Fig. 1: Synthesis strategy and structural characterizations.

a Pauling electronegativity scales of different elements and schematic diagram of the vacancy-engineering strategy for constructing asymmetric single-atom sites. b XRD patterns of Ni1-C/Cl, Zn1-C/Cl, and Nin-C/Cl. c EPR spectra of Ni1-C/Cl, Zn1-C/Cl, and Nin-C/Cl. d Raman spectra of Ni1-C/Cl, Zn1-C/Cl, and Nin-C/Cl. In order to obtain an accurate ID/IG, the two broad peaks in the Raman spectra were deconvoluted into five Lorentzian peaks43. e HAADF-STEM image of Zn1-C/Cl. f HAADF-STEM image of Ni1-C/Cl. g HAADF-STEM image of Nin-C/Cl. Source data are provided as a Source data file.