FUEL CELL SYSTEM
Patent Information
- Authority / Receiving Office
- DE · DE
- Patent Type
- Patents
- Current Assignee / Owner
- TOYOTA JIDOSHA KK
- Filing Date
- 2021-12-06
- Publication Date
- 2026-07-30
AI Technical Summary
The durability of fuel cells is compromised due to the generation of hydrogen peroxide in low-potential environments, leading to chemical decomposition of the electrolyte membrane, which is not effectively controlled by existing catalyst arrangements.
A fuel cell system with a catalyst dispersion section in the electrolyte membrane, controlled by a controller to manage hydrogen and oxygen partial pressures using Equation (1), ensuring the hydrogen partial pressure is maintained at a target level to suppress hydrogen peroxide production.
The system effectively suppresses chemical decomposition of the electrolyte membrane, thereby extending the fuel cell's lifespan by managing hydrogen and oxygen partial pressures based on catalyst position and operating conditions.
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Abstract
Description
TECHNICAL AREA
[0001] The revelation refers to a fuel cell system. BACKGROUND
[0002] A fuel cell (FC) is a power generation device that produces electrical energy through an electrochemical reaction between fuel gas (such as hydrogen) and oxidant gas (such as oxygen and air) in a single elementary fuel cell or a fuel cell stack (hereinafter referred to simply as a "stack") consisting of stacked elementary fuel cells (hereinafter referred to as "cells"). Fuel gas and oxidant gas may, without further distinction, simply be referred to as "reaction gas" or "gas".
[0003] In general, elementary fuel cells comprise a membrane electrode assembly (MEA).
[0004] The membrane electrode assembly has a structure such that a catalyst layer and a gas diffusion layer are sequentially formed on both surfaces of a solid polymer electrolyte membrane (which may be referred to simply as the "electrolyte membrane" below). Accordingly, the membrane electrode assembly can be called a "membrane electrode-gas diffusion layer assembly" (MEGA).
[0005] As required, each elementary fuel cell comprises two separators that sandwich the membrane electrode-gas diffusion layer assembly. Generally, the separators have a structure such that a groove forms a reaction gas flow path on a surface in contact with the gas diffusion layer. The separators also serve as a collector for generated electricity.
[0006] In the fuel cell's anode, hydrogen (H₂) is supplied as fuel gas from the gas flow path and the gas diffusion layer. The supplied hydrogen is protonated by the catalytic action of the catalyst layer, and the protonated hydrogen passes through the electrolyte membrane to the oxidant electrode (cathode). Electrons are generated simultaneously, pass through an external circuit, perform energy conversion, and then travel to the cathode. Oxygen (O₂) is supplied to the cathode as the oxidant gas, and the supplied oxygen reacts with protons and electrons at the cathode, producing water. The produced water moistens the electrolyte membrane, and excess water permeates through the gas diffusion layer and is released to the outside of the system.
[0007] There is significant research on fuel cells designed for installation and use in a fuel cell vehicle (which may be referred to simply as a "vehicle" below).
[0008] For example, patent specification 1 discloses the following technique: A composite membrane with a reinforcing layer is used in an electrolyte membrane, and, to suppress chemical decomposition, an ion exchange material containing carbon particles carrying a Pt-containing catalyst (oxygen reduction reaction (ORR) active catalyst) is arranged at at least one end of the composite membrane. Patent specification 1 also discloses that the catalyst-carried carbon particles are particularly effective when arranged on the cathode side. The ORR active catalyst is a catalyst that accelerates the oxygen reduction reaction (ORR) in the cathode of a fuel cell.
[0009] Patent specification 2 discloses that an electronically isolated catalyst layer is arranged in a membrane and that the catalyst layer is located closer to a cathode side.
[0010] Patent specification 3 discloses that at least one metal catalyst, selected from platinum, gold, palladium, rhodium, iridium and ruthenium, is included in an amount of 0.01 wt% to 80 wt% based on the weight of a polymer solid electrolyte. Patent specification 1: Japanese patent application, published (JP-A) 2014-139939 Patent specification 2: JP-A No. H06-103992 Patent specification 3: JP-A No. H07-090111
[0011] To extend the lifespan of a fuel cell, there is a need for a membrane material with better durability.
[0012] As a result of the durability assessment of the electrolyte membrane in patent specification 1, it was determined that the durability deteriorates depending on the test conditions. It is known that the oxygen reduction reaction of a Pt catalyst alters the selectivity of two-electron reduction with the potential. In a fuel cell using an electrolyte membrane mixed with a dispersed Pt catalyst, some of the oxygen promotes the generation of hydrogen peroxide in a (hydrogen-rich) low-potential environment, thereby accelerating the chemical decomposition of the electrolyte membrane. That is to say, patent specification 1 anticipates that control measures to suppress hydrogen peroxide generation at the location of the catalyst-supported carbon particles are not feasible. SUMMARY
[0013] One objective of the present disclosure is to provide a fuel cell system designed to extend the lifetime of a fuel cell.
[0014] In a first embodiment, a fuel cell system is provided, comprising: a fuel cell an oxidizer gas supply device designed to supply oxygen-containing oxidizer gas to a cathode of the fuel cell, a fuel gas supply device designed to supply hydrogen-containing fuel gas to an anode of the fuel cell, an oxygen partial pressure estimation function designed to estimate the oxygen partial pressure at the cathode of the fuel cell, a hydrogen partial pressure estimation function designed to estimate the hydrogen partial pressure of the fuel cell anode, and a control system wherein the fuel cell comprises one or more elementary fuel cells; wherein the one or more fuel cells comprise a membrane electrode arrangement; wherein the membrane electrode arrangement comprises an anode with an anode catalyst layer, an electrolyte membrane and a cathode with a cathode catalyst layer in that order; wherein the electrolyte membrane comprises a catalyst dispersion section in the electrolyte membrane which is arranged in a layered manner parallel to the anode catalyst layer and the cathode catalyst layer; wherein the catalyst dispersion section comprises a catalyst; where the control system calculates a target hydrogen partial pressure using the following equation (1): Target hydrogen partial pressure = [2 × oxygen permeation coefficient in the thickness direction of the electrolyte membrane × {distance from the anode catalyst layer to the catalyst dispersion section ÷ (electrolyte membrane thickness − distance from the anode catalyst layer to the catalyst dispersion section)} × oxygen partial pressure] ÷ hydrogen permeation coefficient in the thickness direction of the electrolyte membrane; and whereby the control system regulates the hydrogen partial pressure of the anode to the target hydrogen partial pressure.
