ion reservoir to said connector said second leg comprises the second electrolyte composition being capable of transporting the anions from said second ion reservoir to said connector; wherein said connector comprises the cation and anion transporting composition in ionic contact with said first and said second legs; and wherein said first electrode comprises a layer of a first electrically conductive polymer composition capable of being oxidized which is in direct contact with said first ion reservoir, and said second electrode comprises a layer of a second electrically conductive polymer composition capable of being reduced which is in direct contact with said second ion reservoir.

9. A method for generating electric current comprising: providing the thermoelectric device according to claim 1, and applying a temperature difference over said first and second electrolyte compositions.

10. The method according to claim 9, comprising providing a first temperature in said connector and a second temperature in first and second ion reservoirs, wherein said first temperature is lower than said second temperature.

11. A method for generating a temperature difference comprising: providing the thermoelectric device according to claim 1, and applying a potential difference between said first and second electrodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein:

(2) FIG. 1 shows a schematic drawing of a thermoelectric generator according to the invention.

(3) FIG. 2 shows a schematic drawing of a thermoelectric cooler according to the invention.

(4) FIG. 3 shows a schematic drawing of a device used for measuring the example devices.

(5) FIG. 4a shows the ion flow direction in vertical device.

(6) FIG. 4b shows the ion flow direction in lateral device.

(7) FIG. 5 is a plot of open potential versus time.

(8) FIGS. 6-7 are plots of ionic conductivity and ionic Seebeck coefficient, respectively.

(9) FIG. 8-11 show different measurement results made on example devices.

(10) FIG. 12 shows a schematic drawing of an embodiment of a multilegged device according to the invention.

(11) FIGS. 13a and 13b show the chemical structure of PSSNa and PMDQUAT, respectively.

(12) FIG. 14 shows a schematic drawing of some of the components of a thermoelectric generator according to the invention.

(13) FIGS. 15a and 15b show a further schematic drawing of some of the components of a thermoelectric generator according to the invention (FIG. 15a) and an example on a multilegged device (FIG. 15b).

(14) FIG. 16 shows a single-leg PSSNa ionic thermoelectric generator.

(15) FIG. 17 shows electrically coupled single-leg PSSNa ionic thermoelectric generators.

(16) FIG. 18 shows the evolution of the open-circuit potential in the single leg PSSNa ionic thermoelectric generator.

(17) FIG. 19 shows the evolution of the output voltage versus time for the single leg PSSNa ionic thermoelectric generator submitted to a temperature gradient T=1.2 and connected to various load resistance (750 Ohms, 2 MOhms, 7.5 MOhms).

(18) FIG. 20 shows the thermogenerated electrical power for the single leg PSSNa device with different load resistance (T=1.2). The solid black symbol corresponds to the same device connected subsequently to various load resistance. The star symbols corresponds to different devices with different load (T=1.2) generated from the output voltage curves in FIG. 19.

(19) FIG. 21a shows the evolution of the ionic conductivity (), Seebeck coefficient () and corresponding power factor (.sup.2) for PSS:Na versus RH. The inset shows the chemical structure of PSS:Na.

(20) FIG. 21b shows the evolution of the thermal conductivity (), the power factor (.sup.2) and ZT versus RH. All measurements are done at room temperature.

(21) FIG. 22a shows the charge-discharge curves recorded after heating-cooling cycles (40 minutes).

(22) FIG. 22b shows the energy density versus temperature gradient.

(23) FIG. 23 shows impedance spectroscopy data. Phase angle and Capacitance for device, PEDOT:PSS/PSS:Na/PEDOT:PSS, with applied AC at 100 mV at saturated water atmosphere.

DETAILED DESCRIPTION

(24) Schematic Description of a Thermoelectric Generator with a Single Leg

(25) A schematic drawing of an thermoelectric device having a single leg is shown in FIG. 16. The single leg comprising a polyelectrolyte is arranged between two electrodes (PEDOT) comprising a conducting polymer. An applied temperature difference between the electrodes drives ions in the single leg towards one of the electrodes (in this case the upper electrode in FIG. 16). In order to transform the ionic current generated in the polyelectrolyte leg into an electronic current in the external circuit, an electrochemical reaction is required at the electrodes. For this purpose, a conducting polymer, more especially PEDOT-PSS, is used as electrodes since such material transports both electronic and ionic charge carriers. Importantly, a conducting polymer, typically oxidized, possesses a vanishingly small band gap and thus undergoes an electrochemical reaction a very small applied voltage. For this reason, conducting polymers are used as electrode in supercapacitors. Here, we expect that the electric potential difference between two electrodes due to the thermo-diffusion of ions in the polyelectrolyte induces an electrochemical reaction at the conducting polymer electrodes. For the polyanion PSSNa, the cations thermo-diffuses to the cold PEDOT-PSS electrode and increase its electrostatic potential, which triggers a reduction of the conducting polymer with an electron coming from the external circuit:
PEDOT.sup.+PSS.sup.+e.sup.+Na.sup.+.fwdarw.PEDOT.sup.0+PSS.sup.Na.sup.+

(26) At the hot electrode, the oxidation of PEDOT-PSS is the opposite direction of the chemical equation. Both reduction and oxidation are possible at the PEDOT-PSS electrodes because the polymer is not fully oxidized in its pristine conducting form. In order to ensure a good ionic conductivity in the polyelectrolyte legs, as well as no limitation due to the amount of mobile cations, two reservoirs of equal NaCl salt concentration are added at the junctions between the PEDOT-PSS electrodes and the polyanion. The reservoirs are designed with PDMS and filled in with an electrolytic gel made of 10% of polyethyleneoxide (Mw=100,000) and an aqueous solution of NaCl (1 M).

(27) For some applications that do not require lot of current but rather a high voltage, it could be advantageous to couple single leg generators electrically, e.g. via a metal or a conducting polymer, see FIG. 17. The single-leg ionic TEG may thus be connected thermally in parallel and electrically in series by bridging the top-electrode of a single-leg ionic TEG with the bottom electrode of the adjacent single-leg ionic TEG via electronic conductor (no need of ionic conduction) preferably of low thermal conductivity and high electrical conductivity (e.g organic conductor such as PEDOT-Tos, polyaniline-CSA, charge transfer salt).

(28) Such application could be for instance to switching a transistor by applying a voltage via the thermoelectric voltage. The transistor is switched OFF or ON depending on the temperature gradient.

