Efficient hydrogen evolution by ternary molybdenum sulfoselenide particles on self-standing porous nickel diselenide foam

Abstract

With the massive consumption of fossil fuels and its detrimental impact on the environment, methods of generating clean power are urgent. Hydrogen is an ideal carrier for renewable energy; however, hydrogen generation is inefficient because of the lack of robust catalysts that are substantially cheaper than platinum. Therefore, robust and durable earth-abundant and cost-effective catalysts are desirable for hydrogen generation from water splitting via hydrogen evolution reaction. Here we report an active and durable earth-abundant transition metal dichalcogenide-based hybrid catalyst that exhibits high hydrogen evolution activity approaching the state-of-the-art platinum catalysts, and superior to those of most transition metal dichalcogenides (molybdenum sulfide, cobalt diselenide and so on). Our material is fabricated by growing ternary molybdenum sulfoselenide particles on self-standing porous nickel diselenide foam. This advance provides a different pathway to design cheap, efficient and sizable hydrogen-evolving electrode by simultaneously tuning the number of catalytic edge sites, porosity, heteroatom doping and electrical conductivity.

There is on-going research into efficient noble metal-free materials for electrocatalytic hydrogen evolution. Here, the authors prepare ternary molybdenum sulfoselenide particles supported on three-dimensional porous nickel selenide foam, and demonstrate the high efficiency of the hydrogen evolving composite.

The large consumption of fossil fuels and its impact on the environment make it urgent to develop environmentally friendly and renewable energy sources. Hydrogen (H₂) is an attractive and promising energy carrier because of its high energy density and no pollution gas emission¹,². One direct and effective route to generate H₂ is based on electrocatalytic hydrogen evolution reaction (HER) from water splitting, in which an efficient catalyst is required to ensure the energy efficiency³,⁴,⁵. Platinum (Pt)-based noble metals are by far the most active catalysts; however, they are not suitable for large-scale applications because of the high cost and scarcity of Pt on earth⁶. Thus, we aim to identify alternative electrocatalysts based on earth-abundant and cost-effective elements⁷,⁸. Until now, various classes of earth-abundant transition metal compounds are confirmed to be promising candidates⁹,¹⁰, such as metal sulfides, selenides, phosphides, carbides and the composites. However, thus far most of the catalysts exhibit inferior efficiency to Pt, while many involve complicated preparation methods and multiple steps that increase costs. Great progress has been obtained for HER based on layered transition metal dichalcogenides (LTMDs) such as molybdenum disulfide (MoS₂) either in the form of crystalline or amorphous states⁹,¹⁰,¹¹,¹², and even in molecular mimics¹³; however, it remains a challenge to get catalytic performance comparable to that of Pt, which is probably due to the low density and reactivity of active sites, poor electrical transport and inefficient electrical contact to the catalyst¹⁴,¹⁵,¹⁶.

On the basis of the above results, fabricating MoS₂ or its derivatives into hybrids or composites might be an interesting strategy to promote the catalytic performance¹⁷. Currently, carbon-based materials are generally used as the catalyst support because of their high surface area and good conductivity¹⁷,¹⁸. However, complex catalyst synthesis procedures are typically required. As an alternative, arranging the catalysts into double-gyroid structures with numerous nanopores might lead to improved HER activity because of preferential exposure of catalytic active edges rather than the inactive basal planes¹⁹. This approach reminds us of the necessity to make three-dimensional (3D) catalysts with high surface area loaded on porous supports, fast proton transfer and greater contact areas with reactants during the catalytic process. Meanwhile, the bottleneck of the double-gyroid structures is the intrinsically poor conductivity of the catalysts. Thus, even though MoS₂ is established as an effective HER catalyst since 2005 (ref. ¹¹), it is still difficult to obtain satisfactory catalysts in low costs on par with the current Pt catalysts. In the light of these challenges, we conclude that the best strategy is to improve the dispersion and electrical conductivity of these catalysts on the supports and to expose a large number of active edge sites. Furthermore, we consider that arranging two different materials into hybrids might lead to synergistic effects²⁰,²¹ that utilize the best properties of each component.

