Systems and methods for performing pressure frequency response analysis in testing of electrochemical devices

Abstract

Systems and methods for diagnostic testing and operation of electrochemical devices using pressure perturbation techniques are provided. In some embodiments, the systems and methods can be used to conduct pressure frequency response analysis (pFRA), such as electrochemical pressure impedance spectroscopy (EPIS) or other techniques, on fuel cell assemblies. In addition to a primary reactant control valve used to control the supply of a reactant to the fuel cell, a secondary adjustable valve is located upstream of the fuel cell under test and downstream of the primary reactant control valve. The secondary adjustable valve can be adjusted so that pressure oscillations applied near the outlet of the fuel cell are also observed at the fuel cell inlet. This can make it possible to obtain reliable pFRA signals at the fuel cell inlet as well as at the outlet. The secondary adjustable valve can also be adjusted to control the linearity of the voltage response.

Description

SYSTEMS AND METHODS FOR PERFORMING PRESSURE FREQUENCY RESPONSE ANALYSIS IN TESTING OF ELECTROCHEMICAL DEVICESCross-Reference to Related Applications

[0001] This application is related to and claims priority benefits from U.S. Provisional Patent Application Serial No. 63 / 525,001 filed July 5, 2023, entitled “Systems and Methods for Performing Electrochemical Pressure Impedance Spectroscopy”. The ‘001 application is incorporated by reference herein in its entirety.Field of the Invention

[0002] The present invention relates to apparatus, systems and methods for diagnostic testing and operation of electrochemical devices using pressure perturbation techniques. In particular, embodiments of the apparatus, systems and methods can be used to conduct pressure frequency response analysis (pFRA) methods, such as electrochemical pressure impedance spectroscopy (EPIS), on fuel cell assemblies.Background of the Invention

[0003] Frequency response analysis (FRA) methods, such as electrochemical impedance spectroscopy (EIS), are powerful diagnostic techniques that can be used in testing of various electrochemical assemblies, including fuel cells and electrolyzers. Such methods can provide valuable insights into the electrochemical behavior, performance, and durability of these devices by separating the dynamic processes in the device under test (DUT) based on their response time. Note that in some FRA methods, an oscillation excitation is imposed, e.g. on the current, and the voltage response is measured. If the amplitude of the current excitation is large, it will typically result in harmonics in the voltage response due to DUT non-linearities. However, if the excitation is sufficiently small, the response does not exhibit higher harmonics and only contains the fundamental frequency. In this case, the DUT can be considered to be behaving in a linear manner, and the FRA response reduces to a single spectrum, such as EIS.

[0004] Other methods that have been developed more recently, and that can be used in testing electrochemical assemblies such as fuel cells are pressure frequency response analysis(pFRA) methods, such as electrochemical pressure impedance spectroscopy (EPIS). These pFRA methods can combine the benefits of FRA and pressure measurements to further enhance the understanding and characterization of fuel cells.

[0005] Such pFRA methods generally involve the measurement of impedance under varying pressure perturbations, and can provide information on electrode kinetics, mass transport, and other processes occurring within fuel cells. Use of pFRA can allow researchers and engineers to gain a deeper understanding of the fundamental electrochemical processes and mechanisms occurring within the fuel cell. An advantage of using pFRA is that it can be used to differentiate and analyze various components and processes that contribute to the overall impedance of a fuel cell.

[0006] Typically, pFRA experiments involve applying pressure perturbations to a fuel cell at different frequencies and measuring the resulting voltage response or current response at a fixed current or potential, respectively. By varying the pressure frequency, an impedance spectrum can be obtained. The obtained data can then be analyzed using mathematical models and fitting algorithms to extract specific parameters and evaluate the performance of the fuel cell.

[0007] This kind of testing and analysis may be particularly useful in fuel cell research and development, as it can provide valuable information regarding the effects of pressure on various fuel cell components, such as the catalyst layers, gas diffusion layers, membraneelectrode assemblies, and / or on flow field conditions. This knowledge can help researchers enhance or optimize the design and operation of fuel cells, leading to improved efficiency, durability, and performance.

[0008] Furthermore, pFRA can be utilized for the diagnosis of fuel cell degradation and failure mechanisms. By monitoring changes in impedance under different pressure conditions over time, researchers can identify performance losses, catalyst degradation, or other issues affecting operation of a fuel cell. This diagnostic capability can facilitate proactive maintenance and enable the development of strategies to enhance fuel cell reliability and lifespan. Test methods involving pFRA can also be used in fuel cells to diagnose or provide information about operating conditions such as reactant flow conditions, humidity conditions,as well as liquid water condensation and accumulation. This can inform adjusting and optimizing fuel cell operating conditions to improve performance and durability of the fuel cell.Summary of the Invention

[0009] In embodiments of a method for testing a fuel cell assembly, where the fuel cell assembly comprises a cathode-side having a cathode inlet and a cathode outlet, and an anodeside having an anode inlet and an anode outlet, the method comprises:

[0010] while supplying an oxidant stream to the cathode inlet via a primary oxidant control valve, adjusting a secondary oxidant control valve located downstream of the primary oxidant control valve and upstream of the cathode inlet to a partially closed position; and

[0011] while operating the fuel cell assembly to supply electrical power to a load, applying pressure oscillations at or downstream of the cathode outlet over a range of frequencies, and measuring pressure-induced voltage oscillations across the fuel cell assembly if the fuel cell is operating at constant current, or measuring pressure-induced current oscillations in the fuel cell assembly if the fuel cell is operating at constant voltage.

[0012] In some embodiments, the adjusting of the secondary oxidant control valve to a partially closed position comprises (prior to the applying pressure oscillations at or downstream of the cathode outlet over the range of frequencies and measuring the pressure- induced voltage oscillations or measuring the pressure-induced current oscillations):

[0013] applying pressure oscillations at or downstream of the cathode outlet at a first frequency;

[0014] measuring the amplitude of associated pressure oscillations at or upstream of the cathode inlet; and

[0015] adjusting the secondary oxidant control valve to a first partially closed position so that the associated pressure oscillations at or upstream of the cathode inlet have a desired amplitude.

[0016] In some embodiments, the first frequency is the highest frequency in the range of frequencies. In some embodiments, the first frequency is higher than the highest frequency in the range of frequencies.

[0017] In some embodiments, the adjusting of the secondary oxidant control valve to the first partially closed position so that the associated pressure oscillations at or upstream of the cathode inlet have a desired amplitude comprises adjusting the secondary oxidant control valve to the first partially closed position so that the associated pressure oscillations at or upstream of the cathode inlet are at least detectable, for example by a pressure sensor or pressure transducer.