(29) Schematic Description of a Thermoelectric Generator with Two Legs

(30) FIG. 1 shows a thermoelectric generator (TEG) (100) according to the invention which comprises a first leg (101) connected to a first electrode (102) and a second leg (103) connected to a second electrode (104), wherein said first and second legs are coupled via a connector (105). In contrast to prior art, the TEG in FIG. 1 is based on the principle of ion transport. It is to be understood that this drawing is non-limiting. For example, the ion reservoirs 106 and 107 may be positioned anywhere as long as they are in ionic contact with the first 101 and second leg 103, respectively.

(31) When applying a temperature gradient, the cations will move from the connector (105), through the first leg (101) to the first ion reservoir (106) and the anions will to move from the connector (105) through the second leg (103) to the second ion reservoir (107)

(32) The first and second legs (101,103) comprise electrolyte compositions. The first leg (101) comprises a first electrolyte composition being capable of transporting cations from said connector (105) to said first ion reservoir (106). Said second leg (103) comprises a second electrolyte composition being capable of transporting anions from said connector (105) to said second ion reservoir (107). The electrolyte may be based on a polyelectrolyte, an ionic liquid, a macromolecule functionalized with an ionic group or a nanoparticle functionalized with an ionic group.

(33) The first and second electrodes (102,104) comprise a composition capable of transforming the ion transport into an electron transport. This is done by using a first electrode (102) comprising a layer of a first electrochemically active material, more especially an electrically conductive polymer composition capable of being reduced, which is in ionic contact with said first ion reservoir (106). Further, said second electrode (104) comprises a layer of a second electrically conductive polymer composition capable of being oxidized which is in ionic contact with said second ion reservoir. It is critical that electrode compositions are electrically conductive and electrochemically active.

(34) According to one example, the electrodes may be composed of a metal electrode such as an Au electrode coated with the conductive polymer composition capable of being oxidized or the conductive polymer composition capable of being reduced. Preferably, the first and second electrodes comprise redox polymer compositions, such as PEDOT, PSS, polypyrol and/or polyaniline.

(35) The connector (105) comprises a composition of mobile cations and mobile anions. As used herein, mobile cations and anions means that when applying a temperature gradient, the cations will move from the connector (105), through the first leg (101) to the first ion reservoir (106) and the anions will to move from the connector (105) through the second leg (103) to the second ion reservoir (107). The mobile cations and the mobile anions may possibly also move in the opposite direction, from the ion reservoirs to the connector, depending on the driving force. It is important that the material in the connector has a high conductivity for both anions and cations.

(36) The ion reservoirs (106,107) are used in the TEG in FIG. 1 in order to form a place where the transported cations and anions can be collected. The first ion reservoir (106) is in ionic contact with the first leg (101), and the first electrode and a second ion reservoir (107) is in ionic contact with the second leg (103) and the second electrode. The ion reservoirs can be arranged as an external storage place for the transported ions. The ion reservoirs can also be integrated in each leg at a distance from the connector or be integrated in each of the two electrodes.

(37) In order to achieve the desired ion transport, it is a requirement that that the first ion reservoir, the first electrode, the first leg, and the connector are arranged to allow direct or indirect ion transport from the connector to the first ion reservoir via the first leg. Likewise, the second ion reservoir, the second electrode, the second leg, and the connector are also arranged to allow direct or indirect ion transport from the connector to the second ion reservoir via the second leg. Hence, said first and second legs are both in ionic contact with the connector.

(38) However, it should be noted that ion transport between the first and second ion reservoirs, between the first and second electrode, and between the first and second legs should be avoided since this would affect the thermoelectric effect of the TEG. For the same reason, the first and second ion reservoirs (106,107) and said connector (105) are spatially isolated from each other. This allows for ion transport between the cold side (electrodes and ion reservoirs) and the hot side (the connector) of the thermoelectric generator.

(39) In FIG. 1, the TEG produces an electric current when the device is exposed to a temperature gradient, where the temperature is higher in the connector than in the electrodes and ion reservoirs. The heat flow is inducing a transport of ions where the ions inside the connector (hot side) tend to move towards the distal part of the legs (cold side). This is called the thermodiffusion or Soret effect. The temperature gradient is hence the driving-force for the ion transport. Therefore, ions will be a transported from the connector to the ion reservoirs.

(40) When an electrolyte composition in the first leg allowing more cations than anions to be transported is used in combination with an electrolyte composition in the second leg allowing more anions than cations to be transported, this does in itself not result in a continuous ion transport as it would not be possible to maintain charge-balance in the ion reservoir. The inventors have found a way to collect the transported ions in the ion reservoir and achieving charge-balance and at the same time convert the ionic transport to an external electric current.

(41) In FIG. 1, a first electrode capable of undergoing a reduction reaction and a second electrode capable of undergoing an oxidation reaction are used. In the reduction reaction anions can be produced and in the oxidation reaction cations and electrons are produced.

(42) As an example, PEDOT may be reduced at one of the electrodes and an immobile polyanion PSS may be produced, wherefrom a cation may arrive to balance. At the other electrode, PEDOT may be oxidized, due to that anions may arrive close to the PEDOT-PSS electrode but may not go into it since PSS is an immobile polyanion. Hence, instead, the anions may stay in the ion reservoir close to the electrode and the mobile cations, normally balancing the polyanion PSS in the electrode, may also move into the reservoir. The lack of cations in the electrode or the excess of PSS immobile polyanions may therefore lead to the oxidation.

(43) The reduction and oxidation reactions are initiated due to the thermodiffusion, because in order for the transport to occur for prolonged times simultaneous production of counter-ions in the electrodes is needed in order to balance the transported cations entering the first ion reservoir and to balance anions entering the second reservoir. Hence, as the mobile cations arrive in the first ion reservoir, charge-balance is obtained by anions formed in the reduction reaction. Similarly, as the mobile anions arrive in the second ion reservoir, charge-balance is obtained by a cation formed in or released due to the oxidation reaction. Thereby, a continuous flow of mobile cations and anions from the connector to each of the reservoirs, respectively, is created. Due to the oxidation and reduction reactions, an external electrical current may be produced in a connected electronic circuit as the oxidation results in free electrons and as electrons are needed for the reduction to take place.

