Cobaloxime anchored MoS₂ nanosheets as electrocatalysts for the hydrogen evolution reaction

Abstract

Molybdenum disulfide (MoS₂) has emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER). However, the catalytic performance of pure MoS₂ is not as good as expected due to the limited active sites at the surface. Herein, cobaloxime anchored MoS₂ nanosheets (MoS₂–Co(dmgBF₂)₂) were successfully developed as electrocatalysts for the HER in acidic media. Remarkably, MoS₂–Co(dmgBF₂)₂ delivered a current density of 10 mA cm⁻² at a low overpotential of approximately 103 mV versus the reversible hydrogen electrode, which is much lower than the values of most reported MoS₂-based HER electrocatalysts. In addition, the Tafel slope and the exchange current density of MoS₂–Co(dmgBF₂)₂ were calculated to be 45 mV dec⁻¹ and 64.5 μA cm⁻², respectively. Such superior HER activity can be attributed to the abundant active sites from both the edges and the modified basal planes. We believe that this approach not only provides a new method to functionalize MoS₂ through coordination, but also offers a new approach to integrate homogeneous and heterogeneous catalysts for the HER.

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Introduction

Hydrogen (H₂) is an abundant and renewable energy carrier that can be stored, distributed, and used on demand, and it is regarded as a promising substitute for traditional fossil fuels in the future.^1,2 The electrocatalytic hydrogen evolution reaction (HER) has tremendous potential as one of the cleanest and most sustainable approaches to large-scale H₂ production.³ To maximize the efficiency, electrocatalysts are commonly used to facilitate chemical reactions by lowering reaction overpotentials and accelerating reaction rates. Although platinum (Pt) and other noble metals have been proven to be the most active and stable HER electrocatalysts, widespread application of noble metals is critically hampered by their scarcity and high cost.⁴ Therefore, developing low-cost and high-performance HER electrocatalysts based on earth-abundant elements is highly imperative. In the past decade, molybdenum disulfide (MoS₂) has received tremendous attention owing to the natural abundance of its constituent elements and low Gibbs free energy for hydrogen adsorption (DGH*) in the Volmer reaction. However, the catalytic performance of pure MoS₂ is not as good as expected given the limited number of active sites. Owing to its anisotropic structure, MoS₂ is prone to form a two-dimensional (2D) morphology that offers a large surface area and 2D permeable channels for ion adsorption and transport.^5,6 Both experimental⁷ and theoretical⁸ studies have concluded that HER activity arises from the sites located along the edges of the 2D MoS₂ layers, while the basal surfaces are catalytically inert. To enhance the HER activity of MoS₂, some research such as increasing the number of defect sites,^9–11 hybridization with graphene,¹² creating active sites in the basal plane¹³ and formation of ternary Mo–M sulfides^14,15 has been widely carried out. By contrast, very few reports have addressed the functionalization of MoS₂ basal surfaces for the HER. Ultrathin MoS₂ nanosheets are excellent candidates for constructing hybrids with high specific surface areas and high flexibility.^16–18 Both sides of MoS₂ nanosheets can be used as templates for growing functional composites.

The active sites of hydrogenases have inspired the design of molecular catalysts for hydrogen evolution and oxidation. In particular, cobaloximes (a cobalt center coordinated to two glyoximato equatorial ligands and two trans axial ligands) have been extensively investigated.¹⁹ Cobaloximes are one of the most active series of molecular catalysts for electrocatalytic and photocatalytic production of H₂,^20,21 displaying desirable catalytic properties, low overpotential,²² and stability toward O₂.²³ However, electrocatalytic studies on such molecular complexes are typically conducted in nonaqueous solvents.^24,25 Homogeneous systems contain a large pool of molecules that are not directly involved in electrocatalysis because they are not present in the reaction-diffusion layer in the immediate vicinity of the electrode.²¹ By contrast, only a few methodologies have been reported under realistic conditions (i.e., in aqueous media).²⁶ For implementation in devices, molecular catalysts must be immobilized on the surface of conducting electrodes. Thus, all catalysts attached to the electrode surface ideally partake in the HER. Progress has been made regarding the integration of highly active molecular catalysts within electrocatalytic nanomaterials such as carbon nanotubes and graphene.^27–31 The catalytic performance of copper complexes has been studied in the heterogeneous phase through π-stacking anchorage to graphene-based electrodes. Upon anchorage, π-stacking interactions with the graphene sheets provide further π-delocalization that improves the catalytic performance.³² However, to the best of our knowledge, such molecular catalysts immobilized on active electrocatalytic substrates (i.e., MoS₂) for hydrogen evolution have rarely been reported.

