Effects of Fuel Cell Operating Conditions on Electrochemical Pressure Impedance Spectroscopy Diagnostics

Electrochemical pressure impedance spectroscopy (EPIS) is a fuel cell diagnostic technique based on pressure alternating frequency response analysis. This technique has similarities to electrochemical impedance spectroscopy except it implements mechanical perturbations via pressure oscillations rather than voltage or current oscillations to achieve an electrochemical impedance response. This study examines pressure-induced electrochemical resistances while independently varying fuel cell inlet operating conditions, such as cathode relative humidity, gas stoichiometry at the cathode, and fuel cell temperature. Results indicated that EPIS resistances increased with decreasing cathode relative humidity, decreasing cathode stoichiometry, and increasing cell temperature. Variation in transport resistance with cathode stoichiometry was attributed to decreased oxygen partial pressures measured at the cathode outlet. Alternatively, variations in resistance with relative humidities were correlated to changes in humidity ratios throughout the cathode. Resistance changes with cell temperature were a function of both phenomena. Mechanical responses were analyzed by measuring inlet pressure response relative to outlet pressure perturbations. Mechanically based variations observed during cathode stoichiometry tests indicated additional down-channel variations from changes in gas flow. In comparison, insignificant variations in mechanical response were observed during relative humidity and temperature tests, suggesting purely through-plane responses attributed to electrochemical resistances within the membrane electrode assembly.