Hydrogen Evolution Reaction Property in Alkaline Solution of Molybdenum Disulfide Modified by Surface Anchor of Nickel–Phosphorus Coating

Molybdenum disulfide (MoS2) is unfavorable for practical application in the hydrogen evolution reaction (HER) process due to its inert basal surface, inferior conductivity, and limited amount of active edge sites. For the purpose of enhancing the HER performance of this catalyst, the HER activity of its basal surface should be increased. Herein, three types of nickel-phosphorus (Ni–P) coatings—namely, low phosphorus (LP), medium phosphorus (MP) and high phosphorus (HP) —were anchored onto the surfaces of MoS2 nanoparticles via an electroless plating process; thus, three Ni–P/MoS2 composites (Ni–LP/MoS2, Ni–MP/MoS2, and Ni–HP/MoS2) were fabricated. Crystal structures, morphologies, chemical components, and HER performances of each in an alkaline solution were characterized. Both Ni–LP/MoS2 and Ni–MP/MoS2 showed a crystal nature, while the amorphous feature for Ni–HP/MoS2 was validated. The three Ni–P/MoS2 composites exhibited a higher HER activity than the pure MoS2. The HER performance of the Ni–MP/MoS2 composite was more outstanding than those of other two composites, which could be attributed to the presence of metastable nickel phosphides, and the excellent conductivity of Ni–MP coating anchored on the basal surface of MoS2.


Introduction
Hydrogen as a charming and clean energy carrier has received tremendous attention to relieve the energy crisis and environmental problems caused by the consumption of traditional fossil fuels [1].In general, the electrocatalytic hydrogen evolution reaction (HER) process is extensively employed for the generation of hydrogen.The well-known platinum (Pt) and Pt-based alloys possess a superior electrocatalytic HER property [2,3], but the limited availability and high price retard their large-scale applications.In view of this situation, the exploitation of cost-effective and earth-abundant alternatives of Pt-based catalysts is very crucial to push the engineering practice of HER.
Until present, carbides [4], borides [5], selenides [6], nitrides [7], phosphides [8], as well as transition metal sulfides [9] have been recommended as HER catalysts.Among them, molybdenum disulfide (MoS 2 ) can be identified as a promising alternative because of the earth-abundant and low-cost advantages, and the passable electrocatalytic property of this material.Nevertheless, the defects of inert basal plane, insufficient edge sites, and the inferior electron-transport feature of MoS 2 restrict its HER performance [10,11].Conventionally, there are three approaches employed to promote the HER activity of MoS 2 : (i) increasing the active number of edge sites, (ii) enhancing the activity of active sites, and (iii) improving electrical conductivity within the interparticle and interdomain.Thus, numerous attempts-including interlayer intercalation [12,13], gentle oxidation [14,15], phase transformation (from 2H-MoS 2 to 1T-MoS 2 ) [16,17], functional structural design [10,18], as well as stabilizing the edge layers with organic molecules [19]-have been made for enhancing the HER property of MoS 2 .Also, the improvement in the electrical contact between the active sites has been attempted, and various promoters such as gold [12], carbon materials [20], core-shell MoO 3 [21], graphene [22,23], graphene oxide [24,25], and nickel-phosphorus powders [26] have been applied as electrical conduction supports to improve the electrical contact between active sites of MoS 2 .
With respect to the reported efforts mentioned above, unquestionably, attempts made for the increase in abundant active sites, the increment in stacked height along the (002) basal plane aiming to gain a maximum value of edge/basal ratio, and the enhancement in electrical conductivity within the interparticle and interdomain are valuable for the enhancement in HER performance of MoS 2 .However, for this catalyst, the enhancement in HER property of its inert basal plane has not been comprehensively attempted.The electroless nickel-phosphorus (Ni-P) coating possesses merits such as facile fabrication, wear resistance, anticorrosion, excellently electrical conductivity, and promising HER activity, so the anchor of Ni-P coating onto the inert basal plane may be competent for the enhancement in HER performance of MoS 2 .To date, according to our best knowledge so far, the electroless Ni-P coating anchored onto the surface of MoS 2 to enhance the HER property of this catalyst has not been attempted.In this sense, this work will offer a novel sight for the enhancement in HER property of MoS 2 .
The purpose of this research is to reveal the effect of Ni-P coatings with different phosphorus content on the HER activity of MoS 2 .Three Ni-P coatings, i.e., low phosphorus (Ni-LP), medium phosphorus (Ni-MP), and high phosphorus (Ni-HP) coatings were anchored onto the surface of MoS 2 by an electroless plating process.Three Ni-P/MoS 2 composites (Ni-LP/MoS 2 , Ni-MP/MoS 2 , and Ni-HP/MoS 2 ) were fabricated.Morphologies, structures, compositions, and crystallinities of these three Ni-P/MoS 2 composites were characterized by scanning electron microscope (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).Also, the HER properties of them in 1 mol/L KOH solution were evaluated using linear sweep voltammetry (LSV), Tafel polarization, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS).

Preparation and Pretreatment of MoS 2
Firstly, 1.41 g of (NH 4 ) 6 Mo 7 O 24 •4H 2 O and 0.26 g of CH 4 N 2 S were dissolved in 20 mL of deionized water, then this mixed solution was vigorously stirred for 5 min and followed by transfer to a 25 mL Teflon-lined stainless steel autoclave placed in an electric cooker (WRN-010, Eurasian, Tianjin, China).Afterward, the temperature of this cooker was increased from room temperature to 200 • C and kept this temperature for 24 h.After the reaction, the temperature of electric cooker was naturally cooled to ambient temperature, then the mixed solution was centrifuged and the obtained MoS 2 powders were washed with deionized water.Finally, MoS 2 powders were dried at 60 • C under a vacuum atmosphere condition.
The pretreatment process of synthesized MoS 2 powder was as follows: 15 mg of MoS 2 powder was dispersed in 75 mL of DMAc by stirring, then the suspend solution was ultrasonically treated for 22 h using a ultrasonic cleaner (KQ-500DB, Kunshan Ultrasonic Apparatus Co. Ltd., Kunshan, China).After that, this suspended solution was added into 10 mL 50 g/L of SnCl 2 solution (pH of the SnCl 2 solution was previously adjusted to 1 using hydrochloric acid), and the mixed solution was ultrasonically dispersed for 10 min.Then the MoS 2 powders were collected and washed with deionized water adequately.Subsequently, the washed powders were ultrasonically dispersed in 5 mL of activation solution (containing 1 mg of PdCl 2 ) for 20 min; pH of this activation solution was previously adjusted to 2 using hydrochloric acid.At last, the treated MoS 2 powders was centrifugally collected and washed five times with deionized water, and followed by dried in a desiccator ( DL-101, Zhonghuan Electric Furnace Co. Ltd., Tianjin, China).

