Synthesis of yolk–shell spheres based on molybdenum diselenide-encapsulated molybdenum oxide for efficient electrocatalytic hydrogen evolution

Abstract

Transition metal dichalcogenides for electrocatalytic water splitting are important in renewable energy research, with studies showing that the edge structure and specific surface area are crucial to their electrocatalytic activity. Herein, the design and construction of yolk–shell nanospheres based on molybdenum diselenide-encapsulated molybdenum oxide are reported. Their structures were characterized using various techniques, which showed that the void space between the interior core and outer shell increased the number of active sites. This yolk–shell structure exhibited enhanced and stable electrocatalytic activity in the hydrogen evolution reaction (HER). The HER current density at −0.5 V vs. the reversible hydrogen electrode was increased by 95% (to −256 mA cm⁻²) compared to that of traditional flower-like molybdenum diselenide. The overpotential of this yolk–shell structure, which was required to drive a current density of −10 mA cm⁻², was 65 mV lower than that of flower-like molybdenum diselenide (286 mV). Furthermore, long-term potential cycling and potentiostatic measurement of this catalyst showed favorable stability and durability of electrocatalytic activity. This yolk–shell structure plays a key role in energy conversion processes central to several renewable energy technologies.

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1. Introduction

In the past decade, steam reformed hydrogen from fossil fuels, including coal and natural gas, has been widely utilized, with hydrogen production from abundant renewable energy sources becoming pivotal in many research areas.¹ Recently, electrochemical methods have gradually attracted the attention of more researchers as environmental friendly and green technologies. Splitting water into hydrogen and oxygen using an electrocatalyst would allow the development of many renewable and sustainable energy technologies.^2,3 Electrocatalytically produced hydrogen offers a high energy conversion efficiency, less pollution, and produces a much lower carbon monoxide content, mitigating the poisoning effect that often decreases the activity of catalysts used in HER.⁴ As previously reported, the change in Gibbs free energy required to split a water molecule is 1.23 V per electron transferred, with water electrolysis typically requiring an overpotential to drive the multistep reduction half-reactions.³ By using a catalyst to decrease the energy consumption and promote these reactions, hydrogen could easily be generated at a lower overpotential. Hydrogen evolution catalysts are a key component that must be able to drive current densities at low overpotential while remaining stable in the chosen electrolyte. However, the development of a single material to meet these requirements represents a significant unmet challenge. One strategy is to meet these various requirements using multiple materials. For example, the activity of silicon photocathodes has already been enhanced by depositing hydrogen evolution catalysts on the Si surface.⁵ The catalytic activity of heterogeneous catalysts in proportion to active surface area, high active surface area, and surface free energy can effectively enhance the catalytic performance.

Advanced materials with yolk–shell structures are ideal catalysts due to their appealing structures and controllable chemical composition. As a material with an interior core, void space, and permeable outer shell, yolk–shell structures have become the focus of intensive research owing to their unique properties, such as low density, large surface area, high permeability, and excellent loading capacity.⁶ More importantly, the void space in yolk–shell structures provides a compartmentalized and confined environment for guest molecules. Furthermore, the properties and functionality, which rely on both the core and shell, can be tailored, making these structures attractive for applications in catalysis and energy storage.⁷ The design and controllable fabrication of sub-micron materials with yolk–shell structures have recently become increasingly important in materials and catalyst research. According to a previous report, one-dimensional yolk–shell nanorods have been successfully constructed and their properties evaluated in lithium batteries.⁸ At present, the most efficient water-splitting catalysts contain scarce metals, such as platinum (for the HER).⁹ Owing to their abundant sources, low price, and safety of storage and transportation, the development of active HER catalysts comprising earth-abundant elements would be a key step in HER development. To date, HER catalysis has been demonstrated using catalysts made from earth-abundant elements such as nickel–molybdenum alloys,^10–12 which require an overpotential of 50 mV to drive the HER at a current density of −10 mA cm⁻². However, nickel–molybdenum alloys are unstable in strongly acidic solutions, where proton-exchange membranes are operational and voltage loss due to the formation of a pH gradient across the membrane can be minimized.^13,14

