JP2010160993A - Polymer electrolyte fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polymer electrolyte fuel cell with a function which allows water produced at power generation to be used for solid polymer film humidification and eliminates any external humidifier, thereby simplifying the construction thereof. <P>SOLUTION: A gas flow channel of a fuel gas is formed in an anode-side separator, whereas a gas flow channel of an oxidant gas is formed in a cathode-side separator. The cathode-side separator includes a movable rib for controlling the flow of an oxidant gas. At the time of stable power generation, the rib is moved to a flow channel location forming a gas flow channel including a separator's center portion, and at the time of drying an electrolyte film, the rib is moved to a flow channel location generating a drift flow in which a flow of an oxidant gas in the separator's center position is diminished, in accordance with the required amount of water. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a polymer electrolyte fuel cell having a movable rib for controlling the flow of an oxidizing gas in a cathode side separator.

  The fuel cell does not have a moving part and operates even in a zero-gravity environment, and its waste is considered to be used as a clean energy source because of its characteristics such as only water that is harmless to the environment. In particular, polymer electrolyte fuel cells that operate at low temperatures have recently been miniaturized to use as a power source for mobile phones and personal computers using MEMS technology.

  A polymer electrolyte fuel cell has a cell stack structure in which a single cell composed of an electrolyte membrane and a membrane-electrode assembly MEA composed of a pair of electrodes disposed on both sides thereof and a pair of separators sandwiching the MEA is laminated. It has become. In a fuel cell having such a structure, power is generated by supplying an oxidizing gas (air) or a fuel gas to each electrode through a gas flow path formed in each separator. A technique for switching according to a high load and a low load is known (see Patent Document 1).

  FIG. 11 is a diagram schematically illustrating air distribution to the separator described in Patent Document 1. (A) shows a flow path at high load, and (B) shows a flow path at low load. Each is shown. When the load is high, the flow path switching mechanism is opened, and the air flow path is a plurality of straight flow paths. Since the gas flow rate is high at high loads, a sufficient differential pressure is ensured between the cell entrances and exits even if there are a plurality of flow paths. For this reason, it is possible to perform a stable operation without causing the generated water generated by the power generation reaction to block the flow path.

  On the other hand, when the load is low, an S-shaped serpentine flow path is formed by two partition walls provided inside the reaction section, and the flow path is configured to be folded twice in the middle. The air flow rate is low at low load. For this reason, at the time of low load, the flow path switching mechanism is closed to switch the flow path in the cell to the serpentine flow path.

  Thus, by switching the flow path shape according to the load, it is possible to perform a stable response even when a load change occurs. In addition, power generation is always possible using all parts in the cell regardless of the load condition, so the humidification conditions of the membrane are constant, and when the full load operation becomes necessary, it can respond quickly to load fluctuations. can do.

  When the polymer electrolyte fuel cell is operated in the daily life temperature range, the water vapor generated on the cathode (oxygen electrode) side condenses and remains in the electrode structure, obstructing the supply of fuel gas to the reaction surface and outputting it. Reduction (flooding, plugging). On the other hand, the solid polymer membrane constituting the fuel cell exhibits high ionic conductivity only in a wet state and maintains its output, and when it dries, the output also decreases (dry out). Therefore, the polymer electrolyte fuel cell is currently operated by humidifying the anode side (hydrogen electrode side) with a humidifier. That is, water management inside the polymer electrolyte fuel cell is an important issue in relation to improving the performance of the fuel cell.

JP 2007-42496 A

  According to the technique described in Patent Document 1, the humidification condition of the film can be made constant for all the parts in the cell plane. However, the water produced by the reaction is “retained” and used to humidify the electrolyte membrane, eliminating the need for an external humidifier. During stable operation, the water produced by the reaction is discharged and the electrolyte membrane is dried. In some cases, it is necessary to “control the generated water” to “retain” this generated water and use it for humidifying the electrolyte membrane.

  Therefore, in the present invention, the solid polymer fuel cell has a function capable of using the water generated during power generation to humidify the solid polymer membrane, and this external humidifier is eliminated to simplify the structure. The objective is to enable automatic and continuous control to make an appropriate amount of water produced by reaction in the cell according to the power generation situation.

