JPH0927327A - Fuel cell electrode and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel cell electrode with high durability to impurities in a fuel gas such as CO even when the concentration of CO in the fuel gas is high, or even when a fuel cell is operated at relatively low temperature. SOLUTION: A catalyst is formed with Pt supported on carbon and noble metal Pd on one surface of a porous carbon plate to form an electrode. The electrode is immersed in the dilute solution of a chloride or a sulfide of a base metal element X which is easy to adsorb oxygen. By adding a liquid phase reducing agent into the dilute solution, the base metal element is deposited by reduction on the electrode surface. The layer of the base metal element is made extremely thin in a single atom level or a fine particle level. Since a CO molecule is not adsorbed on the base metal atom, the adsorption amount of CO is very small. Since a water molecule adsorbed on the base metal element functions as an oxidizing agent of CO, oxidation removing reaction of CO is easy to occur.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode that undergoes an oxidation or reduction reaction in a fuel cell and a method for manufacturing the same, and in particular, it is resistant to CO and other impurities which are poisonous substances for the surface catalyst. And a method for producing the same.

[0002]

2. Description of the Related Art A fuel cell is a device for directly converting the energy released by an oxidation reaction into electric energy by oxidizing the fuel in an electrochemical process, and the chemical energy of the fuel can be effectively used. Therefore, high thermal efficiency of 40 to 50% can be expected even on a relatively small scale. Further, since the power generator does not include a combustion cycle, the emission amount of SOx and NOx, which have become a major social problem as a pollution factor in recent years, is extremely small. Further, it does not require a large amount of cooling water, and has a characteristic that it is extremely excellent in environmental characteristics because of its small noise and vibration. Furthermore, since the heat generated by power generation is easy to use for hot water supply and heating / cooling, there is also an advantage that the cogeneration system can improve the overall energy efficiency.

Such a fuel cell is generally constructed as follows. That is, as shown in FIG. 4, the unit cell 1 is configured by sandwiching the electrolyte layer 1c, which holds the electrolyte, between the pair of the fuel electrode 1a and the oxidant electrode 1b, which are made of a porous material. Since the output voltage per unit cell 1 is low, the unit cell 1 is connected via the gas separation plate 3.
Are stacked and connected in series to each other to form the fuel cell stack 2, which is configured to increase the output voltage as a whole. In such a fuel cell, the fuel gas is brought into contact with the back surface of the fuel electrode as a reaction gas,
An electrochemical reaction is caused by bringing an oxidant gas as a reaction gas into contact with the back surface of the oxidant electrode to take out electric energy from between the electrodes. Examples of electrolytes include acidic solutions, molten carbonates, and alkaline solutions. At present, phosphoric acid fuel cells using phosphoric acid are considered to be the most practical.

The electrode used in such a phosphoric acid type fuel cell is obtained by binding a platinum catalyst dispersed and supported on the surface of carbon black particles with polytetrafluoroethylene (trade name: Teflon) to form an electrolyte of a porous carbon plate. Manufactured by applying to the side surface. The electrode manufactured in this way has gas permeability, but since polytetrafluoroethylene has water repellent property and carbon and platinum have hydrophilic property, the phosphoric acid aqueous solution has a surface inside. It is held by tension and does not permeate to the other side.

The fuel gas of such a phosphoric acid fuel cell is a mixed gas of CO and H ₂ . This mixed gas is a mixture of natural gas (hydrocarbon such as methane) as a raw material, ZnO
Desulfurization treatment is performed using a catalyst such as NiO, and then a steam reforming reaction is performed using a catalyst such as NiO. For example, when methane is a raw material, first, the reaction as shown in the following formula 1 is performed.

[0006]

[Equation 1] Since this reaction is an endothermic reaction and the number of moles of the reactive species increases, high temperature (700 to 750 ° C.) and high pressure (1 to 40)
The reaction proceeds to the right as the temperature becomes atm). Following the reaction of the above formula 1, a shift reaction of oxidizing CO with steam is performed as in the following formula 2.

[0007]

[Equation 2] Alternatively, a methanation reaction, which is a reverse reaction of the reaction of (formula 1) for reducing CO, is performed. The mixed gas of H ₂ and CO ₂ generated by the reaction of the above formula 2 is used as the fuel gas supplied to the fuel electrode of the cell body during the power generation operation.