Recently, transition metal dichalcogenides have been widely studied as HER catalysts, demonstrating good stability in acidic media.^15,16 Transition metal dichalcogenides, such as MoS₂, MoSe₂, WS₂, and WSe₂, which are composed of earth-abundant elements, are stable in acid media,^9,11,15,17 and catalyze the HER with reported overpotentials of 200–350 mV to drive current densities of −10 mA cm⁻². By analysis and comparison, we found that each HER catalyst reported previously has different strengths and weaknesses, such as excellent catalytic activity coupled with high price, poor electrical conductivity, and unreasonable microstructure. To maximize the effects of hydrogen evolution, it is necessary to combine the advantages of various types of HER catalyst. Herein, a yolk–shell structure based on MoSe₂ and MoO₃ has been designed as an earth-abundant HER catalyst with increased active area. As a HER catalyst, this yolk–shell structure had advantages of high specific surface area and comprising cost-effective earth-abundant materials. Moreover, in the yolk–shell structure, the MoSe₂ shell with high catalytic activity and stability in the HER can protect the MoO₃ core from dissolving under HER conditions, while the MoO₃ core, which has better conductivity than MoSe₂, can improve the catalyst conductivity. By combining the MoO₃ core and MoSe₂ shell, we have successfully created a structure with high surface area, catalytic activity, and stability in strong acids.

2. Experimental section

2.1 Materials synthesis

MoO₃ was prepared using a conventional hydrothermal reaction.¹⁸ To synthesize MoO₃, (NH₄)₆Mo₇O₂₄ (Aladdin, analytically pure, 0.49 g) was dissolved in distilled water (20 mL) with continuous stirring. HNO₃ solution (7.5 mL, 3 M) was added to the above solution and the mixture was transferred into a Teflon-lined stainless steel autoclave (100 mL) and heated at 180 °C for 20 h in an electric oven. At reaction completion, the obtained precipitates were separated by filtration, washed with redistilled water and ethanol, and finally dried at 100 °C in an oven. Yolk–shell MoO₃@MoSe₂ spheres were obtained through the hydrothermal reaction of MoO₃. Hydrazine hydrate–Se (0.125 g of selenium dispersed in 2 mL of 80% hydrazine hydrate) was added to an aqueous dispersion of MoO₃ (0.1 g of MoO₃ in 50 mL of distilled water). The mixture was transferred into a Teflon-lined stainless steel autoclave (100 mL), heated at 180 °C for 5 h, and then at 120 °C for 10 h. At reaction completion, the precipitate was washed with distilled water and dried in a vacuum. To perform the contrast experiment, we also synthesized MoSe₂ using a process similar to that of yolk–shell MoO₃@MoSe₂, except that the heating process was conducted at 180 °C for 20 h. Pt/C used in this work was obtained from Alfa Aesar and had a 10% Pt content. Typically, an aqueous dispersion of Pt/C (5 mL, 1 mg mL⁻¹) was drop cast onto the glassy carbon electrode (GCE) using a micro-injector. When not in use, the sensor was preserved at room temperature.

2.2 Characterization

The material morphology was investigated using scanning electron microscopy (SEM, J4800 Japan) and transmission electron microscopy (TEM, Tecnai G2 F30, FEI, USA). The phase structures of the samples were measured by X-ray diffraction (XRD, Rigaku D/max-2400). The chemical composition was determined by energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS, ESCALAB250xi X-ray Monochromatisation 200W Spot/power: Mono 650 μm). Energy calibrations were performed against the C 1s peak to eliminate charging of the sample during analysis. Raman spectra were also measured (HORIBA Jobin Yvon LabRAM HR 800, 532 nm laser, CCD 200–1050 nm probe).

Electrochemical experiments were carried out in an electrochemical cell with a three-electrode configuration on a electrochemical workstation (CHI, USA, CHI660C). A material-modified GCE was used as the working electrode, while Pt wire and a saturated calomel electrode (SCE) served as the counter electrode and reference electrode, respectively. For each material, an aqueous dispersion (5 μL, 1 mg mL⁻¹) was dropcast onto the GCE. All tests were performed in H₂SO₄ electrolyte (80 mL, 0.5 M). The stability and catalytic activity of the materials in the HER were studied using cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The electrode conductivity was determined using electrochemical impedance spectroscopy (EIS). EIS measurements were performed with the same configuration at an open circuit potential of 10⁶ to 0.01 Hz with an AC voltage of 5 mV. A potentiostatic method was used to evaluate the material durability. To measure the electrochemical capacitance, the potential was swept from −0.1 to 0.1 V vs. SCE and back to −0.1 V three times at each scan rate. The obtained voltammograms were calibrated against the reversible hydrogen electrode (RHE).

