JP2000021426A - Fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve power generating efficiency by eliminating the catalyst poisoning material in an electrode. SOLUTION: In this fuel cell, the fuel material and the oxidizing material react electrochemically with each other via the electrolyte 3 arranged between a fuel pole 4 and an air pole 5 so as to obtain the electromotive force, and at least one of the fuel pole 4 and the air pole 5 is provided with the catalyst. In this case, the poisoning material eliminating agent, which reacts with the catalyst poisoning material for eliminating the poisoning material, is mixed in the fuel material or the oxidizing material to be supplied to the fuel pole 4 or the air pole 5 provided with the catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel cell for obtaining an electromotive force by reacting a fuel substance and an oxidizing substance, and more particularly to a fuel cell in which at least one of a fuel electrode and an air electrode has a catalyst. It is.

[0002]

2. Description of the Related Art As is well known, a fuel cell is a device for obtaining an electromotive force by electrochemically reacting a fuel substance and an oxidizing substance. As one example, a fuel cell such as methyl alcohol (hereinafter referred to as methanol) is used. Fuel cells using hydrogen as a fuel material are known. Fuel cells using methanol as fuel
The point is that it is configured to electrochemically react hydrogen and oxygen obtained from methanol via an electrolyte.
Specifically, methanol vapor and water vapor are reacted by a reformer to generate a fuel gas composed of hydrogen gas and carbon dioxide gas, which is supplied to the fuel electrode side of the fuel cell and, at the same time, is supplied to the air electrode side. And hydrogen in the fuel gas is reacted with oxygen in the air via the electrolyte.
This can be called a so-called indirect methanol fuel cell. On the other hand, a fuel of the type in which a mixture of methanol and water is supplied to the fuel electrode side, hydrogen ions and carbon dioxide are generated on the fuel electrode side, and the hydrogen ions react with oxygen on the air electrode side Batteries have also been developed, which can be referred to as so-called direct methanol fuel cells.

In the former indirect methanol fuel cell as well, it is necessary to generate hydrogen ions on the fuel electrode side. Therefore, the fuel electrode is composed of a diffusion layer made of, for example, carbon cloth and a catalyst made of platinum or the like. And a reaction layer supported by a porous material. Also,
In the latter direct methanol fuel cell as well, in order to decompose methanol as a fuel substance into hydrogen ions and carbon dioxide, a reaction layer and a diffusion layer in which a catalyst such as platinum is supported on a porous material such as carbon are used. It constitutes the fuel electrode. An example of this type of direct methanol fuel cell is described in Japanese Patent Application Laid-Open No. 9-161810.

[0004]

The above-described fuel cell using a hydrocarbon such as methanol as a fuel uses an electrolyte having a positive ion permeability such as a cation exchange membrane.
Generate hydrogen from fuel material. In the indirect methanol fuel cell, the reaction occurs in the reformer, and in the direct methanol fuel cell, the reaction occurs in the reaction layer on the fuel electrode side. In that case, methanol is basically decomposed into hydrogen and carbon dioxide, but carbon monoxide is inevitably generated at a rate of several percent.

[0005] As described above, carbon monoxide contained in the fuel substance is adsorbed by platinum as a catalyst, and causes a so-called poisoning which lowers its activity. On the other hand, a platinum (Pt) -ruthenium (Ru) alloy catalyst is known as a catalyst for oxidizing carbon monoxide in preference to hydrogen. FIG. 7 shows Pt-R
4 schematically shows a process of eliminating platinum poisoning when a u alloy is used as a catalyst. Of water supplied together with methanol as a fuel substance, ionized OH ⁻ is extracted by ruthenium, and this is converted to platinum. Reacts with carbon monoxide adsorbed on to produce hydrogen and carbon dioxide,
Platinum poisoning is eliminated.

