Full paperElectrocatalytically inactive SnS2 promotes water adsorption/dissociation on molybdenum dichalcogenides for accelerated alkaline hydrogen evolution
Graphical abstract
Molybdenum dichalcogenide heterostructures with the SnS2 quantum dots decorated on the basal plane were synthesized via a universal wet chemical process as efficient alkaline HER electrocatalysts. DFT calculations also verify that incorporating SnS2 actually promotes water adsorption capability of MoSe2 both on the basal plane and edge sites.
Introduction
Our severe energy and environmental crisis makes it imperative to search for clean and sustainable energy sources as alternatives to traditional fossil fuels [[1], [2], [3]]. Owing to its having the highest gravimetric energy density and carbon-free emissions, hydrogen produced by renewable energy sources is considered to be the most promising energy carrier for our future society's energy [4,5]. Currently, hydrogen is mostly produced from fossil fuels by steam reforming [6]. Alternatively, photocatalytic, photoelectrocatalytic, or electrocatalytic water splitting driven by renewable energy would make hydrogen a real carrier for clean energy [[7], [8], [9]]. With regards to electrocatalytic water splitting, electrocatalytic performance remains unsatisfactory for both the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER), although numerous research efforts have been devoted to developing efficient electrocatalysts. Currently, precious metal-based materials are the state-of-the-art catalysts for both the HER (e.g., Pt) and the OER (e.g., IrO2), but they suffer from high cost and scarcity [[10], [11], [12], [13], [14], [15], [16], [17]]. Therefore, developing earth-abundant and low-cost alternatives, such as transition metal chalcogenides, metal oxides/hydroxides, and metal alloys, is critically necessary to address this challenge for practical water splitting systems [[18], [19], [20], [21]].
Molybdenum dichalcogenides, in particular MoS2 and MoSe2, are very promising nonprecious-metal-based electrocatalysts for the HER [[22], [23], [24]]. Both density functional theory (DFT) calculations and experimental findings have demonstrated that the HER catalytic activity of molybdenum dichalcogenides is mainly derived from their edge sites [[25], [26], [27], [28], [29]]. In this regards, various strategies have been focused on increasing the number of exposed active sites of molybdenum-dichalcogenide-based electrocatalysts through building various nanostructures, engineering surface defects, or heteroatom doping [[30], [31], [32]]. Unfortunately, although the molybdenum dichalcogenide-based electrocatalysts thus developed display impressive catalytic activity in acidic media, they exhibit inferior HER activity in alkaline media due to the sluggish water dissociation kinetics. Basically, alkaline HER involves water adsorption, water dissociation, and hydrogen recombination and release [33]. Water adsorption and dissociation take place at the beginning of the alkaline HER process and are considered to represent the rate-determining step for the alkaline HER [[34], [35], [36], [37]]. Therefore, designing electrocatalysts with enhanced water adsorption and dissociation capability is the key for the promotion of alkaline HER catalytic activity. Recently, molybdenum-dichalcogenide-based heterostructures with an additional phase (e.g., Ni(OH)2) anchored on MoS2 or MoSe2 nanosheets were reported as efficient alkaline HER catalysts [[38], [39], [40]]. The second phases usually possess strong water affinity and water adsorption capability, which are of great significance for accelerating the water dissociation kinetics of the heterostructured catalysts [41]. Meanwhile, in some cases, the presence of the second phase can also modulate the electronic structure of Mo and optimize the hydrogen adsorption energy [[42], [43], [44]]. On the other hand, heteroatom doping (e.g., Ni, Co) was also demonstrated to be an effective strategy for enhancing alkaline HER kinetics [45,46].
In this work, we propose a new heterostructured design concept in order to improve the alkaline HER activity of molybdenum dichalcogenides. MoSe2/SnS2 and MoS2/SnS2 heterostructures with SnS2 quantum dots decorated on the basal planes are synthesized by a universal wet-chemical strategy for enhanced alkaline HER. DFT calculations reveal that the incorporation of SnS2 brings in the substantial enhancement of water adsorption capability of MoSe2 both on the edge sites and basal planes. Benefiting from the improved water adsorption/dissociation capability, the well-defined heterostructures delivered significantly enhanced hydrogen evolution kinetics in alkaline media.
Section snippets
Materials synthesis
Synthesis of MoSe2 nanosheets. MoSe2 nanosheets were synthesized by a modified hydrothermal process [32]. Briefly, 241.95 mg Na2MoO4·2H2O was added to 20 ml deionized water (DI water) under magnetic stirring as the Mo precursor. Then, 0.1 g NaBH4 was dissolved in 15 ml Ar saturated DI water in a three-neck bottle. Subsequently, 0.16 g Se powders were dispersed into the NaBH4 aqueous solution under Ar flow with mild shaking until Se powders were fully dissolved to form a homogeneous transparent
Results and discussion
The molybdenum dichalcogenide heterostructures were prepared via a two-step hydrothermal method, as illustrated in Scheme 1. The molybdenum dichalcogenide nanosheets were first prepared, and then SnS2 quantum dots were uniformly anchored on the nanosheet surfaces via an in situ precipitation process (See Experimental Section for more details). The X-ray diffraction (XRD) pattern of MoSe2/SnS2 (Fig. S1, Supporting Information) presents typical diffraction peaks that can be well indexed to SnS2
Conclusion
In summary, molybdenum dichalcogenide heterostructures with SnS2 quantum dots decorated on the basal planes were designed and synthesized as efficient alkaline HER electrocatalysts. The optimal MoSe2/SnS2 heterostructured catalyst delivered a substantially lower overpotential of 285 mV as compared with MoSe2 (367 mV) at 10 mA cm−2. The significant improvement in alkaline HER activity is mainly due to the accelerated water adsorption/dissociation kinetics. The DFT calculations reveal that the
Acknowledgements
This work was financially supported by the Australian Research Council (ARC) DECRA Grant (DE160100596), ARC Discovery Project (DP160102627), AIIM FOR GOLD Grant (2018, 2019), and UOW's Vice-Chancellor's Postdoctoral Research Fellowship Funding (X.W.). Y. C., K.R. and X. Z. are sincerely thankful for the funding support from the China Scholarship Council (CSC). We are grateful to the High Performance Computing Center of Nanjing University for providing the IBM Blade cluster system. We also
References (54)
- et al.
Electrochim. Acta
(2017) - et al.
Joule
(2017) - et al.
Nano Energy
(2017) - et al.
Joule
(2018) - et al.
Nature
(2012) - et al.
Angew. Chem. Int. Ed.
(2015) - et al.
Adv. Mater.
(2017) - et al.
Adv. Mater.
(2018) Science
(2004)- et al.
Nat. Chem.
(2013)
Chem. Soc. Rev.
Energy Environ. Sci.
J. Am. Chem. Soc.
Angew. Chem. Int. Ed.
ACS Energy Lett
Angew. Chem. Int. Ed.
Angew. Chem. Int. Ed.
Nat. Nanotechnol.
ACS Nano
J. Am. Chem. Soc.
Chem. Rev.
Adv. Funct. Mater.
Chem. Rev.
Angew. Chem. Int. Ed.
Chem. Soc. Rev.
Nanoscale
Adv. Funct. Mater.
Cited by (0)
- 1
These authors contributed equally to this work.