Synthesis of Ni-P/MoS 2 Composites
The composition of the electroless Ni-P bath was shown as that: 30 g/L of NiSO 4 •6H 2 O, 20 g/L of C 6 H 8 O 7 •H 2 O, 15 g/L of NaAc, 5 g/L of C 4 H 6 O 4 , 5 g/L of NH 4 HF, and 7.5 g/L of NaH 2 PO 2 •H 2 O for Ni-LP plating (15 g/L for Ni-MP and 30 g/L for Ni-HP).When all added reagents were dissolved in deionized water, pH of the solution was adjusted to 4.8-5.0 by the dropwise addition of ammonia solution.After that, the solution temperature was increased from room temperature to 85 ± 2 • C, and 5 mg of pretreated MoS 2 powders were put into 200 mL of Ni-P bath; the plating process lasted for 30 min.As the plating process finished, the prepared Ni-P/MoS 2 powders were collected and washed with deionized water.Finally, the collected powders were dried at 60 • C in a vacuum oven (DZF-6050, Boxun Industrial Co. Ltd., Shanghai, China) Thus, three Ni-P/MoS 2 composites (denoted as Ni-LP/MoS 2 , Ni-MP/MoS 2 and Ni-HP/MoS 2 ) were fabricated.

Characterization
Crystal structures of the synthesized Ni-P/MoS 2 composites were characterized by an X-ray diffractometer (XRD, SmartLab, Tokyo, Japan) using Cu Kα radiation (λ = 1.5418Å) with a scan rate of 2 • (2θ)/s.Morphologies of the composites were observed using a scanning electron microscope (SEM, S-4800, Hitachi, Tokyo, Japan) with an accelerating voltage of 5 kV.Compositions of the three Ni-P/MoS 2 samples were determined by an energy-dispersive X-ray spectrometry (EDS) attached to the scanning electron microscope mentioned above.The X-ray photoelectron spectra (XPS, ESCALAB MK II, Thermo Fisher Scientific, Waltham, MA, USA) of the fabricated composites were characterized with Mg Kα as the excitation source; the binding energy was calibrated by that of the C 1s peak at 284.6 eV derived from the graphitic carbon.

Electrochemical Measurements
5 mg of Ni-P/MoS 2 catalyst and 50 µL of Nafion solution were ultrasonically dispersed in 1mL of solution consisting of 250 µL ethanol and 750 µL deionized water for 30 min to obtain a homogeneous slurry.Subsequently, 5 µL of the slurry was dropped onto a glassy carbon electrode (GCE, 3 mm in diameter, the catalyst load was ~0.337 mg/cm 2 ).The well-known Pt/C catalyst was also loaded to another GCE for comparison in HER property.Before the electrochemical tests, all electrodes naturally dried at room temperature.
All electrochemical tests were conducted by an electrochemical workstation (CHI 650C, Chenhua Co. Ltd, Shanghai, China) in a 1.0 mol/L of KOH electrolyte.A standard three-electrode system was employed, where the GCE bearing the synthesized Ni-P/MoS 2 composite (MoS 2 or Pt/C) acted as the working electrode, a platinum foil served as the counter electrode and a Ag/AgCl electrode employed as the reference electrode.The pure N 2 gas was purged into the solution for 30 min to degas the dissolved oxygen before the tests.The obtained potentials related to the Ag/AgCl electrode were calibrated to the reversible hydrogen electrode (RHE) scale: E RHE = E Ag/AgCl + 0.059 pH + 0.209 V. Linear sweep voltammetry (LSV) was carried out at a scan rate of 2 mV/s with the potential ranging from −0.3 to 0.8 V (vs.RHE).Tafel polarization curves were obtained with a potential window of 0.3~1.5 V (vs.RHE) at a scan rate of 10 mV/s.Cyclic voltammetry (CV) measurements were conducted at a scan rate of 10 mV/s with the potential in the range of −0.6 and 0.2 V (vs.RHE).The electrochemical impedance spectroscopy (EIS) measurements were conducted at a cathodic overpotential of 0.7 V (vs.RHE) with the frequency ranging from 10 5 to 0.01 Hz; 5 mV of the sinusoidal signal was employed.To guarantee the reproducibility of the experimental data, all measurements were performed in triplicate.

X-ray Diffraction (XRD) Analysis
The XRD patterns of the pure MoS 2 and three Ni-P/MoS 2 composites were displayed in Figure 1.For the pure MoS 2 sample, three characteristics peaks at 2θ = 9.8 • , 33.5 • , and 58.4 • corresponding to (002), (100), and (110) crystal planes of hexagonal MoS 2 can be observed.The diffraction peak at 2θ = 9.8 • is correlated to the stacked layers of MoS 2 along the c axis, indicating the cumulated structure for the (002) plane during the hydrothermal process.In contrast with that of the pure MoS 2 , the dispersive peaks for Ni-LP/MoS 2 composite are identified as follows: 44.4  (200), and (220) planes of the fcc nickel become very weak, which may be due to the thin thickness of Ni-LP coating and the insufficient covering of nickel phosphides.However, for the Ni-MP/MoS 2 composite, these three peaks mentioned above can be easily identified.In addition, for Ni-LP/MoS 2 and Ni-MP/MoS 2 composites, the trace signal of MoS 2 can still be found, suggesting that Ni-LP and Ni-MP coatings do not cover the surface of MoS 2 completely.Thus, Ni-LP/MoS 2 and Ni-MP/MoS 2 composites show the mixed crystal characteristics (consisting of crystalline or microcrystalline, and amorphous phases).The presence of mixed crystal phases will be helpful for the electrocatalytic HER activity [10,28].For the Ni-HP/MoS 2 composite compared with other two composites, only one obvious broad peak at 2θ = 44.7 • is easily observed, so the amorphous structure of this composite is validated.The (111) reflection of fcc nickel becomes broadened with the increase in the phosphorus content of Ni-P coating (0.05 at % of P for Ni-LP, 7.37 at % of P for Ni-MP, and 11.25 at % of P for Ni-HP).The increasing phosphorus content of Ni-P coating will lead to the increase in lattice disorder and the decrease in crystallinity of the composite.

Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometry (EDS)
SEM images of the pristine MoS2 and the three Ni-P/MoS2 composite samples are presented in Figure 2. The MoS2 sample shows a nanoflower-like morphology (Figure 2a).As for the Ni-LP/MoS2 composite, the morphology (Figure 2b) of this sample is similar to that of MoS2, but some scattered agglomerates stacking on the surfaces of MoS2 particles can be found.The nanoflower-like morphology corresponding to MoS2 can still be detected, suggesting that the Ni-LP coating does not cover the MoS2 surface completely.For the Ni-MP/MoS2 composite, the nanoflower-like structure of MoS2 (labeled by a blue rectangle in Figure 2c) cannot readily identified; many plates can be found (described by red circles in Figure 2c

Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometry (EDS)
SEM images of the pristine MoS 2 and the three Ni-P/MoS 2 composite samples are presented in Figure 2. The MoS 2 sample shows a nanoflower-like morphology (Figure 2a).As for the Ni-LP/MoS 2 composite, the morphology (Figure 2b) of this sample is similar to that of MoS 2 , but some scattered agglomerates stacking on the surfaces of MoS 2 particles can be found.The nanoflower-like morphology corresponding to MoS 2 can still be detected, suggesting that the Ni-LP coating does not cover the MoS 2 surface completely.For the Ni-MP/MoS 2 composite, the nanoflower-like structure of MoS 2 (labeled by a blue rectangle in Figure 2c) cannot readily identified; many plates can be found (described by red circles in Figure 2c

X-ray Photoelectron Spectroscopy (XPS)
The elemental compositions and chemical states of the three Ni-P/MoS2 composites were characterized by XPS, and results are depicted in Figure 3.
The Mo 3d XPS spectra (Figure 3a) are deconvoluted into six peaks, the two peaks appearing at 226.

X-ray Photoelectron Spectroscopy (XPS)
The elemental compositions and chemical states of the three Ni-P/MoS 2 composites were characterized by XPS, and results are depicted in Figure 3.
The Mo 3d XPS spectra (Figure 3a) are deconvoluted into six peaks, the two peaks appearing at 226.  [26,27].For the P 2p spectrum (Figure 3d), the presence of peak at 128.4 eV for both Ni-LP/MoS 2 and Ni-MP/MoS 2 composites represents the formation of phosphide [32]; another peak appearing at 133.4 eV for all three Ni-P/MoS 2 composites suggests the formation of phosphate [33].Finally, for the three composites, only one peak at 531.7 eV in correlation to the O 1s can be observed (Figure 3e), indicating the bond formation of Mo(IV)-O [15] because of the oxidation.[26,27].For the P 2p spectrum (Figure 3d), the presence of peak at 128.4 eV for both Ni-LP/MoS2 and Ni-MP/MoS2 composites represents the formation of phosphide [32]; another peak appearing at 133.4 eV for all three Ni-P/MoS2 composites suggests the formation of phosphate [33].Finally, for the three composites, only one peak at 531.7 eV in correlation to the O 1s can be observed (Figure 3e), indicating the bond formation of Mo(IV)-O [15] because of the oxidation.

Linear Sweep Voltammetry (LSV)
LSV curves of all samples (MoS2, Ni-LP/MoS2, Ni-MP/MoS2, Ni-HP/MoS2, and Pt/C) measured at room temperature with a scan rate of 2 mV/s are presented in Figure 4a; the obtained onset  The HER activity of Ni-LP/MoS 2 is smaller than those of other two composites, this can be due to the fact that the low sodium hypophosphite addition in the plating bath will result in the poor Ni-P deposition, so the enhancement in conductivity of the Ni-LP/MoS 2 composite is inconsiderable.With the increase in the sodium hypophosphite addition (from 7.5 to 15 g/L), the Ni-P coating will be easily obtained and the coating becomes thick, thereby giving rise to a high conductivity of the Ni-MP/MoS 2 composite.Also, the existence of metastable nickel phosphides, particularly the presence of Ni 2 P, will be beneficial to the enhancement in electrocatalytic property [34].Thus, the sample of Ni-MP/MoS 2 reflects a profound HER activity.As the addition of sodium hypophosphite in the bath increases from 15 to 30 g/L, the amorphous feature of Ni-HP/MoS 2 composite will exert a detrimental effect on the HER performance (by a comparison of curves 4 and 5 in Figure 4a).overpotentials (OVP) are tabulated in Table 1.Besides, for the purpose of comparison, the LSV curves of Ni-LP, Ni-MP, and Ni-HP samples anchored on the GCE electrodes were measured within a potential window between −0.4 V and 0.8 V (vs.RHE), and the results are presented in Figure 4b.Among the eight samples, the commercial Pt/C shows the best HER activity with a current density of 143 mA/cm 2 (cathodic overpotential is ~−70 mV vs. RHE) and a positive OVP (360 mV vs. RHE).By contrast, the pure MoS2 exhibits an inferior HER activity because of the more negative OVP than other samples.The HER activities of three Ni-P samples are smaller than those of Ni-MP/MoS2 and Ni-HP/MoS2 composites because of their more negative OVP (vs.RHE) (196 mV for Ni-LP, 214 mV for Ni-MP, 188 mV for Ni-HP).Although HER performances of the three Ni-P/MoS2 composites are lower than that of the commercial Pt/C, they show a more notable HER activity than the sample of MoS2.Of all three Ni-P/MoS2 composites, the Ni-MP/MoS2 sample exhibits the best HER activity; the current density of this sample at the cathodic overpotential of −70 mV (vs.RHE) is 100 mA/cm 2 , about 70% as that of Pt/C.Also, the OVP value of this sample is higher 475 and 20 mV than those of Ni-LP/MoS2 and Ni-HP/MoS2.Thus, the Ni-MP/MoS2 composite will be competent for being a candidate employed in HER.
The HER activity of Ni-LP/MoS2 is smaller than those of other two composites, this can be due to the fact that the low sodium hypophosphite addition in the plating bath will result in the poor Ni-P deposition, so the enhancement in conductivity of the Ni-LP/MoS2 composite is inconsiderable.With the increase in the sodium hypophosphite addition (from 7.5 to 15 g/L), the Ni-P coating will be easily obtained and the coating becomes thick, thereby giving rise to a high conductivity of the Ni-MP/MoS2 composite.Also, the existence of metastable nickel phosphides, particularly the presence of Ni2P, will be beneficial to the enhancement in electrocatalytic property [34].Thus, the sample of Ni-MP/MoS2 reflects a profound HER activity.As the addition of sodium hypophosphite in the bath increases from 15 to 30 g/L, the amorphous feature of Ni-HP/MoS2 composite will exert a detrimental effect on the HER performance (by a comparison of curves 4 and 5 in Figure 4a).

