JP2005174748A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the corrosion of the catalyst-carrying carbon of an oxidizer electrode at the start and stop of a fuel cell system. <P>SOLUTION: At the time of starting and stopping a fuel cell system, a pressure control valve 8 at an oxidizer electrode exit 4b is closed when at least the supply of oxidizer gas is stopped, then a flow rate control valve 10 of carbonated water is opened and a carbonated water pump 11 is driven, and the carbonated water is introduced from the carbonated water tank 9 into the oxidizer electrode 4 of a fuel cell stack 1. At this time, the carbonated water is supplied from the oxidizer electrode exit 4b to the oxidizer electrode 4, and thereby the carbonated water is supplied from the portion of the oxidizer electrode closer to the fuel electrode exit. Thereby, the corrosion of the catalyst-carrying carbon of the oxidizer electrode on the side closer to the fuel electrode exit where oxygen is easily introduced by the invasion of air from the outside can be effectively prevented. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a fuel cell system, and more particularly to a fuel cell system using carbon as a catalyst carrier.

  In a fuel cell, a fuel gas such as hydrogen gas and an oxidizing gas containing oxygen are electrochemically reacted through an electrolyte, and electric energy is directly taken out between electrodes provided on both surfaces of the electrolyte. In particular, solid polymer fuel cells using solid polymer electrolytes are attracting attention as power sources for electric vehicles because of their low operating temperature and easy handling. That is, a fuel cell vehicle is equipped with a hydrogen storage device such as a high-pressure hydrogen tank, a liquid hydrogen tank, or a hydrogen storage alloy tank in the vehicle, and reacts by supplying hydrogen supplied therefrom and air containing oxygen to the fuel cell. This is the ultimate clean vehicle that drives the motor connected to the drive wheels with the electric energy extracted from the fuel cell, and the only exhaust material is water.

  In the polymer electrolyte fuel cell, the following reaction occurs, whereby electrons flow from the fuel electrode through the external circuit to the oxidant electrode.

(Chemical formula 1)
Anode (fuel electrode): H ₂ → 2H ⁺ + 2e ⁻ (1)
Cathode (oxidant electrode): 2H ⁺ + 2e ⁻ + (1/2) O ₂ → H ₂ O (2)

  By the way, even if the supply of hydrogen and air is stopped when the operation of the fuel cell is stopped, hydrogen and air remain in the piping connecting the supply means and the fuel cell and in the fuel cell. If the electrical connection between the fuel cell and the external circuit is interrupted at this time, a potential difference close to 1 [V] is generated between the fuel electrode and oxidant electrode of the single cell, resulting in deterioration of the electrode catalyst and battery components. Corrosion etc. occur. For example, when a carbon separator is used for a fuel cell, a reaction represented by the following formula (where COs represents a carbon surface oxide) may occur.

(Chemical formula 2)
C + [O ²⁻ ] → COs + 2e ⁻ (3)
COs + [O ²⁻ ] → CO ₂ + 2e ⁻ (4)
Further, when oxygen in the remaining air reacts with hydrogen, 2 [mol] of hydrogen is consumed with respect to 1 [mol] of oxygen, so that the pressure drop of the fuel electrode is larger than the pressure drop of the oxidizer electrode. There may be a pressure difference between the electrode and the oxidizer electrode, which may deteriorate the solid electrolyte membrane.

