Fuel cell utilizing solidous electrolyte

Abstract: A cell unit structure for stacking to form a fuel cell is described, composed of a porous substrate (21), a hydrogen electrode film (22) formed of fine particles of nickel powder (41, 62, 72) stacked on the porous substrate to form fine voids of homogeneous diameter, a film of solidous electrolyte (23) stacked on the hydrogen electrode film, and an oxygen electrode film (24) stacked on the electrolyte film.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to a fuel cell utilizing solid electrolyte. Specifically, the present invention relates to a fuel cell which is composed of stacking solid electrolyte thin film and a process for forming same. Description of the Background Art Fuel cells, such as plate type fuel cells are well known cell products utilizing porous substrates. Generally, fuel cells for the production of electrical energy from a fuel and oxidant are well known in the art. Simply put, such cells are composed of a plurality of cell unit structures comprising sequentially stacked porous substrate, an anode electrode film, an electrolyte layer and a cathode electrode film, the cell units are connected in series. Hydrogen gas as fuel is provided in the anode side of the fuel cell body and air (Oxygen gas) as oxidant is provided in the cathode side thereof, then hydrogen and oxygen are reacted for producing electromotive force and water as a by-product. The electrolyte can be a solid, a molten paste, a free-flowing liquid, or a liquid trapped in a matrix. This invention is concerned with the solid type of electrolyte which is preferred for many applications. It is also well known in the art that voltage dropping in a solid electrolyte may be indicated by following formula: V = iRt x 10⁻² Volts wherein V:voltage drop i:electric current density (Am⁻²) R:specific resistance (Ωm) t:thickness of solid electrolyte (µm) From the above relationship, the voltage drop of the fuel cell can be reduced by reducing the thickness of the solid electrolyte. On the other hand, as a solid electrolyte is formed on a thin electrode film over the porous substrate, the thickness of the electrolyte is determined by its coverage ability but must be thick enough to support a stacked cell structure, therefore, thinner is better but the minimum thickness is restricted. In view of these points, it has been assumed that thickness in a range of 10 to 50 µm is preferrable for a solid electrolyte layer. Porous substrates generally in use have dispersed void sizes in a range of 0.5 to 40 µm. Numbers of pin holes are formed in both the electrode film and the solidous electrolyte where it is stacked on portions of the substrate having relatively large void sizes, when the thin electrode film with the solidous electrolyte are stacked on the substrate. As well known in the art, electromotive force is produced at both sides of the solidous electrolyte when a concentration difference is created by a partial pressure difference of oxygen at each side. That is, the partial pressure difference forms a kind of concentration cell. The electromotive force of a fuel cell may be derived by the following formula: E₀ = (RT/4F) x ln(P₁/P₂) wherein R:gas constant T:absolute temperature F:Faraday constant P₁,P₂:partial pressure of oxygen at either side of the solidous electrolyte As shown in the above formula, electromotive force E₀ proportionally increases corresponding to the ratio of the oxygen partial pressure. Therefore, any pin holes formed in the solid electrolyte reduce the partial pressure difference of oxygen to zero, it causes loss of electromotive force or may even make the production thereof impossible. The avoidance of the formation of pin holes is greatly important for forming fuel cells. SUMMARY OF THE INVENTION It is therefore the principal object of the present invention to provide a fuel cell utilizing solid electrolyte inducing a minimum of voltage drop. It is another object of the present invention to provide a fuel cell utilizing a thin solid electrolyte in which the electrode surface to which the electrolyte is applied has fine voids of homogeneous diameter. It is a further object of the present invention to provide a fuel cell utilizing solid electrolyte which is free from pin holes. It is a furthermore object of the present invention to provide a method for forming a fuel cell utilizing a thin film of solid electrolyte. The patent document CH-A-515 622 