Elsevier

Science Bulletin

Volume 65, Issue 8, 30 April 2020, Pages 640-650
Science Bulletin

Article
In situ interfacial engineering of nickel tungsten carbide Janus structures for highly efficient overall water splitting

https://doi.org/10.1016/j.scib.2020.02.003Get rights and content

Abstract

Regulating chemical bonds to balance the adsorption and disassociation of water molecules on catalyst surfaces is crucial for overall water splitting in alkaline solution. Here we report a facile strategy for designing Ni2W4C-W3C Janus structures with abundant Ni–W metallic bonds on surfaces through interfacial engineering. Inserting Ni atoms into the W3C crystals in reaction progress generates a new Ni2W4C phase, making the inert W atoms in W3C be active sites in Ni2W4C for overall water splitting. The Ni2W4C-W3C/carbon nanofibers (Ni2W4C-W3C/CNFs) require overpotentials of 63 mV to reach 10 mA cm−2 for hydrogen evolution reaction (HER) and 270 mV to reach 30 mA cm−2 for oxygen evolution reaction (OER) in alkaline electrolyte, respectively. When utilized as both cathode and anode in alkaline solution for overall water splitting, cell voltages of 1.55 and 1.87 V are needed to reach 10 and 100 mA cm−2, respectively. Density functional theory (DFT) results indicate that the strong interactions between Ni and W increase the local electronic states of W atoms. The Ni2W4C provides active sites for cleaving H–OH bonds, and the W3C facilitates the combination of Hads intermediates into H2 molecules. The in situ electrochemical-Raman results demonstrate that the strong absorption ability for hydroxyl and water molecules and further demonstrate that W atoms are the real active sites.

Graphical abstract

The Ni2W4C-W3C Janus structures with abundant superficial Ni–W metallic bonds have been demonstrated. The inserting of Ni atoms into the W3C crystals generate a new Ni2W4C phase, making the inert W atoms in W3C be active sites in Ni2W4C for the overall water splitting.

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Introduction

Rational design of highly efficient, low cost and durable electrocatalysts is fundamentally crucial for developing new energy technologies [1]. For the sake of sustainable and efficient hydrogen production, electrochemical water splitting is paramount significant due to its inherent advantages, including feasibility of large-scale production and highly pure product [2]. The involved two half reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) usually perform well at different electrolyte [3]. It is known that even the individual state-of-the-art Pt/C and IrO2 catalyst cannot effectively catalyze HER and OER in overall water splitting cells simultaneously. It is because the Pt/C catalysts are intrinsically highly active for HER and in contrast, the IrO2 catalysts are only excellent for OER [4]. For industrial condition, the HER and OER involved in alkaline electrolyte always suffer from sluggish reaction kinetics for the multistep proton coupled electron transfer process [5]. Therefore, it is highly desirable to develop the alternative materials featured with earth abundant and high electrocatalytic activity, which is comparable to state-of-art noble metals [6].

To meet the increased demands of overall water splitting, it requires to design bifunctional catalysts to catalyze the HER and OER in the same electrolyte simultaneously. However, transition metal sulfides and nitrides usually exhibit superior activity for HER, while transition metal oxides and hydroxides/oxyhydroxides are active materials for OER [7]. In addition, many OER and HER catalysts are only active in either acidic or alkaline electrolytes [8], [9]. Transition-metal carbides, such as tungsten carbide featured with platinum-like catalytic behavior, have been extensively investigated in HER due to the low price, high corrosion resistance, and superior electronic conductivity [10]. In order to realize the application in both HER and OER in alkaline, it requires researchers to tackle a few big challenges. Pure tungsten carbide has too strong adsorption for H* and thus exhibits low activity for HER in alkaline media [11]. An ideal HER electrocatalyst should have fast water adsorption and H* releasing ability [12]. Therefore, developing essential strategies to modify the electronic structure of tungsten carbide to promote the activity is highly desirable. The integration of active component with tungsten carbide could be an effectively feasible strategy to improve the electrocatalytic activity.

The general reported approaches to improve the HER activity of transition-metal carbide are introducing a second transition metals (Co, Ni, and Fe) as additives [13], [14], [15]. However, the doping different components with carbide into one architecture without forming new phases and interfaces have limited effects on the improvement of overall efficient OER and HER in one electrolyte. The integration of different phase’ features with different functionalities can induce remarkable synergistic effects, including electronic regulation, interfacial stabilization, atomic arrangement etc. [16]. One approach is the adoption of Janus structures particle which surface exhibits distinct composition/physical properties and therefore exhibits a distinct interface. Particularly, the abundant interfaces between different phases play crucial roles in binding, transforming, and transporting the surface species such as adsorbents, electrons and intermediates [14], [15], [16]. Designing and constructing of novel Janus structures with multi-interfaces could be a fundamental strategy to maximize the advantages of hybrid catalysts towards the overall water splitting [17].

