Photoelectrochemical Nitrate‐to‐Ammonia Conversion: Cocatalyst Design Principles and Mechanistic Insights from Electrocatalysis

ABSTRACT Photoelectrochemical reduction of nitrate to ammonia (PEC NO 3 RR) offers a solar‐driven route to NH 3 synthesis under ambient conditions, converting an environmental pollutant into a value‐added product. However, the overall performance of a PEC photoelectrode has been largely governed by the cocatalyst deposited on the light absorber since NO 3 RR conversion is kinetically demanding. Electrochemical NO 3 RR research has produced an extensive library of high‐performance catalysts together with a mature mechanistic understanding. Yet this knowledge has rarely been translated into photoelectrode design. This review summarizes recent advances in PEC NO 3 RR, spanning light‐absorber engineering, photoelectrode–cocatalyst integration, intrinsic semiconductor catalytic activity, and architectures that decouple light absorption from catalysis. On this basis, we reframe the electrochemical NO 3 RR catalyst library as a design resource for PEC cocatalysts and identify key evaluation criteria, such as catalytic activity, interfacial charge‐transfer, optical shading, and stability under illumination and electrically biased conditions. By bridging the mechanistic maturity of electrochemistry with the energetic advantages of photoelectrochemistry, this review provides a coherent design framework for advancing PEC NH 3 synthesis.