[0006] However, even if carbon monoxide, which is a substance causing poisoning of the catalyst, is oxidized by water, water is originally an inert substance and is hardly decomposed, and OH
^Since the concentration of-is ^10-7 mol / L at pH 7 and is extremely small, it is difficult to promote the oxidation of carbon monoxide poisoning platinum. Therefore, in the conventional fuel cell, there is a disadvantage that the oxidation of carbon monoxide is a limiting factor and it is difficult to improve the output of the fuel cell.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a fuel cell capable of improving output by eliminating poisoning of a catalyst provided in an electrode. Things.

[0008]

SUMMARY OF THE INVENTION In order to achieve the above objects, the present invention provides a method for removing a poisoning substance capable of reacting with a substance causing poisoning of a catalyst to remove the substance. The composition is characterized in that the agent is mixed with a fuel substance or an oxidizing substance. More specifically, the invention of claim 1 provides an electrochemical reaction between a fuel substance and an oxidizing substance via an electrolyte disposed between a fuel electrode and an air electrode to obtain an electromotive force, And a fuel cell in which at least one of the air electrode is provided with a catalyst, wherein a fuel substance or an oxidizing substance supplied to the fuel electrode or the air electrode provided with the catalyst reacts with a substance poisoning the catalyst. A poisoning substance removing agent for removing the substance is mixed.

In the present invention, the fuel substance may be methanol, the catalyst may include platinum, and an oxidizing agent that releases oxygen atoms or oxygen ions may be used as the poisoning substance removing agent.

Further, in the present invention, carbonic acid can be used as the poisoning substance removing agent.

Therefore, according to the present invention, the poisoning substance removing agent is supplied to the electrode side provided with the catalyst together with the fuel substance or the oxidizing substance, and the substance poisoning the catalyst is supplied to the electrode side at the electrode side. And is removed
Since the activity of the catalyst is maintained, the output as a fuel cell is improved. In particular, when methanol is used as the fuel substance and carbon dioxide is used as the poisoning substance remover, the poisoning substance remover can be obtained by using carbon dioxide and water generated by the reaction between the fuel substance and the oxidizing substance. Therefore, the reaction product can be effectively used.

[0012]

Next, the present invention will be described more specifically with reference to the drawings. FIG. 1 is a diagram schematically showing an example of the present invention. A fuel cell 1 shown here is configured to generate power using methanol as a fuel substance and air as an oxidizing substance. The fuel cell 1 is shown in FIG.
Are stacked in a large number. Describing this unit cell 2, a fuel electrode 4 and an air electrode 5 are formed on both sides of an electrolyte 3 made of a cation exchange membrane. Each of these electrodes 4 and 5 has the same configuration as that of a conventional methanol fuel cell, and includes a catalyst layer and a diffusion layer.

That is, a catalyst layer is formed facing the electrolyte 3, and a diffusion layer is formed on the surface of the catalyst layer.
The diffusion layer has a porous structure made of, for example, carbon cloth, and the catalyst layer has a structure in which a catalyst material is supported on a porous material such as carbon. As a catalyst for the fuel electrode, platinum or a platinum-ruthenium alloy is used. As a catalyst in the air electrode 5, an alloy of platinum and chromium, titanium, tungsten, or the like is used.

Further, on the outer surfaces of the respective electrodes 4 and 5,
Separators 6 and 7 made of carbon or the like are provided. A fuel flow path 8 composed of a plurality of grooves is formed in the separator 6 on the fuel electrode 4 side in the vertical direction in FIG. An air flow path 9 composed of a plurality of grooves is formed in the separator 7 on the side of the air electrode 5 in a direction perpendicular to the plane of FIG.

Each fuel passage 8 communicates with a fuel supply manifold and a discharge manifold (each not shown) formed on the side surface of the fuel cell 1. The fuel cell 1 is connected to a supply manifold and a discharge manifold (each not shown) formed on both upper and lower sides of the fuel cell 1. An air supply source 11 is connected to the air supply manifold via an air supply pipe 10. The air supply source 11 includes, for example, a compressor that pressurizes air and a humidifier (not shown). A discharge pipe 12 is connected to the discharge manifold for air, and the discharge pipe 12
It is connected to the mixer 13. The air supply pipe 10 and the discharge pipe 12 are provided with control valves (not shown). Further, an exhaust pipe 14 is connected to the mixer 13. The exhaust pipe 14 is open to the outside air via, for example, a gas-liquid separator (not shown).