In this study, we demonstrate the fabrication of cobaloxime-immobilized MoS₂ nanosheets through the coordination of cobalt on 4-cyanobenzyl-functionalized MoS₂ nanosheet templates. Unique MoS₂–Co(dmgBF₂)₂ nanosheets with abundant active sites on both the edges and modified basal planes were fabricated. Modifying the catalytically inert basal surfaces with electrochemically active cobaloxime boosted the HER performance. As a result, MoS₂–Co(dmgBF₂)₂ delivered a current density of 10 mA cm⁻² at a low overpotential of approximately 103 mV versus the reversible hydrogen electrode (RHE), a small Tafel slope of 45 mV dec⁻¹, and a high exchange current density of 64.5 μA cm⁻². We demonstrate a novel strategy to increase the number of active sites in 2D structured HER electrocatalysts and enhance their catalytic activity.

Experimental

All reagents, unless otherwise stated, were purchased from Sigma Aldrich, Alfa Aesar and Acros and used without further purification.

Synthesis of chemically exfoliated MoS₂ (CE-MoS₂)

Chemically exfoliated MoS₂ was prepared from bulk MoS₂ through chemical Li-intercalation and ultrasonic exfoliation according to a previously reported procedure with minor modification.¹⁷ Briefly, 1.5 mL of a 2.0 M n-butyl lithium solution in hexane were added to 1.5 g bulk MoS₂ powder under a nitrogen atmosphere. After addition of 10 mL of dry hexane, the dispersion was heated up to 25 °C and further stirred for 3 days. The mixture was then filtered and washed with hexane to remove the excess of n-butyl lithium. The intercalated powder was then re-dispersed in water at a concentration of 1 mg mL⁻¹ and sonicated to exfoliate for 1 h (300 W). After centrifugation to remove lithium cations as well as the non-exfoliated materials, the chemically exfoliated MoS₂ was produced (denoted as CE-MoS₂).

Synthesis of the 4-cyanobenzyl diazonium salt

The 4-cyanobenzyl diazonium salt that is used for the functionalization of CE-MoS₂ was prepared from 4-aminobenzonitrile. Typically, 4-aminobenzonitrile (1.50 g, 12.7 mmol) was dissolved in 40% HBF₄ (5.6 mL) in a round bottom flask by dropwise addition of a minimum amount of deionized water. The resulting solution was kept at 0 °C in an ice bath for 1 h. Then, NaNO₂ (0.88 g, 12.8 mmol) in deionized water (5 mL) was added dropwise to the above solution under vigorous stirring. The transparent solution changes to a suspension with the formation of the corresponding diazonium salt. The crude product was filtered and washed with plenty of diethyl ether. After air drying, fine white 4-cyanobenzyl diazonium salt (yield: 2.0 g, 9.2 mmol, 73%) was achieved.

Synthesis of 4-cyanobenzyl functionalized MoS₂ (MoS₂–PhCN)

4-Cyanobenzyl diazonium salt dissolved in deionized water was added to the dilute brownish CE-MoS₂ dispersion dropwise under exposure to light. After addition of only a few drops, a black precipitate was already formed. The reaction mixture was further stirred for 2 hours and filtered through a reinforced cellulose membrane filter (0.22 μm). Thorough washing was carried out with isopropanol and deionized water to remove organic side-products and impurities and 4-cyanobenzyl functionalized MoS₂ (denoted as MoS₂–PhCN) was produced after vacuum drying at 60 °C.

