Photoelectrochemical N₂-to-NH₃ 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 (NH₃). Herein, an optimum PEC system is reported with an Si-based hierarchically-structured PdCu/TiO₂/Si photocathode and well-thought-out trifluoroethanol as the proton source for lithium-mediated PEC NRR, achieving a record high NH₃ yield of 43.09 µg cm⁻² h⁻¹ and an excellent faradaic efficiency of 46.15% under 0.12 MPa O₂ and 3.88 MPa N₂ at 0.07 V versus lithium(0/+) redox couple (vs Li^0/+). PEC measurements coupled with operando characterization reveal that the PdCu/TiO₂/Si photocathode under N₂ pressures facilitate the reduction of N₂ to form lithium nitride (Li₃N), which reacts with active protons to produce NH₃ 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 O₂ or CO₂ under pressure by accelerating the decomposition of Li₃N. 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 N₂-to-NH₃.