Fuel cell assemblies and related methods of operation

Abstract

A method of operating a fuel cell assembly comprising a plurality of fuel cells connected together to collectively provide power to a load, each fuel cell comprising an anode and a cathode, the method comprising selectively providing an electrical connection between the anode and cathode of at least one of the fuel cells of the assembly to reduce the voltage of the fuel cell independently of the load.

Description

[0001] This application is a divisional application of Chinese patent application 201580066402.4 (PCT / GB2015 / 053737) filed on December 7, 2015 and entitled “Fuel Cell Assembly and Related Operating Methods”. Technical Field

[0002] The present invention relates to a fuel cell assembly and a method of operating a fuel cell assembly. In particular, the present invention relates to a fuel cell assembly configured to regulate the individual fuel cells forming the assembly by controlling the voltage of the fuel cells independently of the load applied to the assembly. Background Art

[0003] Conventional electrochemical fuel cells convert fuel and oxidant into electrical energy and reaction products. Common types of electrochemical fuel cells include membrane electrode assemblies (MEA), which include polymer ion (proton) transfer membranes and gas diffusion structures between anodes and cathodes. Fuel (e.g., hydrogen) and oxidant (e.g., oxygen from air) pass through the respective sides of the MEA to generate electrical energy and water as reaction products. A stack can be formed, which includes a plurality of such fuel cells arranged with independent anode and cathode fluid flow paths. Such a stack is typically in the form of a block, which includes a plurality of individual fuel cell plates held together by end plates at either end of the stack. Summary of the invention

[0004] According to a first aspect, we provide a fuel cell assembly comprising a plurality of fuel cells connected together so as to collectively provide electrical power to a load, the assembly comprising one or more regulating switches, each regulating switch being associated with one or more of the plurality of fuel cells and being configured to selectively provide an electrical connection between the anode and cathode of the fuel cell to which it is associated, or when associated with a fuel cell stack, selectively provide an electrical connection between the anode of the first fuel cell and the cathode of the last fuel cell in the stack so as to control the voltage of the fuel cell or the fuel cell stack independently of the load.

[0005] This is advantageous because the assembly can regulate the fuel cell or its component parts, such as proton exchange membrane or catalyst layer, for effective operation or for mitigating the degradation mechanism of the fuel cell. The electrical connection provided by the regulating switch can provide a short circuit of the associated fuel cell or the fuel cell group connected in series, because it can produce a lower impedance electrical path relative to the electrical path for powering the load. Therefore, the fuel cell can be electrically connected together to power the load and the actuation of the regulating switch can provide additional or alternative electrical connections from the anode and / or cathode of the battery. In particular, the electrical connection can reduce the potential on the associated fuel cell or fuel cell group. During operation (i.e. during the period when the fuel cell assembly is provided with reactants), particularly when the fuel cell assembly is started or shut down (i.e. the fuel cell assembly may be provided or not provided with reactants or when the supply of reactants starts or stops), controlling the voltage of a separate fuel cell has been found to have many beneficial effects, as will be described below.

[0006] Optionally, all fuel cells of the plurality of fuel cells, either individually or as part of a series connected group, are associated with a regulating switch, enabling the voltage of each fuel cell in the assembly to be controlled independently of the load.

[0007] Optionally, each fuel cell of the plurality of fuel cells comprises a corresponding regulating switch for selectively providing the electrical connection. Thus, the voltage of each individual fuel cell can be controlled. It may be advantageous to control the voltage or short circuit of individual fuel cells in the assembly. The regulating switches may be configured to be actuated individually and sequentially. Alternatively, more than one regulating switch may be configured to be actuated simultaneously.

[0008] A fuel cell assembly may include a fuel cell stack.

[0009] Optionally, the one or more regulating switches include semiconductor devices such as transistors. Alternatively, the one or more regulating switches include inductor coupled relays.

[0010] Optionally, each fuel cell includes a fuel cell plate configured to abut the membrane electrode assembly of the fuel cell, the fuel cell plate including at least one connector for connecting to the regulating switch. The fuel cell plate that can separate the individual fuel cells in the fuel cell stack provides a convenient body to form a connection with the regulating switch. Optionally, the fuel cell plate may include a protrusion protruding from the plate for connecting to the regulating switch, or may include a cavity for receiving a connection with the regulating switch. One or more regulating switches may be located within the body of the fuel cell assembly or outside the body of the fuel cell assembly.

