Anode aging in polymer electrolyte membrane fuel Cells I: Anode monitoring by ElectroChemical impedance spectroscopy

Degradation of polymer electrolyte membrane fuel cells (PEMFC) is investigated through an accelerated stress test (AST) consisting of load-induced humidity cycling combined with open circuit voltage. This combined stressor-AST was designed to mimic real operating conditions. Commercially available membrane-electrode assemblies with an initial voltage of about 0.7 V at 0.5 A cm(-2) showed a performance drop of about 900 mu V h(-1). Their operation was followed by monitoring various parameters such as polarization plots, electrode electrochemical surface area, hydrogen permeation and electrochemical impedance spectra. The results demonstrate that, although initially, the anode may be ignored to model the impedance data, this is no longer possible during the AST. Experimental data show that, beyond classical cathode and membrane degradations, the cell undergoes pronounced anode degradations, that significantly affect the cell performances. Local potential measurements excluded the anode degradation to be linked to electrode potential cycling, the latter remaining always between 0 and 0.2 V vs reference hydrogen electrode. Classical mechanisms of Pt/C degradation may thus not be at stake here, but rather mechanical destabilization of the anode microstructure under wet-dry cycling. The temperature elevation at high current density, known to entail local membrane dehydration may be an aggravating factor.