UC Berkeley

We show that the pH gradient at the catalyst–ion exchange membrane interface in a seawater bipolar-membrane electrolyzer can be mitigated by reducing the catalyst–membrane distance. We further show how water transport can be balanced at steady state. Bipolar membrane (BPM) electrolyzers offer advantages in the electrolysis of impure-waters by controlling ion flux, yet still suffer from performance and durabilty limitations. Here, we investigate the impact of NaCl electrolyte (nominally simulated seawater) on BPM electrolyzer operation and identify local pH gradients at electrode–membrane interfaces, arising from coupled ion transport and electrode reactions, as one origin of performance loss and degradation. NaCl in the catholyte induces pronounced pH gradients at the cathode|cation-exchange-layer interface, leading to increased voltage, while partial Cl − crossover to the anode becomes detrimental under locally OH − -deficient conditions, promoting the chlorine evolution reaction and accelerating degradation. Direct physical integration of the cathode and anode catalysts onto the cation and anion exchange layers through spray coating and electrodeposition, respectively, mitigates these effects by minimizing the membrane–catalyst distance. The BPM electrolyzer built in this way achieves 0.50 A cm −2 at 2.8 V and shows a degradation rate of 5.5 mV h −1 over 120 h in 0.50 M NaCl electrolyte, compared to degradation of 71 mV h −1 with a typical porous-transport-layer device structure. This work thus establishes control of local pH gradients as a design principle for BPM electrolysis with impure-water feeds.