[0011] Optionally, the fuel cell board includes a plurality of connectors, and the regulating switch correspondingly includes a plurality of switch elements, which are configured to be connected to the plurality of connectors and operate together to selectively provide electrical connections for the fuel cells associated therewith. Each regulating switch can be provided by a plurality of semiconductor devices. A plurality of connectors on the fuel cell board can advantageously provide connection points for a plurality of semiconductor devices. This is advantageous if a large current needs to be passed through the electrical connection formed by the switch elements. Therefore, a plurality of semiconductor devices can be configured to be actuated together to provide the function of the regulating switch.

[0012] Optionally, each fuel cell board includes at least two connectors, and the at least two connectors are formed on different sides of the fuel cell board. The at least two connectors can be located on the same side of the fuel cell board, different sides of the fuel cell board or opposite sides. If a large current needs to be passed by electrical connection, providing connectors on different parts of the fuel cell board is conducive to reducing the peak current density in the area of ​​the connector. In other embodiments, a single connector on each fuel cell board is provided for connection to a regulating switch.

[0013] Optionally, the plurality of fuel cells are arranged together in a stack, the stack comprising a manifold arranged with the fuel cell stack so as to deliver reactants to each of the plurality of fuel cells in the stack or receive an exhaust flow from each of the plurality of fuel cells in the stack, the assembly comprising a substrate extending between the fuel cell stack and the manifold, the substrate comprising the one or more regulating switches. Optionally, each fuel cell comprises a fuel cell plate configured to abut against a membrane electrode assembly of the fuel cell, the substrate being configured to engage with each fuel cell plate. A common substrate for each fuel cell may be provided. The substrate is advantageous because the regulating switches for a plurality of fuel cells may be mounted on a common substrate, the common substrate being positioned so that the regulating switches carried by it are connected to the fuel cells in the stack.

[0014] Optionally, the electrical connection includes a fuel cell voltage control element, which is configured to control the voltage of the fuel cell associated therewith to a predetermined voltage. The fuel cell voltage control element may include a controllable impedance. The fuel cell voltage control element may be configured to independently control the voltage at the anode and the voltage at the cathode of the fuel cell associated therewith. The fuel cell voltage control element may be configured to control the fuel concentration / fuel amount / anode flooding level to control the anode voltage. The fuel cell voltage control element may be configured to control the oxidant concentration / oxidant amount / cathode flooding to control the cathode voltage.

[0015] Optionally, the fuel cell voltage control element is controllable so that the predetermined voltage is configurable during operation of the fuel cell assembly. Thus, the voltage to which each fuel cell or its anode / cathode is reduced / raised can be actively controlled. A regulating switch can provide a connection to the fuel cell voltage control element. Alternatively, the regulating switch can form part of the fuel cell voltage control element. In either case, an electrical connection is formed between the anode and cathode of its associated fuel cell.

[0016] Optionally, a plurality of regulating switches are provided, and the regulating switch controller is configured to control the regulating switches. Thus, the fuel cells of the assembly may each have an associated regulating switch and / or fuel cell voltage control element. The controller may advantageously actuate the regulating switch and / or control the fuel cell voltage control element as required, for example sequentially or together.

[0017] Optionally, the controller is configured to actuate the regulating switch to selectively provide the electrical connection at the start-up or shutdown of the fuel cell assembly. At startup, the actuation of the regulating switch can protect the fuel cell of the assembly from degradation caused by the reactant pressure front moving through the assembly. At shutdown, the actuation of the switch helps to consume residual reactants. Therefore, outside the startup period, for example, within a period of time after the supply of the reactant is started, the selective actuation of the regulating switch can be reduced or stopped in frequency.

[0018] Optionally, the controller is configured to actuate the regulating switch to selectively provide the electrical connection sequentially across the plurality of fuel cells. Optionally, less than 10% or 5% of the fuel cells in the assembly or only a single fuel cell is provided with electrical connection at any time. It will be appreciated that although each regulating switch may be actuated individually, the controller may actuate more than one at any time.

[0019] Optionally, the controller is configured to actuate the regulating switch to selectively provide the electrical connection in response to the voltage of the fuel cell assembly exceeding a predetermined threshold.Operating the fuel cell assembly close to the open circuit voltage may cause degradation, which may be mitigated by the selective use of the regulating switch.

[0020] Optionally, the controller is configured to actuate the regulating switch to selectively provide the electrical connection in response to one or more measures of fuel cell performance exceeding an associated threshold.

[0021] Optionally, the measure of fuel cell performance includes:

[0022] i) a measure of a pollutant, such as carbon monoxide, in said fuel cell component; or

[0023] ii) a measure of the voltage of any one (or more than one) of the plurality of fuel cells; or

[0024] iii) a measure of flooding an individual fuel cell (or more than one fuel cell) with coolant; or

[0025] iv) a measure of the environmental conditions close to the fuel cell assembly, such as temperature or humidity.