[0015] The controller can determine whether the target hydrogen partial pressure is lower than a specified lower limit. If it is determined that the target hydrogen partial pressure is lower than the lower limit, the controller can increase the oxygen partial pressure at the cathode. If it is determined that the target hydrogen partial pressure is at the lower limit, the controller can maintain the oxygen partial pressure at the cathode and maintain the hydrogen partial pressure at the anode. If it is determined that the target hydrogen partial pressure is higher than the lower limit, the controller can adjust the hydrogen partial pressure at the anode to achieve the target hydrogen partial pressure.
[0016] According to the fuel cell system of the present disclosure, the lifetime of a fuel cell can be extended. List of characters
[0017] The following applies to the associated drawings: Fig. Figure 1 is a curve showing the relationship between the amount of omitted fluorine ions and time in a high-potential shelf-life test; Fig. Figure 2 is a diagram illustrating an example of oxygen and hydrogen concentrations in an electrolyte membrane; Fig. Figure 3 is a schematic representation illustrating an example of a conventional membrane electrode arrangement; Fig. Figure 4 is a schematic representation illustrating an example of the membrane electrode arrangement used in the disclosed embodiments; Fig. Figure 5 is a curve showing an example of the relationship between the position of the catalyst in the thickness direction from the anode in the electrolyte membrane and the gas permeation flow of the electrolyte membrane; Fig. Figure 6 is a curve showing an example of the case where the position of the catalyst dispersion section is optimal with respect to the operating conditions of the fuel cell; Fig. Figure 7 is a curve showing an example of the case where the catalyst dispersion section is located on the cathode side, compared to the case where the position of the catalyst dispersion section is the optimal position with respect to the operating conditions of the fuel cell; Fig. Figure 8 is a curve showing an example of the case where the catalyst dispersion section is located on the anode side, compared to the case where the position of the catalyst dispersion section is the optimal position with respect to the operating conditions of the fuel cell; Fig. Figure 9 is a schematic representation showing an example of the cell level; Fig. Figure 10 is a curve showing an example of the relationship between the position of the catalyst in the thickness direction from the anode in the electrolyte membrane, the oxygen partial pressure and the hydrogen partial pressure; Fig. Figure 11 is a curve showing an example of the relationship between the oxygen partial pressure and the target hydrogen partial pressure; Fig. Figure 12 is a schematic diagram illustrating an example of the fuel cell system of the disclosed embodiments; and Fig. Figure 13 is a flowchart showing an example of the control carried out by the fuel cell system of the disclosed embodiments. DETAILED DESCRIPTION
[0018] The fuel cell system of the disclosed embodiments is a fuel cell system comprising: a fuel cell an oxidizer gas supply device designed to supply oxygen-containing oxidizer gas to a cathode of the fuel cell, a fuel gas supply device designed to supply hydrogen-containing fuel gas to an anode of the fuel cell, an oxygen partial pressure estimation function designed to estimate the oxygen partial pressure at the cathode of the fuel cell, a hydrogen partial pressure estimation function designed to estimate the hydrogen partial pressure of the fuel cell anode, and a control system wherein the fuel cell comprises one or more elementary fuel cells; wherein the one or more fuel cells comprise a membrane electrode arrangement; wherein the membrane electrode arrangement comprises an anode with an anode catalyst layer, an electrolyte membrane and a cathode with a cathode catalyst layer in that order; wherein the electrolyte membrane comprises a catalyst dispersion section in the electrolyte membrane which is arranged in a layered manner parallel to the anode catalyst layer and the cathode catalyst layer; wherein the catalyst dispersion section comprises a catalyst; where the control system calculates a target hydrogen partial pressure using the following equation (1): Target hydrogen partial pressure = [2 × oxygen permeation coefficient in the thickness direction of the electrolyte membrane × {distance from the anode catalyst layer to the catalyst dispersion section ÷ (electrolyte membrane thickness − distance from the anode catalyst layer to the catalyst dispersion section)} × oxygen partial pressure] ÷ hydrogen permeation coefficient in the thickness direction of the electrolyte membrane; and the control system regulates the hydrogen partial pressure of the anode to the target hydrogen partial pressure.
[0019] Fig. Figure 1 is a curve showing the relationship between the amount of omitted fluorine ions and time in a high-potential shelf-life test.
[0020] A durability assessment of an electrolyte membrane incorporating a conventional Pt catalyst was performed. As described in Fig. As shown in Figure 1, one result was that the durability deteriorated depending on the test conditions. This is understood to be due to the following reasons. It is known that the oxygen reduction reaction of the Pt catalyst changes the selectivity of a two-electron reduction with the potential. In a fuel cell using an electrolyte membrane mixed with a dispersed catalyst, in a (H2-rich) low-potential environment, some of the oxygen is converted to hydrogen peroxide, a substance that causes the chemical decomposition of the electrolyte membrane.
[0021] Fig. Figure 2 is a diagram illustrating an example of oxygen and hydrogen concentrations in an electrolyte membrane.
[0022] As in Fig. As shown in Figure 2, under the condition of a hydrogen concentration of 100%, a portion of the area with added catalyst in the electrolyte membrane comprises a region where the hydrogen concentration is higher than the oxygen concentration. The lifespan of the fuel cell appears to decrease for this reason.
[0023] It has been determined that in a fuel cell where a catalyst is added to an electrolyte membrane to suppress chemical decomposition, a significant difference in the decomposition suppression effect occurs depending on the position of the catalyst addition with respect to the membrane thickness and the operating environment (hydrogen partial pressure and oxygen partial pressure). Accordingly, in the disclosed embodiments, the upper limit of the hydrogen partial pressure at the anode is calculated and used as the control target value to maximize the decomposition suppression effect.
[0024] In the disclosed embodiments, the hydrogen partial pressure in the electrolyte membrane is controlled based on the target hydrogen partial pressure, wherein the position of the catalyst-containing catalyst dispersion section arranged in the electrolyte membrane (a distance from the catalyst layer) is used as a parameter.