3. Results and discussion

3.1 Morphology characterization

By partially reducing MoO₃ using hydrazine under thermal annealing using Se powder as a source of selenium atoms, MoO₃@MoSe₂ was synthesized successfully. Fig. 1(a) shows that the obtained MoO₃@MoSe₂ particles were spherical with an average radius of 200 nm and a bumpy surface. Fig. 1(b) shows the surface morphology of a broken sphere, with the surface holes indicating that the spheres had a hollow structure. This hollow structure allows the electrolyte to contact both the inner and outer surface of the hollow spheres, with the void space between the interior core and outer shell improving the adsorption and HER catalysis capabilities. This structural feature increased the effective active surface area of the catalyst. Fig. 1(c) is an enlarged SEM image of a broken sphere, showing a microspheric core encapsulated in the hollow sphere with space between the core and shell, indicating that this material had a yolk–shell structure. The bumpy shell was about 50 nm thick, and seemed to be composed of many vertical nanosheets of different orientations. The radius of this microspheric core was about 100 nm. High-resolution SEM images of the MoO₃@MoSe₂ spheres and MoSe₂ shell are shown in Fig. S1,† where the bumpy surface was more clearly observed and seemed to be composed of many vertical nanosheets of different orientations. Fig. 1(d) shows the morphology of the contrast material, single MoSe₂, which has a flower-like structure consistent with MoSe₂ reported previously.¹¹ Compared with the yolk–shell structure, the flower-like structure had an insufficient effective active surface area to act as a catalyst for the HER. In the follow-up experiment, this conclusion was confirmed by electrochemical investigation. The chemical compositions of the shell and core in the yolk–shell structure were characterized by EDS. The EDS results are shown in Fig. S2 and S3,† with the peak position and area indicating the element type and proportion. This confirmed that the shell was MoSe₂ and the core was MoO₃, indicating that the yolk–shell structure was MoO₃@MoSe₂.

Fig. 1 Typical SEM images of (a) MoO₃@MoSe₂ spheres and (b) broken MoO₃@MoSe₂ spheres. (c) Enlarged SEM image of a broken yolk–shell MoO₃@MoSe₂ sphere. (d) SEM image of flower-like MoSe₂.

As shown in Fig. 2(a), the yolk–shell structure was observed more clearly in the TEM image of MoO₃@MoSe₂. The microspheric cores were encapsulated in the hollow spheres, with space between the core and shell, and holes on the spheres were also observed. Fig. 2(b) shows a TEM image of an empty MoSe₂ shell, in which a hole in the MoSe₂ shell can be observed clearly. When the hole is large enough, the encapsulated core can escape from the shell, leaving only an empty shell. This shell is formed by the aggregation of small sheets with a wave-like shape. This structure has a surface area (including inner and outer surfaces of the shell) that is larger than the surface area of traditional flower-like MoSe₂ (Fig. 2(d)). This yolk–shell MoO₃@MoSe₂ can drive the HER on the inner and outer surface of the shell, making its catalytic performance for HER more satisfactory than that of flower-like MoSe₂. In the follow-up experiment, the electrocatalytic properties of these two materials were studied to prove the above conclusion. The enlarged TEM image of the MoSe₂ shell in Fig. 2(c) shows vertically aligned 2D layers with exposed edge sites. This structure was consistently observed in all enlarged TEM images collected from random areas of the shell, confirming the vertical growth of MoSe₂ nanosheets. Fig. 3(a) is the high resolution transmission electron microscopy (HRTEM) image of the MoSe₂ shell, in which the spacing between adjacent 2D layers was determined from its HRTEM image and measured to be 0.63 nm, corresponding to the interplanar distance of (002) planes in MoSe₂.^19,20 The 2D molecular layers of MoSe₂ were vertically assembled to preferably expose the edge-sites of the layers rather than basal planes, which is useful for the HER owing to the enhanced chemical reactivity of the preferably exposed dangling bonds on the edge sites. No other lattice parameters were observed, except for the spacing between adjacent 2D layers, which indicated that MoSe₂ in the yolk–shell structure had poor crystallinity.