On the other hand, a fuel mixer 16 is connected to the fuel supply manifold via a mixed fuel supply pipe 15.
Further, the fuel mixer 16 is connected to a fuel tank 18 via a fuel supply pipe 17. This fuel tank 1
8 stores methanol or an aqueous methanol solution as a fuel substance. A discharge pipe 19 is connected to the discharge manifold on the side of the fuel electrode 4, and the discharge pipe 19 is connected to the mixer 13. Therefore, the cathode 5
The reaction product and excess air on the fuel electrode 4 side and the reaction product and excess fuel on the fuel electrode 4 side are guided to the mixer 13. Then, the mixer 13 and the fuel mixer 16
It is connected by the mixture supply pipe 20. A control valve such as an on-off valve or a flow control valve is appropriately interposed in each pipeline. Further, an external load 22 such as a motor or a battery is connected to the fuel cell 1 via a conductor 21.

Next, the operation of the fuel cell having the above configuration will be described. Methanol is used as a fuel substance of the fuel cell 1, and the methanol is supplied from the fuel tank 18 to the fuel mixer 16 by adjusting the opening of a control valve (not shown) in appropriate amounts. In the fuel mixer 16, methanol, water and carbonic acid are mixed. These water and carbonic acid are stored in the mixer 13, and are supplied to the fuel mixer 16 by a proper amount through a mixture supply pipe 20 and a control valve interposed in the mixture supply pipe 20.

Here, the water mixed with methanol reacts with methanol by the catalyst of the reaction layer in the fuel electrode 4 to generate hydrogen and carbon dioxide, and poisons platinum by a part of ionized OH ⁻ . The carbon monoxide is mixed to oxidize it to carbon dioxide.
Carbonic acid is obtained by bubbling carbon dioxide generated by power generation in the fuel cell 1 against water, as described later, and oxidizes carbon monoxide poisoning platinum to carbon dioxide. Mixed for. Therefore, carbonic acid mixed with methanol is the poisoning remover in the present invention.

Although the mixing ratio of methanol, water and carbonic acid is not particularly specified in the present invention, the mixing ratio of methanol and water is determined by changing the mixing ratio of methanol and water to that of a conventional direct methanol fuel cell. The mixing ratio may be set in the same manner as above, and carbon dioxide may be generated by blowing as much carbon dioxide as possible into the water.

The liquid mixture of methanol, water and carbonic acid thus prepared is supplied to the fuel electrode 4 side of the fuel cell 1. On the other hand, an appropriate amount of air is supplied from the air supply source 11 to the air electrode 5 with the flow rate adjusted by a control valve (not shown). This air is for oxidizing hydrogen generated by decomposing methanol,
This corresponds to the oxidized substance of the present invention. In supplying air, the air may be humidified by steam.

The mixed solution of methanol, water and carbonic acid supplied to the fuel electrode 4 is mixed with CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ under the platinum catalyst of the reaction layer constituting the fuel electrode 4. Is generated to generate hydrogen ions (protons) and carbon dioxide. The hydrogen ions permeate the electrolyte 3 and react with oxygen on the air electrode 5 side to generate water. The electrons are
Through the external load 22 via the conducting wire 21 to the cathode 5, in the process of generating power at the external load 22,
Or stored as electrical energy, and
In the oxidation of hydrogen.

Further, a part of methanol is not oxidized by water, but causes the following reaction under the platinum catalyst in the fuel electrode 4 to generate carbon monoxide and hydrogen ions. _{CH 3 OH → CO + 4H +} + 4e - so formed carbon monoxide is to poison the platinum catalyst adsorbed on platinum, since mixed carbonate in methanol and reaction with carbon dioxide and carbon monoxide platinum Poisoning is eliminated.

