JP2770541B2 - Solid electrolyte fuel cell - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a solid oxide fuel cell, and more particularly to improving its reliability.

[Prior art] A fuel cell using a general solid electrolyte (hereinafter, referred to as a fuel cell)
The operating principle of the SOFC will be described. Fig. 4
FIG. 4 is an explanatory diagram showing the operation principle of a unit cell (10) constituting the SOFC. In the figure, (11) is a solid electrolyte part made of yttria-stabilized zirconia (YSZ) and has oxygen ion conductivity, and (12) and (13) are a pair of electrode reactions formed by interposing the electrolyte part (11). Department. The fuel electrode portion (12) H ₂ as a fuel gas (a), the oxygen electrode portion (13) is supplied with air or O ₂ as the oxidant gas (b), causing an electrochemical reaction. (16) is an electric circuit for extracting electric energy to an external load (17). Arrow (c) indicates the flow of oxygen ions moving in the solid electrolyte (11) during the electrochemical reaction, and arrow (d) indicates the flow of electrons extracted to the external load (17) of the SOFC.

Next, the operation will be described. The oxidizing gas is supplied with electrons at the oxygen electrode section (13), becomes oxygen ions (reaction 1), and is taken into oxygen ion vacancies in the solid electrolyte section (11). Oxygen ions diffuse in the solid electrolyte (11), react with hydrogen at the fuel electrode (12), and generate water vapor and electrons (reaction 2). Each of these reactions is represented by the following formula.

Reaction ^{_{1: O 2 + 4e - →}} 2O 2- reaction ^{_{2: 2H 2 + 2O 2- →}} 2H 2 O + 4e - Furthermore, if the synthesis reaction 1 and reaction _{2, 2H 2 + O 2 →} 2H 2 O , and the this hydrogen It is similar to the combustion reaction. In this case, the device is usually operated at a high temperature of about 1000 ° C. in order to conduct oxygen ions.

Next, a practical battery configuration will be described. FIGS. 5 and 6 show, for example, the publication “IEEJ Transactions on Papers B, 106, 8
No., pp. 693 to 700, pp. 693 to 700, which is a perspective view and an enlarged cross-sectional view of what is called a conventional cylindrical horizontal stripe SOFC. In FIG. 5, (10) is a cylindrical unit cell, and (20) is a connection unit for connecting the unit cell (10). As described above, a plurality of unit cells (10) are connected in series to one cylinder to form a fuel cell stack. In this example, the fuel gas (a) is supplied to the inside of the cylindrical tube, and the oxidizing gas (b) is supplied to the outer surface of the cylinder. FIG. 6 is a partially enlarged cross-sectional view of FIG. 5, in which (11) is a solid electrolyte portion made of yttria-stabilized zirconia (YSZ);
2) Fuel electrode part composed of cermet of Ni and YSZ, (13)
Is an oxygen electrode portion made of LaCoO ₃ , (14) is an intermediate connector made of Ni, and (15) is a porous substrate tube made of alumina (Al ₂ O ₃ ). On the surface of the base tube (15), a solid layer constituting each of the above battery elements and the like is formed as a thin film by a thermal spraying method or the like.
FIG. 7 is a perspective view including a cross section of an embodiment of a cylindrical SOFC called a vertical stripe type described in, for example, JP-A-57-130381. In the figure, (21) is a solid electrolyte part made of yttria-stabilized zirconia (YSZ), (22) is a fuel electrode part made of a cermet of nickel (Ni) and zirconia (YSZ), and (23) is strontium, for example. An oxygen electrode portion made of LaMnO ₃ or the like doped with, and (24) is a base layer made of calcia-stabilized zirconia (CSZ), on which a solid layer constituting a battery element or the like is formed as a thin film. (2
5) is an intermediate connection layer made of LaCrO ₃ doped with magnesium (Mg) or the like, and (26) is an arcuate layer made of the same material as the fuel electrode section (22). FIG. 8 is a side view showing an example of a stack in which the unit cells formed of one cylinder are arranged and connected via a Ni metal felt (27). In this example, since the metal felt (27) is Ni and a reducing atmosphere is desirable, the oxidizing gas (b) is supplied inside the cylinder and the fuel gas (a) is supplied outside the cylinder. Calcia stabilized zirconia (CSZ) is used as the base layer (24).