Fig. 2 Low magnification TEM images of (a) MoO₃@MoSe₂ and (b) the MoSe₂ shell of a yolk–shell sphere. (c) Enlarged TEM image of the MoSe₂ shell. (d) TEM image of flower-like MoSe₂.

Fig. 3 (a) HRTEM image of the MoSe₂ shell, and (b) XRD pattern of MoO₃@MoSe₂.

The XRD pattern of the yolk–shell structure is shown in Fig. 3(b), which exhibits three XRD diffraction peaks that do not present good peak shapes. The diffraction peaks appearing at 12.8°, 32.8°, and 55.2° corresponded to the (002), (100), and (110) planes of MoSe₂, respectively.²⁰ Compared with the standard XRD pattern of MoSe₂, the XRD diffraction peaks of the yolk–shell structure are broad, indicating that MoSe₂ in the yolk–shell structure had poor crystallinity. This result was in good agreement with the results of HRTEM analysis, further evidencing that MoSe₂ contained in the yolk–shell structure had poor crystallinity. No diffraction peak was observed for MoO₃ in the XRD pattern, probably due to the MoO₃ core having an amorphous structure.

3.2 X-ray photoelectron spectroscopy and Raman spectra

High-resolution XPS was used to investigate the chemical states of Mo, O, and Se contained in yolk–shell MoO₃@MoSe₂. As shown in Fig. 4(a)–(c), the high-resolution XPS of Mo 3d, Se 3d, and O 1s regions provided information about both stoichiometry and bonding. According to a previous study,²¹ the Mo 3d region is fitted by doublets with fixed spectroscopic parameters, such as spin–orbit separation, Mo 3d_3/2 to Mo 3d_5/2, intensity ratio, and full width at half maximum, but with independent and variable positions and intensities as optimized by the program. The Mo 3d XPS spectrum of this yolk–shell structure was decomposed with the Gaussian distribution, as shown in Fig. 4(a). The results showed a perfect fit to two sets of doublet peaks corresponding to Mo contained in the MoSe₂ shell and MoO₃ core, respectively, indicating a mixture of Mo oxidation states. The lower binding energy doublet located at 228.3 and 231.4 eV, corresponding to Mo 3d_5/2 and Mo 3d_3/2, respectively, originated from Mo⁴⁺ in MoSe₂.²² The other doublet located at higher binding energies of 232.5 and 235.6 eV, corresponding to Mo 3d_5/2 and Mo 3d_3/2, respectively, was attributed to Mo⁶⁺ in MoO₃.²³ The existence of the Mo⁶⁺ peak indicated that Mo⁶⁺ was only partially reduced to Mo⁴⁺. Based on the peak areas of Mo⁴⁺ and Mo⁶⁺, obtained by fitting with XPS analysis software MDI Jade 5.0, the ratio of MoSe₂ : MoO₃ was 0.4 : 1. The ratio of Mo : Se : O was 1 : 0.6 : 4.4 and, when excluding the content of chemically absorbed oxygen, the ratio of MoSe₂ : MoO₃ obtained from Se²⁻ and O²⁻ spectra was consistent with that obtained from the Mo⁴⁺ and Mo⁶⁺ spectra. The XPS spectrum of Se was also decomposed with the Gaussian distribution, as shown in Fig. 4(b), and was divided into Se 3d_5/2 and 3d_3/2, with peak positions at 53.9 and 54.7 eV, respectively, which were associated with Se²⁻ species in the MoSe₂.²⁴ The XPS spectrum of O 1s also decomposed with the Gaussian distribution, as shown in Fig. 4(c). The lower binding energy component located at 530.4 eV originated from the lattice oxygen in MoO₃, while the peak located at 532.3 eV was related to the chemically absorbed oxygen site.^24,25 Therefore, the XPS results further demonstrated that the chemical composition of this yolk–shell structure was MoO₃@MoSe₂.

Fig. 4 High-resolution XPS of (a) Mo 3d, (b) Se 3d, and (c) O 1s regions of the yolk–shell structure. (d) Raman spectrum of yolk–shell MoO₃@MoSe₂.