In this work, we propose a strategy of growing ternary molybdenum sulfoselenide MoS_2(1−x)Se_2x particles with vertically aligned layers on a 3D porous HER-active conductive nickel diselenide (NiSe₂) scaffold, which takes advantage of the merits of highly conductive support, double-gyroid structures (3D, porous and lots of exposed edge sites) and synergistic effects between two different catalysts. Indeed, we measure excellent HER performance for this hybrid catalyst that is superior to most reported transition metal dichalcogenides (MoS₂, cobalt diselenide CoSe₂ and so on).

Results

Preparation of 3D porous hybrid electrocatalyst

To the best of our knowledge, the majority of HER catalysts reported thus far are based on nanostructures (nanoparticles, nanosheets and so on); thus, binder polymers (for example, nafion solution) are necessary to fasten the catalysts on the conducting substrates such as glassy carbon electrodes, which somewhat increases the cost. This problem can be avoided by growing the active catalysts directly on self-standing conducting skeletons as the current collectors²²,²³,²⁴. The key challenge is to find a suitable 3D supports with high surface area, high porosity and good conductivity. Graphene or carbon nanotube is not feasible because of their high costs. Instead, nickel (Ni) foam is suitable because of its low price, commercial availability and 3D skeleton structure²⁵ (Fig. 1a). However, Ni foam is not stable in acidic electrolytes because of corrosion. Interestingly, our previous work shows that direct selenization in Ar atmosphere can convert Ni foam to porous NiSe₂ foam (Fig. 1b,c, Supplementary Fig. 1 and Supplementary Note 1) that is HER-active and very stable in acid²⁶. We find that numerous additional pores are generated in the NiSe₂, which provides preferential sites for growing LTMD catalysts with high-density active edges²⁷. Thus, we propose using 3D porous NiSe₂ foam as a conductive skeleton to load ternary MoS_2(1−x)Se_2x catalysts (Supplementary Note 2), thereby utilizing the excellent electrical conductivity, porous structures and high surface area of the NiSe₂ foam (Fig. 1d,e). Indeed, scanning electron microscopy (SEM; Fig. 1d,e and Supplementary Note 3) images clearly show the uniform distribution of small ternary particles on porous NiSe₂ foam, which is important for the electrocatalytic performance of LMDT catalysts.

Figure 1. The schematic diagram and morphology characterizations.

(a) The procedures for growing ternary MoS_2(1−x)Se_2x particles on porous NiSe₂ foam. (b,c) Typical SEM images showing the surface roughness of the NiSe₂ foam grown at 600 °C from commercial Ni foam. (d,e) Typical SEM images showing the morphologies of ternary MoS_2(1−x)Se_2x particles distributed on porous NiSe₂ foam grown at 500 °C. (b,d) Scale bar, 50 μm. (c,e) Scale bar, 1 μm.

Structural characterizations of the electrocatalyst

The chemical composition of the as-grown particles was examined using high-resolution transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and energy-dispersive X-ray spectroscopy. TEM images (Fig. 2a,b and Supplementary Fig. 2) clearly resolve a large amount of vertically aligned MoS_2(1−x)Se_2x layers, suggesting that many active edge sites are exposed at the surface of MoS_2(1−x)Se_2x particles. It is reasonable since the porous structure of NiSe₂ foam with high surface area is favourable for the growth of layered materials with vertically aligned layers²⁷. Meanwhile, XPS spectra in the hybrid reveal the presence of Ni, Mo, S and Se elements (Fig. 2c–e). However, since the Se in NiSe₂ foam has a similar state to that in MoS_2(1−x)Se_2x, it is difficult to demonstrate the selenization of MoS₂ on porous NiSe₂ foam. Instead, to confirm the chemical composition of the molybdenum compound, we put a precursor-decorated Si substrate underlying the NiSe₂ foam during the second selenization. It is clear that the (NH₄)₂MoS₄ precursor has been converted to a distinctive ternary alloy phase at 500 °C from the prominent Mo, S and Se signals in the XPS spectra²⁸ (Fig. 2c–e). Especially in the Raman spectra (Fig. 2f), in comparison with pure MoS₂ that exhibits two prominent peaks at 380 cm⁻¹ (E_1g) and 406 cm⁻¹ (A_2g), there is another obvious peak located at 264 cm⁻¹ for the samples with a ternary phase, which can be ascribed to the A_1g mode of the Mo–Se bond²⁹. Compared with the Raman mode of the bulk MoSe₂ crystals (∼242 cm⁻¹), the blueshifts of this peak to 264 cm⁻¹ suggest a ternary MoS_2(1−x)Se_2x compound rather than a mixture of two solid phases. This Raman feature is also observed from the ternary phase grown on porous NiSe₂ foam, which is consistent with previously reported results on ternary MoS_2(1−x)Se_2x single crystals²⁹. By comparing the relative peak intensity between 264 and 380 cm⁻¹, we estimate that the atomic ratio between S and Se is ∼1, which is further supported by the energy-dispersive X-ray spectroscopy analysis (Supplementary Fig. 3).

