Introduction

Owing to their diffuse nature, electricity from renewable but intermittent energy (for example, solar and wind) must be stored durably for off-grid applications. Electrochemical water splitting to produce hydrogen (H2) offers a promising and sustainable solution for this purpose by converting such electricity energy into stable chemical bonds1,2. Appropriate electrocatalysts, such as platinum (Pt) and its alloys, play a vital role in the H2 evolution reaction (HER) because they can catalyse the conversion from a pair of protons and electrons to H2 at high reaction rates and low overpotentials (η)1,2,3. However, the prohibitive cost and scarcity of Pt pose tremendous limitations to widespread use. Therefore, finding robust and efficient alternative catalysts that are geologically abundant is crucial to the future of ‘hydrogen economy’.

Molybdenum disulfide (MoS2), a widely used industrial catalyst for hydrodesulfurization4, has recently demonstrated promise as effective HER catalyst based on both computational and experimental studies5,6. The HER activity was discovered to arise from the exposed (10–10) planes on edges of MoS2, whereas the (0001) basal planes are catalytically inactive5,6,7. This understanding has led to great efforts to develop highly nanostructured MoS2-based HER catalysts to maximize the number of edge sites, including crystalline8,9,10,11,12 and amorphous materials13,14,15, MoS2-based hybrid materials16,17,18,19 and molecular mimics7. Despite significant success, the design and fabrication of MoS2-based HER electrocatalysts with satisfactory activity and stability remain a big challenge.

In recent years, we have been making efforts to explore efficient electrocatalysts by using Earth-abundant 3d metal (Co, Ni and so on) chalcogenides20,21,22,23,24,25,26,27. New NiSe nanofibres24 and lamellar mesostructured CoSe2/DETA (DETA, diethylenetriamine) nanobelts25 were found to show decent HER activity in acidic electrolyte. Kong et al.28 also observed good HER activities from various polycrystalline transition metal dichalcogenide (ME2, M=Fe, Co, Ni; E=S, Se) films, especially from the CoSe2. Recently, we found that the HER activity of CoSe2 nanobelts can be improved greatly after anchoring Ni/NiO nanoparticles onto their surfaces25. The synergetic chemical coupling effects between CoSe2 and grafted Ni/NiO were believed to contribute to the enhancement. Similar promoted performances have also been observed on MoS2/graphene16,17, MoO3/MoS2 (ref. 18) and MoS2/Au (ref. 19) composite catalysts for H2 production. These works point to the possibility to access new and efficient HER catalysts by combining the promising CoSe2 and MoS2.

We report here that a HER electrocatalyst based on quasi-amorphous MoS2-coated CoSe2 (denoted as MoS2/CoSe2) hybrid is highly active and stable in acidic electrolyte. Notably, without any noble metals, the MoS2/CoSe2 hybrid catalyst shows an onset potential close to commercial Pt catalyst (Johnson-Matthey, 20 wt% Pt/XC-72) and a small Tafel slope of ~36 mV per decade as well as no current loss after long-term chronoamperometry measurement, performing the best among the noble-metal-free HER electrocatalysts. These results suggest a strategy for designing non-noble metal catalysts with enhanced HER performance that is comparable to the state-of-the-art Pt-based catalysts.

Results

MoS2/CoSe2 hybrid catalyst

The MoS2/CoSe2 hybrid was prepared directly in a closed N,N-dimethylformamide (DMF)/hydrazine solvothermal system, where (NH4)2MoS4 was used as a precursor for growing MoS2 around the freshly made CoSe2/DETA nanobelt substrates (Fig. 1a; Supplementary Fig. 1; see Methods for details of the synthesis). The MoS2-coated CoSe2 hybrid was shown by means of scanning electron microscopy and transmission electron microscopy (TEM; Fig. 2a–c), which revealed the compact graphene-like MoS2 nanosheets grown on the surface of CoSe2 with a partially free-standing branch-like feature (Supplementary Fig. 2). Substantial amino groups on the CoSe2/DETA surface (Supplementary Fig. 3) serve as nucleation sites for coupling Mo precursor and subsequently reduced to MoS2 on CoSe2 (refs 21, 23). A control experiment performed under identical synthesis conditions, but without CoSe2 nanobelts, produced three-dimensional (3D) aggregates of MoS2 sheets (Fig. 1b; Supplementary Fig. 4), suggesting that CoSe2 could be a useful support for mediating the growth of loaded materials and constructing novel functional hybrids.

