Bipolar membranes (BPMs) are ionic analogues of semiconductor pn junctions and consist of an anion-selective ionomer membrane laminated with a cation-selective ionomer membrane. BPMs generate pH gradients under an applied bias by driving water dissociation (WD, H2O -&gt; H+ + OH- which is apparently accelerated by the strong local electric field). In BPM water electrolysis, this feature enables devices that drive proton reduction in locally acidic conditions, where the kinetics are fast, and water oxidation in locally basic conditions where efficient earth-abundant catalysts are stable. In electrodialysis, BPMs generate acid and base from salt water on demand and are useful for water treatment, CO2 capture, and other applications. As the predominant H+/OH- ion flow is out from the center of the BPM, their use in electrosynthesis mitigates unwanted cross-over of reactants or products. The key factor traditionally limiting the use of BPMs to niche applications, however, has been the low operating currents and high voltage losses (~0.4 V at 0.1 A/cm2). We have isolated the voltage loss in BPMs to kinetics of the WD reaction. We invented physical electrochemical platforms to study and separate (non-faradaic) proton-transfer catalysis process from those driving by the strong local electric fields and how together these mechanisms control the kinetics of WD. We discovering how tuned metal-oxide nanoparticles provide surfaces with (controllable) proton-absorption sites that catalyze WD while also focusing the interfacial electric field across the BPM junction and thus further speeding the WD rate (Science 2020, Nature Comm. 2022). Temperature-dependent measurements show the WD catalysts don’t lower the activation energy for WD, but may increase the number of water configurational microstates poised for the proton-transfer elementary steps in WD. These discoveries enable BPMs that operate at ~10-times better voltage efficiency than the commercial state of the art, and at currents up to ~4 A/cm2, opening new applications which will be highlighted.

Electric-field effects on chemical (non-faradaic) water dissociation in bipolar membranes

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