[0026] Optionally, the controller is configured to actuate the regulating switch to selectively provide the electrical connection at least between the start of an increase in the flow of reactants through the fuel cell assembly of the assembly and the reactant flow rate or pressure reaching a predetermined level. The increase in reactant flow may result in a pressure front, which may be significant at fuel cell startup. Any adverse effects of the pressure front may be mitigated by using the regulating switch.

[0027] Optionally, the controller is configured to actuate the regulating switch to selectively provide electrical connection for a particular one of the fuel cells in response to an indication that the voltage of the particular one of the fuel cells has become or is about to become negative relative to at least one other fuel cell in the assembly. Optionally, the controller is configured to monitor the voltage of each of the fuel cells in the assembly.

[0028] Optionally, the controller is configured to provide control of the temperature of at least one fuel cell in the fuel cell assembly by actuating its associated regulating switch to selectively provide an electrical connection for the at least one fuel cell. This may be advantageous if the fuel cell assembly is operated in sub-zero conditions to maintain a predetermined threshold temperature on a plurality of fuel cells. When the fuel cell stack is operated at an ambient temperature below a threshold or when the temperature of the fuel cell stack is below a threshold, the controller may be configured to actuate the regulating switch of the fuel cell at the end of the fuel cell stack in preference to the fuel cell at the center of the fuel cell stack.

[0029] Optionally, the controller is configured to balance the temperature of the fuel cells in the fuel cell assembly by selectively actuating the regulating switches of specific fuel cells to increase their temperature.

[0030] Optionally, the controller is configured to actuate the regulating switch of at least one of the fuel cells to provide the electrical connection upon startup of the fuel cell assembly, and in response to the current through the fuel cell reaching a threshold, actuate its associated regulating switch to disconnect the electrical connection. Optionally, the controller is configured to actuate the regulating switches of at least two of the fuel cells to provide the electrical connection upon startup of the fuel cell assembly, and in response to the current through a particular fuel cell of the at least two fuel cells reaching a threshold, actuate its associated regulating switch to disconnect the electrical connection. Thus, when the current through each fuel cell reaches its threshold, the regulating switch may be actuated to remove the electrical connection.

[0031] Optionally, the controller is configured to actuate the regulating switches of most of the fuel cells in the fuel cell assembly to provide the electrical connection at the start-up of the fuel cell assembly, and in response to the current through a specific fuel cell reaching a threshold, actuate its associated regulating switch to disconnect the electrical connection. In another example, electrical connections can be provided for all fuel cells in the assembly, and when each fuel cell reaches a predetermined voltage, its associated regulating switch can be opened to disconnect.

[0032] According to a second aspect, we provide a method of operating a fuel cell assembly, the fuel cell assembly comprising a plurality of fuel cells connected together to collectively provide power to a load, the method comprising:

[0033] An electrical connection is selectively provided between the anode and cathode of at least one of the fuel cells of the assembly, or, for a fuel cell stack, between the anode of the first fuel cell and the cathode of the last fuel cell in the stack, so as to control the voltage of the fuel cell or the fuel cell stack independently of the load.

[0034] Optionally, the method comprises the step of actuating a regulating switch to selectively provide the electrical connection at startup or shutdown of the fuel cell assembly. Outside of the startup period or shutdown period, the switch may be actuated less frequently or only in response to a measured variable of the fuel cell assembly, such as its temperature or a single fuel cell voltage.

[0035] Optionally, the method comprises the step of actuating a regulating switch to selectively provide said electrical connection sequentially across said plurality of fuel cells. Optionally, less than 10%, 5% of the fuel cells or only a single fuel cell are provided with said electrical connection at any time.

[0036] Optionally, the method comprises the step of actuating a regulating switch to selectively provide the electrical connection in response to the voltage of the fuel cell assembly exceeding a predetermined threshold. The threshold may comprise a voltage based on an open circuit voltage of the assembly.

[0037] Optionally, the method comprises the step of actuating a regulating switch to selectively provide said electrical connection in response to one or more measures of fuel cell performance exceeding an associated threshold value.

[0038] Optionally, the measure of fuel cell performance includes:

[0039] i) a measure of a pollutant, such as carbon monoxide, in said fuel cell component; or

[0040] ii) a measure of the voltage of any one (or more than one) of the plurality of fuel cells; or

[0041] iii) a measure of flooding an individual fuel cell (or more than one fuel cell) with coolant; or

[0042] iv) a measure of the environmental conditions close to the fuel cell assembly, such as temperature or humidity.

[0043] Optionally, the method comprises the step of actuating a regulating switch to selectively provide the electrical connection at least between the start of an increase in reactant flow through the fuel cell assembly of the assembly and the reactant flow rate or pressure reaching a predetermined level. The predetermined level may be a nominal operating pressure for a given reactant flow rate.