discloses an electrode-electrolyte assembly for a fuel cell with a solid electrolyte designed to operate at high temperatures. The electrode-electrolyte assembly comprises in the following order a porous support having coarse pores, a layer of electrically conductive material having fine pores and a layer of solid impermeable electrolyte. To form this electrode-electrolyte assembly, the pores of the porous support in the vicinity of its surface are at first filled. The excess filling material is then treated to smooth off the surface of the porous support. A thin intermediate layer is deposited followed by an electrolyte layer. Finally, the material filling the pores of the support and also a part of the material in the thin intermediate layer are removed. In accordance with the invention, a fuel cell is provided which is constructed from a stack of cell unit structures connected in series. The cell unit structures comprise a porous substrate; a hydrogen electrode film formed on the porous substrate and comprising three nickel films: a first nickel film formed on the porous substrate and made from a mixture of a first nickel powder having a particle diameter of less than or equal to 1µm and a second nickel powder having a particle diameter of approximately 3µm, wherein the mixture comprises equal volumes of the first and second nickel powders; a second nickel film formed on the first nickel film and made from the second nickel powder; and a third nickel film formed on the second nickel film and made from the first nickel powder; a coating of platinum formed on the third nickel film of the hydrogen electrode film; a solid electrolyte film formed on the coating of platinum; and an oxygen electrode film formed on the solid electrolyte film. In the above fuel cell in accordance with the invention, the porous substrate can be advantageously formed from one of stainless steel, nickel powder or copper powder and the solid electrolyte film can advantageously have a thickness of 10µm. The invention also provides a process for forming a fuel cell in accordance with the invention which uses sputtering, in particular high frequency sputtering, to form the coating of platinum. Furthermore, in an advantageous variant of the process, the first, second and third nickel films are laminated one after another by the steps of: pressing and sintering a mixture of the first nickel powder and the second nickel powder, wherein the mixture comprises equal volumes of the first and second nickel powders, to form a first nickel film; polishing the surface of the first nickel film; applying a quantity of the second nickel powder to the first nickel film and then pressing and sintering the second nickel powder to form a second nickel film; rubbing a quantity of the first nickel powder into the surface of the second nickel film and then pressing and sintering the first nickel powder to form a third nickel film, this third nickel film having fine voids of homogeneous diameter on its surface. It can be advantageous when the solid electrolyte film is formed from single crystals of LaF₃. There are also several techniques suitable for forming the solid electrolyte film, for instance high frequency sputtering, electron beam sputtering, resistance heating evaporation, magnetron sputtering or metallo-organic chemical vapour deposition. When high frequency sputtering is used for forming the solid electrolyte film, LaF₃, cerium oxide or zirconia stabilized by yttria are suitable targets. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments which are given for explanation and understanding only and are not intended to imply limitation to the invention. In the drawings; Fig. 1 is a plan view of a conventional stacked cell; Fig. 2 is a partial longitudinal sectional view of stacked unit structures of a conventionally used fuel cell; Figs. 3 to 7 are enlarged cross sectional views showing a process of forming a thin film for a hydrogen electrode according to a procedure of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will now be described in detail referring to the accompanying drawings. Referring now to Fig. 1, a fuel cell body 10 is composed of a plurality of cell unit structures S, a pair of fixing plates 11a and 11b to connect the stacked cell units S in series, a hydrogen path 12 for providing hydrogen to an anode plate side of each cell unit S, an oxygen path 13 for providing oxygen to a cathode plate side of each cell unit