Insights into Interfacial and Bulk Transport Phenomena Affecting Proton Exchange Membrane Water Electrolyzer Performance at Ultra‐Low Iridium Loadings

Interfacial and bulk properties between the catalyst layer and the porous transport layer (PTL) restrict the iridium loading reduction for proton exchange membrane water electrolyzers (PEMWEs), by limiting their mass and charge transport. Using titanium fiber PTLs of varying thickness and porosity, the bulk and interface transport properties are investigated, correlating them to PEMWEs cell performance at ultra-low Ir loadings of ≈0.05 mg_Ir cm^-2 . Electrochemical experiments, tomography, and modeling are combined to study the bulk and interfacial impacts of PTLs on PEMWE performance. It is found that the PEMWE performance is largely dependent on the PTL properties at ultra-low Ir loadings; bulk structural properties are critical to determine the mass transport and Ohmic resistance of PEMWEs while the surface properties of PTLs are critical to govern the catalyst layer utilization and electrode kinetics. The PTL-induced variation in kinetic and mass transport overpotential are on the order of ≈40 and 60 mV (at 80 A mg_Ir ^-1 ), respectively, while a nonnegligible 35 mV (at 3 A cm^-2 ) difference in Ohmic overpotential. Thus at least 150 mV improvement in PEMWE performance can be achieved through PTL structural optimization without membrane thickness reduction or advent of new electrocatalysts.