[0044] Optionally, the method comprises the step of actuating a regulating switch to selectively provide electrical connection for a particular one of the fuel cells in response to an indication that the voltage of the particular one of the fuel cells has become or is about to become negative relative to at least one other fuel cell in the assembly.

[0045] Optionally, the method comprises the step of monitoring the voltage of each of the fuel cells in the assembly.

[0046] Optionally, the method comprises the step of providing control of the temperature of at least one fuel cell in the fuel cell assembly by actuating a regulating switch to selectively provide an electrical connection for the at least one fuel cell.

[0047] Optionally, the method comprises the step of balancing the temperature of the fuel cells in the fuel cell assembly by selectively actuating the regulating switches of specific fuel cells to increase their temperature.

[0048] Optionally, the method comprises the steps of actuating the regulating switches of at least two of the fuel cells to provide the electrical connection upon startup of the fuel cell assembly, and actuating its associated regulating switch to disconnect the electrical connection in response to the current through a particular fuel cell reaching a threshold value.

[0049] Optionally, the method includes the following steps: actuating the regulating switches of most of the fuel cells in the fuel cell assembly to provide the electrical connection when the fuel cell assembly is started, and actuating its associated regulating switch to disconnect the electrical connection in response to the current through a specific fuel cell reaching a threshold value.

[0050] Optionally, the step of selectively providing the electrical connection comprises continuously providing the electrical connection for less than 30 seconds, less than 10 seconds, less than 5 seconds, less than 3 seconds or less than 1 second.

[0051] According to another aspect, we provide a computer program or computer program product comprising instructions for execution on a computing device having a processor and a memory for performing the method of the second aspect.

[0052] It will be appreciated that the processor and / or memory may be embedded with the fuel cell assembly, and that the processor may include a microprocessor or microcontroller. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Embodiments of the present invention will now be described in detail below, by way of example only, with reference to the accompanying drawings, in which:

[0054] Figure 1 An exploded schematic diagram of a first exemplary fuel cell assembly having a regulating switch module is shown;

[0055] Figure 2 Shows Figure 1 A more detailed schematic diagram of a fuel cell assembly of FIG. 1 ; and

[0056] Figure 3 A first flow chart showing a method of operating a fuel cell assembly is shown. DETAILED DESCRIPTION

[0057] Figure 1A fuel cell assembly 1 is shown, which includes a fuel cell stack in this example. The fuel cell stack includes a plurality of fuel cells 2 connected together so as to provide electricity to a load (not shown) together. Therefore, the individual fuel cells 2 are electrically connected together to provide a combined power output that can be used to drive a motor. For example, the fuel cell assembly can be configured to provide a power source for a vehicle. The fuel cell 2 may include a membrane electrode assembly 3, which includes a proton exchange membrane, a catalyst layer, and a gas diffusion layer, and is arranged between fuel cell plates 4. The fuel cell plates are bipolar plates in this example, and are formed on the anode side of the fuel cell on one side and the cathode side of the adjacent fuel cell on the other side thereof. Therefore, each fuel cell includes an anode 5 and a cathode 6. The plate may alternatively include a unipolar plate. The fuel cell plate 3 may include or define a channel for receiving a reactant stream to be guided on its associated membrane electrode assembly. The channel may extend from one end of the plate 3 to the other end.

[0058] refer to Figure 2 , one or more of the plurality of fuel cells 2 include a regulating switch 7. The regulating switch 7 is configured to selectively provide an electrical connection 13 or a shunt path between the anode 5 and the cathode 6 of its associated fuel cell 2 for short-circuiting its associated fuel cell 2. Thus, the regulating switch controls the voltage of the battery independently of the load by selectively connecting the electrical connection 13. The electrical connection short-circuits its associated fuel cell by selectively providing a low impedance path (e.g., a low impedance path relative to the fuel cell that powers the load as part of the assembly). Active control of the regulating switch 7 during operation of the fuel cell stack 1, in particular, active control of the voltage of the individual fuel cells in the stack independent of the load, is beneficial to the performance of the fuel cell stack. This effective individual fuel cell voltage control can be advantageously used for all fuel cell technologies that employ polymer electrolyte membranes, i.e., fuel cells based on acid or alkaline polymer electrolyte membranes or phosphoric acid fuel cells. Although not wishing to be bound by theory, it is believed that the selective short-circuiting of the individual fuel cells in the assembly can improve the electrochemical health of the membrane and / or catalyst layer.

