The Effect of FELTMETAL™ Porous Transport Layer Structure on Performance of Anion Exchange Membrane Water Electrolyzers

Abstract The porous transport layer (PTL), or gas diffusion layer (GDL), is a critical component in the performance of anion exchange membrane water electrolyzers (AEMELs) for hydrogen production and hydrogen supply. This work focuses on how the structure of FELTMETAL™ (FM) affects the critical functions of the oxygen evolution reaction (OER) electrode in an AEMEL. As a sintered-particle material with high levels of open pores, FM has high total porosity which positively affects mass transport of the gas and water from the PTL but increases the electrical resistivity and contact resistance. Due to the nature of the high-aspect ratio particles in FM, it also has high surface area which improves electrical properties and enables high current density during AEMEL operation. However, sintered materials typically have a trade-off between total porosity and pore size which can lead to mass transport issues if the pore size and distribution are too tight. Previous experimental electrolysis work determined that increasing the %-density of FM from 18%-density to 60%-density favorably increased current density at a given applied voltage, but it was likely that mass transport issues related to bubble formation and removal caused variation in performance and a decrease in current density over time. This work ultimately aimed to determine the effect of particle size on the structure of the FM PTL (pore size, total porosity, surface area) on AEMEL performance (current density, applied voltage, voltage stability over time, overpotentials). The study evaluated OER PTL FM with distinctly different %-densities and particle size distributions of Ni-alloy HastelloyX. The structures were characterized by metallography and bubble point testing. The AEMEL performance was characterized by polarization and electrochemical imprudence spectroscopy (EIS). Because the work focuses on the OER PTL structure, other key components of the experimental AEMEL were not varied, using standard commercial materials. It was found that the using a finer particle size distribution had a significant positive effect on polarization responses, specifically voltage at 1A/cm2, steady state voltage, and activation overpotential. The finer particle structure likely has increased total pore number and area-% open pores. It is possible that FM PTL materials comprised of finer fiber can enable performance improvement at a lower %-density, reducing the material required to make the PTL.