Abstract: A fuel cell system including a fuel cell stack having a plurality of fuel cells, each of the fuel cells including an electrolyte membrane disposed between an anode and a cathode, an anode supply manifold in fluid communication with the anodes of the fuel cells, the anode supply manifold providing fluid communication between a source of hydrogen and the anodes, an anode exhaust manifold in fluid communication with the anodes of the fuel cells, and a fan in fluid communication with the anodes of the fuel cells, wherein the fan controls a flow of fluid through the anodes of the fuel cells after the fuel cell system is shutdown.

Abstract: A graded electrode is described. The graded electrode includes a substrate; and at least two electrode layers on the substrate forming a combined electrode layer, a composition of the at least two electrode layers being different, the combined electrode layer having an average level of the property that changes across the substrate. Fuel cells using graded electrodes and methods of making graded electrodes are also described.

Abstract: A system and method for breaking-in and humidifying membrane-electrode-assemblies (MEAs) in a fuel cell stack. The method includes performing voltage cycling and humidification of the MEAs in the stack, including one or more temperature steps wherein current density of the stack is cycled within a predetermined range for each of the one or more temperature steps. The method also includes maintaining a fuel cell stack voltage within a predetermined range, and maintaining anode and cathode reactant flows at an approximate set-point during the current density cycling of the one or more temperature steps to break-in and humidify the MEAs in the stack so that the stack is able to operate at a predetermined threshold for a fuel cell stack voltage output capability.

Abstract: A method for determining the health of fuel cells in a fuel cell stack. The method includes maintaining a constant flow of hydrogen to the anode side of the fuel cell stack, shutting off a flow of air to a cathode side of the fuel cell stack when a predetermined concentration of hydrogen in the anode side has been achieved, and identifying a catalyst surface area and a catalyst support surface area for catalyst layers in the fuel cell stack. The method also includes determining the total parasitic current of the fuel cell stack to determine a cross-over parasitic current and a shorting resistance of the fuel cell stack. The method further includes calculating the catalyst surface area and the catalyst support surface area of the catalyst layers and comparing the difference between the identified catalyst surface area and the calculated catalyst surface area to estimate the change in the catalyst surface area.

Abstract: A system and method for detecting a low performing cell in a fuel cell stack using measured cell voltages. The method includes determining that the fuel cell stack is running, the stack coolant temperature is above a certain temperature and the stack current density is within a relatively low power range. The method further includes calculating the average cell voltage, and determining whether the difference between the average cell voltage and the minimum cell voltage is greater than a predetermined threshold. If the difference between the average cell voltage and the minimum cell voltage is greater than the predetermined threshold and the minimum cell voltage is less than another predetermined threshold, then the method increments a low performing cell timer. A ratio of the low performing cell timer and a system run timer is calculated to identify a low performing cell.

Abstract: A method for determining a failure of a membrane in a fuel cell in a fuel cell stack. The method includes measuring the voltage of each fuel cell in the fuel cell stack, calculating an average cell voltage from all of the cell voltages of the fuel cells in the fuel cell stack, and identifying a minimum cell voltage from all of the cell voltages of the fuel cells in the fuel cell stack. The method then determines an absolute delta voltage value as the difference between the average cell voltage of the fuel cells and the minimum cell voltage of the fuel cells at a plurality of sample points during the sample period. A plurality of absolute delta voltage values determined over a plurality of sample periods, filtered for low current density are used to determine whether there is a membrane failure and, by filtering for high current density, to determine whether there is an electrode failure.

Abstract: A method for determining the health of the membranes in a fuel cell stack. The total parasitic current of the fuel cells in the stack is determined. From the total parasitic current, the shorting resistance and the cross-over parasitic current are determined. The health of the membranes is then determined from the cross-over parasitic current and the shorting resistance.

Abstract: A method for determining the health of fuel cells in a fuel cell stack. The method includes maintaining a constant flow of hydrogen to the anode side of the fuel cell stack, shutting off a flow of air to a cathode side of the fuel cell stack when a predetermined concentration of hydrogen in the anode side has been achieved, and identifying a catalyst surface area and a catalyst support surface area for catalyst layers in the fuel cell stack. The method also includes determining the total parasitic current of the fuel cell stack to determine a cross-over parasitic current and a shorting resistance of the fuel cell stack. The method further includes calculating the catalyst surface area and the catalyst support surface area of the catalyst layers and comparing the difference between the identified catalyst surface area and the calculated catalyst surface area to estimate the change in the catalyst surface area.

Abstract: A system and method for removing contaminants from a fuel cell stack. The method includes exposing the cathode and anode of the stack to an air purge, then exposing the cathode and anode of the stack to a water flush and then again exposing the cathode and anode of the stack to an air purge to dry the stack. In one technique, the stack is removed from the vehicle at a maintenance facility to perform the air purge and water flush, and in another technique, the stack remains in the vehicle and appropriate hoses are connected to the stack for the air purges and water flush.

Abstract: A method for reducing the probability of an air/hydrogen front in a fuel cell stack is disclosed that includes closing anode valves for an anode side of the fuel cell stack to permit a desired quantity of hydrogen to be left in the anode side upon shutdown and determining a schedule to inject hydrogen during the time the fuel cell stack is shutdown. The pressure on an anode input line is determined and a discrete amount of hydrogen is injected into the anode side of the stack according to the determined schedule by opening anode input line valves based on the determined pressure along the anode input line so as to inject the hydrogen into the anode side of the stack.

