Highly efficient porous morphology of cobalt molybdenum sulfide for overall water splitting reaction

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Abstract

Hydrogen produced from water splitting is a good alternative to conventional energy sources such as fossil fuels. A highly porous cobalt molybdenum sulfide (CoMoS4) electrocatalyst was fabricated using a simple chemical synthesis route. The porous CoMoS4 acts as an active electrocatalyst in alkaline medium for the oxygen evolution reaction with a low overpotential of 256 mV to reach a current density of 10 mA/cm2. CoMoS4 also acts as an active catalyst for the hydrogen evolution reaction at a current density of 10 mA/cm2 at an overpotential of 143 mV. An alkaline electrolyzer cell was constructed, where CoMoS4 was used as both the anode and cathode for the overall water splitting reaction with a current density of 10 mA/cm2 at a cell voltage of 1.65 V. This study confirms that CoMoS4 is a potential candidate as a bifunctional electrocatalyst for overall water splitting reaction.

Introduction

Hydrogen is considered a promising, environmentally friendly, clean, and renewable energy source [1]. It is a good alternative for conventional energy sources such as fossil fuels, which are environmentally unfriendly, and are finite resources [1], [2], [3]. An ideal technique to produce hydrogen and oxygen is the electrocatalytic decomposition of water using the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) [2]. For the commercial use of water splitting to produce hydrogen, precious metal-based catalysts such as Pt are conventionally used in the HER, and Ru and Ir catalysts are used in the OER [4]. However, the commercialization of large-scale water electrolysis has been hindered by the high cost and low abundance of precious metals and the limited lifetime of the electrodes [5], [6], [7]. To replace these precious metal catalysts there is interest in metal chalcogenides (TMDs) and metal pnictides such as MoS2 [8], Co3S4/CoP [9], Ni3S2/NiO [10], CuCo2S4 [11], and CoMoS4 [3]. Among these, TMDs are the most studied because of their unique properties and specific structures. MoS2 based catalysts have been widely used in recent years for water splitting [8]. Some reports have shown that the incorporation of transition metals such as Co, Cu, and Ni can improve catalytic properties [12]. Therefore, the insertion of Co atoms into MoS2 enhances its catalytic properties. Various TMDs such as Cu-Mo-S, Ni-Mo-S, and Co-Mo-S compounds have attracted research because of their exotic and controllable properties [13], [14], [15]. In this study, a cobalt molybdenum sulfide (CoMoS4) electrode was fabricated because of their bifunctional properties, i.e. their ability to be used as both the anode and the cathode in water splitting [3].

Various methods have been used to synthesize CoMoS4. Xu et al. [16] fabricated nanosheet-like arrays of CoMoS4 on nickel foam (NF) for high-performance supercapacitors using a simple solvothermal method. Ren et al. [3] prepared CoMoS4 powder by a hydrothermal method, loaded it on a carbon cloth, and used it as an active catalyst for the HER. Sun et al. [17] synthesized CoMoS4 using a hydrothermal method for the overall water splitting reaction. Guo et al. [18] first fabricated a self-sacrificing Co-Mo-S system as an active catalyst in the field of water splitting.

In this study, we developed a highly active amorphous CoMoS4 electrocatalyst on a nickel foam (NF) substrate using a simple hydrothermal method. The as-synthesized CoMoS4 electrode was used as a catalyst in the OER and HER with lower overpotentials of 256 mV and 143 mV, respectively, to achieve a current density of 10 mA/cm2. A two-electrode electrolyzer system was fabricated using a CoMoS4 electrode as both the anode and cathode. For the overall water splitting reaction, the cell voltage required was 1.65 V to reach a current density of 10 mA/cm2. The performance of the CoMoS4 active electrode material was compared with those of their metal sulfides, CoS2 and MoS2.

Section snippets

Materials

Co(NO3)2.6H2O, (NH4)2MoS4, Na2MoO4.6H2O, polyvinylpyrrolidone (PVP), KOH, and RuO2 were all purchased from Sigma-Aldrich and were used without further purification.

Preparation of CoMoS4/ NF electrode

To synthesize the CoMoS4/NF electrocatalyst, 0.01 M Co(NO3)2 .6 H2O and 0.05 g of PVP were dissolved in 20 mL of deionized water (DI) to form a reaction mixture. Then, 5 mL of an aqueous solution of 0.02 M (NH4)2MoS4 was added dropwise to the reaction mixture with continuous stirring. This reaction mixture was transferred into a

Results and discussion

The XRD pattern of CoS2 is shown in Fig. 1(a), along with the standard XRD pattern (JCPDS card No. 19-0366), confirming the formation of CoS2. The peaks observed at 31.0°, 35.7°, 47.2°, and 54.7° correspond to the (204), (220), (306), and (330) planes, respectively [23]. Fig. 1(b) presents the XRD pattern of MoS2, confirmed by comparing the data with JCPDS card No.37-1492. Characteristic peaks are observed at 2θ = 33.7° and 57.8°, which are related to the (100) and (110) planes, respectively,

Hydrogen evolution reaction (HER)

OER studies are mainly conducted in alkaline media, but for the HER, there are many studies showing the use of acidic media [32]. In the study of water electrolyzer cells, it is better to use the same electrolyte medium. Thus, in this study, the HER properties of CoMoS4 were measured in a 1.0 M NaOH medium. The LSV curves for CoMoS4/NF, CoS2/NF, MoS2/NF, Pt/C, and NF were measured at a scan rate of 2 mV/s, as shown in Fig. 5(a). The overpotential required by the CoMoS4/NF electrocatalyst was

Conclusions

In summary, highly porous CoMoS4 nanoparticles were synthesized on the surface of a nickel foam substrate and studied as a highly active bifunctional electrode for water splitting. The electrode shows a low overpotential of 143 mV for the HER to reach a current density of 10 mA/cm2 with a Tafel slope of 54.3 mV/dec. This porous electrode also acts as a highly active electrocatalyst in the OER reaction requiring an overpotential of 256 mV to achieve a current density of 10 mA/cm2 with a Tafel

Author statement

A.A. Yadav: Conceptualization, Investigation, Writing - Original Draft Y.M. Hunge: Methodology, Investigation, Writing - Original Draft Seok-Won Kang: concept, design.

Declaration of Competing Interest

None

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1A5A8080290).

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