Figure 1: Schematic illustration of the preparation of MoS2/CoSe2 hybrid.
figure 1

(a) Solvothermal synthesis with CoSe2/DETA nanobelts as substrates for preparation of MoS2/CoSe2 hybrid. (b) Solvothermal synthesis without CoSe2/DETA nanobelts leads to free MoS2 nanosheet aggregates.

Full size image
Figure 2: Characterization of the MoS2/CoSe2 hybrid.
figure 2

(a) Scanning electron microscopy image of MoS2/CoSe2 hybrid. Scale bar, 800 nm. (b,c) TEM images with different magnifications of MoS2/CoSe2 hybrid. Scale bars, 200 and 50 nm, respectively. The inset in c shows corresponding SAED pattern. (d) HRTEM images of MoS2/CoSe2 hybrid showing distinguishable microstructures of MoS2 and CoSe2. Scale bars, 5 nm. (e,f) XRD patterns and EDX spectrum of the MoS2/CoSe2 hybrid, respectively. (g) STEM-EDX elemental mapping of MoS2/CoSe2 hybrid showing clearly the homogeneous distribution of Co (red), Se (green), Mo (yellow) and S (azure). Scale bars, 200 nm.

Full size image

Figure 2d presents high-resolution TEM (HRTEM) images of the MoS2/CoSe2 hybrid. Layered MoS2 (often less than five layers) nanosheets, with an interlayer separation of 0.63 nm, were grown intimately on the CoSe2 substrate. High-crystalline CoSe2 substrate with d spacing of 0.27 nm can be seen frequently through the interspace of grafted MoS2 (see Supplementary Fig. 5 for additional images). This non-fully covered structure may take advantage of merits from both MoS2 and CoSe2 for catalysing H2 evolution. Selected area electron diffraction (SAED) pattern (inset in Fig. 2c; Supplementary Fig. 6) revealed clear diffraction spots (marked by yellow arrows) from single-crystalline CoSe2 support (JCPDS 9-0234) and also faint diffraction rings from grafted MoS2 (JCPDS 77-1716). These barely recognizable diffraction rings indicate the quasi-amorphous structure of MoS2, consistent with the X-ray diffraction (XRD) data of MoS2/CoSe2 with broadening diffraction peaks (Fig. 2e). Inasmuch as amorphous MoS2 has recently been demonstrated to be effective HER catalysts13,14,15 for its abundant defects and resultant more active edge sites29, we thus infer that the quasi-amorphous MoS2 modification may benefit the hybrid material to catalytically evolve H2. Energy-dispersive X-ray spectrum (EDX; Fig. 2f) analysis further confirmed the formation of MoS2/CoSe2 hybrid with Co, Se, Mo and S as the principal elemental components (Cu and C peaks emanate from the carbon-coated TEM grid), agreeing with the X-ray photoelectron spectroscopy (XPS) results (Supplementary Fig. 7). Scanning TEM (STEM) and EDX elemental mappings revealed uniform spatial distribution of Co, Se, Mo and S over the marked detection range of the constructed hybrid material (Fig. 2g).