  In the present invention, the position of the separator rib that forms the gas flow path is moved to a flow path position that forms a uniform gas flow path with high drainage (for example, serpentine type) during stable power generation, and water retention is maintained during electrolyte membrane drying. A solid polymer fuel cell having a movable separator that moves to a high (for example, straight type) flow path position is provided. As will be described later with reference to FIG. 4, when the power density decreases, the temperature of the fuel cell increases. That is, if this temperature rise is detected, the drying of the electrolyte membrane can be detected.

  A polymer electrolyte fuel cell according to the present invention includes an electrolyte membrane made of a polymer material and a membrane-electrode assembly MEA comprising a pair of electrodes disposed on both sides thereof, a pair of current collector plates and a cathode separator that sandwich the assembly MEA. And a cell stack structure in which single cells composed of an anode separator are stacked. A gas flow path for fuel gas is formed in the anode-side separator, while a gas flow path for oxidizing gas is formed in the cathode-side separator. This cathode-side separator is provided with a movable rib that controls the flow of oxidizing gas. During stable power generation, the rib is moved to the flow path position that forms the gas flow path including the central part of the separator, and when the electrolyte membrane is dried. Corresponding to the required amount of moisture, the rib is moved to a flow path position having high water retention, in other words, generating a drift in which the flow of the oxidizing gas in the central portion of the separator is reduced.

  Electrolyte membrane drying can be detected by detecting a temperature rise in the fuel cell or a change in the amount of generated water. A shape memory alloy spring can be used as the moving mechanism of the rib position. Alternatively, the motor that drives the movable rib can be controlled based on the detected signal.

  The flow path of oxidizing gas at the time of stable power generation is an S-shaped serpentine flow path, and the flow path of oxidizing gas at the time of drying the electrolyte membrane is divided into two straight sections by oxidizing gas flowing in from the cathode separator inlet. It is a straight channel that goes to the outlet. The movable rib has a pair of elongated wall-like shapes, and is configured to be movable in both the length direction and the width direction.

  According to the present invention, since it is possible to realize a fuel cell having a self-moisture management function capable of non-humidifying operation, an auxiliary device such as a humidifier is not required, and the fuel cell can be reduced in size and performance. it can. As a result, full-scale spread of fuel cell technology becomes possible, and the ripple effect on the fuel cell industry becomes extremely large.

It is a figure which shows schematic structure of the polymer electrolyte fuel cell actualized based on this invention. It is a figure explaining the flow of the gas with respect to the assembled cell stack structure. It is a figure explaining a cathode side separator. It is a graph which shows the change of the power density at the time of the driving | running which generate | occur | produced the dryout, and temperature. It is the graph shown for the comparison with the time of the driving | operation which dryout generate | occur | produced. It is a graph which shows the time change of the moisture content in a gas diffusion layer obtained by numerical analysis. It is a figure which shows the condensed water distribution inside the cathode side gas diffusion layer at the time of the steady power generation obtained by numerical analysis. It is a figure which illustrates the movable mechanism by the temperature change of a separator rib position. It is a figure explaining an analysis system. It is a figure which shows the oxygen gas flow by the side of the cathode at the time of the steady power generation obtained by numerical analysis. It is the figure which showed typically about the distribution of the air with respect to the reaction part 70 of patent document 1. As shown in FIG.

  Hereinafter, the present invention will be described based on examples. FIG. 1 is a diagram showing a schematic configuration of a solid polymer fuel cell embodied according to the present invention, and has a normal configuration except for the configuration of a cathode separator, which is a feature of the present invention. An exemplary polymer electrolyte fuel cell includes an electrolyte membrane made of an ion exchange membrane made of a polymer material and a membrane-electrode assembly (MEA) made up of a pair of electrodes (GDL) arranged on both sides thereof, and this MEA. The cell stack structure is formed by laminating a single cell composed of a pair of current collector plates (perforated plates) sandwiching the electrode, a cathode separator, and an anode separator. The cathode and the anode are each composed of a separator and a current collector plate. The current collector plate is a conductive material, and is configured by a porous structure (such as a punching metal or a metal mesh) that is permeable to supply gas to the gas diffusion layer. The gas diffusion layer (GDL) is made of, for example, a porous carbon material to which a catalyst such as platinum is bound. Oxidizing gas such as air or oxidant is supplied to the cathode side GDL, and hydrogen gas as fuel gas is supplied to the other anode side GDL. The two gases cause an electrochemical reaction in the MEA, and the single cell obtains an electromotive force.

