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 (NH 3 ) but also facilitates the distributed development and clean utilization of solar energy. Due to the high NH 3 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 N 2 -to-NH 3 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 NH 3 yield rate of 29.0 μg·h −1 ·cm −2 under 0.9-MPa N 2 /H 2 (97 vol%: 3 vol%) and 0.1-MPa CO 2 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 N 2 and protons to produce NH 3 . Introducing small amount of CO 2 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 N 2 -to-NH 3 fixation by utilizing the cycle of Na/Na + .