As a solution to this, a conventional method of purging residual gas when the fuel cell is stopped is employed. For example, a method is known in which an inert gas cylinder such as nitrogen is provided in a fuel cell system and the inside of the fuel cell is replaced with an inert gas supplied from the inert gas cylinder when the fuel cell is stopped (Patent Document 1).
JP-A-7-272740 (5th page, FIG. 1)

  However, the above purge method mainly purges the gas in the gas path in the fuel cell, and it is difficult to remove the residual gas in the vicinity of the catalyst supported on the porous body. Therefore, when storing the fuel cell system in an air atmosphere after purging, hydrogen and oxygen are locally added to the catalyst by the fuel electrode side catalyst in the process of air replacement of the fuel electrode side and the oxidant electrode side path. There may be mixed states. If oxygen remains in the vicinity of the oxidant electrode side catalyst at this time, a reaction requiring hydrogen ions occurs by a reverse current on the oxidant electrode side opposite to the region where oxygen is present on the fuel electrode side through the membrane. Since there is basically no hydrogen on the oxidizer electrode side, hydrogen ions are extracted from the water, and the remaining oxygen ions react with carbon of the catalyst carrier to generate carbon dioxide. The carbon dioxide gas produced by the reaction is diffused into the oxidant gas passage, causing corrosion of the catalyst-carrying carbon, and there is a problem that the Pt fine particles of the catalyst are aggregated to significantly reduce the effective catalyst surface area. .

  The same phenomenon occurs at the start of hydrogen introduction after the fuel electrode side is sufficiently replaced with air during the stop.

  In order to solve the above problems, the present invention provides a fuel cell system including a fuel electrode and an oxidant electrode sandwiching a solid polymer electrolyte membrane, carbonated water storage means for storing carbonated water, The gist of the present invention is that it comprises carbonated water introduction means for introducing carbonated water from the carbonated water storage means to the oxidizer electrode when the operation is stopped.

  According to the present invention, even when a reaction that requires hydrogen ions occurs on the oxidant electrode side when the fuel cell is started or stopped, the hydrogen ions contained in the supplied carbonated water react to react with each other. There is an effect that corrosion of the electrode catalyst-supporting carbon can be prevented.

  Specifically, when carbon dioxide is dissolved in water, carbonic acid is produced by the following equilibrium reaction to exhibit acidity.

(Chemical formula 3)
_{H 2 O + CO 2 ⇔ H} 2 CO 3 ⇔ H + + HCO 3 - ... (5)
When this carbonated water is introduced into the oxidizer electrode when the fuel cell is stopped, hydrogen ions are supplied to the oxidizer electrode, and even if a reaction requiring hydrogen ions occurs on the oxidizer electrode side, The contained hydrogen ions react.

  Next, embodiments of the present invention will be described in detail with reference to the drawings. In addition, although each Example described below is not specifically limited, it is a fuel cell system suitable for a fuel cell vehicle.

FIG. 1 is a main part configuration diagram for explaining a first embodiment of a fuel cell system according to the present invention, and corresponds to claims 1 and 2.
In the fuel cell stack 1, a plurality of unit cells each having a fuel electrode 2 and an oxidant electrode 4 are stacked with a solid polymer electrolyte membrane 3 interposed therebetween, which are schematically shown in FIG.

  Hydrogen gas, which is fuel, is supplied to the fuel electrode 2 from a fuel supply device (not shown). Air is supplied to the oxidant electrode 4 from a blower, a compressor or the like as an oxidant supply device. Both hydrogen gas and air are supplied to the fuel electrode 2 and the oxidant electrode 4 from the left side in the figure, and are supplied in a cocurrent flow type gas discharged from the right side in the figure.

  Connected to the oxidant electrode inlet 4a is a pressure release valve 7 for releasing the pressure when carbonated water is injected when the operation is stopped. An air pressure adjusting valve 8 that adjusts the air pressure during operation is connected to the oxidant electrode outlet 4b.

  The carbonated water tank 9 is carbonated water storage means for storing carbonated water. A carbonated water supply pipe 12 is joined from the carbonated water tank 9 to the oxidant electrode outlet pipe upstream of the air pressure regulating valve 8 via the flow rate regulating valve 10 and the carbonated water pump 11. The flow rate adjusting valve 10, the carbonated water pump 11, and the carbonated water supply pipe 12 are carbonated water introduction means.