[0018] In some embodiments, the method further comprises comparing the amplitude of the pressure oscillations applied at or downstream of the cathode outlet at the first frequency with the amplitude of the associated pressure oscillations observed at or upstream of the cathode inlet, and the secondary oxidant control valve is adjusted so that the associated pressure oscillations at or upstream of the cathode inlet have a desired amplitude relative to the pressure oscillations applied downstream of the cathode outlet at the first frequency. In some embodiments, this can be so that the amplitude of the associated pressure oscillations at or upstream of the cathode inlet is of the same order of magnitude as the amplitude of the pressure oscillations applied at or downstream of the cathode outlet.

[0019] In some embodiments, the method further comprises supplying fuel to the anode inlet while applying the pressure oscillations at or downstream of the cathode outlet at the first frequency and adjusting the secondary oxidant control valve to the first partially closed position.

[0020] In some embodiments, the method further comprises operating the fuel cell assembly to supply electrical power to a load while applying pressure oscillations at or downstream of the cathode outlet at the first frequency and adjusting the secondary oxidant control valve to the first partially closed position.

[0021] In some embodiments, the method further comprises leaving the secondary oxidant control valve set at the first partially closed position while applying pressure oscillations at or downstream of the cathode outlet over the range of frequencies.

[0022] In some embodiments, the method further comprises further adjusting the secondary oxidant control valve while applying pressure oscillations at or downstream of the cathode outlet over the range of frequencies. In some embodiments, further adjusting the secondary oxidant control valve comprises further adjusting the secondary oxidant control valve based on the frequency of the pressure oscillations being applied. For example, the secondary oxidant control valve can be adjusted for different discrete frequencies in the range of frequencies of the pressure oscillations applied at or downstream of the cathode outlet. In some embodiments, further adjusting the secondary oxidant control valve comprises adjusting the secondary oxidant control valve to adjust the linearity of a pressure-induced voltage response.

[0023] In any of the above-described embodiments of a method for testing a fuel cell assembly, a pressure control valve located at or downstream of the cathode outlet can be used to apply the pressure oscillations at the first frequency and / or over the range of frequencies.

[0024] In some embodiments of the methods described above, the fuel cell assembly is a single fuel cell.

[0025] In some embodiments of the methods described above the fuel cell assembly is a fuel cell stack comprising a plurality of fuel cells, and the method comprises measuring pressure-induced voltage oscillations across each fuel cell in the fuel cell stack.

[0026] In embodiments of a fuel cell testing system for testing a fuel cell assembly comprising a cathode-side having a cathode inlet and a cathode outlet, and an anode-side having an anode inlet and an anode outlet, the system comprises:

[0027] an oxidant supply subsystem for supplying oxidant to the cathode-side via the cathode inlet, the oxidant supply subsystem comprising a primary oxidant control valve located upstream of the cathode inlet;

[0028] a cathode back pressure valve located at or downstream of the cathode outlet;

[0029] a fuel supply subsystem for supplying a fuel stream to the anode-side via the anode inlet, the fuel supply subsystem comprising a primary fuel control valve located upstream of the cathode inlet;

[0030] an anode back pressure valve located at or downstream of the anode outlet;

[0031] a voltage monitoring subsystem for measuring the voltage across one or more fuel cells in the fuel cell assembly;

[0032] an electrical load connectable to receive electrical power generated by the fuel cell assembly; and

[0033] an adjustable secondary oxidant control valve located upstream of the cathode inlet and downstream of the primary oxidant control valve.

[0034] In some embodiments, the fuel cell testing system further comprises a humidifier located upstream of the cathode inlet for humidifying the oxidant stream supplied to the cathode-side via the cathode inlet, where the primary oxidant control valve is located upstream of the humidifier and the adjustable secondary oxidant control valve is located downstream of the humidifier, between the humidifier and the cathode inlet.

[0035] In some embodiments, during operation of the fuel cell testing system, when the electrical load is connected and receiving electrical power generated by the fuel cell assembly and pressure oscillations are being applied at or downstream of the cathode outlet using the cathode back pressure valve, adjusting the adjustable secondary control valve changes the amplitude of the associated pressure oscillations at or upstream of the cathode inlet.

[0036] In some embodiments, during operation of the fuel cell testing system, when the electrical load is receiving electrical power generated by the fuel cell assembly and pressure oscillations are being applied at or downstream of the cathode outlet using the cathode back pressure valve, adjusting the adjustable secondary control valve changes the linearity of a pressure-induced voltage response measured using the voltage monitoring subsystem.

[0037] In some embodiments of the above-described system, the adjustable secondary oxidant control valve is located directly upstream of the cathode inlet. In some embodiments, the adjustable secondary oxidant control valve is a choke valve.

[0038] In some embodiments of the fuel cell testing system, the system can be used for testing a fuel cell stack comprising a plurality of fuel cells, and the voltage monitoring subsystem for measuring the voltage across one or more fuel cells in the fuel cell assembly is configured to measure the voltage across each fuel cell of the plurality of fuel cells. In some embodiments, the voltage monitoring subsystem is configured to measure the voltage across the plurality of fuel cells.

[0039] This summary does not necessarily describe the entire scope of all aspects. Other aspects, features, and advantages will be apparent to those of ordinary skill in the art upon review of the following description of specific embodiments.Brief Description of the Drawings

[0040] FIG. 1 is a schematic illustration of an example of a system that can be used to perform embodiments of the pFRA test methods described herein on a fuel cell.

[0041] FIG. 2 is a flow chart illustrating a known method for conducting pFRA on a fuel cell.

[0042] FIG. 3 is a flow chart illustrating an embodiment of an improved method for conducting pFRA on a fuel cell.

[0043] FIG. 4 illustrates the ratio of the amplitude of pressure oscillations observed at the fuel cell cathode inlet to those applied at the fuel cell cathode outlet over a range of frequencies, with a choke valve located directly upstream of the fuel cell cathode set at three different settings.

[0044] FIG. 5 A shows pressure oscillations applied at the cathode outlet of a fuel cell over a period of time, and the corresponding pressure oscillations observed at the cathodeinlet and the voltage response, when a choke valve located directly upstream of the fuel cell cathode inlet is fully open.

[0045] FIG. 5B shows pressure oscillations applied at the cathode outlet of a fuel cell over a period of time, and the corresponding pressure oscillations observed at the cathode inlet and voltage response, when a choke valve located directly upstream of the fuel cell cathode inlet is partially closed so that there is a 100 kPa pressure drop across the choke valve.