FIG. 9 is a sectional view collectively showing the structure of the substrate of the SOFC as described above. FIG. 9 (a) shows a type having no base. In this type, the electrolyte part (1
1) consisting of a fuel electrode (12) and an oxygen electrode (13)
In order to maintain the mechanical strength of the layer structure, the fuel electrode (1
2) In some cases, one of the oxygen electrode sections (13) may be made thicker. 9 (b) and (c) each have a supporting base (15), and FIG. 9 (b) shows a supporting base (1).
5) is on the side of the fuel electrode section (12), which is the same as the conventional example shown in FIG. In FIG. 9 (c), the support base (15) is on the side of the oxygen electrode portion (13), which is the same as the conventional example shown in FIG. Fig. 9 (a),
Both cases (b) and (c) are consistent in that electrodes or a porous film of the base are present on both sides of the electrolyte part (11).

[Problems to be Solved by the Invention] A conventional solid electrolyte fuel cell is configured as described above, and a porous substrate tube or any one of the electrodes is used as a strength retaining material, and a battery such as an electrolyte portion or an electrode is formed thereon. Although forming a thin film of components, during operation, fuel gas and oxidant gas must be transported across the substrate tube and porous electrode to the electrolyte interface. For this reason, different pressures generally appear at both ends of the porous material, that is, at the interface between the gas flow path and the electrolyte. Therefore, even if the pressure of the fuel gas flow path and the pressure of the oxidizing gas flow path are the same, a large pressure difference occurs at both interfaces of the electrolyte, and if there is a minute defect in the electrolyte, the pressure difference is driven. The flow of gas (leakage) generated by the force occurs, and the fuel gas and oxidant gas, which should be completely separated, mix,
It will burn more. This not only reduces the power generation efficiency, but also causes many problems such as generation of local thermal stress due to heat generated by combustion.

The present invention has been made to solve the above-described problems, and maintains the pressure in the fuel gas flow path and the oxidizing gas flow path at a predetermined appropriate value to thereby reduce the electrolyte portion. The pressure on the electrolyte is eliminated by making the total pressure of the gas at both interfaces substantially equal. Therefore, even if a defect occurs in the electrolyte portion, it is an object of the present invention to obtain a highly reliable solid electrolyte fuel cell in which gas leakage therefrom can be suppressed to a minimum and a decrease in power generation efficiency can be prevented.

[Means for Solving the Problems] A solid electrolyte fuel cell according to the present invention includes a fuel electrode portion and an oxygen electrode portion with an electrolyte portion interposed therebetween, supplies fuel gas to the fuel electrode portion, and oxidizes the oxygen electrode portion. Output current density detecting means for detecting an output current density in a solid electrolyte fuel cell in which an electrochemical reaction is caused by supplying an agent gas, and a fuel electrode section of an electrolyte section based on the current density detected by the output current density detecting means Determining means for determining the pressures of the fuel gas and the oxidizing gas from the current density and the pressure data of the gas flow path, which equalize the total pressure of the gas at both interfaces of the gas electrode and the oxygen electrode part, and this pressure. The apparatus is provided with pressure adjusting means for adjusting the pressure of one or both of the fuel gas and the oxidizing gas in accordance with the determining means.

[Operation] The solid electrolyte fuel cell according to the present invention controls the gas pressure in the fuel gas passage and the oxidizing gas passage so as to maintain the gas pressure at a predetermined value. The total pressure of the gas at the two-sided interface can be equalized.

Embodiment An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a solid oxide fuel cell according to one embodiment of the present invention. In the figure, (11) is a solid electrolyte part (hereinafter, referred to as an electrolyte), and (12) is a fuel electrode part (hereinafter, referred to as an electrolyte).
(Referred to as fuel electrode), (13) is an oxygen electrode part (hereinafter referred to as oxygen electrode), (17) is a load, (40) is a fuel gas flow path,
(41) is an oxidizing gas passage. (50) is the substrate, (60)
Is a pressure regulating valve provided in the fuel supply system, (61) is a pressure regulating valve provided in the air supply system, and (62) is output current density detecting means for detecting an output current density. In this case, it is an ammeter. . (63) is a controller having both functions of a pressure determining means and a pressure adjusting means. That is, based on the current density detected by the ammeter (62), the memory (64) is set in the memory (64) so that the total pressure of the gas at both the fuel electrode side and the oxygen electrode side interface of the electrolyte (11) is made substantially equal. The pressures of the fuel gas and the oxidizing gas are determined based on data stored in advance.
Further, the controller (63) controls the opening of the pressure regulating valves (60) and (61) in order to regulate the pressure of one or both of the fuel gas and the oxidizing gas according to the determined pressure value.

