A 3D graphene-supported MoS₂ nanosphere and nanosheet heterostructure as a highly efficient free-standing hydrogen evolution electrode

Abstract

Cross-distributed MoS₂ nanospheres and nanosheets were uniformly deposited on a 3D graphene foam by a hydrothermal approach. Attributable to their synergistic effect, the as-fabricated free-standing electrodes display excellent electrocatalytic activity and outstanding cycling stability towards hydrogen evolution.

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In recent years, molybdenum sulphide has been identified to be a promising cost-effective catalyst for electrocatalytic hydrogen production.^1–4 To gain more active sites, molybdenum sulphide has appeared in a variety of forms and composites (e.g. defect-rich MoS₂ nanosheets, MoS₂ quantum dots, MoS₂/CoS₂/carbon cloth, MoO₂ nanobelts@nitrogen self-doped MoS₂ nanosheets, and amorphous MoS_x film) have been delicately designed and synthesized.^5–10 The poor conductivity of molybdenum sulphides show that good electrical contact with their active sites becomes a determinant factor for enhancing their electrocatalytic hydrogen evolution efficiency.^11–13

Owning to the extraordinary mechanical, structural, electrical and electrochemical properties, graphene have demonstrated considerable activity in wide range of electrochemical and catalytic processes.^14–16 The use of graphene/molybdenum sulphide composites as high performance hydrogen evolution reaction (HER) catalysts are highly favoured in comparison with bare molybdenum sulphide.^17,18 How to expose and stabilize the active sites of molybdenum sulphide at the external surface of catalysts and electrodes becomes another tough issue for the resultant hybrids, which has significant influence on HER enhancement effect.^19,20 Because of limited working volume and specific surface area, traditional 2D planar electrode configurations (e.g. glassy carbon electrode) have found their limitations in sensing, energy storage and conversion, and catalysis fields.²¹ Further extending the arsenal of MoS₂ hybrids and their potential for HER, to a great extent, depends on the possibility to assemble them into macroscopic and highly conductive 3D architectures.

Herein, we demonstrated novel 3D graphene and MoS₂ nanosphere/nanosheet hybrid foams which can be directly used as efficient free-standing HER electrode without the need for any additional additives. 3D graphene was grown on nickel (Ni) foam by chemical vapour deposition (CVD) using Ar/H₂ bubbled ethanol as carbon source.^22,23 Ni foam was subsequently removed by HCl (3 M) to obtain pure 3D graphene. Typical field-emission scanning electron microscope (FESEM) images of 3D graphene reveal a hierarchical porous architecture with high-quality flat surface (Fig. 1a). Such defect-free surface is further confirmed by the absence of D band in its Raman spectrum (Fig. 1c). Moreover, the intensity ratio of G band located at 1576 cm⁻¹ and 2D band at 2720 cm⁻¹ indicates a few layered structure of obtained 3D graphene, which benefits a good mechanical strength and highly conductive network.

Fig. 1 SEM images of (a) 3D graphene and (b) 3D graphene/MoS₂ composite. (c) Raman spectra and (d) XRD patterns of 3D graphene, 3D graphene/MoS₂ composite and bare MoS₂.

3D graphene/MoS₂ composites were synthesized by a typical hydrothermal approach as shown in ESI.† The resultant foam shows that MoS₂ has been densely deposited on the surface of 3D graphene (Fig. 1b). The hybrid foam clearly retains the form of original 3D graphene, except for the blackened colour change. In Raman profile (Fig. 1c), the presence of E_2g¹ and A_1g mode peak at around 374 cm⁻¹ and 400 cm⁻¹ confirmed the success coupling of MoS₂ with 3D graphene.²⁴ In its XRD pattern, however, only two pronounced graphene diffraction peaks corresponding to (002) and (004) diffraction (JCPDS 75-1621) were observed (Fig. 1d).²² The absence of MoS₂ signal suggests the low crystalline and trivial aggregation state of MoS₂ nanostructures. The crystallographic structure of MoS₂ was further compared with bare MoS₂ synthesized in the absence of 3D graphene. The severe aggregation of bare MoS₂ gives three broad diffraction peaks corresponding to (002), (100) + (101) and (110) planes (Fig. 1d), which is good accordance with reported literature (JCPDS 77-1716) (Fig. 1d and S1, ESI†).¹²

