Application of a contact mode AFM for spatially resolved electrochemical impedance spectroscopy measurements of a Nafion membrane electrode assembly.

A Nafion fuel cell membrane is investigated by means of electrochemical atomic force microscopy in different gas atmospheres. From chronoamperometric experiments with a point contact electrode spatially resolved electrochemical impedance spectra are obtained from which information about electrode processes and proton transport in the membrane is derived. In the first part the oxygen reduction reaction is investigated. Due to the absence of diffusion limitation, which is partly a result of the small electrode size, a low frequency inductive loop is observed, which is normally masked in macroscopic electrochemical impedance spectra. The influence of water formation from the oxygen reduction reaction at the cathode is discussed. The second part focuses on a hydrogen/oxygen fuel cell setup. A qualitative explanation is given for the necessity of an applied voltage in addition to the electrochemical potential. Electrochemical impedance spectra obtained at two different positions are compared and fitted based on a Randles-like equivalent circuit. A strongly inhomogeneous performance is observed which is attributed to the properties of the Nafion membrane. The electrolyte resistance and the Nernst impedance are restrictive parameters which describe the diffusion through the membrane.