(44) The ion transport is a result of thermodiffusion initiated by the temperature gradient present as described above. However, the degree of ion transport is further controlled by for example the ion size of the counter ions in the electrolyte compositions, the ion size and charge of the ion reservoirs, and the structure of any polymers present in the electrolyte compositions, such as the use of substituents, degree of cross-linking, amount of ionic groups etc. This is because the maximum transport rate is limited to the rate of which ions can be transported from the reservoirs to the connector through the electrolyte composition and the energy barriers for reduction and oxidation in the electrodes. Below, different ways of controlling the transport rate is discussed.

(45) In detail, a requirement of achieving ion transport from the ion reservoirs is that the cations and anions of the ion reservoirs have sufficiently small ion radius to be able to move through the electrolyte compositions. Hence, the ion reservoirs should comprise mobile cations and anions, respectively. Since a polymer composition in general is a very porous structure, inorganic ions, preferably monovalent inorganic ions such as H.sup.+, Li.sup.+, Na.sup.+, Cl.sup., Br.sup., I.sup., ClO.sub.4.sup., may move through the electrolyte compositions. Preferably, ions that are not electrochemically active themselves should be used. Some organic ions are also considered to be sufficiently mobile. However, organic ions having more than 15 carbon atoms, preferably not more than 10 carbon atoms, more preferably not more than 5 carbon atoms, would not be realistic to use, since they become too big and too slow to transport.

(46) Preferably, the ion concentration in the connector may be about the same as the ion concentration in the ion reservoirs.

(47) It is also advantageous that the ions are easily leaving the connector. This is achieved by using salts which easily dissociate or which are in a more or less dissociated form. This can be facilitated by using a hydrated or wetted salt, a hygroscopic salt, which form solvated shells around the ions. Another way of obtaining freely movable ions is to use a solution of salt as ion reservoir. Another alternative is to use polar high boiling point solvents, such as propylene carbonate, diethlyene glycol, DEG, PEG or other non volatile materials such as an ionic liquid, succinonitrile, as such or gellified with a polymer.

(48) Further, the ionic contact between the reservoirs and the legs is of importance. In order to achieve high degree of contact, the ion reservoir may be integrated with the electrolyte composition of the leg.

(49) Further, it is advantageous to provide the cationic and anionic polymer with counter ions, which are small and have easily leaving groups to facilitate the ion transport.

(50) Further, the reactivity of the reduction and oxidation agents in the electrodes will influence the transport rate. The electrode may comprise of electrochemically material that conduct both ions and electronic charge carriers. The electrode has advantageously a small band gap in order to undergo an electrochemical reaction for very small voltage, such as the thermo-voltage produced by Soret effect of ions in a polyelectrolyte. Hence, the electrode should undergo a thermo-induced electrochemical reaction

(51) Furthermore, the ion transport may be significantly controlled by adding different additives, for instance polar high boiling point solvents, such as propylene carbonate, diethlyene glycol, DEG, PEG or other non volatile materials such as an ionic liquid, succinonitrile, as such or gellified with a polymer, or zwitter ions, to the electrolyte composition. The inventors have found that the ion transport was significantly enhanced by adding water to the electrolyte composition. Preferably, the water is added to the ion reservoirs and/or the connector, and thereafter it may penetrate into the legs by osmosis. The electrolyte compositions in the legs may comprise up to more than 50% of water regarded to its volume. The addition of water to the electrolyte composition results in that the ions are solvated. As a result, the electrostatic attraction between the mobile ions and the polyions weakens and the activation energy for the ion transport decreases. Increasing further the humidity leads to the creation of water percolation paths, the film is wet, and the ionic conductivity tends to saturate towards the ionic conductivity of the liquid phase. In preferred embodiments of the invention, the first and/or second electrolyte composition is hydrated or wetted. Advantageously, a hygroscopic material can be included in the electrolyte compositions in order to keep the leg hydrated/wetted for prolonged time. Alternatively, a solution of salt can be used as ion reservoirs and connector.

(52) The present invention further relates to a thermoelectric device for producing a temperature gradient or temperature difference.

(53) FIG. 2 shows a thermoelectric device for producing a temperature gradient or temperature difference (200) according to the invention which comprises a first leg (201) connected to a first electrode (202) and a second leg (203) connected to a second electrode (204), wherein said first and second legs are coupled via a connector (205). In contrast to prior art, the device in FIG. 2 is based on the same principle as the thermoelectric device in FIG. 1, namely the utilization of ion transport as charge carriers. Therefore, the device is built on the same general concept as the TEG in FIG. 1 acceding to the invention with some modifications and differences explained below.

(54) In analogy with the device described in FIG. 1, said first leg (201) is connected to said first electrode (202) by being in ionic contact, and said second leg (203) is connected to said second electrode (204) by being in ionic contact. Further, the thermoelectric cooler also comprises a first and second ion reservoir where said first ion reservoir (206) is in ionic contact with said first leg (201) and said first electrode and said second ion reservoir (207) is in ionic contact with said second leg (203) and said second electrode. Furthermore, said first and second ion reservoirs (206,207) and said connector (205) are spatially isolated from each other.

(55) However, in the thermoelectric device in FIG. 2, the first ion reservoir (206) comprises mobile cations, and said second ion reservoir (207) comprises mobile anions which are transported from the ion reservoirs to the connector via the first and second leg. Therefore, said first leg (201) comprises a first electrolyte composition being capable of transporting cations from said first ion reservoir (206) to said connector (205), said second leg (203) comprises a second electrolyte composition being capable of transporting anions from said second ion reservoir (207) to said connector (205). The mobile ions are then collected in the connector of the device so that charge-balance is obtained. The connector therefore comprises a cation and anion transporting composition in ionic contact with said first and said second legs.

(56) Another difference is that said first electrode (202) comprises a layer of a first electrically conductive polymer composition capable of being oxidized which is in direct contact with said first ion reservoir (206), and said second electrode (204) comprises a layer of a second electrically conductive polymer composition capable of being reduced which is in direct contact with said second ion reservoir.

(57) In FIG. 2, a voltage is applied over the electrodes, which initiates an oxidation reaction in the first electrode as the electrode becomes depleted in electrons. Thereby, cations are produced in the first leg. Similarly, a reduction reaction takes place in the second leg due to excess amount of electrons in the second electrode. Thereby anions are produced in said second electrode. The oxidized electrode therefore produces an excess amount of cations and the reduced electrode produces an excess amount of negative ions. This results in the movement of excess ions through each leg of the device to the connector. The driving-force of the ion transport is to obtain charge-balance of the system. This is achieved since by the transport the excess cation and excess anion are collected which results in charge neutralization.