Raman spectroscopy is an effective method for the material characterization. This yolk–shell structure was characterized by Raman spectroscopy using a laser wavelength of 532 nm. Fig. 4(d) shows the Raman spectrum of MoO₃@MoSe₂. The peak at a lower wavelength (241.6 cm⁻¹) was characteristic of the A_1g mode of MoSe₂ (out-of-plane vibration), while the peak at a higher wavelength (284.3 cm⁻¹) was characteristic of the E¹_2g mode of MoSe₂ (in-plane vibration). Compared with the Raman characteristic peaks of monolayer MoSe₂,²⁶ which has the out-of-plane vibrational mode (A_1g) located at 241 cm⁻¹ and in-plane vibrational mode (E¹_2g) located at 287 cm⁻¹, the Raman characteristic peaks of MoSe₂ contained in the yolk–shell structure have shifted. Some reports have checked the thickness-dependent shift of A_1g and E¹_2g from monolayer to bilayer, during which the high-frequency mode shifts from 241 to 241.5 cm⁻¹, whereas the low-frequency mode shifts from 287.3 to 285.3 cm⁻¹. The A_1g was found to be blue-shifted, while the E¹_2g was red-shifted with increasing MoSe₂ thickness, with the shifts in the high-frequency vibrational modes not being as sensitive as those in the low-frequency modes. As expected, the blue-shift of A_1g and red-shift of E¹_2g indicated that MoSe₂ in the yolk–shell structure was not monolayer, but a multilayer. This result was consistent with that obtained from HRTEM analysis. Except the characteristic peaks of MoSe₂, all other characteristic peaks present in the Raman spectrum of the yolk–shell structure were attributed to MoO₃. According to a previous report,^27,28 the Raman spectrum of MoO₃ can be discussed in terms of internal and external modes. The intensity of the Raman peaks varies based on crystal orientation and laser source polarization. The vibration modes in MoO₃ appear at around 1000–600 cm⁻¹, 600–400 cm⁻¹, and below 200 cm⁻¹, which correspond to stretching, deformation, and lattice modes, respectively.²⁹ Under ambient conditions, the positions of all Raman peaks are in good agreement with those described in the literature.²⁸ The peak at 816.4 cm⁻¹ represents symmetric Mo–O₃–Mo stretching (A_g) with the bonding aligning along the axis direction. Two bands at 667.0 and 470.5 cm⁻¹ resulted from antisymmetric νMo–O₂–Mo stretching (B_2g) and bending (A_g), respectively. The peaks at 365.3 cm⁻¹ are the A_g modes corresponding to the δO₂=Mo=O₂ scissor. The band at 337.6 cm⁻¹ was characteristic of δO₃–Mo–O₃ deformation (B_1g). The peaks at 245.7 and 176.4 cm⁻¹ corresponded to B_3g and B_2g modes, respectively, due to the δO₂–Mo–O₂ scissor. Finally, the peak at 146.3 cm⁻¹ was attributed to the δ(O₂Mo₂)_n mode.³⁰