S, and a pair of lead lines 14 and 15 to collect electric power from the cathode and the anode sides of each cell unit S. Both paths 12 and 13 which provide gas are installed on the outside of the cell body 10, and lead lines 14 and 15 are installed on the outside of each cell unit S. Electric power and water (as a by-product) are produced when hydrogen gas and air (oxygen gas), provided respectively, react through an electrolyte, then electric power produced is collected by the lead lines 14 and 15. Fig. 2 is an enlarged longitudinal sectional view of stacked fuel cell bodies as shown in Fig. 1 which utilize a type of solid electrolyte. A cell unit is composed of a lamination of a porous substrate 21 formed of stainless steel, a hydrogen electrode film (a first electrode film) 22, a solid electrolyte film 23, and an oxygen electrode film (a second electrode film) 24, which are laminated sequentially. The hydrogen electrode film 22 is formed on the porous substrate 21 as in the following example; Example 1 As the porous substrate for the base of the hydrogen electrode, SUS316L (JIS standard) having about a 40% void ratio, 0.5 µm nominal void size and a thickness of about 1 mm was used. Although the nominal void size is indicated as 0.5 µm, the actual void sizes are dispersed in a certain range, there being many voids having void sizes of about 10 µm, sometimes up to about 40 µm. The porous substrate 21 was punched to a disc having a diameter of 12.5 mm (0.5 inch), then the disc was subjected to ultrasonic cleaning in a trichlene solution followed by drying. The obtained substrate is shown in Fig. 3 diagramatically. Then nickel powder having a particle diameter of less than or equal to 1 µm (hereinafter, submicron diameter) and nickel powder having a particle diameter of 3 µm were mixed at a volume ratio of 1 : 1, and dispersed in water. The dispersion of the mixed nickel powders was applied to form a uniform coating on the porous substrate 21 as shown in Fig. 4. The applied coating was dried at ambient temperature, then sintered at 1000 °C for 1 hour in a hydrogen atmosphere. Thus, a first nickel film 41 was formed. The surface of the first nickel film 41 was polished as shown in Fig. 5, to remove projections therefrom. As an agent for the polishing, a # 600 grit paper was used. The polished nickel film 41 was subjected to ultrasonic cleaning in deionized water and trichloroethylene solution for 10 min, the cleaned film was thereafter dried at ambient temperature. Then about 50 mg of nickel powder having a particle diameter of 3µm was applied uniformly to the surface of the first nickel film 41 of the porous substrate 21, a pressure of about 6.9 x 10⁷ Pa (700 g kg cm⁻² where g is the acceleration due to gravity) was applied to the film, and the newly obtained film was sintered at 750 °C for 1 hour in a hydrogen atmosphere. Thus, a second nickel film 61 was formed as shown in Fig. 6. Nickel powder having a submicron diameter was rubbed into the surface of the second nickel film 61, and pressure of about 6.9 x 10⁷ Pa (700 g kg cm⁻² where g is the acceleration due to gravity) was applied to the film, nickel powder having a submicron diameter was again rubbed into the surface of the pressed film, and the obtained film was again sintered at 750 °C for 1 hour in a hydrogen atmosphere. Thus, a third nickel film 71 was formed as shown in Fig. 7. According to the above procedure, a hydrogen electrode film 22 having thickness of about 100 µm which was composed of the first nickel film 41, the second nickel film 61 and the third nickel film 71 laminated sequentially on the surface of the porous substrate 21, was obtained. Voids having diameter of 1 to 3 µm opened uniformly at the top surface thereof. After forming the third nickel film 71, platinum (Pt) was laminated on the surface of the third film 71 to a thickness of about 200 µm using sputtering. Sputtering was performed using a high frequency sputtering system targeting Pt under 6.65 Pa (5 x 10 ⁻² mmHg) of pressure for 1 hour in an argon (Ar) gas atmosphere. Coating of the surface of the third nickel film 71 by Pt is effective for promoting a reaction at the electrode indicated as follows, because Pt acts as catalyst; O²⁻ + H₂ = H₂O + 2e⁻ Therefore, high electric power can be produced easily and quickly in the cell when a coating of Pt is applied. Further, the preferred porous substrate may also be formed of nickel or copper. The porous substrate