Engineering water dissociation sites in MoS₂ nanosheets for accelerated electrocatalytic hydrogen production

Abstract

Earth-abundant MoS₂ is widely reported as a promising HER electrocatalyst in acidic solutions, but it exhibits extremely poor HER activities in alkaline media due to the slow water dissociation process. Here we present a combined theoretical and experimental approach to improve the sluggish HER kinetics of MoS₂ electrocatalysts through engineering the water dissociation sites by doping Ni atoms into MoS₂ nanosheets. The Ni sites thus introduced can effectively reduce the kinetic energy barrier of the initial water-dissociation step and facilitate the desorption of the ⁻OH that are formed. As a result, the developed Ni-doped MoS₂ nanosheets (Ni-MoS₂) show an extremely low HER overpotential of ∼98 mV at 10 mA cm⁻² in 1 M KOH aqueous solution, which is superior to those (>220 mV at 10 mA cm⁻²) of reported MoS₂ electrocatalysts.

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Broader context

Enhancing the sluggish kinetics of the electrocatalytic hydrogen evolution reaction (HER) in water–alkali electrolyzers is pivotal for large-scale and sustainable hydrogen production. Earth-abundant MoS₂ is widely reported as a promising HER electrocatalyst in acidic solutions, but it exhibits extremely poor HER activities in alkaline media due to the slow water dissociation process. Here we present a combined theoretical and experimental approach to improve the sluggish HER kinetics of MoS₂ electrocatalysts through engineering the water dissociation sites by doping Ni atoms into MoS₂ nanosheets. The Ni sites thus introduced can effectively reduce the kinetic energy barrier of the initial water-dissociation step and facilitate the desorption of the ⁻OH intermediates that are formed. As a result, the developed Ni-doped MoS₂ nanosheets (Ni-MoS₂) show a highly competitive HER performance as compared to other state-of-the-art HER electrocatalysts. Therefore, this work opens up a favorable direction for exploring efficient and robust water-splitting electrocatalysts.

With its high energy density and environmentally friendly advantages, molecular hydrogen has been widely regarded as one of the most promising energy carriers.¹ The scalable and sustainable production of hydrogen fuel through efficient and cost-effective electrocatalytic/photocatalytic/photoelectrocatalytic water splitting technologies, e.g., water–alkali and chlor-alkali electrolyzers, is highly promising as a means to meet the future global energy demands.² To this end, active, durable, and earth-abundant electrocatalysts are essential to lower the kinetic overpotentials of the hydrogen evolution reaction (HER) and ultimately accelerate hydrogen production in alkaline solutions.³ Platinum (Pt) has been acknowledged as the most active and stable HER electrocatalyst with a near-zero onset overpotential.⁴ Unfortunately, the large-scale utilization of Pt catalysts in H₂-production electrolyzers is seriously hampered by its scarcity and cost.

In regard to its elemental abundance, high activity, and electrochemical stability, molybdenum disulfide (MoS₂) is a promising catalyst for the electrocatalytic and photocatalytic HER.⁵ Recently, both density functional theory (DFT) calculations and experimental results have demonstrated that the electrocatalytic HER activity of crystalline MoS₂ catalysts originates from the unsaturated Mo–S sites along the edges.⁶ Inspired by this fundamental understanding, extensive efforts have been dedicated to increasing the number of exposed active sites on MoS₂ catalysts by engineering the nanostructures, e.g., double-gyroid mesoporous MoS₂ films,⁶ vertically aligned MoS₂ films,⁸ defect-rich MoS₂ nanosheets,⁹ amorphous MoS_x films,¹⁰ [Mo₃S₁₃]²⁻ clusters,¹¹ and CoS_x/MoS_x hybrids.¹² Unfortunately, although the MoS₂-based electrocatalysts thus developed exhibit enhanced HER activities in acidic solutions, the HER kinetics in alkaline electrolytes still suffer from a high overpotential (>220 mV at a current of 10 mA cm⁻²).¹² The high kinetic energy barrier of the initial water dissociation process (the Volmer step) and the strong adsorption of the formed ⁻OH on the surfaces of MoS₂ are responsible for the sluggish HER kinetics in alkaline solutions.¹³