Abstract: An algorithm for determining a polarization curve of a fuel cell stack. When the fuel cell stack is running and certain data validity criteria have been met, the algorithm goes into a data collection mode where it collects stack data, such as stack current density, average cell voltage and minimum cell voltage. When the stack is shut down, the algorithm uses a cell voltage model to solve a least squares problem to estimate predetermined parameters that define the polarization curve. If the estimated parameters satisfy certain termination criteria, then the estimated parameters are stored to be used by a system controller to calculate the polarization curve of the stack.

Abstract: An algorithm for determining the maximum net power available from a fuel cell stack as the stack degrades over time using an online adaptive estimation of a polarization curve of the stack. The algorithm separates the current density range of the stack into sample regions, and selects a first sample region from the far left of the estimated polarization curve. The algorithm then calculates the cell voltage for that current density sample region, and determines whether the calculated cell voltage is less than or equal to a predetermined cell voltage limit. If the calculated cell voltage is not less than the cell voltage limit, then the algorithm selects the next sample region along the polarization curve. When the calculated cell voltage does reach the cell voltage limit, then the algorithm uses that current density for the sample region being analyzed to calculate the maximum power of the fuel cell stack.

Abstract: A graded electrode is described. The graded electrode includes a substrate; and at least two electrode layers on the substrate forming a combined electrode layer, a composition of the at least two electrode layers being different, the combined electrode layer having an average level of the property that changes across the substrate. Fuel cells using graded electrodes and methods of making graded electrodes are also described.

Abstract: A hydrogen concentration sensor for measuring the hydrogen concentration in an anode sub-system of a fuel cell system. The hydrogen concentration sensor includes a membrane, a first catalyst layer on one side of the membrane and a second catalyst layer on an opposite side of the membrane where the sensor operates as a concentration cell. The first catalyst layer is exposed to fresh hydrogen for the anode side of a fuel cell stack and the second catalyst layer is exposed to an anode recirculation gas from an anode exhaust of the fuel cell stack. The voltage generated by the sensor allows the hydrogen partial pressure in the recirculation gas to be determined, from which the hydrogen concentration can be determined.

Abstract: A system and method for detecting and predicting low performing cells in a fuel cell stack. When the fuel cell stack is running and certain data validity criteria have been met, an algorithm collects the data, such as stack current density, average cell voltage and minimum cell voltage. This information is used to estimate predetermined parameters that define the stack polarization curve. The system defines a predetermined minimum current density that is used to identify a low performing cell. The system then calculates an average cell voltage and a minimum cell voltage at the minimum current density set-point, and calculates a cell voltage difference between the two. If the cell voltage difference is greater than a predetermined low voltage threshold and the minimum cell voltage is less than a predetermined high voltage threshold, the algorithm sets a flag identifying a potential for a low performing cell.

Abstract: A method for providing a fast and reliable start-up of a fuel cell system. The method uses a stack voltage response to a load to assess if hydrogen and oxygen are being sufficiently distributed to all of the fuel cells by coupling an auxiliary load to the fuel cell stack until a predetermined minimum cell voltage has been reached or a first predetermined time period has elapsed. The method then determines whether a minimum cell voltage has dropped to a first predetermined voltage and, if so, reduces the maximum power allowed to be below the first predetermined voltage value, determines whether the minimum cell voltage in the stack is below a second predetermined voltage, or determines whether the minimum cell voltage drop rate is greater than a predetermined voltage drop rate. If none of these conditions are met, the method returns to loading the stack with system components.

Abstract: A method that employs a model based approach to determine a maximum anode pressure set-point based on existing airflow in the exhaust gas line. This approach maximizes anode flow channel velocity during bleed events while meeting the hydrogen emission constraint, which in turn increases the amount of water purged from the anode flow channels to increase stack stability.

Abstract: A method for determining a failure of a membrane in a fuel cell in a fuel cell stack. The method includes measuring the voltage of each fuel cell in the fuel cell stack, calculating an average cell voltage from all of the cell voltages of the fuel cells in the fuel cell stack, and identifying a minimum cell voltage from all of the cell voltages of the fuel cells in the fuel cell stack. The method then determines an absolute delta voltage value as the difference between the average cell voltage of the fuel cells and the minimum cell voltage of the fuel cells at a plurality of sample points during the sample period. A plurality of absolute delta voltage values determined over a plurality of sample periods, filtered for low current density are used to determine whether there is a membrane failure and, by filtering for high current density, to determine whether there is an electrode failure.

Abstract: A fuel cell system is provided that includes a fuel cell stack with a plurality of fuel cells and a power converter in electrical communication with the fuel cell stack. The power converter is configured to selectively regulate a power of the fuel cell stack and short circuit the fuel cell stack, as desired. A method for starting the fuel cell stack is also described, including the steps of causing a short circuit of the fuel cell stack by placing the power converter in a short circuit mode; introducing a hydrogen to the anodes of the fuel cell stack to displace a quantity of air on the anodes; and placing the power converter in a power regulation mode. A degradation of the fuel cell stack during start-up is thereby militated against.

Abstract: A method for creating an oxygen depleted gas in a fuel cell system, including operating a fuel cell stack at a desired cathode stoichiometry at fuel cell system shutdown to displace a cathode exhaust gas with an oxygen depleted gas. The method further includes closing a cathode flow valve and turning off a compressor to stop the flow of cathode air.