Synthesis of a MoSx–O–PtOx Electrocatalyst with High Hydrogen Evolution Activity Using a Sacrificial Counter‐Electrode

Abstract Water splitting is considered to be a very promising alternative to greenly produce hydrogen, and the key to optimizing this process is the development of suitable electrocatalysts. Here, a sacrificial‐counter‐electrode method to synthesize a MoSx/carbon nanotubes/Pt catalyst (0.55 wt% Pt loading) is developed, which exhibits a low overpotential of 25 mV at a current density of 10 mA cm−2, a low Tafel slope of 27 mV dec−1, and excellent stability under acidic conditions. The theory calculations and experimental results confirm the high hydrogen evolution activity that is likely due to the fact that the S atoms in MoSx can be substituted with O atoms during a potential cycling process when using Pt as a counter‐electrode, where the O atoms act as bridges between the catalytic PtOx particles and the MoSx support to generate a MoSx–O–PtOx structure, allowing the Pt atoms to donate more electrons thus facilitating the hydrogen evolution reaction process.


Electrode Preparation
Bare glassy carbon electrodes (GCE) (3 mm diameter, CH Instrument Inc.) were polished with different sizes alumina slurry on a microcloth and subsequently rinsed with ultrapure water and ethanol. The electrodes were then sonicated in ethanol, and dried under a gentle nitrogen stream. To prepare the working electrode, a 2 mg CNT sample (CNTs were purchased from Cnano Technology (Beijing) Limited (purity > 95%; diameter 11 nm; length = 10 m (average); synthesis method, CVD)) was ultrasonically dispersed in the mixed solution of ethanol and H 2 O (500 mL), and then 8 L of the resultant suspension was dropped onto the GCE surface and dried at room temperature. For comparison,a commercially available Pt/C-modified GCE (20 wt% Pt supported on carbon black, fuel cell grade from Alfa Aesar) was prepared in the same way.

Synthesis of MoS x /CNTs
The hybrid catalysts were synthesized via electrochemical deposition method, wherein CNT-modified GCEs were soaked in a 2 mM (NH 4 ) 2 MoS 4 aqueous solution containing 0.1M NaClO 4 , and the MoS x was deposited in situ onto CNTs by i-t experiment. At the end of deposition, the working electrode was rinsed with water gently and dried at room temperature overnight. All of the potentials in our paper are calibrated to a reversible hydrogen electrode (RHE) based on the Nernst equation. For comparison, the parallel experiments using various deposition times were also carried out.

Synthesis of MoS x /CNTs/Pt
The MoS x /CNTs catalyst electrode was used as working electrode, a Pt wire as counter electrode, and a SCE (3 M KCl filled) electrode as reference in 0.5 M H 2 SO 4 solutions, and electrodeposited in the potential range from -0.2 to -0.7V for cyclic voltammograms (CV) at a scan rate of 100 mV/s. At the end of deposition, the working electrode was rinsed with water gently and dried at room temperature overnight. The parallel experiments using various deposition cycles were also carried out. For comparison, we also use MoS x /CNTs catalyst electrode as working electrode and the graphite rod as counter electrode, and CV in 0.025 mM H 2 PtCl 6 ·6H 2 O, the everything else is the same.

Synthesis of MoS x /CNTs/W 10k and MoS x /CNTs/Pd 2k
The MoS x /CNTs catalyst electrode was used as working electrode, a W or Pd wire as counter electrode, and a SCE (3 M KCl filled) electrode as reference in 0.5 M H 2 SO 4 solutions, and electrodeposited in the potential range from -0.2 to -0.7V for cyclic voltammograms (CV) at a scan rate of 100 mV/s. At the end of deposition, the working electrode was rinsed with water gently and dried at room temperature overnight. Electrochemical impedance spectroscopy (EIS) measurement was carried out at the open-circuit voltage with an AC voltage of 5 mV. All data were reported without iR compensation. In all measurements, we used SCE as the reference electrode. It was calibrated with respect to RHE. The calibration was performed in the high-purity hydrogen-saturated electrolyte with a Pt foil as the working and counter electrodes.

Structure Characterization
Cyclic voltammetry was run at a scan rate of 1 mV s -1 , and the average of the two potentials at which the current crossed 0 was taken to be the thermodynamic potential for the hydrogen electrode reaction. In 0.5 M H 2 SO 4 , E(RHE)= E(SCE)+0.267 V.

TOF calculation 5
Cyclic voltammetry measurements of our samples were carried out in PBS electrolyte(PH=7) with a potential window from -0.2 to 0.6 V vs RHE and scan rate of 50 mV/s. Assuming one electron redox process, the integrated charge over the whole potential range was divided by two. Then, the value was divided by the Faraday constant to get the number of active sites for different samples. The turnover frequency (s -1 ) can be estimated according to this equation: where I represents the current density for different samples during the LSV measurement in 0.5 M H 2 SO 4 , F is the Faraday constant (C/mol), and n is the number of the active sites (mol) for different samples.

Computational Details
Density functional theory (DFT) calculations were performed using the all-electron, full potential electronic structure code FHI-aims at the level of the generalised gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Periodic boundary conditions were adopted, the structure was modelled as a nanosheet cut out from a monolayer of 2H-MoS 2 , which was separated from each other by at least 12 Å in the y-direction and 20 Å in the z-direction to reduce the electrostatic interactions between them. The default "tight" basis were used in all calculations in this work. To account for the missing long-range tail of van der Waals forces, these functionals were augmented by the van der Waals scheme of Tkatchenko and Scheffler. A Gaussian occupation scheme with a smearing of 0.05 eV was used throughout.

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To quantitatively access the stability of oxygen atoms at various sites of MoS 2 nanosheet, the formation energy of oxidation ( E ox ) could be written as where E ox and E MoS 2 are the total energies of the oxidized system and the clean (unoxidized) MoS 2 nanosheet, n i the number of constituent element i being added/removed from the structure, u i is the atomic chemical potential (u s and u o ).
According to previously published literature, the adsorption energy, which describes the stability of hydrogen adsorption, was defined by Where E MoS 2 +H is the total energy for one hydrogen atom adsorbed on the MoS 2 monolayer with the PtOx catalyst, E MoS 2 is the total energy for the MoS 2 catalytic system without hydrogen adsorption, and E H 2 is the total energy of a separated H 2 molecule as determined from DFT calculations.
The Gibbs free energy for atomic hydrogen adsorption was then calculated as Here, E ZPE is the zero-point energy difference between the adsorbed state of the system and the gas phase state and S H is the entropy difference between the adsorbed state of this system and the gas phase standard state (300 K, 0.1 Mpa).
The electron-density rearrangement, AB , was determined as From our experimental data and previous reports, we knew that the edges exposed in MoS 2 monolayer were mainly Mo edges terminated by disulfide (S 2-2 ) or sulfur (S 2-) 7 ions, and thus the diverse S species (apical S 2-, S 2and S 2-2 edges) could make a tremendous impact on the oxidation and Pt nucleation owing to their different bonding characteristics. Therefore, ab initio calculations were first used to determine the energetics of atomic oxygen substitution in the MoS 2 nanosheets, followed by revealing the location of PtO x clusters on the oxidized system and further to elucidate the triggering mechanism of this complex catalysis for HER.

Faradic efficiency
The Faradic efficiency is defined as the available efficiency of electrons involved in