0428同步年報-2021-全

060 ACTIVITY REPORT 2021 and under extended electrolysis, the first coordination shell of Ni remained nearly the same, while the second scattering shell of Fe appeared. The coordination structure of the Ni–Fe double-atom catalyst (Ni–Fe–N–C) remained stable under OER conditions on a time scale of 5h ( Fig. 1(a) ). As displayed in Fig. 1(b) , the oxidation state of Ni in the dry sample was between +2 and +3. The energy of the absorption edge of Ni increased only slightly on formation of Ni–Fe–N–C and remained nearly the same during extended electrolysis, indicating that only a few Ni 2+ ions in Ni–N–C were oxidized to Ni 3+ on activation. As shown in Fig. 1(d) , after electrolysis for 1 hour, the average oxidation state of the Fe ion was greater than +3, indicating the formation of substantial Fe(IV) species during OER. The distal iron ion was initially surrounded by close to six oxygen ligands in the first shell and exhibited a Fe–Ni path in the second shell ( Fig. 1(c) ). With increasing duration of electrolysis, the gradual increase of the coordination number (CN) of Ni–Fe was related to the formation of more Ni–Fe atom pairs. The process of formation of Ni–Fe double atoms from Ni–N–C is summarized in Fig. 1(e) . Compared with conventional XAS, a coordinated environment- sensitive approach of operando K ß high-energy- resolution fluorescence detected (HERFD)–XAS, as plotted in Fig. 2 , was performed to investigate the characters of frontier orbitals as well as the bonding state of reactive metals. After five cyclic voltammetry cycles, the symmetry of Ni transformed to highly symmetrical octahedral because of the desorption of nitrogen and attachment of oxygen, in which the signal area of the pre-edge decreased further. This result indicates the restoration of a high symmetry at the Ni centre, which is consistent with the formation of the Ni–Fe–N–C catalyst with octahedral symmetry ( Fig. 1(e) ). During further electrolysis, the dipole transition remained largely steady ( Figs. 2(a) and 2(b) ). In the higher-energy region of the HERFD–XAS Ni K-edge spectra, a metal-to-ligand electronic transition (MLET) was observed. On immersion into the electrolyte, the intensity of the MLET substantially declined. This result is consistent with the coordination of some O ligands and the departure of one or more N ligands, as metal–O interaction is weaker than metal–N interaction. On activation with cyclic voltammetry and continuous electrolysis, the intensity of the MLET further decreased until a steady state was reached in 4hours, indicating that iron ions continued to attach to the Ni site during initial electrolysis until an equilibrium was reached in 4hours. As plotted in Fig. 2(e) , the energy split of Ni–Fe–N–C was about 1.3eV, similar to that of Fe(NO 3 ) 3 , which has six coordinated H 2 O ligands of symmetry O h . This result is attributed to a ligand field caused by H 2 O ligands weaker Fig. 2 : Operando HERFD–XAS experiments of Ni–Fe–N–C. (a) Operando Ni K-edge HERFD–XAS of Ni–Fe–N–C, the graph is derived from the intensity of pre-edge spectra under varied conditions. (b) Integrated area of pre-edge region from 8,332 eV to 8,337eV of Ni K-edge HERFD–XAS for Ni–Fe–N–C under varied conditions. (c) Operando Fe K-edge HERFD–XAS of Ni–Fe–N–C at varied durations of activation. (d) Fe:Ni ratio in Ni–Fe–N–C determined with HERFD–XAS. (e) Energy levels E g and T 2g , and the ligand-field- splitting energy in Ni–Fe–N–C, Fe(NO 3 ) 3 , FeOOH and NiFeOOH. [Reproduced from Ref. 1] (a) (b) (c) (d) (e)