Strongly coupled Mo2C and Ni nanoparticles with in-situ formed interfaces encapsulated by porous carbon nanofibers for efficient hydrogen evolution reaction under alkaline conditions

https://doi.org/10.1016/j.jcis.2019.09.102Get rights and content

Highlights

  • A simple synthesis method of novel non-precious metal electrocatalysts for HER.

  • As bimetallic precursor, NiMoO4 can promote the formation of heterostructures.

  • The improved interaction between Ni and Mo2C can generate numerous HER active sites.

  • Strongly coupled Mo2C and Ni nanoparticles on porous carbon fibers as HER catalyst.

Abstract

Herein, strongly coupled Mo2C and Ni nanoparticles with in-situ formed interfaces encapsulated by porous carbon nanofibers (Ni-Mo2C-CNF) have been rationally fabricated via pyrolyzing electrospinning polyvinyl alcohol fibers containing hydrothermally obtained NiMoO4 under Ar atmosphere and applied as high-performance and stable electrocatalyst for HER in alkaline electrolytes. Powered by NiMoO4 as homologous bimetallic precursor, the Ni-Mo2C-CNF possesses numerous in-situ formed Ni-Mo2C interfaces, which facilitates the synergistic effect between Ni and Mo2C, improving the conductivity and thus boosting the electrocatalytic performance towards HER. In the meantime, the porous carbon nanofibers with well encapsulated Ni-Mo2C active components stacks, constituting conductive network, which promotes the mass transport, electron transfer, active sites exposure and electrocatalytic stability. As a result, the Ni-Mo2C-CNF features prominently in HER, as it demands a low overpotential of 196 mV but is able to stably yield the current density of 10 mA cm−2 with a small Tafel plot of 54.7 mV dec−1. The method demonstrated in our work to synthesize bimetallic heterostructured materials will offer valuable inspiration to construct promising non-precious electrocatalysts for diverse vital renewable energy applications.

Introduction

With the depletion of fossil fuels, energy shortage is an important issue that constrains economic and social development [1]. Hydrogen is an ideal alternative energy carrier to fossil fuels in the future which is clean, efficient, and zero pollutant emissions [2], [3], [4], [5]. Up to now, electrocatalytic water splitting has been considered as a simple, effective, and pollution-free hydrogen production process. Although the rare Pt-based materials are found to be the most effective electrocatalysts for hydrogen evolution reaction (HER) [6]. Its large-scale applications are still limited due to the drawback of high consumption and expensive price [7], [8], [9]. Therefore, efficient, non-precious metal electrocatalysts that can be produced on a large scale are needed for the future hydrogen economy [10], [11].

In the past decade, 3d metal based-materials, such as Co, Fe, Ni, Mn, either alone or composites, have been reported as excellent electrocatalysts for hydrogen production, among which Ni-based catalysts attract more attention, owing to their significant availability, low overpotential and excellent corrosion resistance [12], [13], [14], [15]. Meanwhile, molybdenum carbide (Mo2C) has been extensively studied as HER catalysts, in virtue of excellent electrocatalytic performance, such as significant electrical conductivity, wide range pH stability, and similar d-band electronic structure to Pt-group metals [5], [9]. Inspired by the aforementioned results, several outstanding researches have been studied, focusing on Ni-Mo2C based composite catalysts [16], [17], [18], [2]. It has been proven that doping Ni into Mo2C can achieve a better HER performance compared with the bare Mo2C [19]. Although the Ni and Mo2C composite modified materials have exhibited good HER properties, it is convinced that constructing novel Ni-Mo2C based materials with numerous active interfaces between Ni and Mo2C can improve their interaction and further significantly enhance the electrocatalytic performance towards HER.

Herein, we have made it a success to fabricate strongly coupled Mo2C and Ni nanoparticles encapsulated by porous carbon nanofibers (Ni-Mo2C-CNF) through annealing hydrothermally obtained NiMoO4 modified electrospinning polyvinyl alcohol fibers under Ar atmosphere. With NiMoO4 as single metal precursor, the resultant Ni-Mo2C-CNF can be endowed with massive Ni/Mo2C interfaces, which exhibits an outstanding activity for HER with a low overpotential (196 mV) to reach current density of 10 mA cm−2, a small Tafel plot (54.7 mV dec−1) and remarkable stability during operation in basic solution. Therefore, this work will provide positive inspirations for the designable construction of heterostructured composite materials with outstanding electrocatalytic performance for various future energy applications. The Ni-Mo2C-CNF electrocatalysts were prepared by three step procedures, as shown in Scheme 1. The NiMoO4 nanorods, as homologous bimetallic precursor, were firstly synthesized by a hydrothermal method based on the published literature and appropriately characterized in Fig. S1. Then, the nanofibers containing NiMoO4 are prepared by electrospinning the mixture of NiMoO4 and PVA. Finally, the electrospinning resultant was carbonized under the Ar atmosphere to achieve the targeted catalyst. In order to understand whether the homogenous bimetal oxide improves the performance of the catalyst, the Ni-Mo2C-CNF-H (NiAc2·4H2O and (NH4)6Mo7O24·4H2O as precursors), Ni-CNF, and Mo2C-CNF were synthesized in the same methods as described above.

Section snippets

Preparation of NiMoO4 nanorods

The NiMoO4 nanorods were prepared by a hydrothermal method according to the reported literature [20]. Briefly, 0.49 g NiAc2·4H2O and 0.34 g (NH4)6Mo7O24·4H2O were dissolved in 35 ml H2O. The clear solution was transferred into a 50 ml Teflon-lined stainless autoclave and kept at 160 °C for 6 h. After reaction, the solution was centrifuged, washed, and dried to obtain NiMoO4 nanorods.

Preparation of Ni-Mo2C-CNF catalyst

The NiMoO4 nanorods (0.3 g) and 10 wt% PVA aqueous solution (20 g) were mixed together. The obtained precursor

Results and discussion

The crystal phase and morphology of as-prepared materials were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM). As shown in Fig. 1a, the XRD patterns of Ni-Mo2C-CNF and Ni-Mo2C-CNF-H completely coincide with the standard cards of Ni (PDF#04-0850) and Mo2C (PDF#35-0787), which proves the successful synthesis of carbon nanofibers modified by Ni and Mo2C. Furthermore, it is also indicated that the employment of homologous or

Conclusions

Based on previous reports on the interaction between Mo2C and Ni, which can effectively speed up the sluggish HER Kinetics [16], [17], [18], [2]. The present work that application of NiMoO4 as homologous bimetallic precursor can promote the in-situ formation of interfaces between Ni and Mo2C phase, thereby reinforce the corresponding interaction for generating numerous HER active sites and further boost the HER activity of electrocatalyst. The Ni-Mo2C-CNF obtained by pyrolyzing electrospinning

Declaration of Competing Interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China, China (Nos. 21005008 and 21773017 and 21303181), Jilin Provincial Science Research Foundation of China, China (No. 20160520133JH) and Educational Department of Jilin Province of China, China (No. JJKH20170544KJ).

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