Probing electrode structure using electrochemical impedance spectroscopy

In this work, we extend the technique described by Lefebvre et al. to perform impedance measurements in N-2/N-2 using Membrane Electrode Assemblies (MEAs) containing symmetrical electrodes at different relative humidities. The average ionic resistance of the individual catalyst layer can therefore be estimated from one half of the total average ionic resistance. The trends in the ionic resistance with RH can be extrapolated to obtain the value at 100% RH. The value at 100% RH is useful for benchmarking different electrode structures and compositions in terms of maximum possible ionic conductivity and for comparisons with fuel cell data obtained under stationary conditions wherein the RH is typically 100%. This paper discusses the details of the methodology adopted and provides examples wherein the use of impedance measurements allow us to gain valuable insight related to the composition and structure of these electrodes. For certain types of electrodes, or electrodes operating under sub-saturated conditions typical for automotive conditions, lower proton conductivity should have a significant negative effect on fuel cell performance.