3.3 Electrochemical investigation

Cyclic voltammograms (CVs) of MoO₃, MoSe₂, and yolk–shell MoO₃@MoSe₂ were measured to compare their electrochemical stabilities under HER conditions. As shown in Fig. 5(a), MoO₃ showed extreme instability under the testing conditions, immediately producing anodic peaks and cathodic peaks that corresponded to redox behavior of MoO₃. This expected behavior from the Pourbaix diagram of the Mo–O system indicates that MoO₃ has a limited window of stability (only at anodic potentials and in acidic media).³⁰ However, CV of MoSe₂ showed no obvious redox peak, which illustrated that MoSe₂ was more stable than MoO₃ under HER conditions. Furthermore, no redox peaks were observed in CV of yolk–shell MoO₃@MoSe₂, indicating the direct protective effect of MoSe₂ against MoO₃ dissolution through surface coverage of MoSe₂. Comparing CV of MoO₃, MoSe₂, and MoO₃@MoSe₂ showed that yolk–shell MoO₃@MoSe₂ was more stable than MoO₃. As shown in Fig. 5(b), the conductivity of these materials ware studied by EIS. Compared with the bare GCE, a loss of conductivity in the MoSe₂-modified GCE was observed. However, with MoO₃ incorporation, the resistance of the MoO₃@MoSe₂-modified GCE was lower than that of the MoSe₂-modified GCE, which indicated that the presence of MoO₃ in catalyst improved the conductivity of the catalyst effectively. The results shown in Fig. 5(a) and (b) indicate that, for this yolk–shell structure, the MoSe₂ shell, which has high stability in the HER, can protect the MoO₃ core from the dissolving under HER conditions, and that MoO₃, which has better conductivity than MoSe₂, can improve the catalyst conductivity. Linear scanning voltammetry (LSV) images of MoO₃, MoSe₂, and the proposed yolk–shell MoO₃@MoSe₂ were also studied to compare their HER catalytic activity. As shown in Fig. 5(c), no HER current density was observed in the LSV of MoO₃, which suggested that MoO₃ in this study had almost no catalytic activity for the HER. Compared with MoO₃, an obvious HER current density was observed in the LSV of MoSe₂. For yolk–shell MoO₃@MoSe₂, the current density obtained at −500 mV vs. RHE was −256 mA cm⁻², which was almost twice that of flower-like MoSe₂ (−131 mA cm⁻²). The activity of a catalyst is known to increase along the active edge when the active catalyst surface area increases. From structural characterization, this yolk–shell structure was found to have MoSe₂ shells with clear vertically aligned 2D layers with exposed edge sites. As shown in Fig. 6, the HER can be driven on the inner and outer surface of the MoSe₂ shell based on the hollow structure of yolk–shell MoO₃@MoSe₂, while flower-like MoSe₂ can only catalyze the HER on its outer surface. This yolk–shell structure, with a specific surface area about two-fold that of the flower-like structure, led to enhanced electrochemical catalytic activity in the HER. As shown in Fig. 5(c), flower-like MoSe₂ exhibited an overpotential of 286 mV to drive the HER at a benchmark current density of −10 mA cm⁻², which was consistent with previous reports.^20,31 In comparison, the overpotential of yolk–shell MoO₃@MoSe₂ required to drive the HER at a benchmark current density of −10 mA cm⁻² was only 221 mV. As MoO₃ in this paper showed no catalytic activity for HER, the lower overpotential and higher HER current density of yolk–shell MoO₃@MoSe₂ was due to its higher active surface area. This was consistent with results obtained from structural characterization, further proving the effect of active surface area on catalytic activity. As shown in Fig. 5(d), the catalytic activities of flower-like MoSe₂, yolk–shell MoO₃@MoSe₂, and 10% Pt/C were compared. The catalytic activities of MoO₃@MoSe₂ and Pt/C were demonstrably superior to that of flower-like MoSe₂. The LSVs of Pt/C and MoO₃@MoSe₂ were compared to analyze their catalytic activities, with the two curves crossing each other at −283 mV vs. RHE. The overpotential of Pt/C required to drive the HER at a benchmark current density of −10 mA cm⁻² was 133 mV, which represented a decrease of 88 mV compared to that of MoO₃@MoSe₂. In the low overpotential region, the catalytic activity of Pt/C in the HER was higher than that of MoO₃@MoSe₂; however, at higher overpotentials, the catalytic activity of Pt/C in the HER was lower than that of MoO₃@MoSe₂. According to a previous report, in the low overpotential region, H coverage is too low to produce (and release) molecular H₂, while at higher over-potentials, H coverage increases along the edge. As H₂ is released, the local coverage is reduced once again, leaving open sites at which more protons can adsorb and produce H₂. According to this previous work and the LSV results of Pt/C and MoO₃@MoSe₂ herein, the catalytic activities of these two materials was compared and reasons for differences in catalytic activity were analyzed. Based on a previous study,³ the catalyst can drive the HER at a lower overpotential by decreasing the energy consumption of water electrolysis and promoting the multistep reduction half-reactions. In the low overpotential region, the Pt/C catalyst can drive the HER at a lower overpotential than the MoO₃@MoSe₂ catalyst because Pt/C decreased the required Gibbs free energy more easily than MoO₃@MoSe₂. However, at higher overpotentials, the MoO₃@MoSe₂ catalyst showed better catalytic activity than Pt/C due to its higher active surface area.

Fig. 5 (a) Electrochemical stability of MoO₃, MoSe₂, and yolk–shell MoO₃@MoSe₂. (b) Electrochemical impedance spectra of different electrodes. (c) Electrochemical activity of MoO₃, MoSe₂, and yolk–shell MoO₃@MoSe₂ in the hydrogen evolution reaction. (d) Catalytic activity of MoSe₂, MoO₃@MoSe₂, and Pt/C.

Fig. 6 Illustration of the hydrogen evolution reaction on the inner and outer surfaces of the MoSe₂ shell in the yolk–shell structure.