[0025] According to the disclosed embodiments, the hydrogen partial pressure and the oxygen partial pressure can be suitably adjusted in accordance with the catalyst addition position; the generation of hydrogen peroxide in the catalyst added to the electrolyte membrane can be suppressed; and the chemical decomposition of the electrolyte membrane can be effectively suppressed by reducing the oxygen transition to the anode. As a result, the durability of the electrolyte membrane and the service life of the fuel cell are improved.
[0026] The fuel cell system of the present disclosure comprises at least the fuel cell, the fuel gas supply device, the oxidant gas supply device, the oxygen partial pressure estimation function, the hydrogen partial pressure estimation function and the control system.
[0027] In general, the fuel cell system of the present disclosure is installed and used in a fuel cell vehicle which includes an engine as a power source.
[0028] The fuel cell system of the present embodiments can be installed and used in a vehicle that can be powered by a secondary cell.
[0029] The motor is not specifically restricted. It can be a conventionally known drive motor.
[0030] The fuel cell can be a fuel cell consisting of only one elementary fuel cell, or it can be a fuel cell stack consisting of stacked elementary fuel cells.
[0031] The number of stacked elementary fuel cells is not specifically limited. For example, anywhere from two to several hundred elementary fuel cells can be stacked, or from two to 200 elementary fuel cells can be stacked.
[0032] The fuel cell stack can include an end plate at both ends of the stacking direction of each elementary fuel cell.
[0033] Each elementary fuel cell includes at least one membrane electrode arrangement.
[0034] The membrane electrode assembly comprises, in this order, the anode including the anode catalyst layer, the electrolyte membrane, and the cathode including the cathode catalyst layer. The membrane electrode assembly may also include, in this order, an anode-side gas diffusion layer, the anode catalyst layer, the electrolyte membrane, the cathode catalyst layer, and a cathode-side gas diffusion layer.
[0035] The cathode (oxidation electrode) comprises the cathode catalyst layer. The cathode may also include the cathode-side gas diffusion layer.
[0036] The anode contains the anode catalyst layer. The anode may also include the anode-side gas diffusion layer.
[0037] The cathode catalyst layer and the anode catalyst layer are collectively referred to as the "catalyst layer".
[0038] The catalyst layer can, for example, include a metallic catalyst to accelerate an electrochemical reaction, a proton-conducting electrolyte, or electron-conducting carbon particles.
[0039] The catalyst metal can be, for example, platinum (Pt) or an alloy of Pt and another metal (such as a Pt alloy mixed with cobalt, nickel or the like).
[0040] The electrolyte can be fluorinated resin or a similar substance. For example, a Nafion solution or similar can be used as the fluorinated resin.
[0041] The catalyst metal is supported on carbon particles. The electrolyte and the carbon particles supporting the catalyst metal (i.e., the catalyst particles) can be mixed in each catalyst layer.
[0042] For example, water-repellent carbon particles, obtained by improving the water-repellent effect of commercially available carbon particles (charcoal powder) by heating, can be used as the carbon particles for carrying the catalyst metal (i.e., supporting carbon particles).
[0043] The cathode-side gas diffusion layer and the anode-side gas diffusion layer are collectively referred to as the "gas diffusion layer".
[0044] The gas diffusion layer can be a gas-permeable, electrically conductive element or the like.
[0045] Examples of the electrically conductive element include a porous carbon material, such as carbon cloth and carbon paper, and a porous metallic material, such as metal mesh and foam metal.
[0046] The electrolyte membrane includes a catalyst dispersion section.
[0047] The catalyst dispersion section is arranged parallel to and layered with the anode catalyst layer and the cathode catalyst layer in the electrolyte membrane.
[0048] The electrolyte membrane can be a solid polymer electrolyte membrane. Examples of solid polymer electrolyte membranes include hydrocarbon electrolyte membranes and fluorine electrolyte membranes, such as a moisture-containing thin perfluorosulfonic acid membrane. The electrolyte membrane could, for example, be a Nafion membrane (manufactured by DuPont).
[0049] The catalyst dispersion section includes a catalyst. The catalyst can be an oxygen reduction reaction (ORR) active catalyst. The oxygen reduction reaction (ORR) active catalyst can be the catalyst metal mentioned above. The catalyst dispersion section can include the electrolyte mentioned above.
[0050] The arrangement position of the catalyst dispersion section can be adjusted to suit the operating conditions of the fuel cell.
[0051] As required, each elementary fuel cell can comprise two separators that sandwich-like enclose the membrane electrode assembly. One of the two separators is an anode-side separator and the other is a cathode-side separator. In the disclosed embodiments, the anode-side separator and the cathode-side separator are collectively referred to as the "separator".
[0052] The separator can include inlet and outlet ports for releasing the reaction gas and refrigerant in the stacking direction of the elementary fuel cells. The refrigerant can be, for example, a mixed solution of ethylene glycol and water to prevent freezing at low temperatures. The reaction gas is either the fuel gas or the oxidizer gas. The fuel gas can be hydrogen or a similar substance. The oxidizer gas can be oxygen, air, dry air, or a similar substance.
[0053] Examples of feed holes include, among others, a fuel gas feed hole, an oxidant gas feed hole, and a refrigerant feed hole.
[0054] Examples of outlet holes include fuel gas outlet holes, oxidizer gas outlet holes, and refrigerant outlet holes.
[0055] The separator may include one or more fuel gas supply holes, one or more oxidizer gas supply holes, one or more refrigerant supply holes, one or more fuel gas outlet holes, one or more oxidizer gas outlet holes, and one or more refrigerant outlet holes.
[0056] The separator can have a reaction gas flow path on one surface in contact with the gas diffusion layer. On the opposite surface in contact with the gas diffusion layer, the separator can also have a refrigerant flow path to maintain the fuel cell temperature at a constant level.
[0057] If the separator is the anode-side separator, it may include one or more fuel gas supply holes, one or more oxidizer gas supply holes, one or more refrigerant supply holes, one or more fuel gas outlet holes, one or more oxidizer gas outlet holes, and one or more refrigerant outlet holes. On the surface in contact with the anode-side gas diffusion layer, the anode-side separator may have a fuel gas flow path for delivering the fuel gas from the fuel gas supply hole to the fuel gas outlet hole. On the opposite surface to the surface in contact with the anode-side gas diffusion layer, the anode-side separator may have a refrigerant flow path for delivering the refrigerant from the refrigerant supply hole to the refrigerant outlet hole.
