Optimizing Hydrogen Binding on Ru Sites with RuCo Alloy Nanosheets for Efficient Alkaline Hydrogen Evolution

Abstract Ruthenium (Ru)‐based catalysts, with considerable performance and desirable cost, are becoming highly interesting candidates to replace platinum (Pt) in the alkaline hydrogen evolution reaction (HER). The hydrogen binding at Ru sites (Ru−H) is an important factor limiting the HER activity. Herein, density functional theory (DFT) simulations show that the essence of Ru−H binding energy is the strong interaction between the 4dz2 orbital of Ru and the 1s orbital of H. The charge transfer between Ru sites and substrates (Co and Ni) causes the appropriate downward shift of the 4dz2 ‐band center of Ru, which results in a Gibbs free energy of 0.022 eV for H* in the RuCo system, much lower than the 0.133 eV in the pure Ru system. This theoretical prediction has been experimentally confirmed using RuCo alloy‐nanosheets (RuCo ANSs). They were prepared via a fast co‐precipitation method followed with a mild electrochemical reduction. Structure characterizations reveal that the Ru atoms are embedded into the Co substrate as isolated active sites with a planar symmetric and Z‐direction asymmetric coordination structure, obtaining an optimal 4dz2 modulated electronic structure. Hydrogen sensor and temperature program desorption (TPD) tests demonstrate the enhanced Ru−H interactions in RuCo ANSs compared to those in pure Ru nanoparticles. As a result, the RuCo ANSs reach an ultra‐low overpotential of 10 mV at 10 mA cm−2 and a Tafel slope of 20.6 mV dec−1 in 1 M KOH, outperforming that of the commercial Pt/C. This holistic work provides a new insight to promote alkaline HER by optimizing the metal‐H binding energy of active sites.


Experimental section Theoretical calculations
All the DFT calculations were performed using the Vienna ab Initio Simulation package (VASP). [1] Electron-ion interactions were described using standard PAW potentials. [2] For the electron-electron exchange and correlation functional was described through the generalized gradient approximation (GGA) of revised Perdew-Burke-Ernzerhof (RPBE). [3] A plane-wave cutoff energy of 450 eV was used in all the computations. The electronic structure calculations were employed with a Fermi-level smearing of 0.1 eV for the slab systems. A Monkhorst-Pack mesh with 3 × 3 × 1 K-points was used for the Brillouin zone integration. The convergence thresholds of the energy and forces were set to be 1×10 -5 eV and 0.02 eV Å -1 , respectively. The chemical-bonding analysis of the electronic properties of Ru-H is processed using Local Orbital Basis Suite Toward Electronic-Structure Reconstruction (LOBSTER) program. [4,5] To explore the Gibbs free energy of hydrogen adsorption (∆GH*) on Ru site, we calculated the ∆GH on the Ru slab hollow and top site. We found that the ∆GH* for the Ru hollow and top site are -0.22 and 0.13 eV, respectively, indicating weak adsorption of H* on Ru top site restrict HER. To optimize the adsorption of H* on Ru, we construct the model of Ru atom embedded in the Co and Ni slab. A (3×3×4) supercell containing 71 Co atoms and a Ru atom. Vacuum layer is set to 15 Å. The Gibbs free energy of each elementary step was calculated as where ΔE is the reaction energy calculated by the DFT method. ΔZPE and ΔS are the changes in zero-point energies and entropy during the reaction, respectively.

Synthesis of pure Co precursors
This experiment was carried out at room temperature in air. 0.6 mmol Co(NO)3·6H2O was added into 100 mL water (contains 15 mmol CTAB) with magnetic stirring to form solution A. After 20 min, 3 mmol SB in 20 mL water was added into solution A slowly. The system was held for another 1 h with magnetic stirring. The precipitation is washed by water and acetone for 5 times. Those black powders are dried in vacuum oven at 353 K for whole night. The precipitation is collected as Ru1Cox precursors.

Synthesis of RuCo precursors
This experiment was carried out at room temperature in air. 0.6 mmol Co(NO)3·6H2O and 1.2 mmol RuCl3·xH2O were added into 100 mL water (contains 15 mmol CTAB) with magnetic stirring to form solution A. After 20 min, 3 mmol SB in 20 mL water was added into solution A slowly. The system was held for another 1 h with magnetic stirring. The precipitation is washed by water and acetone for 5 times. Those black powders are dried in vacuum oven at 353 K for whole night. The precipitation is collected as Ru1Cox precursors.