In this embodiment, yttria-stabilized zirconia (YSZ) having oxygen ion conductivity is used as the electrolyte (11), hydrogen is used as the fuel gas, and air is used as the oxidant gas. In the figure, arrows (a) and (b) indicate the flow of fuel gas and air, arrows (c), (d) and (e) indicate diffusion of oxygen gas, hydrogen gas and water vapor, and arrow (f) indicates the electrolyte (11). 2) shows the diffusion of oxygen ions inside. This embodiment shows a SOFC corresponding to FIG. 9 (c).

Next, the operation will be described. The basic power generation operation is the same as that of the conventional fuel cell. Hereinafter, pressure control in the controller (63) which is a main part of the present invention will be described. In FIG. 1, P _{A, F} , P _{C, F} indicate the pressure (total pressure) in the gas flow paths (40), (41), and P _A , P _C are the interface between the electrolyte and the fuel electrode (12). It shows the total gas pressure at the oxygen electrode (13) side interface.

P _A as P _A = P _C is satisfied in SOFC according to this _{embodiment, F} and P _{C, F} is the pressure adjusting valve (60) is not adjusted by (61). In a fuel cell using an oxygen ion conductive electrolyte, hydrogen diffuses in the fuel electrode (12) from the fuel gas flow path (40) toward the electrolyte (11) on the fuel electrode (12) side. The following reaction occurs at the interface between the fuel electrode (12) and the electrolyte (11).

H ₂ + O ²⁻ → H ₂ O + 2e ^— The water vapor H ₂ O generated by this reaction diffuses in the fuel electrode (12) in a direction opposite to that of hydrogen and is discharged into the fuel gas flow path (40). On the oxygen electrode (13) side, oxygen gas diffuses through the substrate (50) and the oxygen electrode (13) from the oxidizing gas flow path (41) toward the electrolyte (11), and is reacted by the following reaction. It becomes 2O ^2- and is taken into the electrolyte (11).

O ₂ + 4e ⁻ → 2O ²⁻ At this time, the following relationship holds for the pressure at the interface.

Fuel electrode (12) side: P _{A, F} <P _A oxygen electrode (13) side: P _{C, F>} P _C Therefore, if the pressure of the two gases supplied to the fuel cell (fuel gas and air) is equal to In this case, that is, when P _{A, F} = P _{C, F} , the pressures P _A , P _C at the electrolyte interface are P _A > P _C. At the electrolyte (11) interface, the pressure on the fuel electrode (12) side increases, and if the electrolyte (11) is not completely dense, the fuel gas leaks to the oxygen electrode (13) side and burns there, generating electricity. Efficiency decreases. Therefore, in order to solve the above-mentioned problems, it is necessary to supply two kinds of gases so that P _{A, F} <P _{C, F.}

The following describes how much pressure difference should be applied to the two supply gases. For this prediction, this embodiment uses the theory of non-isobaric diffusion in the pores. FIG. 2 is an explanatory diagram showing a model for explaining non-isobaric diffusion in pores, and shows the transport of two types of gas in a porous material.
(65) indicates a porous material, (66) indicates an electrolyte interface, and arrow g indicates a gas flow. As the two types of gases, oxygen (O ₂ ) and nitrogen (N ₂ ) are considered for the oxygen electrode (13), and hydrogen (H ₂ ) and water vapor (H ₂ O) are considered for the fuel electrode (12). As a preferred material, in the SOFC shown in FIG. 1, a porous fuel electrode (12) is provided on the fuel electrode side, and a porous substrate (50) and a porous oxygen electrode (13) are combined on the oxygen electrode side. In the following description, the suffixes 1 and 2 represent two types of gas, the suffixes 0 and L relate to the coordinate Z taken in the thickness direction of the porous material (65), and Z = 0 represents the gas flow side of the porous material (65). And Z = L represents the interface (66) on the electrolyte side of the porous material (65).
P indicates the total pressure, and J indicates the molar flux of the gas.

Next, a calculation method will be described. The two-component non-equal pressure pore diffusion system shown in FIG. 2 is expressed by the following binary simultaneous differential equation.