[0058] If the separator is the cathode-side separator, it may include one or more fuel gas supply holes, one or more oxidizer gas supply holes, one or more refrigerant supply holes, one or more fuel gas outlet holes, one or more oxidizer gas outlet holes, and one or more refrigerant outlet holes. On the surface in contact with the cathode-side gas diffusion layer, the cathode-side separator may have an oxidizer gas flow path for delivering the oxidizer gas from the oxidizer gas supply hole to the oxidizer gas outlet hole. On the opposite surface to the surface in contact with the cathode-side gas diffusion layer, the cathode-side separator may have a refrigerant flow path for delivering the refrigerant from the refrigerant supply hole to the refrigerant outlet hole.
[0059] The separator can be a gas-tight, electrically conductive element, or the like. The electrically conductive element can be, for example, dense carbon compressed to be gas-tight, or a pressed sheet (e.g., iron, aluminum, stainless steel, or the like). The separator can have a collection function.
[0060] The fuel cell stack can have a distributor, such as an inlet distributor that communicates between the feed holes, and an outlet distributor that communicates between the outlet holes.
[0061] Examples of inlet distributors include an anode inlet distributor, a cathode inlet distributor, and a refrigerant inlet distributor.
[0062] Examples of outlet distributors include an anode outlet distributor, a cathode outlet distributor, and a refrigerant outlet distributor.
[0063] The fuel cell system includes the fuel gas supply device. The fuel gas supply device delivers the hydrogen-containing fuel gas to the anode of the fuel cell. The fuel gas supply device can deliver the fuel gas to the anodes of the fuel cell stack.
[0064] The fuel gas is a gas that mainly contains hydrogen. For example, it could be hydrogen gas.
[0065] Examples of fuel gas supply equipment include fuel gas tanks, such as liquid hydrogen tanks and pressurized hydrogen tanks.
[0066] The fuel gas supply device is electrically connected to the control unit. The fuel gas supply device is controlled by a control signal from the control unit. At least one value, selected from the group containing the flow rate and pressure of the fuel gas supplied to the anode from the fuel gas supply device, can be controlled by the control unit.
[0067] The fuel cell system may include a fuel gas supply flow path.
[0068] The fuel gas supply path connects the fuel gas supply device and the anode inlet of the fuel cell. The fuel gas supply path enables the delivery of fuel gas from the fuel gas supply device to the anode of the fuel cell. The fuel gas supply path enables the delivery of fuel gas from the fuel gas supply device to the anodes of the fuel cell stack.
[0069] The fuel gas supply flow path may include a fuel gas supply valve.
[0070] The fuel gas supply valve allows control of the flow rate, pressure, etc. of the fuel gas supplied to the anode.
[0071] The fuel gas supply valve can be electrically connected to the control unit, and the flow rate of the fuel gas supplied to the anode and the fuel gas pressure (anode pressure) can be controlled by the control unit by controlling the opening and closing of the fuel gas supply valve. By controlling the degree of opening of the fuel gas supply valve, the flow rate of the fuel gas supplied to the anode and the fuel gas pressure (anode pressure) can be controlled.
[0072] The fuel cell system may include a fuel exhaust gas flow path.
[0073] The fuel exhaust gas outlet flow path is connected to the anode outlet of the fuel cell.
[0074] The fuel exhaust gas flow path captures fuel exhaust gas, which is the fuel gas released from the anode of the fuel cell. The fuel exhaust gas flow path can capture the fuel exhaust gas released from the anodes of the fuel cell stack.
[0075] The fuel exhaust gas contains, for example: the fuel gas that has passed through the anode without reacting; water that has been generated in the cathode and then transferred to the anode; corroded substances that have been generated in the catalyst layer, the electrolyte membrane, etc.; and oxidizing agent gas that may have been supplied to the anode during a cleaning process.
[0076] The fuel exhaust gas outlet flow path may include a fuel exhaust gas outlet valve.
[0077] The fuel exhaust valve allows the fuel gas to escape to the outside (the outside of the system). The outside can be the outside of the fuel cell system or the outside of the vehicle. The fuel exhaust valve can be electrically connected to the controller, and the flow rate of the fuel gas being vented to the outside can be controlled by the controller's operation of the valve's opening and closing. By controlling the degree of opening of the fuel exhaust valve, the pressure of the fuel gas supplied to the anode (anode pressure) can be controlled.
[0078] The fuel cell system can include a circulation flow path.
[0079] The circulation flow path branches off from the fuel exhaust gas outlet flow path and connects to the fuel gas supply flow path. The circulation flow path allows the captured fuel exhaust gas to be returned to the anode as recirculation gas. The circulation flow path also allows water contained in the captured fuel exhaust gas to be supplied to the anode.
[0080] The circulation flow path can merge with the fuel gas supply flow path at their confluence. In the fuel cell system, the circulation flow path can be equipped as needed with an ejector, a circulation pump (such as a hydrogen pump) to control the flow rate of the circulating gas, etc.
[0081] The circulation pump can be electrically connected to the controller, and the flow rate of the circulating gas can be controlled by the controller by controlling the on / off switching, speed, etc. of the circulation pump.
[0082] The ejector can be located, for example, at the confluence of the fuel gas supply and recirculation flow paths. The ejector delivers a gas mixture containing the fuel gas and the recirculating gas to the anode of the fuel cell. The ejector can supply this gas mixture to the anodes of the fuel cell stack. A conventionally known ejector can be used for this purpose.
[0083] The fuel cell system may include an anode gas-liquid separator.
[0084] The anode gas-liquid separator is located at the junction of the circulation flow path and the fuel exhaust flow path. The anode gas-liquid separator separates the water contained in the fuel exhaust from the fuel gas, which is the fuel gas discharged from the anode outlet. The anode gas-liquid separator collects these components and feeds at least some of the collected water and fuel gas to the anode. The anode gas-liquid separator can be electrically connected to the control system. The opening and closing of the anode gas-liquid separator's outlet valve can be controlled by the control system. The degree of opening of the anode gas-liquid separator's outlet valve can also be controlled by the control system. The amount of water supplied to the anode, the fuel gas flow rate, and other parameters can be controlled in this way.
[0085] The fuel cell system includes the oxidizer gas supply device.
[0086] The oxidizer gas supply device delivers the oxygen-containing oxidizer gas to the cathode of the fuel cell. The oxidizer gas supply device can deliver the oxidizer gas to the cathodes of the fuel cell stack.
[0087] The oxidizing gas is an oxygen-containing gas. It can be air, dry air, pure oxygen, or the like.