That is, FIG. 3 is a schematic view for explaining the process, in which carbon monoxide is adsorbed on platinum, while carbonate ions are adsorbed on ruthenium forming a reaction layer together with platinum. I have. That ^{_{carbonate, H 2 CO 3 → 2H +}} + CO 3 2- next by ruthenium, _CO ^{3 2-is} adsorbed on the ruthenium, carbon monoxide adsorbed on the platinum and _{^{CO + CO 3 2- → 2CO 2}} + 2e - Eventually, a reaction of CH ₃ OH + H ₂ CO ₃ → 3H ₂ + 2CO ₂ occurs.

Therefore, carbon monoxide poisoning platinum is oxidized and removed by carbonic acid, and as a result, the platinum catalyst is activated. In that case, the concentration of carbonic acid is up to 10
^-4 mol / l, 100 times higher than carbonate ion in water
Since the concentration is about 0 times higher and carbonic acid is easily decomposed, the oxidation of carbon monoxide poisoning platinum is promoted. In other words, by mixing carbonic acid with methanol, carbon monoxide generated by the decomposition of methanol can be effectively removed, and the catalytic function of platinum can be maintained in a good state. As a result, an electrochemical reaction between the fuel substance and the oxidized substance occurs efficiently, so that the power generation efficiency of the fuel cell can be improved. Since water is mixed with methanol as described above, ionized OH ⁻ functions as described with reference to FIG. 7 to oxidize carbon monoxide and eliminate the poisoning of platinum. It is considered that the effect of the occurrence is slightly generated in parallel with the elimination of the poisoning by carbonic acid.

On the other hand, carbon dioxide is generated from the fuel electrode 4 side and water is generated from the air electrode 5 side due to the above-mentioned methanol oxidation reaction.
9 to the mixer 13. Water is stored in the mixer 13 in advance, and discharge pipes 12 and 19 are stored therein.
The carbon dioxide gas is bubbled by supplying the carbon dioxide and the water via the gas. As a result, carbon dioxide reacts with water to produce carbonic acid. Its concentration is at most 10 ^-4 mol / l. That is, carbon dioxide is obtained by using carbon dioxide, which is a reaction product generated by power generation, and sent to the fuel mixer 16 via the mixture supply pipe 20 to be mixed with methanol. Therefore, the effective use of the waste generated by the power generation is being attempted.

Here, an experimental example performed to confirm the effect of the present invention will be described. Bubbling for feeding carbon dioxide gas into pure water was performed to obtain carbonated water, which was mixed with methanol to form a fuel liquid. The fuel liquid was supplied to a fuel cell having the configuration shown in FIGS. 1 and 2 to generate power, and the current density and voltage were measured. Also, for comparison,
Water containing no carbon dioxide and methanol were mixed to prepare a fuel liquid, and using this fuel liquid, power was generated in the same manner, and the current density and voltage were measured. The measurement results are shown in FIG.

As is known from the measurement results, in the fuel cell according to the present invention, since the fuel substance contains carbonic acid, the decrease in current density is small and the output characteristics are improved as compared with the conventional one. Was observed. This is considered to be because the oxidation of carbon monoxide by carbonic acid was promoted, and the activity of the platinum catalyst was favorably maintained.

The specific example described above is an example in which the present invention is applied to a direct methanol fuel cell. However, the present invention relates to a so-called indirect method using a reformed gas obtained by reforming methanol in a reformer. The present invention can also be applied to a methanol fuel cell. Hereinafter, an example thereof will be described. In FIG. 5, a reformer 23 for reforming methanol to generate hydrogen gas is provided. The reformer 23 has substantially the same configuration as a conventionally known one, and is configured to react hydrogen supplied from a fuel tank 24 with water vapor supplied from a water tank 25 to generate hydrogen gas. It is configured. A copper-based catalyst has been used as a catalyst for accelerating the reaction.

Methanol is supplied to the reforming section in the form of steam together with water, and the reforming section has a high catalytic activity of 200 to 3 times.
It is heated to a temperature of 00 ° C., and the reforming reaction proceeds within that temperature range. This reaction is based on CH ₃ O
H + H ₂ O → 3H ₂ + CO ₂ −49.5 kj / mol;
Since the reaction is an endothermic reaction, the fuel cell 1 is heated using the power generated by the fuel cell 1 as a heat source.