  The separator is one of the main members constituting the fuel cell, and has a role of blocking fuel gas and air by being sandwiched between the fuel cells. In addition, each separator is supplied with fuel gas and air. A road is necessary. There is no restriction on the material of each separator. A gas flow path (details will be described later) having the characteristic features of the present invention is provided on the inner surface on the GDL side of the cathode side separator. A gas flow path of hydrogen gas, for example, a serpentine shape, is formed on the inner surface on the GDL side of the anode-side separator according to a normal technique.

  FIG. 2 is a diagram illustrating a gas flow with respect to the assembled cell stack structure. Oxygen gas (air) is supplied to the flow path formed in the cathode separator. At the outlet, water generated together with oxygen gas flows out. Hydrogen is supplied to the flow path formed in the anode separator. Hydrogen is decomposed into electrons and hydrogen ions (protons) by the action of the catalyst (Pt) coated on the carbon black particles of the MEA electrode (anode GDL), and the protons move through the electrolyte membrane with water. It reaches the cathode GDL and combines with oxygen and electrons that have passed through the external circuit on the catalyst to become water.

  FIG. 3 is a diagram for explaining the movement of the rib of the cathode side separator. Oxygen gas (air) is supplied to the cathode separator from the inlet in the figure and discharged to the outlet. The flow of oxygen gas inside the separator is controlled by a pair of movable ribs (separator ribs). Each rib has an elongated wall shape and is configured to be movable in both the length direction and the width direction. Now, it is assumed that the pair of movable ribs is in the type 1 state shown in FIG. At this time, the pair of movable ribs are in a state where their one ends in the length direction are moved so as to be in contact with the mutually opposing ends inside the separator. That is, an S-shaped serpentine flow path is formed by two movable ribs provided inside the separator, and the flow path has a structure that is folded twice in the middle. As a result, a substantially uniform gas flow path is formed on the entire surface including the central portion of the separator. When the electrolyte membrane dries in this state, it moves to the type 2 state shown in FIG.

  In FIG. 3 (b), the pair of movable ribs are arranged so as to form flow paths at both ends in the lengthwise direction and with a predetermined interval between the ribs. It is in. As a result, the pair of movable ribs can move to the straight flow path position with high water retention and can be used for water retention of the solid polymer film. At this time, the oxygen gas flowing in from the inlet is divided into two parts as shown by the arrows in the drawing, straightly going to the outlet, and flows out from there. At this rib position, the flow rate of oxygen gas flowing in the gap between the ribs is greatly reduced. That is, a drift in which the gas flow rate is reduced occurs in the central portion of the separator. For this reason, water accumulates at the center in the figure as indicated by a circle. This water can be used for humidifying the solid polymer membrane. However, instead of shifting to a complete straight type as illustrated in FIG. 3B, in order to generate a drift according to the required amount of water, as illustrated in FIG. 8B described later. A flow path (type 4) having both a serpentine flow path and a straight flow path can be provided.

  Even in the state shown in FIG. 3B, when the drying of the electrolyte membrane is still detected, the ribs are further moved in the direction in which the distance between the pair of ribs is reduced. That is, a larger drift is generated corresponding to the required amount of moisture. Type 3 shown in FIG. 3C shows a state where the gap between the ribs is minimized. The oxygen gas flow between the ribs is effectively cut off, and as shown in a circle in the figure, water is further accumulated, and the electrolyte membrane is dried. However, if flooding occurs and is detected in this state, the rib is moved to the state shown in FIG.

  The state shown in FIG. 3D corresponds to the type 1 state shown in FIG. That is, an S-shaped serpentine flow path is formed by two ribs provided inside the separator. In this state, this state is maintained as long as the optimum amount of water is detected, and when the electrolyte membrane is detected to dry, as described above, the state moves to the type 2 state shown in FIG. To do.

