Elsevier

Electrochimica Acta

Volume 426, 10 September 2022, 140779
Electrochimica Acta

P-block Bi doping stabilized reconstructed nickel sulfide as high-performance electrocatalyst for oxygen evolution reaction

https://doi.org/10.1016/j.electacta.2022.140779Get rights and content

Abstract

Turning the chemical state of in-situ converted oxyhydroxides is essential for the development of efficient oxygen evolution reaction (OER) catalysts. In this work, P-block element Bi is first introduced as a dopant to tune the in-situ reconstruction of Ni3S2 pre-catalyst towards enhanced OER activity. Analysis revealed that Bi doping adjusts the electronic structure of Ni3S2, improves its stability, and leads to mild surface oxidation forming supported Bi-NiOOH species. Theoretical calculations showed that Bi dopant serves as electron depletion center to provide electrons for Ni and O, and reduces the energy barriers of OER intermediates. Consequently, a low OER overpotential of 339 mV is achieved by the Bi-Ni3S2/NF electrode in 1M KOH at 100 mA cm−2. More importantly, Bi-Ni3S2-NF retains a high catalytic stability even at 200 mA cm−2. This work unveils the role of P-block metal doping on the sulfide-based OER pre-catalyst, providing an alternative strategy for the development of high-performance electrocatalysts.

Introduction

Due to its slow reaction kinetics, oxygen evolution reaction (OER) is a limiting process affecting the energy conversion efficiency in water splitting reaction and rechargeable metal-air batteries [1,2]. Thus, it is critical to develop efficient and low-cost OER electrocatalysts to achieve the practical application of these energy conversion technologies [3]. Transition metal (Ni, Co, Fe)-based materials, and their corresponding sulfides, phosphides, carbides, nitrides, and oxides, have shown to be very active towards OER reaction [4], [5], [6]. Among these materials, transition metal sulfides (TMSs) usually exhibit high OER activity and are thus widely used in OER electrocatalysts [7,8]. The most common strategies used to enhance their OER activities include nonporous structural design, heteroatom doping, and defect engineering of the catalyst surface, Wang et al. [9], [10], [11]. Although significant achievements have been made towards the high OER activity of these catalysts, the true catalytic phase or components in the transition metal-based has usually been neglected [12,13].

Recently, a significant number of studies have reported that under the high potential operating condition of OER, oxidation of the transition metal-based catalysts may occur and form oxyhydroxide [14], [15], [16], i.e., the fabricated OER catalysts are pre-catalysts. During the OER reaction, the composition, crystal structure, and morphology of the metal-based catalysis would change dramatically [17,18]. Reports have shown that the post-reconstructed catalyst surface usually considers integrating features of the pristine catalyst and in-situ generated oxyhydroxides, which enhances electrocatalytic performance [19]. This surface oxidation and reconstruction process have targeted induction of the OER pre-catalyst to achieve highly active oxyhydroxide OER catalysts [20,21]. For example, metal cation doping (such as V5+, Ce3+, Fe2+, W6+, and Mo6+ ions) has been used to adjust the local coordination environment and electronic structure to achieve deep self-reconstruction of transition metal-based catalysts towards improved OER catalytic activity [22], [23], [24]. In addition to cation doping, the etching process of anions (B2−, Cl, S2−, and F) can generate more surface vacancies to trigger surface reconstruction of transition metal-based catalysts [25], [26], [27]. In fact, in addition to the transition metals, there is another type of metals: P-block metal elements (Sn, In, Bi, Ge) [28]. P-block metals have single electron pairs, which are different from unfilled valence shell d orbitals in transition metals [29,30]. This feature in P-block metals is expected to provide a unique tuning effect on the chemical state of OER pre-catalyst during its surface reconstruction in catalytic OER reaction [31]. However, to the authors’ knowledge, the introduction of P-block metals in the OER electrocatalysts has been rarely reported. Among the P-block metals, Bi-based materials have adjustable P-electron density and semiconductor properties [32,33]. Therefore, it is expected that the introduction of Bi doping into transition metal-based OER catalyst might result in a unique feature, i.e., tuning of the pre-catalyst, and therefore promote the reconstruction of an active oxyhydroxide OER catalyst.