The resulting gas thus obtained contains about 1% of carbon monoxide gas in addition to hydrogen and carbon dioxide. A premix tank 26 for mixing the reformed gas mainly composed of hydrogen with carbon dioxide is provided. That is, the premixing tank 26 is connected to the reformer 23 via the fuel supply pipe 27 and is connected to the mixer 13 via the mixture supply pipe 20. The other configuration is the same as the configuration shown in FIG. 1, and therefore, the same reference numerals are given to FIG. 5 as in FIG. 1 and the description is omitted.

In the fuel cell 1 having the structure shown in FIG. 5, the methanol and water supplied to the reformer 23 are sent to the reforming section in the form of steam, where a reforming reaction takes place and hydrogen gas and Generated with carbon gas and trace amounts of carbon monoxide gas. This reformed gas is sent to the premix tank 26. In the process, a purification process for removing carbon monoxide gas may be performed as needed. Even in that case, a small amount of carbon monoxide gas is supplied to the premixing tank 26.

The premixing tank 26 further includes a mixer 13
Supplies carbonic acid. In the example shown in FIG.
Carbonic acid is vaporized by a suitable heating means. Therefore, in the premixing tank 26, the reformed gas composed of hydrogen gas and carbon dioxide gas and a trace amount of carbon monoxide gas and carbon dioxide are mixed in a gaseous state. Then, this mixed gas is supplied to the fuel electrode 4 side of the fuel cell 1. On the other hand, air, which is an oxidizing substance, is supplied to the air electrode 5 side.

The hydrogen gas of the mixed gas supplied to the fuel electrode 4 emits electrons to become protons, passes through the electrolyte 3 and reacts with oxygen on the air electrode 5 side to generate water. That is, an electrochemical oxidation reaction between hydrogen and oxygen occurs, and an electromotive force is obtained. The carbon monoxide contained in the gas mixture is
Although it is adsorbed by platinum in the reaction layer constituting the fuel electrode 4 and reduces its catalytic activity, it is oxidized by the action of carbonic acid contained in the mixed gas, and the poisoning of platinum is eliminated. The oxidizing action is as described in the specific example described above. Carbonate ions are adsorbed on ruthenium, which is a kind of catalyst constituting the reaction layer, and the carbonate ions react with carbon monoxide to form monoxide. Carbon is oxidized. In this way, the poisoning of platinum is eliminated by the carbonic acid supplied together with the fuel substance, and the oxidation of carbon monoxide by the carbonic acid occurs quickly and in large quantities, so that the activity of platinum is maintained well and the output of the fuel cell 1 is improved. I do.

An experimental example performed to confirm the effect of the fuel cell 1 shown in FIG. 5 shows that the fuel cell having the configuration shown in FIG. 8
% Of the reformed gas mixed with carbon dioxide was supplied to generate power, and the voltage and current density were measured. For comparison, the reformed gas containing 0.8% of carbon monoxide gas was directly supplied to the fuel cell 1 without mixing carbon dioxide with the reformed gas to generate power, and the voltage and the current density were measured. did. The measurement results are shown in FIG.

As can be seen from the measurement results, in the fuel cell according to the present invention, since the fuel substance contains carbonic acid, the current density is reduced little and the output characteristics are improved as compared with the prior art. Was observed. This is considered to be because the oxidation of carbon monoxide by carbonic acid was promoted, and the activity of the platinum catalyst was favorably maintained.