  FIG. 4 is a graph showing changes in power density and temperature during operation in which dryout occurs. As shown in the figure, when the output density decreases due to the occurrence of dryout, the temperature of the fuel cell increases. That is, if this temperature rise is detected, the drying of the electrolyte membrane can be detected. FIG. 5 is a graph shown for comparison with an operation in which dryout has occurred. In the case of low load, the power density does not decrease, indicating that the fuel cell temperature is constant and stable power generation is in progress.

  In addition to detecting a temperature rise in order to detect the drying of the electrolyte membrane, for example, it is conceivable to detect a change in the amount of water under the rib. Using a material whose shape (volume) changes repeatedly due to a change in the amount of moisture, the shape change can be used for the driving force of the ribs as in the shape memory alloy (see Example 1). Alternatively, a sensor that detects a change in temperature, a moisture content, or an output can be provided, and a motor that drives the movable rib can be controlled in accordance with the sensor output.

  FIG. 6 shows the time change of the moisture content in the gas diffusion layer obtained by numerical analysis. The amount of moisture in the gas diffusion layer during steady power generation is greater in the straight flow path (type 2, type 3) than in the serpentine flow path (type 1).

  FIG. 7 is a diagram showing the distribution of condensed water (low-temperature water particles) inside the cathode-side gas diffusion layer during steady power generation obtained by numerical analysis. A large amount of condensed water in the gas diffusion layer exists in the lower part of the separator rib in each type. That is, it is considered that the separator rib has a lid effect for closing the flow path for drainage, in other words, a water retention effect. Here, in the case of (B) type 2, as compared with (A) type 1, more condensed water is also present in the central gas flow path. This is because the condensed water is not easily drained due to the uneven flow of oxygen gas. That is, even if the area of the separator rib is the same, it is understood that the straight type has a higher water retention effect due to the drift than the serpentine type.

  FIG. 8 is a diagram illustrating a movable mechanism according to a temperature change of the separator rib position. A shape memory alloy spring can be used as a movable mechanism that responds to temperature changes. There is known a bias type bidirectional actuator in which a shape memory alloy spring that is soft (weak) at a low temperature and hard (strong) at a high temperature and a bias spring made of a normal spring material are set to press each other. . By using such a bias type bidirectional actuator (see http://www.sogospring.co.jp/product/product3.html) as the actuator shown in FIG. 8, the temperature changes from a low temperature to a high temperature. Accordingly, the rib position can be continuously moved from FIG. 7 (A) to FIG. 7 (B) and further to the state of (C).

  In FIG. 3, typical positions of the ribs are illustrated as type 1, type 2, and type 3, respectively. However, as shown in FIG. 8B as type 4, the rib positions are different types. It is possible not only to provide a position (for example, type 4) or to control a continuous rib position as well as a stepwise rib position control.

  Numerical analysis was performed on the configuration described with reference to FIGS. 1 and 2. FIG. 9 is a diagram for explaining the analysis system. The 3D simulator for mass transfer in polymer electrolyte fuel cells simulates the flow of a fluid (gas, liquid) as particles by moving them randomly while considering collisions and interactions between virtual particles. “Lattice gas method”. The fuel cell has a laminated structure of cathode and anode separators and gas diffusion layers, and a membrane electrode assembly (MEA) (zero thickness) in which a catalyst layer and a solid polymer membrane are integrated. Note that the analysis system is divided into a regular cubic lattice, and the number of lattices in the entire calculation region is the x direction 864, the y direction 816, and the z direction 72. The separator gas flow channel shape is a serpentine type flow channel on the anode side and three types of serpentine type channel and straight type flow channel 2 type on the cathode side (see FIG. 3). The gas diffusion layer is regarded as a porous layer (thickness t), and virtual solid particles are randomly arranged on the calculation lattice points so that the porosity ε = 0.8. On the MEA surface, virtual particles simulating the catalyst (platinum) are randomly arranged on the calculation lattice point with an arbitrary probability α.

