Facile, cost-effective plasma synthesis of self-supportive FeS_X on Fe foam for efficient electrochemical reduction of N₂ under ambient conditions

The electrochemical N₂ reduction reaction (NRR) in an aqueous medium has recently aroused great attention for the synthesis of NH₃ under ambient conditions. However, this process generally suffers from a low NH₃ production rate and often requires a noble-metal based electrocatalyst with some sophisticated nanosynthesis method. This work reports a new non-precious, self-supportive iron sulfide (FeS_x) NRR electrocatalyst, synthesized by a simple H₂S-plasma treatment on low-cost Fe foam. The H₂S-plasma treatment sulfurizes the Fe surface to afford a self-supportive FeS_x thin layer on the Fe foam (FeS_x/Fe). The synthesized FeS_x/Fe foam can be directly used as the electrode for the NRR, and it is demonstrated to show a remarkable NH₃ production rate of 4.13 × 10^-10 mol s^-1 cm^-2 and a high faradaic efficiency of 17.6%, significantly outperforming many other reported non-precious electrocatalysts. Further material characterization shows that the surface FeS_x converts to the mackinawite FeS after the NRR; the mackinawite FeS is possibly the actual high-activity NRR electrocatalyst, and density functional theory calculation is further employed to elucidate the NRR mechanism. Given the high performance and low cost, we envision that the plasma-synthesized FeS_x/Fe will be of great promise for the electrochemical NH₃ synthesis under ambient conditions.