(115d) Engineering the Water Balance in Hydroxide Exchange Membrane Electrolyzers Under Cathode Dry Conditions

Water electrolysis is critical for the production of green hydrogen for use as a chemical feedstock, transportation fuel, and long-term energy storage solution. However, current electrolyzer technologies, such as alkaline water electrolyzers (AELs), are relatively inefficient due to high internal resistances caused by liquid electrolytes. Proton-exchange membrane electrolyzers (PEMELs), on the other hand, use very efficient solid polymer electrolyte membranes to achieve high hydrogen production rates, but they generate a harsh acidic environment that requires the use of expensive precious metal materials. In this work, we study hydroxide exchange membrane electrolyzers (HEMELs), which use a hydroxide-conducting polymer membrane in an alkaline environment and have the potential to produce hydrogen at high efficiencies and low cost.

In a typical HEMEL, water is fed to the anode side of the cell, while the cathode inlet is kept dry to allow for the easy separation and pressurization of the desired hydrogen product formed by the hydrogen evolution reaction (HER). For the HER to proceed, water must be consumed, and the rate of consumption increases at high operating current densities. As a result, some researchers have reported detrimental “cathode dryout” conditions at high current densities,¹ where transport limitations dominate and prevent any further hydrogen production. In this work, we study the water balance of a HEMEL cathode to improve water transport and delay cathode dryout conditions to higher current densities.

First, we attempt to increase the diffusion of water into the cathode through the membrane by using thinner membranes to decrease the diffusion length scale. We also use different chemistries of membranes with different water diffusivities to make water accessible to the cathode. We then design catalyst layers that can effectively utilize and retain water at the cathode through the use of hydrophilic and hydrophobic additives that alter the water transport pathways through the catalyst-ionomer network. We find that careful tuning of the catalyst layer and porous transport layer wettability can allow for improved utilization of water and easy removal of gaseous hydrogen products to improve HEMEL performance at high current densities. This work could allow for green hydrogen production at improved reaction rates with reduced transport limitations.

(1) Koch, S.; Disch, J.; Kilian, S. K.; Han, Y.; Metzler, L.; Tengattini, A.; Helfen, L.; Schulz, M.; Breitwieser, M.; Vierrath, S. Water Management in Anion-Exchange Membrane Water Electrolyzers under Dry Cathode Operation. RSC Adv 2022, 12 (32), 20778–20784. https://doi.org/10.1039/D2RA03846C.