Photoelectrochemical N₂‐to‐NH₃ Fixation with High Efficiency and Rates via Optimized Si‐Based System at Positive Potential versus Li^0/+

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 (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 Li0/+). 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.

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“…It consumes ≈2% of yearly world energy production and contributes 1% of CO 2 emitted to the atmosphere. [2,3] Recently, researchers have been trying to produce ammonia through nitrogen reduction reactions by using environmentally friendly and low-energy techniques like photocatalysis [4] (PC), electrocatalysis, [5] and photoelectrocatalysis [6] under ambient conditions. Among these, photocatalytic N 2 reduction reaction (N 2 RR) is a better way to produce green ammonia by using a semiconductor, since it absorbs incident photons from light irradiation and generates electron-hole pairs which is quite a well-known concept.…”

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“…Inspired by the structure and fundamental principles of photosynthetic machinery, extensive studies have focused on developing artificial photosynthesis devices. − Examples include PEC cells and photocatalysts for solar water splitting, − carbon dioxide utilization, , and nitrogen fixation. , Despite their potential to sustainably produce fuels and useful chemicals, these systems have faced challenges such as low efficiency and stability, impeding their practical application. Notably, solar water oxidation, also known as the oxygen evolution reaction (OER), is a critical bottleneck , in these processes, as it is responsible for supplying electrons and protons for the reductive production of target chemicals.…”