[0059] In this example, each fuel cell 2 in the fuel cell stack 1 comprises an associated regulating switch. In particular, each fuel cell 2 in the fuel cell stack 1 is provided with a regulating switch 7 comprising a first and a second switching element 8, 9. The first switching element 8 is provided on a first substrate 10 and the second switching element 9 is provided on a second, different substrate 11. The first and second switching elements 8, 9 are connected to the anode and cathode of each fuel cell at different locations. This is advantageous because providing more than one switching element allows a distribution of the current density flowing between the switching element and the anode / cathode.

[0060] The switching element of the regulating switch 7 may be implemented as a semiconductor device, and in this example includes a transistor. Therefore, the first substrate 10 and / or the second substrate 11 may include a printed circuit board (PCB) or a ceramic substrate having the switching element thereon.

[0061] The regulating switch, in particular the switching element, is configured to receive a switching signal from the controller 12. The controller 12 may transmit a control signal to a gate terminal of the transistor-based switching element to control the electrical connection between two other transistor terminals connected to the anode 5 and cathode 6.

[0062] In another example, the electrical connection 13 includes a fuel cell voltage control element (not shown). The fuel cell voltage control element may include an active element, such as a controllable impedance. When the regulating switch forms an electrical connection, this may allow the fuel cell voltage control element to actively control the voltage of the fuel cell independently of the load. The fuel cell voltage control element may be configured so that the voltage at the anode and / or the voltage at the cathode may be controlled separately. The fuel cell voltage control element may be configured to control the fuel concentration / fuel amount / anode flooding level to control the anode voltage. The fuel cell voltage control element may be configured to control the oxidant concentration / oxidant amount / cathode flooding to control the cathode voltage. It may be advantageous to control the electrical connection in combination with the fuel or oxidant level to control the voltage at the anode and / or cathode. The fuel cell voltage control element may be controlled by the controller 12.

[0063] Return to Figure 1 , an inlet manifold 14 is shown, which is configured to deliver one or more reactants to each of the fuel cells 2. The exhaust manifold 15 is configured to receive unused reactants and reaction byproducts from the fuel cells and provide an exhaust flow from the fuel cell assembly. Therefore, the fuel cell plate 4 and / or the fuel cell 2 is configured to receive the reactant flow from the inlet manifold 14, guide the reactants on the active area of ​​each fuel cell, and pass the exhaust flow to the exhaust manifold 15. The first substrate 10 is installed between the stack of fuel cells 2 and the inlet manifold 14. The second substrate is installed between the stack of fuel cells 2 and the outlet manifold 15.

[0064] The fuel cell board 4 may include a connector, which may include a protruding protrusion for connecting to a regulating switch or a switch element thereof. Thus, the first and second substrates 10, 11 may be adapted to connect their regulating switches to the connector / protrusion of the fuel cell board 4, for example by including holes or connecting structures.

[0065] Will understand though Figure 1 and Figure 2 One example of how selective switching of electrical connections may be achieved is shown, but other structures and arrangements are also available. For example, semiconductor devices may be integrated into fuel cell boards.

[0066] Figure 3 A method of operating a fuel cell assembly is shown, which includes selectively providing an electrical connection between the anode and cathode of at least one of the fuel cells of the assembly to control the voltage of the fuel cell independently of the load in step 30. The selective electrical connection is removed after a regulation period of, for example, 10 ms. As shown in step 31, the method may include selectively providing an electrical connection between the anode and cathode of at least another of the fuel cells of the assembly to control the voltage of the fuel cell independently of the load.

[0067] The method may include sequentially providing electrical connections for a plurality of fuel cells in a fuel cell assembly, as shown in step 32. The sequential provision of electrical connections may begin at startup or shutdown of the fuel cell assembly. The step may be performed a limited number of times so that a subset of fuel cells are provided with electrical connections, while the remaining fuel cells are not provided. Alternatively, all fuel cells may be sequentially provided with their associated electrical connections for a predetermined number of times, or until a fuel cell assembly measurement, such as a cathode exhaust temperature, is reached. The method may be periodically called during operation of the fuel cell assembly. Thus, during operation of the fuel cell assembly, two operating modes may be provided, i.e., a first mode in which a fuel cell is provided with electrical connections in order to control its voltage and a second mode in which no electrical connection is provided. The fuel cell assembly may be configured to periodically alternate between a first mode and a second mode. An electrical connection of any one fuel cell may be provided temporarily, for example, for less than 5 seconds, less than 2 seconds, less than 1 second, less than 0.5 seconds, or less than 0.1 seconds. The selection of the first mode or the second mode may be based on a measurement associated with the fuel cell assembly relative to a threshold, such as a temperature or voltage of one or more of the fuel cells.