[0088] An air compressor, for example, can be used as the oxidizing gas supply device.
[0089] The oxidizer gas supply device is electrically connected to the controller. The oxidizer gas supply device is controlled by a control signal from the controller. At least one value, selected from the group consisting of the flow rate and pressure of the oxidizer gas supplied to a cathode from the oxidizer gas supply device, can be controlled by the controller.
[0090] The fuel cell system may include an oxidant gas supply flow path.
[0091] The oxidizer gas supply path connects the oxidizer gas supply device and the cathode inlet of the fuel cell. This path allows the oxidizer gas to be supplied from the oxidizer gas supply device to the cathode of the fuel cell. The oxidizer gas supply path can also allow the oxidizer gas to be supplied from the oxidizer gas supply device to the cathodes of the fuel cell stack.
[0092] The fuel cell system may include an oxidizer exhaust gas outlet flow path.
[0093] The oxidizer exhaust gas flow path is connected to the cathode outlet of the fuel cell. This flow path allows the oxidizer exhaust gas, which is the oxidizer gas released from the fuel cell cathode, to be discharged to the outside. The oxidizer exhaust gas flow path can also allow the oxidizer exhaust gas, which is the oxidizer gas released from the cathodes of the fuel cell stack, to be discharged to the outside.
[0094] The oxidizer exhaust gas outlet flow path can be provided with an oxidizer gas pressure control valve.
[0095] The oxidizer gas pressure control valve is electrically connected to the control unit. When the control unit opens the oxidizer gas pressure control valve, oxidizer gas, which is the reacted oxidizer gas, is released from the oxidizer gas outlet flow path to the outside. By controlling the degree of opening of the oxidizer gas pressure control valve, the pressure of the oxidizer gas supplied to the cathode (cathode pressure) can be controlled.
[0096] The fuel gas supply flow path and the oxidizer gas supply flow path can be connected via a merging flow path. The merging flow path can be provided with a purge valve.
[0097] The purge valve can be electrically connected to the control unit. By opening the purge valve via the control unit, the oxidizer gas in the oxidizer gas supply device can be allowed to flow as purge gas into the fuel gas supply flow path.
[0098] The purge gas is used for purging. It can be fuel gas, oxidizer gas, or a reaction gas mixture containing these.
[0099] The fuel cell system may include a refrigerant supply device and a refrigerant circulation flow path as the cooling system of the fuel cell.
[0100] The refrigerant circulation flow path communicates between the refrigerant inlet port and the refrigerant outlet port provided in the fuel cell, and it allows the refrigerant supplied from the refrigerant supply device to circulate in and out of the fuel cell.
[0101] The refrigerant supply unit is electrically connected to the controller. The refrigerant supply unit is controlled by a control signal from the controller. The flow rate of the refrigerant supplied from the refrigerant supply unit to the fuel cell is controlled by the controller. This allows the temperature of the fuel cell to be controlled.
[0102] Examples of refrigerant supply devices include a cooling element and a cooling water pump.
[0103] The refrigerant circulation flow path can be provided with a radiator for heat dissipation from the cooling water.
[0104] For example, a mixed solution of ethylene glycol and water can be used as the cooling water (refrigerant) to prevent freezing at low temperatures.
[0105] The fuel cell system can include a secondary cell.
[0106] The secondary cell (battery) can be any rechargeable and dischargeable cell. Examples of secondary cells include nickel-hydrogen and lithium-ion cells. The secondary cell can incorporate a power storage element, such as an electrical double-layer capacitor. The secondary cell can have a structure in which multiple secondary cells are connected in series. The secondary cell supplies power to the motor, the oxidizer gas supply device, etc. The secondary cell can be recharged by an external power source, such as a household power source. The secondary cell can be recharged by the output power of the fuel cell. The charging and discharging of the secondary cell can be controlled by the control unit.
[0107] The fuel cell system includes the oxygen partial pressure estimation function. This function estimates the oxygen partial pressure at the fuel cell's cathode.
[0108] The oxygen partial pressure estimation function can be electrically connected to the controller and can be activated according to a control signal from the controller. The oxygen partial pressure estimation function can also be integrated into the controller as a program. Accordingly, the controller can also function as the oxygen partial pressure estimation function.
[0109] The oxygen partial pressure can be estimated, for example, by the following procedure: A data set that specifies the relationship between the operating conditions of the fuel cell and the oxygen partial pressure is created in advance, and the actual operating conditions of the fuel cell are compared with the data set, thereby estimating the oxygen partial pressure.
[0110] The fuel cell system can also include a pressure sensor to measure the pressure of the oxidizer gas at the cathode. The pressure sensor is electrically connected to the controller and provides it with the measured oxidizer gas pressure. The controller can then estimate the oxygen partial pressure based on the oxidizer gas pressure measured by the pressure sensor.
[0111] The timing at which the oxygen partial pressure estimation function estimates the oxygen partial pressure is not specifically restricted, as long as it is after the fuel cell has started generating power.
[0112] The fuel cell system includes the hydrogen partial pressure estimation function. This function estimates the hydrogen partial pressure at the anode of the fuel cell.
[0113] The hydrogen partial pressure estimation function can be electrically connected to the controller and can be controlled according to a control signal from the controller. The hydrogen partial pressure estimation function can also be integrated into the controller as a program. Accordingly, the controller can also function as the hydrogen partial pressure estimation function.
[0114] The hydrogen partial pressure can be estimated, for example, by the following method: A data set that specifies the relationship between the operating conditions of the fuel cell and the hydrogen partial pressure is created in advance, and the actual operating conditions of the fuel cell are compared with the data set, thereby estimating the hydrogen partial pressure.
[0115] The fuel cell system can also include a pressure sensor to measure the pressure of the fuel gas at the anode. The pressure sensor is electrically connected to the controller and provides it with the measured fuel gas pressure. The controller can then estimate the hydrogen partial pressure based on the fuel gas pressure measured by the pressure sensor.
[0116] The timing at which the hydrogen partial pressure estimation function estimates the hydrogen partial pressure is not specifically restricted, as long as it is after the start of power generation by the fuel cell.
[0117] The fuel cell system includes the control unit.
