Ru–W Pair Sites Enabling the Ensemble Catalysis for Efficient Hydrogen Evolution

Abstract Simultaneously optimizing elementary steps, such as water dissociation, hydroxyl transferring, and hydrogen combination, is crucial yet challenging for achieving efficient hydrogen evolution reaction (HER) in alkaline media. Herein, Ru single atom‐doped WO2 nanoparticles with atomically dispersed Ru–W pair sites (Ru–W/WO2‐800) are developed using a crystalline lattice‐confined strategy, aiming to gain efficient alkaline HER. It is found that Ru–W/WO2‐800 exhibits remarkable HER activity, characterized by a low overpotential (11 mV at 10 mA cm−2), notable mass activity (5863 mA mg−1 Ru at 50 mV), and robust stability (500 h at 250 mA cm−2). The highly efficient activity of Ru–W/WO2‐800 is attributed to the synergistic effect of Ru–W sites through ensemble catalysis. Specifically, the W sites expedite rapid hydroxyl transferring and water dissociation, while the Ru sites accelerate the hydrogen combination process, synergistically facilitating the HER activity. This study opens a promising pathway for tailoring the coordination environment of atomic‐scale catalysts to achieve efficient electro‐catalysis.


Synthesis of Ru SACs catalysts:
In a typical synthesis, 3 ml NH4OH, 80 ml ethanol and 180 ml deionized water were mixed by stirring 10 minutes.Following, 10ml pure dopamine solution (40 mg ml -1 ) and 10 ml Ru contained dopamine solution (DA=40 mg ml -1 , Ru=0.43 mg ml -1 ) were slowly alternate dropped into the solution under vigorous stirring.The reaction was further stirred for 12 h and gradually formed brown precipitation under stirring.The obtained product was collected by centrifugation, washed with deionized water and ethanol 3 times.The precursor was dried in an oven at 60 ℃ overnight and heated at 800 ℃ for 5 h in Ar atmosphere at a heating rate of 2 ℃/min.

6.Characterization:
Scanning electron microscopy (SEM) images were obtained on a FEI Apreo S microscope.Transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HR-TEM) images were collected with a Talos F200X microscope.Aberration-corrected scanning transmission electron microscopy (AC HAADF-STEM) images were collected with a JEOL JEM-ARM200F microscope.The X−ray diffraction (XRD, Bruker D8 Advance) was used to analyze crystal structures.X-ray photoelectron spectroscopy (XPS) measurements were conducted on a PHI 5000 Verasa spectrometer using Al Kα radiation.The metal contents in the catalysts were determined by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) on a Agilent 5110.X-ray absorption fine structure (XAFS) measurements for the Ru K-edge, W L3-edge were performed on beamline 1W1B of Beijing Synchrotron Radiation Facility.The acquired EXAFS data were extracted and processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages.
Operando Raman spectra were measured under controlled electrochemical potentials using a threeelectrode epoxy cell with a counter electrode of Pt wire and Ag/AgCl.A controlled active area of 0.384 cm 2 by an insulation layer on carbon paper drop-casted with 0.1 mg catalyst was used as the working electrode.Raman spectra were collected using a Raman spectrometer (WITEC Alpha500) by a 633 nm He-Ne laser (Research Electro-Optics, Inc., USA) at the objective.A 20× microscope objective lens (NA=0.4,Epiplan-Neofluar, Zeiss, Germany) was used, focusing on the sample surface and avoiding the contact to the electrolyte.Acquisition time was set as 20 s for the spectral Raman shift ranging from 500 to 1800 cm -1 window a UHTS300 spectrometer (WITec GmbH, Germany) with a CCD camera (Andor Tech-nology, UK) operating at -60 °C.The downward shift ratio and the theoretical value was calculated based on the equation below: Where γ represents the downward shift ratio, ν represents the Raman shift, m represents the relative molecular mass.

