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 process. Currently, most electrocatalytic NO₃⁻-to-NH₃ transformations are achieved in alkaline electrolytes, which not only require a large power supply but also pose additional challenges for accurate quantification and large-scale separation of NH₃. Herein, the silicon nanowire (black silicon) uniformly modified with Cu nanoparticles (Cu–Si NWs) is designed for the photoelectrochemical nitrate reduction reaction (PEC NO₃RR) in strong acidic electrolyte. Under AM 1.5G illumination, the Cu–Si NWs achieve a remarkably positive onset potential of 0.3 V vs. the 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 appropriate Schottky contact in Cu/Si interfaces, which 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, this PEC system exhibits excellent stability and great potential for environmental remediation in simulated industrial wastewater treatment experiments. This work introduces a strategy for fabricating highly efficient PEC devices for removing nitrate contaminants in strong acidic media.