e Electron Occupancy as a Descriptor for Designing Iron Single-Atom Electrocatalysts.

A quantitative electronic structure-performance relationship is highly desired for the design of single-atom catalysts (SACs). The Fe single-atom catalysts supported by ordered mesoporous carbon with the e electron occupancy from 1.7 to 0.7 are synthesized. A linear relationship has been established between the e electron occupancy of the Fe site and the catalytic activity/activation entropy of oxygen-related intermediates. Fe SAC with an e electron occupancy of 0.7 alters the rate determining step from OH desorption to OOH formation. The value of the turn-over frequency is ≈28 times that of the Fe SAC site with an e electron occupancy of 1.7 e, and the mass activity is ≈6.3 times that of commercial Pt/C. When used in a zinc-air battery, the Fe SAC gives a remarkable power density of 196.3 mW cm and a long-term stability exceeding 1500 h. The discovery of e electron occupancy descriptor provides valuable insights for designing single-atom electrocatalysts.