Cooperative oxidation of NH3 and H2O to selectively produce nitrate via a nearly barrierless N–O coupling pathway

Abstract

Photoelectrochemical (PEC) direct ammonia oxidation is a sustainable alternative to the industrial production of nitrate or nitrite (NO_x⁻), while the highly selective NO_x⁻ synthesis remains challenging due to the intricate kinetics and the inherent competition from the water oxidation reaction (WOR). Herein, we report a high-performance Si-based photoanode modified with Ni–Cu bimetallic nanosheets (NiCuO_x/Ni/n-Si), which presents a benchmark faradaic efficiency of 99% for PEC NO_x⁻ synthesis together with a record partial photocurrent density of ∼12 mA cm⁻² at a low bias of 1.38 V_RHE under AM 1.5G illumination. This results in a NO_x⁻ production rate of 59 μmol h⁻¹ cm⁻² and simultaneously a H₂ production rate of 214 μmol h⁻¹ cm⁻² on the counter electrode. Operando PEC spectroscopic measurements combined with theoretical calculations reveal that water molecules activated at Ni sites produce Ni^IVO species, which bond with the activated ammonia at Cu sites via a nearly barrierless N–O coupling pathway. Such a bimetallic synergetic mechanism presents a first-order hole-transfer kinetics, which kinetically circumvents the competing O₂ production and thus significantly promotes the selectivity to NO_x⁻ on the NiCuO_x/Ni/n-Si photoanode. This work provides a promising strategy for designing advanced photoanodes for cooperative oxidation reactions.