Figure 2. Characterization of the ternary MoS_2(1−x)Se_2x/NiSe₂ foam hybrid catalysts.

(a,b) TEM images showing the vertical layer orientation of MoS_2(1−x)Se_2x particles grown on different regions of porous NiSe₂ foam. Scale bar, 5 nm. (c–e) Detailed XPS analysis of the Mo 3d, S 2p and Se 3d spectra in different samples, such as binary MoS₂ particles on Si, MoS_2(1−x)Se_2x particles on Si and MoS_2(1−x)Se_2x particles on porous NiSe₂ foam. (f) Raman spectra measured on different samples.

Hydrogen evolution catalysis

Considering the metallic and porous feature in the NiSe₂ foam, and the good dispersion and preferential layer orientation of ternary MoS_2(1−x)Se_2x particles, it is expected that this 3D architecture should have outstanding HER activity, such as low overpotentials, low Tafel slopes and large exchange current densities. To evaluate the catalytic performance of these ternary MoS_2(1−x)Se_2x particles on 3D porous NiSe₂ foam, we performed detailed electrocatalytic measurements via a standard three-electrode set-up in a 0.5 M H₂SO₄ electrolyte de-aerated with high-purity N₂. The loading of MoS_2(1−x)Se_2x catalysts is ∼4.5 mg cm⁻². Figure 3a shows that the self-standing porous hybrid catalyst can afford a geometric current density of −10 mA cm⁻² at a very low overpotential of −69 mV for the ternary MoS_2(1−x)Se_2x/NiSe₂ hybrid electrode (Supplementary Table 1). In contrast, for binary MoS₂ on NiSe₂ foam and pure NiSe₂ foam, overpotentials of −118 and −153 mV are needed to achieve −10 mA cm⁻², respectively. The catalytic overpotential (−69 mV) of the MoS_2(1−x)Se_2x/NiSe₂ hybrid is also much lower than those of the best catalysts thus far based on LTMDs MoS₂ (−110 mV)¹⁸, WS₂ (−142 mV)¹⁵ and WS_2(1−x)Se_2x (−170 mV)³⁰, and first-row transition metal dichalcogenides CoSe₂ (−139 mV)²⁴, NiSe₂ (−136 mV)²⁶ and CoS₂ (−142 mV)³¹, suggesting that our ternary MoS_2(1−x)Se_2x particles/NiSe₂ foam hybrid is an outstanding HER catalyst. Meanwhile, a Tafel slope, which is an inherent property of the catalyst, can be obtained by extracting the slopes from the linear regions in Tafel plots (Fig. 3b). We find that the ternary electrode possesses a smaller Tafel slope of 42.1 mV per decade than that of binary MoS₂ on NiSe₂ foam (58.5 mV per decade) and pure NiSe₂ foam (46.4 mV per decade). In addition, our hybrid catalyst leads to a Tafel slope much lower than many previously reported cheap and efficient HER catalysts in the same electrolyte (Supplementary Table 2). More interestingly, based on the intercept of the linear region of the Tafel plots, the exchange current densities (j₀,_geometrical) at the thermodynamic redox potential (η=0) can be calculated to be 299.4 μA cm⁻² for the ternary-phase hybrid catalysts. This exchange current density is one to two orders of magnitude larger than those of well-known LTMDs MoS₂ and WS₂, or first-row transition metal dichalcogenides CoSe₂ and CoS₂ catalysts (Supplementary Table 2). Thus, considering the small overpotential (−69 mV to reach 10 mA cm⁻²), low Tafel slope (∼42.1 mV per decade) and large exchange current density (∼299.4 μA cm⁻²), it is worth pointing out that the catalytic performance of our as-prepared catalyst is superior to most of the MoS₂-based catalysts.

