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

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 ). Herein, an optimum PEC system is reported with an Si-based hierarchically-structured PdCu/TiO /Si photocathode and well-thought-out trifluoroethanol as the proton source for lithium-mediated PEC NRR, achieving a record high NH yield of 43.09 µg cm h and an excellent faradaic efficiency of 46.15% under 0.12 MPa O and 3.88 MPa N at 0.07 V versus lithium(0/+) redox couple (vs Li ). PEC measurements coupled with operando characterization reveal that the PdCu/TiO /Si photocathode under N pressures facilitate the reduction of N to form lithium nitride (Li N), which reacts with active protons to produce NH 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 or CO under pressure by accelerating the decomposition of Li 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 -to-NH .