Photoelectrochemical Coupling of Waste-Nitrogen Oxidation Reactions with Hydrogen Evolution for Sustainable Energy Conversion.

Photoelectrochemical (PEC) water splitting offers a sustainable method for hydrogen production, but is limited by slow oxygen evolution reaction (OER) kinetics and the low economic value of oxygen (O2). Alternative anodic oxidation reactions have been developed to replace OER, enhancing energy efficiency and producing valuable products. This review analyzes recent advancements in photoanodes for the selective oxidation of urea, ammonia, and nitrogen oxides under solarlight into valuable chemicals,such as nitrogen (N2), carbon dioxide (CO2), and nirtates, by utilizing alternative oxidation pathways alongside the hydrogen evolution reaction (HER). This review focuses on the mechanistic pathways of oxidation, highlighting strategies to tackle challenges such as incomplete oxidation and nitrate buildup through optimized catalyst design, nanostructuring, and interfacial engineering. Key systems include nickel phosphide (Ni2P)-sensitized titanium dioxide (TiO2) nanotubes, silicon (Si) photoanodes with Ni-based cocatalysts, and amorphous Ni-Mo-O layers, all showing better charge separation, lower overpotentials, and strong long-term stability. Additionally, PEC NO oxidation provides a low-temperature, selective approach for transforming trace NO pollutants into nitrates suitable for fertilizer, supported by reactor-scale innovations in gas-phase PEC systems. This review examines catalyst stability, selectivity, and device design, suggesting future directionsfor scalable, durable, and affordable PEC systems that promote clean energy and environmental sustainability.