Synthesis of N,N′,N′′,N′′′-(tetrafluorodiborato)bis[μ-(2,3-butanedionedioximato)]cobalt(II)

N,N′,N′′,N′′′-(Tetrafluorodiborato)bis[μ-(2,3-butanedionedioximato)]cobalt(II) (denoted as Co(dmgBF₂)₂) was prepared according to a previously reported procedure.³³ The suspension resulting from the addition of BF₃·Et₂O (10 mL, 78.0 mmol) to Co(OAc)₂·4H₂O (2.0 g, 8.0 mmol) and dimethylglyoxime (1.9 g, 16.0 mmol) in diethyl ether (150 mL) was stirred at room temperature for 6 h and filtered. The solid Co(dmgBF₂)₂(H₂O)₂ was washed with ice-cold water and recrystallized from methanol (yield: 2.0 g, 65%). MS (EI) (provided in the ESI†): m/z calcd 384.75; found 408.02 (with Na⁺ and H⁺). IR: ν = 1621, 1445, 1387, 1209, 1165, 1093, 1008, 825 cm⁻¹; Atom Conc %: C, 61.23%; N, 15.73%; Co, 2.63%; B, 8.25%; F, 12.14%.

Coordination of Co(dmgBF₂)₂ on the surface of MoS₂–PhCN

Typically, MoS₂–PhCN (50 mg) was firstly dispersed in dry acetonitrile (10 mL). Dissolved oxygen was removed from the dispersion by bubbling nitrogen gas for 30 min. Then, the Co(dmgBF₂)₂ complex (100 mg) was added in one portion. The resulting mixture was stirred overnight under a nitrogen atmosphere. After centrifugation, the solid was washed with fresh acetonitrile (3 × 5 mL) and dried under vacuum. The Co(dmgBF₂)₂ coordinated MoS₂–PhCN (denoted as MoS₂–Co(dmgBF₂)₂) was obtained as a brown powder. IR: ν = 2231, 1642, 1621, 1400, 1387, 1084, 829, 627 cm⁻¹; Atom Conc %: Mo, 3.71%; S, 7.53%; C, 66.49%; N, 9.64%; Co, 1.58%; B, 5.05%; F, 6.01%.

Characterization

SEM measurements were performed on a FEI Sirion-200 field emission scanning electron microscope. Transmission electron microscopy (TEM) images were acquired using a Tecnai G2F20 S-TWIN transmission electron microscope (FEI) operated at 200 kV. The Raman spectra were obtained on Lab-RAM HR800 with excitation by an argon ion laser (532 nm). TGA of the samples was performed using a Q5000IR (TA Instruments, USA) thermogravimetric analyzer at a heating rate of 20 °C min⁻¹ under nitrogen flow. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI-5000C ESCA system; the C 1s value was set at 284.6 eV for charge corrections.

Electrochemical measurements

The electrochemical experiments for the HER were carried out in a conventional three-electrode cell using a CH Instrument (model 660D) at room temperature. A Ag/AgCl (3 M KCl) electrode and platinum wire were used as the reference and counter electrodes, respectively. For working electrodes, 2 mg of catalyst was blended with 200 μL of Nafion solution (0.05 wt%) and sonicated for 1 h, producing catalyst ink; then 9 μL of catalyst ink was pipetted onto the glassy carbon surface (0.2471 cm⁻²). The electrodes were dried at room temperature before measurement. The polarization curves were obtained in 0.5 M H₂SO₄ at a scan rate of 5 mV s⁻¹ at room temperature. All potentials in this study were referenced to a reversible hydrogen electrode (RHE) via calibration measurement in N₂-saturated electrolyte. EIS measurements were carried out from 1000 kHz to 0.02 Hz with an amplitude of 10 mV at the open-circuit voltage. The electrochemical double-layer capacitances (C_dl) of catalysts were calculated from CV curves. The CV curves were performed at scan rates varying from 20 to 200 mV s⁻¹ in the region from 0.10 to 0.20 V vs. the RHE. The capacitive currents of ΔJ (J_anodic − J_cathodic) are plotted as a function of the CV against the scan rate. The slope of the fitting line is equal to twice the C_dl, which is linearly proportional to the electrochemically active surface area of the electrode.

