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

Jian Zhang a, Tao Wang b, Pan Liu cd, Shaohua Liu a, Renhao Dong a, Xiaodong Zhuang a, Mingwei Chen cd and Xinliang Feng *a
aCenter for Advancing Electronics Dresden (cfaed) & Department of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany. E-mail: xinliang.feng@tu-dresden.de
bUniversité Lyon, Ens de Lyon, CNRS, Université Lyon 1, Laboratoire de Chimie, UMR 5182, F-69342, Lyon, France
cWPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
dCREST, JST, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan

Received 21st June 2016 , Accepted 19th July 2016

First published on 19th July 2016


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.



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 MoSx films,10 [Mo3S13]2− clusters,11 and CoSx/MoSx 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 Ni0.13Mo0.87S2 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-MoS2G(H2O) = 0.66 eV) < Co-MoS2G(H2O) = 0.76 eV) < Fe-MoS2G(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.


image file: c6ee01786j-f1.tif
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 NixMo1−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 (Co0.03Mo0.97S2) and Fe-MoS2 (Fe0.12Mo0.88S2) 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 Ni0.13Mo0.87S2 (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).


image file: c6ee01786j-f2.tif
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 Ni0.12Mo0.88S2, which is consistent with inductively coupled plasma mass spectrometry (ICP-MS) analysis (Ni0.13Mo0.87S2) (Fig. S9–S11, ESI). The peaks of Ni 2p3/2 and Ni 2p1/2 were observed at 857.9 eV and 876.0 eV, respectively (Fig. 2e). The binding energies of Mo 3d5/2 and Mo 3d3/2 in the Ni-MoS2 nanosheets shifted to 228.8 and 232.1 eV, respectively (Fig. 2f), in contrast to those (Mo 3d5/2 at 228.5 and Mo 3d3/2 at 231.8 eV) in the pristine MoS2 nanosheets. Likewise, the S 2p3/2 and S 2p1/2 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 A1g and E12g modes of Ni-MoS2 shifted to 406 and 376 cm−1, relative to the MoS2 nanosheets (A1g at 412 cm−1; E12g 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 MoSx 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 CoSx/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 MoCx 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).


image file: c6ee01786j-f3.tif
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 (Cp).24 For comparison, the Cp of the MoS2 catalysts was approximately 0.54 F. However, the Ni-MoS2, Co-MoS2, and Fe-MoS2 catalysts showed low Cp 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.


image file: c6ee01786j-f4.tif
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-MoS2−x: x is expressed in NixMo1−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

This journal is © The Royal Society of Chemistry 2016