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

Abstract

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.

1. 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 (MoS₂) 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 MoS₂ restrict its HER performance [10,11]. Conventionally, there are three approaches employed to promote the HER activity of MoS₂: (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–MoS₂ to 1T–MoS₂) [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 MoS₂. 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 MoO₃ [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 MoS₂.

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 MoS₂. 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 MoS₂. To date, according to our best knowledge so far, the electroless Ni–P coating anchored onto the surface of MoS₂ 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 MoS₂.

The purpose of this research is to reveal the effect of Ni–P coatings with different phosphorus content on the HER activity of MoS₂. 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 MoS₂ by an electroless plating process. Three Ni–P/MoS₂ composites (Ni–LP/MoS₂, Ni–MP/MoS₂, and Ni–HP/MoS₂) were fabricated. Morphologies, structures, compositions, and crystallinities of these three Ni–P/MoS₂ 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).

2. Experimental Details

2.1. Material

The analytical grade reagents including ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O, 99 wt %), thiourea (CH₄N₂S, 99 wt %), nickel sulfate hexahydrate (NiSO₄·6H₂O), citric acid monohydrate (C₆H₈O₇·H₂O), sodium hypophosphite monohydrate (NaH₂PO₂·H₂O), succinic acid (C₄H₆O₄), ammonium hydrogen fluoride (NH₄HF), sodium acetate trihydrate (NaAc·3H₂O), ammonia solution (25 wt %), potassium hydroxide (KOH), hydrochloric acid (36.5 wt %), N, N–dimethylacetamide (DMAc), palladium chloride (PdCl₂) powder, and stannous chloride dehydrate (SnCl₂·2H₂O) were purchased from Jingchun Scientific Co. Ltd. (Shanghai, China). All of them were used as received and without further purification. The 5 wt % of Nafion solution was supplied by Alfa Aesar Chemicals Co. Ltd. (Shanghai, China). The Pt/C power (20 wt % Pt on Vulcan XC–72R) was provided by Yu Bo Biotech Co. Ltd. (Shanghai, China).

2.2. Preparation and Pretreatment of MoS₂

Firstly, 1.41 g of (NH₄)₆Mo₇O₂₄·4H₂O and 0.26 g of CH₄N₂S 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 MoS₂ powders were washed with deionized water. Finally, MoS₂ powders were dried at 60 °C under a vacuum atmosphere condition.

The pretreatment process of synthesized MoS₂ powder was as follows: 15 mg of MoS₂ 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 SnCl₂ solution (pH of the SnCl₂ solution was previously adjusted to 1 using hydrochloric acid), and the mixed solution was ultrasonically dispersed for 10 min. Then the MoS₂ 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 PdCl₂) for 20 min; pH of this activation solution was previously adjusted to 2 using hydrochloric acid. At last, the treated MoS₂ 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).

2.3. Synthesis of Ni–P/MoS₂ Composites

The composition of the electroless Ni–P bath was shown as that: 30 g/L of NiSO₄·6H₂O, 20 g/L of C₆H₈O₇·H₂O, 15 g/L of NaAc, 5 g/L of C₄H₆O₄, 5 g/L of NH₄HF, and 7.5 g/L of NaH₂PO₂·H₂O 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 MoS₂ 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/MoS₂ 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/MoS₂ composites (denoted as Ni–LP/MoS₂, Ni–MP/MoS₂ and Ni–HP/MoS₂) were fabricated.

2.4. Characterization

Crystal structures of the synthesized Ni–P/MoS₂ composites were characterized by an X-ray diffractometer (XRD, SmartLab, Tokyo, Japan) using Cu Kα radiation (λ = 1.5418 Å) 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/MoS₂ 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.

2.5. Electrochemical Measurements

5 mg of Ni–P/MoS₂ 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/cm²). 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/MoS₂ composite (MoS₂ 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 N₂ 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 10⁵ 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.

