Amorphous/Crystalline Heterojunction Ni(OH) 2 /Cu 2 O(111) for Photoelectrocatalysis NO 3 − ‐to‐NH 3 : Positive Onset Potential, High Selectivity, and Complete Conversion

ABSTRACT Photoelectrocatalytic selective nitrate (NO 3 − ) reduction to ammonia (NH 3 ) offers a sustainable route for the green nitrogen cycle, overcoming the inherent limitations of standalone electrocatalysis and photocatalysis. The central challenge is developing an efficient photoelectrocatalyst that simultaneously reduces overpotential and maximizes NH 3 selectivity. Herein, an amorphous/crystalline heterojunction Ni(OH) 2 /Cu 2 O bonded by Cu‐O v ‐Ni, is constructed via photoelectrocatalytic interface regulation for synergistic photoelectrocatalysis (SPEC) NH 3 synthesis. Results demonstrate ∼100% light absorption efficiency and ∼90% high charge separation efficiency, enabling almost completely rapid 98.80% NO 3 − conversion with stable NH 3 FE (> 99%) and high selectivity (> 97%). A superior NH 3 yield of 1316.1 µmol h −1 cm −2 is attained, alongside an excellent positive onset potential of 0.322 V versus RHE. In situ experiments further reveal that crystalline Cu 2 O activates carriers to accelerate the rate‐determining step (N +5 → N +3 ), while amorphous Ni(OH) 2 significantly enhances electron extraction and *H accumulation for protonation (N +3 → N −3 ). Increasing potential shifts the system from electro‐assisted photocatalysis (activating Cu‐O···*NO 3 − sites) to photo‐assisted electrocatalysis (Cu 2 O → Ni(OH) 2 electron transfer), then to SPEC (efficient *H coupling) process. Further application to actual fertilizer plant wastewater achieves ∼90% NO 3 − removal efficiency and > 85% NH 3 selectivity, demonstrating promising scalability for efficient photoelectrocatalysis NO 3 − ‐to‐NH 3 with great environmental remediation potential.