[0046] FIG. 6A shows pressure oscillations applied at the cathode outlet of a fuel cell over a period of time, and the corresponding pressure oscillations observed at the cathode inlet and the voltage response with no choke valve located directly upstream of the fuel cell cathode inlet.

[0047] FIG. 6B shows pressure oscillations applied at the cathode outlet of a fuel cell over a period of time, and the corresponding pressure oscillations observed at the cathode inlet and the voltage response with a choke valve located directly upstream of the fuel cell cathode inlet when the valve is open.

[0048] FIG. 6C shows pressure oscillations applied at the cathode outlet of a fuel cell over a period of time, and the corresponding pressure oscillations observed at the cathode inlet and voltage response, when a choke valve located directly upstream of the fuel cell cathode inlet is partially closed so that there is a 10 kPa pressure drop across the choke valve.

[0049] FIG. 6D shows pressure oscillations applied at the cathode outlet of a fuel cell over a period of time, and the corresponding pressure oscillations observed at the cathode inlet and voltage response, when a choke valve located directly upstream of the fuel cell cathode inlet is partially closed so that there is a 25 kPa pressure drop across the choke valve.

[0050] FIG. 7A. is a Nyquist plot showing example EPIS test results obtained when a choke valve located directly upstream of the fuel cell cathode inlet is partially closed so that there is a 10 kPa pressure drop across the choke valve.

[0051] FIG. 7B. is a Nyquist plot showing example EPIS test results obtained when a choke valve located directly upstream of the fuel cell cathode inlet is partially closed so that there is a 15 kPa pressure drop across the choke valve.

[0052] FIG. 8 is a bode plot amplitude obtained when a choke valve located directly upstream of a fuel cell cathode inlet is partially closed, and the fuel cell is operated at two different oxidant stoichiometries, and the plot shows that the EPIS amplitude increases as the oxidant stoichiometry at the cathode is reduced from 2.0 to 1.6.

[0053] FIG. 9A-9F show the Fast Fourier Transform (FFT) magnitude calculated for the outlet pressure, inlet pressure, and cell voltage for a single-cell fuel cell assembly operated at constant current when a choke valve located directly upstream of the fuel cell cathode inlet is open and when it is partially closed, illustrating that partially closing the choke valve can reduce harmonics and improve linearity of the voltage response.Detailed Description of Preferred Embodiment(s)

[0054] Pressure Frequency Response Analysis (pFRA) methods, which can include, for example, electrochemical pressure impedance spectroscopy (EPIS) and other techniques, are fuel cell test methods and diagnostic tools that can be used to analyze the voltage or current response of a fuel cell as a function of an applied pressure signal in the frequency domain. In some embodiments, pressure oscillations are applied at or downstream of the cathode outlet, and corresponding oscillations in fuel cell voltages at a fixed current during operation of the fuel cell are measured. The pressure-voltage amplitude ratio and phase difference can then be used to decipher information about the conditions and processes in the fuel cell.

[0055] One challenge with some existing methods that use pFRA or EPIS is that when pressure oscillations are applied by controlling a back pressure valve located downstream of the cathode outlet, the pressure oscillations are typically significantly dampened at the cathode inlet due to the volume, hydraulic resistance and capacitive effect of process lines and components located upstream of the cathode inlet (e.g. between the cathode inlet and the oxidant mass flow controller or primary oxidant control valve). The dampening effect means that corresponding pressure oscillations may not be observable or apparent at the cathodeinlet of the fuel cell, especially during high frequency pressure oscillations, and in this case a meaningful pFRA signal may be obtained only for the cathode outlet.

[0056] A pressure oscillation applied at or downstream of the cathode outlet propagates in the cathode flow field from the cathode outlet towards the cathode inlet, and has a different amplitude and phase at different locations along the flow field channel or path. Not having information about the pressure oscillation at the cathode inlet makes it more difficult to interpolate and determine pressure conditions along the cathode flow field, limiting the useful information that can be provided by pFRA testing.

[0057] Even if the pressure oscillations are observable at the cathode inlet, in existing methods one can still not control the oscillation amplitude. However, the propagation of the pressure oscillation amplitude back along the channel affects the voltage oscillations and contains diagnostic information. Therefore, the capability to control the propagation and amplitude of pressure oscillations along the flow field channel from the cathode outlet toward the cathode inlet can provide diagnostic information regarding the dynamic processes involved in the voltage response.

[0058] In embodiments of the systems and methods described herein, a valve is added upstream of the cathode inlet, downstream of the usual primary oxidant control valve. This secondary oxidant control valve is adjustable so that the flow resistance through it increases as it is adjusted from open to closed (it is not a simple on-off valve or shutoff valve). The adjustable valve can function as a variable flow resistor which can be adjusted to reduce or eliminate the capacitive effect of process lines and components located upstream of the cathode inlet, hence allowing the pressure oscillations at the cathode inlet (associated with the pressure oscillations applied at or downstream of the cathode outlet) to be controlled and amplified so that they are more easily detected. This can make it possible to obtain reliable pFRA signals at the cathode inlet of the fuel cell, which can allow for interpolation of the pressure oscillation along the flow field and the extraction of diagnostic information about the fuel cell.

[0059] In some embodiments of the systems and methods described herein, the secondary oxidant control valve is located directly upstream of the cathode inlet, for example, withoutany lines or components that house a significant volume of fluid located between the secondary oxidant control valve and the cathode inlet. In some embodiments in which there is a humidifier between the primary oxidant control valve and the cathode inlet, the secondary oxidant control valve is located between the humidifier and the cathode inlet. Preferably the secondary oxidant control valve is closer to the cathode inlet than to the oxidant humidifier, in terms of the fluid volume between the components. In other words, there is less fluid volume between the secondary oxidant control valve and the downstream cathode inlet than between the secondary oxidant control valve and the upstream humidifier. In some embodiments, the secondary oxidant control valve is a choke valve.