Electrochemical measurement:
All the electrochemical measurements were carried out in a conventional three-electrode

Computational Method:
We have employed the first-principles to perform density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) formulation. [1- 3] e have chosen the projected augmented wave (PAW) potentials to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 520 eV. [4,5]Partial occupancies of the Kohn−Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV.The electronic energy was considered self-consistent when the energy change was smaller than 10 −5 eV.A geometry optimization was considered convergent when the energy change was smaller than 0.05 eV Å −1 .In our structure, the U correction is used for W and Ru atoms.The Brillouin zone integration is performed using 2×2×1 Monkhorst-Pack k-point sampling for a structure.Finally, the adsorption energies (Eads) were calculated as Eads= Ead/sub -Ead -Esub, where Ead/sub, Ead, and Esub are the total energies of the optimized adsorbate/substrate system, the adsorbate in the structure, and the clean substrate, respectively.The free energy was calculated using the equation:

G=Eads+ZPE-TS
where G, Eads, ZPE and TS are the free energy, total energy from DFT calculations, zero-point energy and entropic contributions, respectively.The U correction for W and Ru were used as 4.18 and 4.92 eV for d orbital, respectively.

Supplementary Figures
cell using the CHI 760E electrochemical workstation at room temperature.A rotating disk electrode (RDE) with a glassy carbon (GC) electrode (diameter: 5 mm; area: 0.196 cm 2 ) was utilized as the working electrode (WE), and the Pt plate was used as the counter electrode.The Ag/AgCl reference electrode calibrated with RHE in 1 M KOH was used as a reference electrode for long-time stability measurement.For electrode preparation,10 mg of Ru-W/WO2-800 catalyst was dispersed in the mixture solution of IPA (400 μl) and 5 wt.%Nafion (20 μl) by sonication for over 30 minutes.Then, 10 μl of the catalyst ink was drop-cast onto the surface of the GC electrode and dried in the air.Linear sweep voltammetry (LSV) plots were carried out in an Ar saturated 1.0 M KOH with a sweep rate of 1 mV s −1 at 1600 rpm.All of the potentials in LSV are 100% iR-corrected.The resistance for iR-compensation was tested at the open circuit potential, Ru-W/WO2-800 (9.3 Ω), Ru1/WO2-800 (8.3 Ω), Pt/C (8.0 Ω), Ru/C (8.4 Ω), Ru SACs (8.9 Ω) and WO2-800 (8.3 Ω).All the potentials were converted to the reversible hydrogen electrode (RHE) by equation.VRHE=E(VS Ag/AgCl)+0.197+0.0592*pHThe Tafel slope was obtained the LSV curve using the equation of η=a+blogj, where a refers to the intercept, b is the Tafel slope and η denotes the overpotential.Electrochemical impedance spectroscopy (EIS) measurements were collected at η=20 mV in the frequency range from 10 kHz to 0.01 Hz, The Mass activity is calculated based on equation of Mass Activity=I/m, where I(A) is the measured current, m (mg) is the mass of Ru loaded on the GC electrode.The turnover frequency (TOF) is calculated based on the equation of TOF=I/(4Fn), where I(A) is the measured current.F is the Faraday constant (96485 C mol −1 ).n=m/M, n (mol) is the molar amount of Ru loaded on the GC electrode, m is the mass of Ru, and M is the molecule weight.For the double-layer capacitor (Cdl) data, cycling voltammetry (CV) curves were recorded in the non-Faradic region with scanning rate of 5,10, 15, 20, 25 and 30 mV s -1 , and the Cdl can be obtained by plotting the current difference (Δj) against the scanning rate.
Figure S1.Illustration of the synthetic process of dual-atom Ru-W catalyst.

Figure S2 .
Figure S2.SEM image of as obtained mixture of polydopamine, W and Ru.

Figure S3 .
Figure S3.XRD pattern of the as obtained mixture of polydopamine, W and Ru.Only the diffraction peaks belonging to polydopamine were observed.

Figure S9 .
Figure S9.Dark and bright filed STME images and corresponding line intensity profiles of WO2 clusters in Ru-W/WO2-800.

Figure S14 .
Figure S14.The turnover frequency of Ru-W/WO2-800 and Pt/C for HRE in 1 M KOH.

Figure S20 .
Figure S20.Simulative structure of OH adsorbed on the Ru SACs and WO2-800 and the corresponding adsorption energy.

Figure S22 .
Figure S22.Different adsorption models and the corresponding adsorption energy of water dissociation intermediate (H-OH) on Ru-W/WO2-800.

Figure S23 .
Figure S23.Simulative H adsorption model and corresponding adsorption energy on Ru and W sites