To further validate the increase in active surface area from flower-like structure to yolk–shell structure, electrochemical capacitance surface area measurements were performed, as shown in Fig. 7(a) and (b). The non-faradic capacitive current associated with an electrochemical double layer charging upon repeated potential cycling was measured to study the electrochemically active surface area of the materials. A potential range of 0.15–0.35 V vs. RHE was selected for the capacitive current measurements because no obvious electrochemical features corresponding to faradic current were observed in this region. The double layer charging current, i_c, is proportional to both the scan rate, ν, and the electrochemically active surface area of the electrode, A_echem, as shown below:³²

i_c ∝ v × A_echem (1)

Fig. 7 Electrochemically active surface area measurement of (a) yolk–shell MoO₃@MoSe₂ and (b) flower-like MoSe₂ (from (a–e) the curves are 20, 40, 80, 160, and 320 mV s⁻¹, respectively). (c) Capacitive current measured at 0.25 V vs. RHE plotted as a function of scan rate. (d) Tafel plot of yolk–shell MoO₃@MoSe₂.

Electrochemical capacitance was measured to determine the active surface area of the different catalysts and the capacitive current density measured at 0.25 V vs. RHE was plotted as a function of scan rate for flower-like MoSe₂ and yolk–shell MoO₃@MoSe₂ (Fig. 7(c)). The slope of the linear regression equation reflects the relative size of the electrochemically active surface area of the material to a certain extent. Therefore, comparing the slopes allows rough determination of the size of the active area of a material. As shown in Fig. 7(c), the dependence of the current density on the scan rate in this region was linear for both materials, which was consistent with the capacitive charging behavior. Yolk–shell MoO₃@MoSe₂ had a slope of 4.9 × 10⁻³, which was about twice that of flower-like MoSe₂ (2.5 × 10⁻³). This result could explain why the HER current density of the yolk–shell structure was larger than that of the flower-like structure. From flower-like to yolk–shell structure, the active surface area increases with the changes in the structure. The increase in active surface area is related to improved HER performance, and is a very useful approach to improving catalytic performance.

3.4 Tafel analysis and stability study

Tafel analysis was performed on the Tafel polarization curves of yolk–shell MoO₃@MoSe₂, as shown in Fig. 7(d). The Tafel equation, which relates the current and overpotential for electrocatalytic reactions such as hydrogen evolution, as shown below:

η = a + b × log(i) (2)

where, η is the overpotential, i is the catalytic current, a is an intercept, and b is the Tafel slope. Tafel analysis has been used to distinguish different mechanistic pathways. In acidic solutions, hydrogen evolution on an electrode surface mainly involves three reactions.³³ The primary discharge step (Volmer step) is the common first step, followed by either combination (Tafel step) or ion–atom reaction (Heyrovsky step) to give H₂. When the Volmer step is fast and followed by the Heyrovsky reaction, the Tafel slope is 40 mV dec⁻¹ and the symmetric factor, α, is estimated to be 0.5. Assuming a small surface coverage of hydrogen, a fast discharge reaction (3) followed by a rate-determining combination reaction (5) will result in a theoretical Tafel slope of 30 mV dec⁻¹. If the Volmer step is rate-determining, or the surface coverage is close to unity, the Tafel slope is 120 mV dec⁻¹.

Discharge reaction (Volmer step):

H₃O⁺ + e⁻ → H_ad + H₂O, b = 120 mV (3)

Ion–atom reaction (Heyrovsky step):

H_ad + H₃O⁺ + e⁻ → H₂ + H₂O, b = 40 mV (4)

Combination reaction (Tafel step):

H_ad + H_ad → H₂, b = 30 mV (5)

Therefore, for many electrodes, Tafel slopes of about 30, 40, or 120 mV dec⁻¹ should be observed. However, deviations from these values are also common. Many factors can cause such deviation. For example, the surface coverage of hydrogen might be intermediate and potential dependent, or the discharge reaction may have a significant activation barrier.^30,33 According to the Tafel slope calculated from Fig. 7(d), we estimated the rate-determining steps for the yolk–shell material. Due to the Tafel slope of MoO₃@MoSe₂ being 56 mV dec⁻¹, the main HER mechanism was the Heyrovsky step. The low HER overpotential and Tafel slope showed that there were adequate active sites for the reaction.^34,35 These parameters indicated that yolk–shell MoO₃@MoSe₂ was a good HER catalyst.