(58) If the potentials at the electrodes are reversed compared to a reference state, the ion motion becomes the opposite in both legs. As the ions may transport heat, an ion motion in the opposite direction implies an inverse heat flow compared to the heat flow in the reference state.

(59) Schematic Description of a Thermoelectric Generator with Multiple Legs

(60) FIG. 12 shows a multilegged thermoelectric assembly, i.e. a thermoelectric assembly comprising more than one pair of legs. The multilegged thermoelectric assembly is arranged as described in relation to the device above, except that there are two more connectors and two more legs. The first leg is arranged of a cation transporting material, and ionically connects said first ion reservoir and a first one of said connectors; the second leg is arranged of an anion transporting material, and ionically connects said first one of said connectors and a second one of said connectors; the third leg is arranged of a cation transporting material, and ionically connects said second one of said connectors and a third one of said connectors; the fourth leg is arranged of an anion transporting material, and ionically connects said third one of said connectors and said second ion reservoir.

(61) When a temperature difference or a potential difference is applied, an electric current or a temperature difference, respectively is produced an analogy with the description above. Providing a multilegged device is advantageous, as a higher voltage can be produced by the same temperature difference.

(62) Naturally, the device may be extended by further connectors and pair of legs

(63) Dimensions

(64) The thermoelectric device according to the present invention may have various dimensions.

(65) The efficiency of a thermoelectric device depends on more factors than only the maximum ZT of a material. This is primarily due to the temperature dependence of all the materials properties (ionic conductivity, , ionic Seebeck coefficient, , and thermal conductivity, ) that make up ZT(T) as a function of temperature.

(66) A small letter z is used for the figure-of-merit of a thermoelectric device in order to distinguish it from the material's figure of merit ZT=.sup.2/. The maximum efficiency () of a thermoelectric device is used to determine zT. Like all heat engines, the maximum power-generation efficiency of a thermoelectric generator is thermodynamically limited by the Carnot efficiency (T/Th). If the temperature is assumed to be independent and n-type and p-type thermoelectric properties are matched (, and ), (an unrealistic approximation in many cases) the maximum device efficiency is given by Equation (1) with Z=z.

(67) $\begin{array}{cc}=\frac{T}{T_h}.Math.\frac{\sqrt{1+\mathrm{zT}}-1}{\sqrt{1+\mathrm{zT}}+\frac{T_c}{T_h}}& \mathrm{Eq}.\left(1\right)\end{array}$

(68) In order to obtain the maximum efficiency of the TEG, dimensions of the legs need to be optimized such that the lengths L.sub.n and L.sub.p and the cross section areas S.sub.n and S.sub.p of the legs satisfy the Equation (2), wherein n stands for n-type and p stands for p-type. The lengths L.sub.n and L.sub.p are the lengths of the legs extending from the cold side to the hot side. The cross section areas S.sub.n and S.sub.p of the legs are the cross section areas through which the ions are moving when they diffuse by the temperature difference along the lengths of the legs.

(69) $\begin{array}{cc}\frac{I_n{S}_p}{I_p{S}_n}=\left(\frac{{}_n{}_n}{{}_p{}_p}\right)& \mathrm{Eq}.\left(2\right)\end{array}$

(70) As far as the device architecture is concerned, miniaturization is known to improve the efficiency of Peltier coolers (large heat flow).

(71) In the following, reference is made to FIG. 14, which shows a schematic drawing of some components of thermoelectric device 140 comprising connectors 145 ionically coupled via legs 141 and 143. The legs 141 and 143 of the device may be provided as a film comprising the electrolyte composition. The thickness of the film of electrolyte composition L in the legs may vary. The thickness of the film, i.e. length L, may be in the range of from 1 nm to 5 dm, for instance in the range of from 10 nm to 5 cm, or from 10 m to 5 mm, such as in the range of from 1 nm to 500 m, as for example in the range of from 10 m to 500 m, such as in the range from 50 m to 500 m, or in the range of from 1 nm to 10 m, such as in the range from 1 nm to 1 m.

(72) If screen printing is used, the film of electrolyte composition, i.e. length L, may be in the range of from 1 m to 500 m. If spin coating is used, the film of electrolyte composition may be in the range of from 1 nm to 10 m. If liquid handling robot is used, the film of electrolyte composition may be in the range of from 1 mm to 5 dm.

(73) Preferably, the thermoelectric device may have dimensions of the legs allowing for relatively low resistance and relatively high power.

(74) The electrolyte composition in the legs may have a thickness of about 1 cm. The temperature difference over such a leg may be up to e.g. about 100 K.

(75) The electrolyte composition in the legs may be of a given material having a thickness of about 50-100 m. The temperature difference over such a leg may be up to e.g. about 30 K, given a temperature below 200 C. on the hot side.

(76) The figure-of-merit may be in the range of from 0.1 to 2, or in the range of from 0.5 to 1.8, or in the range from 0.8 to 1.5. A relatively high Seebeck voltage is preferable.

(77) Both the dimension of the connector 145 and the dimension of the legs 141, 143 influences the performance of the device.

(78) If the connector has a relatively large length compared to its width/thickness, the resistance of the connector is very large since the ions need to travel a long distance, which may limit the power of the device.

(79) If the connector 145 has a length Lc in about the same range as its width/thickness, typically a length which is not more than 10 times larger than the width/thickness, more typically not more than 5 times larger than the width/thickness, the resistance of the connector 145 is small, which is favorable in term of internal resistance.

(80) However, considering a relatively thick connector 145 (Lc>>), typically larger than twice the thickness L of a leg 141, 143, also implies that there is a large temperature drop across the connector that cannot be used for thermoelectric generation. So, in a relatively thick connector the voltage will drop, which will decrease the power of the device. However, there might be areas of applications where this is acceptable.

(81) Theoretically, there will be an optimal thickness of the connector 145 depending on its ionic conductivity and thermal conductivity. The ratio of the distance between the legs (d.sub.int) to the width of the legs (W) may preferably be less than 1. The thickness of the connector Lc may preferably be in the same order of magnitude as the thickness of the legs L (assuming the ionic conductivity and the thermal conductivity of the legs and connector are of the same order of magnitude).

