Targeted Assembly of Ultrathin NiO/MoS2 Electrodes for Electrocatalytic Hydrogen Evolution in Alkaline Electrolyte

The development of non-noble metal catalysts for hydrogen revolution in alkaline media is highly desirable, but remains a great challenge. Herein, synergetic ultrathin NiO/MoS2 catalysts were prepared to improve the sluggish water dissociation step for HER in alkaline conditions. With traditional electrode assembly methods, MoS2:NiO-3:1 exhibited the best catalytic performance; an overpotential of 158 mV was required to achieve a current density of 10 mA/cm2. Further, a synergetic ultrathin NiO/MoS2/nickel foam (NF) electrode was assembled by electrophoretic deposition (EPD) and post-processing reactions. The electrode displayed higher electrocatalytic ability and stability, and an overpotential of only 121 mV was needed to achieve a current density of 10 mA/cm2. The improvement was ascribed to the better catalytic environment, rather than a larger active surface area, a higher density of exposed active sites or other factors. DFT calculations indicated that the hybrid NiO/MoS2 heterostuctured interface is advantageous for the enhanced water dissociation step and the corresponding lower kinetic energy barrier—from 1.53 to 0.81 eV.


Synthesis of MoS2 Nanosheets
20 mg bulk MoS2 was dispersed in 20 ml dimethyl formamide (DMF) and stripped by ultrasonic cell disruptor for 3 h. Subsequently, the resulting dispersion was centrifuged for 10 min at 10000 rpm and the supernatant was decanted gently, the remaining sediment was cleaned by deionized water until reaching neutral and dried in a vacuum at 70 °C for 6 hours. After cooling naturally, the MoS2 nanosheets were obtained.

Synthesis of Different Molar Ratio NiO/MoS2 Hybrid Materials
Firstly, specific mass (8 mg, 16 mg, 24 mg, 32 mg) of MoS2 nanosheets above-mentioned were dissolved into 50 mL 1 mmol/L Ni(NO3)2 solution respectively. Next, these solutions were ultrasonic mixed for 10 min separately. Then the above solution was transferred to Teflon-lined stainless-steel autoclaves and heated in an electric oven at 120 °C for 6 h. After cooling naturally, the black precipitates were obtained by ultrasonic cell disruptor, washed with distilled water and ethanol several times, and dried in a vacuum at 70 °C for 3 h. Subsequently, the samples were heated to 500 °C for 2 h under Ar atmosphere with a heating rate of 4 °C/min. After cooling to ambient temperature, the different mixed (the molar ratio of MoS2 : NiO was 1:1, 2:1, 3:1, 4:1) NiO/MoS2 materials was obtained.

Preparation of NiO/MoS2/NF Electrode
After the electrochemical measurements of different molar ratio, we found that the catalyst performed best when the molar ratio was 3:1. So we fabricated the NiO/MoS2/NF electrode with NiO/MoS2-3:1 to explore effects of the assembly method on HER electrochemistry activity. In detail, 2 mg NiO/MoS2-3:1 power was dispersed in 10 mL DMF and vibrated with ultrasonic wave for 1 h to fabricate homogeneous solution. Then 1 mL 50 µg/mL DMF solution of Ni(NO3)2 was added in above-mentioned dispersion to use for electrophoresis. After electrophoresising with a steady current of 5 mA for 5 min, The NF was heated to 500 °C for 2 h under Ar atmosphere with a heating rate of 4 °C/min. Cooling down to room temperature, then we obtained the NiO/MoS2/NF electrode.