[0060] FIG. 1 is a schematic illustration of an example of a system that can be used to perform embodiments of the test methods described herein, including diagnostic pFRA testing on a fuel cell. FIG. 1 shows fuel cell 100 connected to equipment for testing of the fuel cell. Fuel cell 100 comprises an anode-side 112 and a cathode-side 114, with a membrane electrolyte 116 (shown with shading) therebetween. Anode-side 112 can comprise an anode electrode, typically comprising a porous gas diffusion layer (GDL) and an anode electrocatalyst layer between the GDL the membrane electrolyte 116. Similarly, cathode-side 114 can comprise a cathode electrode, typically comprising a porous gas diffusion layer (GDL) and a cathode electrocatalyst layer between the GDL and membrane electrolyte 116. The anode and cathode electrocatalyst layers can be deposited on opposing sides of the membrane electrolyte or on the respective GDLs. The membrane, sandwiched by the anode and cathode electrocatalysts and GDLs forms the membrane electrode assembly (MEA). There is typically a flow field plate adjacent to each of the anode and cathode GDLs for directing reactant streams to the anode and cathode, respectively, and removing the produced heat and water, as well as conducting electricity. Fuel cell 100 also comprises an anode inlet 120 an anode outlet 122, a cathode inlet 124 and a cathode outlet 126. Fuel cell 110 can be an individual fuel cell. In other embodiments, fuel cell 100 can represent a fuel cell assembly, such as a fuel cell stack, comprising multiple fuel cells. In this case, anode-side 112 represents the anode side of the stack, cathode-side 114 represents the cathode side of thestack, and membrane electrolyte 116 represents the electrolyte layers between adjacent electrodes in the stack.

[0061] Anode inlet 120 is connected to a source of hydrogen via a mass flow controller comprising valve 130 and mass flow meter 132, and optionally to a source of nitrogen (or another inert gas) via a mass flow controller comprising valve 134 and mass flow meter 136. The mass flow controllers allow for mixing of the gases and reducing the concentration of hydrogen supplied to fuel cell 100, as desired.

[0062] Cathode inlet 124 is connected to a source of oxidant (e.g. an oxygen source, or air supplied via an air compressor) via a mass flow controller comprising valve 140 and mass flow meter 142, and optionally to a source of nitrogen (or another inert gas) via a mass flow controller comprising valve 144 and mass flow meter 146. The mass flow controllers allow for mixing of the gases supplied to fuel cell 100, as desired.

[0063] Humidifiers 150 and 155 are optional, and can be used to control the humidity of fuel and oxidant gases supplied to fuel cell 100, respectively. In some systems on / off by-pass valves (not shown in FIG. 1) can be used to bypass the humidifiers. In some systems a cooling subsystem (not shown in FIG. 1) comprising a coolant loop can be operated to circulate coolant through the fuel cell to control temperature during testing.

[0064] The volume of gas and hydraulic resistance within the process lines and humidifier 155 located between cathode inlet 124 and valve 140 can cause dampening of pressure oscillations applied downstream of cathode outlet 126 as they move toward the cathode inlet. For this reason, an additional adjustable valve - choke valve 160 in the embodiment of FIG. 1 - is located upstream of cathode inlet 124 and downstream of cathodeside humidifier 155.

[0065] Pressure and temperature sensors 170 and 172 can be used to monitor temperature and pressure at or near anode inlet 120 and anode outlet 122, respectively. Pressure and temperature sensors 174 and 176 can be used to monitor temperature and pressure at or near cathode inlet 124 and cathode outlet 126, respectively. Pressure and temperature sensor 178can be used to monitor temperature and pressure upstream of choke value 160, between cathode-side humidifier 155 and choke valve 160.

[0066] Fuel cell 100 can be connected to an electrical load 180 by closing switch 182. A voltmeter or voltage monitoring system 184 can be used to monitor or measure the voltage across fuel cell 100, (for example, across a single cell or across individual cells or groups of cells in a fuel cell stack comprising multiple fuel cells).

[0067] Back pressure valves 190 and 195 located downstream of anode and cathode outlets 122 and 126, respectively, can be used to control the pressure at the anode and cathode respectively.

[0068] In conventional fuel cells and fuel cell test systems there is typically a primary oxidant control valve upstream of the fuel cell cathode inlet to control the oxidant flow (e.g. valve 140 in FIG. 1) and a valve downstream of the fuel cell cathode outlet to control cathode pressure (e.g. back pressure valve 195 in FIG. 1). Embodiments of the systems described herein have an extra adjustable valve (e.g. choke valve 160 in FIG. 1) located upstream of the cathode inlet and downstream of the primary oxidant control valve. This is not a simple on- off valve. This secondary oxidant control valve can be operated to mitigate the capacitive effect of the upstream process lines and components, and / or to reduce harmonics and control the linearity of the voltage response.

[0069] FIG. 2 is a flow chart illustrating a known method 200 for conducting pFRA on a fuel cell. Block 210 of method 200 comprises supplying an oxidant stream (typically air) to the fuel cell cathode and a fuel stream (typically hydrogen) to the fuel cell anode, each at a given pressure, temperature, humidity, and flow rate. Block 220 of method 200 comprises operating the fuel cell to supply electrical power to a load. In method 200, the fuel cell is operated at constant current. Anode and cathode input flow rates can be controlled, for example, using mass flow controllers located upstream of the fuel cell.

[0070] Block 220 of method 200 further comprises applying pressure oscillations to the cathode-side over a range of frequencies using a pressure control valve located downstream of the cathode outlet (for example, back pressure valve 195 in FIG. 1) while the fuel cell is operating and supplying power to the load. Block 230 of method 200 comprises monitoringor measuring pressure-induced voltage oscillations across the fuel cell while the pressure oscillations are being applied to the cathode-side.

[0071] FIG. 3 is a flow chart illustrating an embodiment of an improved method 300 for conducting pFRA tests on a fuel cell. Method 300 may involve additional elements (or blocks) that are not illustrated in FIG. 3. Some of the elements shown in method 300 of FIG. 3, and of various embodiments of the methods described herein, are optional. Furthermore, some elements of method 300 illustrated in FIG. 3, or of various embodiments of the methods described herein, may be performed in a different sequence from that illustrated or described, or may be performed in parallel with one or more other elements, or maybe be combined with one or more other elements.

[0072] Block 310 of method 300 comprises supplying an oxidant stream (typically air) to fuel cell cathode, for example, at a given pressure, temperature, humidity, and flow rate. Optionally Block 310 can also comprise supplying fuel (typically hydrogen) to the fuel cell anode. Block 320 of method 300 comprises applying pressure oscillations to the cathode-side at a first frequency, for example, using a pressure control valve located downstream of the cathode outlet (for example, back pressure valve 195 in FIG. 1). In some embodiments the first frequency is the highest frequency that is to be used in testing the fuel cell using method 300. Block 330 of method 300 is optional, and comprises comparing the amplitude of the pressure oscillations applied at or near the cathode outlet with the amplitude of the pressure oscillations observed at or near the cathode inlet.

