Defect-Rich Heterogeneous MoS2/rGO/NiS Nanocomposite for Efficient pH-Universal Hydrogen Evolution

Molybdenum disulfide (MoS2) has been universally demonstrated to be an effective electrocatalytic catalyst for hydrogen evolution reaction (HER). However, the low conductivity, few active sites and poor stability of MoS2-based electrocatalysts hinder its hydrogen evolution performance in a wide pH range. The introduction of other metal phases and carbon materials can create rich interfaces and defects to enhance the activity and stability of the catalyst. Herein, a new defect-rich heterogeneous ternary nanocomposite consisted of MoS2, NiS and reduced graphene oxide (rGO) are synthesized using ultrathin αNi(OH)2 nanowires as the nickel source. The MoS2/rGO/NiS-5 of optimal formulation in 0.5 M H2SO4, 1.0 M KOH and 1.0 M PBS only requires 152, 169 and 209 mV of overpotential to achieve a current density of 10 mA cm−2 (denoted as η10), respectively. The excellent HER performance of the MoS2/rGO/NiS-5 electrocatalyst can be ascribed to the synergistic effect of abundant heterogeneous interfaces in MoS2/rGO/NiS, expanded interlayer spacings, and the addition of high conductivity graphene oxide. The method reported here can provide a new idea for catalyst with Ni-Mo heterojunction, pH-universal and inexpensive hydrogen evolution reaction electrocatalyst.


Synthesis of αNi(OH)2 Nanowires
The αNi(OH)2 nanowires was synthesized by the hydrothermal method [1]. Briefly, 20 mmol of NiSO4·6H2O and 20 mmol of NaOH were dissolved in deionized water (40 mL) and stirred vigorously for 30 min to obtain homogeneous solution. Then, the NaOH solution was slowly poured into the NiSO4·6H2O aqueous solution with stirring for 5 min. Well-mixed solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and kept at 120 °C for 24 h (heat rate 3 °C min -1 ). After naturally cooled at room temperature, the as-prepared αNi(OH)2 was collected by centrifugation and washed several times with distilled water. The obtained products were dispersed in 200 mL deionized water to form a homogeneous suspension of nickel hydroxide with concentration of 4mg mL −1 .

Synthesis of MoS2/rGO/NiS Nanocomposite
For the synthesis of MoS2/rGO/NiS catalyst, 5 mL αNi(OH)2 suspension was added into 60 mL of ethanol/water (V:V = 1:1) solution. After stirring for 5 min, 2 mL GO solution was dispersed in the αNi(OH)2 solution and treated with ultrasonication for 30 min. Subsequently, 281 mg of L-ch and 78 mg Na2MoO4·2H2O were added into the αNi(OH)2/GO solution and kept at stirring for 30 minutes. The final solution was then transferred into a 100 mL Teflon-lined stainless autoclave. After reactions at 200 °C for 24h, the black precipitates were centrifuged and washed with ethanol and deionized water several times, then dried at 80 °C for 12h. The sample is labeled as MoS2/rGO/NiS-X (X=0, 3, 5, 7) according to the volume of αNi(OH)2 added (0, 3, 5 and 7 wt%). Pristine MoS2 is prepared with Lch and Na2MoO4·2H2O with a molar ratio (L-ch:Na2MoO4 = 2:1) under the same hydrothermal conditions.

Characterization
Morphologies and microstructures of the as-prepared samples were analyzed using field emission scanning electron microscope (SEM, SU8020, Tokyo, Japan) operating at a voltage of 20 kV, and high-resolution transmission electron microscope (HR-TEM, Tecnai G2 F30, Oregan, USA) at 300 kV. Energy disperse X-ray (EDS) mapping attached to TEM was conducted to estimate the elemental composition and distribution of catalysts. The elemental microanalysis and atom binding states were examined by X-ray photoelectron spectroscope (XPS, ESCALAB 250XI, Waltham, USA) with an Al Kα radiator. X-ray diffraction (XRD, SMARTLAB, Tokyo, Japan) pattern of catalysts were recorded on an X-ray diffractometer with a Cu Kα radiation (λ = 1.5406 Å). Nitrogen sorption isotherms was carried out using an Automatic specific surface area analyzer at −196 °C (TriStarⅡ3020, Georgia, USA). All samples were outgassed at 150 °C for 3h in a dynamic vacuum before measurement. The Brunauer Emmett Teller (BET) specific surface area was calculated from adsorption data. Raman spectra of as-obtained sample were examined with a 6 mW laser power at a 532 nm laser excitation (inVia Reflex, London, UK).

Electrochemical Measurements
The HER performance of catalyst was performed via an electrochemical workstation (CHI 660E, ChenHua Instruments Co. Ltd., Shanghai, China) in a three-electrode system. Saturated calomel electrode (in 0.5 M H2SO4 and 1 M PBS) and Hg/HgO (1 M KOH) electrode were used as reference electrode. A graphite rob was used as a counter electrode for all electrochemical experiments. A glassy carbon electrode/rotating disc electrode (RDE) loaded with various catalysts was used as a working electrode at a rotation speed of 1600 rpm during the experiment. The homogeneous catalyst ink was prepared by dispersing 5 mg catalysts in 1 mL of mixture solvent containing 5 μL Nafion solution, 780 μL DI-water and 200 μL ethanol followed by ultrasonication for 30 min. Then 5 μL catalyst ink was loaded onto a GC electrode (mass loading～0.353 mg cm −2 ) and dried naturally at an ambient temperature. Before the electrochemical measurement, the electrolyte solution was bubbled with nitrogen for at least 30 min to remove dissolved oxygen. Linear sweep voltammetry (LSV) was performed in H2SO4 (pH = 0.4), KOH (pH = 14) and PBS (pH = 7.4) solution with a scan sweep rate of 5 mV S −1 from 0 to −0.6 V vs. RHE. Tafel plot was acquired by fitting the linear portion of the polarization curve. Cyclic voltammetry (CV) measurements for the stability of the catalysts was performed by taking 1000 CV cycles at a scan rate of 100 mV S −1 (potential windows in H2SO4 and PBS solution are −0.4-0.2V vs SCE and in KOH solution is −0.4-0.1V vs Hg/HgO), and then LSV was measured again under same initial conditions. Electrochemical impedance spectra (EIS) was performed at various overpotentials with frequency from 0.1 to 100,000 Hz at the amplitude voltage of 5 mV. To evaluate the electrochemical active surface area of the catalysts, the double-layer capacitances (Cdl) was measured by CV with scan rates of 40, 80, 120, 160, 200 mV S −1 (potential windows in acidic, alkaline and neutral media are 0.36-0.46V vs RHE, 0.6-0.7V vs RHE and 0.7-0.8V, respectively). All potentials data were calibrated to a reversible hydrogen electrode (RHE): E(RHE)=E(SCE) + E θ + 0.059PH V (E θ = 0.244 in 0.5 M H2SO4 and 1 M PBS solution, E θ = 0.098 in 1 M KOH). All the ohmic potential drop caused by solution resistance has been corrected with 95% iRcorrection. Figure S1.        Tables   Table S1. The electrochemical impedance fitting results of as-prepared samples in 0.5 M H2SO4.

Sample Name
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