_{J 1 = (1 / e 1} ) · [- (P / RT) · D 1K · (dx 1 / dz) - (x 1 / RT) · {D 12 + D 2K + D 2K (x 1 / D 1K + x 2 / D _2K ) ・ (B ₀ P / η)｝ ・ ∂P / ∂Z] J ₂ = (1 / e ₂ ) ・ [-(P / RT) ・ D _2K・ (dx ₂ / dz)-(x ₂ / RT) · ｛D ₁₂ + D _1K + D _1K (x ₁ / D _1K + x ₂ / D _2K ) · (B ₀ P / η)｝ · ∂P / ∂Z] where R is the gas constant and T Is the absolute temperature, η is the viscosity coefficient of the mixed gas, and x ₁ and x ₂ are the mole fraction of gas 1 and 2 (x ₁ + x ₂ =
1), D _1K, D _2K is effective Knudsen diffusion coefficient of the pores of the gas _{1,2, D iK = (d /} 3) · (ε / τ) · (8RT / πM 1) 1/2 (i = 1,2). Here, d, ε, τ are characteristic parameters of the porous material (65), d is a representative pore diameter, ε is a porosity, and τ is a maze factor. Further, M _i is the molecular weight of the gas i. D ₁₂ is the effective mutual diffusion coefficient of the gas 1, 2, B ₀ is a parameter relating to viscosity flow of the porous material (65). Further, e ₁ = x ₂ + x ₁ (D _2K / D _1K ) + (D ₁₂ / D _1K ) e ₂ = x ₁ + x ₂ (D _1K / D _2K ) + (D ₁₂ / D _2K ). This simultaneous differential equation is defined as a boundary condition, that is, the total pressure P _{0 at} Z = ₀ (this corresponds to the pressure in the gas flow),
And Z = L by solving for a given molar flux J ₁ , J ₂ under the mole fraction x _1,0 , x _2,0 (= 1−x _1,0 ) at Z = 0. total pressure P _L is obtained in the.

Next, how to give J ₁ and J ₂ will be described. First, the case of the fuel electrode (12) will be described. In the fuel electrode (12), the suffix 1 is hydrogen and the suffix 2 is steam. At the time of power generation, equimolar hydrogen and water vapor should have diffused in the opposite directions, so the following equation holds.

J ₂ = −J ₁ The value of J ₁ is given from the current density i, which is one of the important operating parameters of the SOFC. That is, J ₁ = i / 2F, where F is a Faraday constant.

Next, the case of the oxygen electrode (13) will be described. In the oxygen electrode (13), the subscript 1 is oxygen and the subscript 2 is nitrogen. During power generation, oxygen according to the current density is diffused in the porous material (65) from the gas flow to the electrolyte interface (66). However, since nitrogen does not participate in the reaction, this net molar flux is zero.

J ₂ = 0, and J ₁ is given by J ₁ = i / 4F similarly to the fuel electrode (12).

Next, an example of a calculation result will be described. FIG. 3 shows the pressure at the electrolyte interface (66) when two kinds of porous base tubes of alumina (Al ₂ O ₃ , solid line in the figure) and calcia-stabilized zirconia (CSZ, dotted line in the figure) were used for SOFC. It is a thing. The horizontal axis is the current density (A / cm ² ), and the vertical axis is the pressure (atm) at the electrolyte interface. The properties of this porous substrate tube are shown in the table. The temperature is set to 1000 ° C., and the physical properties of various gases are also determined from the main peaks for estimating predetermined physical constants.

In FIG. 2, Case 1 is a case where the porous tube is on the fuel electrode (12) side (corresponding to FIG. 9 (b)), and Case 2 is a case where the porous tube is on the oxygen electrode (13) side (FIGS. 1 and 2). (Corresponds to Fig. 9 (c))
Is shown. The calculation conditions are as follows: the pressure in the gas flow is 1 atm, and the mole fraction in the gas flow is _xH2 = 0.
425 (x _H2O = 0.575), in the case of _{Case2 x O2 = 0.21 (x N2} =
0.79). Electrolyte interface with increasing current density (66)
It can be seen that the pressure at the point deviates from the pressure (= 1) in the gas flow. Thus, for example, in an SOFC in which an Al ₂ O ₃ tube is provided as a base tube on the fuel electrode (12) side, the current density is reduced.
When the pressure is 0.5 A / cm ² , the pressure at the electrolyte interface (66) on the fuel electrode (12) side is higher than that in the fuel gas flow channel by about 0.03 atm. The same calculation is performed on the oxygen electrode (13) side to calculate how much the pressure drops in the oxidizing gas channel at the electrolyte interface (66) on the oxygen electrode (13) side. Based on these values, it is known how to set the pressure in the flow path of the two supply gases so that the pressure difference at both interfaces of the electrolyte (11) can be made substantially zero.