[0073] Block 340 of method 300 comprises adjusting or setting a secondary oxidant control valve located downstream of the primary oxidant control valve and upstream of the cathode inlet to a first position. In some embodiments this involves adjusting the secondary oxidant control valve to control or improve the linearity of the voltage response, and / or adjusting the secondary oxidant control valve to control or adjust the amplitude of the pressure oscillations observed at or near the cathode inlet. In some situations, this will involve adjusting the secondary oxidant control valve from a fully open position to a partially closed position. In some embodiments of method 300, the secondary oxidant control valve is adjusted so that the pressure oscillations observed at or near the cathode inlet are detectable and have a desired amplitude, or so that they have a desired relative amplitude whencompared to the amplitude of the pressure oscillations applied at or near the cathode outlet. In some embodiments of method 300, the secondary oxidant control valve is adjusted so that the amplitude of the pressure oscillations observed at or near the cathode inlet is of approximately the same order of magnitude as the amplitude of the pressure oscillations applied at or near the cathode outlet. In some embodiments of method 300, the secondary oxidant control valve is adjusted so that the pressure oscillations at or near the cathode inlet are at least detectable (e.g. by a pressure sensor located at or near the cathode inlet). In some embodiments, pressure oscillations observed “near” the cathode inlet are pressure oscillations observed at a location between the secondary oxidant control valve and the cathode inlet. In some embodiments, pressure oscillations applied “near” the cathode outlet are pressure oscillations applied using pressure control valve located downstream of the cathode outlet. In some embodiments, pressure oscillations observed “near” the cathode outlet are pressure oscillations observed at a location between the cathode outlet and the pressure control valve located downstream of the cathode outlet.

[0074] In some embodiments of method 300, the fuel cell is not operated to supply electrical power to a load during Blocks 320, 330 and 340. Even if the fuel cell is not operated to supply electrical power to a load during Blocks 320, 330 and 340, optionally the fuel cell anode may be supplied with fuel during these blocks, for example to reduce the pressure differential across the membrane electrolyte. In some embodiments of method 300, Block 310 comprises supplying fuel to the fuel cell anode, and the fuel cell is operated to supply electrical power to a load during Blocks 320, 330 and 340. Block 350 of method 300 comprises supplying fuel and oxidant to the fuel cell and operating the fuel cell to supply electrical power to a load. In some embodiments of method 300, the fuel cell is operated at constant current. In other embodiments of method 300, the fuel cell is operated at constant voltage. Anode and cathode input flow rates can be controlled, for example, using mass flow controllers located upstream of the fuel cell.

[0075] Block 350 of method 300 comprises, while operating the fuel cell to supply electrical power to a load, applying pressure oscillations to the cathode-side over a range of frequencies, for example up to the first (high) frequency used in Blocks 320 and 340. This can be done, for example using a pressure control valve downstream of the cathode outlet (forexample, back pressure valve 195 in FIG. 1) while the fuel cell is operating and supplying power to the load. In some embodiments, applying pressure oscillations to the cathode-side over a range of frequencies means applying pressure oscillations to the cathode-side at multiple different discrete frequencies. For example, in some embodiments pressure oscillations are applied to the cathode-side at multiple discrete frequencies in the range of 0.1 to 0.001 Hz, or in the range of 1.0 to 0.001 Hz.

[0076] In some embodiments of method 300, during Block 350 the secondary oxidant control valve remains set at the first position to which it was adjusted in Block 340. In other embodiments of method 300, the secondary oxidant control valve is adjusted to different positions during Block 350. For example, the secondary oxidant control valve may be adjusted to different positions in order to reduce harmonics and improve linearity in the voltage response, as the frequency of the pressure oscillations applied to the cathode-side is altered.

[0077] Block 360 of method 300 comprises monitoring or measuring pressure-induced voltage oscillations across the fuel cell if the fuel cell is operating at constant current; or monitoring or measuring pressure-induced current oscillations in the fuel cell if the fuel cell is operating at constant voltage, while the pressure oscillations are being applied to the cathode-side.

[0078] Embodiments of the improved method provide an extra degree of freedom in terms of controlling the pressure oscillations in the fuel cell, compared to existing methods, by adding an extra adjustable valve to the cathode process line downstream of the primary oxidant control valve and humidifier, and upstream of the cathode inlet. This valve can be adjusted to control the amplitude of pressure oscillations observed at or upstream of the fuel cell cathode inlet caused by pressure oscillations applied at or downstream of the fuel cell cathode outlet, and / or can be adjusted to improve the linearity of the voltage response.

[0079] FIG. 4 shows an example of the effect of adjusting a choke valve directly upstream of the fuel cell cathode inlet on the amplitude of pressure oscillations observed at the fuel cell cathode inlet, for a single-cell fuel cell assembly, when pressure oscillations over a range of frequencies are applied using a back pressure valve / controller located downstreamof the fuel cell cathode outlet. FIG. 4 shows the ratio of the amplitude of the pressure oscillations observed at the cathode inlet to the amplitude of the pressure oscillations applied at the cathode outlet, over a range of pressure oscillation frequencies, for three different settings of the choke valve with 0.5 NLPM cathode flow (air). When the ratio is close to 1 it indicates that the pressure oscillations observed at the cathode inlet are close in amplitude to those observed at the cathode outlet at a given pressure oscillation frequency. Plot 410 shows that, with the choke valve fully open, the amplitude of the pressure oscillations observed at the cathode inlet is similar to the amplitude of the pressure oscillations observed at the cathode inlet at low frequencies (e.g. 0.001 Hz), but as the frequency is increased the amplitude of the pressure oscillations at the cathode decreases dramatically so that the pressure oscillation applied at the cathode outlet is almost undetectable at the cathode inlet at higher frequencies (e.g. the amplitude drops ~90 times at 0.4 Hz). Plot 420 shows that when the choke valve is partially closed - so that it is set with a pressure drop of about 14kPa across it - the amplitude of the pressure oscillations observed at the cathode inlet relative to the pressure oscillation applied cathode outlet decreases slightly as the frequency is increased to 0.1Hz, but they are still detectable. Plot 430 shows that when the choke valve is closed further - so that it is set with a pressure drop of about 25 kPa across it - the amplitude of the pressure oscillations observed at the cathode inlet is almost the same as the amplitude of the pressure oscillations applied and observed at the cathode outlet over the range of frequencies tested (0.001 to 0.4 Hz).

[0080] Thus, FIG. 4 shows that with the valve fully open, the amplitude of the associated pressure oscillations observed at the cathode inlet drops with increasing frequency of the pressure oscillations applied at the cathode outlet. However, when the valve is partially closed to a certain pressure drop across it, then the pressure oscillation amplitude at the cathode inlet remains approximately constant over a range of frequencies, allowing for reliable measurement of the pFRA signal at the cathode inlet and outlet.