Catalytic hydrogen evolution

To assess the HER electrocatalytic activity, thin film of various catalysts was prepared on glassy carbon (GC) electrodes for cyclic voltammetry in H2-saturated 0.5 M H2SO4 electrolyte (see Methods for experimental details). Potentials were measured vs saturated calomel electrode (SCE) and are reported vs reversible hydrogen electrode (RHE). The electrode was kept rotating (1,600 r.p.m.) during the measurements to remove in situ-emerged H2 bubbles. Figure 3a shows that GC electrode guarantees a minimal background activity for H2 evolution. A reductive sweep of MoS2/CoSe2 hybrid showed a low η of 11 mV for the HER, beyond which a sharp increase in cathodic current was observed, corresponding to catalytic H2 evolution (Fig. 3a; Supplementary Fig. 9). By contrast, pure CoSe2 nanobelts exhibited inferior HER activity with a larger onset potential of ~50 mV and lower catalytic current, while pure MoS2 nanosheet aggregates only affected little HER activity. The HER kinetics of the above catalysts was probed by corresponding Tafel plots (log j~η) (Fig. 3b). Tafel slope of ~36 mV per decade was measured for MoS2/CoSe2 hybrid, which is close to the value of 30 mV per decade for Pt/C catalyst and lower than that of 48 mV per decade for CoSe2 and 101 mV per decade for 3D MoS2 aggregates (Table 1). This Tafel slope is also comparable to or lower than that of all the reported noble-metal-free HER catalysts in the literature (Supplementary Table 1), demonstrating the superior HER kinetics of MoS2/CoSe2 hybrid. At high current densities (for example, 10 mA cm−2 with η of 68 mV), MoS2/CoSe2 hybrid also represents a more efficient catalyst relative to other noble-metal-free HER catalysts (Supplementary Table 1). The HER inherent activity of these catalysts was evaluated by the exchange current density (j0). The j0 of 7.3 × 10−2 mA cm−2 for MoS2/CoSe2 hybrid outperforms the values of 8.4 × 10−3 mA cm−2 for pure CoSe2 and 9.1 × 10−4 mA cm−2 for pure MoS2 (Table 1) and also the j0 value for most of the reported noble-metal-free HER catalysts (Supplementary Table 1). The high electrode kinetic metrics (including onset potential of −11 mV and the Tafel slope of 36 mV per decade) and large j0 (only ~1 order of magnitude lower than the value of 7.1 × 10−1 mA cm−2 for Pt) highlight the exceptional H2 evolving efficiency of the new MoS2/CoSe2 hybrid catalyst.

Figure 3: Electrocatalytic hydrogen evolution of different catalysts.
figure 3

(a) Polarization curves for HER on bare GC electrode and modified GC electrodes comprising MoS2/CoSe2 hybrid, pure MoS2, pure CoSe2 and a high-quality commercial Pt/C catalyst. Catalyst loading is about 0.28 mg cm−2 for all samples. Sweep rate: 2 mV s−1. (b) Tafel plot for the various catalysts derived from a. (c) Chronoamperometric responses (j~t) recorded on MoS2/CoSe2 hybrid and pure MoS2 at a constant applied potential of −0.7 V vs SCE. The catalysts were deposited on CFP with the same loading of 1 mg cm−2. Inset digital photos show the H2 bubbles formed on MoS2-modified CFP (up) and MoS2/CoSe2-modified CFP (down) at the time point of 20 h. All the measurements were performed in H2-saturated 0.5 M H2SO4 electrolyte. dec, decade.

Full size image
Table 1 Comparison of catalytic parameters of different HER catalysts.
Full size table

Material stability

The long-term stability of MoS2/CoSe2 hybrid catalyst was assayed by means of chronoamperometry measurement (j~t) with a high catalyst loading of 1 mg cm−2 on carbon fibre paper (CFP). This quasi-electrolysis process was performed at a constant potential of −0.7 V vs SCE in 0.5 M H2SO4 for 24 h. As shown in Fig. 3c, the current density of MoS2/CoSe2-modified CFP electrode decreased gradually at the initial 3 h, which then increased quickly over 24 h of continuous operation. We hypothesize that the more efficient HER-active sites in our hybrid materials are the interfaces between MoS2 and CoSe2, where Co from the support materials could promote the HER kinetics by lowering the Gibbs free energy of adsorbed hydrogen (ΔGH)8,29,30. The severe reducing condition at the initial 3 h caused degradation of external MoS2 and allowed more electrolyte to access the MoS2–CoSe2 interfaces (Supplementary Fig. 11; Supplementary Note 1; Supplementary Table 2), yielding the increased HER current density. Remarkably, after 24 h of operation, XPS studies revealed no obvious chemical state change of HER-active S (Fig. 4) and the homogeneous elemental distribution was maintained (Supplementary Fig. 12; Supplementary Note 1), suggesting the robustness of the hybrid catalyst. By comparison, under the exact same condition, the pure MoS2 catalyst exhibited a slow but continuous decrease in HER activity. Furthermore, digital photo (inset in Fig. 3c) taken from MoS2/CoSe2-modified CFP electrode showed vigorous effervescence at 20 h (also Supplementary Movie 1), comparing favourably with the H2 bubbles formed on free MoS2-modified electrode. The exceptional long-term durability of our MoS2/CoSe2 hybrid catalyst suggests the promise for implementing this new catalyst into realistic hydrogen evolution electrode.

