Sodium-mediated photoelectrochemical nitrogen-to-ammonia fixation

Photoelectrochemical nitrogen reduction reaction (PEC NRR) not only is a viable route for the synthesis of green ammonia (NH₃) but also facilitates the distributed development and clean utilization of solar energy. Due to the high NH₃ production and faradaic efficiency, lithium (Li)-mediated PEC NRR system has attracted many attentions of the researchers. However, some issues such as limited Li resources, high cost, harsh use requirements and high energy consumption strictly impede the large-scale applications of the system. Here, we have developed cheap and abundant sodium (Na) as an alternative mediator in the process of PEC N₂-to-NH₃ fixation. An optimum Na-mediated PEC NRR system is demonstrated with an WO_x/Si hierarchical photocathode and methanol as a fast proton shuttle, achieving a good NH₃ yield rate of 29.0 μg·h⁻¹·cm⁻² under 0.9-MPa N₂/H₂ (97 vol%: 3 vol%) and 0.1-MPa CO₂ at 0.8 V versus Na redox couple (vs. Na⁺/Na). The addition of WO_x layer accelerates the reduction of Na⁺ and enriches a Na layer on the photocathode surface, part of which reacts with N₂ and protons to produce NH₃. Introducing small amount of CO₂ can reduce the accumulation of “dead” Na and release the Na⁺ to refresh the activity of Na and support the cycle of Na-mediated process. Our work opens a new avenue for efficient solar-powered N₂-to-NH₃ fixation by utilizing the cycle of Na/Na⁺. © 2024 Elsevier B.V.