Figure 4. a) Ru 3p and b) N 1s high-resolution XPS spectra of the
ACP/RuSAC+C. c) Comparison of Ru 3p peaks of the
ACP/RuSAC+C and the ACP/RuC. XAS
results: d) Ru K-edge XANES profile and e) EXAFS spectra in R space for
ACP/RuSAC+C, Ru foil and RuO2 are used
as references. f) WT-EXAFS plots of Ru foil, RuO2,
ACP/RuSAC+C and ACP/RuC.
2.2. Electrochemical HER performance
To validate the synergy between the Ru single atoms and the Ru
nanoclusters, the HER electrocatalytic performances were performed in an
acidic electrolyte (0.5 M H2SO4). For
comparison, the HER activities of Pt, bare CP, and
ACP/RuC were also examined under the same conditions.
Prior to measurement, the prepared electrodes underwent stabilization by
cyclic voltammetry (CV). Figure 5 a shows the linear sweep
voltammetry (LSV) polarization curves of the electrocatalysts. As
expected, Pt demonstrated highly efficient HER performance, exhibiting
the lowest onset potential close to zero and requiring only an
overpotential (η10) of 61 mV to acquire a current
density of 10 mA cm−2. Notably,
ACP/RuSAC+C exhibited high HER performance compared to
those for CP and ACP/RuC. Specifically,
ACP/RuSAC+C obtained a significantly low
η10 value of 87 mV, which is much lower than that of
ACP/RuC (160 mV). The overpotentials required for all
three samples to reach 10, 50, and 100 mA cm−2 are
summarized in Figure 5b for convenient comparison. To gain further
insights into the catalytic reaction kinetics and mechanism of the HER,
the corresponding Tafel slope was determined by linear fitting of the
polarization curve (Figure 5c). Remarkably, the Tafel slope of
ACP/RuSAC+C was determined as 85 mV
dec−1, indicating that ACP/RuSAC+Cfollows the Volmer-Heyrovsky HER mechanism. In contrast,
ACP/RuC exhibited a significantly larger Tafel slope of
140 mV dec−1, clearly demonstrating that
ACP/RuSAC+C possesses more favorable reaction kinetics
for the HER process. Electrochemical double-layer capacitance
(Cdl) was measured using CV at various scan rates within
the non-faradaic region (Figure S7, Supporting Information). As the
Cdl value is proportional to the electrochemical active
surface area (ECSA), the highest value of Cdl of
ACP/RuSAC+C (10.7 mF cm–2) implies a
greater number of exposed active sites during HER operation (inset of
Figure 5d). As demonstrated in Figure 5d, the self-standing electrode
with the coexistence of Ru nanoclusters and single atoms, possessed a
high ECSA value of 305.7 cm2 compared to that of
ACP/RuC (100 cm2). Furthermore, the
turnover frequency (TOF) was calculated from the current densities
obtained from the LSV curves by assuming that all Ru atoms are active
sites (determined by inductively coupled plasma analysis (ICP) (Table
S4, Supporting Information)) contributing to the HER performance. As
depicted in Figure 5e, the TOF values for ACP/RuSAC+Care higher than those for ACP/RuC in a wide range of
overpotential. For example, at 100, 150, and 200 mV, the TOF values are
3.96, 9.7, and 18 s-1, respectively for
ACP/RuSAC+C, which are comparable or even superior to
the values of the previously reported noble metal-based electrocatalysts
(Figure 5f and Table S5, Supporting Information). The LSV polarization
curve was normalized by the ECSA to further evaluate the intrinsic
activity enhancement of the electrodes, as shown in Figure S8,
Supporting Information. Moreover, electrochemical impedance spectroscopy
(EIS) was conducted to assess the charge-transfer resistance
(Rct) of the electrodes. As depicted,
ACP/RuSAC+C exhibited a significantly low
Rct value of 15 Ω, which suggests that
ACP/RuSAC+C possess the fastest electron transport
capability at the interface of electrode and electrolyte, thereby
significantly improving the overall performance of the catalyst (Figure
S9, Supporting Information). Mass activity (MA) is an important
quantitative factor of how effectively a catalyst can generate hydrogen
gas per unit mass. A higher MA indicates that a catalyst is more
efficient at catalyzing the HER, resulting in a higher rate of hydrogen
gas production. Figure 5g demonstrates the series of MA values for Pt/C,
ACP/RuSAC+C, and ACP/RuC, under
different operating potentials. The ACP/RuSAC+C exhibits
notably higher mass activity at all overpotentials, compared to
ACP/RuC and Pt. Additionally, hydrogen production was
analyzed using gas chromatography, demonstrating that the Faradaic
efficiency (FE) of ACP/RuSAC+C approached approximately
100% at various applied potentials (Figure S10, Supporting
Information). Long-term durability serves as another key parameter for
determining the catalytic performance of electrodes. The stability of
ACP/RuSAC+C was evaluated by CV cycles and a
chronoamperometric (CA) method in 0.5 M
H2SO4. Figure S11, Supporting
Information, presents the LSV polarization curves of
ACP/RuSAC+C before and after 1000 CV cycles.
Impressively, it exhibited a marginal decline after the 1000 cycles,
signifying its robust stability. Moreover, the current density was
maintained at approximately 10 mA cm-2 during the CA
test for at least 50 h, with only a slight decrease, confirming its
superior stability in acidic media (Figure 5h). These multiple
evaluation results confirm the excellent activity and substantial
stability of ACP/RuSAC+C for HER. After 50 h of
stability testing, the electronic state of ACP/RuSAC+Cwas analyzed by XAS. XANES analysis revealed that the electronic state
of ACP/RuSAC+C remained mostly unchanged throughout the
extended period of electrocatalytic stability testing, indicating the
robust chemical stability of the catalyst (Figure S12, Supporting
Information). To emphasize the significance of CP activation, the HER
performances of ACP/RuSAC+C and CP/Ru were also
compared. The key distinction between these two electrodes was the
presence of an activation process before immersion in the Ru solution.
By conducting this comparison, we aimed to highlight the impact of CP
activation on electrocatalytic properties. The LSV measurements and the
corresponding Tafel slopes of ACP/RuSAC+C and CP/Ru are
shown in Figure S13a,b, Supporting Information. As expected, CP/Ru
exhibits a significantly poor HER performance with an
η10 of 463 mV and a Tafel slope of 234 mV
dec–1. Moreover, from the Nyquist plot and ECSA,
CP/Ru evidently exhibited decidedly poor catalytic performance,
indicating that the activation process has an unprecedented impact on
the HER performance (Figures S14-S17, Supporting Information).