The above calculation is performed in advance, and the pressure of the gas flow path with respect to the current density is stored in the memory (64). During power generation, the controller (63) inputs the current density detected by the ammeter (62), knows the corresponding pressure value from the memory (64), controls the pressure regulating valves (60) and (61), The pressure in the flow path is controlled to a predetermined pressure. Thereby, the gas pressure in the fuel gas flow path and the oxidizing gas flow path can be maintained at a predetermined pressure, and the total pressure of the gas at both interfaces of the electrolyte on the fuel electrode side and the oxidizing electrode side can be equalized. Therefore, even when the electrolyte (11) is not completely dense, gas leakage through the electrolyte (11) can be minimized, combustion due to gas mixing can be reduced, reliability can be improved, and efficiency can be improved. Drop can be prevented.

In the above embodiment, the pressure regulating valves are provided in both the fuel gas and the oxidizing gas supply channels. However, the pressure regulating valve may be provided only in the other channel while keeping one pressure constant.
Further, in the above-described embodiment, the configuration is such that both gas flow paths are maintained at a preset pressure calculated in advance. However, the pressure in both gas flow paths may be dynamically controlled according to a load change.

[Effects of the Invention] As described above, according to the present invention, a fuel electrode portion and an oxygen electrode portion are provided with an electrolyte portion interposed therebetween, a fuel gas is supplied to the fuel electrode portion, and an oxidant gas is supplied to the oxygen electrode portion. Output current density detecting means for detecting an output current density in a solid electrolyte fuel cell causing an electrochemical reaction, and a fuel electrode side of an electrolyte part and an oxygen electrode based on the current density detected by the output current density detecting means. Pressure determining means for determining the pressures of the fuel gas and the oxidizing gas from the current density and the pressure data of the gas flow path, which equalize the total pressure of the gas at both interfaces on the side of the unit, and the fuel according to the pressure determining means Gas pressure adjusting means for adjusting the pressure of one or both of the gas and the oxidizing gas minimizes gas leakage through the electrolyte even when the electrolyte is not completely dense. Thus, there is an effect that a solid electrolyte fuel cell can be obtained, in which combustion due to gas mixing is reduced, reliability can be improved, and a decrease in efficiency can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a solid oxide fuel cell according to an embodiment of the present invention, and FIG. 2 is for explaining calculation of a set pressure in a pressure determining means according to an embodiment of the present invention. FIG. 3 is a graph showing a calculation result according to one embodiment of the present invention as a relationship between a current density and a pressure at an electrolyte interface. FIG. 4 is an explanatory diagram showing an operation principle of a solid electrolyte fuel cell. 5 and 6 are a perspective view and a cross-sectional view showing a conventional cylindrical horizontal stripe type solid electrolyte fuel cell, and FIGS. 7 and 8 are a perspective view and a side view showing a conventional cylindrical horizontal stripe type solid electrolyte fuel cell. FIG. 9 is a sectional view showing a solid oxide fuel cell by type. (11) ... solid electrolyte, (12) ... fuel electrode, (13) ...
... oxygen electrode, (60), (61) ... pressure regulating valve, (62) ...
... ammeter, (63) ... controller. The same reference numerals indicate the same or corresponding parts.

Claims (1)

(57) [Claims] 1. A solid material comprising a fuel electrode portion and an oxygen electrode portion with an electrolyte portion interposed therebetween, supplying a fuel gas to the fuel electrode portion, and supplying an oxidizing gas to the oxygen electrode portion to cause an electrochemical reaction. In the electrolyte fuel cell, output current density detecting means for detecting the output current density, based on the current density detected by the output current density detecting means, at both interfaces of the electrolyte part on the fuel electrode part side and the oxygen electrode part side Pressure determining means for determining the pressures of the fuel gas and the oxidizing gas from the current density and the pressure data of the gas flow path which make the total pressure of the gas equal, and the fuel gas and the oxidizing agent according to the pressure determining means. A solid electrolyte fuel cell comprising gas pressure adjusting means for adjusting the pressure of one or both of the agent gases.