Volume 4, Issue 2, Pages 285-297 (February 2018) Graphene Defects Trap Atomic Ni Species for Hydrogen and Oxygen Evolution Reactions  Longzhou Zhang, Yi Jia, Guoping Gao, Xuecheng Yan, Ning Chen, Jun Chen, Mun Teng Soo, Barry Wood, Dongjiang Yang, Aijun Du, Xiangdong Yao  Chem  Volume 4, Issue 2, Pages 285-297 (February 2018) DOI: 10.1016/j.chempr.2017.12.005 Copyright © 2017 Elsevier Inc. Terms and Conditions

Chem 2018 4, 285-297DOI: (10.1016/j.chempr.2017.12.005) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 1 Preparation and Structure Characterizations of A-Ni@DG (A) The fabrication schematic of A-Ni@DG. (B) STEM-EDS elemental mapping of A-Ni@DG. (C) HADDF-STEM image of A-Ni@DG. (D) Bright-field STEM image of A-Ni@DG with high resolution. The defective sites (vacant and with Ni single atom trapped) are marked with red arrows. (E) The corresponding HADDF-STEM image of A-Ni@DG of (D). (F) The zoomed-in image of the defective area (vacancy) marked with the yellow dashed frames in the bottom left of (E). (G) The zoomed-in image of the defective area (with atomic Ni trapped) marked with the yellow dashed frames in the top left of (E). The Di-vacancy is marked with the red dashed line. (H and I) Ni K-edge XANES spectra (H) and the K2-weighted Fourier transform spectra (I) of Ni@DG, A-Ni@DG, and the Ni foil reference samples. (J) The LCF analysis of XANES theoretical modeling. The red line is the superposition of the theoretical model simulations of the three configurations and fits the experimental data well. Chem 2018 4, 285-297DOI: (10.1016/j.chempr.2017.12.005) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 2 Electrochemical Hydrogen and Oxygen Evolution Activities (A) HER polarization curves of DG, Ni@DG, A-Ni@DG, and Pt/C performed in 0.5 M H2SO4 electrolyte. (B) The turnover frequency curve of A-Ni@DG and other catalysts reported in the literature for hydrogen evolution. Data were collected from Kibsgaard et al.,27 Zhang et al.,28 Ma et al.,29 Chen et al.,30 Pu et al.,31 Popczun et al.,32 Pan et al.,33 McKone et al.,34 and Li et al.35 (C) OER polarization curves of DG, Ni@DG, A-Ni@DG, and Ir/C performed in 1 M KOH electrolyte. (D) The turnover frequency curve of A-Ni@DG and other catalysts reported in the literature for oxygen evolution. Data were collected from Gong et al.,36 Lu and Zhao,37 Yeo and Bell,38 Godwin and Lyons,39 Jiang et al.,40 Trotochaud et al.,41 Zhao et al.,42 Ahn and Bard,43 Ma et al.,44 and Song and Hu.45 (E) Photos of a working electrode using A-Ni@DG as catalyst during CV measurement from −0.1 to −0.3 V versus RHE are shown to observe the HER. The photos were taken at 5 s intervals. Chem 2018 4, 285-297DOI: (10.1016/j.chempr.2017.12.005) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 3 Comparison of HER and OER Activities and Durabilities on DG and G as Supports for Trapping Single Ni Atoms (A and B) HER (A) and OER (B) polarization curves of A-Ni@DG, A-Ni@G, and Pt/C performed in 0.5 M H2SO4 electrolyte and 1 M KOH electrolyte, respectively. (C) Comparison of the Tafel slope, the specific activity obtained at a potential of −0.1 V versus RHE for HER and 1.5 V versus RHE for OER, and the Ni content obtained by ICP and TG analysis between A-Ni@DG and A-Ni@G. The heights of the A-Ni@DG bars were normalized and the heights of the A-Ni@G bars were altered accordingly. (D) The chronopotentiometry curves of A-Ni@DG and A-Ni@G at a cathodic current density of 5 mA/cm2 in 0.5 M H2SO4 electrolyte. (E) The chronopotentiometry curves of A-Ni@DG and A-Ni@G at an anodic current density of 5 mA/cm2 in 1 M KOH electrolyte. Chem 2018 4, 285-297DOI: (10.1016/j.chempr.2017.12.005) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 4 Mechanistic Study of Catalytic Active Sites on A-Ni@DG (A–C) Illustrations of three different types of catalytic active sites corresponding to a single Ni atom supported on (A) perfect hexagons, (B) D5775, and (C) Di-vacancy. (D–F) The projected densities of state (PDOS) with respect to the three configurations: (D) perfect hexagons, (E) D5775, and (F) Di-vacancy. (G and H) Energy profiles of the three configurations (A–C) for HER (G) and OER (H), respectively. Chem 2018 4, 285-297DOI: (10.1016/j.chempr.2017.12.005) Copyright © 2017 Elsevier Inc. Terms and Conditions