Herein, we demonstrate a novel strategy to efficiently speed up the sluggish HER kinetics of MoS₂ electrocatalysts through doping Ni atoms into crystalline MoS₂ nanosheets. The DFT calculations reveal the fact that the kinetic energy barrier of the initial water dissociation step and the adsorption interaction of ⁻OH are substantially reduced on Ni-doped MoS₂ catalysts (Ni-MoS₂). Accordingly, Ni-MoS₂ nanosheets with a chemical composition of Ni_0.13Mo_0.87S₂ were prepared on carbon cloth via a one-pot hydrothermal reaction. The resultant Ni-MoS₂ nanosheets exhibit an excellent electrochemical HER activity in 1 M KOH aqueous solution with an extremely low overpotential of ∼98 mV at a current density of 10 mA cm⁻². The achieved overpotential is much lower than those of reported MoS₂ electrocatalysts (overpotential is >220 mV at 10 mA cm⁻² in basic solutions and ≥110 mV at 10 mA cm⁻² in acidic solutions).^12,14

The kinetic energy barriers of the prior water dissociation step (ΔG(H₂O), Volmer step), the Gibbs free energy of adsorbed ⁻OH (G(OH)), and the concomitant combination of H* intermediates into molecular hydrogen (ΔG(H), Tafel step) were firstly investigated using the DFT calculations according to the as-built catalyst models including MoS₂, Ni-MoS₂, Co-doped MoS₂ (Co-MoS₂), and Fe-doped MoS₂ (Fe-MoS₂) (Fig. S1, ESI†). As shown in Fig. 1 and Table S1 (ESI†), MoS₂ exhibits a very high ΔG(H₂O) up to 1.17 eV and an extremely low G(OH) (−5.24 eV). Substituting a Mo atom with a metal (Ni, Co, or Fe) atom along the edge of MoS₂ dramatically decreases the ΔG(H₂O) value in the order of: Ni-MoS₂ (ΔG(H₂O) = 0.66 eV) < Co-MoS₂ (ΔG(H₂O) = 0.76 eV) < Fe-MoS₂ (ΔG(H₂O) = 0.96 eV). In contrast to MoS₂, the G(OH) value is reduced to −3.46 eV for Ni-MoS₂, −3.46 eV for Co-MoS₂, and −3.36 eV for Fe-MoS₂. These greatly reduced ΔG(H₂O) and G(OH) values on Ni-MoS₂ suggest that the kinetics of the initial water dissociation step and the concomitant desorption of the formed ⁻OH can be effectively promoted after the doping of a Ni atom into the edge of MoS₂. In addition, the ΔG(H) is −0.06 eV for Ni-MoS₂, 0.13 eV for Co-MoS₂, and −0.10 eV for Fe-MoS₂, which are much lower than 0.60 eV for MoS₂. The negative value of ΔG(H) for Ni-MoS₂ catalysts shows that the subsequent Tafel step towards molecular hydrogen can spontaneously occur in thermodynamics.

Fig. 1 The results of the DFT calculations and the corresponding mechanisms of the electrocatalytic HER on the surfaces of different catalysts under alkaline conditions. ΔG(H₂O) and ΔG(H) are the related kinetic energy barriers for the Volmer and Tafel steps on the catalysts, respectively. G(OH) is the Gibbs free energy of the adsorbed ⁻OH on the surfaces of catalysts. E (eV) in the diagram represents the free energies of the different reactive stages. The yellow, blue and red spheres represent S, Mo and Ni, respectively.

Encouraged by these DFT results, we prepared metal-doped MoS₂ nanosheets (M-MoS₂), where the metal (Ni, Co, or Fe) atoms are homogeneously doped into the crystalline MoS₂ nanosheets, as schematically illustrated in Fig. S2 (ESI†). Specifically, the Ni-MoS₂ nanosheets were constructed on carbon cloth (1 × 3 cm²) through a one-pot hydrothermal reaction at 200 °C for 24 h, involving NiSO₄·6H₂O, Na₂MoO₄·2H₂O, and L-cysteine in 15 mL deionized water. The molar content (x, expressed in Ni_xMo_1−xS₂) of Ni in the as-obtained Ni-MoS₂ nanosheets could be tuned from 6.2% to 19.1% by adjusting the dosage of NiSO₄·6H₂O. The loading weight of Ni-MoS₂ nanosheets on the carbon cloth was approximately 0.89 mg cm⁻². Under the same hydrothermal conditions, Co-MoS₂ (Co_0.03Mo_0.97S₂) and Fe-MoS₂ (Fe_0.12Mo_0.88S₂) nanosheets on carbon cloth were also prepared utilizing CoSO₄·7H₂O and FeSO₄·7H₂O as Co and Fe sources, respectively. For comparison, pristine MoS₂ nanosheets were synthesized through the same process without involving NiSO₄·6H₂O.

