Elsevier

Applied Surface Science

Volume 491, 15 October 2019, Pages 590-594
Applied Surface Science

Full length article
Influence of the S:Ni ratio in raw materials on the NixSy electrocatalysts

https://doi.org/10.1016/j.apsusc.2019.06.124Get rights and content

Highlights

  • This paper were set out to explore the influence of the S:Ni ratio in raw materials on the different nickel sulfides

  • The relevance between S:Ni in raw materials and stoichiometric ratio, microscopic structure and OER performance were discussed.

  • The electrochemical measurements and theoretical calculations indicate that Ni3S4 has higher effective active sites than other nickel sulfides

Abstract

Nickel sulfides have been considered to be among the most effective oxygen evolution reaction (OER) electrocatalysts. Until now, there are many different nickel sulfides, such as Ni3S2, NiS, Ni3S4, NiS2, etc. In this paper, we set out to explore the influence of the S:Ni ratio in raw materials on the different nickel sulfides. With the characterizations of XRD, XPS, SEM, TEM and BET, the relevance between the molar ratio of S:Ni in raw materials and stoichiometric ratio, microscopic structure and OER performance are discussed. According to the results of the experiments, the Ni3S4 derived from the 3:1 M ratio of S:Ni in raw material exhibits the smallest overpotential (245 and 314 mV to achieve 10 and 20 mA/cm2) and Tafel slope (81.5 mV/dec). Meanwhile, theoretical calculation of ΔG(H2O*) indicate that the adsorption energy in Ni3S4 is much closer to the IrO2 than other nickel sulfides, which are in good agreement with the OER performance of electrochemical measurements.

Introduction

The increasing depletion of fossil fuels which include coal, oil and natural gas, and the environmental problems have started an urgent demand for clean and sustainable alternative energy sources [1,2]. As an effective way to produce clean fuel, the electrolysis of water is attracting more and more attentions [[3], [4], [5], [6], [7]]. Among the electrochemical processes, the oxygen evolution reaction (OER, 2H2O →O2 + 4H+ + 4e) is the rate-limiting step of whole water splitting reaction owing to the complex four-electron redox process [[8], [9], [10]]. Presently, precious metal oxides, such as RuO2 and IrO2, are recognized to be the best OER eletrocatalysts because of their low overpotentials at practical current densities [[11], [12], [13]]. However, the high cost limits their widespread applications in electrochemical water splitting [14,15]. Accordingly, it is one of the most critical requirements for researchers to find efficient, stable, abundant and inexpensive OER electrocatalysts.

Nickel sulfides benefitting from their low-cost metal sources, easy-to-operate synthesis process and excellent OER activity have attracted a great attentions [16,17]. To get nickel sulfides, solution-based strategies which are developing their advantages in mild and high yield are often used to get the product [18]. For instance, our team has been exploring the use of sulfur powder [19] and cysteine [20] as sulfur source in the synthesis of nickel sulfides for studying their performance in water splitting. Considerable progress has been made in nickel sulfides, but most of them are discrete samples in reported literatures and it is difficult to establish any connection between different nickel sulfides in synthesis method, composition, microscopic structure and electrocatalytic performance. Therefore, some ingenious and universal laws need to be explored to provide theoretical guidance for the design of new and more active nickel-sulfide electrocatalysts. Unfortunately, such research work is still in its infancy, and it is a great challenge to explore imperceptible laws in nickel sulfides.

Over past few years, there are a variety of different stoichiometric ratios of S and Ni in the reported nickel sulfides, such as Ni3S2 [[21], [22], [23], [24], [25], [26], [27]], NiS [28], NiS2 [29] and Ni3S4 [[30], [31], [32], [33], [34]], etc. Among all the nickel sulfides, although their molecular formulas, microscopic structure, physical and chemical properties are different, their constituent elements are S and Ni. In view of the above, if we can design an experiment in which the nickel source remains unchanged and the sulfur source increases gradually in the raw materials, will the stoichiometric ratio of the final product change? Will there be a series of regular changes in the microscopic structure and OER properties of the nickel sulfides? Urged by these questions, the nickel sulfides with different stoichiometric ratios of S and Ni were explored by changing the ratio of S:Ni in raw materials. According to the experimental results, we found that when the molar ratio of S:Ni in raw materials increased, a series of regular changes had taken place among the stoichiometric ratio of S:Ni in the final product, the microscopic structure, and the OER performance. Furthermore, results from electrochemical measurements and theoretical calculations indicate that the Ni3S4 derived from the 3:1 M ratio of S:Ni in raw material has the most excellent performance in OER, suggesting that Ni3S4 has higher effective active sites than other nickel sulfides.

Section snippets

Chemicals

Nickel acetate tetrahydrate (Ni(OAc)2·4H2O), 0.333 g polyvinyl pyrrolidone (PVP, Mr. = 10,000), monoethanolamine (MEA), sodium thiosulfate were purchased from Aladdin Inc. All chemical reagents were used as received without further purification.

Synthesis of nickel sulfides

The as-synthesized nickel sulfide of sample a was obtained as following: firstly, solution A was made by adding 1.5 mmol Ni(OAc)2·4H2O (0.373 g) and 0.0333 mmol PVP (0.333 g) to the 5 mL MEA, and then this reagents were uniform mixing; similarly,

Relevance between the molar ratio of S:Ni in raw materials and the stoichiometric ratio of nickel sulfides

The as-obtained different nickel sulfides were firstly studied by XRD. As shown in Fig. 1, the XRD pattern for the finally collected powder of sample a could be readily indexed to hexagonal Ni3S2, corresponding to JCPDS card No. 44-1418. Meanwhile, all the diffraction peaks of sample b could be well indexed to hexagonal NiS (JCPDS card No. 12-0041) and without any additional diffraction peaks of other impurities, indicating the high quality of as-obtained NiS material. When the ratio of S:Ni in

Conclusion

In summary, a series of nickel sulfides were successfully synthesized by changing the ratio of S:Ni in raw materials and characterized by XRD, XPS, SEM and TEM. In addition, the relevance between the molar ratio of S:Ni in raw materials and stoichiometric ratio, microscopic structure and OER performance were discussed in this paper. All of these researches about the nickel sulfide materials will be promising for applications of water splitting. This investigation could open up new approach to

Acknowledgments

The authors are grateful for the financial aid from the National Natural Science Foundation of China (Grants 21,401,203 and 21,702,045) and the Education Department of Henan Province (Grant 15A150035).

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