[0118] The controller physically comprises a processing unit, such as a central processing unit (CPU), a memory device, such as read-only memory (ROM) and random-access memory (RAM), and an input / output interface. The ROM is used to store a control program, control data, etc., to be processed by the CPU, and the RAM is primarily used as various workspaces for control processing. The controller can also be a control device, such as an electronic control unit (ECU).
[0119] The control unit can be electrically connected to an ignition switch, which may be installed in the vehicle. The control unit can also be powered by an external power source, even when the ignition switch is off.
[0120] The controller calculates the target hydrogen partial pressure using the following equation (1). The controller adjusts the hydrogen partial pressure at the anode to the target hydrogen partial pressure. Equation (1): Target hydrogen partial pressure = [2 × oxygen permeation coefficient in the thickness direction of the electrolyte membrane × {distance from the anode catalyst layer to the catalyst dispersion section ÷ (electrolyte membrane thickness - distance from the anode catalyst layer to the catalyst dispersion section)} × oxygen partial pressure] ÷ hydrogen permeation coefficient in the thickness direction of the electrolyte membrane pH2r=2k02kH2(tcattmem−tcat)p02
[0121] The meaning of each symbol in the above equation (1) is as follows. k_H2: Hydrogen permeation coefficient (mol / (m·s·kPa)) in the thickness direction of the electrolyte membrane Pr_H2: Target anode hydrogen partial pressure (kPa) k_O2: Oxygen permeation coefficient (mol / (m·s·kPa)) in the thickness direction of the electrolyte membrane p_O2: Cathode oxygen partial pressure (kPa) t_mem: electrolyte membrane thickness (m) t_cat: Distance (m) from the anode catalyst layer to the catalyst dispersion section
[0122] By calculating the target hydrogen partial pressure using equation (1) above and controlling the hydrogen partial pressure at the anode to this target, chemical decomposition of the electrolyte membrane can be effectively suppressed. If the hydrogen partial pressure at the anode exceeds the target hydrogen partial pressure from equation (1), hydrogen peroxide (H₂O₂) is produced, which is a substance that causes the decomposition of the electrolyte membrane within the catalyst contained in the electrolyte membrane.
[0123] Fig. Figure 3 is a schematic representation illustrating an example of a conventional membrane electrode arrangement. Fig. 3 means “t_mem” is the electrolyte membrane thickness (m).
[0124] Fig. Figure 4 is a schematic representation illustrating an example of the membrane electrode arrangement used in the disclosed embodiments. Fig. 4 means “t_mem” is the electrolyte membrane thickness (m), and “t_cat” means the distance (m) of the catalyst dispersion section from the anode catalyst layer.
[0125] In Fig. 3 O2, which has permeated through the electrolyte membrane, undergoes two-electron reduction in the anode catalyst layer, thereby becoming hydrogen peroxide (H2O2).
[0126] The hydrogen peroxide is converted into free radicals by Fe ions and the like, and these free radicals oxidize and decompose the polymer of the electrolyte membrane. Fig. 4. H2 and O2, which have permeated from both electrodes, react in an oxygen reduction reaction active catalyst (ORR active catalyst) which has been added to the electrolyte membrane. If the H2 flux and O2 flux at the ORR active catalyst position is 2:1, the O2 that has permeated to the anode catalyst layer is essentially zero.
[0127] Fig. Figure 5 is a curve showing an example of the relationship between the position of the catalyst in the thickness direction from the anode in the electrolyte membrane and the gas permeation flow of the electrolyte membrane. Fig. Figure 5 shows the flux (j_H2) of H2 permeating from the anode at the ORR catalyst position shown below, and the flux (j_O2) of O2 permeating from the cathode at the ORR catalyst position. The reaction formula for the formation of water from hydrogen and oxygen is as follows: 2H2 + O2 = H2O
[0128] Accordingly, the ideal hydrogen flux and the ideal oxygen flux at the position where the catalyst dispersion section is located are described as follows: 2jH2 = jO2 jH2=KH2tcatpH2 j_H2: hydrogen flow (mol / (m 2 ·s)) k_H2: Hydrogen permeation coefficient (mol / (m·s·Pa)) in the thickness direction of the electrolyte membrane p_H2: Anode hydrogen partial pressure (kPa) t_cat: Distance (m) from the anode catalyst layer to the catalyst dispersion section j02=K02tmem−tcatp02 j_O2: Oxygen flux (mol / (m 2 ·s)) k_O2: Oxygen permeation coefficient (mol / (m·s·kPa)) in the thickness direction of the electrolyte membrane p_O2: Cathode oxygen partial pressure (kPa) t_mem: electrolyte membrane thickness (m) t_cat: Distance (m) from the anode catalyst layer to the catalyst dispersion section
[0129] It is described how the optimal position for adding the ORR active catalyst to the electrolyte membrane is determined with respect to the operating conditions of the fuel cell (the H2 partial pressure and the O2 partial pressure), which is described below.
[0130] Fig. Figure 6 is a curve showing an example of the case where the position of the catalyst dispersion section is optimal with respect to the operating conditions of the fuel cell.
[0131] Among the in Fig. Under the conditions shown in Figure 6, the hydrogen and oxygen fluxes reacting in the catalyst dispersion section are described as follows. Hydrogen flow × 2 = oxygen flow
[0132] Accordingly, no O2 permeates through the anode catalyst layer and no hydrogen peroxide is generated in the anode catalyst layer.
[0133] Fig. Figure 7 is a curve showing an example of the case where the catalyst dispersion section is located on the cathode side, compared to the case where the position of the catalyst dispersion section is the optimal position with respect to the operating conditions of the fuel cell.
[0134] Among the in Fig. Under the conditions shown in Figure 7, the hydrogen and oxygen fluxes reacting in the catalyst dispersion section are described as follows. Hydrogen flow × 2 = oxygen flow
[0135] Accordingly, some of the O2 that permeates through the electrolyte membrane is converted to hydrogen peroxide (H2O2) in the anode catalyst layer. However, the amount of hydrogen peroxide produced is smaller than with an electrolyte membrane that does not include an ORR active catalyst.
[0136] Fig. Figure 8 is a curve showing an example of the case where the catalyst dispersion section is located on the anode side, compared to the case where the position of the catalyst dispersion section is the optimal position with respect to the operating conditions of the fuel cell.
