ZIF-8-derived ZnS–Ni3Fe–Ni co-loaded N-doped porous carbon for efficient hydrogen evolution reaction catalysis

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Abstract

Herein, N-doped porous carbon (NC) decorated with ZnS, Ni3Fe, and metallic Ni, and denoted as ZnFeNiS/NC, was fabricated via the direct carbonization of a zeolitic imidazolate framework (ZIF-8), that is, a Zn-based metal–organic framework, combined with absorption and sulfurization processes. The synthesized material was used as an electrocatalyst for the hydrogen evolution reaction (HER) in alkaline solutions. Compared with Zn/NC, ZnFe/NC, and ZnFeNi/NC, ZnFeNiS/NC exhibited excellent HER activity with the lowest potential (162.57 mV) and Tafel slope (65.48 mV decade−1) at a current density of 10 mA cm−2. This better performance can be attributed to the synergistic effects of the ZnS, Ni3Fe, and Ni dopants; the ZnS, Ni3Fe, and Ni nanoparticles embedded in NC not only provided many active reaction sites but also ensured long-term stability in alkaline media. The new method presented here could allow the fabrication of non-noble-metal electrocatalysts with excellent electrocatalytic activity for HER.

Introduction

Hydrogen is an ideal alternative to traditional energy sources such as coal, oil, and kerosene due to its eco-friendly, renewable, and sustainable properties [1]. Water splitting is the most promising technique to produce high-purity hydrogen in large quantities via electrochemical methods. This approach requires electrocatalysts with superior hydrogen evolution reaction (HER) activity. To date, Pt-, Ir-, and Ru-based materials are the most efficient HER catalysts, with low Tafel slope and onset overpotential [[2], [3], [4]]; however, their high cost and low abundance hamper their widespread applications. Thus, they should be replaced with novel non-noble-metal catalysts having excellent HER performance.

Many researchers recently investigated transition-metal dichalcogenides, including MoS2 [5], NiS [6], and FeS [7], as HER electrocatalysts due to their low cost and high abundance. Many efforts have been devoted to optimize the HER activity of such materials, to replace Pt-, Ir-, and Ru-based electrocatalysts. Ma et al. fabricated self-supported, three-dimensional, well-mixed, and highly-dense NiS–CoS nanorod arrays, which showed exceptional HER performance with excellent durability in both acidic and alkaline solutions [8]. Liu et al. prepared hierarchical ZnS@C@MoS2 core–shell nanostructures with a bottom-up strategy via simple selective etching, achieving excellent HER activity [9]. Shwetharani et al. synthesized nanostructured FeSe2 through solvothermal reduction, reporting a lower overpotential for HER [10]. However, the transition-metal dichalcogenide materials developed so far have still lower conductivity and less active sites compared to noble-metal HER electrocatalysts.

Metal–organic frameworks (MOFs), coordinated by metal ions and organic ligands, have recently attracted much attention due to their tunable metal center, high specific surface area, and excellent physicochemical stability [11]. They are widely used in many fields, including heterogeneous catalysis [12], drug delivery [13], gas separation [14], and chemical sensing [15]. MOFs are also utilized as templates or precursors to fabricate metal-doped carbon materials via thermolysis. Zhou et al. selected Co-based MOFs to synthesize CoSe2-decorated carbon nanotubes via a carbonization/oxidation/selenylation procedure [16]. Li et al. adopted reduced graphene oxide-wrapped MOFs to prepare cobalt-doped porous carbon polyhedrons at 600 °C under Ar atmosphere [17]. Liu et al. realized porous carbon polyhedrons via pyrolysis by using copper benzenetricarboxylate (HKUST-1) as the precursor [18]. Carbon-based materials can enhance the electrical conductivity of the catalysts and nitrogen doping can highly improve their electrocatalytic activity for HER [[19], [20], [21]]. A MOF structure presents several nitrogen-containing linkers; therefore, transition-metal-decorated N-doped carbon can be derived from MOFs.

In this study, we developed a MOF-derived strategy to fabricate N-doped porous carbon (NC) codecorated with ZnS, Ni3Fe, and Ni, and denoted as ZnFeNiS/NC, via the direct carbonization of a Zn-based MOF, that is, a zeolitic imidazolate framework (ZIF-8), combined with absorption and sulfurization processes under Ar atmosphere. The Zn, Fe, Ni, and S dopants could act as active sites during HER; besides, the NC structure could avoid the aggregation among the ZnS, Ni3Fe, and Ni nanoparticles and effectively enhance the electrical conductivity of the catalyst. Due to the synergistic effect of the ZnS, Ni3Fe, and Ni decorations and the NC matrix, ZnFeNiS/NC exhibited outstanding electrocatalytic performance for HER.

Section snippets

Materials

Zn(NO3)2·6H2O, Ni(NO3)2·6H2O, thiourea, 2-methylimidazole, methanol, ethanol, and polyvinylpyrrolidone K30 (PVP) were purchased from Sinopharm Chemical Reagent Co., Ltd. All the reagents were of analytical grade and used without further purification. Deionized water was also utilized.

Fabrication of PVP/ZIF-8 nanoparticles

First, 2-methylimidazole (3.8 g) was dissolved in a methanol solution (100 mL) containing PVP (8.0 g). Then, another methanol solution (100 mL) with Zn(NO3)2·6H2O (4.5 g) was slowly poured into the as-obtained

Results and discussion

Fig. 2 displays the XRD patterns of Zn/NC, ZnFe/NC, ZnFeNi/NC, and ZnFeNiS/NC. All the samples showed peaks at about 26.1° and 44.3° corresponding to the (002) and (101) planes, respectively, of graphitic carbon [22]. The Fe loading made both peaks stronger and resulted in the appeared of a new one with lower intensity at 44.7°, ascribed to the (220) plane of α-Fe [23]; this indicates that the presence of Fe improved the graphitization degree of the electrocatalyst. Compared to ZnFe/NC,

Conclusions

ZnS–Ni3Fe–Ni-codecorated N-doped porous carbon was successfully fabricated via facile pyrolysis, absorption, and sulfurization and used as an HER electrocatalyst. Compared with Zn/NC, ZnFe/NC, and ZnFeNi/NC, ZnFeNiS/NC exhibited excellent HER performance in 1.0 M KOH with the lowest overpotential (162.57 mV) and Tafel slope (65.48 mV decade−1) at a current density of 10 mA cm−2. It also showed outstanding stability, with a current decay of only 4.74% after 10 h of continuous measurements. The

Credit author statement

1. Yanqiu Jing, Yide Yang and Hongfei Yin contributed equally to this work (Writing-Original Draft).

2. Jian Yang, Yang Yang, Min Yan and Qili Zhang provided the measurements.

3. Yanqiu Jing, Dongsheng Luo, Qingbin Zeng and Bin Li provided the idea for this experiment.

4. Qingbin Zeng has great contributions to the revised manuscript.

Note: All materials, used in this experiment, were provided by all authors.

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.

Acknowledgements

The authors wish to acknowledge the eceshi (www.eceshi.cn) for the XPS analysis.

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    Yanqiu Jing, Yide Yang and Hongfei Yin contributed equally to this work.

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