Local Spin-State Tuning of Iron Single-Atom Electrocatalyst by S-Coordinated Doping for Kinetics-Boosted Ammonia Synthesis.

The electrochemical nitrogen reduction reaction (e-NRR) is envisaged as alternative technique to the Haber-Bosch process for NH synthesis. However, how to develop highly active e-NRR catalysts faces daunting challenges. Herein, a viable strategy to manipulate local spin state of isolated iron sites through S-coordinated doping (Fe -NSC) is reported. Incorporation of S in the coordination of Fe -NSC can induce the transition of spin-polarization configuration with the formation of a medium-spin-state of Fe (t 6 e 1), which is beneficial for facilitating e electrons to penetrate the antibonding π-orbital of nitrogen. As a consequence, a record-high current density up to 10 mA cm can be achieved, together with a high NH selectivity of ≈10% in a flow cell reactor. Both experimental and theoretical analyses indicate that the monovalent Fe(I) atomic center in the Fe -NSC after the S doping accelerates the N activation and protonation in the rate-determining step of *N to *NNH.