(82) The dimension of the conducting polymer electrodes will limit the maximum amount of charges Q (integrated current dQ/dt) that can be generated by the ionic thermoelectric generator. The dimension of the conducting electrodes will fully limit given that the generated ion concentration gradient, when running the device, is not the limiting phenomena thanks to ion reservoirs that are large enough.

(83) The amount of charges that may be generated by a given temperature gradient before the device must be recharged and the process must be reversed, increases with the thickness of the PEDOT-PSS electrodes. Hence, it is advantageous to consider geometries where the conducting polymer electrode has a large volume, but still a close distance to the legs in order to avoid a large ionic resistance in the reservoir.

(84) Possible architecture includes for instance a conducting polymer electrode surrounding the electrolytic reservoir, with an additional insulating layer to avoid contact with the legs.

(85) Vertical Versus Lateral Structures

(86) The thermoelectric device according to the invention may be a vertical devices or a lateral device. In a lateral device, FIG. 4a, the connector is normally arranged to the side of the reservoirs, and the mobile ions move in a lateral direction, substantially parallel with the surface area of the leg. In a vertical device, FIG. 4b, the connector, leg and reservoir normally at least partially covers each other, and the surface area of the electrolyte film makes up the cross section of the device. In more detail, in a vertical device the connector, leg and reservoir are normally arranged in substantially in the same plane on a substrate, and the ion flows in a direction substantially parallel with said substrate. Further, in a vertical device the connector, leg and reservoir are normally arranged on top of each other on a substrate, each element in a different plane, and the ions flow in a direction substantially normal to said substrate.

(87) The ionic thermogenerator demonstrated in the examples is fabricated as proof of concept. It is not intended to show the design optimum to get the maximum power out of the device.

(88) In the device presented in the examples, the length of the legs are about L=1 cm, the thickness about T=150 microns, and the width about W=1 mm. Because it is a lateral device, the cross section for the ion current (W*T) is little, the length is long and thus the resistance is high.

(89) Thus, for a conductivity of about 1 S/m, the resistance R=1/conductivity*L/(W*T) will be about 0.1 MOhms. Assuming the resistance of the legs is the largest resistance in the device, for device with 2 legs, the internal resistance is thus 0.2 MOhms.

(90) The maximum power of the device is obtained when the load resistance R is similar to internal resistance Rint: Pmax=Vload2/Rint.=(Voc/2)2/Rint. The load voltage across the load resistance Vload is half the open-circuit voltage Voc when the load resistance is equal to the internal resistance. The open circuit voltage can be simply estimated by the Seebeck coefficient of the two legs and the temperature gradient used. The sum of the Seebeck coefficient for the polycation and polyanion legs is of the order of 50 mV/K. So, assuming T=1 degree, the open circuit voltage is 50 mV, and the load voltage is 25 mV. The maximum power is 25.sup.2*10.sup.11W=6.25 nW.

(91) In another example, a device is constructed vertically (the temperature gradient across the thickness of the polyelectrolyte film) with the following dimensions of the legs: L=10.sup.4 m, W=10.sup.3 m, T=10.sup.2 m. In this case, the cross-section area, S, is given by W times T, and the length is given by L.

(92) Given, an ionic conductivity of 1 S/m, the resistance of the leg is R=10 Ohms. Hence, simply by going to a vertical architecture, the resistance of the leg decreases by 4 orders of magnitude. Assuming the same temperature gradient, the maximum power is then 62.5 W.

(93) In order to further increase the power for the same temperature gradient, the number of legs in the thermoelectric modules may be increased. N legs connected in series in a thermoelectric module will increase the open circuit voltage by N times, but the internal resistance will also increase by N times. Thus, the maximum will also increase by N times.

(94) The limitation for a vertical architecture is in the achievable temperature gradient. A thin film will lead to a small temperature gradient, thus a small Seebeck voltage and small Voc and small power output.

(95) However for some applications, it is desirable to have thin devices, such as to put on the body (flexible) and use the heat from the body to generate electricity to power some other devices.

(96) Thicknesses in the sub-microns are possible to use for high ZT materials such as for applications in nanoelectronics.

(97) Most of the applications envisaged will consider a polyelectrolyte leg with a thickness of 1 micron or larger. A typical thickness of a polyelectrolyte leg is in the range of from 10 to 1000 microns.

(98) Manufacturing Techniques

(99) In term of manufacturing, the advantage of using a polyelectrolyte is the ability to process it starting from a solution. Hence, low cost manufacturing technique such as printing can be used. Screen printing technique is ideal to create patterns of a thickness in the range of from 1 micron to 500 micron.

(100) In more detail, the manufacturing may be performed by means of a technique selected from a group comprising screen printing, wire-bar coating, knife coating, bar coating, spin coating, dip coating or spray coating. This is advantageous as it normally allows for short manufacturing times.

(101) Legs of a thickness in the range larger than 1 mm might be needed for some specific applications using large temperature gradient, reaching the upper limit of what can stand the materials (max temperature 300 C.). Thicker legs can be envisaged in application with a cold side at low temperature, such as in airplane, where the temperature at 10 km altitude is 70 C. and the temperature in the plane or close to the motor is from room temperature to several hundred degrees Celsius.

(102) In order to fabricate thick legs, in the range of from 1 mm to 10 dm, liquid handling robot can be used to fill-in plastic cavities with polyelectrolytes. It is not excluded that the polyelectrolyte can be blended with more conventional plastics such that standard manufacturing techniques for plastic, like extrusion or injection, are used.

(103) If spin coating is used, the film of electrolyte composition may be in the range of from 1 nm to 10 m.

(104) FIG. 15a further illustrates how the components of a thermoelectric device 150 according to the present disclosure may be positioned. The electrode 154, reservoir 157, legs 151 and 152 and a connector 155 of the device is illustrated in FIG. 15a. It may be advantageous to consider geometries where the conducting polymer electrode 154 has a large volume, but still a close distance to the legs 151, 152 in order to avoid a large ionic resistance in the reservoir 157. Possible architecture includes for instance a conducting polymer electrode 154 surrounding the electrolytic reservoir 157, with a additional insulating layer 158 to avoid contact with the legs 152.

