Photoelectrochemical nitrate denitrification towards acidic ammonia synthesis on copper-decorated black silicon

Abstract

Nitrate electroreduction to ammonia has broad prospects as a complementary route to the energy-intensive Haber-Bosch industry. Currently, most electrocatalytic NO3−-to-NH3 transformations are achieved in alkaline electrolyte, which not only requires a large power supply but also poses additional challenges for accurate quantitation and large-scale separation of NH3. Herein, the silicon nanowire (black silicon) uniformly modified with Cu nanoparticles (Cu-Si NWs) is designed for photoelectrochemical nitrate reduction (PEC NO3RR) in strong acid electrolyte. Under AM 1.5 G illumination, the Cu-Si NWs achieves a remarkably positive onset potential of 0.3 V vs. RHE and an impressive saturated photocurrent density of −34.29 mA cm−2 in 0.5 M H2SO4. More importantly, the Faradaic efficiency of ammonium (NH4+) and corresponding solar-to-NH4+ efficiency reach up to 97.03% and 51.07%, respectively. Mechanistic investigations uncover the proper Schottky contact in Cu/Si interfaces facilitates charge transfer effectively, contributing to the low onset potential and high photocurrent density. In-situ experiments and theoretical analysis have further confirmed that the incorporation of Cu effectively accelerates the activation and protonation steps of NO3–. Moreover, the PEC system exhibits excellent stability and great potential in environmental remediation in simulated industrial wastewater treatment. This work introduces a strategy for fabricating highly efficient PEC devices for diminishing nitrate contaminant in strong acid media.