Bipolar Palladium Membrane Enabling Crossover-Free Selective Proton Transport

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 N₂ 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.