Next, the operation of the fuel cell system having the above configuration will be described.
At the time of starting or stopping the fuel cell system, at least when the oxidant gas supply is stopped, the pressure control valve 8 at the oxidant electrode outlet 4b is closed, then the carbonated water flow rate control valve 10 is opened and the carbonated water pump 11 is driven. Then, carbonated water is introduced from the carbonated water tank 9 into the oxidant electrode 4 of the fuel cell stack 1. At this time, since carbonated water is supplied from the oxidant electrode outlet 4b to the oxidant electrode 4, carbonated water is supplied from the portion of the oxidant electrode close to the fuel electrode outlet.

  Thereby, it is possible to effectively prevent the corrosion of the catalyst-supporting carbon of the oxidant electrode on the side close to the fuel electrode outlet where oxygen is easily introduced due to air intrusion from the outside. Further, the oxidant electrode pressure can be controlled by monitoring the pressure of the oxidant electrode and adjusting the pressure release valve 7 branched from the oxidant electrode inlet 4a as necessary.

  According to the present embodiment described above, even when a reaction that requires hydrogen ions occurs on the oxidizer electrode side when the fuel cell is started or stopped, the hydrogen ions contained in the supplied carbonated water react. As a result, the corrosion of the catalyst-supporting carbon of the oxidizer electrode can be prevented.

FIG. 2 is a main part configuration diagram for explaining Example 2 of the fuel cell system according to the present invention, and corresponds to claims 3 and 4.
In the fuel cell stack 1, a plurality of unit cells each having a fuel electrode 2 and an oxidant electrode 4 are stacked with a solid polymer electrolyte membrane 3 interposed therebetween, which are schematically shown in FIG. On the opposite side of the oxidizer electrode 4 from the solid polymer electrolyte membrane 3 side, a pure water path 6 is provided via a porous separator 5.

  The pure water path 6 is intended to improve the power generation performance in a high current region by cooling the fuel cell stack 1 and recovering the generated water at the oxidant electrode 4. The generated water can be recovered at the oxidant electrode 4 by setting the differential pressure so that the pressure of the pure water passage 6 is lower than the pressure of the oxidant electrode 4.

  Hydrogen gas, which is fuel, is supplied to the fuel electrode 2 from a fuel supply device (not shown). Air is supplied to the oxidant electrode 4 from a blower, a compressor or the like as an oxidant supply device. Both hydrogen gas and air are supplied to the fuel electrode 2 and the oxidant electrode 4 from the left side in the figure, and are supplied in a cocurrent flow type gas discharged from the right side in the figure.

  Connected to the oxidant electrode inlet 4a is a pressure release valve 7 for releasing the pressure when carbonated water is injected when the operation is stopped. An air pressure adjusting valve 8 that adjusts the air pressure during operation is connected to the oxidant electrode outlet 4b.

  The carbonated water tank 9 is carbonated water storage means for storing carbonated water. The pure water tank 20 for storing pure water and the carbonated water tank 9 can be selectively communicated with the inlet side of the pump 22 by a three-way valve 21. The outlet side of the pump 22 is connected to the pure water path inlet 6 a of the fuel cell stack 1 via the pure water supply pipe 26.

  The pure water passage outlet 6b is connected to the inlet of the pure water tank 20 via a pressure adjusting valve 25 that adjusts the pressure of pure water. A bypass path 23 that branches from the pure water supply pipe 26 connected to the outlet of the pump 22 and returns to the pure water tank 20 and a flow rate adjustment valve 24 that adjusts the flow rate of the bypass path 23 are provided.

  The flow of pure water during normal operation is sucked into the pump 22 from the pure water tank 20 through the three-way valve 21, and from the outlet of the pump 22 through the pure water supply pipe 26 to the fuel cell stack 1 from the pure water path inlet 6 a. To be supplied. Pure water in the pure water path 6 exchanges moisture with the oxidant electrode 4 via the porous separator 5.

