In situ modulating coordination fields of single-atom cobalt catalyst for enhanced oxygen reduction reaction

Single-atom catalysts, especially those with metal−N4 moieties, hold great promise for facilitating the oxygen reduction reaction. However, the symmetrical distribution of electrons within the metal−N4 moiety results in unsatisfactory adsorption strength of intermediates, thereby limiting their performance improvements. Herein, we present atomically coordination-regulated Co single-atom catalysts that comprise a symmetry-broken Cl−Co−N4 moiety, which serves to break the symmetrical electron distribution. In situ characterizations reveal the dynamic evolution of the symmetry-broken Cl−Co−N4 moiety into a coordination-reduced Cl−Co−N2 structure, effectively optimizing the 3d electron filling of Co sites toward a reduced d-band electron occupancy (d5.8 → d5.28) under reaction conditions for a fast four-electron oxygen reduction reaction process. As a result, the coordination-regulated Co single-atom catalysts deliver a large half-potential of 0.93 V and a mass activity of 5480 A gmetal−1. Importantly, a Zn-air battery using the coordination-regulated Co single-atom catalysts as the cathode also exhibits a large power density and excellent stability.


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Supplementary Fig. 3. XRD patterns of CR-Co/ClNC, Co/NC, and NC.N2 physisorption isotherms reveal a slightly higher specific surface area of CR-Co/CINC and similar pore architecture in comparison with Co/NC.It shows that the accessibility of Co sites and mass transport to active sites for CR-Co/ClNC is slightly superior to Co/NC during ORR process, suggesting the significantly enhanced ORR performance of CR-Co/ClNC should therefore derive from local optimization of electronic and coordination structures.Binding energy (eV)

Supplementary Fig. 5 .
(a) XPS survey spectra of CR-Co/ClNC and Co/N and (b) Cl 2p XPS spectra of CR-Co/ClNC.

Supplementary Fig. 8 .
(a, c) The fitting curve of the K-edge k 3 -weighted EXAFS spectrum and (b, d) the Re(k 3 χ(k)) oscillation curve for CR-Co/ClNC and Co/NC, respectively.

Supplementary Fig. 9 . 1 S11Supplementary Fig. 10 .
Proportions of different N species in the CR-Co/ClNC and Co/NC catalysts.(a) Co K-edge XANES spectra and (b) Normalized derivative curves for CR-Co/ClNC and reference samples.

Supplementary Fig. 11 .Supplementary Fig. 12 .Supplementary Fig. 13 .
Calculated oxidation state of Co at CR-Co/ClNC and Co/NC based on the absorption edge of Co K-edge XANES spectra.(a) Polarization curves for CR-Co/ClNC and Pt/C under 0.1 M O2saturated KOH, 1600 rpm.(b) The onset potential can be determined based on the LSV curves when the ORR current is 5% of the diffusion-limited current.The cyclic voltammetry curves of CR-Co/ClNC catalyst at different scan rates (5-25 mV/s) (a) first, (c) second, (e) third and corresponding electrochemical double-layer capacitance (Cdl) (b) first, (d) second, (f) third.

Supplementary Fig. 15 .Supplementary Fig. 16 .Supplementary Fig. 17 .Supplementary Fig. 18 . 19 .
(a) Cdl results from three repeated tests for CR-Co/ClNC and Co/NC.(b) ECSA values calculated from Cdl.The error bar is obtained by three repeated tests.(a) The cyclic voltammetry curves of Pt/C catalyst at different scan rates (5-25 mV/s) and (b) corresponding electrochemical double-layer capacitance (Cdl).(a) The cyclic voltammetry curves of NC catalyst at different scan rates (5-25 mV/s) and (b) corresponding electrochemical double-layer capacitance (C dl ).ECSA values calculated from potential cycling.The ECSA (m 2 g -1 ) can be estimated as the specific value from the gravimetric capacitance Cdl by the equation: ECSA = /(s × ), where Cs is the double layer capacitance (F m -2 ) of the glassy carbon electrode surface, for which the typical value of 0.4 F m -2 was used in KOH solution, and L is the mass of catalyst deposited on the electrode.LSV curves at various rotation rates of (a) CR-Co/ClNC, (b) Co/NC, (c) Pt/C, and (d) NC.

Fig. 21 .Supplementary Fig. 26 .Supplementary Fig. 27 .Supplementary Fig. 28 .Supplementary Fig. 29 .Supplementary Fig. 30 .Supplementary Fig. 31 .
(a) RRDE curves and (b) electron number and H2O2 yield for CR-Co/ClNC and reference catalysts.(a) In situ XAFS device diagram and in situ cell.(b) k 3 χ(k) curves of Co K-edge EXAFS oscillation functions (c) Corresponding k 3 -weighted FT of Co K-edge EXAFS oscillation functions for CR-Co/ClNC under different working conditions (ex situ, 1.00 V, 0.90 V and 0.75 V).(a) The fitting curve of the Co K-edge k 3 -weighted EXAFS spectrum and (b) the Re(k 3 χ(k)) oscillation and fitting curve for CR-Co/ClNC under ex situ condition.(a) The fitting curve of the Co K-edge k 3 -weighted EXAFS spectrum and (b) the Re(k 3 χ(k)) oscillation and fitting curve for CR-Co/ClNC under 1.00 V. (a) The fitting curve of the Co K-edge k 3 -weighted EXAFS spectrum and (b) the Re(k 3 χ(k)) oscillation and fitting curve for CR-Co/ClNC under 0.90 V. (a) The fitting curve of the Co K-edge k 3 -weighted EXAFS spectrum and (b) the Re(k 3 χ(k)) oscillation and fitting curve for CR-Co/ClNC under 0.75 V. (a) The calculated oxidation state and (b) The fitted average formal d-band electron counts of Co at CR-Co/ClNC under ex situ, 1.00 V, 0.90 V and 0.70 V conditions based on the absorption edge of Co K-edge XANES spectra.The error bars are the standard deviations of three replicate calculation.32.(a) XANES spectra of Co K-edge recorded at different applied potentials during the ORR process for Co/NC.Inset, magnified absorption edge region.(b) The fitted average formal d-band electron counts of Co at Co/NC under ex situ, 1.00 V, and 0.90 V conditions based on the absorption edge of Co K-edge XANES spectra.The error bars are the standard deviations of three replicate calculation.