In the present study, the P-block element Bi was introduced as doping agent to tune the chemical state of Ni3S2 pre-catalysts during its self-reconstruction. The Ni3S2-based OER catalyst was selected because Ni3S2 is a typical representative of TMSs due to its low cost and high electrical conductivity [10,34]. The Bi-Ni3S2 nanorods were loaded on conductive nickel foam substrate through a simple one-step hydrothermal method. The analysis of the chemical state of Bi-Ni3S2 reveals that the doping of Bi enhances the stability of Ni3S2 towards electrochemical oxidation during OER reaction. Accordingly, the self-reconstruction of Bi-Ni3S2 into oxyhydroxide (Bi-NiOOH) only occurs on the catalyst surface, forming a supported Bi-NiOOH@Ni3S2/NF catalyst. That is, the bulk Bi-Ni3S2 phase was preserved during the electrochemical reconstruction process. Hence, the reconstructed outer layer of Bi-NiOOH prevents further corrosion of Bi-Ni3S2, resulting in an effective synergistic effect to further improve the OER reaction performance. Theoretical calculations reveal that the remarkably enhanced OER activity originated from Bi atoms as the electron loss center to effectively adjust the catalyst electronic structure, improve the conductivity of Bi-NiOOH active phase, and reduce the adsorption free energy and reaction energy barrier. The OER overpotential of the Bi-Ni3S2/NF self-supporting electrode is 339 mV at the current density of 100 mA cm−2, which is much lower than that of the Ir-C/NF electrode (380 mV). The electrode shows no apparent decay after 24h stability test at a high current density around 200 mA cm−2, showing its good stability performance. This work unveils the role of P-block Bi doping in tuning the dynamic evolution of transition metal sulfide OER catalyst, offering a new direction for the development of efficient self-reconstructed OER electrocatalysts.

Section snippets

Preparation of Bi-Ni3S2/NF

First, a 2 cm × 2 cm nickel foam (NF) was pretreated in dilute hydrochloric acid solution, distilled water, and ethanol under ultrasonication for 15 min, respectively. 20 mL Na2S•9H2O solution (2 mmol) was then mixed with the solution prepared by dissolving Bi(NO3)3•5H2O (0.1 mmol) and Ni(CH3COO)2•4H2O (0.9 mmol) into 20 mL ethylene glycol. After stirring for 20 min, the above solution was transferred into a 50 mL Teflon-lined autoclave containing the pretreated nickel foam. The autoclave was

Preparation and characterizations of Bi-Ni3S2/NF

Fig. 1 depicts the preparation procedure of Bi-Ni3S2/NF pre-catalyst and its dynamic evolution during OER reaction. Briefly, the Bi-doped Bi-Ni3S2 arrays are fabricated on nickel foam substrate through a facile hydrothermal reaction. As will be discussed in the following paragraphs, the doping of Bi improves the stability of Ni3S2, which avoids the complete reconstruction of bulk Ni3S2. Therefore, during OER reaction, only the Bi-Ni3S2 surface is oxidized and reconstructed into a Bi-NiOOH

Conclusion

In the present study, a self-supported P-block Bi-doped Ni3S2 OER pre-catalyst was fabricated on nickel foam substrate using a one-step hydrothermal method. In-situ Raman and pre- and post OER reaction characterizations of the Bi-doped Ni3S2 revealed that the Bi doping has a stabilization effect on Ni3S2. This ensures a slight oxidation of the Ni3S2 surface during the OER reaction, which resulted in a unique regulation of the surface oxyhydroxide formed on the bulk Ni3S2 crystal. In addition,

CRediT authorship contribution statement

Bowen Du: Investigation, Writing – original draft. Yuhong Luo: Visualization, Data curation. Guihua Liu: Visualization, Data curation. Wei Xue: Visualization, Data curation. Yanji Wang: Writing – review & editing. Jingde Li: Conceptualization, Methodology, Supervision, Writing – review & editing. Luis Ricardez-Sandoval: Conceptualization, Methodology, Supervision, Writing – review & editing.

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No.21908039), Outstanding Young Talents Project of Hebei High Education Institutions (BJ2019013), Natural Science Foundation of Hebei Province (B2019202289, B2019202199), and the “Hundred Talents Program” of Hebei Province (E2019050013). The calculations were performed using the facilities of the Shared Hierarchical Academic Research Computing Network and Compute/Calcul Canada.

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