Results and discussion

The strategy for synthesizing MoS₂–Co(dmgBF₂)₂ is illustrated in Scheme 1. First, chemically exfoliated MoS₂ (CE-MoS₂) was prepared from bulk MoS₂ through Li intercalation and ultrasonic exfoliation. CE-MoS₂ can be dispersed in polar solvents (Fig. S1†), for example, water and dimethylformamide. Detailed chemical and morphological characterization is provided in Fig. S1–S4.† Then, CE-MoS₂ was functionalized with 4-cyanobenzyl diazonium salt in aqueous media to produce 4-cyanobenzyl-functionalized MoS₂ (denoted as MoS₂–PhCN). Subsequently, MoS₂–PhCN and N,N′,N′′,N′′′-(tetrafluorodiborato)bis[μ-(2,3-butanedionedioximato)]cobalt(II) (denoted as Co(dmgBF₂)₂) were dispersed in acetonitrile and stirred overnight in an inert atmosphere for preparing Co(dmgBF₂)₂-coordinated MoS₂ nanosheets (denoted as MoS₂–Co(dmgBF₂)₂), the successful functionalization of CE-MoS₂ was evidenced by several analytical techniques, the results are presented in Fig. S5–S12.† Co(dmgBF₂)₂ and MoS₂ are homogeneous³⁴ and heterogeneous⁷ catalysts, respectively, for the HER. This approach not only provides a new method to functionalize MoS₂ through coordination but also offers a new approach to integrate homogeneous and heterogeneous catalysts for the HER.

Scheme 1 Preparation of MoS₂–Co(dmgBF₂)₂. (i) Sonication: 1 h, 300 W. (ii) Deionized water, 0 °C, 2 h. (iii) Acetonitrile, nitrogen, 10 h.

The morphology of the materials was studied through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The TEM image of CE-MoS₂ (Fig. 1a) illustrates a typical nanosheet morphology with a large aspect ratio and size of up to 200 nm × 200 nm. The semitransparent and uniform morphology of CE-MoS₂ indicates the presence of ultrathin MoS₂ nanosheets with basal planes. After functionalization, MoS₂–PhCN and MoS₂–Co(dmgBF₂)₂ show similar wrinkled sheet morphologies (Fig. 1b–e) without obvious cobaloxime particles (Fig. 1f, uniform thickness: 6.0 nm, roughness: 0.5 nm), suggesting that Co(dmgBF₂)₂ was uniformly coordinated onto the MoS₂ surface. A distance of 1.62 ± 0.1 nm is determined for the interlayer spacing of MoS₂–Co(dmgBF₂)₂ (Fig. 1d), which is larger than that of 0.63 nm in most bare MoS₂ due to the functionalization.¹⁴

Fig. 1 TEM images of (a) CE-MoS₂ and (b) MoS₂–PhCN. (c) and (d) TEM, (e) SEM, and (f) AFM images of MoS₂–Co(dmgBF₂)₂.

