Local Electric\nField Effects on Water Dissociation\nin Bipolar Membranes Studied Using Core–Shell Catalysts

The local electric field strength is thought to affect\nthe rate\nof water dissociation (WD) in bipolar membranes (BPMs) at the catalyst–nanoparticle\nsurfaces. Here, we study core–shell nanoparticles, where the\ncore is metallic, semiconducting, or insulating, to understand this\neffect. The nanoparticle cores were coated with a WD catalyst layer\n(TiO<sub>2</sub> or HfO<sub>2</sub>) via atomic layer deposition (ALD),\nand the morphology was imaged with transmission electron microscopy.\nIrrespective of the core material, these core–shell catalysts\ndisplayed comparable WD overpotentials at optimal mass loading, despite\nthe hypothesized differences in the electric field strength across\nthe catalyst particle suggested by continuum electrostatic simulations.\nSubstantial atomic interdiffusion between the core and shell was ruled\nout by X-ray absorption spectroscopy, X-ray photoelectron spectroscopy,\nand diffuse reflectance optical measurements. However, the optimal\nmass loading of catalyst was roughly 1 order of magnitude higher for\nthe conductive and high dielectric core materials than for the low\ndielectric insulating cores. These findings are consistent with the\nhypothesis that electric field screening within the core material\nfocuses the electric field drop between particles such that larger\nfilm thicknesses can be tolerated. Collectively, these data support\nthe idea that it is the local electric field at the molecular level\nthat controls proton-transfer rates and that the metal core/dielectric-shell\nconstructs introduced here modulate that field. Further materials\nand synthetic design may enable optimization of the electric field\nstrength across the proton-transfer trajectory at the material surface.