formed of nickel is formed by sintering the nickel powder, the void size of the obtained substrate becomes dispersed in a range of 3 to 50 µm as an irregular surface which is formed according to configuration of nickel powder. As nickel powder has high adherence to the nickel powder electrode film, it has higher water resistance than a substrate formed of stainless steel. Alternatively, the porous substrate can also be formed of copper powder. As the configuration of copper powder is round, configuration of the voids can be rounded and the void size thereof becomes 3 to 40 µm which is similar to nickel powder. The porous substrate using the copper powder becomes an insulator when contacted with oxygen, therefore it cannot be used as the oxygen electrode but is most preferrably used for the hydrogen electrode. The solid electrolyte 23 in Fig. 2 is formed as described in the following examples; Example 2 A 10 µm thick solid electrolyte film was formed on the Pt coated hydrogen electrode film 22 which was laminated on the porous substrate 21. An electron beam sputtering technique was used for forming the film, using a turbo-pump to sustain a vacuum of 1.3 x 10⁻⁶ Pa (10⁻⁸ mmHg). The temperature of the substrate was varied in a range of ambient to 580 °C, sputtering speed was controlled by a controller. As the material of the solidous electrolyte, single LaF₃ crystals were used, the solid electrolyte film was formed under conditions of 500 °C of substrate temperature, 2 nm/sec (20 Å/sec) of sputtering speed and -3.0 kV of accelerating voltage. Thus, a solid electrolyte film having no pin holes was obtained. Example 3 Resistance heating was used for forming the layer, using a turbo-pump as mentioned in the first procedure in a vacuum of 1.3 x 10⁻⁶ Pa (10⁻⁸ mmHg). The temperature of the substrate was determined at 400°C, with a sputtering speed of 0.3 to 0.5 nm/sec (3 to 5 Å/sec). The forming of the film was performed over about 5 to 6 hours. A 10 µm thick solidous electrolyte film having no pin holes was obtained. La 1-x SrF 3-x can also be used as material for the solid electrolyte. When X-ray diffraction analysis was applied to the film obtained from La 0.95 Sr 0.95 F 2.95 , only a peak of LaF₃ was found. From this result, the obtained film of solid electrolyte was not formed of a mixture of LaF₃ and SrF₂ but only of LaF₃. Example 4 A magnetron sputtering technique was applied to the forming, at a 400 °C substrate temperature under a pressure of 0.7 Pa (5.3 x 10 ⁻³ mmHg) in an Ar atmosphere. Sputtering was performed over 40 hours using a target of LaF₃ powder. A 10 µm film thickness was obtained. It was concluded from the results of X-ray diffraction tests, that the obtained film was formed an LaF₃ polycrystal having poor crystaline quality. As the solid electrolyte material it is preferrable to use the following compounds La 0.95 Sr 0.05 F 2.95 , La 0.90 Sr 0.10 F 2.90 , La 0.95 Ba 0.05 F 2.95 , or La 0.90 Ba 0.10 F 2.90 . The composition of solid electrolyte films obtained by sputtering techniques are similar to the composition of the original materials before sputtering even if materials having complex composition are used. Therefore, the sputtering method is preferred to thinner lamination qualities of materials having complex compositions such as La 0.95 Sr 0.05 F 2.95 . Example 5 A metal organic chemical vapor deposition (MOCVD) technique was applied to forming the film of solid electrolyte. Metal organic compounds including La and F in its molecule were heat-decomposed, then the obtained film of LaF₃ was laminated on the Pt coated surface of the third nickel film 71 formed on the porous substrate 21 as shown in Fig. 7. The structural formula of the compound is following; The film forming was accomplished under the following conditions : the temperature of the substrate was determined at 600°C, the temperature of the metal organic compounds was maintained at 230 °C, with argon gas (Ar) as a carrier gas at flow rate of 100 ml/min. The stream of the metal organic compounds in the reactor was moved to the surface of the porous substrate 21 to react with it, thus a film of LaF₃ was obtained. Example 6 A high frequency sputtering technique was used to form the film of solid electrolyte, using a high frequency sputtering system at a substrate temperature of 800 °C, under an Ar pressure of 0.7 Pa (5.3 x 10⁻³ mmHg). For a target, zirconia stabilized by addition of yttria was used. The sputtering was