CELL WITH SOLID ELECTROLYTE

Abstract: Solid electrolyte containing as major component heter­ opolytungstic acid and/or heteropolymolybdic acid contain­ ing carbon as hetero atom. The heteropolyacids are stable, and can be easily synthesized and easily formed into a shaped material.

Description: The present invention relates to an electrochemical cell utilising (generally hydrated) hydrogen ion-conductive solid electrolyte. Particularly, it relates to an electrochemical cell which is suitably used as hydrogen-oxygen fuel cell, hydrogen sensor or pH sensor. It has been reported by, for example, O. Nakamura et al. in Materials Research Bulletin 17 , 231 (1982) that a heteropoly-acid (solid) such as dodecatungsto-phosphate (H₃PW₁₂0₄₀.nH₂0) is a proton-conductive solid electrolyte. However, no heteropolyacids containing carbon as a heteroatom have been known as yet. NATURE, 312 , 537-538 (1984) discloses a substance believed to be a 12-heteropolytungstic acid with carbon as the heteroatom. It is prepared by dissolving tungsten carbide in a solution of hydrogen peroxide and evaporating to dryness at 100°C. US-A-4,024,036 discloses a proton permselective solid-state member formed by a heteropolyacid, and its use as a solid electrolyte in an electrochemical cell. The present invention provides an electrochemical cell utilising a solid electrolyte comprising a heteropolyacid, characterised in that the principal component of which is a heteropoly-tungstic acid and/or heteropolymolybdic acid containing carbon as a heteroatom. It has been found that the heteropolyacid containing carbon as a heteroatom to be used in the electrochemical cell of the present invention exhibits a remarkable proton conductivity, that is, it is a proton-conductive solid electrolyte. The heteropolyacid has advantages in that its preparation is simple and that it can be molded more easily than other heteropolyacids. Further, the heteropolyacid is still advantageous in that a firm thin film can be produced from an aqueous solution thereof. A heteropolyacid solid electrolyte is a solid comprising a heteropolyanion represented by the general formula, M' x M y O z m- , and a hydrated proton represented by the general formula, H(m' H₂0)⁺, wherein proton is conducted. M is generally tungsten (W) or molybdenum (Mo) and P,Si,As and the like are known as the heteroatom M'. The solid electrolyte used in the cell according to the present invention is represented by the above formula wherein M' is carbon (C) and M is tungsten (W) or molybdenum (Mo). Further though the heteropolyacids of the prior art have been prepared by liquid phase reaction between polytungstic acid (or polymolybdic acid) and an acid containing a heteroatom (for example, dodeca acid), the solid electrolyte of the present invention is prepared by the reaction between tungsten carbide and/or molybdenum carbide and a solution of hydrogen peroxide. Desirably a cell according to the invention is manufactured using an electrolyte which is prepared by preparing a solution by reaction of tungsten carbide and/or molybdenum carbide with a solution of hydrogen peroxide; and evaporating the resulting solution to produce said electrolyte in solidified form. Preferably said evaporation is effected by blowing air through the solution. The heteropolyacid used in the present invention is a solid heteropolyacid containing carbon as a heteroatom which comprises a heteropolyanion represented by the general formula, M' x M y O z m- (wherein the ratio of x to y is between 1: 12 and 4:12) and a hydrated proton represented by the formula, H(m' H₂0)⁺. In the above general formula, M' is C and M is at least one element selected from W and Mo. Further, the above formula can be also represented by the formula: WO₃.aCO₂.bH₂0₂ cH₂0 (wherein 0.083 ≦ a ≦ 0.25, 0.05 ≦ b ≦ 1 and 0.16 ≦ c ≦ 4). Some embodiments of the present invention will now be described with reference to the accompanying drawing in which: Fig. 1 shows a cross sectional view of a cell using a solid electrolyte according to the present invention. Examples of the present invention will now be described. Example 1 100ml of a 15% aqueous solution of hydrogen peroxide (H₂0₂) was added to 16 g of tungsten carbide (WC) powder. When gas evolution was reduced, an additional 5 ml of 3% H₂0₂ was further added. This procedure was repeated 5 times. The resulting mixture was allowed to stand at a room temperature for 24 hours or over to dissolve the WC, thus obtaining a pale yellow strongly acid solution. This solution was filtered to remove sooty carbon. The excess H₂O₂ contained in the solution was removed by decomposition with a platinum gauze having adherent platinum black. The obtained solid electrolyte stock solution was placed in a flat-bottomed glass vessel, heated until just before its solidification and allowed to stand at a room temperature. By allowing to stand for 24 hours or over, a glassy plate was obtained. This solid did not exhibit any X-ray diffraction pattern, that is, it was amorphous. Further, the solid had a molar ratio of C to W of about 1 : 12. When the solid was heated, it first lost moisture and was decomposed with release of CO₂ at about 300°C into WO₃. Infrared or Raman spectroscopic analysis of the solid showed the presence of a strong band of about 950 to 1000 cm⁻¹. Further, the infrared spectroscopic analysis thereof showed the presence of an absorption of about 1300 to 1400 which was inherent in the substance. ¹³C- NMR spectroscopic analysis of the above solid electrolyte stock solution showed the presence of a resonance absorption at about 167 ppm. It is apparent from the above results that the obtained solid is a kind of heteropolyacids containing carbon as a heteroatom. A cell shown in Fig. 1 was produced by the use of the above solid plate. In this figure, numeral 1 refers to the above solid plate. Platinum springs 4 and 5 press platinum electrodes 