  As virtual particles in the lattice gas method, hydrogen gas particles as fuel, oxygen gas particles as oxidant, water particles generated by reaction (power generation), and nitrogen gas particles filled in the fuel cell as initial conditions are considered. The water particles are high-temperature water particles (image of water vapor) and low-temperature water particles (image of condensed water), and a total of 5 particles are analyzed. Hydrogen gas particles and oxygen gas particles flow in from the opening on the inlet side of the bipolar separator at an almost constant number of particles (inflow probability parameter: V, corresponding to oxygen gas, hydrogen gas flow rate), and from the opening on the opposite side of the separator It flows out with the outlet pressure taken into consideration (the outflow is the same for the initially filled nitrogen gas particles and produced water particles). The most basic HPP model is applied as the collision rule for all five particles.

  In this analysis, when hydrogen gas particles at the hydrogen electrode and oxygen gas particles at the oxygen electrode pass simultaneously on platinum catalyst particles randomly arranged (probability α) on the zero-thickness MEA surface that partitions the oxygen electrode and the hydrogen electrode. Suppose that hydrogen gas particles disappear and oxygen gas particles are replaced by high-temperature water particles (water vapor). This extinction / replacement is simulated by a reaction in the catalyst layer, that is, power generation. The number of high-temperature water particles (1 to 6) existing on each lattice point is considered to simulate the water vapor partial pressure there. Therefore, in this analysis, the number of particles corresponding to the saturated vapor pressure (dew point temperature) relative to the fuel cell temperature during power generation is set, and the high-temperature water particles that exceed the set number of particles are replaced with low-temperature water particles (condensation). The image). The number of particles corresponding to this saturated vapor pressure is defined as the number of saturated particles Nsat. That is, when Nsat is small, the fuel cell relatively simulates power generation under a low temperature environment, and when it is large, power generation under a high temperature environment is simulated. Between the low-temperature water particles, an LG model, which is a phase change model that takes into account the interaction (attraction) with particles at positions separated by several lattices (distance r), is applied. Also, a hydrophilic surface and a hydrophobic (water repellent) surface can be simulated, respectively, depending on whether or not an attractive force (distance rw) is considered between the low-temperature water particles and the structure surface. In this analysis, the separator (gas flow path) surface was given a hydrophilic surface (rw / r = 1), and the gas diffusion layer was given water-repellent conditions (rw / r = 0).

FIG. 10 is a diagram illustrating the oxygen gas flow on the cathode side during steady power generation obtained by numerical analysis. The size of the arrow is changed in proportion to the flow rate. In the case of the straight type (type 2, type 3), oxygen gas does not flow in the center of the cell, and a drift occurs.

Claims (5)

A single cell composed of an electrolyte membrane of a polymer material and a membrane-electrode assembly MEA comprising a pair of electrodes disposed on both sides thereof, and a pair of cathode side separator and anode side separator sandwiching the assembly MEA was laminated. In a polymer electrolyte fuel cell having a cell stack structure,
The anode-side separator forms a fuel gas gas flow path, while the cathode-side separator forms an oxidizing gas gas flow path,
The cathode-side separator is provided with a movable rib for controlling the flow of oxidizing gas. During stable power generation, the rib is moved to a channel position that forms a gas channel including the central part of the separator, and the electrolyte membrane is dried. Corresponding to the required amount of water, sometimes the rib is moved to a flow path position that generates a drift in which the flow of the oxidizing gas in the center of the separator is reduced.
A polymer electrolyte fuel cell comprising: 2. The polymer electrolyte fuel cell according to claim 1, wherein the electrolyte membrane drying is detected by detecting a temperature rise, output change, or produced water amount change of the fuel cell. 3. The polymer electrolyte fuel according to claim 2, wherein a motor that drives a movable rib according to a sensor output such as a shape memory alloy spring or the temperature rise is used as the movable mechanism according to the temperature change of the rib position. battery. The flow path of the oxidizing gas at the time of stable power generation is a serpentine type flow path, and the flow path of the oxidizing gas at the time of drying the electrolyte membrane is divided into two parts by the oxidizing gas flowing from the cathode side separator inlet. 2. The solid polymer fuel cell according to claim 1, wherein the solid polymer fuel cell is a straight flow path extending straight toward the outlet. 2. The polymer electrolyte fuel cell according to claim 1, wherein each of the movable ribs has a pair of configurations having an elongated wall shape, and is configured to be movable in both a length direction and a width direction.

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