Figure 4: S 2p XPS spectrum analysis.
figure 4

S 2p XPS spectra for pure MoS2, MoS2/CoSe2 hybrid and MoS2/CoSe2 hybrid after stability test. The top right corner demonstrates the structure model of MoS2/CoSe2 hybrid.

Full size image

HER-enhanced mechanism

The experimentally observed high HER intrinsic activity (j0=7.3 × 10−2 mA cm−2) of MoS2/CoSe2 hybrid catalyst prompted us to probe the enhanced mechanism. Generally, the value of ΔGH is considered as a reasonable descriptor of HER activity for a wide variety of catalysts3,30,31. An optimum HER activity is suggested to be achieved at value of ΔGH≈0 (ref. 30). Lower ΔGH will lead to very high surface coverage of Hads, while higher ΔGH will make the protons bonded too weakly on the catalyst surface, which both lead to the slow HER kinetics3,30,31. Previous density functional theory (DFT) calculations5 showed that MoS2 edge sites with unsaturated sulfur atoms can lower the ΔGH approaching to 0 and thus are active for HER, which was later proved by Jaramillo et al.6 experimentally. Recently, Hu and co-workers13,14 found that amorphous MoS2 films are particularly HER active and the increased coordinately and structurally unsaturated sulfur atoms led to the enhancement. Furthermore, first-row transition metal ions, especially Co, can promote the HER activity of MoS2 by coupling with S-edges to lower their ΔGH from 0.18 to 0.10 eV to afford a faster proton adsorption kinetics8,29,30. As to MoS2/CoSe2 hybrid catalyst, the quasi-amorphous MoS2 around the CoSe2 may bring in more active edge sites. Meanwhile, the CoSe2 chemically interacts with MoS2 by forming S–Co bond (Fig. 4 and inset), similar to the XPS peak observed in Co-promoted MoS3 film32, which can further improve the HER activity of MoS2. By contrast, no such XPS peak was found in Fig. 4 for pure MoS2 catalyst. XPS analyses of the S 2p region also exhibit a dramatically decreased electron-binding energy (by ~1.3 eV) after growing MoS2 on the CoSe2, suggesting the formation of more terminal S22− and S2− ions, which are HER active (Fig. 4)14,19. Therefore, the CoSe2 nanobelts, a material with decent HER activity by itself, can not only chemically couple with MoS2 to promote the HER activity, but also serve as an effective support for mediating the growth of MoS2 to form more terminal S22− and S2−. Meanwhile, anchored MoS2 may also boost the HER-active sites of CoSe2 and these electrocatalytic synergistic effects33 together lead to the high HER performance of MoS2/CoSe2 hybrid catalyst.

For the HER in acidic media, two separate pathways (the Volmer–Tafel or the Volmer–Heyrovsky mechanism) have been proposed for reducing H+ to H2 (refs 30, 34). Specifically, the two distinct mechanisms involve three principal steps, referring to the Volmer (electrochemical hydrogen adsorption: H3O++e−→Hads+H2O), the Heyrovsky (electrochemical desorption: Hads+H3O++e−→H2↑+H2O) and the Tafel (chemical desorption: Hads+Hads→H2↑) reactions30,34. Tafel slope, an intrinsic property of electrocatalysts, could be used to probe the elementary steps involved in the H2 evolution. For example, HER kinetic models suggest that Tafel slope of about 120, 40 or 30 mV per decade will be obtained if the Volmer, Heyrovsky or Tafel reaction is the rate-determining step, respectively34. The Tafel slope down to ~36 mV per decade for MoS2/CoSe2 in 0.5 M H2SO4 is the lowest value measured till now for MoS2-based HER catalysts (Supplementary Table 1), even approaching to that of ~30 mV per decade for Pt/C catalysts. This Tafel slope suggests a Tafel-step-determined Volmer–Tafel mechanism works in the MoS2/CoSe2 catalyst, where the synergistic CoSe2 substrate with decent HER activity presumably contributes to this HER mechanism.