By the way, when the shift reaction of the formula (2) does not proceed efficiently due to a malfunction of the catalyst or when a hydrocarbon having a large number of carbon atoms is used due to the diversification of the fuel gas, the shift reaction of the formula (1) is used. As the required amount of water vapor increases, (CO ₂ + C
The ratio of O) / H ₂ increases, and the proportion of CO gas mixed in the fuel gas increases.

When the fuel gas containing a large amount of CO gas is supplied to the fuel electrode as described above, CO is strongly adsorbed on the platinum catalyst of the fuel electrode, so that the surface area on which H ₂ is adsorbed is crushed by CO. Poisoning of the catalyst that deteriorates the characteristics of the fuel electrode will occur. Therefore, there is a need for a catalyst that can suppress the deterioration of the fuel electrode characteristics due to poisoning even when a large amount of CO is mixed in the fuel gas.

As such a catalyst, conventionally, an alloy catalyst of platinum and other noble metal elements, for example, Pt / R is used.
u ¹⁾ , Pt / Rh ²⁾ , Pt / Pd ^{3) and the} like have been used. Regarding 1), please refer to P. N. Ross, K .; Ki
noshita, A .; J. Scarpellino a
nd P.N. Stonehart, J. et al. Electroa
nal. Chem 63 97 (1975), 2), idem, ibid, 59 177 (197).
Regarding 5) and 3), P. Stonehart, In
t. J. Hydrogen Energy 9 921
(1984), idem, USP 4,407,90.
6 (1983).

These alloy catalysts are used because CO is not adsorbed on the noble metal element portion, so that the amount of CO adsorbed decreases, while water molecules, such as acids adsorbed on the alloy compound, are not adsorbed on the catalyst surface. This is because it serves as an oxidant for the CO absorbed by and is promoted to remove CO by oxidation. The reason why noble metal is used as an element to be alloyed with platinum is that it exhibits the same high activity as hydrogen with respect to the oxidation reaction of hydrogen. Furthermore, the use of such an alloy catalyst has the advantage that the amount of platinum, which is very expensive, is reduced, and the production cost of the catalyst is reduced.

[0012]

However, in recent years,
CO and NH in fuel gas due to diversification of fuel
The concentration of impurities such as ₃ , H ₂ S and the like increases more and more, and catalysts having higher resistance than conventional alloy catalysts using two elements have been demanded.

On the other hand, the so-called on-side type fuel cell, which is installed in places such as hotels, hospitals and office buildings where there is a demand for electricity and heat, is smaller than the fuel cell for electric power of the thermal power alternative type. Because of its capacity, it is operated at normal pressure and at a slightly lower temperature of around 190 ° C. In such an operation at normal temperature and low temperature, the reaction rate at the electrode decreases,
The influence of poisoning of the fuel electrode catalyst by impurities in the fuel gas becomes large. Therefore, in particular, in the on-site type fuel cell, a catalyst having high resistance is required as a catalyst for the fuel electrode.

The present invention has been proposed in order to solve the above problems of the prior art, and even when the CO concentration in the fuel gas is high or the fuel cell is at a relatively low temperature. An object of the present invention is to provide a fuel cell electrode having high resistance to impurities such as CO in fuel gas even when it is operated, and a method for manufacturing the same.

[0015]

To achieve the above object, the invention according to claim 1 provides a fuel cell electrode in which an alloy catalyst of platinum and a noble metal element is formed on one surface of a porous carbon plate. A layer of a base metal element that easily adsorbs oxygen is formed on the surface of the alloy catalyst.

According to the present invention having the above-mentioned structure, when the fuel gas is supplied to the fuel cell, the adsorption of impurities of the fuel gas is suppressed by the base metal element layer, and the adsorption of oxygen is adsorbed. The water molecules and the like adsorbed on the layer of the base metal element, which is easy to perform, serve as an oxidant for the impurities adsorbed on platinum, so that the oxidation and removal of the impurities are promoted.

According to a second aspect of the invention, in the fuel cell electrode according to the first aspect, the base metal element is As or S.
It is characterized in that it is at least one of b, Ge, Sn, Bi and Pb.