[0068] The instantaneous increase in fuel cell current and the reduction in fuel cell potential due to the electrical connection may have many potential benefits to fuel cell operation. Many advantageous implementations of the method are described below. It will be appreciated that the method described below can be implemented by a switch controller and / or a fuel cell voltage control element configured to perform the described method.

[0069] First, selective electrical connections can be used to quickly adjust the membrane electrode assembly of the fuel cell. Therefore, when receiving the reactant stream, selective electrical connections can be provided one or more times for the fuel cells in the fuel cell assembly in sequence or together. Using selective electrical connections, rapid voltage and load cycling of the fuel cell can be safely caused, regardless of the balance of the behavior of each fuel cell. Although not wishing to be bound by theory, it is believed that the process of rapidly cycling the fuel cell voltage and load quickly promotes the oxidation and evaporation of the solvent / impurities used in the manufacture of the fuel cell, while causing temperature and humidity cycles, which are intended to promote changes in the membrane and ionomer structure required to adjust the fuel cell.

[0070] In addition, selective electrical connections may be provided to quickly condition a fuel cell after an extended period of inactivity. Thus, the assembly may be configured to determine a period of time since last operation of the fuel cell assembly and, if it exceeds a time threshold of inactivity, provide selective electrical connections of one or more of the plurality of fuel cells of the assembly.

[0071] The fuel cell voltage control element can be configured to safely manipulate the current, cathode / anode potential, temperature and hydration of the fuel cell assembly through selective electrical connections. These parameters can be controlled to debug the fuel cell assembly. Therefore, we provide a method of debugging a fuel cell assembly, which includes selectively providing an electrical connection between the anode and cathode of at least one of the fuel cells of the assembly to control the voltage of the fuel cell independently of the load. The debugging of the fuel cell assembly can include operating the fuel cell assembly for the first time with reactant flow after its assembly.

[0072] It has been found that the method is advantageous in maintaining a predetermined hydration level of a fuel cell membrane and / or catalyst layer. The use of the method can avoid membrane dehydration during a long period at or near an open circuit voltage (OCV). It has been found that in some cases, operating a fuel cell assembly with a high impedance load or under open circuit conditions, the membrane and catalyst forming the active area of ​​each fuel cell and the electrochemical fuel cell reaction thereon may be dehydrated. This may lead to a reduction in the working life of the fuel cell assembly or a reduction in performance. Without wishing to be bound by theory, it is believed that providing an electrical connection allows the electrochemical reaction of the fuel cell to proceed at a greater rate, which can generate water (in a hydrogen and oxygen fuel cell) as a reaction byproduct to hydrate the membrane.

[0073] Thus, the method may include a method of hydrating a fuel cell assembly, comprising selectively providing an electrical connection between an anode and a cathode of at least one of the fuel cells of the assembly so as to control the voltage of the fuel cell independently of the load. The method may be performed periodically or continuously to hydrate the fuel cell membrane and / or catalyst, thereby extending the service life by avoiding degradation mechanisms associated with low humidity conditions. The method may be performed if the voltage of the fuel cell of the assembly or a subgroup of the fuel cells of the assembly is above a predetermined voltage threshold, such as close to an OCV value. Thus, the above-mentioned first and second operating modes may be determined based on the above-mentioned fuel cell assembly voltage or another measure of fuel cell hydration.

[0074] Hydration of fuel cell assemblies using selective electrical connections is particularly advantageous in fuel cell assemblies that operate without a stream of humid fluid introduced into the fuel cell assembly (ie, assemblies that generate moisture within the fuel cell assembly via electrochemical reaction byproducts).

[0075] The method has been found to be advantageous for removing blocking species, such as fuel cell catalyst poisons (i.e., air pollutants) and / or surface oxides contained in the fuel cell oxidant. In particular, operation of the fuel cell voltage control element can be used to manipulate the potential at the cathode of each fuel cell to mitigate the effects of cathode poisoning.

[0076] In particular, a fuel cell voltage control element can be used to cause starvation of the fuel cell oxidant (i.e., cathode starvation) so that the fuel cell cathode potential drops toward the anode potential, and a low fuel cell potential below 0.2V can be obtained regardless of the oxidant supply and / or power demand. In a hydrogen and oxygen PEM fuel cell assembly, the fuel cell cathode potential is nominally above 0.6V. RHE (reversible hydrogen electrode), but can be reduced to below 0.5, 0.4, 0.3, 0.2 or 0.1 V RHE .