The crystalline structure of the M-MoS₂ nanosheets was first confirmed by X-ray diffraction (XRD) measurements. As shown in Fig. S3 (ESI†), the Ni-MoS₂ nanosheets show the diffraction peaks at diffraction angles similar to semiconducting MoS₂.¹⁵ To probe the morphologies of the as-obtained Ni-MoS₂, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) were employed. Fig. 2a and Fig. S4 (ESI†) reveal numerous sheet-like nanostructures, which are vertically aligned and inter-connected on the carbon cloth. The thickness and length of the Ni-MoS₂ nanosheets are approximately 5–10 nm and 40–100 nm, respectively. Elemental mappings of field-emission SEM (FE-SEM) reveal the homogenous distributions of Ni, Mo, and S elements over the Ni-MoS₂ nanosheets (Fig. S5a–e, ESI†). The corresponding energy dispersive X-ray spectroscopy (EDS) analysis further suggests a chemical elemental composition of Ni_0.13Mo_0.87S₂ (Fig. S5f, ESI†). Fig. 2b and c show the HRTEM images of the Ni-MoS₂ nanosheets. Lattice fringes with lattice distances of 0.75 and 0.28 nm correspond to the (002) edge and (100) plane facets of the Ni-MoS₂ nanosheets, respectively.¹⁶ Scanning TEM (STEM)-EDS characterization was utilized to analyze the elemental distributions in the Ni-MoS₂ nanosheets (Fig. 2d). Apparently, the Ni atoms are homogeneously distributed in the Ni-MoS₂ nanosheets. Similarly, the morphologies and chemical compositions of the as-prepared MoS₂, Co-MoS₂, and Fe-MoS₂ nanosheets were also investigated by SEM, XRD and EDS analyses (Fig. S6–S8, ESI†).

Fig. 2 Morphology and structural characterization of the Ni-MoS₂ catalysts. (a) SEM and (b and c) HRTEM images of the Ni-MoS₂ nanosheets; (d) corresponding STEM-EDS chemical mappings of Ni, Mo, and S elements in the Ni-MoS₂ nanosheets; high-resolution XPS spectra of (e) Ni 2p, (f) Mo 3d, and (g) S 2p in the MoS₂ and Ni-MoS₂ nanosheets. The insets in (a and b) are the low-magnification SEM image of the Ni-MoS₂ nanosheets coated on the carbon cloth and the HRTEM image of the edges of the Ni-MoS₂ nanosheets, respectively.

X-ray photoelectron spectroscopy (XPS) was applied to probe the composition and valence state of the Ni-MoS₂ nanosheets. The survey spectrum demonstrates a chemical composition of Ni_0.12Mo_0.88S₂, which is consistent with inductively coupled plasma mass spectrometry (ICP-MS) analysis (Ni_0.13Mo_0.87S₂) (Fig. S9–S11, ESI†). The peaks of Ni 2p_3/2 and Ni 2p_1/2 were observed at 857.9 eV and 876.0 eV, respectively (Fig. 2e). The binding energies of Mo 3d_5/2 and Mo 3d_3/2 in the Ni-MoS₂ nanosheets shifted to 228.8 and 232.1 eV, respectively (Fig. 2f), in contrast to those (Mo 3d_5/2 at 228.5 and Mo 3d_3/2 at 231.8 eV) in the pristine MoS₂ nanosheets. Likewise, the S 2p_3/2 and S 2p_1/2 signals in the Ni-MoS₂ also have a shift of ∼0.3 eV, relative to those in the MoS₂ nanosheets (Fig. 2g and Fig. S9, ESI†). Raman spectroscopy was further used to survey the Ni-MoS₂ nanosheets. The characteristic Raman bands of the A_1g and E¹_2g modes of Ni-MoS₂ shifted to 406 and 376 cm⁻¹, relative to the MoS₂ nanosheets (A_1g at 412 cm⁻¹; E¹_2g at 388 cm⁻¹) (Fig. S12, ESI†). These XPS and Raman results highlight the strong influence of Ni atom doping on the electronic structure of MoS₂.

