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.