Synthesis of RuCo ANSs
Rotate glassy disk carbon electrode (GCE) (3 mm) is the support of active materials to be used as working electrode. Ag/AgCl (saturated with 4 M KCl) and graphite rod were used as counter electrode and reference electrode, respectively. Electrolyte is 1 M KOH. Working electrode for Ru1Cox precursors was prepared by drop 5 uL dispersion (catalyst ink (2 mg/mL) with water containing 10 uL 5 wt. % Nafion solution) on a GCE (polished by Al2O3). The working electrode was applied in cyclic voltammetry (CV) from -1.0 to -1.824 V vs. RHE for 600 cycles with scan rate of 0.05 V/s. The working electrode was gently washed by water for several times and used for following characterizations.

Synthesis of RuNi ANSs
Similar to the synthesis process of Ru1Cox precursors without Co use, Ni(NO3)2·6H2O is used as metal source for RuNi synthesis. Feeding ratio of Ru/Ni is 1:2.

Structural characterization
The morphology and microstructure characterization of the as-prepared samples were investigated by scanning electron microscopy (SEM) (FEI, Helios Nanolab 600i, 2kV), Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) (FEI, ETEM, G2, 200kV), scanning transmission electron microscopy with an energy dispersive X-ray spectroscopy attachment (STEM-EDS, FEI Titan Themis, 300 kV). X-ray diffraction (XRD) (Bruker ECO D8 power X-ray diffractometer with Cu Kα radiation) were used to determine the crystal structure of the samples. The in-situ formed samples were dissolved into 10 mL nitric acid (1 M), and then the mixed solution was sonicated for 3 min. This solution was heated to 180 ºC for 2 h, and then the pink solution was collected for ICP-OES test. Hydrogen sensor test is conducted at room-temperature using a home-made instrument. In the Ar saturated container, the current response curves at different potentials were collected. 5 % H2/Ar was injected into container to get a H2/Ar saturated container, in which, the H2 adsorption/desorption response curves were collected. The TPD was conducted on PCA-1200.

Electrochemical characterization
All experiments were carried out using CHI760e and a three-electrode system in a PTFE bottle (100 mL). The in-situ formed working electrode (Ru1Cox ANSs) was tested at a spin rating of 1,600 rpm (Pine). Linear sweep voltammetry (LSV) and chronoamperometry were measured at 298 K, in air, and at 10 mV/s (scan rate) using the CHI 760e potentiostat. After tested at various density current, the catalyst was washed with water and ethanol to be measured by STEM, XPS, and Raman. The electrolyte was collected for ICP-OES testing. The cyclic voltammograms used for stability test were recorded using a scan rate of 0.05 V/s. The EIS was conducted on a Princeton 4000A at 100 mV overpotential from 100,000 to 0.01 Hz.

TOF calculations
The TOF value is calculated based on the ICP results, using the following equations: where is the number of catalytic atoms, t is time and 2 is the number of H2 molecules, that are obtained from the electrochemical current (i) as: where i is the current, t is the time, F is the Faraday constant (96485 C/mol), 2 represents the number of electrons involves and n is the amount of catalysts loading.          Figure S9. Raman spectra of the as-prepared RuCo precursor (bottom, black), RuCo ANSs (middle, red) and RuCo ANSs after HER (top, blue).The peaks located at 473 and 677 cm -1 belong to Co(OH)2. [6] The small peak at 195 cm -1 belongs to Co3O4 phase, which may be caused by air oxidation. The peak located at 523 cm -1 is assigned to the Eg modes of Ru-O. [7] This peak vanishes after HER due to the metallic Ru formation. Figure S10. The STEM-EDS mapping of as-prepared RuCo precursor.    Figure S13c, we can see that no peak belongs to pure Co can be detected. By comparing the bulk Ru and RuCo ANSs in Figure S13d, we conclude that the peak located at radial distance of 2 Å belongs to Ru-Co peaks.

Figures and Tables
Futhermore, in the Co K-edge, the Co-Ru peak can also be observed. In short, the EXAFS results demonstrate the RuCo alloy formation.  Figure S14. Nyquist representation of impedance spectra recorded for the as-prepared Co precursor (blue triangles), RuCo ANSs (black squares) and Ru/C (red circles).