High-magnification SEM images of 3D graphene/MoS₂ composite were obtained to show the particular morphology of MoS₂ nanostructure (Fig. 2a–c). Apparently, MoS₂ nanospheres and nanosheets with flower-like structures can be uniformly grown on the surface of 3D graphene to form a large-scale conformal coating. Moreover, the uniformly sized nanospheres dispersed among nanosheets exhibit a hierarchical array feature with rare aggregation. Such cross-distributed heterostructure is helpful to maximize the exposure of active edge sites of MoS₂ and benefit the penetration of electrolyte, which may contribute to the optimization of HER performance. Lower the precursor (ammonium tetrathiomolybdate, ATTM) concentration demonstrated only MoS₂ nanospheres in size of 200–300 nm (Fig. S2a and c, ESI†). By contrast, further increase of precursor concentration brought a severe aggregation of MoS₂ nanostructures on 3D graphene surface (Fig. S2b and d, ESI†). We speculate that the high-quality 3D graphene surface can effectively inhibit the aggregation growth of MoS₂. The fluffy MoS₂ nanostructures are further characterized by high resolution transmission electron microscope (HRTEM). As shown in Fig. 2d, most nanosheets are well stacked with MoS₂ layers <10. Meanwhile, the crisscrossed lines reveal the abundant folded edges of flow-like MoS₂ nanostructures.

Fig. 2 Low and high magnification FESEM images (a–c) of 3D graphene/MoS₂ composite prepared at ammonium tetrathiomolybdate concentration of 2.2 mg mL⁻¹. (d) HRTEM image of MoS₂ nanospheres.

We then directly applied 3D graphene/MoS₂ foam as free-standing working electrode for HER in 0.5 M H₂SO₄ electrolyte using a typical three-electrode setup. Its electrocatalytic performance was firstly evaluated by linear sweep voltammetry at a scan rate of 2 mV s⁻¹. For a comparison, Pt plate and pristine 3D graphene foam were examined as reference free-standing electrodes. Fig. 3a shows typical polarization curves of working electrodes normalized by the projected area. As expected, Pt plate electrode is highly active for HER with an overpotential close to zero. In comparison with bare 3D graphene, all hybrid foams exhibit significant enhancement in electrocatalytic activity, which confirmed the superior HER performance of MoS₂. Among them, 3D graphene/MoS₂ electrode C prepared at ATTM concentration of 2.2 mg mL⁻¹ exhibits the best activity, which has a small onset potential of ca. −110 mV and a current density of 50 mA cm⁻² at −200 mV. This optimal performance compare favourably with, in some cases superior to, the behaviour of other molybdenum sulphide catalysts in acidic electrolyte (Table S1, ESI†). The enhanced performance is believed to be exerted from the synergistic effect of MoS₂ heterostructure and highly conductive framework of 3D graphene. Further increase of precursor concentration does not lead to the expected promotion in onset potential and current density. Instead, an unstable polarization curve at high overpotential indicates the formation and release of large bubbles on electrode surface, which may block the electrolyte diffusion and electron transport at the electrode and electrolyte interface. Apparently, the well-distributed and cross-dispersed MoS₂ nanospheres and nanosheets are more beneficial for H₂ production and release, as confirmed by the vapour-like bubbles shown in optical photograph of 3D graphene/MoS₂ electrode C during I-time scan at a potential of −0.6 V vs. Ag/AgCl (Fig. S3, ESI†).

Fig. 3 (a) Polarization curves of various samples and (b) corresponding Tafel plots. 3D graphene/MoS₂ A, B, C and D samples represents the hybrid foam prepared at ATTM concentration of 0.55, 1.1, 2.2 and 4.4 mg mL⁻¹, respectively. (c) Polarization curve before and after 1000 CV sweeps, and (d) chronoamperometric response of 3D graphene/MoS₂ electrode C.

A Tafel slope is intimately related to the reaction mechanism during the HER process. Therefore, Tafel plot analyses were further performed to evaluate the HER performance of as-fabricated electrodes. As shown in Fig. 3b, 3D graphene/MoS₂ electrode C yields a Tafel slope of 47 mV dec⁻¹ over the potential range of −122 to −181 mV, which is comparable or even superior to many reported molybdenum sulphide catalysts (Table S1, ESI†). Tafel slope represents the potential needed to increase the resulting current density by one order of magnitude. Classical hydrogen evolution theory denotes that a Tafel slope of ≈40 mV indicates a low H_ads coverage, that is the hydrogen evolution is achieved by a rapid Volmer reaction followed by a rate-limiting Heyrovsky reaction (as shown in eqn (1) and (2)). This differs from the well-known Volmer–Tafel mechanism of Pt electrode (eqn (1) and (3)) and Volmer mechanism (eqn (1)) of 3D graphene with Tafel slope of ≈30 and 120 mV dec⁻¹, respectively.² The exchange current density of 3D graphene/MoS₂ is calculated to be 3.5 μA cm⁻², which is slightly higher than MoS₂/mesoporous graphene/GCE electrode (Table S1, ESI†).

