Hierarchical C–MoCSx @MoS2 nanoreactor as a chainmail catalyst for seawater splitting

https://doi.org/10.1016/j.apcatb.2022.121832Get rights and content

Highlights

  • Hierarchical C–MoCSx@MoS2 nanoreactor is elaborately constructed.

  • X-ray absorption spectroscopy confirms the chemical integration of MoC and MoS2.

  • C-MoCSx @MoS2 nanoreactor shows superior HER activity and stability in sea water.

  • Chainmail effect of defective MoS2 shell protect the active sites from deposition of Ca2+/Mg2+ during seawater splitting.

Abstract

Hierarchical structures with a complex interior and a functional exterior are highly advantageous for energy-related electrocatalysis. Herein, a carbon-supported molybdenum carbide/sulfide heterostructure interior is covered with a defect-rich MoS2 nanosheet exterior. The resulting hierarchical structure (C–MoCSx @MoS2) is a promising nanoreactor for seawater splitting. X-ray absorption spectroscopy demonstrates that MoC and MoS2 are chemically integrated into the C–MoCSx core, providing abundant C–Mo–S sites for hydrogen evolution. Experimental studies and theoretical calculations show that the defect-rich MoS2 exterior exhibits a high capability for repelling salt deposition, allowing the penetration of low-saline water into the inner C–MoCSx to accelerate the water-splitting reaction. As expected, the obtained C–MoCSx @MoS2 is remarkably active and exceptionally stable in natural seawater. This work opens new avenues for developing effective electrocatalysts for seawater splitting and other energy-related applications.

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A nanoreactor consisting of a carbon-supported molybdenum carbide/sulfide heterostructure interior and a defect-rich MoS2 nanosheet exterior was constructed and applied as a model system for examining the correlation between structured design and electrocatalytic HER performance. Benefiting from the chainmail effect of the defect-rich MoS2 shell, the prepared C–MoCSx @MoS2 nanoreactor demonstrates high activity and stability in seawater splitting.

Introduction

Hydrogen is a promising alternative to fossil fuels and is gaining increasing attention owing to its high energy density (142 MJ kg−1) and zero pollutant emissions [1], [2]. Renewable energy-coupled water electrolysis is among the most attractive means of generating high-purity hydrogen [3], [4], [5]. Over the past decade, low-cost and highly efficient electrocatalysts have been well developed in electrolytes containing freshwater, some of which have outperformed the state-of-the-art Pt electrocatalysts [6], [7], [8]. Nevertheless, freshwater accounts for less than 1% of the earth’s water. Conversely, seawater comprises 96.5% of the water resources [9], [10], therefore, it is abundantly available for electrolysis. Although direct electrolysis of seawater is desired, the hydrogen evolution reaction (HER) is much more challenging in natural seawater than in freshwater because of the highly corrosive nature and ion poisoning from a range of undesirable ions [11], [12].

Molybdenum (Mo) is an earth-abundant transition metal with rich redox chemistry. For these reasons, Mo has attracted extensive attention in designing HER catalysts [13], [14]. Mo-based compounds (e.g., MoS2, Mo2C/MoC, MoP, and MoN/Mo2N/Mo5N6) exhibit Pt-like behavior toward the HER over a wide pH range [15], [16]. The intrinsic HER activity is further enhanced by constructing ultrafine dual-phase Mo-based compounds that may create numerous accessible active sites [17], [18]. However, the complex ionic chemistry of seawater brings additional challenges to electrolysis, such as interferences of side reactions, ionic poisoning, and corrosion, which degrade the cell performance [19]. A nanoreactor that protects the active sites from deposition or poisoning by alkali metal ions (e.g., Na+, K+, Ca2+, and Mg2+) is challenging to construct but highly demanded.