Considering that at least one decade of linearity in Tafel extrapolation at relatively large η is desirable to ensure an accurate Tafel analysis, the calculated Tafel slopes (Fig. 3b) and derived HER mechanism may be inconclusive. To prove the proposed HER mechanism, we performed a computational study on the new MoS2/CoSe2 catalyst to gain detailed insights into the adsorption, activation and reaction processes (Fig. 5; see Supplementary Methods for computational details). DFT calculations suggested that reactant (R) contains two H atoms adsorbed on different sites of the optimized MoS2/CoSe2 model with H–H distance of 2.134 Å. Two approached H atoms on the side S atoms of MoS2 cluster then formed a transition state (TS) with H–H distance of 0.945 Å and imaginary vibration frequency of 828i cm−1. After crossing the TS, product (P) with a weakly absorbed H2 molecule (0.745 Å) was formed. The calculated activation barrier of 1.13 eV (30.7 kcal mol−1) for the Tafel-step reaction on MoS2/CoSe2 hybrid, which can be overcome at a slightly higher η, approaches that of Pt (111) electrode35, agreeing well with our experimentally observed fast HER kinetics.

Figure 5: HER mechanism.
figure 5

Reaction pathway of HER on MoS2/CoSe2 hybrid according to the Volmer–Tafel route. The calculated distance of two hydrogen atoms and energies are displayed in Å and eV. Blue, orange, azure, yellow and pink indicate Co, Se, Mo, S and H atoms, respectively.

Full size image

Discussion

In conclusion, we demonstrate that an effective and robust hydrogen evolution catalyst can be made by marrying inexpensive transition metal chalcogenides. The new MoS2/CoSe2 catalyst shows exceptional HER catalytic properties in acidic electrolyte with onset potential of mere −11 mV, a small Tafel slope of 36 mV per decade and a high exchange current density, representing the first non-noble metal catalyst that approaches the performance of state-of-the-art Pt/C catalyst. Inspired by the Nature using transition metals as catalytically active site to construct effective HER catalysts (for example, hydrogenase enzymes)5,36, we believe that our study here will facilitate the development of newly efficient HER catalysts based on transitional metal chalcogenides.

Methods

Synthesis of CoSe2/DETA nanobelts and MoS2/CoSe2 hybrid

All chemicals are of analytical grade and were used as received without further purification. First, ultrathin lamellar mesostructured CoSe2/DETA nanobelts were prepared by our recently developed method20. Briefly, 0.249 g Co(OAc)2·H2O and 0.173 g Na2SeO3 were added into a mixed solution (40 ml) with a volume ratio of VDETA/VDIW=2:1 (DIW, deionized water). The obtained wine solution was then transferred into a 50 ml Taflon-lined autoclave, which was sealed and maintained at 180 °C for 16 h. The resulting black floccules were collected by centrifugation (4,000 r.p.m. for 5 min) and washed by absolute ethanol for three times, and the resulting CoSe2/DETA nanobelts were dried for next use. To prepare the MoS2/CoSe2 hybrid, 10 mg freshly made CoSe2/DETA nanobelts and 10 mg (NH4)2MoS4 were dispersed in 10 ml DMF and sonicated for 15 min under ambient conditions. Then, 0.05 ml N2H4·H2O was added into the suspension. After sonicating for another 15 min to dissolve completely, the mixed solution was transferred into a 50 ml Teflon-lined autoclave, which was sealed and heated in an oven at 200 °C for 10 h and then cooled to room temperature naturally. The resulting black product was collected by centrifugation (7,000 r.p.m. for 8 min), then washed at least four times by distilled water and absolute ethanol to remove ions and possible remnants, and dried under vacuum at 80 °C for 6 h.

Synthesis of free 3D MoS2 sheet aggregates

The synthetic procedure of free 3D MoS2 sheet aggregates is the same with that for preparing MoS2/CoSe2 hybrid, the only difference is that no CoSe2/DETA nanobelt substrates were added during the synthesis.

