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 3 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 SA -NSC) is reported. Incorporation of S in the coordination of Fe SA -NSC can induce the transition of spin-polarization configuration with the formation of a medium-spin-state of Fe (t 2g 6 e g 1), which is beneficial for facilitating e g electrons to penetrate the antibonding π-orbital of nitrogen. As a consequence, a record-high current density up to 10 mA cm -2 can be achieved, together with a high NH 3 selectivity of ≈10% in a flow cell reactor. Both experimental and theoretical analyses indicate that the monovalent Fe(I) atomic center in the Fe SA -NSC after the S doping accelerates the N 2 activation and protonation in the rate-determining step of *N 2 to *NNH.