Tafel Polarization
The linear Tafel polarization curves are shown in Figure 5, the relation between overpotential (η) and current density (log j) is illustrated using Equation ( 1) where, b represents the Tafel slope, a is a constant.Moreover, the symmetry factor (β) reflecting the HER activity can be calculated by the Tafel slope via Equation ( 2) where, parameters of R, T, n e , and F are corresponding to the gas constant (8.314J/(K•mol)), absolute temperature, number of exchanged electrons, and the Faraday constant (96,485 C/mol), respectively.The values of b and β derived from the analysis of Figure 5 are also summarized in Table 1.
A small Tafel slope indicates a high HER activity; as shown by Figure 5, the HER performance of the pure MoS 2 is inferior because of the large value of Tafel slope (144 mV/decade).Among

Tafel Polarization
The linear Tafel polarization curves are shown in Figure 5, the relation between overpotential (η) and current density (log j) is illustrated using Equation ( 1) where, b represents the Tafel slope, a is a constant.Moreover, the symmetry factor (β) reflecting the HER activity can be calculated by the Tafel slope via Equation ( 2) where, parameters of R, T, ne, and F are corresponding to the gas constant (8.314J/(K•mol)), absolute temperature, number of exchanged electrons, and the Faraday constant (96,485 C/mol), respectively.The values of b and β derived from the analysis of Figure 5 are also summarized in Table 1.
A small Tafel slope indicates a high HER activity; as shown by Figure 5, the HER performance of the pure MoS2 is inferior because of the large value of Tafel slope (144 mV/decade).Among the three Ni-P/MoS2 samples, the Ni-MP/MoS2 composite possesses a minimum b value (57 mV/decade) very close to that of the Pt/C sample (52 mV/decade), thereby indicating the promising HER performance of this composite.However, values of b for Ni-LP/MoS2 and Ni-HP/MoS2 composites are 108 and 70 mV/decade, respectively, so their HER performances are lower than that of Ni-MP/MoS2 sample [35].

Cycle Voltammograms (CV)
CV curves of the three Ni-P/MoS2 samples were tested within a potential window of −0.6 to 0.2 V (vs.RHE) at a scan rate of 10 mV/s (Figure 6a).As indicated by Figure 6a, the oxidation peaks locate at the range from −0.35 to −0.32 V, while the reduction peaks appear at the potential in the range of −0.46~−0.37V.The detected quasi-reversible redox peaks may be related to the processes of electrochemical hydrogen adsorption and electrochemical hydrogen desorption.The CV curves of the pure MoS2 and Pt/C samples (inset in Figure 6a) show a similar potential window as those of Ni-P/MoS2 composites.The peak current density for all samples follows the trend: MoS2 < Ni-LP/MoS2 < Ni-HP/MoS2 < Ni-MP/MoS2 < Pt/C, revealing the prominent HER performances of the commercial

Cycle Voltammograms (CV)
CV curves of the three Ni-P/MoS 2 samples were tested within a potential window of −0.6 to 0.2 V (vs.RHE) at a scan rate of 10 mV/s (Figure 6a).As indicated byFigure 6a, the oxidation peaks locate at the range from −0.35 to −0.32 V, while the reduction peaks appear at the potential in the range of −0.46~−0.37V.The detected quasi-reversible redox peaks may be related to the processes of electrochemical hydrogen adsorption and electrochemical hydrogen desorption.The CV curves of the pure MoS 2 and Pt/C samples (inset in Figure 6a) show a similar potential window as those of Ni-P/MoS 2 composites.The peak current density for all samples follows the trend: MoS 2 < Ni-LP/MoS 2 < Ni-HP/MoS 2 < Ni-MP/MoS 2 < Pt/C, revealing the prominent HER performances of the commercial Pt/C and three Ni-P/MoS 2 composites.In addition, CV curves of three Ni-P samples were demonstrated in Figure 6b.These three samples show an oxidation peak at ~0.32 V and a reduction peak at ~0.38 V.The reduction peak may be assigned to the hydrogen adsorption and desorption processes.The parameter (n, the number of active sites, mol/cm 2 ) embodying the HER activity can be obtained using Equation (3) [36] where Q is the voltammetric charge (C/cm 2 ), F is the Faraday constant, and the coefficient of 2 represents the fact that two electrons are required to form a hydrogen molecule from two water molecules.In comparison with the value of n, HER performances for all samples in Pt/C and three Ni-P/MoS2 composites.In addition, CV curves of three Ni-P samples were demonstrated in Figure 6b.These three samples show an oxidation peak at ~0.32 V and a reduction peak at ~0.38 V.The reduction peak may be assigned to the hydrogen adsorption and desorption processes.The parameter (n, the number of active sites, mol/cm 2 ) embodying the HER activity can be obtained using Equation (3 where Q is the voltammetric charge (C/cm 2 ), F is the Faraday constant, and the coefficient of 2 represents the fact that two electrons are required to form a hydrogen molecule from two water molecules.In comparison with the value of n, HER performances for all samples in Figure 6a

