Figure 5. Electrochemical HER performance in 0.5 M H2SO4 a) LSV curves. b) Comparison of overpotential at typical current densities. c) Tafel plots gained from the polarization curves in a). d) ECSA measurement. The inset shows the corresponding Cdl values of electrodes. e) Calculated TOF and f) Comparison of TOF values between ACP/RuSAC+Cand other noble metal based HER electrocatalysts. g) MA values of ACP/RuSAC+C and ACP/RuC. h) CA measurement of the obtained ACP/RuSAC+C for about 50 h.
2.3. DFT calculations
We performed a theoretical analysis using DFT calculations to understand the effect of a Ru single atom on the HER catalytic activity of Ru clusters. We modeled a cuboctahedral Ru nanocluster consisting of 55 Ru atoms supported on the graphene, which represents the ACP substrate. The Ru-N4 single-site was then embedded in the support adjacent to the Ru cluster. We conducted additional calculations with the aim of theoretically probing the effect of the distance between single Ru sites and nanoclusters on the HER activity. Three models were constructed with distances of 4.3, 6.1, and 7.9 Å. The obtained results suggest that the HER activity is maximized when the Ru single atoms are located at approximately 4.3 Å from the nanoclusters, which are correlated with the statistical results shown in Figure 3f,g (Figure 6 a,b). Therefore, this optimized structural model is considered for further analysis.
The binding energies of H* vary significantly depending on the adsorption sites,[33] thus all possible sites were investigated (Figure S18, Supporting Information). Our DFT calculations suggested that H* binding at the bottom (111) facet of both ACP/RuC and ACP/RuSAC+C is most favorable for HER with ∆G_(H*) closest to 0 eV; in contrast, the edge sites, upper (111) facet, and (100) facets exhibited stronger H* binding strength, leading to the higher overpotential for HER (Figure S19, Supporting Information). The Bader charge analysis demonstrated that the Ru atoms located close to the support exhibited more positive charges, with an average value of +0.14 e, compared to the other Ru atoms (Figure S20, Supporting Information). This phenomenon resulted in a weaker interaction with the partially positively charged H* adsorbate, which is in agreement with the literature (Figure S20a,b, Supporting Information).[32,34] Additionally, the presence of RuSAC in ACP/RuSAC+C made ∆G_(H*) of the bottom (111) facet site of RuC is optimal (∆G_(H*) = –0.17 eV in ACP/RuSAC+C vs. –0.25 eV in ACP/RuC(Figure 6c). Interestingly, we found substantially strong H* binding on the RuSAC site (∆G_(H*) = –0.59 eV), thus suggesting that this site can easily be covered by H* under the operation conditions.[35] Considering the H* pre-adsorbed RuSAC, a substantial promotion of the HER was expected at the bottom (111) facet by weakening the H* binding from –0.17 eV to –0.09 eV (Figure 6d). To further investigate the electronic state of RuC, we additionally modeled ACP/RuSAC, which consists of only the isolated RuSAC on ACP (Figure S20a, Supporting Information). By comparing the Bader charge of RuC in the presence or absence of RuSAC, we discovered that the presence of RuSAC nearby led to the oxidation of RuC from +1.55e to +1.60e (Figure S21 and Table S6, Supporting Information). Furthermore, the pre-adsorption of H* at the RuSAC led to the further oxidation of RuC to +1.63e, confirming that the presence of RuSAC and the pre-adsorbed H* on the RuSAC site resulted in better HER performance (Table S6, Supporting Information). This finding is also corroborated by d-band theory, where a downshift in the d-band center of transition metal active sites results in a weakening of adsorbate binding due to higher occupancy of antibonding sites.[36,37] It was observed that introducing RuSAC and pre-adsorbed H* downshifted the d-band center of the Ru cluster sequentially (Figure 6e). In summary, our DFT calculations demonstrate that introducing an N-coordinated Ru single atom site adjacent to the Ru cluster provides a synergistic effect to achieve optimal H* binding strength at the Ru cluster.