Bipolar Electrolysis Cells with Hydride Ion-proton Conductor Heterejunctions

Protonic solid oxide electrolysis cells are pivotal for environmentally sustainable hydrogen production via water splitting but suffer from efficiency losses due to partial hole conductivity. Here, we introduce a device architecture based on a hydride-ion (H− )/proton (H+ ) bipolar electrolyte, which exploits electrochemical rectification at a heteroionic interface to overcome this limitation. The perovskite-type BaZr0.5In0.5O2.75 electrolyte undergoes an in situ transformation under electrolysis conditions, forming an H+ -conducting hydrate layer adjacent to the anode and an H− -conducting oxyhydride layer near the cathode, governed by competitive thermodynamic equilibria of hydration and hydrogenation. This bipolar configuration enables high Faradaic currents through the superior H− ion conductivity of the oxyhydride phase, stabilized by cathodic potentials, while facilitating continuous H+ /H− interconversion at the interface. Furthermore, electrochemical hydrogenation generates an electron-depleted interfacial layer that effectively suppresses hole conduction. Consequently, the cells achieve efficiencies of ∼95% at 1.0 A cm− 2 , surpassing conventional H+ unipolar designs.