AC Impedance Investigations of Proton Conduction in Nafion[trademark sign

AC impedance spectroscopy has been employed to study the conduction of protons in Nafion 117 polymer electrolyte membranes. Both two- and four-electrode geometries have been used to uniquely distinguish between the membrane impedance and the interfacial impedances. The results show that the impedance of Nation for frequencies up to 100 kHz is characterized by a pure resistance, similar to conventional liquid electrolytes. The frequency dependent features observed using a two-electrode geometry are shown to be consistent with well-characterized interracial impedances and do not arise from ionic conduction in the membrane. These results show that previous two-electrode studies reported in the literature have misinterpreted the impedance of the electrode interfaces as belonging to the conduction process in the electrolyte. It is essential for the understanding of membrane fuel cells that we have a detailed knowledge of the conduction mechanism in Nafion and other ionomer materials. This becomes of critical impor-tance when we consider questions relating to ionic mobility, includ-ing the question of whether the Grotthus (rotational relaxation f the solvent after hydrogen ion transfer) mechanism is active within the polymer electrolyte as it s in liquid acid and base electrolytes.