Atomic Interstitial Oxygen in a CuNi Alloy Achieving *NO Bridged-Adsorption Reversal for Photoelectrocatalytic Ammonia Synthesis

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Photoelectrochemical nitrate reduction to ammonia (PEC NO3RR) is a promising route for solar-driven ammonia synthesis. Yet, high ammonia yields in PEC NO3RR are largely confined to highly negative bias, which limits the overall energy utilization efficiency, whereas efficient operation under positive bias remains elusive. Herein, we constructed a CuNi-O/Si photoelectrocatalyst by introducing atomically dispersed interstitial oxygen atoms into the CuNi alloy lattice on silicon nanoarrays. The atomic interstitial oxygen induces local charge polarization, directing electron migration toward coordinated Cuδ+ and Niδ+ sites, thereby enhancing interfacial charge transfer. Crucially, the interstitial oxygen tunes a configuration inversion of the *NO intermediate from a strongly bound N-top configuration to a moderately adsorbed O-bridge configuration, mitigating overbinding and accelerating subsequent hydrogenation. Consequently, the CuNi-O/Si photoelectrocatalyst with an onset potential (+0.6 V vs RHE) exhibits a photocurrent density of −13.8 mA cm−2 and an ammonia yield rate of 27 μmol cm−2 h−1 at +0.3 V vs RHE under AM 1.5 illumination, corresponding to an incident photon-to-current efficiency of 58.5%. Additionally, this photoelectrocatalyst achieves continuous ammonia synthesis for 50 h using air and water under ambient conditions combined with nonthermal plasma. This work provides an atomic-level strategy of nonmetal interstitial tuning to develop functional materials for solar energy conversion.