Photoelectrochemical N 2 -to-NH 3 Fixation with High Efficiency and Rates via Optimized Si-Based System at Positive Potential versus Li 0/ .

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, we report an optimum PEC system with a 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 (versus Li 0/+ ). PEC measurements coupled with operando characterization revealed 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 . This article is protected by copyright. All rights reserved.