[0077] Therefore, we provide a method for mitigating the effects of cathode poisoning of a fuel cell in a fuel cell assembly, which includes selectively providing an electrical connection between the anode and cathode of at least one of the fuel cells of the assembly so as to control the voltage of the fuel cell independently of the load. The voltage at the cathode electrode can be reduced superior to the control of the anode potential. As described above, this can be achieved by fluid manipulation of the oxidant supply or cathode operating conditions by, for example, reducing the oxidant concentration, so that the cathode limiting current (i.e., limited by the available oxidant) is reduced to a value significantly lower than the anode limiting current (i.e., limited by the available fuel). When selective electrical connection is made to cause an "oxidant starvation condition", the cathode electrochemical potential can be caused to decrease towards the anode electrochemical potential.

[0078] This approach has been found to be advantageous in proton exchange membrane hydrogen-oxygen fuel cell components, promoting the removal of dissolved platinum Pt 2+ In particular, by periodically manipulating the cathode potential to drop below approximately 0.4 V, any dissolved Pt 2+ The ions are redeposited back onto the active catalyst particles in the catalyst layer of each fuel cell, thereby improving the durability of the fuel cell. Without wishing to be bound by theory, it is suspected that during normal fuel cell operation, the surface atoms of the platinum particles dissolve in the formation of Pt 2+ In the presence of hydrogen, the intermediates reprecipitate as platinum particles in the membrane electrolyte of the fuel cell membrane. However, since these particles reprecipitate in the membrane rather than in the catalyst layer adjacent to the membrane, they are inactive for catalyzing the fuel cell reaction.

[0079] Thus, the method may include a method of controlling the level of catalyst particles in a catalyst layer of at least one of a plurality of fuel cells forming a fuel cell assembly, comprising selectively providing an electrical connection between an anode and a cathode of at least one of the fuel cells of the assembly so as to control the voltage of the fuel cell independently of the load. The voltage at the cathode electrode may be reduced in preference to control of the anode potential.

[0080] The method has been found to be advantageous in maintaining a predetermined fuel cell assembly (or fuel cell stack) temperature. The method may be performed, for example, in response to a period of low power demand (relative to a threshold power demand) to maintain stack temperature during operation in a sub-zero ambient environment, regardless of fuel cell load demand.

[0081] Thus, the method may include a method of controlling the temperature of a fuel cell in a fuel cell assembly comprising a plurality of fuel cells, the method comprising selectively providing an electrical connection between an anode and a cathode of at least one of the fuel cells of the assembly so as to control the voltage of the fuel cell independently of the load. Therefore, the electrical connection may allow current to flow, which in turn may heat the fuel cell. The method may be configured to maintain a predetermined fuel cell temperature. During operation of the fuel cell assembly, two operating modes may be provided, namely a first mode in which the fuel cell is selectively provided with the electrical connection (e.g. sequentially) so as to increase its temperature and a second mode in which no electrical connection is provided, the first mode operating if the temperature of the fuel cell assembly is below a predetermined temperature threshold (or if the power output is below a "low power" output threshold), and the second mode operating if the temperature is above a predetermined temperature threshold. The predetermined temperature threshold may include 4°C (or 0, 1, 2, 3, 5, 6°C).

[0082] The method has been found to be advantageous for manipulating the anode potential to promote oxidation and / or desorption of fuel cell catalyst poisoning contaminants present in the anode fluid stream, such as contaminants in the fuel (eg, carbon monoxide, CO).

[0083] The fuel cell voltage control element may be configured to induce a fuel starvation condition by increasing the anode potential to an overpotential, to the extent that fuel contaminants such as CO are oxidized / desorbed to restore fuel cell performance.

[0084] Therefore, the method can include a method for removing contaminants from the anode of a fuel cell in a plurality of fuel cells forming a fuel cell assembly, which includes selectively providing an electrical connection between the anode and cathode of at least one of the fuel cells of the assembly so as to control the voltage of the fuel cell independently of the load. In particular, the method can include a step of controlling the anode potential in preference to the cathode potential. This can be achieved by fluid manipulation of the fuel supply or anode operating conditions by, for example, reducing the fuel concentration, so that the anode limiting current (i.e., limited by the available fuel) is reduced to a value significantly lower than the cathode limiting current (i.e., limited by the available oxidant). When selective electrical connection is made to cause a "fuel starvation condition", the anode electrochemical potential can be caused to increase toward the cathode electrochemical potential.

[0085] In another example, the controller can be configured to actuate a regulating switch to selectively provide electrical connection at least between the increase of the reactant flow of the fuel cell through the assembly and the reactant flow rate or pressure reaching a predetermined level. The increase of the reactant flow may cause the reactant "front" of the increased pressure to move through the fuel cell assembly. The pressure front may be obvious when the fuel cell assembly is started from the closed state. The pressure front that reduces the pressure when the fuel cell assembly is closed may also move through the fuel cell assembly, and the controller may also be configured to actuate the regulating switch in such a case.