In the invention having the above-mentioned structure, when the fuel gas is supplied to the fuel cell, the impurities of the fuel gas are adsorbed by As, Sb, Ge, Sn, Bi, P.
As, Sb, G while being suppressed by layers such as b
Since the layers such as e, Sn, Bi, and Pb easily adsorb oxygen, the water molecules and the like adsorbed on this layer function as an oxidizing agent for the impurities adsorbed on platinum, so that the impurities are promoted to be oxidized and removed. To be done.

According to a third aspect of the present invention, in the method for producing an electrode for a fuel cell in which an alloy catalyst of platinum and a noble metal is formed on one surface of a porous carbon plate, the electrode is a chloride or sulfide of a base metal element. It is characterized in that the base metal element is adsorbed on the electrode surface by immersing the base metal element in the dilute solution of (1) and reducing and precipitating the base metal element by adding a liquid phase reducing agent to the dilute solution.

In the invention of claim 3 as described above, since the base metal element can be deposited on the electrode surface in the state of being immersed in the solution, steps such as reduction in a hydrogen stream after drying are taken. There is no need, and manufacturing becomes easy.

According to a fourth aspect of the present invention, in a method for producing an electrode for a fuel cell, which comprises forming an alloy catalyst of platinum and a noble metal on one surface of a porous carbon plate, the electrode is a chloride or sulfide of a base metal element. The base metal element is precipitated by repeating the potential sweep in the dilute solution, and the base metal element is adsorbed on the electrode surface.

In the invention described in claim 4 as described above, the base metal element can be deposited on the surface of the electrode in a state of being immersed in the solution, and it is formed by adjusting the potential sweep voltage and the number of cycles. The thickness of the base metal element layer formed can be freely adjusted.

[0023]

Embodiments of the present invention will be described below with reference to the drawings.

(1) First Embodiment A fuel cell electrode and a method of manufacturing the same, which are embodiments corresponding to the invention described in claims 1 to 3, will be described below as a first embodiment. In this embodiment, the present invention is applied to the fuel electrode.

(Structure) First, the structure of the fuel electrode of the present embodiment will be described according to the procedure of the manufacturing method thereof. That is, a catalyst is formed on one surface of a porous carbon plate by an alloy of platinum bearing carbon and a noble metal. Such a fuel electrode is immersed in a dilute solution of a base metal element chloride or sulfide that easily adsorbs oxygen. Then, by adding the liquid phase reducing agent to the dilute solution, the base metal element is reduced and precipitated on the surface of the fuel electrode. The base metal element layer is formed extremely thin at the single atom level or the fine particle level.

(Operation) The operation of this embodiment as described above is as follows. FIG. 1 is a schematic diagram showing the surface of the catalyst layer (A) of the fuel electrode according to the present embodiment and the surface of the catalyst layer (B) of the conventional fuel electrode.

That is, as shown in FIGS. 1A and 1B, when the fuel gas is supplied to the fuel electrode, CO is adsorbed to the fuel electrode. However, as shown in FIG. 1 (A), a very small amount (Q _pb <1) of base metal atoms X are deposited on the surface of the catalyst layer in the present embodiment. CO molecules are not adsorbed on the base metal atom X thus deposited. Therefore, in the fuel electrode according to the present embodiment, the amount of CO adsorbed is further reduced as compared with the fuel electrode in which the conventional alloy catalyst of two elements shown in FIG. 1B is formed.

Oxidation of the adsorbed CO is carried out as shown in the following formula 3 by the action of water molecules as the CO oxidant at a positive potential higher than the hydrogen region.

[0029]

(Equation 3) Since oxygen is easily adsorbed on the base metal atom X in the present embodiment, many water molecules are adsorbed, as shown in FIG. Therefore, in the fuel electrode according to the present embodiment, as compared with the conventional fuel electrode in which the alloy catalyst of two elements is only formed as shown in FIG.
Oxidation and removal of O are more likely to occur.

Since the radius of hydrogen atoms used as fuel gas is small and the base metal atom X layer is formed to be extremely thin at a single atom level or a fine particle level, hydrogen atoms are more internal than the base metal atom X layer. It enters and is adsorbed by platinum fine particles. Therefore, the base metal atom X layer does not affect the catalytic function of platinum.

(Effects) The effects of the present embodiment as described above are as follows. That is, the amount of CO adsorbed on the surface of the fuel electrode becomes smaller than that of the conventional fuel electrode on which the alloy catalyst of two elements is formed, and the adsorbed C
O can be easily removed by oxidation. Therefore, it is possible to provide a fuel cell having high resistance to CO even when a fuel gas having a high CO concentration is used or when the fuel gas is operated at a temperature lower than the standard time.

