Cell resistances of poly(2,5-benzimidazole)-based high temperature polymer membrane fuel cell membrane electrode assemblies: Time dependence and influence of operating parameters

Time-dependent measurements of cell impedance of a HT-PEFC based on ABPBI were performed at constant frequencies close to the high-frequency (h.f.) intercept of the corresponding Nyquist plots with the real axis. The h.f. impedances approximate the ohmic resistance of the cell and they decrease, when current (140 mA cm−2) is switched on. Steady-state values are attained after 10 min. Vice versa, when current is switched off (OCV), the h.f. impedances instantaneously increase but reach steady-state values only after about 1 h. These values rise with increasing gas flow rates. The results are discussed in terms of hydration/dehydration processes, changing the equilibrium between orthophosphoric and pyrophosphoric acid and thus the conductivity of the electrolyte as well as the mobility of molecules and charge carriers. Impedance spectra were recorded after each time-dependent measurement under OCV conditions. The fit of these impedance data based on an equivalent circuit revealed ohmic resistances corrected by h.f. inductances and low frequency impedances associated with the cathode oxygen exchange reaction. The charge transfer resistances deduced from the low frequency impedances strongly depend on both air and hydrogen flow rates.