[0081] FIGS. 5A and 5B show the effect of pressure oscillations applied at the cathode outlet (~1 kPa pressure oscillation at a frequency of 0.1Hz) over a period of 100 seconds when a choke valve located directly upstream of the fuel cell cathode inlet is fully open (FIG.5 A), and when the choke valve is partially closed so that there is 100 kPa pressure dropacross it (FIG. 5B). In both cases the fuel cell, which was a single-cell fuel cell assembly, was operated at constant current (15 A; 0.3 A / cm2) with 1.25 NLPM cathode flow (air) and hydrogen as fuel. In FIG. 5A the pressure oscillation applied at the cathode outlet is shown in plot 510a, the corresponding pressure measured at the cathode inlet is shown in plot 520a, and the fuel cell voltage is shown in plot 530a. In FIG. 5B the pressure oscillation applied at the cathode outlet is shown in plot 510b, the corresponding pressure measured at the cathode inlet is shown in plot 520b, and the fuel cell voltage is shown in plot 530b. It can be seen that the amplitude of the pressure oscillation at the cathode inlet is much less (and therefore more difficult to detect) when the choke valve is fully open (FIG. 5A) than when the choke valve is partially closed (FIG. 5B). In this case the corresponding voltage oscillation is much less when choke valve is fully open than when the choke valve is partially closed (FIG. 5B). It is possible to tune or adjust the choke valve so that the amplitude of the pressure oscillation at the cathode inlet is approximately the same as the amplitude of the pressure oscillation applied at the cathode outlet (as shown in FIG. 5B, for example), or so that they have a desired relative amplitude, and / or so that the pressure oscillation applied at the cathode outlet is at least detectable at the cathode inlet.

[0082] FIGS. 6A-6D show the effect of pressure oscillations applied at the cathode outlet (~2.0 kPa pressure oscillation at a frequency of 0.1Hz) over a period of 100 seconds with no choke valve (FIG. 6A), when a choke valve located directly upstream of the fuel cell cathode inlet is fully open (FIG. 6B), when the choke valve is partially closed so that there is 10 kPa pressure drop across it (FIG. 6C), and when the choke valve is partially closed so that there is 25 kPa pressure drop across it (FIG. 6D) . The fuel cell operating conditions were different than in the example shown in FIGS. 5A and 5B. The fuel cell, which was a single cell fuel cell assembly, was operated at constant current (15 A; 0.3 A / cm2) with 0.5 NLPM cathode flow (air), and hydrogen as fuel.

[0083] In FIG. 6A the pressure oscillation applied at the cathode outlet is shown in plot 610a, the corresponding pressure measured at the cathode inlet is shown in plot 620a, and the fuel cell voltage is shown in plot 630a. In FIG. 6B the pressure oscillation applied at the cathode outlet is shown in plot 610b, the corresponding pressure measured at the cathode inlet is shown in plot 620b, and the fuel cell voltage is shown in plot 630b. In both cases theamplitude of the pressure oscillation at the cathode inlet is much less than at the outlet, but the oscillation is visible in plots 620a and 620b and is somewhat detectable. In FIG. 6C the pressure oscillation applied at the cathode outlet is shown in plot 610c, the corresponding pressure measured at the cathode inlet is shown in plot 620c, and the fuel cell voltage is shown in plot 630c, and in FIG. 6D the pressure oscillation applied at the cathode outlet is shown in plot 61 Od, the corresponding pressure measured at the cathode inlet is shown in plot 620d, and the fuel cell voltage is shown in plot 630d. When the valve is partially closed (as shown in FIGS. 6C and 6D), the amplitude of the pressure oscillation at the cathode inlet increases (see plots 620c and 620d) and is much easier to detect.

[0084] FIGS. 7A and 7B are Nyquist plots showing some example EPIS test results obtained using embodiments of the systems and methods described herein, in which a secondary oxidant control valve located directly upstream of the cathode inlet is set at two different pressure drops: partially closed with a 10 kPa pressure drop across it (FIG. 7A) versus a 15 kPA pressure drop across it (FIG. 7B). The fuel cell, which was a single-cell fuel cell assembly, was operated at constant current (0.2 A / cm2) with 0.27 NLPM cathode flow (air) with hydrogen as fuel, and with a 2 kPa pressure oscillation applied using a backpressure valve downstream of the cathode outlet. FIG. 7A shows the inlet pressure impedance plot 710a, and the outlet pressure impedance plot 720a, and FIG. 7B shows the inlet pressure impedance plot 710b, and the outlet pressure impedance plot 720b, illustrating that as the pressure drop across the choke valve is adjusted from 10 kPa to 15 kPa the inlet and the outlet pressure impedances come closer to one another, with the size difference between the plots getting smaller. Furthermore, the size of both the inlet and outlet pressure impedance plots gets smaller with increasing pressure drop across the secondary oxidant control valve. This means that under these operating conditions, the voltage response reduces as the pressure drop across the secondary oxidant control valve is increased. These correlations can be used to provide insights into the dynamics of the fuel cell.

[0085] Diagnostic information is key to understanding the fundamental processes in fuel cells and thus improving performance, lifetime, and cost, which is important for their successful commercialization. In this context, frequency-based diagnostic tools offer the advantage of enabling the separation of dynamic processes based on their characteristicfrequency and response time. The systems and methods described herein can enhance the value of pFRA and EPIS in diagnostic testing of fuel cells, as it is now possible to also adjust and control the signal conditions at the fuel cell inlet, in addition to the outlet, and thus decipher the dynamic processes occurring along the fuel cell reactant flow field path from inlet to outlet. For example, it can enhance understanding of mass transport properties, water distribution conditions and reactant flow conditions in the flow field. This is turn can aid with developing improved flow field designs, as well as adjusting the operating parameters for improved or optimal operation of the fuel cell.

[0086] The results shown in FIG. 8 illustrate the effect of oxidant stoichiometry on an EPIS signal, for a single-cell fuel cell assembly operated at constant current (0.3 A / cm2) with 0.5 NLPM cathode flow (air), with hydrogen as fuel, and with a 2 kPa pressure oscillation applied using a back pressure valve located downstream of the cathode outlet, where the pressure drop across a choke valve located directly upstream of the cathode inlet was set at 20 kPa. FIG. 8 is a bode plot amplitude, showing the EPIS amplitude at the cathode inlet and outlet when the fuel cell was operating at an oxidant stoichiometry of 2.0 and at an oxidant stoichiometry of 1.6. It can be seen from these results that the EPIS amplitude increases as the oxidant stoichiometry at the cathode drops from 2.0 to 1.6. This illustrates that the EPIS signal can be used, for example, as a diagnostic tool to detect low oxidant stoichiometry conditions in the fuel cell.