Electrochemical Impedance Spectroscopy (EIS)
The obtained Nyquist plots for the three Ni-P/MoS2 samples are shown in Figure 7.An equivalent circuit model of Rs(QRct) (where, Rs, and Rct are the solution resistance, and charge transfer resistance; Q presents the constant phase element, CPE) was employed to analyze the electrochemical impedance data.The value of double layer capacity (Cdl) is calculated using Equation (4).
where, Y0 (S s/cm 2 ) is the CPE coefficient in line with the double layer capacitance; m, related to the constant phase angle, is the CPE exponent.The EIS data were analyzed by the free-charged Zsimpwin software, and the results are summarized in Table 2. Based on the value of m (ranging from 0.835 to 0.871), CPE shows a capacity nature; the low Rs for all samples indicates the excellent conductivity of the electrolyte.The smaller Rct and the higher Cdl for the sample of Ni-MP/MoS2 manifests that this composite owns a more excellent HER property than other two composites.
In addition, another parameter of exchange current density (j0) can also be calculated using Equation ( 5)

Electrochemical Impedance Spectroscopy (EIS)
The obtained Nyquist plots for the three Ni-P/MoS 2 samples are shown in Figure 7.An equivalent circuit model of R s (QR ct ) (where, R s , and R ct are the solution resistance, and charge transfer resistance; Q presents the constant phase element, CPE) was employed to analyze the electrochemical impedance data.The value of double layer capacity (C dl ) is calculated using Equation (4).
where, Y 0 (S s/cm 2 ) is the CPE coefficient in line with the double layer capacitance; m, related to the constant phase angle, is the CPE exponent.The EIS data were analyzed by the free-charged Zsimpwin software, and the results are summarized in Table 2. Based on the value of m (ranging from 0.835 to 0.871), CPE shows a capacity nature; the low R s for all samples indicates the excellent conductivity of the electrolyte.The smaller R ct and the higher C dl for the sample of Ni-MP/MoS 2 manifests that this composite owns a more excellent HER property than other two composites.
In addition, another parameter of exchange current density (j 0 ) can also be calculated using Equation ( 5) RT n e FR ct (5) where, n e is the number of transferred electrons, and parameters of R, T, and F are already defined by Equation (2).As ascertained by the data (Table 2), without any doubt, the Pt/C sample shows the best HER performance followed by the Ni-P/MoS 2 composites and last by MoS 2 sample.The three Ni-P/MoS 2 samples show a larger j 0 than the virgin MoS 2 , consistent with the analyses of R ct and C dl .The value of j 0 for the Ni-MP/MoS 2 sample is larger than those of other two composite samples, so the charming HER behavior of Ni-MP/MoS 2 sample is further validated [37].
where, ne is the number of transferred electrons, and parameters of R, T, and F are already defined by Equation (2).As ascertained by the data (Table 2), without any doubt, the Pt/C sample shows the best HER performance followed by the Ni-P/MoS2 composites and last by MoS2 sample.The three Ni-P/MoS2 samples show a larger j0 than the virgin MoS2, consistent with the analyses of Rct and Cdl.The value of j0 for the Ni-MP/MoS2 sample is larger than those of other two composite samples, so the charming HER behavior of Ni-MP/MoS2 sample is further validated [37].