To evaluate the electrocatalytic HER activities of the catalysts, a three-electrode configuration in Ar-saturated 1 M KOH aqueous solution was applied using a Hg/HgO electrode and a graphite rod as the reference and counter electrodes, respectively (Fig. S13, ESI†). All potentials are referenced to the reversible hydrogen electrode (RHE) and the ohmic potential drop caused by the solution resistance has been deducted unless noted (Fig. S14, ESI†). As shown in Fig. 3a, although MoS₂ can act as a HER catalyst, the hydrogen evolution reaction occurred at an overpotential of ∼197 mV and the cathodic current density reached 10 mA cm⁻² at a high overpotential of ∼308 mV. Noticeably, the doping of Ni, Co, or Fe atoms into MoS₂ nanosheets leads to profound enhancements of the HER activities. Specifically, Co-MoS₂ and Fe-MoS₂ catalysts show overpotentials of only 203 and 163 mV at a current density of 10 mA cm⁻², respectively. Remarkably, the onset overpotential of Ni-MoS₂ catalysts was as low as 45 mV and a current density of 10 mA cm⁻² was delivered at an extremely low overpotential of ∼98 mV, which is much lower than those of the as-prepared MoS₂ catalysts (∼308 mV at 10 mA cm⁻²) and the reported MoS₂-based catalysts (regardless of whether in basic and acidic solutions) including amorphous MoS_x film (∼500 mV at 4 mA cm⁻²),¹⁰ MoS₂ nanoparticles grown on graphene (∼155 mV at 10 mA cm⁻²),¹⁷ defect-rich MoS₂ nanosheets (∼190 mV at 10 mA cm⁻²),⁹ double-gyroid mesoporous MoS₂ films (∼235 mV at 10 mA cm⁻²),⁷ Li-MoS₂ films (∼168 mV at 10 mA cm⁻²),¹⁸ and CoS_x/MoS₂ hybrids (∼220 mV at 5 mA cm⁻²)¹² (Table S2, ESI†). Moreover, the HER overpotential achieved by the Ni-MoS₂ catalysts is comparable to those of highly active NiO/Ni heterostructures (∼80 mV at 10 mA cm⁻²),¹⁹ CoP nanowires on carbon cloth (∼209 mV at 10 mA cm⁻²),²⁰ porous MoC_x nano-octahedra (∼151 mV at 10 mA cm⁻²),²¹ cobalt–sulfide films (∼180 mV at 10 mA cm⁻²),²² and CoO/Co/N-doped carbon hybrids (∼232 mV at 10 mA cm⁻²)²³ (Table S3, ESI†).

Fig. 3 Electrochemical HER measurements of different catalysts. (a) Polarization curves and (b) corresponding Tafel slopes of the MoS₂, Ni-MoS₂, Co-MoS₂, Fe-MoS₂, and commercial Pt/C catalysts; (c) CV stability and (d) 100 h operating durability of the Ni-MoS₂ catalysts in 1 M KOH aqueous solution. Scan rate: 1 mV s⁻¹.

Fig. 3b displays Tafel plots of the corresponding polarization curves, which provide further insights into the HER reaction pathways on the surfaces of the catalysts. The Tafel slope of the MoS₂ catalysts is as high as 201 mV per decade. However, the Tafel slope of the Ni-MoS₂ catalysts is significantly decreased to 60 mV per decade. Compared with the MoS₂ catalysts, the greatly decreased Tafel slope highlights that the kinetics of the water dissociation step is effectively facilitated on the Ni-MoS₂ catalysts. On the basis of the Tafel analysis, the exchange current density of the Ni-MoS₂ catalysts was estimated to be ∼0.98 mA cm⁻² (Fig. S15, ESI†). Meanwhile, the turnover frequency (TOF) of the Ni-MoS₂ catalysts was up to 0.32 s⁻¹ at an overpotential of 150 mV (Fig. S16, ESI†). In addition, the electrochemical impedance spectroscopy (EIS) analyses also confirmed a faster HER kinetic process on the Ni-MoS₂ catalysts than on the MoS₂ catalysts (Fig. S17, ESI†).