2 and 3 against the both surfaces of the solid plate 1, respectively. The other ends of the springs 4 and 5 are taken out as electric leads 6 and 7. Glass tubes 8 and 9 are bonded to the solid electrolyte 1 with an epoxy adhesive 10. Steam-saturated hydrogen was fed at 10°C via inlets 11 and 12 and discharged via outlets 13 and 14, respectively. The electromotive force generated between the electric leads 6 and 7 in this step was O V. Then, steam-saturated hydrogen was fed via the inlet 11 at 10°C, while a gaseous mixture comprising 10 % of steam-saturated hydrogen and 90 % of argon was fed via the inlet 12 at 10°C. When the temperature of the cell was 20°C, an electromotive force of 29.1 mV was generated between the lead 6 and the lead 7. The hydrogen concentration of the gas to be fed via the inlet 12 was varied and the electromotive force between the leads 6 and 7 was measured at each of the concentration. The electromotive force E(mV) was varied depending upon the equation: wherein is hydrogen concentration of the gas fed via the inlet 12. When oxygen was fed via the inlet 12, an electromotive force of about 1 V was generated. It was confirmed with an ammeter 16 that an electric current flowed from the lead 7 to the lead 6, when a switch 15 was on. The resistance of the solid electrolyte was estimated based on the response of the voltage given when a pulse of an electric current was applied on the cell shown in Fig. 1. The resistance was about 500Ω at 20°C. Since the solid electrolyte had a thickness of 0.2 cm and the electrode area was 0.2 cm², the specific resistance was 500Ω cm. Example 2 A solid electrolyte stock solution was prepared in a similar manner as the one described in Example 1. The stock solution was placed in a flat-bottomed glass vessel. Air of a room temperature (about 20°C) was blown into the vessel to thereby evaporate the moisture, thus solidifying the solution. The obtained solid plate was amorphous and had a molar ratio of C to W of 1 : 9. When the solid plate was heated, it began to release CO₂ at a temperature of about 200°C and decomposed into WO₃. The other characteristics thereof were similar to those of the solid obtained in Example 1. Accordingly, it was apparent that the solid obtained in Example 2 was a heteropolyacid (solid) containing carbon as a heteroatom. A similar cell as the one produced in Example 1 was produced by the use of the above solid plate. The relationship between the electromotive force generated between the leads 6 and 7 and the hydrogen concentration was examined in a similar manner as the one described in Example 1. As a result of this examination, the concentration dependence represented by the equation 1 was confirmed and the solid was found to be a hydrogen ion-conductive solid electrolyte. The specific resistance of the solid electrolyte was 250Ω cm. Example 3 The same procedure as the one described in Example 1 was repeated except that molybdenum carbide (Mo₂C) was used instead of WC to obtain a solid electrolyte stock solution. The solution was solidified by blowing air to obtain a heteropolyacid having a ratio of C to Mo of about 1 : 11. A similar cell as the one shown in Fig. 1 was produced by the use of this solid plate. The relationship between the electromotive force generated between the leads 6 and 7 and the hydrogen concentration was examined in a similar manner as the one described in Example 1. As a result of this examination, the concentration dependence represented by the equation 1 was observed. The specific resistance of the solid electrolyte was 300Ω (at 20°C). Example 4 The same procedure as the one described in Example 1 was repeated except that a mixture comprising WC and MO₂C at a molar ratio of 18 : 1 instead of WC was reacted with H₂0₂ to obtain a solid electrolyte stock solution. The stock solution was placed in a flat-bottomed glass vessel and solidified by blowing air. The obtained solid was a heteropolyacid having a molar ratio of C to metal (W + Mo) of about 1 : 9. A similar cell as the one shown in Fig. 1 was produced by the use of the above plate. The relationship between the electromotive force generated between the leads 6 and 7 and the hydrogen concentration was examined to observe the concentration dependence represented by the equation 1, thus confirming that the obtained heteropolyacid containing carbon, tungsten and molybdenum was also a hydrogen ion-conductive solid electrolyte. The solid electrolyte had a specific resistance of about 300Ω (at 20°C). As described above, the use of the specified type of solid electrolyte for preparing cells has excellent effects.

Claims: An electrochemical cell utilising a solid electrolyte comprising a heteropolyacid, characterised in that the principal component of which is a heteropolytungstic acid and/or heteropolymolybdic acid containing carbon as a heteroatom. An electrochemical cell according to claim 1, wherein the heteropolyacid is a heteropolytungstic acid. An electrochemical cell according to claim 1, wherein the heteropolyacid is one which comprises a heteropolyanion represented by the general formula, C x M y O z m- (wherein M is at least one element selected from the group consisting of W and Mo and the ratio of x to y is between 1 : 12 and 4:12) an a hydrated proton represented by the formula H(m' H₂O)⁺. A method of manufacturing a cell according to any preceding claim wherein said electrolyte is prepared by preparing a solution by reaction of tungsten carbide and/or molybdenum carbide with a solution of hydrogen peroxide; and evaporating the resulting solution to produce said electrolyte in solidified form. A method according to claim 4 wherein said evaporation is effected by blowing air through the solution.

Inventors: ISHIKAWA, AKIRA, KAWAMURA, GO, KUDO, TETSUICHI
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: C01G39
IPC codes: H01B1, G02F1, H01M8, C01G41, C01G39, G01N27