Characterization

The samples were characterized by different analytic techniques. X-ray powder diffraction (XRD) was carried out on a Rigaku D/max-rA X-ray diffractometer with Cu Ka radiation (λ=1.54178 Å); TEM images, HRTEM images, SAED and an energy-disperse X-ray spectrum (EDS) were taken with a JEOJ-2010 transmission electron microscope with an acceleration voltage of 200 kV. STEM and EDX elemental mapping were performed on JEOL ARM-200F. The X-ray photoelectron spectra (XPS) were recorded on an ESCALab MKII XPS using Mg Ka radiation exciting source. The Fourier transform infrared spectra were measured on a Bruker Vector-22 FT-IR spectrometer at room temperature. Nitrogen sorption was determined by Brunauer, Emmett and Teller measurements with an ASAP-2020 surface area analyzer (see Supplementary Fig. 8 for results). The degradation of external MoS2 around CoSe2 was determined by the inductively coupled plasma–atomic emission spectrometry analysis using an Atomscan Advantage (Thermo Ash Jarrell Corporation, USA) spectrometer.

Electrocatalytic study

Electrochemical measurements were performed at room temperature using a rotating disk working electrode made of GC (PINE, 5 mm diameter, 0.196 cm2) connected to a Multipotentiostat (IM6ex, ZAHNER elektrik, Germany). The GC electrode was polished to a mirror finish (No. 40-6365-006, Gamma Micropolish Alumina, Buehler; No.40-7212, Microcloth, Buehler) and thoroughly cleaned before use. Pt wire and SCE were used as counter and reference electrodes, respectively. The potentials reported in our work were vs the RHE through RHE calibration described below.

The preparation method of the working electrodes containing investigated catalysts can be found as follows. In short, 5 mg of catalyst powder was dispersed in 1 ml of 3:1 v/v DIW/isopropanol mixed solvent with 40 μl of Nafion solution (5 wt%, Sigma-Aldrich), then the mixture was ultrasonicated for about 30 min to generate a homogeneous ink. Next, 10 μl of the dispersion was transferred onto the GC disk, leading to the catalyst loading ~0.28 mg cm−2. Finally, the as-prepared catalyst film was dried at room temperature. For comparison, bare GC electrode that has been polished and cleaned was also dried for electrochemical measurement.

Before the electrochemical measurement, the electrolyte (0.5 M H2SO4) was degassed by bubbling pure hydrogen for at least 30 min to ensure the H2O/H2 equilibrium at 0 V vs RHE at a rotation rate of 1,600 r.p.m. The polarization curves were obtained by sweeping the potential from −0.7 to −0.2 V vs SCE at room temperature and 1,600 r.p.m. (to remove the in situ-formed H2 bubbles on the RDE), with a sweep rate of 2 mV s−1. The electrochemical impedance spectroscopy measurement was performed in the same configuration at open circuit potential over a frequency range from 100 kHz to 5 mHz at the amplitude of the sinusoidal voltage of 5 mV and room temperature (see Supplementary Fig. 10 for results). MoS2/CoSe2 and pure MoS2-coated CFPs (Toray, 1 cm2, catalyst loading 1 mg) were used as working electrodes to collect chronoamperometry data at the applied potential of −0.7 V vs SCE. The polarization curves were replotted as overpotential (η) vs log current (log j) to get Tafel plots for assessing the HER kinetics of investigated catalysts. By fitting the linear portion of the Tafel plots to the Tafel equation (η=b log (j)+a), the Tafel slope (b) can be obtained. All data were reported without iR compensation.

RHE calibration

In all measurements, we used SCE as the reference electrode. It was calibrated with respect to RHE. The calibration was performed in the high-purity hydrogen-saturated electrolyte with a Pt foil as the working electrode. Cyclic voltammetry was run at a scan rate of 1 mV s−1, and the average of the two potentials at which the current crossed 0 was taken to be the thermodynamic potential for the hydrogen electrode reaction. In 0.5 M H2SO4 solution, ERHE=ESCE+0.28 V.

DFT calculations

The computational modelling of the adsorption, activation and reaction processes involved in HER on MoS2/CoSe2 was performed by periodic DFT with the Vienna Ab-initio Simulation Package (VASP). MoS2/CoSe2 model-building details (Supplementary Figs 13–15), HER mechanism and relevant references are provided in the Supplementary Methods.

Additional information

How to cite this article: Gao, M.-R. et al. An efficient molybdenum disulfide/cobalt diselenide hybrid catalyst for electrochemical hydrogen generation. Nat. Commun. 6:5982 doi: 10.1038/ncomms6982 (2015).