[0086] This approach can be implemented to advantageously shunt current during the presence of a hydrogen / air front flowing through the anode / cathode, which can provide the benefit of reducing reverse current decay mechanisms and / or rapid fuel cell shutdown.

[0087] A reverse current decay mechanism occurs when a fuel cell in a fuel cell assembly is driven by current generated by other fuel cells in the assembly so as to produce a reverse current compared to normal operation. This may reduce the performance of the associated fuel cell. By providing an electrical connection and / or controlling the voltage of the fuel cell by means of the electrical connection, a low cell voltage (e.g., less than 0.2V for a hydrogen-oxygen polymer membrane-based fuel cell) can be obtained during the reactant pressure front.

[0088] It has been found that the degradation that normally occurs during unmitigated start / stops where a pressure front occurs can be almost completely eliminated. This significantly improves fuel cell durability, particularly in applications that require many complete fuel cell start / shutdown events, such as automotive applications.

[0089] Thus, the method may include the following steps: in response to an increase in pressure and / or flow rate of a reactant (e.g., fuel) through a fuel cell assembly, actuating one or more of the regulating switches in the fuel cells in the assembly. The regulating switches may be actuated together or in groups and sequentially or individually and sequentially.

[0090] When shutting down a hydrogen fuel cell assembly, residual hydrogen may be present in the fuel cell assembly. It is important to reduce the hydrogen concentration to a predetermined level. The method can be used to consume any remaining hydrogen in the fuel cell assembly when the assembly is shut down. By actuating the regulating switch, the hydrogen can be consumed quickly.

[0091] Thus, the method may include the step of actuating one or more of the regulating switches in the fuel cells in the assembly when the fuel cell assembly is closed. The regulating switches may be actuated together or in groups and sequentially or individually and sequentially. This step may be performed in response to cessation of the flow of fuel and / or oxidant through the fuel cell assembly, for example when a compressor driving reactants through the assembly is deactivated.

[0092] It has been found that in some cases, when the regulating switch is actuated, the residual hydrogen present in the anode chamber / fuel supply connection is consumed so quickly that the fuel cell stack is completely vented and left in a safe dormant state only a few seconds later. Similarly, oxygen may be consumed in the cathode chamber to vent the cathode chamber.

[0093] In other embodiments (not shown), a single regulating switch (which may be formed of one or more switching elements) may provide a voltage for controlling a group of series-connected fuel cells in a fuel cell assembly. Thus, the regulating switch may be used to provide an electrical connection to form an electrical circuit with a group of series-connected fuel cells. So, rather than providing an electrical connection for a single fuel cell, an electrical connection is provided for a group of fuel cells.

[0094] The fuel cell assembly may form one of a plurality of fuel cell assemblies in a fuel cell system. Actuation of the regulating switch may be performed on each fuel cell assembly in turn. Thus, the regulating switch of a first fuel cell assembly may be actuated, followed by the regulating switch of a second fuel cell assembly. This may be advantageous because the fuel cell assembly that is not subjected to actuation of the regulating switch may be operated in a normal operating mode.

Claims

1. A fuel cell assembly comprising a plurality of fuel cells connected in series so as to collectively provide power to a load, the assembly comprising: a plurality of regulating switches, each regulating switch being associated with one or more of the series-connected fuel cells and being configured to selectively provide electrical connection between an anode and a cathode of its associated fuel cell for controlling a voltage across the associated fuel cell independently of the load; wherein each of the regulating switches selectively provides a corresponding electrical connection in a first operating mode of the fuel cell assembly and deactivates the corresponding electrical connection in a second operating mode of the fuel cell assembly; a controller programmed to, in response to a measure of fuel cell performance exceeding a threshold, actuate the regulating switch associated with the fuel cell to selectively provide electrical connection thereof in the first operating mode; and The controller is programmed to actuate the regulating switch to selectively provide its respective electrical connection between the start of reactant flow increase through the fuel cell assembly and the reactant flow rate or pressure exceeding a predetermined level.

2. The fuel cell assembly of claim 1 further comprising a fuel cell plate in each fuel cell, the fuel cell plate being configured to abut against the membrane electrode assembly of its fuel cell, the fuel cell plate comprising at least one connector for connecting to its regulating switch.

3. A fuel cell assembly according to claim 1, wherein the plurality of fuel cells are arranged together in a stack and include a manifold for delivering reactants to one or more of the plurality of fuel cells in the stack or receiving exhaust gas flows from one or more of the plurality of fuel cells in the stack, and the assembly includes a substrate having the plurality of regulating switches.

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