  Although the water exchange between the pure water path 6 and the oxidant electrode 4 depends on the operating conditions, the recovery of the produced water from the oxidant electrode 4 to the pure water path 6 is dominant on the oxidant electrode outlet 4b side. On the oxidant electrode inlet 4a side, pure water consumption due to humidification from the pure water path 6 to the oxidant electrode 4 is dominant. Excess pure water is discharged from the pure water passage outlet 6 b and returns to the pure water tank 20 via the pressure regulating valve 25.

  During normal operation, in the fuel cell stack 1, fuel gas and air flow from left to right in the figure, and pure water flows from right to left in the figure. That is, the gas supply to the fuel cell stack is a parallel flow type, and the pure water that exchanges moisture with the gas is a counter flow type that faces the gas flow direction.

Next, the operation of the fuel cell system having the above configuration will be described.
At the time of starting or stopping the fuel cell system, at least when the oxidant gas supply is stopped, the three-way valve 21 upstream of the pump 22 is switched so that the carbonated water tank 9 and the pump 22 communicate with each other. To the pure water path 6. Then, by adjusting the pressure control valve 25 that adjusts the pressure of the pure water path 6, carbonated water is supplied to the oxidant electrode 4 side by controlling to reduce the differential pressure between the oxidant electrode 4 and the pure water path 6. It becomes possible. The oxidant electrode pressure can be controlled by monitoring the pressure of the oxidant electrode 4 and adjusting the pressure release valve 7 branched from the upstream side of the oxidant electrode 4 as necessary.

  According to this example, as in Example 1, even when a reaction requiring hydrogen ions occurs on the oxidizer electrode side when the fuel cell is started or stopped, the hydrogen ions contained in the supplied carbonated water By reacting, there is an effect that corrosion of the catalyst-supporting carbon of the oxidant electrode can be prevented.

  Further, the pump for circulating pure water can be shared for introducing carbonated water, and the number of parts for newly constructing the carbonated water introduction path can be reduced.

  Furthermore, according to the present embodiment, since the flow directions of the oxidant gas channel and the pure water channel are opposed to each other, when the pressure of the pure water channel increases, the pressure easily rises and carbonated water penetrates into the oxidant gas channel Thus, there is an effect that carbonated water can be effectively supplied from the pure water inlet, which is easy to perform, to the oxidant gas channel outlet where the catalyst-supported carbon is likely to corrode.

It is a principal part block diagram explaining Example 1 of the fuel cell system which concerns on this invention. It is a principal part block diagram explaining Example 2 of the fuel cell system which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Fuel cell stack 2 ... Fuel electrode 3 ... Solid polymer electrolyte membrane 4 ... Oxidant electrode 4a ... Oxidant electrode inlet 4b ... Oxidant electrode outlet 7 ... Pressure release valve 8 ... Air pressure regulating valve 9 ... Carbonated water tank 10 ... Flow control valve 11 ... Carbonated water pump

Claims (4)

In a fuel cell system having a fuel electrode and an oxidant electrode sandwiching a solid polymer electrolyte membrane,
Carbonated water storage means for storing carbonated water;
Carbonated water introduction means for introducing carbonated water from the carbonated water storage means to the oxidant electrode when the fuel cell is started or stopped; and
A fuel cell system comprising:   2. The fuel cell system according to claim 1, wherein carbonated water is introduced into the oxidizer electrode close to the fuel electrode outlet. In a fuel cell system comprising a fuel electrode and an oxidant electrode sandwiching a solid polymer electrolyte membrane, an oxidant gas channel of the oxidant electrode, and a pure water channel adjacent through a porous body,
Carbonated water storage means for storing carbonated water;
Carbonated water introduction means for introducing carbonated water into the pure water flow path at a pressure higher than the pressure of the oxidant gas flow path when starting or stopping the fuel cell;
A fuel cell system comprising:   The fuel cell system according to claim 3, wherein the flow directions of the oxidant gas channel and the pure water channel oppose each other.

Images