Figure 3. a) XRD patterns of ACP/RuSAC+C and
ACP/RuC. b) Magnified zone of the (002) reflection. c)
H2-TPD measurements of ACP/RuSAC+C and
bare CP. d, e) AC HAADF-STEM image of ACP/RuSAC+C. f)
Statistical distance between Ru single atoms and the surface of Ru
nanoclusters in AC HAADF-STEM images. g) Intensity profiles obtained in
area 1 to 4 in Figure e). h) HAADF-STEM image and corresponding element
mapping images of C, N, and Ru elements (red, blue, and pink colors
represent C, N, and Ru, respectively).
XPS was conducted to analyze the compositions and chemical states of the
electrodes. The overall spectrum of ACP/RuSAC+C is
depicted in Figure S4, Supporting Information, from which the elements
Ru, N, O, and C can be observed. In the Ru 3p spectrum, peaks
corresponding to both Ru0 (462.2 eV, 484.4 eV) and
Run+ (464 eV, 487.7 eV) were observed for
ACP/RuSAC+C (Figure 4 a). Additionally, the N 1s
spectrum in Figure 4b provides evidence of the presence of a Ru-N bond
at 399 eV, further confirming the presence of both single atoms and
nanoclusters in ACP/RuSAC+C. For the
ACP/RuC, the analysis revealed two peaks corresponding
to Ru0 at 462.08 and 484.2 eV, as depicted in Figure
S5, Supporting Information. A comparative analysis of the Ru 3p spectra
of the two electrocatalysts in Figure 4c demonstrates a positive shift
in the binding energy of the metallic Ru species on
ACP/RuSAC+C by approximately ~0.2 eV.
This shift suggests electron transfer from Ru clusters to Ru single
atoms, due to the electronegativity of the Ru-N bond in the Ru single
atoms.[14,32] This synergistic effect between the
Ru nanoclusters and Ru single atoms of ACP/RuSAC+Ccontributes to its superior performance in the HER, and this will be
further explored through theoretical density functional theory (DFT)
calculations. Synchrotron-based X-ray absorption spectroscopy (XAS) was
conducted to investigate the electronic configurations and atomic
coordination environments of the samples. Figure 4d presents the Ru
K-edge X-ray absorption near-edge structure (XANES) spectra of
ACP/RuSAC+C. The absorption edge of
ACP/RuSAC+C lies between those of the Ru foil and
RuO2, indicating that the chemical valence of Ru is
between 0 and +4. As observed from the corresponding Ru K-edge
Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS)
spectra of the ACP/RuSAC+C in Figure 4e, two prominent
peaks were identified at approximately 1.56 and 2.53 Å, which belong to
the Ru-N, and Ru-Ru coordination, respectively. Additionally, it can be
obtained from the EXAFS fitting in the R-space that the coordination
number of Ru-N is 4 (Figure S6, Supporting Information). The wavelet
transform (WT) of EXAFS in k-space was further employed to confirm the
coordination information on the Ru atoms (Figure 4f). The WT analysis of
ACP/RuSAC+C showed two prominent maximum values at 4.0
and ~8 Å–1, where the former peak
corresponds to Ru-N coordination, while the latter peak corresponds to
Ru-Ru bond. In addition, compared with the WT signals of
ACP/RuSAC+C, only Ru-Ru coordination was observed in
ACP/RuC.