The chemical structure of the functionalized MoS₂ was further analyzed through Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The transmittance peak at 2231 cm⁻¹ in the FTIR spectrum of MoS₂–PhCN (Fig. 2a) can be ascribed to the C≡N stretching vibration. The peak observed at approximately 630 cm⁻¹ can be assigned to the S–C stretching vibration, strongly suggesting that 4-cyanobenzyl was successfully anchored to the surface of MoS₂. After cobaloxime coordination, the peaks at 1093 and 1008 cm⁻¹, which originate from Co(dmgBF₂)₂, appear in the spectrum of MoS₂–Co(dmgBF₂)₂, indicating the presence of cobaloxime. In the Raman spectrum (Fig. 2b), bulk MoS₂ exhibits peaks at 377.9 and 404.9 cm⁻¹, which can be attributed to the in-plane optical vibration of the Mo–S bond in the E¹_2g mode and the out-of-plane optical vibration of the S atoms in the A¹_g mode, respectively.³⁵ The A¹_g mode of CE-MoS₂ blueshifts by approximately 2.5 cm⁻¹, while the E¹_2g mode redshifts by approximately 1.3 cm⁻¹, suggesting that CE-MoS₂ is ultrathin after exfoliation. However, the J₃ mode of CE-MoS₂ could not be observed for MoS₂–PhCN. Moreover, the E¹_2g and the A¹_g modes of MoS₂–PhCN show a slight blueshift and broadening because of covalent bonding between 4-cyanobenzyl and CE-MoS₂ after functionalization. No apparent peak changes between MoS₂–PhCN and MoS₂–Co(dmgBF₂)₂ were found, indicating that the coordination between cobaloxime and the cyano group has a very limited effect on MoS₂ nanosheets.

Fig. 2 a) FTIR spectra and (b) Raman spectra of CE-MoS₂, MoS₂–PhCN, Co(dmgBF₂)₂, and MoS₂–Co(dmgBF₂)₂. (c) TGA profiles. (d) XPS survey spectra of MoS₂–PhCN, Co(dmgBF₂)₂, and MoS₂–Co(dmgBF₂)₂.

Thermogravimetric analysis was performed to evaluate the stability of the materials. MoS₂–PhCN shows little mass loss below 220 °C and 20% mass loss in the temperature range of 220–500 °C (Fig. 2c). This result suggests that MoS₂–PhCN has good stability.³⁶ For Co(dmgBF₂)₂·2H₂O, 21.8% mass loss was observed before 180 °C. The mass loss of 32.9% in the temperature range of 250–350 °C can be ascribed to the decomposition and recombination of Co(dmgBF₂)₂. By contrast, MoS₂–Co(dmgBF₂)₂ shows 25% mass loss in the temperature range of 250–350 °C and only 7% mass loss in the temperature range of 100–180 °C, indicating the improved thermal stability of MoS₂–Co(dmgBF₂)₂.

XPS spectra were measured to confirm the chemical composition and valence states of the as-prepared samples. The XPS survey spectra of MoS₂–Co(dmgBF₂)₂ indicate the existence of C, Mo, S, N, Co, F, B, and O (Fig. 2d and S13†). The Mo 3d spectrum (Fig. 3a) shows two pairs of peaks (Mo 3d_5/2 and 3d_3/2) at binding energies of 235.0 and 232.6 eV and 231.3 and 228.1 eV, which can be ascribed to surface-oxidized MoO_x and MoS₂, respectively. The peaks of Mo⁴⁺ 3d_5/2 and 3d_3/2 are split into the 1T phase and 2H phase, suggesting the presence of both phases. In comparison with the S 2p core level spectrum of CE-MoS₂ (Fig. S14b†), peaks at 163.7 eV and 162.7 eV were observed for both MoS₂–PhCN (Fig. S14d†) and MoS₂–Co(dmgBF₂)₂ (Fig. 3b), because functionalization gives rise to a component at higher binding energies in the S 2p core level spectra.¹⁷ In the N 1s spectrum of MoS₂–Co(dmgBF₂)₂ (Fig. 3c), a nitrogen species at the binding energy of 400.6 eV is evident and can be assigned to cyano nitrogen, which coordinates with the cobalt centers of Co(dmgBF₂)₂. The peak at 399.8 eV can be assigned to the CoN₄ in Co(dmgBF₂)₂. The Co 2p_3/2 and 2p_1/2 peaks shift from 780.8 and 795.8 eV to 778.9 and 794.0 eV after coordination between Co(dmgBF₂)₂ and the cyano group (Fig. 3d), providing additional evidence of successful coordination. Moreover, the percentage of Co(dmgBF₂)₂ coordination on MoS₂–PhCN was calculated according to the intensity ratios of Co and N (Table S1†). The results revealed that 47.6% of the cyano groups are coordinated to Co(dmgBF₂)₂. The molar ratios of Mo/S/Co of MoS₂–PhCN and MoS₂–Co(dmgBF₂)₂ were determined through inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis (Table S2†). The ICP-AES measurements revealed that the molar ratio of Mo/S/Co is 1 : 1.79 : 0.24. According to this result, MoS₂–Co(dmgBF₂)₂ contains approximately 3.78 wt% Co.

