Photoelectrocatalytic NO_x Reduction to Ammonia: Advances and Challenges

Ammonia (NH₃) is an essential chemical for agriculture and an emerging carbon-free energy carrier, yet its conventional Haber-Bosch synthesis remains energy-intensive and carbon-emissive. Photoelectrochemical (PEC) NO_x reduction (NO_xRR), encompassing the conversion of nitrate (NO₃^-), nitrite (NO₂^-), and related NO_x species (NO, N₂O, etc.), offers a sustainable alternative by coupling solar energy conversion with the valorization of nitrogenous pollutants into NH₃ under ambient conditions. This topical review summarizes recent progress in PEC NO_xRR, with particular emphasis on interfacial engineering strategies for photocathode design and mechanistic insights into multielectron, proton-coupled reduction pathways. We first outline the fundamental electrochemical and photophysical principles governing NO_xRR, highlighting key challenges such as sluggish kinetics, intermediate control, and competition with the hydrogen evolution reaction. We then discuss recent developments in photocathode materials, including copper-based oxides, bismuth vanadate, and defect-engineered titania, as well as hybrid (organic-inorganic) architectures that integrate molecular catalysts, plasmonic components, or cocatalysts to enhance activity and selectivity. We also provide a critical comparison of the two different strategies for solar-driven NO_x reduction to ammonia: the pure photoelectrochemical configuration and the photovoltaic-electrochemical-coupled system. Finally, we identify persistent challenges related to photocorrosion, product quantification, and scalability and outline future directions toward integrated, solar-driven NO_x-to-NH₃ conversion. By bridging materials design, interfacial catalysis, and device engineering, PEC NO_xRR emerges as a promising pathway for decentralized and carbon-neutral ammonia production.