Local impact of cell degradation on transport and structural properties in the cathode catalyst layer of a polymer electrolyte membrane fuel cell

Fuel cells can be a solution for carbon-free transport, but are affected by high price and limited lifetime. Material level degradation mechanisms are understood but structure changes require more analysis. This study investigates the degradation impact on material, transport and structural properties in the cathode catalyst layer. A 500 h AST using automotive load cycle and conditions (80 °C, 2.5/2.3 bar abs , 50%/30% relative humidity, 1.3/1.5 stoichiometry at anode and cathode) is applied and alterations of these properties are discussed. Whilst the membrane's influence appears to be minor, Pt/C degradation mainly caused the performance decay on the material level (−37% active area). The structural analysis revealed local pore size reduction and a 0.6 μm thick layer at the interface cathode catalyst layer (CCL)/membrane with collapsed porosity. Besides carbon and catalyst degradation, ionomer migration within the catalyst layer during wet/dry cycles cause the pores to fill with ionomer. The local ionomer concentration increased more at humid conditions at air outlet (+59%) and results in decreased proton transport resistance (−38%) and increased oxygen transport resistance (+10%). Consequently, this work reveals that the ionomer macroscopically migrates into the CCL pores during fuel cell operation and this represents an important additional degradation mechanism.