Understanding the microstructure of a core–shell anode catalyst layer for polymer electrolyte water electrolysis

Reducing precious metal loading in the anodic catalyst layer (CL) is indispensable for lowering capital costs and enabling the widespread adoption of polymer electrolyte water electrolysis. This work presents the first three-dimensional reconstruction of a TiO 2 -supported IrO 2 based core shell CL (3 mg IrO2 /cm 2 ), using high-resolution X-ray ptychographic tomography at cryogenic temperature of 90 K. The high data quality and phase sensitivity of the technique have allowed the reconstruction of all four phases namely pore space, IrO 2 , TiO 2 support matrix and the ionomer network, the latter of which has proven to be a challenge in the past. Results show that the IrO 2 forms thin nanoporous shells around the TiO 2 particles and that the ionomer has a non-uniform thickness and partially covers the catalyst. The TiO 2 particles do not form a percolating network while all other phases have high connectivity. The analysis of the CL ionic and electronic conductivity shows that for a dry CL, the ionic conductivity is orders of magnitudes lower than the electronic conductivity. Varying the electronic conductivity of the support phase by simulations, reveals that the conductivity of the support does not have a considerable impact on the overall CL electrical conductivity.