The characterization of proton exchange membrane fuel cell electrodes is essential for understanding the electrode performance. In this paper, mercury intrusion porosimetry and the nitrogen adsorption method were used to measure pore size distributions and porosities (ϵ) of various electrodes, which were made with either platinum supported on amorphous carbon (Pt/VA) or platinum supported on graphitized carbon (Pt/VG), and had ionomer‐to‐carbon weight ratios (I/C) of 0.5, 1.0, and 1.5. The oxygen effective diffusivity ($ D_{{\rm O}_2}^{\rm eff} $) in electrodes was measured as a function of relative humidity (RH) in an apparatus that was previously described [Z. Yu, R. N. Carter, J. Power Sources 195 (2010) 1079–1084]. The tortuosity of electrodes at the dry condition (80 °C and 0% RH) was then determined from the measured porosities and $ D_{{\rm O}_2}^{\rm eff} $. For a given catalyst, as the I/C ratio increased, it was found that the electrode's mean pore size, porosity, and $ D_{{\rm O}_2}^{\rm eff} $ all decreased, but the tortuosity increased. For a given I/C ratio, the Pt/VA electrode exhibited larger mean pore size, larger porosity, larger $ D_{{\rm O}_2}^{\rm eff} $, and smaller tortuosity compared with the Pt/VG electrode. The contrast between Pt/VA and Pt/VG electrodes with the same I/C ratio indicates different ionomer distribution on the catalyst surface.

A technique is described for determining the source and degree of electrodes degradation in membrane electrode assemblies (MEAs) from proton exchange membrane fuel cells. This technique is nondestructive and provides spatial resolution across the MEA's active area. The degradation modes that are considered are the oxygen reduction activity loss through either electrochemically active area or mass activity loss, increased gas transport resistance from corrosion of the catalyst's carbon support, and the increased protonic transport loss through chemical degradation of the electrode layer's ion‐conducting polymer. A current distribution tool is used with cyclic voltammetry and polarization curves to isolate each of the primary sources of voltage degradation. Results of new MEAs are presented first to demonstrate the approximate resolution of the test method. Then results are presented for MEAs that were aged in a model test to cause carbon corrosion in the cathode layer via localized anode starvation. These include follow‐up materials characterization, where it is shown that the results of this electrochemical diagnostic correlate well with postmortem materials characterization. Finally, results are shown for a part aged in a model start‐up/shutdown aging test and compared with modeling results to explain the spatial characteristics of the observed degradation.