3. Results and Discussion

3.1. Characterizations of Ni–P/MoS₂ Composites

3.1.1. X-ray Diffraction (XRD) Analysis

The XRD patterns of the pure MoS₂ and three Ni–P/MoS₂ composites were displayed in Figure 1. For the pure MoS₂ sample, three characteristics peaks at 2θ = 9.8°, 33.5°, and 58.4° corresponding to (002), (100), and (110) crystal planes of hexagonal MoS₂ can be observed. The diffraction peak at 2θ = 9.8° is correlated to the stacked layers of MoS₂ along the c axis, indicating the cumulated structure for the (002) plane during the hydrothermal process. In contrast with that of the pure MoS₂, the dispersive peaks for Ni–LP/MoS₂ composite are identified as follows: 44.4° for Ni₁₂P₅, 30.1° and 36.1° for Ni₅P₄; 41.9°, 43.2°, and 68.1° for Ni₅P₂; and 46.6° and 51.8° for Ni₃P. As for the XRD pattern of Ni–MP/MoS₂, the detected peaks are ascribed as that: 28.9° and 32.0° for Ni₅P₄; 36.8° for NiP; 40.8° and 55.0° for Ni₂P; and 46.6° for Ni₃P. The presence of nickel phosphides would be helpful for the enhancement in HER of the composite [27]. Both Ni–LP/MoS₂ and Ni–MP/MoS₂ composites remain the crystal features of the corresponding Ni–P coatings; but the crystallinity of Ni–LP/MoS₂ is higher than that of Ni–MP/MoS₂. For the Ni–LP/MoS₂ composite, intensities of peaks at 2θ = 44.7°, 51.9°, and 76.3° corresponding to (111), (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/MoS₂ composite, these three peaks mentioned above can be easily identified. In addition, for Ni–LP/MoS₂ and Ni–MP/MoS₂ composites, the trace signal of MoS₂ can still be found, suggesting that Ni–LP and Ni–MP coatings do not cover the surface of MoS₂ completely. Thus, Ni–LP/MoS₂ and Ni–MP/MoS₂ 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/MoS₂ 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.

3.1.2. 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), 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.

3.1.3. 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 226.4 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 3d_5/2. 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 3d_3/2, 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 3d_5/2 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 MoO₄²⁻, 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 2p_3/2 and S 2p_1/2 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 S₂O₃²⁻, 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 S⁴⁺ state in SO₃²⁻, which tends to locate at edges of MoS₂ layers [30,31], and also suggesting the oxidation of MoS₂.

As for the 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 2p_3/2 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 2p_1/2 line [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.

3.2. Hydrogen Evolution Reaction (HER) Performance of the Ni–P/MoS₂ Composite

3.2.1. 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 overpotentials (OVP) are tabulated in

Table 1. The obtained values of onset overpotentials (OVP), Tafel slope (b), symmetry factor (β), and active site (n).

Samples	OVP/(mV)	b/(mV/decade)	β	n/(×10−8 mol/cm2)
MoS2	−320	144	0.41	0.97
20 wt % Pt/C	360	52	1.14	16.8
Ni–LP/MoS2	−155	108	0.55	1.94
Ni–MP/MoS2	320	57	1.04	2.69
Ni–HP/MoS2	300	70	0.84	2.28

. 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/cm² (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/cm², 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).

3.2.2. 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)

$$\eta =b\times \log j+a$$

(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)

$$\beta =\frac{2.303RT}{n_{e}bF}$$

(2)

where, parameters of R, T, n_e, and F are corresponding to the gas constant (8.314 J/(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].

3.2.3. 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 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~−0.37 V. 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 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/cm²) embodying the HER activity can be obtained using Equation (3) [36]

$$n=\frac{Q}{2F}$$

(3)

where Q is the voltammetric charge (C/cm²), 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 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/cm² for Ni–LP, 2.42 × 10⁻⁸ mol/cm² for Ni–MP, and 2.19 × 10⁻⁸ mol/cm² 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.

3.2.4. 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 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).

$$C_{\mathrm{dl}}=Y_0\times {(R_{\mathrm{s}}^{-1}+R_{\mathrm{ct}}^{-1})}^{\mathrm{m}-1}$$

(4)

where, Y₀ (S s/cm²) 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. 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 (j₀) analyzed from the EIS spectra.

Samples	Rs/(Ω·cm2)	Rct/(Ω·cm2)	m	Cdl/(×10−4·F/cm2)	j0/(×10−5·A/cm2)
MoS2	0.46	3042	0.850	2.84	0.84
Pt/C	0.42	19.2	0.849	51.2	133
Ni–LP/MoS2	0.43	2758	0.845	6.41	0.93
Ni–MP/MoS2	0.60	43.2	0.871	46.0	59.4
Ni–HP/MoS2	0.50	239.3	0.835	7.85	8.75

. 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/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)

$$j_0=\frac{RT}{n_{\mathrm{e}}F{R}_{\mathrm{ct}}}$$

(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/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 R_ct and C_dl. 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].

3.3. 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. 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.

4. Conclusions

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 property, the fabricated Ni–P/MoS₂ composite shows a more remarkable HER activity than the pure MoS₂.

(2)

Among the three Ni–P/MoS₂ composites, the HER performance of Ni–MP/MoS₂ sample is higher than those of other two samples, which is validated by positive onset potential (~320 mV), large cathodic current (100 mA/cm² at η = −70 mV vs. RHE), and small Tafel slope (~57 mV/decade).

(3)

The excellent HER property of Ni–MP/MoS₂ composite can be due to the excellent conductivity of Ni–MP coating, as well as the coexistence of some nickel phosphides.