Design principles for water dissociation catalysts in high-performance bipolar membranes

Water dissociation (WD, H 2 O → H + + OH − ) is the core process in bipolar membranes (BPMs) that limits energy efficiency. Both electric-field and catalytic effects have been invoked to describe WD, but the interplay of the two and the underlying design principles for WD catalysts remain unclear. Using precise layers of metal-oxide nanoparticles, membrane-electrolyzer platforms, materials characterization, and impedance analysis, we illustrate the role of electronic conductivity in modulating the performance of WD catalysts in the BPM junction through screening and focusing the interfacial electric field and thus electrochemical potential gradients. In contrast, the ionic conductivity of the same layer is not a significant factor in limiting performance. BPM water electrolyzers, optimized via these findings, use ~30-nm-diameter anatase TiO 2 as an earth-abundant WD catalyst, and generate O 2 and H 2 at 500 mA cm −2 with a record-low total cell voltage below 2 V. These advanced BPMs might accelerate deployment of new electrodialysis, carbon-capture, and carbon-utilization technology. It is important yet challenging to elucidate the mechanism of water dissociation in bipolar membrane electrolysers. Here the authors show how water dissociation is accelerated by electric-field-focusing and catalytic effects and uncover design principles to optimize the performance.