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 mgIr 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 mgIr -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.

Pathway to Complete Energy Sector Decarbonization with Available Iridium Resources using Ultralow Loaded Water Electrolyzers.

We present ultralow Ir-loaded (ULL) proton exchange membrane water electrolyzer (PEMWE) cells that can produce enough hydrogen to largely decarbonize the global natural gas, transportation, and electrical storage sectors by 2050, using only half of the annual global Ir production for PEMWE deployment. This represents a significant improvement in PEMWE's global potential, enabled by careful control of the anode catalyst layer (CL), including its mesostructure and catalyst dispersion. Using commercially relevant membranes (Nafion 117), cell materials, electrocatalysts, and fabrication techniques, we achieve at peak a 250× improvement in Ir mass activity over commercial PEMWEs. An optimal Ir loading of 0.011 mgIr cm-2 operated at an Ir-specific power of ∼100 MW kgIr-1 at a cell potential of ∼1.66 V versus RHE (85% higher heating value efficiency). We further evaluate the performance limitations within the ULL regime and offer new insights and guidance in CL design relevant to the broader energy conversion field.