(105) For a multi-legs device, the conducting polymer electrode can be considered up to 10 times thicker than overall thickness of the leg+connector, i.e. Z.sub.polymer may be up to 10 times thicker than Z.sub.a. However, the conducting polymer electrode may also be up to about 100 times the thickness of the leg+connector

(106) A multi-legged device is further illustrated in FIG. 15b, in which several connectors 155 and legs 151, 152 are coupled in series to an electrode 157. This device also comprises an insulator layer 155.

EXPERIMENTAL EXAMPLES

Example 1

Thermoelectric Properties of Polyelectrolytes

(107) The thermoelectric properties of the polyanion poly(styrene sulfonate) (PSS) with mobile sodium cations (Na.sup.+) and the polycation poly-2-[(methacryloyloxy)-ethyl]trimethylammonium (PMADQUAT) with mobile chloride anions (Cl.sup.) are measured in the device illustrated in FIG. 3. This device can be considered as the elementary power generator for a polyelectrolyte.

(108) A glass substrate 352 with two pre-patterned gold electrodes 353, 354 by thermal evaporation (1 mm in width, 53 mm in length, approx. 100 nm in thickness for each and 1 mm apart from each other). Solution polyelectrolytes 351 (PSSNa or PMADQUAT, 2 wt % in distilled (DI) water, 40 l) were drop-casted on the prepared substrate and dried naturally. The obtained films give the thickness as 1.66 m for PSSNa and 1.16 m for PMADQUAT.

(109) A temperature difference is then applied between the two gold electrodes by a heater 355 and cooler 366 positioned below the glass substrate. An electric potential can be measured between the two gold electrodes.

(110) Without temperature gradient the potential between the two gold electrodes is small and fluctuates, see also FIG. 5. Upon heating, a temperature difference arises and the open circuit voltage, V.sub.OC, increases with time, and stabilizes at well defined values: V.sub.oc=24.84 mV/K for PSSNa and 1.96 mV/K for PMADQUAT when the measurement is performed at a relative humidity of 50% RH. The Seebeck coefficient is defined as the measured open circuit voltage, V.sub.OC, divided by the temperature difference at the two gold electrodes, T.sub.Au.

(111) By the term relative humidity, it is herein meant the ratio of the actual amount of water vapor (absolute humidity) present in the air to the saturation point at the same temperature.

(112) The ionic conductivity, , and ionic Seebeck coefficient, , can be systematically measured for different values of relative humidity. The ionic conductivity is typically low in dry films (for instance, 10.sup.3 S/m at 10% RH) and increases drastically up to 0.74 S/m for PSSNa and 5.57 S/m for PMADQUAT at 80% RH, as can be seen in FIGS. 6 and 7, respectively.

(113) When a temperature gradient, T.sub.Au, of 1.2 K is imposed between the two gold electrodes coated by the polyelectrolyte films, the mobile ions are expected to thermo-diffuse towards the cold electrode and generate a measurable thermo-voltage. For humid films, it is clear that the sign of the mobile charged ions dictates the sign of the thermo-voltage. The polyanion PSSNa possesses a positive ionic Seebeck coefficient of +50 mV/K at 80% RH; while the polycation PMADQUAT displays a negative ionic thermopower of 9 mV/K at 80% RH. Both the thermovoltage and the conductivity increase with the humidity, which supports the direct involvement of mobile ions in the thermo-voltage.

(114) Compared to electronic thermoelectrics, the power factors (.sup.2) of these ionic thermoelectrics are surprisingly high: 1830 Wm.sup.1K.sup.2 for PSSNa and 410 WK.sup.2 m.sup.1 for PMADQUAT at 80% RH. Assuming a thermal conductivity of =0.3 WK1m1 as typical for a polymer gel, thermoelectric figure-of-merit (ZT) is 1.8 for PSSNa and 0.4 for PMADQUAT, equivalent to the best electronic thermoelectric materials.

Example 1bis

Electric Power Generation from One Leg Device or from Multiple Single-Leg Devices

(115) The open-circuit voltage of the device increases linearly with the temperature gradient (FIG. 18) and its value is about 55 mV for 1 K, which is close to the measured ionic Seebeck coefficient with the Au electrodes

(116) The device is then connected to a load resistance and the output voltage across the load is followed versus time. At the origin of the time axis, there is no temperature difference, but a temperature gradient is increased until it reaches a constant value of T=1.2 K at about 1500 seconds. The initial output voltages are smaller than the open-circuit voltage, as expected for a generator connected to a load resistance, but it increases steadily to become larger than the open-circuit voltage to reach a maximum at 64 mV (R=7.5 MOhms), 53 mV (R=2 MOhms), 32 mV (750 kOhms). This increase in the output voltage corresponds to an induced thermo-generated current of 8.53 nA, 26.5 nA, and 42.6 nA and a maximum electrical power of 0.546 nW, 1.40 nW and 1.36 nW, for respectively R=7.5 MOhms, 2 MOhms and 750 kOhms. The total charge stored electrochemically in the two PEDOT-PSS electrodes is Q=0.0001 coulombs. Like any electric power generator, the power output depends on the load resistance and possesses a maximum when the load resistance is equal to the internal resistance, here about 2.5 MOhms (FIG. 20.

Example 2

Point of the Electrochemically Active Electrodes

(117) The strategy to increase the thermo-voltage is to connect polycation and polyanion legs electrically in series and thermally in parallel, since they have Seebeck voltages of opposite sign. PSSNa has a positive ionic Seebeck coefficient, , while PMADQUAT shows a negative Seebeck voltage at high humidity level. PSSNa may be defined as a P-leg and PMADQUAT as a N-leg.

(118) Device 4, arranged generally as described in relation to FIG. 1, and in more detail a connector to conduct ions comprising an aqueous solution of NaCl, two ion reservoirs comprising a NaCl solution of the same concentration as the solution in the connector, a first and a second leg, respectively, and electrochemically active PEDOT-PSS electrodes. The reservoirs are in contact with the legs and the electrodes.

(119) Onto a glass substrate, two PEDOT:PSS electrodes are prepared by drop-casting the solution and baked at 50 C. (L: 18 mm, W: 15 mm and T: 8.6 um). PSSNa and PMADQUAT legs are fabricated with the their solution (2 wt % in DI water mixed with Silquest-187A silane (5 wt %)) and baked at 110 C. for 5 minutes, with the help of sticky tape frame. Then, the sticky tapes are removed. Continuously, frames for the ionic conductor (L: 11 mm and W: 6 mm) and reservoir (L: 38 mm and W: 5 mm) are fabricated thermally cross linking with SU-8 by using modes and baked at 100 C. for 4 hours. The resulting frames have the thickness as around 500 m. Each device has the channels 1-mm-wide and 11-mm-long.