HER Stability of the Ni-P/MoS2 Hybrid Composites
From an engineering perspective, besides the HER activity, the stability is another essential requirement to guarantee the durability of Ni-P/MoS2 composite.Hence, continuous CV tests for 1000 cycles were carried out at a rate of 50 mV/s (Figure 8) to evaluate the stabilities of Ni-LP/MoS2, Ni-MP/MoS2, and Ni-HP/MoS2 composites.As shown in Figure 8, by a comparison of the current density at the initial time and after the long-term test, electrocatalytic behaviors of the three Ni-P/MoS2 composites just show a very slight decay, and thereby indicate an excellent HER durability of the composite.For example, the cathodic current density of Ni-MP/MoS2 composite only shows a decrease of 4% (η = 80 mV vs. RHE).In addition, EDS spectra of the Ni-MP/MoS2 composite before and after the stability tests were measured (Figure 9) to monitor the leaching of phosphorus element.From an engineering perspective, besides the HER activity, the stability is another essential requirement to guarantee the durability of Ni-P/MoS 2 composite.Hence, continuous CV tests for 1000 cycles were carried out at a rate of 50 mV/s (Figure 8) to evaluate the stabilities of Ni-LP/MoS 2 , Ni-MP/MoS 2 , and Ni-HP/MoS 2 composites.As shown in Figure 8, by a comparison of the current density at the initial time and after the long-term test, electrocatalytic behaviors of the three Ni-P/MoS 2 composites just show a very slight decay, and thereby indicate an excellent HER durability of the composite.For example, the cathodic current density of Ni-MP/MoS 2 composite only shows a decrease of 4% (η = 80 mV vs. RHE).In addition, EDS spectra of the Ni-MP/MoS 2 composite before and after the stability tests were measured (Figure 9) to monitor the leaching of phosphorus element.As indicated by data reported in Figure 9, about 16.7% phosphorus element is leached out; thus, in the further research, the enhancement in the stability for the Ni-MP/MoS 2 sample will be considered.As indicated by data reported in Figure 9, about 16.7% phosphorus element is leached out; thus, in the further research, the enhancement in the stability for the Ni-MP/MoS2 sample will be considered.As indicated by data reported in Figure 9, about 16.7% phosphorus element is leached out; thus, in the further research, the enhancement in the stability for the Ni-MP/MoS2 sample will be considered.The anchor of Ni-P coatings on the surface of MoS 2 is helpful in the enhancement of its HER property, the fabricated Ni-P/MoS 2 composite shows a more remarkable HER activity than the pure MoS 2 .(2) Among the three Ni-P/MoS 2 composites, the HER performance of Ni-MP/MoS 2 sample is higher than those of other two samples, which is validated by positive onset potential (~320 mV), large cathodic current (100 mA/cm 2 at η = −70 mV vs. RHE), and small Tafel slope (~57 mV/decade).(3) The excellent HER property of Ni-MP/MoS 2 composite can be due to the excellent conductivity of Ni-MP coating, as well as the coexistence of some nickel phosphides.
), which can be assigned to the deposited Ni-MP particles.Unlike the sample of Ni-LP/MoS2, defects (such as void and segregation) existing on the surface of Ni-MP/MoS2 composite are reduced.The morphology of Ni-MP/MoS2 different from that of the Ni-LP/MoS2 could be attributed to the medium crystalline nature of the Ni-MP coating.The smooth, uniform, and cauliflower-like morphology of Ni-HP/MoS2 composite (Figure 2d) can be observed, indicating the amorphous nature of this composite sample.This can be due to the uniform Ni-HP coating anchored to the surface of MoS2.EDS spectra of the three Ni-P/MoS2 composites were measured, and results are also given in Figure 2. Compared with that of MoS2, as demonstrated by Figure 2, except for S, Mo, and O elements, the existence of Ni and P elements for the three Ni-P/MoS2 composites reveals the anchor of Ni-P coatings on the surface of MoS2.The presence of O element may be assigned to the oxidation of MoS2 particles and Ni-P coatings in air and/or the hydrothermal synthesis process [29,30].The P and Ni contents (at %) for both Ni-MP/MoS2 and Ni-HP/MoS2 samples are very higher than that of Ni-LP/MoS2 sample, and they follow the order of Ni-LP/MoS2 < Ni-MP/MoS2 < Ni-HP/MoS2; however, S and Mo contents for these three composites are in a reverse trend (Ni-LP/MoS2 > Ni-MP/MoS2 > Ni-HP/MoS2).This can be explained as the plating rate of Ni-LP bath system being smaller than those of the Ni-MP and Ni-HP systems, because of the low addition of sodium hypophosphite in the Ni-LP bath solution.Generally, plating rates of the three Ni-P plating systems follow the sequence: Ni-HP > Ni-MP > Ni-LP, the thickness of Ni-LP coating will be thinner than those of other two Ni-P coatings.Thus, the overlapped percentage of MoS2 by the Ni-LP coating is smaller than those by Ni-MP and Ni-HP coatings, that is, the exposed sites of MoS2 for Ni-LP/MoS2 are larger than those for Ni-MP/MoS2 and Ni-HP/MoS2 composites.Based on this analysis, for the three Ni-P/MoS2 composites, we can confirm that the contents of P, Ni, S, and Mo elements follow the order mentioned above.
), which can be assigned to the deposited Ni-MP particles.Unlike the sample of Ni-LP/MoS 2 , defects (such as void and segregation) existing on the surface of Ni-MP/MoS 2 composite are reduced.The morphology of Ni-MP/MoS 2 different from that of the Ni-LP/MoS 2 could be attributed to the medium crystalline nature of the Ni-MP coating.The smooth, uniform, and cauliflower-like morphology of Ni-HP/MoS 2 composite (Figure 2d) can be observed, indicating the amorphous nature of this composite sample.This can be due to the uniform Ni-HP coating anchored to the surface of MoS 2 .EDS spectra of the three Ni-P/MoS 2 composites were measured, and results are also given in Figure 2. Compared with that of MoS 2 , as demonstrated by Figure 2, except for S, Mo, and O elements, the existence of Ni and P elements for the three Ni-P/MoS 2 composites reveals the anchor of Ni-P coatings on the surface of MoS 2 .The presence of O element may be assigned to the oxidation of MoS 2 particles and Ni-P coatings in air and/or the hydrothermal synthesis process [29,30].The P and Ni contents (at %) for both Ni-MP/MoS 2 and Ni-HP/MoS 2 samples are very higher than that of Ni-LP/MoS 2 sample, and they follow the order of Ni-LP/MoS 2 < Ni-MP/MoS 2 < Ni-HP/MoS 2 ; however, S and Mo contents for these three composites are in a reverse trend (Ni-LP/MoS 2 > Ni-MP/MoS 2 > Ni-HP/MoS 2 ).This can be explained as the plating rate of Ni-LP bath system being smaller than those of the Ni-MP and Ni-HP systems, because of the low addition of sodium hypophosphite in the Ni-LP bath solution.Generally, plating rates of the three Ni-P plating systems follow the sequence: Ni-HP > Ni-MP > Ni-LP, the thickness of Ni-LP coating will be thinner than those of other two Ni-P coatings.Thus, the overlapped percentage of MoS 2 by the Ni-LP coating is smaller than those by Ni-MP and Ni-HP coatings, that is, the exposed sites of MoS 2 for Ni-LP/MoS 2 are larger than those for Ni-MP/MoS 2 and Ni-HP/MoS 2 composites.Based on this analysis, for the three Ni-P/MoS 2 composites, we can confirm that the contents of P, Ni, S, and Mo elements follow the order mentioned above.
4 and 232.3 eV (Ni-LP/MoS2), and 226.1 and 232.0 eV (Ni-MP/MoS2) can be attributed to the S 2s of MoS2; the peaks at 229.0 eV (Ni-LP/MoS2) and 228.8 eV (Ni-MP/MoS2) correspond to Mo 3d5/2.Peaks at 232.4 eV (Ni-LP/MoS2), 232.6 eV (Ni-MP/MoS2) and 232.3 eV (Ni-HP/MoS2) can be assigned to Mo 3d3/2, which suggests the existence of Mo(IV) [16].Moreover, the peaks at 233.2 eV for Ni-LP/MoS2, 232.7 eV for Ni-MP/MoS2, and 233.3 eV for Ni-HP/MoS2 are ascribed to Mo 3d5/2 component [21].The peaks with a binding energy of 235.7 eV for Ni-LP/MoS2, and 235.6 eV for the other two composites may be assigned to MoO3 or MoO4 2− , indicating the oxidation of these samples [29].As shown by Figure 3b, with respect to S 2p spectrum, four peaks can be observed for the Ni-LP/MoS2 and Ni-MP/MoS2; the peaks at 161.8 and 163.7 eV for Ni-LP/MoS2 (163.5 eV for Ni-MP/MoS2) correspond to the S 2p3/2 and S 2p1/2 lines of MoS2, indicating a −2 oxidation state of sulfur.The peak appearing at 162.9 eV (Ni-LP/MoS2) and 162.5 eV (Ni-MP/MoS2) may be indexed to S2O3 2− , which can be attributed to the oxidation of sulfur element during the hydrothermal process.The peaks at 170.1 eV (Ni-LP/MoS2) and 169.1 eV (for Ni-MP/MoS2 and Ni-HP/MoS2) are identified to the S 4+ state in SO3 2− , which tends to locate at edges of MoS2 layers[30,31], and also suggesting the oxidation of MoS2.
4 and 232.3 eV (Ni-LP/MoS 2 ), and 226.1 and 232.0 eV (Ni-MP/MoS 2 ) can be attributed to the S 2s of MoS 2 ; the peaks at 229.0 eV (Ni-LP/MoS 2 ) and 228.8 eV (Ni-MP/MoS 2 ) correspond to Mo 3d 5/2 .Peaks at 232.4 eV (Ni-LP/MoS 2 ), 232.6 eV (Ni-MP/MoS 2 ) and 232.3 eV (Ni-HP/MoS 2 ) can be assigned to Mo 3d 3/2 , which suggests the existence of Mo(IV) [16].Moreover, the peaks at 233.2 eV for Ni-LP/MoS 2 , 232.7 eV for Ni-MP/MoS 2 , and 233.3 eV for Ni-HP/MoS 2 are ascribed to Mo 3d 5/2 component [21].The peaks with a binding energy of 235.7 eV for Ni-LP/MoS 2 , and 235.6 eV for the other two composites may be assigned to MoO 3 or MoO 4 2− , indicating the oxidation of these samples [29].As shown by Figure 3b, with respect to S 2p spectrum, four peaks can be observed for the Ni-LP/MoS 2 and Ni-MP/MoS 2 ; the peaks at 161.8 and 163.7 eV for Ni-LP/MoS 2 (163.5 eV for Ni-MP/MoS 2 ) correspond to the S 2p 3/2 and S 2p 1/2 lines of MoS 2 , indicating a −2 oxidation state of sulfur.The peak appearing at 162.9 eV (Ni-LP/MoS 2 ) and 162.5 eV (Ni-MP/MoS 2 ) may be indexed to S 2 O 3 2− , which can be attributed to the oxidation of sulfur element during the hydrothermal process.The peaks at 170.1 eV (Ni-LP/MoS 2 ) and 169.1 eV (for Ni-MP/MoS 2 and Ni-HP/MoS 2 ) are identified to the S 4+ state in SO 3 2− , which tends to locate at edges of MoS 2 layers [30,31], and also suggesting the oxidation of MoS 2 .As for the Ni 2p spectrum (Figure 3c), peaks observed at 852.9 eV (Ni-LP/MoS 2 and Ni-MP/MoS 2 ) and 852.7 eV (Ni-HP/MoS 2 ) are corresponding to the fcc Ni.For samples of Ni-MP/MoS 2 and Ni-HP/MoS 2 , the peak at 856.3 eV can be attributed to the oxidized Ni species with a 2+ oxidation state.The peaks with binding energy of 859.8 eV (Ni-LP/MoS 2 ), 861.7 eV (Ni-MP/MoS 2 ), and 861.9 eV (Ni-HP/MoS 2 ) can be ascribed to the satellite of the Ni 2p 3/2 line [26,27].Meanwhile, peaks at 873.7 eV (Ni-MP/MoS 2 ) and 873.8 eV (Ni-HP/MoS 2 ) can be assigned to oxidized Ni species.Peaks at 879.4 eV for Ni-LP/MoS 2 (879.8eV for Ni-MP/MoS 2 , and 880.0 eV for Ni-HP/MoS 2 ) are corresponding to the satellite of the Ni 2p 1/2 line Ni 2p spectrum (Figure 3c), peaks observed at 852.9 eV (Ni-LP/MoS2 and Ni-MP/MoS2) and 852.7 eV (Ni-HP/MoS2) are corresponding to the fcc Ni.For samples of Ni-MP/MoS2 and Ni-HP/MoS2, the peak at 856.3 eV can be attributed to the oxidized Ni species with a 2+ oxidation state.The peaks with binding energy of 859.8 eV (Ni-LP/MoS2), 861.7 eV (Ni-MP/MoS2), and 861.9 eV (Ni-HP/MoS2) can be ascribed to the satellite of the Ni 2p3/2 line [26,27].Meanwhile, peaks at 873.7 eV (Ni-MP/MoS2) and 873.8 eV (Ni-HP/MoS2) can be assigned to oxidized Ni species.Peaks at 879.4 eV for Ni-LP/MoS2 (879.8 eV for Ni-MP/MoS2, and 880.0 eV for Ni-HP/MoS2) are corresponding to the satellite of the Ni 2p1/2 line

