Design Principles for Hydroxide Exchange Membrane Water Electrolyzers

Abstract

Green hydrogen, produced by via water splitting using renewable electricity, will play a crucial role as a renewable energy carrier and in decarbonizing hard-to-decarbonize sectors in the future.1 Hydroxide-exchange-membrane water electrolyzers (HEMWE) have the potential to be more cost effective compared to the incumbent technology, proton-exchange-membrane water electrolyzers (PEMWE), through the use of cheaper catalysts and cell component materials.2 However, HEMWE are at much earlier stage of development compared to PEMWE and, in part, generally operate at lower efficiencies due to a lack of established guiding design and operating principles. Herein, we study how membrane electrode assembly (MEA) component design and cell operating conditions influence the performance of HEMWE. A custom design three-electrode MEA is used to breakdown the overpotential contributions from the anode and cathode, showing that anode OER still dominates the kinetic loss while cathode mass transport limitations is more severe compared to anode. A significant improvement in cell performance is observed for more porous anode porous transport electrode (PTE) compared to dense ones, due to a higher anode catalyst utilization. On the cathode, the use of a hydrophobic gas diffusion layer (GDL) improves performance at high current densities by facilitating the removal of hydrogen gas. The impact of feed configurations including electrolyte fed to both sides, anode fed only, and cathode fed only, on HEMWE performance is thoroughly investigated. In various electrolytes including KOH, KHCO3, K2CO3 and DI water, HEMWEs perform the best with anode fed only, signifying that back diffusion of water through the membrane is sufficient to supply enough reactant to the cathode. These learned principles were used to assemble a completely PGM-free HEMWE that reached 1.6 A cm-2 at 2 V in 1 M KOH, and exhibited stable performance in 0.1 M KOH at 1.5 A cm-2 for over 500 h.AcknowledgementsThis work was funded under the HydroGEN Consortium by the Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231.References B. Pivovar, N. Rustagi and S. Satyapal, The Electrochemical Society Interface, 27, 47 (2018).K. Ayers, N. Danilovic, R. Ouimet, M. Carmo, B. Pivovar and M. Bornstein, Annual review of chemical and biomolecular engineering, 10, 219 (2019).