(542a) Understanding and Managing Multi-Ion Interactions in a Non-Ideal Transport Model of Bipolar Membranes for Water Electrolysis

Water electrolysis provides a means of producing H2 gas without carbon emissions. However, H2 produced by water electrolysis is currently more expensive than H2 produced using fossil fuels. One of the key drivers of the cost of water electrolysis is the need to purify water upstream of the electrolyzer to prevent chloride oxidation at the anode and fouling of commonly-used monopolar membranes (i.e., cation and anion exchange membranes). BPMs, composed of an anion exchange layer and a cation exchange layer with a water dissociation catalyst at the interface, enable higher control over co-ion transport and impurities than monopolar membranes because each layer can be tuned independently. However, it is challenging to rationally-design BPMs for impure water electrolysis due to a lack of fundamental understanding of the coupled interactions between water, mobile ions, and fixed charge groups.

In this study, we use continuum modeling to systematically investigate the use of bipolar membranes (BPMs) for water electrolysis with impure feedstocks (e.g., seawater). The model defines the flux of each species using a modified Nernst-Planck-Poisson framework and the contributions of ion-ion, ion-membrane, and swelling interactions to the electrochemical potential. The simulations quantify the effect of these interactions on ion transport and how the membrane properties could be modulated to reduce voltage losses while mitigating deleterious effects of impurities and co-ions. Our analysis shows that simple dilute-solution theory is not capable of predicting the effect of impurities on BPM water electrolysis and identifies the dominant electrochemical potential interaction terms that dictate transport in BPMs. This study provides guidelines for modeling water electrolysis under non-ideal conditions and material design strategies for impure water electrolyzers.