[0137] Among the in Fig. Under the conditions shown in Figure 8, the hydrogen and oxygen fluxes reacting in the catalyst dispersion section are described as follows. Hydrogen flow × 2 = oxygen flow
[0138] Accordingly, no O₂ permeates the anode catalyst layer, and therefore no hydrogen peroxide is generated in the anode catalyst layer. However, the environment surrounding the catalyst dispersion section becomes H₂-rich. Consequently, some of the O₂ is reduced by two electrons, generating hydrogen peroxide. Furthermore, the amount of hydrogen peroxide generated in the catalyst dispersion section is greater than the amount generated in the anode catalyst layer of a membrane electrode assembly that uses an electrolyte membrane without an ORR active catalyst.
[0139] In the cells of the fuel cell stack, the oxygen partial pressure and the hydrogen partial pressure decrease from the inlet holes to the outlet holes. Accordingly, it is not easy to satisfy equation (1) over the entire surface of the cells.
[0140] Even if the cells are present in some form, there is also an area of severe chemical decomposition in the plane.
[0141] Various gas flow patterns are conceivable in each elementary fuel cell, such as a straight flow path, a serpentine flow path, a counterflow, a parallel flow, etc. Accordingly, the oxygen partial pressure distribution on the electrode surface also varies. Consequently, the target hydrogen partial pressure value also varies depending on the position of each power-generating surface.
[0142] Accordingly, it is not easy to control the hydrogen partial pressure to a local target hydrogen partial pressure.
[0143] The local target hydrogen partial pressure can also be brought close to the actual hydrogen partial pressure by varying the catalyst additive position (t_cat) of the electrolyte membrane beneath the fuel cell stack cells. However, this complicates the electrolyte membrane production process, leading to increased costs.
[0144] On the other hand, in an actual usage environment, chemical decomposition in the electrode surface of each elementary fuel cell does not progress uniformly, and the position where a leak occurs is often predetermined.
[0145] If it is possible to control the hydrogen partial pressure relative to the oxygen partial pressure at the location where leakage is likely to occur, it is possible to suppress chemical decomposition at that location. Accordingly, the stack's durability can be improved.
[0146] In the disclosed embodiments, an estimated value for a region where the stress due to chemical decomposition on the electrode surface is relatively large can accordingly be used as an oxygen partial pressure, which is inserted into equation (1).
[0147] The oxygen partial pressure estimation function can estimate the oxygen partial pressure of a region where the stress from chemical decomposition in the cathode is relatively large, and the controller can insert the oxygen partial pressure estimated by the oxygen partial pressure estimation function into equation (1) to calculate the target hydrogen partial pressure.
[0148] Fig. Figure 9 is a schematic representation showing an example of the cell level. Fig. 9 means “Air_in” is the oxidizer gas supply hole; „ “H2_in” indicates the fuel gas inlet hole; and “LLC_in” indicates the refrigerant inlet hole. An area surrounded by a dashed line is an area susceptible to chemical decomposition.
[0149] Fig. Figure 10 is a curve showing an example of the relationship between the position of the catalyst in the thickness direction from the anode in the electrolyte membrane, the oxygen partial pressure and the hydrogen partial pressure.
[0150] For example, if the fuel cell system of the disclosed embodiments is a fuel cell system in which the position where leakage always occurs after durability testing in an actual usage environment is the cathode inlet area, the target hydrogen partial pressure is determined based on the oxygen partial pressure at the position and the hydrogen partial pressure at the position can be controlled to the target hydrogen partial pressure.
[0151] For example, the area immediately surrounding the cooling water outlet is easily degraded due to high temperatures. Accordingly, the target hydrogen partial pressure can be determined based on the oxygen partial pressure at that location, and the hydrogen partial pressure at that location can be controlled to achieve the target hydrogen partial pressure.
[0152] By using the estimated value of the area where the stress from chemical decomposition is relatively high on the electrode surface as the oxygen partial pressure, which is inserted into equation (1), the cathode can exert its most effective action in the region where chemical decomposition is relatively severe. Accordingly, the occurrence of leakage due to chemical decomposition can be suppressed and the durability of the fuel cell can be improved.
[0153] The control system determines whether the target hydrogen partial pressure is lower than the lower limit of the hydrogen partial pressure or not.
[0154] If the target hydrogen partial pressure is determined to be lower than the lower limit of the hydrogen partial pressure, the controller increases the oxygen partial pressure at the cathode. If the hydrogen partial pressure is lower than the lower limit of the hydrogen partial pressure, the target hydrogen partial pressure can be increased by increasing the oxygen partial pressure at the cathode. Accordingly, the oxygen partial pressure at the cathode is increased so that the target hydrogen partial pressure is at or above the lower limit of the hydrogen partial pressure.
[0155] If it is determined that the target hydrogen partial pressure is the lower limit of the hydrogen partial pressure, the controller receives the oxygen partial pressure of the cathode and it receives the hydrogen partial pressure of the anode.
[0156] If the target hydrogen partial pressure is determined to be higher than the lower limit of the hydrogen partial pressure, the controller adjusts the hydrogen partial pressure at the anode to achieve the target hydrogen partial pressure. If the target hydrogen partial pressure is higher than the lower limit of the hydrogen partial pressure, suppressing electrolyte membrane degradation is prioritized, even if this results in reduced fuel efficiency.
[0157] The target hydrogen partial pressure can be defined as the hydrogen partial pressure at which electrolyte membrane decomposition does not occur.
[0158] The lower limit of the hydrogen partial pressure can be set as the minimum partial pressure to prevent the occurrence of hydrogen deficiency, taking into account the variance in the fuel gas distribution between the cells or in each cell and the reaction distribution of the fuel gas.
[0159] The lower limit of the hydrogen partial pressure can be set appropriately, depending on the operating conditions of the fuel cell; it can also be a constant. Alternatively, the lower limit of the hydrogen partial pressure can be preset to a value corresponding to the position of the catalyst dispersion section, eliminating the need to calculate the lower limit of the hydrogen partial pressure each time.
[0160] Fig. Figure 11 is a curve showing an example of the relationship between the oxygen partial pressure and the target hydrogen partial pressure.
[0161] As in Fig. As shown in Figure 11, for example, if the fuel cell is operated at an oxygen partial pressure of 20 kPa, the hydrogen partial pressure suitable for suppressing electrolyte membrane decomposition is 22 kPa. However, if the lower limit of the hydrogen partial pressure to prevent hydrogen deficiency is 40 kPa, the hydrogen partial pressure cannot be reduced to 40 kPa or less, and electrolyte membrane decomposition therefore continues. By increasing the oxygen partial pressure, the hydrogen partial pressure suitable for suppressing electrolyte membrane decomposition is also increased. Accordingly, electrolyte membrane decomposition can be suppressed, thus preventing hydrogen deficiency in the fuel cell.