[0087] FIGS. 9A-9F show the Fast Fourier Transform (FFT) magnitude calculated for the outlet pressure, inlet pressure, and cell voltage for a single-cell fuel cell assembly operated at constant current (0.5 A / cm2) with 1.04 NLPM cathode flow (air), hydrogen as fuel, and with a 7 kPa pressure oscillation at a frequency of 0.1Hz applied using a back pressure valve located downstream of the cathode outlet. FIGS. 9A, 9C and 9E show the FFT magnitude calculated for the outlet pressure, inlet pressure and cell voltage, respectively, when a choke valve located directly upstream of the cathode inlet was open (with 0 kPa pressure drop across it). FIGS. 9B, 9D and 9F show the FFT magnitude calculated for the outlet pressure, inlet pressure and cell voltage, respectively, when the choke valve was partially closed with the pressure drop across it set at 70 kPa. It can be seen that in both cases (FIG. 9E and 9F) the voltage exhibits harmonics, however partially closing the upstream valve results in significantsuppression of the higher harmonics. More specifically, the ratio of the second to first harmonic FFT magnitude for the case with the open valve is 25% (see FIG. 9E), whereas the ratio of the second harmonic to first harmonic FFT magnitude when the choke valve was partially closed is 7% (see FIG. 9F). Partially closing the valve significantly reduced the second harmonic effect. This illustrates that a secondary oxidant control valve (such as a choke valve) located upstream of the cathode inlet and downstream of the primary oxidant control valve can be used and adjusted to improve pFRA linearity to result in EPIS. This can be very useful, especially for modelling and diagnostic purposes, as linear approximation simplifies the modelling requirements that can be used to decipher dynamic processes in the fuel cell that contribute to the pFRA / EPIS signal.

[0088] Embodiments of the systems and methods described herein can be used for testing fuel cell assemblies, such as individual fuel cells or fuel cell stacks. In a fuel cell stack, voltage oscillations can be measured across individual cells, across pairs or groups of cells, or across the entire stack. The systems and methods described herein are particularly suitable for testing polymer electrolyte fuel cells designed for operation on air or oxygen and hydrogen, however the systems and methods may be adopted or adapted for other types of fuel cells and / or fuel cells operating on other reactants, or indeed for other types of electrochemical devices such as electrolyzers.

[0089] Generally, during the testing described herein, the fuel cell assembly under test is operated to supply power to an electrical load at constant current, while the voltage oscillations associated with the applied pressure oscillations are measured. However, in other embodiments of the method (for example, during testing of a single cell) the fuel cell assembly can be operated at constant voltage and the associated oscillations in fuel cell current can be measured instead.

[0090] In other embodiments of the methods described herein, the adjustable valve located directly upstream of the cathode inlet can be used to apply pressure oscillations in thefuel cell cathode (instead of using the back pressure valve at the cathode outlet for this purpose).

[0091] In other embodiments of the methods described herein, pressure oscillations are applied at the outlet on the anode-side instead of on the cathode-side, and an adjustable valve located upstream of the anode inlet (and downstream of the primary fuel control valve) can be used to reduce or eliminate the capacitive effect of upstream process lines and components on the anode-side, hence enabling detectability and controllability of pressure oscillations at the anode inlet.

[0092] Some embodiments of the methods described herein can involve applying other pressure perturbations such as a pressure impulse. The correlation of these other pressure perturbations with their voltage or impedance response can provide additional diagnostic information about the fuel cell being tested. Furthermore, pressure perturbations can also be used to improve operational conditions in the fuel cell, such as water removal from the cathode or anode as a result of the pressure oscillation / impulse.

[0093] One application for the technology described herein is in fuel cell test stations, where diagnostic tools are utilized by researchers to understand dynamic processes in fuel cells and improve their design. Another potential application for this technology is to assist fuel cell system integrators in developing operating strategies that enhance or optimize fuel cell system performance. Furthermore, embodiments of the methods described herein could also be used in fuel cell systems to diagnose and improve the functionality of an operational fuel cell system in real time.

[0094] As used in this description and in the appended claims, "fuel" means a substantially pure hydrogen gas stream or a gas stream comprising hydrogen, such as areformate gas stream, for example, and "oxidant" means a substantially pure oxygen gas stream or a gas stream comprising oxygen, such as air, for example.

[0095] "Inert gas" means a gas stream that is substantially unreactive in a fuel cell, such as nitrogen, argon, or helium, or any combination thereof, for example.

[0096] “Reactant” is used to refer to a material that is reactive in a fuel cell, such as hydrogen or oxygen, for example.

[0097] Some acts of the methods (also referred to as blocks, elements or steps of the methods) may be performed in an order other than that which is described and illustrated herein, or may be performed in parallel with one or more other acts, or maybe be combined with one or more other acts. To the extent that some of the acts rely on the completion of other acts of the method, these may need to be performed in a particular sequence. Also, it should be appreciated that in some embodiments of the methods not all of the acts described in the flowcharts are required to be performed, that additional acts may be added, and / or that some of the illustrated acts may be omitted or substituted with other acts.

[0098] It is contemplated that part of any aspect or embodiment discussed in this specification can be implemented or combined with part of other aspects or embodiments discussed in this specification.

[0099] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.

Claims

What is claimed is:

1. A method for testing a fuel cell assembly, the fuel cell assembly comprising a cathode-side having a cathode inlet and a cathode outlet, and an anode-side having an anode inlet and an anode outlet, the method comprising: while supplying an oxidant stream to the cathode inlet via a primary oxidant control valve, adjusting a secondary oxidant control valve located downstream of the primary oxidant control valve and upstream of the cathode inlet to a partially closed position; while operating the fuel cell assembly to supply electrical power to a load: applying pressure oscillations at or downstream of the cathode outlet over a range of frequencies; and measuring pressure-induced voltage oscillations across the fuel cell assembly if the fuel cell is operating at constant current, or measuring pressure-induced current oscillations in the fuel cell assembly if the fuel cell is operating at constant voltage.

2. The method of claim 1 wherein the step of adjusting the secondary oxidant control valve to a partially closed position comprises, prior to the applying pressure oscillations at or downstream of the cathode outlet over the range of frequencies and measuring the pressure-induced voltage oscillations or measuring the pressure-induced current oscillations: applying pressure oscillations at or downstream of the cathode outlet at a first frequency; measuring the amplitude of associated pressure oscillations at or upstream of the cathode inlet; and adjusting the secondary oxidant control valve to a first partially closed position so that the associated pressure oscillations at or upstream of the cathode inlet have a desired amplitude.

