Heterogeneous Ni₃S₂@FeNi₂S₄@NF nanosheet arrays directly used as high efficiency bifunctional electrocatalyst for water decomposition

Abstract

Developing and designing bifunctional electrocatalysts are very important for the production of hydrogen from water electrolysis. The reasonable interface modulation can effectively lead to the optimization of electronic configuration through the interface electron transfer in the heterostructures and thus resulting in the enhanced efficiency. In this work, self-supported and heterogeneous interface-rich Ni₃S₂@FeNi₂S₄@NF electrocatalyst for overall water splitting was designed and prepared through a controllable step-wise hydrothermal process. Density functional theory calculations suggest that heterogeneous interface formed between Ni₃S₂ and FeNi₂S₄ can optimize the Gibbs free energy for H* adsorption (ΔG_H*). Benefiting from the open structure of the nanosheet arrays, the abundant heterogeneous interfaces in Ni₃S₂@FeNi₂S₄@NF composite, the positive synergistic effect between Ni₃S₂ and FeNi₂S₄, and the good conductivity of foamed nickel (NF) substrate, the optimized Ni₃S₂@FeNi₂S₄@NF nanoarray catalyst displayed excellent electrocatalytic activities, the overpotential is only 83 mV and 235 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at 10 mA cm⁻², respectively. Importantly, an alkaline electrolyser directly using the Ni₃S₂@FeNi₂S₄@NF as both the anode and cathode achieved an ultralow cell voltage of 1.46 V, accompanied by outstanding stability. The performance is better than that of most other transition-metal sulfides electrocatalysts. This work may provide a useful strategy for reasonably regulating heterogeneous interfaces to effectively improve the performance of materials, thus accelerating the practical application of transition-metal sulfides electrocatalysts for overall water splitting.

Introduction

With the continuous consumption of fossil fuels, the problem of environmental pollution is becoming more and more serious. Hydrogen as renewable and clean energy has been identified as the most ideal and potential alternative to fossil fuels [1], [2], [3]. The electrocatalytic water decomposition to produce H₂ is recognized as the most ideal, clean and environmental pathway [4], [5], [6]. The whole process of electrolysis of water involves two half-reactions: anodic multi-electron OER and cathodic two-electron HER [7], [8]. Theoretically, the cell voltage of electrocatalytic water decomposition is only 1.23 V, however, the commercial electrolysers usually need the voltage about 1.8 to 2.0 V due to the high overpotential in the process [9]. The application of effective and stable electrocatalysts can lower the energy barrier and accelerate the reaction kinetics, thereby decreasing the overpotential [10], [11], [12]. Therefore, over the past few decades, much research has focused on developing effective and stable electrocatalysts to reduce the overpotential of HER and OER. It is widely known that precious metal catalysts are still the most efficient electrocatalysts due to their excellent activities [13], [14]. Nevertheless, the rareness and expensive cost of these precious metal catalysts severely impede their extensive applications [15], [16]. Consequently, it is indispensable to develop inexpensive and high-efficiency electrocatalysts to overcome this shortcoming. Up to now, various nonprecious metal electrocatalysts have been widely researched for water electrolysis, among them, transition metal sulfide has become a research hotspot because of their natural abundance and outstanding performance for the past few years [17], [18], [19]. However, few of these transition metal sulfides are highly active for both HER and OER. For instance, many literatures have reported that nickel sulfides and cobalt sulfides display activities highly to HER. However, their OER activities are unsatisfactory [20]. On the contrary, NiFe composites display outstanding OER activities, but their HER activities are relatively poor [21]. This inspired us to further enhance the electrocatalytic activities of these transition metal sulfides and develop high efficient bifunctional electrocatalysts for both HER and OER.