[0162] Fig. Figure 12 is a schematic diagram showing an example of the fuel cell system of the disclosed embodiments.
[0163] The in Fig. Figure 12 illustrates the fuel cell system 100 comprising a fuel cell 10, a fuel gas supply device 20, a fuel gas supply valve 21, a fuel exhaust valve 22, a circulation pump 23, an oxidant gas supply device 30, an oxidant gas pressure control valve 31, a cooling element 40, a cooling water pump 41, a controller (not shown), an oxygen partial pressure estimation function (not shown), and a hydrogen partial pressure estimation function (not shown).
[0164] Fig. Figure 13 is a flowchart showing an example of the control carried out by the fuel cell system of the disclosed embodiments.
[0165] First, the fuel cell is started.
[0166] The oxygen partial pressure estimation function then estimates the oxygen partial pressure at the cathode of the fuel cell.
[0167] The controller calculates a target hydrogen partial pressure based on the determined oxygen partial pressure using equation (1).
[0168] The control determines whether the calculated target hydrogen partial pressure (Pr_H2) is lower than a preset lower limit of the hydrogen partial pressure (P_H2_Llim) or not.
[0169] If it is determined that the target hydrogen partial pressure is less than the lower limit of the hydrogen partial pressure, the controller increases the oxygen partial pressure at the cathode.
[0170] If it is determined that the target hydrogen partial pressure is the lower limit of the hydrogen partial pressure, the controller receives the oxygen partial pressure of the cathode and it receives the hydrogen partial pressure of the anode.
[0171] If it is determined that the target hydrogen partial pressure is higher than the lower limit of the hydrogen partial pressure, the controller adjusts the hydrogen partial pressure at the anode to achieve the target hydrogen partial pressure. The hydrogen partial pressure can be the value estimated by the hydrogen partial pressure estimation function.
[0172] The hydrogen partial pressure in the fuel cell can be controlled, for example, by the following methods.
[0173] If the opening degree of the fuel exhaust valve is reduced, the concentration of nitrogen permeating from the cathode increases. Consequently, the hydrogen partial pressure can be reduced. If the fuel exhaust valve is an ON-OFF valve, reducing the opening frequency increases the concentration of nitrogen permeating from the cathode. Consequently, the hydrogen partial pressure can be reduced.
[0174] When the circulation flow rate is increased by the circulation pump, the hydrogen concentration gradient generated from the upstream to the downstream side of the cell becomes smaller. Consequently, the hydrogen partial pressure on the inlet side can be reduced, and the hydrogen partial pressure on the outlet side can be increased.
[0175] Reducing the opening degree of the fuel gas supply valve decreases the anode gas pressure. Consequently, the hydrogen partial pressure can be reduced. If the fuel gas supply valve is an ON-OFF valve, decreasing the opening frequency reduces the anode gas pressure. Consequently, the hydrogen partial pressure can be reduced.
[0176] These control methods can be combined.
[0177] The oxygen partial pressure in the fuel cell can be controlled, for example, by the following methods.
[0178] If the opening degree of the oxidizer gas supply valve is reduced, the cathode gas pressure is increased. Consequently, the oxygen partial pressure can be increased.
[0179] When the flow rate of air supplied from the air compressor is increased, the oxygen concentration gradient generated from the upstream to the downstream side of the cell becomes smaller. Consequently, the partial pressure of oxygen on the outlet side can be increased.
[0180] The control methods mentioned above can be combined. Reference symbol list 1 anode catalyst layer 2 Electrolyte membrane 3 Cathode catalyst layer 4 Catalyst dispersion section 10 Fuel cell 20 Fuel gas supply device 21 Fuel gas supply valve 22 Fuel exhaust valve 23 Circulation pump 30 Oxidizing agent gas supply device 31 Oxidizing agent gas pressure control valve 40 cooling elements 41 Cooling water pump 100 fuel cell systems
Claims
[1] Fuel cell system comprising: a fuel cell, an oxidant gas supply device designed to supply oxygen-containing oxidant gas to a cathode of the fuel cell, a fuel gas supply device designed to supply hydrogen-containing fuel gas to an anode of the fuel cell, an oxygen partial pressure estimation function configured to estimate an oxygen partial pressure of the cathode of the fuel cell, a hydrogen partial pressure estimation function configured to estimate a hydrogen partial pressure of the anode of the fuel cell, and a control, wherein the fuel cell comprises one or more elementary fuel cells; wherein the one or more fuel cells comprise a membrane electrode assembly; wherein the membrane electrode assembly comprises an anode having an anode catalyst layer, an electrolyte membrane, and a cathode having a cathode catalyst layer in that order; wherein the electrolyte membrane comprises a catalyst dispersion section in the electrolyte membrane arranged in a layered manner parallel to the anode catalyst layer and the cathode catalyst layer; wherein the catalyst dispersion section comprises a catalyst; wherein the controller calculates a target hydrogen partial pressure by the following equation (1): Target hydrogen partial pressure = [2 × oxygen permeation coefficient in the thickness direction of the electrolyte membrane × {distance from the anode catalyst layer to the catalyst dispersion section ÷ (electrolyte membrane thickness − distance from the anode catalyst layer to the catalyst dispersion section)} × oxygen partial pressure ÷ ] hydrogen permeation coefficient in the thickness direction of the electrolyte membrane; and wherein the controller controls the hydrogen partial pressure of the anode to the target hydrogen partial pressure. [2] Fuel cell system according to claim 1, wherein the controller determines whether or not the target hydrogen partial pressure is lower than the lower limit of the hydrogen partial pressure; wherein, when it is determined that the target hydrogen partial pressure is lower than the lower limit of the hydrogen partial pressure, the controller increases the oxygen partial pressure of the cathode; wherein, when it is determined that the target hydrogen partial pressure is the lower limit of the hydrogen partial pressure, the controller obtains the oxygen partial pressure of the cathode and obtains the hydrogen partial pressure of the anode; and wherein, when it is determined that the target hydrogen partial pressure is higher than the lower limit of the hydrogen partial pressure, the controller controls the hydrogen partial pressure of the anode to achieve the target hydrogen partial pressure.