Photocatalytic nitrate reduction is limited not simply by photon absorption, but by whether the reaction interface can store electrons, polarize nitrate, and sustain proton-coupled N-O bond cleavage against competing oxygen reduction. To address this limitation, a recoverable photovoltage-gated membrane microreactor is constructed from a cesium tungsten bronze/silver vanadate heterojunction confined in a polyacrylonitrile nanofiber scaffold and gated by a thin polyaniline interphase. The membrane preserves a surface-accessible oxide junction while creating a hydrated, near-neutral, anion-permissive reaction microenvironment. Multiscale photophysical measurements show that polyaniline converts the oxide/polymer boundary into an electron-buffering interface, thereby lowering charge-transfer resistance, amplifying the surface photovoltage, increasing the extracted photocurrent, and extending photoinduced-state persistence in complementary nanosecond TRPL/TAS and microsecond TRMC measurements. Under visible light, the optimized membrane reduces 20.0 mg L-1 N O 3 − -N by 97.8% within 180 min, reaching 0.44 mg L-1 residual N O 3 − -N and an apparent rate constant of 0.0198 min-1, while real-water tests retain 88.6% removal. Product-resolved 15N analysis directly identifies N2 as the dominant nitrogen product (79.6% selectivity), with minor N 2 O formation (2.7%) and nitrate-derived N H 4 + (15.7%), while the combined gas/liquid inventory closes the endpoint nitrogen balance to approximately 100%. In situ Raman and photochromic spectroscopy track surface nitrate consumption, formation of the NOX intermediate, and reversible W5+ electron storage during the reaction. DFT predicts a multi-contact W/Ag/PANI nitrate-binding motif that is consistent with the experimentally observed surface-bound nitrate conversion and interfacial electron enrichment, and further indicates charge transfer to adsorbed nitrate, single bond N-O bond elongation, weaker O2 competition, and a lower thermodynamic free-energy requirement for the NO → NOH step. These results demonstrate interfacial photovoltage gating within the present membrane architecture and define a testable design framework for evaluating electron-directed nitrate-conversion interfaces in other material systems.