(120) The edge electrodes of Device 4 are connected to a load resistance (50 kOhms) when a temperature gradient of 1.2 C. is applied. The output voltage measured over the resistance is recorded versus time. The Device 4 shows an increase in potential versus time upon applying the temperature difference. A true electrical power is generated from the temperature difference. The potential reaches a plateau indicative that the temperature gradient is now constant. In this specific experiment, the output voltage suddenly drops after 4500 s because the reservoirs are dried due to that the water of the NaCl solution has evaporated at the hot side.

(121) In Device 4, the temperature difference per mm is 1.2 K, and the Voltage output is illustrated in FIG. 8.

Example 3

Improvement of the Connector and Reservoir

(122) The Device 5 is fabricated in the same way as Device 4 but the reservoir and ionic connectors are composed of a gel comprising NaCl 1M with 10% polyethyleneoxide (PEO). The presence of polyethyleneoxide in the gel slow down the evaporation of water and the output potential can be maintained and recorded for a longer time than for Device 4 (given the same load resistance and the same applied temperature difference), as is illustrated in FIG. 9.

(123) Any power generator has its own internal resistance R.sub.in. When the load resistance is equal to the internal resistance, the power of the generator is maximum. This is observed also for our device, see also FIG. 10.

Example 4

Charge and Discharge

(124) The output voltage over a resistance of 50 kOhm versus time may be followed in a series of three cycles, H1+C1, H2+C2 and H3+C3, respectively, of charge (T=1.2C) and discharge (T=0C), illustrated in FIG. 11. The three heating-cooling cycles (obtained for Device 5 in N.sub.2 atmosphere with 80% RH) are explained in detail below:

(125) Heating half-cycle: Apply a temperature difference of T=1.2C between the electrodes, connect the device with the load resistance and record the output voltage (H1 for 10 min, H2 for 20 min, H3 for 30 min).

(126) Cooling half cycle: Stop heating the device and disconnect the out load R. When the temperature difference T between the electrodes gets to 0, connect the load and record the output voltage for some time (C1 for 10 min, C2 for 20 min, C3 for 30 min).

(127) For each cycle, the out load is only connected to the device when temperature difference T between the electrodes is in equilibrium.

(128) When T=1.2C, an output power is measured, electrical current is generated. This leads to an electrochemical reaction in the PEDOT-PSS electrodes: one is reduced, one is oxidized. As a result, the two PEDOT-PSS electrodes are not at the same electrical potential even if no temperature gradient is applied. In other words, the heat charges a PEDOT-PSS battery cell.

(129) When T=0C, there is still an output potential across the load resistance since the two PEDOT-PSS electrodes are not at the same potential. But this potential drops during time indicating that a current discharge is measured. When this current is zero, the two PEDOT-PSS electrodes have the same oxidation level. This is equivalent to discharge a PEDOT battery.

(130) When a temperature difference is applied to the device, the two electrodes undergo reduction and oxidation, respectively. If PEDOT is used, the PEDOT at one electrode becomes more reduced than the pristine PEDOT at the same electrode was, and the PEDOT at the other electrode becomes more oxidized than the pristine PEDOT at that electrode was. If the temperature difference is no longer applied, there is still an electric potential difference between the electrodes, in other words a charged battery. If a resistance is connected to the PEDOT electrodes, a discharge current will flow through the resistance and the ions will move in opposite direction inside the legs compared to when the temperature difference was applied.

(131) The amount of charges that may be stored in the electrodes of this device is related to the capacitance of the electrodes and the thickness of the PEDOT-PSS layer among others.

Example 5

Multiple Legs Ionic Thermogenerators

(132) The voltage increases with the size of the electrolyte films in the legs. Therefore, the voltage may increase by adding a number of legs. The power also increases with the size.

(133) As an example, 1 cm.sup.2 may correspond to about 1 V. If one leg corresponds to 1 V, then a million of legs would correspond to 1 V.

(134) In Device 6, which is a device arranged as described in relation to FIG. 12, and manufactured in an analogous manner as described above, the temperature difference per mm is 1.5 K.

(135) For a multi-legs device, the conducting polymer electrode can be considered up to at least 10 times thicker than the overall thickness of the leg and the connector. For such large dimension of the electrode, the ionic resistance in the electrode will be limiting the internal resistance.

(136) In term of power generation, a too large electrode will lead to a low power, but a larger total amount of charge generated, that is a longer time per a cycle.

(137) The multiple legged device may be charged slowly, and discharged rapidly.

(138) For applications requiring higher peak power, a thermoelectric generator slowly charging a supercapacitor, which can then deliver a large current or peak power, is a solution.

Example 6

Reduction and Oxidation of PEDOT-PSS Electrodes

(139) In general, a heat flow caused by a temperature difference between a so-called cold and hot part of the device, respectively, is inducing a transport of Na.sup.+ and Cl.sup. ions where the ions inside the connector (hot side) tend to move towards the distal part of the legs (cold side) in relation to the connector.

(140) The Na.sup.+ tend to move through the first leg and the Cl.sup. tend to move through the second leg. The sodium ions tend to move through the first leg as the first leg comprises an immobile anionic polymer which constitutes a negatively charged path on which the ions can move on. The chloride ions tend to move through the second leg as the second leg comprises an immobile cationic polymer which constitutes a positive charged path on which Cl can be transported.

(141) As Na.sup.+ enters an ion reservoir, situated at the distal part of the first leg, there will be a driving force for obtaining charge balance which causes a reduction reaction at the first electrode. The first electrode is composed of PEDOT.sup.+PSS.sup. and the reduction reaction which occurs is:
PEDOT.sup.+PSS.sup.+e.fwdarw.PEDOT.sup.0+PSS.sup.

(142) During the reduction reaction, a PSS.sup. is released and transported to the first ion reservoir to provide for charge-balance in the first ion reservoir where the Na.sup.+ ion has entered due to thermodiffusion.

