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.

Impedance Spectroscopic Investigation of Proton Conductivity in Nafion Using Transient Electrochemical Atomic Force Microscopy (AFM).

Spatially resolved impedance spectroscopy of a Nafion polyelectrolyte membrane is performed employing a conductive and Pt-coated tip of an atomic force microscope as a point-like contact and electrode. The experiment is conducted by perturbing the system by a rectangular voltage step and measuring the incurred current, followed by Fourier transformation and plotting the impedance against the frequency in a conventional Bode diagram. To test the potential and limitations of this novel method, we present a feasibility study using an identical hydrogen atmosphere at a well-defined relative humidity on both sides of the membrane. It is demonstrated that good quality impedance spectra are obtained in a frequency range of 0.2-1,000 Hz. The extracted polarization curves exhibit a maximum current which cannot be explained by typical diffusion effects. Simulation based on equivalent circuits requires a Nernst element for restricted diffusion in the membrane which suggests that this effect is based on the potential dependence of the electrolyte resistance in the high overpotential region.