Salt Ion Accumulation in Bipolar Membranes Limits the Maximum Rate of Neutralization

Bipolar membranes (BPMs) emerge as a valuable component in novel energy conversion devices utilizing a water-splitting reaction within BPMs. However, the opposite process, proton and hydroxide recombination (forward bias), remains challenging to control due to its strong dependence on the electrolyte composition. Even minor contamination of acid and base solutions by salt can significantly compromise the BPM performance. This study examines the impact of salt contamination on the BPM performance under forward bias. The results reveal that, during neutralization, salt ions accumulate near the BPM junction, hindering H⁺ and OH^- transport toward the catalytic interface. Notably, the anion-exchange layer exhibits a high sensitivity to salt contamination in the base solution, with active site swapping between OH^- and anions emerging as the rate-determining step. The extent of this transport limitation depends on the acid/base-to-salt ratio. To address this issue, mitigation strategies are explored, including asymmetric BPMs. Reducing the thickness of the anion-exchange layer significantly enhances OH^- mobility, thereby increasing the limiting current density of neutralization in salt-contaminated electrolytes. These insights offer a deeper understanding of mass-transport limitations in BPMs and highlight pathways to optimize performance in energy conversion applications.