Figure 6. a) Atomic configurations of ACP/RuSAC+C, where
the distance between RuN4 and Ru cluster is 4.3 Å, 6.1 Å
and 7.9 Å. The distance is measured between Ru hollow site of
RuC and RuSAC. b) The binding Gibbs free
energy of H* (ΔGH*) at the hollow site
of RuC with varying distances. For comparison,
ΔGH* in the absence of RuSAC is also
presented. c) Atomic structures of ACP/RuC (left) and
ACP/RuSAC+C (right), where the RuSAC and
RuC correspond to Ru single atom catalyst and cluster,
respectively. d) The Gibbs free energy diagram for HER. Note that
ACP/RuSAC+C (H*) indicates the system
with pre-adsorbed H* due to the favorable adsorption
of H* on the Ru single atom site of
ACP/RuSAC+C. e) Projected density of states (PDOS) for
different Ru cluster models. Red dashed line indicates the d-band center
of Ru atoms in the cluster of each model.
3. Conclusion
We developed a facile and cost-effective method to synthesize a unique
self-standing electrode through a series of steps (acid treatment,
immersion, and high-temperature pyrolysis). AIMD simulations revealed
that the defects formed during acid treatment played a crucial role in
accelerating the adsorption of Ru ions and extending their binding
duration. This resulted in increased incorporation of Ru atoms into the
support, even when utilizing an identical amount of Ru loading. The
coexistence of Ru single atoms and Ru nanoclusters of
ACP/RuSAC+C was further confirmed by AC HAADF-STEM, XPS,
and XAS analysis. Moreover, DFT calculations confirmed the cooperative
interaction between coupled Ru single atoms and nanoclusters, resulting
in a synergistic effect that may contribute to attaining an optimal
H* binding strength. Consequently, the
ACP/RuSAC+C electrode exhibited relatively good HER
performance in acidic electrolyte. Particularly, it demonstrated an
ultrahigh TOF of 3.96, 9.7, and 18 s-1 at
overpotentials of 100, 150, and 200 mV, respectively. Moreover,
ACP/RuSAC+C achieved an enhanced mass activity,
outperforming that of commercial Pt, demonstrating the intrinsic
catalytic capability of ACP/RuSAC+C.
4. Experimental Section
Synthesis of ACP : The bare CP was cleaned by sequential
sonication in acetone, DI water, and ethanol for 15 min each.
Afterwards, it was immersed in a solution consisting of 45 mL
H2SO4 and 15 mL HNO3 for
2 h at 60 ℃ with continuous stirring. Following this step, it was
adequately soaked in deionized water and dried at 60 ℃ to obtain the
ACP.
Synthesis of ACP/RuSAC+C : ACP (1 \(\times\) 0.5
cm2) was immersed in a well-dispersed 3.0 mg
mL−1 Ru solution for 15 h. The
ACP-Ru3+ was then rinsed multiple times with DI water
and dried at room temperature. Subsequently, it was carbonized under a
N2 atmosphere with 1.2 g dicyanamide as the nitrogen
source. It was calcined at 800 ℃ for 1 h at a heating rate of 10 ℃/min.
The final sample was designated as ACP/RuSAC+C.
Synthesis of ACP/RuC : ACP/RuC was
prepared using the same method as that for ACP/RuSAC+C,
with the only difference being the concentration of the Ru solution
used. Instead of using a 3.0 mg mL−1 Ru solution, a 30
mg mL−1 Ru solution was employed for the preparation
of ACP/RuC.
Synthesis of CP/Ru : CP/Ru was synthesized by the same route as
that for ACP/RuSAC+C; however, bare CP was used instead
of ACP.
Electrochemical Characterization: The HER performance
measurements were performed using an Ivium potentiostat V55630 and a
three-electrode electrolytic setup. The saturated calomel electrode
(SCE) was used as the reference electrode and graphite rod was used as
the counter electrode. The as-prepared electrodes were directly used as
working electrodes (Geometric area: 0.25 cm2 ). The
HER performance of the corresponding electrodes was tested in
0.5 M H2SO4 solutions. Linear sweep
voltammetry (LSV) curves were performed at a scan rate 5 mV
s−1 from 0 V to \(-\)0.84 V vs. SCE. Electrochemical
surface area (ECSA) was determined from the double layer capacitance
(Cdl) of the catalyst surface. The Cdlwas determined by measuring Cyclic Voltammetry (CV) in the non-Faradaic
potential region at multiple scan rates ranging from 20 to 100 mV
s−1. Electrochemical impedance spectroscopy (EIS) data
were measured in the frequency range of 100 kHz\(-\)0.1 Hz. The
stability of ACP/RuSAC+C was examined by Cyclic
Voltammetry (CV) with a scan rate of 50 mV s−1 for
1000 cycles. Thereafter, the stability of the prepared
ACP/RuSAC+C was continuously evaluated viachronoamperometry (CA) for 50 h. The Faradaic efficiency of electrode
was measured in a typical H-type cell. Prior to the testing, the
electrolyte was bubbled with N2 for at least 30 min. The
gas product was analyzed using gas chromatography (GC, Agilent
Technologies).
Computational details: We performed density functional theory
(DFT) calculations using Vienna Ab initio Simulation Program code (VASP
version 5.4.4.) with projector augmented wave (PAW) pseudopotentials and
Perdew–Burke–Ernzerhof (PBE) exchange-correlation
functional.[38-40] A plane wave cutoff energy of
400 eV was employed, and convergence tolerances for energy and force
were set to 10-4 eV and 0.05 eV/Å, respectively. We
modelled Ru nanocluster and Ru single-atom catalyst on a (4 \(\times\)8) supercell of graphene. To avoid an artificial interaction between
periodic images in z -direction, a vacuum layer of 15\(\mathring{\mathrm{A}}\) was added.
The Gibbs free energy of HER was calculated considering two consecutive
protonation steps.
\(*+\left(H^{+}+e^{-}\right)\leftrightarrow\ H*\) (1)
\(H*+\left(H^{+}+e^{-}\right)\leftrightarrow\ *+H_{2}\) (2)
where the asterisk (*) indicates the adsorption site. To convert
DFT-calculated total energies into Gibbs free energy, correction values
for adsorbates and H2 molecule were calculated using the
harmonic oscillator approximation and the ideal gas approximation,
respectively, as implemented in atomic simulation environment (ASE)
(Table S1).[41] The effect of the electrode
potential was included using the computational hydrogen electrode (CHE)
method, which assumes the equivalent chemical potential of 0.5
H2 (g) and a proton-electron pair (H++ e-) at 0 V vs. RHE and standard
conditions.[42] When the potential \(U\) is
applied, the chemical potential of the electron is shifted by \(-eU\),
i.e., \(G\left(H^{+}+e^{-}\right)=0.5G\left(H_{2}\right)-eU\),
thus enabling the potential-dependent calculation of Gibbs free
energies. We performed ab initio molecular dynamics (AIMD) simulations
to investigate a reactivity between Ru precursors and defective/pristine
graphene supports. The simulations were sampled by the canonical
ensemble (NVT) employing the Nose–Hoover thermostat with a time step of
1.0 fs for 10 ps at 330 K.[43,44] Based on the
experimental synthesis process and ID/IGratio results, we modelled pristine CP and defective CP (ACP) using a (4\(\times\) 3) supercell of graphene. To consider an aqueous solution
containing RuCl3, 58 water molecules and 1
RuCl3 molecule were added to the computational cell,
setting the water density to be approximately 1 g cm−3(Figures S2 and S3, Supporting Information).