Nitrogen Fixation by Ru Single-Atom Electrocatalytic Reduction Hengcong Tao, Changhyeok Choi, Liang-Xin Ding, Zheng Jiang, Zishan Han, Mingwen Jia, Qun Fan, Yunnan Gao, Haihui Wang, Alex W. Robertson, Song Hong, Yousung Jung, Shizhen Liu, Zhenyu Sun  Chem  Volume 5, Issue 1, Pages 204-214 (January 2019) DOI: 10.1016/j.chempr.2018.10.007 Copyright © 2018 Elsevier Inc. Terms and Conditions

Chem 2019 5, 204-214DOI: (10.1016/j.chempr.2018.10.007) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 1 Surface Characterization by XPS (A) C 1s and Ru 3d XPS spectra of Ru@NC. (B) Zr 3d XPS spectrum of Ru@ZrO2/NC. Chem 2019 5, 204-214DOI: (10.1016/j.chempr.2018.10.007) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 2 Morphology and Structure Characterization of Ru@ZrO2/NC (A) SEM image. (B) Low-magnification HAADF-STEM image along with EDS maps of C, N, Ru, Zr, and O. (C) EDX spectrum of the region shown in image (B). (D and E) Low-magnification (D) and high-magnification (E) HAADF-STEM images. (F) Enlarged HAADF-STEM image of the region encased by the dotted square in (E). The inset shows the size-distribution histogram of Ru objects. (G) The image was subjected to a bandpass filter to remove the variations in intensity due to the differing carbon support thicknesses, allowing for clearer distinguishing of the Ru single atoms. (H) HAADF-STEM image after tuning color contrast. Chem 2019 5, 204-214DOI: (10.1016/j.chempr.2018.10.007) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 3 Electrochemical Nitrogen Reduction Activities (A) UV-visible absorption spectra of the electrolytes after electrolysis at −0.21 V for 2 hr with Ar-saturated electrolyte (Ar gas), without Ru@ZrO2/NC catalyst (carbon paper), or at an open circuit (open circuit). (B) The FE and yield rate of NH3 over 5% Ru@C, Ru@FLG (NaBH4), Ru@ZrO2/NC, Ru@NC (removal of ZrO2), Ru@ZrO2/C (in the absence of –NH2 groups), and Ru@C (in the absence of –NH2 groups and removal of ZrO2). The results of Au catalysts7–9 reported earlier are also provided for comparison. The NH3 yield rates were normalized by dividing corresponding noble metal mass. (C–E) The FEs (C), yield rates (D), and partial current densities (E) of NH3 over Ru@NC, Ru@C, Ru@ZrO2/NC, and Ru@ZrO2/C at various applied potentials. The catalytic results of NC were also added in (C) and (E). (F) The long-term durability test at −0.21 V over Ru@ZrO2/NC at ∼10°C. During the NH3 yield rate calculation in (B), (D), and (F), contributions of ZrO2/NC and ZrO2/C in Ru@ZrO2/NC and Ru@ZrO2/C, respectively, and NC in Ru@NC were subtracted. Chem 2019 5, 204-214DOI: (10.1016/j.chempr.2018.10.007) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 4 Morphology and Structure Characterization of the Ru@ZrO2/NC after Six Cycle Tests of NRR (A) Low-magnification HAADF-STEM image. (B–F) EDS maps of C (B), N (C), Ru (D), Zr (E), and O (F) of the region shown in (A). (G) HAADF-STEM image showing the presence of single Ru sites annotated with yellow dotted circles. Chem 2019 5, 204-214DOI: (10.1016/j.chempr.2018.10.007) Copyright © 2018 Elsevier Inc. Terms and Conditions

Figure 5 Calculation Models and Free-Energy Diagrams for NRR (A) ΔGPDS for NRR on various reaction sites. (B) Calculation models for Ru@Zr32O63 and Ru/NC2. (C) Free-energy diagram for NRR on Ru@Zr32O63. (D) Free-energy diagram for NRR on Ru@NC2. The horizontal dashed line in (A) represents the ΔGPDS of Ru (0001) (0.61 eV). Stoichiometric ZrO2 and ZrO2 with O vacancy are modeled with Zr32O64 and Zr32O63, respectively. The asterisk (*) represents a surface site for adsorption. Black and red lines indicate the free-energy change for NRR and hydrogen adsorption, respectively. The free-energy change at the PDS is denoted in the figure. After the first desorption of NH3, it is omitted for clarity. Chem 2019 5, 204-214DOI: (10.1016/j.chempr.2018.10.007) Copyright © 2018 Elsevier Inc. Terms and Conditions