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

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 mu g 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 Li0/+). PEC measurements coupled with operando characterization reveal that the PdCu/TiO2/Si photocathode under N-2 pressures facilitate the reduction of N-2 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 O-2 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 N-2-to-NH3.

Automated summary

A Si-based hierarchically-structured PdCu/TiO2/Si photocathode with trifluoroethanol as the proton source was reported for lithium-mediated photoelectrochemical nitrogen reduction reaction (PEC NRR), achieving a record high NH3 yield of 43.09 mu g cm(-2) h(-1) and an excellent faradaic efficiency of 46.15%. The PEC measurements coupled with operando characterization revealed that the PdCu/TiO2/Si photocathode under N-2 pressures facilitated the reduction of N-2 to form lithium nitride, which reacted with active protons to produce NH3. The Li-mediated PEC NRR process was further enhanced by introducing small amounts of O-2 or CO2 to accelerate the decomposition of Li3N.