Electrochemical Impedance Spectroscopy of PEM Fuel Cells at Elevated Temperature and Various Relative Humidities

摘要

PEM fuel cells operated at elevated temperatures have significant advantages over low temperature PEM fuel cells: increased CO tolerance, more efficient heat rejection, and more useful waste heat. At ambient pressure, however, the increase in cell temperature results in fuel cell operations at low relative humidity (RH). The temperature and RH changes influence many parameters of a PEM fuel cell, such as membrane, electrode and charge transfer resistances. Electrochemical impedance spectroscopy was used to study these resistances at elevated temperature and various RHs. At low RH, the impedance spectroscopy for a thick cathode (0.5mg/cm2 Pt loading) displays a 45 deg slope at the high-frequency intercept, which is associated with ionic resistance of the cathode. The ionic resistance may be obtained by a transmission line model that simulates H_2/N_2 operation or eliminated by using a thin cathode with 0.05mg/cm Pt loading. Change of the charge transfer resistance with the RH using the thin cathode shows that the catalytic activity of oxygen reduction increases significantly with RH in the range of 0 approx 60 percent . Dependence of catalytic activity on RH is not apparent beyond this range.