(2) Second Embodiment A method for manufacturing a fuel cell electrode, which is an embodiment corresponding to the invention described in claim 4, will be described below as a second embodiment. In this embodiment, the present invention is applied to the fuel electrode.

(Procedure) A method for manufacturing the fuel cell electrode according to the present embodiment will be described according to the procedure. That is,
In this embodiment, similarly to the first embodiment, a catalyst is formed on one surface of a porous carbon plate by an alloy of platinum carrying carbon and a noble metal. Such an electrode is immersed in a dilute solution of a base metal element chloride or sulfide that easily adsorbs oxygen. Then, the base metal element is deposited on the electrode surface by repeating the potential sweep several times in the dilute solution. The base metal element layer is formed to be extremely thin at the level of a single atom or at the level of fine particles by adjusting the voltage and the number of cycles.

(Effects) The effects of this embodiment as described above are as follows. That is, since a base metal element layer that easily adsorbs oxygen can be formed extremely thin on the surface of the fuel electrode, it is possible to provide a fuel cell having high resistance to CO, as in the first embodiment.

Further, a base metal element layer can be deposited on the electrode surface in a state of being immersed in the solution, and the thickness of the base metal element layer to be formed can be controlled by adjusting the potential sweep voltage and the number of cycles. Since it can be freely adjusted, it is possible to easily manufacture an electrode having a constant quality.

(3) Other Embodiments The material of each member, the element of the composition and the like in the present invention can be appropriately changed. That is, the noble metal used for the alloy catalyst with platinum may be one of Pd, Rh, Ru, Te, In, Os and the like, or a combination thereof. Also,
Base metal elements that are adsorbed or precipitated on the catalyst surface are Pb, A
It may be one of s, Sb, Ge, Sn, Bi, etc., or a combination thereof.

The liquid phase reducing agent according to the third aspect of the present invention can form a base metal element layer well even if any of sodium formate, formaldehyde, methanol and ethanol is used.

Then, the electrode is immersed in a dilute solution of a base metal element chloride or sulfide, and then dried in air,
After that, the base metal element can be adsorbed on the electrode surface by subjecting the electrode to a reduction treatment in a hydrogen gas flow of about 300 ° C. as a reducing agent.

Furthermore, although the above-described embodiments are applied to the fuel electrode of a fuel cell, the present invention can also be applied to an air electrode of a fuel cell and a manufacturing method thereof.

[0040]

Embodiments of the present invention as described above will be described below with reference to the drawings.

(1) First Example An example corresponding to the first embodiment will be described as a first example.

(Structure) First, the structure of the fuel cell electrode of the present embodiment will be described in accordance with the procedure of its manufacturing method. That is, P dispersedly carried on the surface of carbon black particles
A t / Pd alloy (Pt 7 wt%, Pd 4 wt%) is bound with polytetrafluoroethylene and applied to one surface (electrolyte side) of the porous carbon plate. Use this electrode for PbCl ₂
By immersing in a mM aqueous solution and dissolving sodium formate, which is a liquid phase reducing agent, in this aqueous solution for reduction treatment, P
b is adsorbed on the electrode surface.

(Effects) The effects of this embodiment having the above-mentioned structure will be described with reference to cyclic voltagrams and IV characteristics.

Comparison of cyclic voltammograms before and after CO adsorption FIG. 2 shows that the fuel electrode of this example and the fuel electrode of the prior art in room temperature 1M sulfuric acid were 0.1 V / s, and the potential range was 0 to 1.4 V /.
vs. It is a representation of a cyclic voltammogram in RHE. In the figure, the solid line shows the voltammogram waveform before introduction of CO gas, the alternate long and short dash line shows the conventional fuel electrode after introduction of CO gas, and the broken line shows the fuel electrode tomogram of this embodiment after introduction of CO.

The CO gas adsorption condition is CO (0.5%)
+ CO ₂ (29.5%) + H ₂ (70%), 5 mV / v
s. Bubble for 5 minutes with RHE, CO purge; 1 with N ₂ .
Bubbling for 0 minutes, and then 0-1.V at 0.1 V / s.
The sweep is performed up to 4V.