performed over 40 hours, and a 10 µm thickness of film of solid electrolyte having no pin holes was obtained thereafter. Compounds such as cerium oxide may also be used. The oxygen electrode film 24 is formed as in the following examples; Example 7 The oxygen electrode film was formed of perovskite compound. A perovskite compound of La 0.6 Sr 0.4 CoO x was obtained by mixing powder of cobalt acetate (CH₃COO)₂Co 4H₂O , lanthanum acetate (CH₃COO)₂La and strontium acetate (CH₃COO)₂Sr, which amounts thereof were weighed corresponding to a composition ratio of La 0.6 Sr 0.4 CoO x . The mixture of powder as above described was baked at 1000 °C for 5 hours in an oxygen atmosphere. The electric resistance ratio of the obtained perovskite compound was 4.4 ohm·cm. The perovskite compound was dissolved in propylene glycol, then this solution was applied to the surface of the solidous electrolyte film 23, baked under pressure at 300 °C for 8 hours in an oxygen atmosphere over 8 hours. Thus, an oxygen electrode film 24 as shown in Fig. 2 was obtained. Example 8 A perovskite compound was formed as shown in Example 7. The obtained compound was mixed with platinum black at a ratio of 3:1, then dissolved in propylene glycol. The mixture was applied to the surface of the solid electrolyte film 23, then baked under the same conditions described in Example 7, thus an oxygen electrode film 24 was obtained. Example 9 A perovskite compound was formed as shown in Example 7. The obtained compound was then sputtered on the surface of the solid electrolyte film 23 using a high frequency sputtering system. The sputtering was accomplished in an Ar atmosphere under a pressure of 1.3 Pa (1 x 10⁻² mmHg), at 0.5 µm/hour sputtering speed for 2 hours, thus about a 1 µm thickness of oxygen electrode film 24 was obtained. The perovskite compound as above mentioned has similar characteristics to platinum but is much less expensive. Example 10 Ag powder was dissolved in propylene glycol then the obtained solution was applied to the surface of the solid electrolyte film 23. The film was baked under pressure at 300 °C in an oxygen atmosphere for 8 hours. Thus, an oxygen electrode film 24 was obtained. Example 11 Chloroplatinic acid (H₂PtCl₆) was dissolved in propylene glycol. The obtained solution was treated as described in Example 10, thus, an oxygen electrode film 24 was obtained. Referring now again to Fig. 2, the porous substrate, the hydrogen eletrode film, the solidous electrolyte film, and the oxygen electrode film formed as previously described are stacked sequentially to form a cell unit structure. The cell unit is installed in a cell casing 25 having conductivity to electrically connect the hydrogen electrode film 22 of the cell unit with the casing 25. Concurrently, the oxygen electrode film 24 is connected electrically with a separator 27 which is adhered to the cathode side. A insulator 26 is installed between the cell casing 25 and the separator 27. Thus, a fuel cell body 40a is constructed. At the side of the porous substrate 21 of the cell body 40a, a conductive separator 27′ is connected electrically with the cell casing 25 concurrently with the oxygen electrode film 24′ at both sides of the separator 27′. A insulator 26′ is installed between the separator 27′ and the casing 25′, thus a fuel cell body 40b is constructed and stacked with the cell body 40a. Similarly, fuel cell bodies 40c, 40d...(not shown in Fig. 2) are constructed and stacked sequentially. Therefore, a plurality of the cell units can be stacked connected in series. Oxygen, as an oxidant, is provided from gas inducing paths 29 and 29′ to the cells, and hydrogen as fuel is provided from gas inducing paths 30 and 30′ to the cells, thus, electric power is produced. According to the present invention, as the hydrogen electrode is formed of a fine mixture of powders of nickel then sintered, void sizes can be homogenized. Thus, formation of pin holes on the films laminated on the electrode can be avoided even if a very thin solidous electrolyte film is laminated. Accordingly, the voltage drop can be reduced by reducing the thickness of the solid electrolyte. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principles thereof. Therefore, the invention should be understood to include all possible embodiments and modifications to shown embodiments which can be embodied without departing from the principle of the invention as set out in the appended claims.