Reasonable constructing and interface engineering of the heterogeneous nanostructure is a very useful method to further improve the activity of the materials. Because the heterogeneous nanocomposites not only possess the intrinsic activity of each component but also displays some new activity and obviously enhanced activities due to the synergistic interactions among each component [22], [23], [24]. Moreover, the increase of charge/electron transfer rate at the heterogeneous interface and the increase of active sites will significantly improve the activities of the material [25], [26]. Zang et al. prepared hierarchical a-MoS₂-Ni₃S₂/NF nanosheet arrays, because of the synergistic effect between a-MoS₂ and Ni₃S₂ and the increased active surface area and active sites, the a-MoS₂-Ni₃S₂/NF electrocatalyst displayed outstanding activities for electrolysis of water, only required a decomposition voltage of 1.54 V at 100 mA cm⁻² [27]. Shao et al. prepared an efficient heterostructures (Ni, Fe)S₂@MoS₂. Compared with (Ni, Fe)S₂ and MoS₂, the prepared heterostructures (Ni, Fe)S₂@MoS₂ electrocatalyst displayed outstanding electrocatalytic activity for HER and OER due to the presence of abundant heterogeneous interfaces [28]. Li et al. prepared foam–like Co₉S₈/Ni₃S₂ nanowire arrays, due to the abundant lattice defects in the Co₉S₈/Ni₃S₂ heterogeneous interfaces, the as-prepared Co₉S₈/Ni₃S₂ exhibited efficient activities for HER and OER [29].

Many reported literatures prove that the morphology of the materials has a significant effect on its electrocatalytic activities [30], [31], [32]. The oxygen and hydrogen produced by the electrolysis of water form bubbles, which attached to the surface of the electrode. If these bubbles don't escape from the electrode in time, they will become bigger and bigger and cover the surface of the electrode to prevent further reactions, which will seriously affect the performance of electrocatalyst. Therefore, it is very indispensable to reduce the size and adhesive force of bubbles in electrolysis of water. In a variety of nanostructured materials, two-dimensional (2D) nanoarray structure can provide superaerophobic surface, which can reduce the size and adhesive force of bubbles, accelerate the release of gas, and thus enhance the activity and stability of the electrocatalysts [33]. Also, 2D nanoarray structure is composed of 2D nanosheets, which possess faster electron and mass transport, especially high surface-to-volume ratio. It can provide a large number of surface catalytic active sites and superior mechanical flexibility, which can also enhance the catalytic performance and stability [34]. Furthermore, the nanosheets grown in situ on the conductive substrate (for instance Ni foam, carbon cloth and stainless steel) can further improve electrochemical performance of the catalysts because of the following reason: The direct growth of the nanosheet on the conductive substrate can not only avoid the employment of polymer binder to reduce the equivalent series resistance but can further improve an efficient pathway for charge and electronic and provide more active sites.

Inspired by the above consideration, we used the interface engineering to design and prepare heterogeneous nanoarray Ni₃S₂@FeNi₂S₄ electrocatalyst directly grown on NF (Ni₃S₂@FeNi₂S₄@NF). The whole preparation process consists of three steps. Firstly, NiFe Layered double hydroxides (LDHs) nanosheets were grown on NF through a one-step hydrothermal process. Then, NiFe LDHs nanosheet arrays were further used as a substrate to grow NiOOH to construct an interface-rich core–shell heterogeneous structure. Finally, the LDHs was converted to sulfides by a hydrothermal sulfuration method. 2D nanoarray core–shell structure can provide more active sites at the heterointerface and facilitate the timely release of bubbles, the good synergistic effect between Ni₃S₂ and FeNi₂S₄ results in the superior performance of the Ni₃S₂@FeNi₂S₄@NF. Therefore, as predicted, compared to pure Ni₃S₂ and FeNi₂S₄, the prepared Ni₃S₂@FeNi₂S₄@NF showed outstanding catalytic performance for HER and OER. It delivers a low overpotential of 83 mV and a small Tafel slope of 53 mV dec⁻¹ for HER. Meanwhile, it also delivers an overpotential of 235 mV and a Tafel slope of 92 mV dec^-1 for OER. In addition, the Ni₃S₂@FeNi₂S₄@NF nanoarray as a bifunctional electrocatalyst in an electrolyser delivers ultralow decomposition voltage of 1.46 V at 10 mA cm⁻² with outstanding durability.