Fig. 3 (a) Mo 3d, (b) S 2p, (c) N 1s, and (d) Co 2p XPS spectra of MoS₂–Co(dmgBF₂)₂ and Co(dmgBF₂)₂.

Both Co(dmgBF₂)₂ and MoS₂ have been widely studied as electrocatalysts for the HER. In this work, the performance of the as-prepared MoS₂–Co(dmgBF₂)₂ in HER electrocatalysis was examined in 0.5 M H₂SO₄ by using a typical three-electrode system. The polarization curves of the samples are shown in Fig. 4a, along with the benchmark Pt/C as a reference. The MoS₂–Co(dmgBF₂)₂ shows an onset potential (P_onset) of 41 mV, which is much lower than those of CE-MoS₂ (132 mV) and MoS₂–PhCN (81 mV) (Table 1). MoS₂–Co(dmgBF₂)₂ can reach a current density of 10 mA cm⁻² at 103 mV, which is much lower than those of CE-MoS₂ (296 mV) and most reported MoS₂-based HER electrocatalysts (Table S3†). The Tafel slope is an experimental kinetic parameter often used for evaluating HER electrocatalysts. The linear regions of the Tafel plots can be fitted by using the Tafel equation η = a + b log(j), where η is the overpotential, a is the Tafel constant, b is the Tafel slope, and j is the current density.³⁷ The Tafel plots shown in Fig. 4b have a small Tafel slope of 44.8 mV dec⁻¹ for MoS₂–Co(dmgBF₂)₂, which is also much lower than those for MoS₂–PhCN (119.7 mV dec⁻¹) and CE-MoS₂ (113.5 mV dec⁻¹), suggesting that the kinetics of the water molecule dissociation step are efficiently facilitated on the surface of MoS₂–Co(dmgBF₂)₂. According to the classic theory of the HER in acidic media, the Volmer–Heyrovsky HER mechanism is responsible for catalysis of the electrochemical HER by MoS₂–Co(dmgBF₂)₂, and electrochemical recombination with an additional proton is the rate-limiting step.^38,39 All these results demonstrate that the as-prepared MoS₂–Co(dmgBF₂)₂ exhibits superior HER electrocatalysis performance in 0.5 M H₂SO₄ compared with the reported MoS₂-based electrocatalysts (Fig. 4c). The over-potential and Tafel slope from repeat samples are provided in the ESI (Fig. S15 and Table S4†).

Fig. 4 HER performance of MoS₂–Co(dmgBF₂)₂ in 0.5 M H₂SO₄. (a) Polarization curves, (b) corresponding Tafel plots, (c) cyclic voltammogram (CV) curves obtained at a scan rate of 200 mV s⁻¹ in the region of 0.1–0.2 V versus the RHE, and (d) capacitive current at 0.15 V as a function of scan rate for the as-prepared materials in 0.5 M H₂SO₄. (e) Nyquist plots of different samples over the frequency range of 1000 kHz to 0.01 Hz at an overpotential of 103 mV versus the RHE. (f) Comparison with state-of-the-art MoS₂-based HER electrocatalysts.

