Unveiling a Surface-Bulk Electronic Dichotomy in Co3O4/ZnO Heterojunctions for Enhanced Electrocatalytic Nitrate-to-Ammonia Conversion.

Constructing heterojunctions is a powerful strategy to optimize electrocatalysts, yet interfacial electronic reconstruction is often oversimplified as unidirectional charge transfer. Herein, we report Co3O4/ZnO heterostructures that exhibit a unique surface-bulk electronic dichotomy for nitrate reduction to ammonia. Surface-sensitive XPS reveals electron deficiency at Co sites, while bulk-sensitive XANES and DFT calculations demonstrate net electron accumulation within the Co3O4 lattice. This distinctive feature, arising from interfacial Co─O─Zn bonding and enriched oxygen vacancies, establishes a "surface-capture and bulk-activation" motif in which electron-deficient surface Co sites enhance nitrate adsorption, while electron-enriched bulk Co centers optimize *H and *NOH adsorption, thereby lowering the rate-determining step barrier to 0.41 eV. The optimized Co3O4/ZnO catalyst achieves a Faradaic efficiency of 98.36% at -0.25 V (vs. RHE) and a NH3 yield of 9.35 mg h-1 cm-2. Furthermore, a membrane electrode assembly flow cell integrating glycerol oxidation at the anode delivers a current density of 600 mA·cm-2 at 2.5 V with stable operation over 40 h. This work unveils surface-bulk electronic decoupling as a new paradigm for heterojunction design, demonstrating both exceptional catalytic performance and practical application potential for energy-efficient ammonia electrosynthesis.