Figure 3. Electrocatalytic performance of different catalysts.

(a) The polarization curves recorded on MoS_2(1-x)Se_2x/NiSe₂ foam hybrid, MoS₂/NiSe₂ foam hybrid and pure NiSe₂ foam electrodes compared with a Pt wire. (b) Tafel plots recorded on the catalysts in a. (c) Polarization curves showing negligible current density loss of ternary MoS_2(1−x)Se_2x/NiSe₂ hybrid electrodes initially, after 1,000 CV cycles and after the stability test. (d) Time dependence of current densities −10 and −140 mA cm⁻² recorded on the MoS_2(1−x)Se_2x/NiSe₂ hybrid electrode under given potentials of −69 and −121 mV, respectively. (e) Plot showing the extraction of the C_dl from different electrodes. (f) Electrochemical impedance spectroscopy (EIS) Nyquist plots of different electrocatalysts. The data were fit to the simplified Randles equivalent circuit shown in the inset. The loading of MoS_2(1−x)Se_2x catalyst is 4.5 mg cm⁻².

Aside from a stringent requirement for high HER activity, stability is another important criterion in evaluating the performance of an electrocatalyst. In our experiment, a long-term cyclic voltammetry (CV) test between −0.20 and 0.07 V versus RHE shows no significant degradation of cathodic current densities for the hybrid catalyst after 1,000 cycles (Fig. 3c). Particularly, the cathodic current density for the hybrid catalyst remains stable and exhibits no obvious degradation for electrolysis at a given potential (−69 or −121 mV) for over a long period (>16 h; Fig. 3d), suggesting the potential use of this catalyst over a long time in an electrochemical process. Even after long-term stability and cyclability tests, the catalytic performance of this hybrid catalyst still shows no degradation compared with its initial state (Fig. 3c). In addition, the Faradaic efficiency for hydrogen evolution of this hybrid catalyst was evaluated (Supplementary Note 4). The efficiency is determined to be nearly 100% during 60 min of electrolysis (Supplementary Fig. 4).

To elucidate the origin of the differences in the overall catalytic performance among different catalysts, a simple CV method¹⁵,²⁰,²⁴ was utilized to measure the corresponding electrochemical double-layer capacitances (C_dl) for evaluation of the electrochemically effective surface areas (Supplementary Fig. 5). Taking consideration of the direct proportion between the effective surface area and double-layer capacitance, we just need to compare the capacitance values C_dl. By plotting the positive and negative current density differences (Δj=j_a−j_c) at a given potential (0.15 V versus RHE) against the CV scan rates, we can directly get the C_dl, which is equal to half the value of the linear slopes of the fitted lines in the plots. As shown in Fig. 3e, the MoS_2(1−x)Se_2x/NiSe₂ hybrid electrode exhibits a C_dl value of 319.15 mF cm⁻², which is one order of magnitude larger than that of the pure MoS₂/NiSe₂ foam (30.88 mF cm⁻²), and ∼43 times larger than that of pure NiSe₂ foam (7.48 mF cm⁻²), demonstrating the proliferation of active sites in the porous hybrid catalyst, which accordingly results in the improved catalytic performance. From these capacitance values, we can roughly calculate the electrochemically effective surface area, and thus the turnover frequency per site (0.030 s⁻¹ at 100 mV and 0.219 s⁻¹ at 150 mV, see Supplementary Table 3) by using a similar calculation method developed by Jaramillo et al.³² (Supplementary Note 5). The turnover frequency values are larger than many values reported on MoS₂-like catalysts, and close to that of transition metal phosphide-based electrocatalysts (Supplementary Table 3). On the other hand, electrochemical impedance spectroscopy was carried out to examine the electrode kinetics under the catalytic HER-operating conditions (Fig. 3f). According to the Nyquist plots and data fitting to a simplified Randles circuit, our results clearly reveal that the charge-transfer resistance (R_ct∼0.5 Ω) for the MoS_2(1−x)Se_2x/NiSe₂ hybrid is much smaller than that for pure MoS₂/NiSe₂ (R_ct∼8 Ω) or for porous NiSe₂ foam alone (R_ct∼22 Ω). In addition, all the catalysts have very small series resistances (R_s∼0.6−1.2 Ω), suggesting high-quality electrical integration of the catalyst with the electrode.