Electrocatalytic stability is another important criterion for the HER catalysts. For the Ni-MoS₂ catalysts, after 2000 cyclic voltammetry (CV) cycles in 1 M KOH aqueous solution, the overpotential required for a current density of 10 mA cm⁻² increased by only 5 mV (Fig. 3c). A long-term HER process was performed at a current density of 10 mA cm⁻² (Video S1, ESI†). Fig. 3d manifests that the Ni-MoS₂ catalysts retained a steady HER activity and no noticeable increase in potential was observed for hydrogen production over a period of 100 h. After the above HER durability assessment, the structural information of the Ni-MoS₂ catalysts was scrutinized using SEM. The morphology of the Ni-MoS₂ catalysts showed no structural variations, suggesting their superior structural stability during the HER process (Fig. S18, ESI†). The electrochemical stability of the Ni-MoS₂ catalysts was further confirmed by element mapping, EDX, and XPS analysis (Fig. S19 and S20, ESI†).

To clarify the influence of the active surface area on the electrocatalytic HER activity, the active surface areas of the as-synthesized catalysts were analyzed through their electrochemical double layer capacitances (C_p).²⁴ For comparison, the C_p of the MoS₂ catalysts was approximately 0.54 F. However, the Ni-MoS₂, Co-MoS₂, and Fe-MoS₂ catalysts showed low C_p values of ∼0.35 F, 0.32 F, and 0.27 F, respectively (Fig. 4a and Fig. S21, ESI†). These results demonstrate that the excellent HER activity of the Ni-MoS₂ catalysts originates from the improved HER kinetics, rather than the active surface area. As illustrated in the reported volcano plots, under acidic conditions, the HER kinetics of a catalyst is strongly correlated with its hydrogen adsorption ability.^6,25 To further understand the roles of hydrogen adsorption and water dissociation in electrocatalytic HER kinetics of the Ni-MoS₂ catalysts, the electrochemical HER activities of the catalysts were tested under acidic conditions. As shown in Fig. 4b and c, in comparison with the MoS₂ catalysts, the HER overpotential of the Ni-MoS₂ catalysts at 10 mA cm⁻² was decreased by only 43 mV in 0.5 M H₂SO₄ aqueous solution, which was far less than 209 mV in 1 M KOH aqueous solution. These studies clearly manifest that the greatly enhanced HER activity of the Ni-MoS₂ catalysts in 1 M KOH aqueous solution is mainly attributed to the initially accelerated water dissociation, rather than the hydrogen adsorption properties.

Fig. 4 (a) Electrochemical double layer capacitances of the as-achieved catalysts in 1 M KOH aqueous solution; (b) polarization curves of the catalysts in 0.5 M H₂SO₄ aqueous solution and (c) the alterations of the HER overpotentials of the catalysts under acidic and basic conditions.

We also investigated a series of Ni-MoS₂ catalysts with different molar contents of Ni (Ni-MoS_2−x: x is expressed in Ni_xMo_1−xS₂) in 1 M KOH aqueous solution (Fig. S22, ESI†). As shown in Fig. S23 (ESI†), the HER polarization curves of the Ni-MoS₂ catalysts dramatically shifted towards lower overpotentials along with the increased Ni content. When the molar content of Ni was ∼13.3%, the Ni-MoS₂ catalysts exhibited the lowest HER overpotential (∼98 mV at 10 mA cm⁻²). The overpotential increased if the molar content of Ni was more than 13.3% and even the overpotential of the NiS₂ catalysts at 10 mA cm⁻² was approximately 201 mV.

In summary, we have demonstrated a novel strategy to greatly accelerate the sluggish HER kinetics of MoS₂ electrocatalysts through engineering the water dissociation sites in alkaline environments. The combined DFT and experimental results show that the doping of Ni atoms into crystalline MoS₂ nanosheets can efficiently lower the kinetic energy barrier of the initial water dissociation step and facilitate the desorption of the formed ⁻OH from the surface of the Ni-MoS₂ catalysts. Therefore, this work opens up a favorable direction for exploring efficient and robust water-splitting catalysts, which have promising applications in alkali electrolyzers and solar-driven photocatalytic/photoelectrocatalytic devices.

Acknowledgements

This work was financially supported by an ERC grant on 2DMATER and EC under Graphene Flagship (No. CNECT-ICT-604391). We also acknowledge the Cfaed (Center for Advancing Electronics Dresden) and the Dresden Center for Nanoanalysis (DCN) at TU Dresden. P. L. and M. C. were supported by JST-CREST “Phase Interface Science for Highly Efficient Energy Utilization”, Japan Science and Technology Agency.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ee01786j

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