From the above experimental results shown in FIG. 2, when the fuel electrode of this example was used, the hydrogen region (0 to 0
The degree of decrease in the current value of 0.4 V) is smaller than that in the case where the conventional fuel electrode is used, and the current due to the oxidation of CO starts to flow from the lower potential side. This indicates that the amount of CO adsorbed was reduced and CO was more easily oxidized.

IV Characteristics of Single Cell when CO is Mixed in Fuel Gas A single cell was assembled using the fuel electrode according to this example, and CO
IV when operating with fuel gas mixed with
The (current-voltage) characteristics were examined in comparison with those using a conventional fuel electrode. The test conditions are 190 ° C. normal pressure, fuel gas is CO (1%) + CO ₂ (29%) + H ₂ (70%)
And the oxidizing gas is air.

First, when the CO concentration is low or the gas utilization rate is low, no remarkable difference in characteristics is found as compared with the case where the conventional fuel electrode is used. However, when the CO concentration is low or the gas utilization rate is high, it is apparent from FIG. 3 showing the results of this test that the fuel cell using the fuel electrode of the present invention shows better characteristics.

(2) Second Example An example corresponding to the second embodiment will be described as a second example. That is, in the present example, as in the method of manufacturing the fuel electrode of the first example, the carbon-supported Pt / Pd alloy was applied to one surface of the porous carbon plate, and then the PbCl _{2 in} a low concentration solution (C ≦ 1 mM). Then, in this low concentration solution, -0.05 to 1.4 V / vs. PbCl ₂ is deposited on the electrode surface by repeating the potential cycling a dozen or more times in the RHE potential region.

[0050]

As described above, according to the present invention, a layer of a base metal element that easily adsorbs oxygen is formed on the surface of the alloy catalyst of the electrode, so that the CO concentration in the fuel gas is high. Also, even when the fuel cell is operated at a relatively low temperature, it is possible to provide a fuel cell electrode having high resistance to impurities such as CO in fuel gas, and a method for producing the same.

[Brief description of drawings]

FIG. 1 is a schematic view showing a catalyst layer surface of a fuel cell electrode according to a first embodiment of the present invention, where (A) is a fuel electrode of the present embodiment and (B) is a conventional fuel electrode. Is.

FIG. 2 is a diagram showing a graph comparing cyclic voltammograms before and after CO adsorption in the fuel electrode of the first embodiment of the present invention and the fuel electrode of the conventional example.

FIG. 3 is a graph showing IV characteristics when CO is mixed in the fuel gas in the unit cell to which the fuel electrode of the first embodiment of the present invention is applied and the unit cell to which the fuel electrode of the conventional example is applied. FIG.

FIG. 4 is an exploded perspective view showing a general phosphoric acid type fuel cell.

[Explanation of symbols]

 X ... Base metal atom 1 ... Single cell 1a ... Fuel electrode 1b ... Oxidizer electrode 1c ... Electrolyte layer 2 ... Fuel cell stack 3 ... Gas separation plate

Claims (4)

[Claims] 1. A fuel cell electrode having an alloy catalyst of platinum and a noble metal element formed on one surface of a porous carbon plate, wherein a layer of a base metal element that easily adsorbs oxygen is formed on the surface of the alloy catalyst. An electrode for a fuel cell, which is characterized in that 2. The base metal element is As, Sb, Ge,
The fuel cell electrode according to claim 1, wherein the electrode is at least one of Sn, Bi and Pb. 3. A method for producing an electrode for a fuel cell, comprising forming an alloy catalyst of platinum and a noble metal on one surface of a porous carbon plate, wherein the electrode is a base metal element chloride or sulfide which easily adsorbs oxygen. An electrode for a fuel cell characterized by being immersed in a dilute solution, and adsorbing the base metal element on the electrode surface by adding a liquid-phase reducing agent to the dilute solution to reduce and precipitate the base metal element. Manufacturing method. 4. A method for producing a fuel cell electrode, comprising forming an alloy catalyst of platinum and a noble metal on one surface of a porous carbon plate, wherein the electrode is a chloride or sulfide of a base metal element that easily adsorbs oxygen. A method for producing a fuel cell electrode, comprising immersing in a dilute solution, repeating the potential sweep in the dilute solution to precipitate the base metal element, and adsorbing the base metal element on the electrode surface.