Volmer step H₃O⁺ + e⁻ → H_ads + H₂O (1)

Heyrovsky step H₃O⁺ + H_ads + e⁻ → H₂ + H₂O (2)

Tafel step H_ads + H_ads → H₂ (3)

High durability is another significant criterion to evaluate HER catalysts. To test the stability of as-fabricated free-standing electrode in acidic electrolyte, a long-term cycling test was conducted among the potential range of −0.3 to 0.2 V at a scan rate of 10 mV s⁻¹. After 1000 sweeps, the electrode exhibit little current lost in the i–V curve (Fig. 3c). Potentiostatic electrolysis was carried out at a potential of −0.19 V, a long term current stability at ca. 40 mA cm⁻² was measured (Fig. 3d). After the long-term cycling measurements, the morphology of remained electrode was observed by SEM, which revealed negligible change apart from few cracks possibly formed during drying process (Fig. S4, ESI†). These aspects confirm the excellent stability of as-obtained electrodes for long-term applications, which also reflect the high-quality 3D graphene surface is promising for MoS₂ coupling.

Finally, we speculate that the rich active edge sites and the excellent intrinsic conductivity are the major reasons for the superior HER performance of the as-prepared electrodes. Electrochemical impedance spectroscopy (EIS) was measured to further evaluate these electrodes. As shown in Fig. 4a, MoS₂ coating slightly increased the faradaic impedance of 3D graphene foam, but all the hybrid electrodes still demonstrate comparable faradaic impedance with Pt plate, suggesting a small series resistance in the hybrid electrodes. This possibly results from the coherent interface between 3D graphene and MoS₂ heterostructure and from the three-dimensional and highly conductive pathways of 3D graphene, both of which ensure efficient electrical communication between the active sites of MoS₂ and conductive 3D graphene electrode. EIS of 3D graphene-MoS₂ C electrode at the potential of −150 mV was further carried out and only one semicircle is observed (Fig. 4b), which suggests a one-time constant for the electrocatalysis process. EIS data is simulated according to the equivalent circuit given in Fig. 4b (inset), in which Warburg resistance is ignored because of the low overpotential polarization. 3D graphene-MoS₂ C electrode gives a charge transfer resistance (R_ct) of 25 Ω, which is much smaller than that of previously reported molybdenum sulfide-based catalysts (e.g. amorphous molybdenum sulfide, MoS₂ nanorose/3D rGO composite).^25,26 This suggests the remarkable electrocatalytic performance of 3D graphene-MoS₂ C electrode.

Fig. 4 (a) Nyquist plots of as-prepared electrodes at open circuit potentials. (b) Nyquist plot of 3D graphene-MoS₂ C electrode at the potential of −150 mV. Inset is the equivalent circuit, where R_s, R_ct and CPE represent the series resistance, charge transfer resistance and constant phase element of the electrode, respectively.

In summary, highly active HER free-standing electrode was fabricated by hydrothermally depositing MoS₂ on 3D graphene. CVD-grown 3D graphene is found to be able to effectively inhibit the aggregation growth of MoS₂, resulting well-distributed, low crystalline MoS₂ nanospheres and nanosheets heterostructure. Meanwhile, the high-quality flat surface of 3D graphene afford MoS₂ strong coupling, which benefits efficient electrical communication and long-term stability. The multiplexed and conductive network of 3D graphene also ensures the rapid charge transfer of the electrocatalytic process. The synergistic integration of 3D graphene and MoS₂ heterostructure provide abundant well-exposed active edge sites for efficient HER with a small onset potential of around −110 mV and Tafel slope of 47 mV dec⁻¹. This efficient free-standing foam electrode exhibit significant potential for commercialization application without further modification.

Acknowledgements

We thank the support from Ministry of Education of Singapore under an AcRF Tier 2 grant (MOE2014-T2-2-003, ARC31/14).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05074c

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