Disentangling the cooperative roles of tungsten and copper oxides in three-dimensional heterojunction photocathodes for nitrate conversion to ammonia

Valdivia-Alvarez D.; Casas K.R.; Aranda-Aguirre A.; de Oca J.M.; Pastrana E.C.; Alejo F.; Garcia-Segura S.; Cerrón-Calle G.A.; Alarcon H. · Journal of Alloys and Compounds (2026) · DOI: 10.1016/j.jallcom.2026.191093

Resumen

Photoelectrochemical reduction of nitrate (PERN) has emerged as a promising strategy for coupling solar energy conversion with sustainable nitrogen recovery. However, the individual roles of photoactive and electrocatalytic phases during nitrate conversion remain poorly understood. In this work, three-dimensional Ni/WO3, Ni/CuOx, and Ni/WO3/CuOx photocathodes were synthesized through hydrothermal growth and electrodeposition to investigate cooperative effects between light harvesting and catalytic NO3− reduction sites. Structural characterization confirmed the formation of WO3 and CuOx phases and the successful integration of both semiconductors within the ternary photoelectrode architecture. Electrochemical measurements revealed that CuOx sites dominate NO3− activation under dark conditions, whereas WO3 exhibits limited intrinsic electrocatalytic activity toward nitrate reduction. However, under illumination the Ni/WO3/CuOx photocathode displayed enhanced photoelectrochemical activity arising from the synergistic interaction between WO3 and CuOx. Mott-Schottky analysis demonstrated the formation of an n-p heterojunction that promotes charge separation and interfacial electron transfer. Band-edge alignment derived from Mott-Schottky and Tauc analyses supports a Z-scheme charge-transfer pathway in which WO3 conduction band electrons recombine with CuOx valence band holes. Product distribution and Faradaic efficiency analyses showed that the photoelectrochemical contribution increased by 1.5-fold at low overpotentials (−0.8 V vs. Ag/AgCl), whereas electrocatalytic pathways dominated at higher cathodic applied potentials. Nitrite measurements during NO3− electrolysis revealed that CuOx sites promote the reduction of NO3− to NO2−, while photogenerated electrons from WO3 accelerate the subsequent reduction of NO2−, thereby suppressing NO2− accumulation under illumination. These findings provide mechanistic insights into the complementary roles of photoactive and catalytic phases during PERN and establish a rational design strategy for n-p heterojunction photoelectrodes toward enhanced NH3 production.

Instituciones: Universidad Nacional Mayor de San Marcos; Universidad Nacional de San Agustín de Arequipa; Universidad Nacional de Ingeniería