Table 1 Catalytic HER performance of samples in 0.5 M H₂SO₄^a

Catalysts	P onset (mV)	η 10 (mV)	Tafel slope (mV dec^−1)	J 0 (μA cm^−2)	C dl (mF cm^−2)
a P onset: onset potential, η10: overpotential required to reach the current density of 10 mA cm^−2, J0: exchange current density, and Cdl: double-layer capacitance.
CE-MoS2	132	296	113.54	29.78	0.9
MoS2–PhCN	81	251	119.68	97.00	0.8
MoS2–Co(dmgBF2)2	41	103	44.77	64.49	11.5
Pt/C	0	26	34.99	147.2	n/a

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To assess the effective active areas of MoS₂–Co(dmgBF₂)₂, a series of cyclic voltammetry measurements were performed at scan rates varying from 20 to 200 mV s⁻¹ in the region from 0.10 to 0.20 V versus the RHE (Fig. 4d and S16†). To more clearly understand the origin of the enhanced electrocatalytic performance, we compared the electrochemically active surface area of MoS₂–Co(dmgBF₂)₂, MoS₂–PhCN, and CE-MoS₂ by measuring the double-layer capacitance.^40,41 The double-layer capacitance (Fig. 4e) of MoS₂–Co(dmgBF₂)₂ was calculated to be 11.5 mF cm⁻², which is approximately 14 times and 13 times higher than those of MoS₂–PhCN (0.8 mF cm⁻²) and CE-MoS₂ (0.9 mF cm⁻²), respectively, indicating the presence of considerably more active sites in MoS₂–Co(dmgBF₂)₂. In addition, electrochemical impedance spectroscopy analysis (Fig. 4f) was performed to provide further insight into the electrode kinetics. Fig. 4f shows the obtained Nyquist plots, which could be fitted by using an equivalent circuit (Fig. S17, and Table S5†). The charge transfer resistance (R_ct) at the material/electrolyte interface is usually used to probe the electrocatalytic activity.⁴² The R_ct value of 443 Ω for MoS₂–Co(dmgBF₂)₂ was lower than those for MoS₂–PhCN (880 Ω) and CE-MoS₂ (1247 Ω), indicating a faster faradaic process and superior HER kinetics. The lower R_ct can probably be attributed to the modulation of the electronic structure through surface modification and immobilization of cobaloxime. Stability is another crucial parameter reflecting durable operation of HER electrocatalysts. For a current density–time (i–t) test, the working electrode was operated at η = 110 mV for 9 h, while the current density was continuously monitored (Fig. S18†), demonstrating the excellent durability of MoS₂–Co(dmgBF₂)₂. All of the preceding results demonstrate that MoS₂–Co(dmgBF₂)₂ exhibits superior HER activity as well as excellent stability.

Based on the preceding results and discussion, the superior electrocatalytic activity of MoS₂–Co(dmgBF₂)₂ for the HER can be attributed to the following aspects: (i) ultrathin MoS₂ nanosheet templates have good conductivity; (ii) owing to the immobilization of cobaloxime on the catalytically inert basal planes, abundant active sites from both the edges and the modified basal planes are formed; (iii) moderate interaction between cobaloxime and H₂O makes the initial Volmer step and Heyrovsky steps easy, thus reducing the energy barrier for the HER; (iv) surface modification and the expanded interlayer can be used to further optimize the electronic structure; and (v) synergistic effects between cobaloxime and MoS₂ may contribute to improved HER performance in acidic media.

Conclusions

In summary, we synthesized cobaloxime-immobilized MoS₂ nanosheets via the coordination of cobalt on 4-cyanobenzyl-functionalized MoS₂ nanosheet templates. The as-prepared material exhibited excellent catalytic activity for the HER in acidic media. By immobilizing cobaloxime on the basal planes of MoS₂, we modified the catalytically inert basal surfaces, improving the performance. Research efforts along these directions could offer opportunities to develop surface-modified MoS₂ or other 2D structures that are comparable in performance to the most effective noble metal catalysts (such as Pt for the HER) available today.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China for Excellent Youth Scholars (51722304), 973 Program of China (2013CBA01602), National Natural Science Foundation of China (61306018, 21574080, 21774072, 21720102002), Shanghai Committee of Science and Technology (15JC1490500, 16JC1400703), German Research Foundation (DFG) within the Cluster of Excellence “Center for Advancing Electronics Dresden” (cfaed), EU Graphene Flagship, and Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment (SKLPEE-KF201702, Fuzhou University).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta08684a

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