To evaluate the stability of yolk–shell MoO₃@MoSe₂ in the HER, we compared it with commercial 10% Pt/C. Their stability was investigated by sequential LSV scans and a potentiostatic method. Fig. 8(a) and (b) show sequential scanned LSVs of MoO₃@MoSe₂ and Pt/C, respectively. In the 1st cycle, compared with MoO₃@MoSe₂, Pt/C had a lower hydrogen evolution overpotential and higher hydrogen evolution current density. However, with increasing scan time, the overpotential increased and current density decreased obviously. At the 1000th cycle, the overpotential required by Pt/C to drive the HER at a benchmark current density of −10 mA cm⁻² was 502 mV, which represented an increase of 369 mV compared to the 1st cycle (133 mV). However, Fig. 8(a) shows little change in the overpotential and current density of MoO₃@MoSe₂ with continuous LSV scanning. After 1000 cycles, the overpotential required to drive the HER at a benchmark current density of −10 mA cm⁻² had increased only 15% compared with the 1st cycle (221 mV). MoO₃@MoSe₂ retained its activity throughout extended reductive potential cycling. A potentiostatic method was also used to compare the stability of the two materials. As shown in Fig. 8(c), for MoO₃@MoSe₂, the current density remained almost unchanged for 7200 s, while the current density of Pt/C decreased obviously with time (Fig. 8(d)). The results in Fig. 8 show that, compared with Pt/C, yolk–shell MoO₃@MoSe₂ had better stability under acid conditions. Furthermore, the scarcity and high cost of the noble metal materials prohibit the wide-scale deployment of Pt/C. Therefore, for practical application under acid conditions, yolk–shell MoO₃@MoSe₂ is a more suitable and promising as a HER catalyst. To compare this catalyst with those from previous studies, the characteristics of different HER catalysts are shown in Table 1. The catalyst prepared in this study offers reasonable electrochemical activity for the HER, and is an appropriate catalyst for electrocatalytic hydrogen generation.

Fig. 8 Comparison of electrochemical stability: sequential linear sweep voltammograms of (a) MoO₃@MoSe₂ and (b) Pt/C, and current density vs. time behavior of (c) MoO₃@MoSe₂ and (d) Pt/C.

Table 1 Catalytic properties of transition metal dichalcogenides and their composites for H₂ evolution

Catalysts	Synthetic method	Onset potential [mV vs. RHE]	overpotential [mV] at −10 mA cm^−2	Tafel slope [mV dec^−1]	Ref.
MoS2	Hydrothermal method	−54	200	100	34
MoS2/carbon	Hydrothermal method	−130	250	83	36
MeSe2	Operando method	−150	250	105	31
MoSe2/graphene	Hydrothermal method	−50	175	69	19
MoO3/Ag	Operando method	−48	145	43	37
MoSe2/NG	High-temperature annealing	−30	106	57	38
CNT@MoSe2	One-step solvothermal reaction	−70	178	58	39
MoSe2/graphene	In situ growth	−50	159	61	40
MoO3/MoSe2	Hydrothermal method	−52	221	56	This work

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4. Conclusions

In this work, yolk–shell MoO₃@MoSe₂ has been synthesized and efficiently drove the HER due to its high active surface area and good electrocatalytic properties. Electrochemical capacitance measurements showed that this yolk–shell structure had an active surface area almost twice that of the traditional flower-like structure. In this yolk–shell structure, the MoSe₂ shell can drive the HER efficiently and protect the MoO₃ core from dissolving under HER conditions, while the MoO₃ core can improve the electrical conductivity of the catalyst. This proposed yolk–shell MoO₃@MoSe₂ catalyst exhibited excellent electrocatalytic activity in the HER, with a current density reaching −256 mA cm⁻² at −0.5 V vs. RHE, which was 1.9 times higher than that of the traditional flower-like structure. The overpotential required to drive a current density of −10 mA cm⁻² was 221 mV, which was 65 mV lower than that of the traditional flower-like structure (286 mV). Further long-term potential cycling and potentiostatic measurements of this catalyst demonstrated the favorable stability and durability of electrocatalytic activity. This yolk–shell structure with a high active surface area is a promising candidate for enabling the widespread deployment of cost-effective systems for electrochemical hydrogen production.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Foundation of China under Grant No. 51372106.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7se00466d

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