Bipolar Palladium Membrane Enabling Crossover‐Free Selective Proton Transport

ABSTRACT Establishing a proton‐selective membrane, capable of completely blocking the crossover of other chemical species, has been regarded as a long‐sought‐after goal for the stable and efficient operation of electrochemical devices. Conventional polymeric membranes suffer from an inherent trade‐off between conductivity and selectivity, imposed by their water‐mediated proton‐conduction mechanism, which precludes crossover‐free proton transport. Herein, we demonstrate a unique proton shuttling mechanism of the bipolar palladium membrane that enables selective proton transport while suppressing molecular crossover. Through bipolar electrochemical proton absorption and desorption, coupled with the lattice‐channeled hydrogen diffusion in palladium, the conductivity–selectivity relationship can be fundamentally circumvented. Using the bipolar electrochemical palladium membrane, we construct a water‐fed Li‐mediated N ₂ reduction system, suppressing water crossover between an aqueous anolyte and a non‐aqueous catholyte even under continuous flow and enabling water to serve as a sustainable proton source. We achieve an ammonia Faradaic efficiency of 51% and stable operation for 12 h under potential cycling. This study introduces a new ion transport mechanism, expanding the design space for next‐generation electrochemical systems that require strict compartmentalization.