Photoelectrochemical N2‐to‐NH3 Fixation with High Efficiency and Rates via Optimized Si‐Based System at Positive Potential versus Li0/+

Abstract As a widely used commodity chemical, ammonia is critical for producing nitrogen‐containing fertilizers and serving as the promising zero‐carbon energy carrier. Photoelectrochemical nitrogen reduction reaction (PEC NRR) can provide a solar‐powered green and sustainable route for synthesis of ammonia (NH 3 ). Herein, an optimum PEC system is reported with an Si‐based hierarchically‐structured PdCu/TiO 2 /Si photocathode and well‐thought‐out trifluoroethanol as the proton source for lithium‐mediated PEC NRR, achieving a record high NH 3 yield of 43.09 µg cm −2 h −1 and an excellent faradaic efficiency of 46.15% under 0.12 MPa O 2 and 3.88 MPa N 2 at 0.07 V versus lithium(0/+) redox couple (vs Li 0/+ ). PEC measurements coupled with operando characterization reveal that the PdCu/TiO 2 /Si photocathode under N 2 pressures facilitate the reduction of N 2 to form lithium nitride (Li 3 N), which reacts with active protons to produce NH 3 while releasing the Li + to reinitiate the cycle of the PEC NRR. The Li‐mediated PEC NRR process is further enhanced by introducing small amount of O 2 or CO 2 under pressure by accelerating the decomposition of Li 3 N. For the first time, this work provides mechanistic understanding of the lithium‐mediated PEC NRR process and opens new avenues for efficient solar‐powered green conversion of N 2 ‐to‐NH 3 .