In each of the above specific examples, an example in which carbon dioxide is used as a poisoning substance removing agent has been described. However, if carbon dioxide is used, carbon dioxide as a raw material is generated with power generation in a fuel cell. Therefore, the effective use of the raw materials can be achieved, the power generation cost can be reduced accordingly, and the generation of new emissions accompanying the removal of the poisoning substances can be avoided. However, the poisoning substance removing agent in the present invention is not limited to carbonic acid, and a substance that releases oxygen atoms or oxygen ions such as hydrogen peroxide, ozone, and formic acid can be used as the poisoning substance removing agent. is there. Further, in each of the above specific examples, the Pt-Ru alloy is shown as the catalyst on the fuel electrode side, but the catalyst in the fuel cell targeted by the present invention is a Pt-Ti alloy, a Pt-Re alloy, a Pt-Mo alloy. , A platinum-based alloy such as a Pt-Sn alloy, or another appropriate catalyst. Further, the present invention, besides the fuel cell using methanol as a fuel material,
The present invention can be applied to a fuel cell using other hydrocarbons or flammable gas as fuel. In each of the above specific examples, air is used as the oxidizing substance. However, other oxidizing substances (gas or liquid) may be used instead of air.

Each of the specific examples described with reference to the above-described drawings includes inventions other than the inventions described in the appended claims. The other is a fuel cell that obtains an electromotive force by electrochemically reacting a fuel substance and an oxidizing substance through an electrolyte that transmits ions generated by reacting the fuel substance using a catalyst, A fuel cell comprising a poisoning substance removing agent mixing means for mixing a poisoning substance removing agent that reacts with a substance that inactivates the catalyst to activate the catalyst in the fuel substance. is there. This poisoning substance removing agent mixing means corresponds to the above-described fuel mixer 16, premix tank 26, or a configuration in which the mixer 13 is added to these.

Still another aspect of the present invention is that the poisoning substance removing agent mixing means includes a substance generated after the fuel substance electrochemically reacts and a substance generated after the oxidizing substance electrochemically reacts. And a means for collecting and supplying a poisoning substance removing agent from at least one of the following. The mixer 13 corresponds to this poisoning substance removing agent collecting / supplying means.

[0039]

As described above, according to the present invention,
The poisoning substance removing agent is supplied to the electrode side provided with the catalyst together with the fuel substance or the oxidizing substance, and the substance poisoning the catalyst is removed by reacting with the poisoning substance removing agent at the electrode side. The activity of the fuel cell can be maintained in a good state to improve the output of the fuel cell. In particular, when methanol is used as the fuel substance and carbon dioxide is used as the poisoning substance remover, the poisoning substance remover can be obtained by using carbon dioxide and water generated by the reaction between the fuel substance and the oxidizing substance. Therefore, the reaction product can be effectively used.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of the fuel cell of the present invention in principle.

FIG. 2 is a schematic sectional view showing a part of a unit cell.

FIG. 3 is an explanatory diagram for explaining a process of oxidizing carbon monoxide with carbonic acid.

FIG. 4 is a diagram showing measurement results of current-voltage characteristics of a specific example of the present invention and a comparative example.

FIG. 5 is a schematic view showing another embodiment of the present invention in principle.

FIG. 6 is a diagram showing measurement results of current-voltage characteristics of the specific example shown in FIG. 5 and a comparative example.

FIG. 7 is an explanatory diagram for explaining an oxidation process of carbon monoxide by OH in a Pt—Ru alloy catalyst.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Fuel cell, 2 ... Single cell, 3 ... Electrolyte, 4 ... Fuel electrode, 5 ... Air electrode, 13 ... Mixer, 16 ... Fuel mixer, 18 ... Fuel tank, 26 ... Premix tank.

Claims (3)

[Claims] 1. An electromotive force is obtained by electrochemically reacting a fuel substance and an oxidizing substance via an electrolyte disposed between a fuel electrode and an air electrode to obtain an electromotive force. In a fuel cell provided with a catalyst, a poisoning substance that removes a fuel substance or an oxidizing substance supplied to a fuel electrode or an air electrode provided with the catalyst by reacting with a substance poisoning the catalyst. A fuel cell, comprising a remover. 2. The method according to claim 1, wherein the fuel substance is methanol, the catalyst contains platinum, and the poisoning substance removing agent is an oxidizing agent that releases oxygen atoms or oxygen ions. The fuel cell device according to claim 1. 3. The fuel cell device according to claim 1, wherein the poisoning substance removing agent is carbonic acid.