Characterizations
Crystallographic structure of all as-prepared samples was investigated with X-ray powder diffraction (XRD, X'Pert PRO MPD, CuKR) at a scanning rate of 1 °C/min. XRD data were collected in the 2θ ranges from 10º to 80º. The morphology of the samples were examined with field-emission scanning electron microscopy (SEM, Hitachi, S-4800). Transmission electron microscopy (TEM) images were collected on HRTEM, JEM-2100UHR with an accelerating voltage of 200 kV. The samples were prepared by dropping the ethanol solution of samples on the Cu grids and were observed at 100 kV. The X-ray photoelectron spectroscopy (XPS) measurements were performed in an ESCALAB 250 spectrometer. EDX elemental mapping were performed on JEOL ARM-200F and atomic force microscopy (AFM) were implemented by Bruker Dimension Icon system. We found that the catalyst performed best and its performance was the nearest with platinum after electrochemistry tests when the molar ratio was 3:1, so we tested characterizations of this specific proportion powder.

Electrochemical measurements
The electrochemical performance of electrodes were obtained in a standard three-electrode electrochemical cell by AUTOLAB PGSTAT302N electrochemical workstation (Metrohm, Switzerland). The prepared integrated electrodes, carbon rods and saturated Ag/AgCl electrode were used as working electrode, counter electrode and reference electrode, respectively. All the electrochemical tests were carried out in KOH (1.0 mol/L, pH = 13.6) electrolyte.
All potentials reported in this work were calibrated to reversible hydrogen electrode (RHE) according to the Nernst equation (Evs.RHE=0.059×pH+E θ Hg/HgO + Evs. Hg/HgO (0.098) or Evs.RHE=0.059×pH+E θ Ag/AgCl + Evs.Ag/AgCl (0.197)). The potentials reported in this work were corrected for the ohmic losses according to Ecorrected = Emeasured -j ×Rs ×A/1000, where j (mA/cm 2 ) is the geometric current density, Rs (Ω) is the equivalent series resistance that can be determined from the EIS, and A (cm 2 ) is the geometric area of the electrode.
The electrochemical activities of samples towards HER were examined by linear sweep voltammetry (LSV) with a scan rate of 5 mV/s at room temperature. It is worth noting that Hg/HgO was used as reference electrode in the stability test for stability consioderation. Tafel slopes were evaluated based on a steady-state current density method and were calculated by plotting overpotential against Log (j, current density), the Tafel equation: Z = blog j + a, where b is the Tafel slope and a is a constant. The electrochemical impedance spectroscopy (EIS) measurements of samples were performed out in the same configuration from 10 5 to 0.01 Hz with an AC voltage of 5 mV with electrochemical workstation.
Cyclic Voltammetry (CV) taken at various scan rates (30, 60, 90, 120 and 150 mV/s) were recorded in the non-faradic potential range of 0.10-0.23 V vs RHE and were used to estimate the double-layer capacitance (Cdl). Cdl was determined as the linear slope by plotting anodic current density at 0.165 V against the scan rate. Cdl = ic/ν, Where ic represents the charging current, ν is the scan rate.

Computations Methods
The DFT calculations were performed to investigate the adsorption and dissociation of water on MoS2 and MoS2/NiO(111) by using the Vienna ab initio simulation package (VASP). 1 The ion-electron interaction is described with the projector augmented wave (PAW) method. 2 Electron exchange-correlation is represented by the functional of Perdew, Burke and Ernzerhof (PBE) of generalized gradient approximation (GGA). 3 A cutoff energy of 450 eV was used for the plane-wave basis set. The MoS2 edge was modeled in a zigzag nanoribbon with width of three Mo-S chains, and the Mo edge is 100%-saturated by S. The NiO(111) surface slab was modeled in a rectangular 3√3×3 unit mesh with six-layer thickness. The bottom three layers were kept fixed at the optimized bulk positions during all the computations, and the thickness of the vacuum layer was set to be 12 Å to ensure the decoupling between periodic images. The Brillouin zone was sampled by a Monkhorst-Pack k-point mesh of 2×4×1 grid. The convergence threshold for structural optimization was set to be 10 -5 eV in energy and 0.01 eV/Å in force. The climbing-image nudged elastic band (CI-NEB) method 4 was used to determine the minimum energy pathways for H2O dissociation, and the transition states were obtained by relaxing the force below 0.05 eV/Å.

Characterization Atlas
Scheme S1. Schematic of the synthesis of synergetic NiO/MoS2 electrode.