(143) The second electrode is composed of PEDOT.sup.0. As the Cl.sup. enters an ion reservoir, situated at the distal part of the second leg, there will be a driving force for an oxidation to occur at the second electrode:
PEDOT.sup.0.fwdarw.PEDOT.sup.++e.sup.

(144) In more detail, the reaction in the second electrode can be described as follows:
PEDOT.sup.0+PSSNa.fwdarw.PEDOT+PSS.sup.+e.sup.+Na.sup.+

(145) In this case the Na.sup.+ ion is released and transported to the second ion reservoir to provide for charge-neutralization as Cl.sup. is entering the second ion reservoir.

(146) By the oxidation reaction an electron is transported in an external circuit, built up between the two electrodes, from the second electrode to the first electrode where the reduction reaction occurs.

Example 7

Crosslinked Polyelectrolyte

(147) At humidity levels higher than 80% RH, the polyelectrolyte films can lose their mechanical integrity and dissolve in the absorbed water. To prevent this a polyelectrolyte PSS:Na film was cross-linked using a polysiloxane crosslinker (Silquest-187A). This allows for characterization of the thermoelectric properties of the polyelectrolyte in wet conditions. When a reservoir of 1M NaCl is connected to the cross-linked polyelectrolyte film, the polymer film swells to reach water saturation. The ionic conductivity of this wet and salt-doped polyelectrolyte film reaches 1.2 S/m (FIG. 21a) which is close to the ionic conductivity of a liquid aqueous electrolyte. The crosslinked wet PSS:Na film possesses a large and positive ionic Seebeck coefficient of +47 mV/K (FIG. 21a); thus leading to an very high power factor (.sup.2) of 2680 Wm.sup.1K.sup.2 (FIG. 21a).

(148) The thermal conductivity of the wet crosslinked PSS:Na film soaked in NaCl solution reaches 0.49 Wm.sup.1K.sup.1 (FIG. 21b). The cross-linked PSS film soaked with the salt solution reaches a very high ZT value of 1.6 due to the advantageous ionic conductivity as compared to the film exposed at 80% RH.

Example 8

The Thermoelectric Device as a Supercapacitor

(149) In this example the conducting polymer poly(3,4-ethylene dioxythiophene)-polystyrene sulfonate (PEDOT:PSS) used as supercapacitor electrodes. Conducting polymers transport both electronic and ionic charge carriers such that the surface notion of the electric double layer capacitor becomes a bulk notion and the specific capacitance reaches 10.sup.5-10.sup.6 F/kg. The charge-discharge for this supercapacitor is measured by the output voltage vs. time as presented in FIG. 22 for one heating-cooling cycle in N.sub.2 atmosphere at 80% RH. When a temperature gradient (T=1.2 K) is applied, the thermo-electric effect acts like a generator that is charging a PEDOT-PSS/PSS/PEDOT-PSS supercapacitor. When a stable temperature gradient is reached, the device is connected to the a load resistance. The current generated is recorded for 40 minutes and corresponds to an integrated charge of Q=3.5510.sup.5 coulombs stored in the PEDOT:PSS electrodes. Directly after the heating half-cycle, the device is disconnected from the load resistor and the temperature gradient gradually falls to zero. When T=0 K, the open circuit voltage of the charged supercapacitor is V.sub.oc(T=0)=55.3 mV, which is of the same order of magnitude but slightly lower than the Seebeck voltage (110 mV) measured at open circuit for T=1.2 K (see inset of FIG. 22). This indicates that the supercapacitor is not fully charged in 40 min at this temperature gradient (as expected from the charging curve that has not reached it minimum current as in FIG. 22a). The specific capacitance is C.sub.s=QV.sub.oc.sup.1M.sup.1=10.sup.5 Fkg.sup.1 (the mass of the PEDOT:PSS electrode is 0.32 mg). This value is in good agreement with the measured specific capacitance by impedance spectroscopy (C.sub.s=610.sup.4 F/kg, see FIG. 23) and corresponds to the values reported in the literature for conducting polymers. The charged supercapacitor is then connected to a small load resistor (50 k) and its discharge is characterized. The output voltage decreases versus time. When the current level drowns within the noise level, the two PEDOT:PSS electrodes can be assumed to be back at the same oxidation level as before charging. The charge released during discharge is Q=3.4710.sup.5 coulombs, thus smaller than the charge produced by the thermoelectric effect. This is attributed to the self-discharge of the polarized PEDOT:PSS electrodes. For the supercapacitor, the energy density is 173 J/kg for T=1.2K. The energy density increases quadratically with the temperature gradient according to E=C.sub.s(T.sub.I).sup.2 (see FIG. 22b). The extrapolated energy density for T=10 K and T=100 K are 12 kJ/kg (3.32 Wh/kg) and 1.2 MJ/kg (332 Wh/kg). Importantly, the energy density for larger temperature gradient can be high because the ionic Seebeck voltage of the polyelectrolyte is exceptionally large. For the sake of comparison, Li-polymer batteries have an energy density of about 200 Wh/kg.

Example 9

Measurement of Specific Capacitance by Impedance Spectroscopy

(150) Devices were fabricated by sandwiching a 50 m thick solid-state PSS:Na film between two PEDOT:PSS film electrodes, PEDOT:PSS/PSS:Na/PEDOT:PSS. When applying an alternating current (AC) voltage across the supercapacitor, the polarization characteristics of the electrolyte strongly depends on the frequency. The device was is kept in a home-made climate chamber with saturated water atmosphere overnight. The device was connected to an impedance spectrometer through PEDOT:PSS electrodes and the frequency was swept from 10.sup.4 Hz to 10.sup.2 Hz. The device was scanned with different AC varied from 1 V, 100 mV, 10 mV to 1 mV, respectively, which display similar capacitive behavior. The phase angle and capacitance versus frequency with AC at 100 mV are given in FIG. 23. The phase angle of the device reaches low angle (<45) at low frequency, implying that device showing a dominant capacitive behavior at those frequencies. For the capacitance, it increases with the frequency and saturates at low frequencies, around 710.sup.4 F. With the mass of the PEDOT:PSS, the specific capacitance for the PEDOT:PSS was calculated as about 610.sup.4 FKg.sup.1.

(151) It should be noted that the invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

(152) It is further noted that, in the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. A single apparatus or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain features or method steps are recited in mutually different dependent claims does not indicate that a combination of these features or steps cannot be used to an advantage.