Quantum mechanics calculations

To understand the improvement on the catalytic hydrogen evolution of the MoS_2(1−x)Se_2x/NiSe₂ hybrid catalyst, quantum mechanics calculations at the density functional theory (DFT) level (PBE-D3 flavor, see Supplementary Note 6) were performed to calculate the binding free energies of hydrogen on the Mo atom¹¹,²³. Although it was originally suggested that the edge S atom is the catalytic atom in hydrogen evolution on MoS₂ (ref. ¹¹), we find that H₂ formation going through the Mo atom via the Heyrovsky reaction³³ has a lower barrier than the Heyrovsky and Volmer³⁴ reaction on the S atom. Therefore, we use a lower hydrogen-binding energy on the Mo atom as the indicator of a lower barrier in the Heyrovsky step. Since there are various exposed facets in our as-prepared NiSe₂ foam (Supplementary Fig. 1), we modelled the reaction on the simple low-index (100), (110) and (111) surfaces of NiSe₂. Molybdenum dichalcogenide with Se:S ratios of 0:1, 1:1, 1:0 are modelled, and, in the 1:1 case, the S and Se alternate above and below the plane to avoid strain. As shown in Fig. 4a, ΔG_H* is 8.4 kcal mol⁻¹ for hydrogen adsorbed on MoS_2(1−x)Se_2x/MoS_2(1−x)Se_2x, which is more reactive than MoS₂/MoS₂ with a ΔG_H* of 10.6 kcal mol⁻¹, agreeing with the reported experimental results (Supplementary Figs 6 and 7)³⁵,³⁶. In contrast, once the MoS_2(1−x)Se_2x particles are hybridized with porous NiSe₂ foam, the relevant ΔG_H* on MoS_2(1−x)Se_2x/NiSe₂ (100) and MoS_2(1−x)Se_2x/NiSe₂ (110) are further decreased to 2.7 and 2.1 kcal mol⁻¹, making these hybrid catalysts much more active than MoS₂/MoS₂ and MoS_2(1−x)Se_2x/MoS_2(1−x)Se_2x in the HER process. To understand the reason for the improved reactivity of MoS_2(1−x)Se_2x/NiSe₂ hybrid catalysts, we examined the intermediate structures (Fig. 4b). When the MoS_2(1−x)Se_2x particles are placed on top of the NiSe₂ substrate, they relax to form a chemically bonded hybrid on the (100) and (110) surfaces of NiSe₂, while remaining unbonded from the (111) surface of NiSe₂. Thus, DFT calculations corroborate that the MoS_2(1−x)Se_2x/NiSe₂ hybrid is a promising electrocatalyst (Fig. 4).

Figure 4. Density functional theory calculations.

(a) Calculated adsorption free energy diagram for hydrogen (H*) adsorption at the equilibrium potential for MoS_2(1−x)Se_2x/NiSe₂ hybrid, binary MoS₂ and ternary MoS_2(1−x)Se_2x catalysts. (b) Intermediate structures of hydrogen-bound MoS_2(1−x)Se_2x /NiSe₂ (100), MoS_2(1−x)Se_2x/NiSe₂ (110) and MoS_2(1−x)Se_2x /NiSe₂ (111).

