Correlating nanostructure features to transport properties of polymer electrolyte membrane electrolyzer anode catalyst layers

Abstract

In this work, polymer electrolyte membrane water electrolyzer catalyst layers are stochastically generated with α pore regions (defined as regions with pores <250 nm in size), β pore regions (defined as regions with pores >250 nm in size) and an ionomer layer of uniform thickness. Pore network modelling simulations are performed to correlate structural characteristics (porosity, mean pore diameter, and β pore volume fraction) to single and two-phase mass transport properties. Porosity and two-phase mass permeability are well correlated, but order of magnitude variations are observed due to inhomogeneous pore size distributions at the same porosity. The effect of varying ionomer and iridium oxide material mass fractions on ion and electron transport properties is also explored. Electrical conductivity is well correlated with catalyst volume fraction, while ionic conductivity is less correlated, likely due to changes in connectivity resulting from the inhomogeneous structure of the porous phase. Due to the non-homogeneity of the pore sizes in the catalyst layer, existing correlations for porous media are insufficient predictors. As such, we recommend that both porosity and mean pore diameter are considered when estimating transport properties. It is additionally recommended that both α pore and β pore regions must be incorporated into catalyst layer designs to allow both enhanced connectivity and mass transport characteristics.