3. 2 .
Hydrogen Evolution Reaction (HER) Performance of the Ni-P/MoS 2 Composite 3.2.1.Linear Sweep Voltammetry (LSV) LSV curves of all samples (MoS 2 , Ni-LP/MoS 2 , Ni-MP/MoS 2 , Ni-HP/MoS 2 , and Pt/C) measured at room temperature with a scan rate of 2 mV/s are presented in Figure 4a; the obtained onset overpotentials (OVP) are tabulated in Table 1.Besides, for the purpose of comparison, the LSV curves of Ni-LP, Ni-MP, and Ni-HP samples anchored on the GCE electrodes were measured within a potential window between −0.4 V and 0.8 V (vs.RHE), and the results are presented in Figure 4b.Among the eight samples, the commercial Pt/C shows the best HER activity with a current density of 143 mA/cm 2 (cathodic overpotential is ~−70 mV vs. RHE) and a positive OVP (360 mV vs. RHE).By contrast, the pure MoS 2 exhibits an inferior HER activity because of the more negative OVP than other samples.The HER activities of three Ni-P samples are smaller than those of Ni-MP/MoS 2 and Ni-HP/MoS 2 composites because of their more negative OVP (vs.RHE) (196 mV for Ni-LP, 214 mV for Ni-MP, 188 mV for Ni-HP).Although HER performances of the three Ni-P/MoS 2 composites are lower than that of the commercial Pt/C, they show a more notable HER activity than the sample of MoS 2 .Of all three Ni-P/MoS 2 composites, the Ni-MP/MoS 2 sample exhibits the best HER activity; the current density of this sample at the cathodic overpotential of −70 mV (vs.RHE) is 100 mA/cm 2 , about 70% as that of Pt/C.Also, the OVP value of this sample is higher 475 and 20 mV than those of Ni-LP/MoS 2 and Ni-HP/MoS 2 .Thus, the Ni-MP/MoS 2 composite will be competent for being a candidate employed in HER.
the three Ni-P/MoS 2 samples, the Ni-MP/MoS 2 composite possesses a minimum b value (57 mV/decade) very close to that of the Pt/C sample (52 mV/decade), thereby indicating the promising HER performance of this composite.However, values of b for Ni-LP/MoS 2 and Ni-HP/MoS 2 composites are 108 and 70 mV/decade, respectively, so their HER performances are lower than that of Ni-MP/MoS 2 sample [35].Metals 2017, 7, 211 9 of 15
Figure 6a with a descending order of Pt/C > Ni-MP/MoS 2 > Ni-HP/MoS 2 > Ni-LP/MoS 2 > MoS 2 can be attested.For comparison, we also calculated the active number of sites of three Ni-P samples (2.04 × 10 −8 mol/cm 2 for Ni-LP, 2.42 × 10 −8 mol/cm 2 for Ni-MP, and 2.19 × 10 −8 mol/cm 2 for Ni-HP).Among the three Ni-P samples, the Ni-MP coating shows a higher HER performance than Ni-LP and Ni-HP samples because of the small m values of them; but m values of these three Ni-P samples are smaller than that of Ni-MP/MoS 2 .Thus, except for that of the well known commercial Pt/C, the HER activity of Ni-MP/MoS 2 is most noticeable.
with a descending order of Pt/C > Ni-MP/MoS2 > Ni-HP/MoS2 > Ni-LP/MoS2 > MoS2 can be attested.For comparison, we also calculated the active number of sites of three Ni-P samples (2.04 × 10 −8 mol/cm 2 for Ni-LP, 2.42 × 10 −8 mol/cm 2 for Ni-MP, and 2.19 × 10 −8 mol/cm 2 for Ni-HP).Among the three Ni-P samples, the Ni-MP coating shows a higher HER performance than Ni-LP and Ni-HP samples because of the small m values of them; but m values of these three Ni-P samples are smaller than that of Ni-MP/MoS2.Thus, except for that of the well known commercial Pt/C, the HER activity of Ni-MP/MoS2 is most noticeable.

