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).