Discussion

In general, the as-prepared hybrid catalysts possess the merits of all the MoS₂ catalysts that have ever been reported on improving the relevant catalytic performance¹⁸,¹⁹. Namely, the outstanding HER activity as well as good stability for ternary MoS_2(1−x)Se_2x particles/porous NiSe₂ foam can be attributed to the synergistic effects from the dense catalytic edge sites at the MoS_2(1−x)Se_2x surface, good dispersion of the MoS_2(1−x)Se_2x particles on NiSe₂ foam, good electrical contact and chemical bonding between MoS_2(1−x)Se_2x and NiSe₂ catalysts, and 3D porous structures of HER-active NiSe₂ foam: first, similar to MoS₂, the catalytic property of MoS_2(1−x)Se_2x is greatly related to the number of exposed edge sites¹¹,³⁷. Indeed, in our experiments, because of the porous structure and curved surface of as-grown NiSe₂ foam, ternary MoS_2(1−x)Se_2x layers tend to exhibit vertical orientation on the NiSe₂ surface as demonstrated using high-resolution TEM, indicating that abundant active edge sites are exposed in these MoS_2(1−x)Se_2x particles²⁷. Second, the electrical conductivity of MoS_2(1−x)Se_2x catalysts is another crucial factor to the electrocatalytic activity since the intrinsic conductivity is extremely low between two adjacent van der Waals bonded S–Mo–S layers¹⁴,¹⁶. In our case, the MoS_2(1−x)Se_2x layers are vertically oriented, which enhances the electron transfer from the electrode to the MoS_2(1−x)Se_2x layers. Furthermore, the underlying NiSe₂ foam is metallic and is composed of lots of 3D porous structures that ensure rapid electron transport from the less-conducting MoS_2(1−x)Se_2x to the electrodes, and easy diffusion of the electrolyte into the active sites¹⁸. Finally, the Gibbs free energy for hydrogen adsorption on MoS_2(1−x)Se_2x edges plays significant roles in this HER process, which is much lower compared with MoS₂, leading to higher coverage of hydrogen adsorption at the active sites. Thus, by using this strategy, we can simultaneously engineer the catalysts with high surface area, 3D porous structures, good electrical conductivity and a large number of exposed active edge sites.

In summary, we propose and validate a simple and efficient strategy to synthesize a robust and stable self-standing hydrogen-evolving catalyst by simply growing ternary MoS_2(1−x)Se_2x particles on 3D porous and metallic NiSe₂ foam. According to our experimental results and quantum mechanics DFT calculation, these MoS_2(1−x)Se_2x/NiSe₂ hybrid catalysts exhibit an outstanding catalytic performance superior to that of the widely reported LTMD catalysts (especially MoS₂, WS₂ and so on) and first-row transition metal pyrites (CoSe₂, CoS₂ and so on). Our catalysts are very effective in catalysing hydrogen production by integrating metal dichalcogenides and pyrites into 3D hybrid architectures that possess high surface area, porous structures, good electrical conductivity and abundant active edge sites, making it promising to realize large-scale water splitting.

Methods

Material synthesis

3D porous NiSe₂ foam was directly synthesized by thermal selenization of commercial Ni foam in a tube furnace. Then, the as-prepared NiSe₂ foam was immersed in (NH₄)₂MoS₄ solution in dimethylformamide (DMF) solvent (5 wt% (NH₄)₂MoS₄ in DMF) and dried on the hot plate, followed by thermolysis or second selenization at 500 °C in the tube furnace. The details are shown in the Supplementary Information.

Electrochemical measurements

The electrochemical measurements were conducted in a three-electrode setup with an electrochemical station (Gamry, Reference 600). The polarization curves were collected by linear sweep voltammetry with a scan rate of 0.5 mV s⁻¹ in 82 ml of 0.5 M H₂SO₄, so as to suppress the capacitive current due to the high surface area and high porosity of the porous samples³⁸. A saturated calomel electrode was used as the reference electrode, a Pt wire (CH Instruments Inc.) as the counter electrode and as-prepared hybrid catalysts as the self-supported working electrodes. During the electrochemical measurements, high-purity N₂ gas was continually bubbled throughout the whole electrochemical measurement. Potentials versus RHE can be calculated compared with saturated calomel electrode by adding a value of 0.263 V after calibration. The electrochemical stability of the catalyst was evaluated by continuously cycling the catalyst for 1,000 times at a scan rate of 50 mV s⁻¹. Chronoamperometry was performed under a given potential for the MoS_2(1−x)Se_2x/NiSe₂ hybrid electrode. The electrochemical impedance spectroscopy test was carried out in the same device configuration at a potential of −0.15 V versus RHE with the frequency ranging from 10 mHz to 1 MHz with a 10 mV AC dither. All the potentials used here were referred to RHE.

Data availability

The data that support the findings of this study are available from the corresponding author upon request.

Additional information

How to cite this article: Zhou, H. et al. Efficient hydrogen evolution by ternary molybdenum sulfoselenide particles on self-standing porous nickel diselenide foam. Nat. Commun. 7:12765 doi: 10.1038/ncomms12765 (2016).

Supplementary Material

Supplementary Information

Supplementary Figures 1-7, Supplementary Tables 1-3, Supplementary Notes 1-6 and Supplementary References.