A fuel cell converts chemical energy to electrical energy by promoting a chemical reaction between two reactants; namely, a fuel (such as hydrogen) and an oxidant (such as oxygen in the air). One common type of fuel cell is the solid polymer electrolyte fuel cell which typically includes a cathode flow field plate, an anode flow field plate and a membrane electrode assembly (MEA) disposed between the cathode flow field plate and the anode flow field plate. The MEA includes a solid polymer electrolyte, typically a proton exchange membrane (PEM), between a first or anode catalyst and a second or cathode catalyst. Each flow field plate has an inlet, an outlet and open-faced channels connecting the inlet to the outlet and for distributing the reactants to, and removing products from, the MEA.
The anode catalyst interacts with the anode reactant to catalyze the conversion of the anode reactant to reaction intermediates. The reaction intermediates include cations (e.g., protons) and electrons. When the anode reactant is hydrogen gas, the anode catalyst breaks the hydrogen gas into protons and electrons (H2→2H++2e−). The electrolyte provides a barrier to the flow of the electrons from the anode-side of the PEM to the cathode-side, while permitting the cations to pass through to the cathode-side of the PEM. Instead, the electrons flow from the anode-side of the PEM to the cathode-side of the PEM through an external load (thus providing useable electrical current).
At the cathode-side of the PEM, the cathode catalyst interacts with the cathode reactant and the reaction intermediates to catalyze the conversion of the cathode reactant to the chemical product(s) of the fuel cell reaction. More specifically, the cathode catalyst interacts with the cathode reactant, cations that have passed through the PEM, and electrons which have traveled around the PEM via the external load. When hydrogen and oxygen are used as the anode and cathode reactants, respectively, the cathode catalyst aids in the reaction of oxygen with protons and electrons to form water (½O2+2H++2e−→H2O).
In addition to forming chemical products, such as water, the fuel cell reaction also produces heat. Thus, a fuel cell may also include one or more coolant flow field plates disposed adjacent to the anode flow field plate and/or the cathode flow field plate. Such coolant flow field plates typically have an inlet, an outlet and channels that provide fluid communication between the inlet and outlet. A coolant enters the coolant flow field plate at the inlet and flows through the channels towards the outlet, where it exits the coolant flow field. As the coolant flows through the channels of the coolant flow field plate, it absorbs heat produced within the fuel cell and, upon exiting the fuel cell, removes such heat from the fuel cell.
To increase electrical output voltage, fuel cells are typically arranged in series to form a fuel cell stack. In such arrangements, one side of a flow field plate may function as the anode flow field plate for one fuel cell within the stack, while the opposite side of the same flow field plate serves as the cathode flow field plate for an adjacent fuel cell within the stack (i.e., a bipolar plate). The stack may also include other plates such as, for example, anode and/or cathode coolant flow field plates, as well various bus plates, end plate hardware, and other components that are well known in this field.
During operation, a fuel cell stack is susceptible to loss of heat to the environment (e.g., conductive heat loss through attached hardware), particularly at the ends of the fuel cell stack. This loss of heat results in the temperature of the fuel cell stack being non-uniform along its length, with the ends of the fuel cell stack not being maintained at a desired operating temperature. For example, the temperature of the end cells of the fuel cell stack can be relatively lower than the temperature of the remainder of the cells in the fuel cell stack. As relatively hot reactant(s) or products containing water pass through the inlets and outlets extending through the end plate to enter the fuel cell stack, and experience a temperature drop, water from the reactant(s) or products may condense in colder areas in the fuel cell stack. For example, hydrogen coming from a reformer can be relatively hot and saturated with water, such saturation being desirable in order to prevent drying out of the solid polymer electrolyte in the fuel cell stack. In some situations, water may be added to the reactant(s) in a separate step (e.g., pre-humidification) for this same purpose. As this hot, water-saturated hydrogen enters the fuel cell stack, water may condense in the relatively cooler cells at the end of the fuel cell stack.
Condensation of water within the fuel cells at the end of the fuel cell stack is problematic since water can block, for example, the flow channels and flood the fuel cell. Such flooding decreases voltage of the affected fuel cell(s), and overall performance of the fuel cell stack decreases. In addition, flooding may also result in a partial or complete fuel starvation condition which in turn leads to corrosion of one or both electrodes in the affected fuel cell(s).
Prior attempts to solve this problem have primarily involved the incorporation of an electric heater into the fuel cell stack. For example, published Japanese Patent Publication No. 8-167424 discloses a resistive heating element connected in series to, and located between, the fuel cell and the adjacent collecting electrode plate at the end of a fuel cell stack. Current from the fuel cell stack is directed through the resistive heater, which results in increased heating of the fuel cell at the end of the fuel cell stack in response to increased current output from the fuel cell stack. Thus, heating is greatest under conditions of high current output from the fuel cell stack.
Similarly, U.S. Publication No. US 2001/0036568 A1 is directed to a fuel cell stack assembly having a heating element (e.g., a resistive heater) that heats the end plate of the fuel cell stack. A temperature sensor detects the temperature of the fuel cell at the end of the fuel cell stack and sends a signal to a controller which, in turn, controls the power source that supplies power to the resistive heater. At some predetermined temperature, the controller senses that the temperature of the fuel cell at the end of the fuel cell stack is too low and sends an appropriate signal to the power source, thus turning on the resistive heater. This patent publication also discloses the use of a single or multiple sensors interfaced to detect one or more operating parameters of the fuel cell system. All such detected parameters are then inputted to the controller, which can then control appropriate heating of the fuel cell stack.
While advances have been made in this field, there remains a need for improved fuel cell systems generally, including systems and methods for heating or maintaining the temperature of fuel cell(s) within a fuel cell stack. The present invention fulfills these and/or other needs, and provides further related advantages.