3. The method of claim 2 wherein the step of adjusting the secondary oxidant control valve to the first partially closed position so that the associated pressure oscillations at or upstream of the cathode inlet have a desired amplitude comprises adjusting the secondaryoxidant control valve to the first partially closed position so that the associated pressure oscillations at or upstream of the cathode inlet are at least detectable.

4. The method of claim 2 further comprising comparing the amplitude of the pressure oscillations applied at or downstream of the cathode outlet at the first frequency with the amplitude of the associated pressure oscillations observed at or upstream of the cathode inlet; and wherein the step of adjusting the secondary oxidant control valve so that the associated pressure oscillations at or upstream of the cathode inlet have a desired amplitude comprises adjusting the secondary oxidant control valve to the first partially closed position so that the associated pressure oscillations observed at or upstream of the cathode inlet have the desired amplitude relative to the pressure oscillations applied downstream of the cathode outlet at the first frequency.

5. The method of claim 4 wherein the step of adjusting the secondary oxidant control valve to the first partially closed position so that the associated pressure oscillations observed at or upstream of the cathode inlet have the desired amplitude relative to the pressure oscillations applied downstream of the cathode outlet at the first frequency comprises adjusting the secondary oxidant control valve so that the amplitude of the associated pressure oscillations at or upstream of the cathode inlet is of the same order of magnitude as the amplitude of the pressure oscillations applied at or downstream of the cathode outlet.

6. The method of any of claims 2 to 5 wherein the first frequency is the highest frequency in the range of frequencies.

7. The method of any of claims 2 to 6 further comprising supplying fuel to the anode inlet while applying the pressure oscillations at or downstream of the cathode outlet at the first frequency and adjusting the secondary oxidant control valve to the first partially closed position.

8. The method of any of claims 2 to 7 further comprising operating the fuel cell assembly to supply electrical power to a load while applying pressure oscillations at or downstream of the cathode outlet at the first frequency and adjusting the secondary oxidant control valve to the first partially closed position.

9. The method of any of claims 2 to 8 wherein the method further comprises leaving the secondary oxidant control valve set at the first partially closed position while applying pressure oscillations at or downstream of the cathode outlet over the range of frequencies.

10. The method of any of claims 2 to 8 wherein the method further comprises further adjusting the secondary oxidant control valve while applying pressure oscillations at or downstream of the cathode outlet over the range of frequencies.

11. The method of claim 10 wherein the step of further adjusting the secondary oxidant control valve comprises further adjusting the secondary oxidant control valve based on the frequency of the pressure oscillations being applied.

12. The method of claim 10 wherein the step of further adjusting the secondary oxidant control valve comprises adjusting the secondary oxidant control valve to adjust the linearity of a pressure-induced voltage response.

13. The method of any of claims 1 to 12 wherein the step of applying pressure oscillations at or downstream of the cathode outlet comprises using a pressure control valve located at or downstream of the cathode outlet to apply the pressure oscillations.

14. The method of any of claims 1 to 13 wherein the fuel cell assembly is a single fuel cell.

15. The method of any of claims 1 to 13 wherein the fuel cell assembly is a fuel cell stack comprising a plurality of fuel cells, and the method comprising measuring pressure- induced voltage oscillations across each fuel cell in the fuel cell stack.

16. A fuel cell testing system for testing a fuel cell assembly comprising a cathode-side having a cathode inlet and a cathode outlet, and an anode-side having an anode inlet and an anode outlet, the system comprising: an oxidant supply subsystem for supplying oxidant to the cathode-side via the cathode inlet, the oxidant supply subsystem comprising a primary oxidant control valve located upstream of the cathode inlet; a cathode back pressure valve located at or downstream of the cathode outlet; a fuel supply subsystem for supplying a fuel stream to the anode-side via the anode inlet, the fuel supply subsystem comprising a primary fuel control valve located upstream of the cathode inlet; an anode back pressure valve located at or downstream of the anode outlet; a voltage monitoring subsystem for measuring the voltage across one or more fuel cells in the fuel cell assembly; an electrical load connectable to receive electrical power generated by the fuel cell assembly; and an adjustable secondary oxidant control valve located upstream of the cathode inlet and downstream of the primary oxidant control valve.

17. The fuel cell testing system of claim 16 further comprising a humidifier located upstream of the cathode inlet for humidifying the oxidant stream supplied to the cathode-side via the cathode inlet, where the primary oxidant control valve is located upstream of the humidifier and the adjustable secondary oxidant control valve is located downstream of the humidifier, between the humidifier and the cathode inlet.

18. The fuel cell testing system of any of claims 16 to 17 wherein during operation of the system, when the electrical load is connected and receiving electrical power generated by the fuel cell assembly and pressure oscillations are being applied at or downstream of the cathode outlet using the cathode back pressure valve, adjusting the adjustable secondary control valve changes the amplitude of the associated pressure oscillations at or upstream of the cathode inlet.

19. The fuel cell testing system of any of claims 16 to 17 wherein during operation of the system, when the electrical load is receiving electrical power generated by the fuel cell assembly and pressure oscillations are being applied at or downstream of the cathode outlet using the cathode back pressure valve, adjusting the adjustable secondary control valve changes the linearity of a pressure-induced voltage response measured using the voltage monitoring subsystem.

20. The fuel cell testing system of any of claims 16 to 19 wherein the adjustable secondary oxidant control valve is located directly upstream of the cathode inlet.

21. The fuel cell testing system of any of claims 16 to 20 wherein the adjustable secondary oxidant control valve is a choke valve.

22. The fuel cell testing system of any of claims 16 to 21 wherein the fuel cell assembly is a fuel cell stack comprising a plurality of fuel cells, and the voltage monitoring subsystem for measuring the voltage across one or more fuel cells in the fuel cell assembly is configured to measure the voltage across each fuel cell of the plurality of fuel cells.

23. The fuel cell testing system of any of claims 16 to 21 wherein the fuel cell assembly is a fuel cell stack comprising a plurality of fuel cells and the voltage monitoring subsystem for measuring the voltage across one or more fuel cells in the fuel cell assembly is configured to measure the voltage across the plurality of fuel cells.

24. An apparatus having any new and inventive feature, combination of features, or sub-combination of features as described herein.

25. Methods having any new and inventive steps, acts, combination of steps and / or acts or sub-combination of steps and / or acts as described herein.

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