A laminate MoS2 membrane with multimodal vacancies has demonstrated excellent ability to repel ion deposition in high-salinity water [20]. More than 88% of the metal ions can be repelled using a layered MoS2 membrane with Mo-edge nanopores during the seawater penetration process [21]. Therefore, a functional MoS2 shell can serve as a chainmail that protects the reactive center of a nanoreactor from metal-ion deposition or poisoning during the seawater splitting process. Realizing such a nanoreactor is quite challenging and, to the best of our understanding, has not been reported in the literature. Constructing hybrids with the designed function and catalytic performance is one holy grail in the synthesis of catalysts, for example, achieved by a controllable assembly of hierarchical nanoreactors with the particular configuration [22], [23].

Recently, Zn- and Mo-based hybrid zeolitic imidazolate frameworks (HZIF–Zn/Mo) have been reported as the ideal reactive precursors of well-defined Mo-based hybrids with high electrochemical functionality [24], [25]. Adopting the HZIF-engaged strategy, an elegant hybrid structure comprising a carbon-supported Mo-based carbide/sulfide heterostructure with a defect-rich MoS2 box (C–MoCSx @MoS2) exterior has been constructed herein. The ultrafine dual-phase MoCSx reactive centers benefit the active sites exposure and defect-rich MoS2 membrane repels salt deposition, conferring the obtained C–MoCSx @MoS2 with boosted HER activity in seawater splitting. Experimental results and density-functional theory (DFT) simulations reveal that C–MoCSx @MoS2 functions as a nanoreactor with a chainmail effect from the defective MoS2 shell.

Section snippets

Preparation of HZIF-Zn/Mo

HZIF-Zn/Mo microcubes were synthesized according to a previously reported method [24]. Specifically, 3.3 g polyvinylpyrrolidone (PVP, K88–96, Aladdin Reagents Ltd) was first dissolved in 60 mL N, N-dimethylformamide (DMF, Sinopharm Group Chemical Reagent), followed by adding 90 mg zinc acetate dehydrate (Zn(CH3COO)20.2 H2O, 99.99%, Aladdin Reagents Ltd), 54 mg 2-methylimidazole (2-mim, 98%, Aladdin Reagents Ltd), and 22 mg H2MoO4. Thereafter, the mixture was heated at 160 °C in a 100 mL

Synthesis and structural characterization of C–MoCSx@MoS2

The fabrication process of C–MoCSx @MoS2 is shown in Fig. 1. First, the HZIFs cubes are prepared using a surfactant-assisted method. The as-synthesized HZIFs cubes are then mixed with thioacetamide (TAA) in ethanol, followed by refluxing at an elevated temperature. As the reflux reaction proceeds, the HZIFs and TAA interact to gradually form a complex shell around the HZIFs cubes, which is organized into crystallographic ZnS and amorphous MoOx phases (HZIFs@MoOx/ZnS). As shown in the scanning

Conclusion

In summary, C–MoCSx @MoS2 nanoreactors are constructed using a Mo-based HZIF-engaged strategy. This rational synthetic strategy, involving an initial controlled solvothermal sulfidation, subsequent thermal annealing, and selective etching processes, could effectively tailor the compositions of the final nanoreactor. X-ray absorption spectrum and electrochemical tests show that the C–MoCSx core with its abundant C–Mo–S sites and defective MoS2 shell endow C–MoCSx @MoS2 with the desired HER

CRediT authorship contribution statement

Yang Li: Data curation, Investigation, Formal analysis, Writing – original draft. Shouwei Zuo: Formal analysis, Data curation. Qiaohong Li: Formal analysis, Validation. Huawei Huang: Formal analysis. Xin Wu: Formal analysis. Jing Zhang: Formal analysis, Data curation. Huabin Zhang: Project administration, Conceptualization, Funding acquisition, Writing – review & editing. Jian Zhang: Project administration, Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by the National Key Research and Development Program of China (2021YFA1501500, 2018YFA0208600, 2017YFA0403400), the NSFC (21935010), and King Abdullah University of Science and Technology.

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