Figure 9 .
Figure 9. EDS spectra of the Ni-MP/MoS2 composite before and after the stability test.
In summary, three types Ni-P coatings (Ni-LP, Ni-MP, and Ni-HP) were anchored to the surface of MoS2 via an electroless plating process.Structures, morphologies, chemical components of the fabricated Ni-LP/MoS2, Ni-MP/MoS2, and Ni-HP/MoS2 composites were characterized.Also, the HER performances of them in 1.0 mol/L KOH solution were evaluated.The main conclusions are shown as follows: (1) The mixed crystal features for both Ni-LP/MoS2 and Ni-MP/MoS2, and the amorphous characteristic for Ni-HP/MoS2 are validated.The phosphorous content of Ni-P coating shows a remarkable influence on the crystallinity and the morphology of the Ni-P/MoS2 composite.The anchor of Ni-P coatings on the surface of MoS2 is helpful in the enhancement of its HER

Figure 9 .
Figure 9. EDS spectra of the Ni-MP/MoS2 composite before and after the stability test.
In summary, three types Ni-P coatings (Ni-LP, Ni-MP, and Ni-HP) were anchored to the surface of MoS2 via an electroless plating process.Structures, morphologies, chemical components of the fabricated Ni-LP/MoS2, Ni-MP/MoS2, and Ni-HP/MoS2 composites were characterized.Also, the HER performances of them in 1.0 mol/L KOH solution were evaluated.The main conclusions are shown as follows: (1) The mixed crystal features for both Ni-LP/MoS2 and Ni-MP/MoS2, and the amorphous characteristic for Ni-HP/MoS2 are validated.The phosphorous content of Ni-P coating shows a remarkable influence on the crystallinity and the morphology of the Ni-P/MoS2 composite.The anchor of Ni-P coatings on the surface of MoS2 is helpful in the enhancement of its HER

Figure 9 .
Figure 9. EDS spectra of the Ni-MP/MoS 2 composite before and after the stability test.
In summary, three types Ni-P coatings (Ni-LP, Ni-MP, and Ni-HP) were anchored to the surface of MoS 2 via an electroless plating process.Structures, morphologies, chemical components of the fabricated Ni-LP/MoS 2 , Ni-MP/MoS 2 , and Ni-HP/MoS 2 composites were characterized.Also, the HER performances of them in 1.0 mol/L KOH solution were evaluated.The main conclusions are shown as follows: (1) The mixed crystal features for both Ni-LP/MoS 2 and Ni-MP/MoS 2 , and the amorphous characteristic for Ni-HP/MoS 2 are validated.The phosphorous content of Ni-P coating shows a remarkable influence on the crystallinity and the morphology of the Ni-P/MoS 2 composite.
• for Ni 12 P 5 , 30.1 • and 36.1 • for Ni 5 P 4 ; 41.9 • , 43.2 • , and 68.1 • for Ni 5 P 2 ; and 46.6 • and 51.8 • for Ni 3 P.As for the XRD pattern of Ni-MP/MoS 2 , the detected peaks are ascribed as that: 28.9 • and 32.0 • for Ni 5 P 4 ; 36.8 • for NiP; 40.8 • and 55.0 • for Ni 2 P; and 46.6 • for Ni 3 P.The presence of nickel phosphides would be helpful for the enhancement in HER of the composite [27].Both Ni-LP/MoS 2 and Ni-MP/MoS 2 composites remain the crystal features of the corresponding Ni-P coatings; but the crystallinity of Ni-LP/MoS 2 is higher than that of Ni-MP/MoS 2 .For the Ni-LP/MoS 2 composite, intensities of peaks at 2θ = 44.7 • , 51.9 • , and 76.3 • corresponding to (111),

Table 2 .
Parameters of solution resistance (Rs), charge transfer resistance (Rct), double-layer capacitance (Cdl), the dimensionless CPE exponent m, and exchange current density (j0) analyzed from the EIS spectra.

Table 2 .
Parameters of solution resistance (R s ), charge transfer resistance (R ct ), double-layer capacitance (C dl ), the dimensionless CPE exponent m, and exchange current density (j 0 ) analyzed from the EIS spectra.