Claims: A fuel cell constructed from a stack of cell unit structures (40a, 40b,..) which are connected in series, the cell unit structures comprising: a porous substrate (21); a hydrogen electrode film (22) formed on the porous substrate (21) and comprising three nickel films: a first nickel film (41) formed on the porous substrate (21) and made from a mixture of a first nickel powder having a particle diameter of less than or equal to 1µm and a second nickel powder having a particle diameter of approximately 3µm, wherein the mixture comprises equal volumes of the first and second nickel powders; a second nickel film (61) formed on the first nickel film (41) and made from the second nickel powder; and a third nickel film (71) formed on the second nickel film (61) and made from the first nickel powder; a coating of platinum formed on the third nickel film (71) of the hydrogen electrode film (22); a solid electrolyte film (23) formed on the coating of platinum; an oxygen electrode film (24) formed on the solid electrolyte film (23). A fuel cell as set forth in claim 1, wherein the porous substrate (21) is formed from one of stainless steel, nickel powder or copper powder. A fuel cell as set forth in claim 1, wherein the solid electrolyte film (23) has a thickness of 10µm. A process for forming a fuel cell constructed from a stack of cell unit structures (40a, 40b,...) which are connected in series, the process comprising the steps of: preparing a porous substrate (21) which forms the base of the cell unit structure; forming a first nickel film (41) on the porous substrate (21), the first nickel film (41) being made from a mixture of a first nickel powder having a particle diameter of less than or equal to 1µm and a second nickel powder having a particle diameter of approximately 3µm, wherein the mixture comprises equal volumes of the first and second nickel powders; forming a second nickel film (61) made from the second nickel powder on the first nickel film (41) forming a third nickel film (71) made from the first nickel powder on the second nickel film (61); sputtering platinum onto the third nickel film (71) to form a coating of platinum for promoting electron generation in the working fuel cell; forming a solid electrolyte film (23) on the coating of platinum; and forming an oxygen electrode film (24) on the solid electrolyte film (23). A process for forming a fuel cell as set forth in claim 4, wherein the porous substrate (21) is formed from one of stainless steel, nickel powder or copper powder. A process for forming a fuel cell as set forth in claim 4, wherein the first, second and third nickel films (41, 61, 71) are laminated one after another by the steps of: pressing and sintering a mixture of the first nickel powder and the second nickel powder, wherein the mixture comprises substantially equal volumes of the first and second nickel powders, to form a first nickel film (41); polishing the surface of the first nickel film (41); applying a quantity of the second nickel powder to the first nickel film (41) and then pressing and sintering the second nickel powder to form a second nickel film (61); rubbing a quantity of the first nickel powder into the surface of the second nickel film (61) and then pressing and sintering the first nickel powder to form a third nickel film (71), this third nickel film (71) having fine voids of homogeneous diameter on its surface. A process for forming a fuel cell as set forth in claim 4, wherein the sputtering of platinum is accomplished by a high frequency sputtering technique. A process for forming a fuel cell as set forth in claim 4, wherein the solid electrolyte film (23) is formed from single crystals of LaF₃. A process for forming a fuel cell as set forth in claim 4, wherein the solid electrolyte film (23) is formed using one of the following techniques: high frequency sputtering, electron beam sputtering, resistance heating evaporation, magnetron sputtering or metallo organic chemical vapour deposition. A process for forming a fuel cell as set forth in claim 4, wherein the solid electrolyte film (23) is formed using high frequency sputtering, characterized in that the sputtering uses a target selected from LaF₃, cerium oxide and zirconia stabilized by yttria. A process for forming a fuel cell as set forth in claim 4, wherein the solid electrolyte film (23) has a thickness of 10µm.

Inventors: FUSHIMI KAZUO, KAWAKAMI KAZUHIKO, KITAKIZAKI KAORU, FUSHIMI, KAZUO, KAWAKAMI, KAZUHIKO, KITAKIZAKI, KAORU
Applicants: MEIDENSHA ELECTRIC MFG CO LTD, KABUSHIKI KAISHA MEIDENSHA
Application year: 1990
Main classification: H01M8
IPC codes: H01M8, H01M4, C23C16, C23C14