Measurements of Pore Size Distribution, Porosity, Effective Oxygen Diffusivity, and Tortuosity of PEM Fuel Cell Electrodes

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/V A ) or platinum supported on graphitized carbon (Pt/V G ), and had ionomer‐to‐carbon weight ratios (I/C) of 0.5, 1.0, and 1.5. The oxygen effective diffusivity ( D_ O₂ᵉᶠᶠ ) 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_ O₂ᵉᶠᶠ . For a given catalyst, as the I/C ratio increased, it was found that the electrode's mean pore size, porosity, and D_ O₂ᵉᶠᶠ all decreased, but the tortuosity increased. For a given I/C ratio, the Pt/V A electrode exhibited larger mean pore size, larger porosity, larger D_ O₂ᵉᶠᶠ , and smaller tortuosity compared with the Pt/V G electrode. The contrast between Pt/V A and Pt/V G electrodes with the same I/C ratio indicates different ionomer distribution on the catalyst surface.