Study of pH Gradients and Carbonation of Hydroxide Exchange Membrane Electrolyzers

Hydroxide exchange membrane electrolyzers (HEMELs) can produce hydrogen at scale with high efficiencies. However, like their HEM fuel cell counterparts, the alkaline membrane and ionomer of HEMELs may be susceptible to CO 2 contamination and cause performance losses. CO 2 can form (bi)carbonates which increase ohmic resistances due to their reduced conductivity compared to hydroxide. More importantly, the potential gradient across the membrane drives a self-purging mechanism which can lower the anode pH, causing thermodynamic overpotentials. We used modeling and experiments to study these phenomena in HEMELs in order to understand CO 2 -related losses through the conductivity and pH effects of different CO 2 concentrations over a range of current densities. We found that pH gradients are the more significant barrier to cell performance and controlled them using three supporting electrolytes. We found that operating HEMELs at high current densities >1000 mA cm −2 can recover >200 mV of overpotential due to self-purging of (bi)carbonates, but there is still some unrecoverable overpotential from the generated pH gradients. KOH electrolytes can be used to reduce this pH gradient, but K 2 CO 3 and KHCO 3 supporting electrolytes are susceptible to the same detrimental effects of carbonation and should not be used to minimize CO 2 contamination.