3D V–Ni3S2@CoFe-LDH core-shell electrocatalysts for efficient water oxidation
Graphical abstract
Introduction
The rapid development of society has led to the excessive consumption of fossil fuels, which has caused a series of environmental problems (such as acid rain, greenhouse gases, etc.), so it is particularly important to find clean and sustainable energy [1,2]. Hydrogen (H2) is considered to be an ideal clean energy source that can replace fossil fuels due to its high energy density, storability and non-pollution [3,4]. Catalytic reactions have been widely used in the field of energy [5,6], so the different synthesis methods and evaluation systems of catalysts have emerged [7,8]. Electrocatalytic water splitting is an effective and green way to produce hydrogen energy. During the electrocatalytic hydrolysis process, the anodic oxygen evolution reaction (OER) of the four-electron process has slow reaction kinetics, requires greater overpotential and more power consumption, which is not conducive to the cathodic hydrogen evolution reaction (HER) and sustainable development [9,10]. In this case, finding a suitable electrocatalyst to greatly improve the reaction kinetics and reduce the OER overpotential is highly urgent in the practical application of overall water splitting. It is known that RuO2 and IrO2 are ideal electrocatalysts and are considered the benchmark for OER reaction [11,12]. However, their high price and rarity limit practical commercial applications. Therefore, it is of significance to develop efficient, stable and inexpensive OER electrocatalysts to replace precious metal catalysts.
Nowadays, various OER electrocatalysts, such as transition-metal oxides [13], hydroxides [14], phosphide [15], and sulfide [16] has been widely studied. Especially, layered double hydroxides (LDHs), consisting of positively charged layers and charge-balancing anions between layers, has attracted much attention for OER due to their unique 2D lamellar and electronic structure [17,18]. Since Dai et al. reported that LDHs exhibit excellent electrocatalytic OER activity [19], a variety of LDHs and their derivatives have been studied as OER electrocatalysts [20,21]. However, inferior electrical conductivity and sparse exposed active sites of LDHs greatly restrict their catalytic ability. Many strategies, such as preparation of ultrathin LDH nanosheets [22,23], defect engineering [24,25] and hybridizing engineering [26,27], have been adopted to overcome these difficulties. Among them, hybridizing LDHs with other conductive materials is a simple, cheap and efficient method to significantly improve the electrochemical performance. For example, Zhu et al. combined CoNi-LDH with Ti3C2Tx MXene to form CoNi-LDH/Ti3C2Tx, which exhibited prominent OER activity with the overpotential of 257.4 mV at the current density of 100 mA/cm2 [28]. Furthermore, constructing LDHs-based core-shell hybrid nanostructures might achieve better electrocatalytic performance [[29], [30], [31]]. The core material can ensure a fast channel for electron transfer, while the LDHs shell can provide rich active sites effectively. For instance, Ren et al. reported a self-standing 3D core-shell Cu@NiFe-LDH electrocatalyst with a low overpotential of 281 mV at the current density of 100 mA/cm2 [31]. Recently, Ni3S2 has been proved to be a remarkable electrocatalyst because of its enhanced conductivity and great electron transfer ability [32,33]. And V doped can further improve the conductivity and catalytic ability of Ni3S2 [34,35]. Therefore, V-doped Ni3S2 nanorod arrays can be selected as core to combine with ultrathin CoFe-LDH shell to form V–Ni3S2@CoFe-LDH core-shell structure, which would obtain outstanding electrocatalytic OER activity.
Based on the above point of view, we adopted a simple hydrothermal-electrodeposition method to synthesize V–Ni3S2@CoFe-LDH structure on nickel foam (NF). Herein, one-dimensional (1D) V-doped Ni3S2 nanorod arrays as core provides high conductivity ensuring fast electron transport, and the outer ultrathin CoFe-LDH nanosheets offer rich exposed active sites for OER. Furthermore, the hierarchical 3D nanostructure with loose structure facilitates the electrolyte ion entry and gas escaping. Resulting from the above advantages, the 3D V–Ni3S2@CoFe-LDH core-shell electrode shows remarkable OER property with low overpotentials of 190 mV and 240 mV at current densities of 10 mA/cm2 and 50 mA/cm2, respectively, along with a superior long-term stability in 1 M KOH. Our work offers an easy and effective idea to design cheap and highly active electrocatalysts for water splitting, and this idea would be also used in other energy conversion fields.
Section snippets
Synthesis of V–Ni3S2/NF and Ni3S2/NF
Nickel foam (NF) was pretreated before hydrothermal treatment. A piece of 2 cm × 4 cm NF was washed in acetone, 6 M HCl solution, ethanol and deionized water for 10 min, respectively, then dried at 60 °C in a vacuum oven. 50 mg sodium orthovanadate and 125 mg thioacetamide was dissolved in 30 mL deionized water and stirred continuously for 40 min at room temperature. Afterward, the solution was transferred to a 50 mL Teflon-lined autoclave which contained a preprocessed NF. Then the autoclave
Structure characterizations of the catalysts
The synthesis process of V–Ni3S2@CoFe-LDH/NF is shown in Fig. 1, which mainly includes two steps (hydrothermal and electrodeposition), excluding the high-temperature calcination step. Firstly, V–Ni3S2 nanorods arrays were grown on the NF by a hydrothermal way. Then CoFe-LDH nanosheets were decorated on the V–Ni3S2 nanorods via one-pot electrodeposition method to form the unique 3D core-shell V–Ni3S2@CoFe-LDH structure. The unique core-shell structure not only accelerates charge transfer but
Conclusions
In summary, we designed and prepared the 3D V–Ni3S2@CoFe-LDH/NF core-shell electrode for OER by a simple hydrothermal-electrodeposition method. Due to the outstanding electrical conductivity of V–Ni3S2 nanorod cores, rich exposed catalytic sites from the CoFe-LDH nanosheets interconnected with each other, fast gas release and electrolyte transport from the 3D hierarchical structure, the V–Ni3S2@CoFe-LDH/NF electrocatalyst exhibits excellent OER activity with a low overpotential of 190 mV at the
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
This work was supported by the Ph.D. Research Startup Fund (BK202013) from Hubei University of Automotive Technology, the Natural Science Foundations of Hubei Province and the Open Fund (QCCLSZK2021A02 and QCCLSZK2021A01) from Hubei Key Laboratory of Critical Materials of New Energy Vehicles (Hubei University of Automotive Technology).
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