Bismuth activated succulent-like binary metal sulfide heterostructure as a binder-free electrocatalyst for enhanced oxygen evolution reaction

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Abstract

Oxygen evolution reaction (OER) is the key to achieve highly efficient hydrogen production during water splitting. Herein, flexible nanorods-integrated succulent-like Bi2S3/Ni3S2/NF heterostructure has been prepared by a facile solvothermal method and applied for OER. We highlight the unique nonequivalent sp3 hybridization of P-region metal based sulfides, which makes a possibility of electronic inductive effect and enhanced electrocatalytic performance. The Bi2S3/Ni3S2/NF catalyst shows low overpotential 268 mV at 10 mA cm−2 and low Tafel slope of 82 mV dec−1. Long-term stability evaluated at high current density suggests that succulent-like Bi2S3/Ni3S2 could be a promising alternative to noble-metal based electrocatalysts for water oxidation reaction in alkaline medium.

Introduction

Nowadays, energy supply and environment pollution are the extremely urgent issues around the world [1]. Many efforts have been devoted to develop abundant and environmental-friendly energy sources [2]. In this regard, electrocatalytic water splitting is recognized as one kind of effective ways to achieve clean and renewable energy, which is significant to energy storage and conversion [3]. Hydrogen can be obtained by electrocatalytic water splitting based on two half-reactions: hydrogen evolution reaction (HER) at cathode and oxygen evolution reaction (OER) at anode [4], [5]. However, the currently commercial electrolyzers usually operate at a high voltage region of 1.8–2.0 V because of the sluggish kinetics of Odouble bondO bond formation with four electrons transferred at anode, leading to high energy consumption and limited industrialization [6].

A catalyst plays an essential role in heterogeneous water splitting system, especially in the bottle-neck process of OER. It is well-known that the precious-metal based materials, such as IrO2 and RuO2, are considered as benchmark catalysts due to the intrinsic electrocatalytic activity, but their large-scale application is hindered by the scarcity, high cost and poor stability [7], [8]. Therefore, intense efforts have been made to develop unfilled 3d-orbital transition metal based materials, for instance, transition metal oxides [9], layered double hydroxides [10], sulfides [11], phosphides [12] and borides [13], etc., due to the tunable electronic structure, controllable morphology, good stability, earth-abundant and low cost.

Transition metal sulfides (TMSs), including MoS2 [14], Co9S8 [15], Ni3S2 [16], CuCo2S4 [17], NiCo2S4 [18], have aroused enormous attention in the field of energy science. Among the TMSs materials, Ni3S2 is outstanding with intrinsic metallic behavior and higher conductivity than many of others (e.g., MoS2) [19], [20]. Meanwhile, the low cost makes Ni3S2 pretty available for numerous electrochemical applications [21], [22], [23]. However, the electrochemical activity of Ni3S2 is inferior to benchmark Pt/C and RuO2 catalysts. Numerous efforts have been carried out to improve the electrocatalytic performance of Ni3S2 by coupling with sulfides, phosphides and hydroxides hybrids [24], [25]. Feng et al. reported MoS2/Ni3S2 heterostructures for electrochemical overall-water-splitting to overcome the inherently poor conductivity of MoS2 [26]. Xi et al. prepared Ni3S2/Ni2P heteronanorods on nickel foam (NF) for HER in a wide pH range [27]. However, to our best knowledge, no research has focused on directly growth of Ni3S2 coupled with bismuth sulfide (Bi2S3) materials for OER.

Bi2S3, the typical laminar structured semiconductor with direct band gap (1.3 eV), has been largely applied to lithium/sodium storage [28], solar cells [29], photocatalysis [30] and electrochemical sensors [31] on account of environmental benignity, biocompatibility, rapid electron transfer, large absorption efficient and photoelectric properties. Unlike transition metal sulfides, Bi2S3 has rarely been investigated in OER, but its unique hybrid orbital makes it possible to generate electronic modulation on the host metal, resulting promoted electroactivity [32]. Hereinto, the combination of Bi2S3 and Ni3S2 is highly expected to fabricating environmental friendly electrocatalyst with high-performance for water splitting.

Furthermore, facile synthesis of a catalyst or electrode is also important for the practical use. Conventional electrode preparation is usually carried out by means of ink loading on a planar electrode based on polymer binders, which may cause aggregation and be detrimental to gas adsorption and desorption process [33]. Differently, in-situ growth of a catalyst on 3D substrate, like NF, can not only alleviate the issues mentioned above but also offers large surface area and more active sites exposure [34]. A recent work by Ho et al. demonstrated Ni3S2 superstructures on NF, showing low overpotential of 340 mV at the current density of 10 mA cm−2 [35]. Komarneni et al. constructed Ni3S2 nanosheets on NF through multiple hydrothermal electrodeposition method [36]. As discussed above, NF substrate serves as both current collector and Ni source interacting with sulfides. However, some reports have emphasized that the self-standing TMSs are so fragile that they cannot be used directly as electrode [37]. Therefore, the synthesis of flexible TMSs electrode with favorable electrocatlytic performance is significant to water oxidation reaction.

In this work, a self-assembled succulent-like Bi2S3/Ni3S2/NF catalyst is developed via mild solvothermal method and applied to the OER. Unique nonequivalent sp3 hybridization of Bi2S3 gives rise to electronic modulation of heterostructure and resultant electrocatalytic performance. Bi2S3/Ni3S2/NF, as a flexible electrode, shows low overpotential of 268 mV at 10 mA cm−2 and low Tafel slope (82 mV dec−1) in alkaline medium. Moreover, Bi2S3/Ni3S2/NF displays good stability at high current density for 12 h. Succulent-like Bi2S3/Ni3S2/NF can be a promising substitute for noble-metal based OER catalysts.

Section snippets

Reagents and materials

KOH, HCl, thiourea (CH4N2S), ethylene glycol (EG), ethanol (EA), N,N-dimethylformamide (DMF) and acetone are analytical grade and purchased from Chengdu Kelong Chemical Reagent Factory (Chengdu, China). Bi(NO3)3·5H2O is purchased from Adamas-beta Reagent Co., Ltd. (Shanghai, China). Ruthenium Oxide (RuO2) is purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Nafion solution (5 wt%) is purchased from Shanghai Aladdin Biochemical Technology Co., Ltd (Shanghai, China). NF is

Morphology and chemical structure analysis

SEM images and EDX spectrum of Ni3S2/NF are shown in Fig. S1. Ni3S2/NF presents a harsh film mainly on the large radius of curvature of NF. As for Bi2S3/Ni3S2/NF (Fig. 1a), Bi2S3 tends to anchor on the the small radius of curvature of 3D framework. With the nucleation proceeding, Bi2S3 nanorods integrate into unique succulent-like morphology with a size of nearly 2 μm (Fig. 1b and c). The resultant microstructures increase surface areas, which offer more accessible active sites for

Conclusions

In this paper, self-assembly flexible Bi2S3/Ni3S2/NF catalyst is successfully prepared via facile solvothermal method. We demonstrate the significance of nonequivalent sp3 hybridization of P-region metal sulfides, which leads to electronic inductive effect and synergistic effect of Bi2S3/Ni3S2/NF heterostructure. The nanorods-integrated succulent-like microstructures offer notably increased ECSA. With the aid of electronic modification, as-obtained Bi2S3/Ni3S2/NF catalyst exhibits an enhanced

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.

Acknowledgments

This work was financially supported by Sichuan Science and Technology Program (Grant No. 2018G20130 and 2018GZ0543).

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