Herein, we provide a facile strategy for designing the unique Ni2W4C-W3C Janus structures with abundant interfaces and superficial Ni–W metallic bonds through in situ interfacial engineering by combining the electrospinning technology and graphitization process. The advantages of this approach would be the new formation of Ni2W4C phase, making inert W atoms in W3C be active sites in Ni2W4C for the overall water splitting. In the Janus structures, the Ni2W4C provides active sites for cleaving H–OH bonds, and the W3C facilitates the combination of Hads intermediates into H2 molecules. The in situ electrochemical-Raman results demonstrated the strong absorption ability for hydroxyl and water molecules and the in situ Raman spectra for HER and OER process further demonstrated that W atoms is the real active sites.

Section snippets

Synthesis of Ni2W4C-W3C/CNFs hybrid

In a typical procedure, 0.17 g nickel nitrate hexahydrate and 0.43 g ammonium met tungstate were dissolved in 12 mL polyacrylonitrile/dimethylformamide (PAN/DMF) solution with a mass fraction of 13% PAN and stirred by magnetic stirring apparatus to obtain homogeneous solution. Then the mixed solution was transferred to a syringe with a stainless copper needle at the tip. The electrospinning experiment was performed in a home-made electrostatic spinning machine with a flow rate of 0.6 mL h−1 and

Results and discussion

As shown in Fig. 1a, large amounts of small nanoparticles uniformly distribute on the surfaces of CNFs. The CNFs host provides a stable bracket and reaction zone for the spontaneous formation of the Ni2W4C-W3C nanoparticles and exhibits a unique 3D networks, which are beneficial for the efficient charge separation and fast change transfer [18]. The Ni2W4C-W3C nanoparticles display pea-like morphology and the size ranges from 10 to 80 nm (Figs. 1b and S1 (online)). Fig. 1b and c exhibit the

Conclusions

In summary, the unique Ni2W4C-W3C Janus structures with abundant interfaces and superficial Ni–W metallic bonds have been demonstrated through the in situ interfacial engineering by combining the electrospinning technology and graphitization process. The in situ temperature XRD results strongly reveal the in situ formation process of Ni2W4C by inserting the Ni atoms into the crystal lattice of WCx. The Ni2W4C-W3C/CNFs showed outstanding electrocatalytic performance, requiring overpotentials of

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51803077, 51872204), the National Key Research and Development Program of China (2017YFA0204600), the Natural Science Foundation of Jiangsu Province (BK20180627), Postdoctoral Science Foundation of China (2018M630517, 2019T120389), the Ministry of Education (MOE) and the State Administration for Foreign Expert Affairs (SAFEA), 111 Project (B13025), the National First-Class Discipline Program of Light Industry

Songge Zhang received his Bachelor degree from Zhejiang Sci-Tech University in 2018 under the supervision of Prof. Mingliang Du and Dr. Han Zhu. In 2019, he joined Prof. Mingliang Du’s lab in Jiangnan University as a Research Assistant. His research interests mainly focus on the design of high-entropy alloy nanoparticles and electrospun nanomaterials for electrocatalytic CO2 reduction.

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    Songge Zhang received his Bachelor degree from Zhejiang Sci-Tech University in 2018 under the supervision of Prof. Mingliang Du and Dr. Han Zhu. In 2019, he joined Prof. Mingliang Du’s lab in Jiangnan University as a Research Assistant. His research interests mainly focus on the design of high-entropy alloy nanoparticles and electrospun nanomaterials for electrocatalytic CO2 reduction.

    Guohua Gao received his Ph.D. degree from Tongji University in 2010. He was promoted to Associate Professor of Physics in 2014 at Tongji University. His current research mainly engages in the study of effect of ionic diffusion, material lattice distortion, band structure and phonon regulation on 2D and porous materials by theoretical and experimental methods for new energy applications.

    Han Zhu received his Ph.D. degree from Zhejiang Sci-Tech University in 2016 under the supervision of Prof. Mingliang Du. In 2017, he joined the School of Chemical and Materials Engineering in Jiangnan University as an Associate Professor. His current research interests mainly focus on design and synthesis of electrospun nanofiber based novel nanostructured materials applied in electrocatalysis, heterogeneous catalysis, electrochemical sensors and environmental catalysis.

    Yang Chai is an Associate Professor at the Hong Kong Polytechnic University. He is a member of Hong Kong Young Academy of Sciences, and the vice president of Physical Society of Hong Kong. His current research interest includes low-dimensional materials for electron devices and energy applications.

    Mingliang Du received his Ph.D. degree in 2007 at South China University of Technology and worked as a Postdoctoral Research Fellow. In 2009, he joined the Zhejiang Sci-Tech University in Hangzhou, as Associate Professor, and was promoted to Professor in 2014. In 2017, he joined the Department of Chemistry and Materials Engineering, Jiangnan University. His current research focuses on synthesis of nanomaterials and carbon materials and their applications in clean energy, electrochemical biosensing, etc.; electrospinning and its applications.

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    These authors contributed equally to this work.

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