0D/3D MoS2-NiS2/N-doped graphene foam composite for efficient overall water splitting

https://doi.org/10.1016/j.apcatb.2019.04.072Get rights and content

Highlights

  • Low cost melamine sponge was used to prepare N-doped graphene foam (NGF).

  • MoS2-NiS2/NGF composite was prepared by hydrothermal and CVD methods.

  • NGF facilitates the transport of electrons, ions, and gases.

  • Heterointerfaces in MoS2-NiS2 boost the dissociation of H2O molecules.

  • MoS2-NiS2/NGF composite with enhanced electrocatalytic activity.

Abstract

Electrochemical water splitting is strongly dependent on mass transport and active sites, however, the difficulty in facilitating mass transport and exposing sufficient active sites is the major bottleneck for both half reactions of the overall water splitting, i.e., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). To address these two issues, a facile and economical strategy is demonstrated for the preparation of the bimetallic sulfides anchored three-dimensional (3D) nitrogen-doped graphene foam (MoS2-NiS2/NGF) hybrid for efficient overall water splitting. As a result, strong interactions occur between MoS2-NiS2 nanoparticles and NGF with unique 3D interconnected tubular hollow structure, leading to the superior performance towards HER and OER. The overpotential and charge transfer resistance of the hybrid are much lower than those of the bare NGF, MoS2/NGF, NiS2/NGF, and physically mixed MoS2-NiS2 + NGF, which can be attributed to the synergistic effect of NGF and bimetallic sulfides with hetero interfaces, thus endowing MoS2-NiS2/NGF abundant active sites and diversified pathways for highly-efficient transport of mass and electron. This bifunctional catalyst also exhibits excellent overall water splitting capability with a current density of 10 mA cm−2 at 1.64 V, which provides a platform for the synthesis of large-scale and cost-efficient catalysts for water splitting.

Introduction

Electrocatalytic water splitting to produce hydrogen and oxygen is an appealing yet challenging task. The development of the two half reactions of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with low overpotential over electrocatalysts is of paramount importance for the successful application of overall water splitting [[1], [2], [3], [4]]. At present, precious noble metal-based electrocatalysts (Pt, RuO2 and IrO2) have been regarded as the highly-efficient and ideal HER and OER catalysts to lower the overpotential and reach considerable energy conversion efficiency [[5], [6], [7]]. However, the deficient and high costs are standing in the way of the large-scale applications for these noble metal-based materials. To turn around this adverse situation, massive research on seeking for advanced, economical, and earth-abundant bifunctional electrocatalysts for overall water splitting is underway, which is still a grand challenge thus far.

In this context, some transition-metal sulfides have been widely employed as bifunctional HER and OER electrocatalysts [[8], [9], [10], [11], [12], [13]], simplifying and optimizing the water splitting system, as well as lowering the product costs. Particularly, hybrid of bimetallic sulfides show superior catalytic performance compared to their monometallic counterparts [14,15]. Much more active sites can be exposed when combining two or more metal species, thus improves the electrochemical reaction kinetics. More importantly, bimetallic sulfides tend to form the heterointerfaces which is able to facilitate the dissociation of H2O molecules and then boost the electrocatalytic performances, as exemplified by the previous studies reported by our group [16] and others [14,17]. In the effort to achieve higher performance, one effective way recently adopted is to incorporate these transition-metal sulfides with carbon additives, which can improve the conductivity and further facilitate the electrocatalytic activity. Among them, two-dimensional (2D) graphene has been attracting increasing attention [[18], [19], [20], [21], [22], [23], [24]]. However, when compositing 2D graphene with electrocatalysts, one of the major problems is the tendency of the 2D graphene materials to aggregate and/or restack due to the existence of van der Waals interactions and/or strong π-π stacking between the graphene sheets [25]. Such irreversible aggregation and stacking not only hamper the homogeneous dispersion of the hybrid catalysts to lower the quality of the catalyst ink, but also make their surface partially inaccessible to reactants and electrolyte. Moreover, it is also detrimental to the evolution and release of gas products of electrolysis, forming aggregated bubbles on the surface of electrocatalysts to block active sites from the electrolyte. These structural drawbacks inevitably cause poor conductivity and compromise the electrocatalytic activity. A desirable electrocatalyst should possess not only rich micropores to supply abundant active sites, but also unimpeded channels and high conductivity for improving electron and mass transfer [26]. Therefore, it is necessary to consider the impact of structure on the reaction activity of HER and OER in a rational design for highly active electrocatalysts [27].

Recently, three-dimensional (3D) graphene with interconnected tubular structure has been successfully prepared by various methods, including template-assisted synthesis [28], self-assembly [[29], [30], [31]], and direct deposition [32,33], etc. It has been demonstrated to be a promising additive for electrocatalyst because of its remarkable electrical properties, diversified pore structure, and fast kinetics for mass and electron transfer [31,34]. The high macroporosity of 3D graphene can greatly increase the accessible surface area of the loaded catalysts, provide multiplex electron transfer network, and facilitate the desorption of gas products [35]. These unique properties strongly suggest that 3D graphene is an ideal scaffold for improving the electrocatalytic activity of the supported materials. For example, Zou et al. [36] prepared 2D NixSy nanowalls decorated 3D nitrogen-doped graphene foam by using the template-assisted method and further employed it as a trifunctional catalyst in unassisted artificial photosynthesis. Zhao et al. [31] reported 3D graphene aerogel supported layered MoS2 nanosheets catalyst via self-assembly approach, which exhibited high catalytic activity and excellent durability for HER application. Motivated by the above studies, herein, a designed template-assisted method for the fabrication of bimetallic sulfides of MoS2 and NiS2 (MoS2-NiS2) anchored 3D nitrogen-doped graphene foam (NGF) as a bifunctional catalyst for HER and OER is reported. We choose melamine sponge as the raw material to prepare NGF due to its low cost and continuous 3D network macroscopic structure. Of special interest to this work, the resulting MoS2-NiS2/NGF is capable of fully combining the merits of both 3D graphene networks and bimetallic sulfides, achieving more active sites and lower overpotential than the corresponding counterparts. Such synergistic effect states clearly that strong interactions exist between the components, which ultimately realizes the superb activity and durability for overall water splitting. The results can offer a new methodology for rational design and exploration of highly-efficient electrocatalysts towards water electrolysis.

Section snippets

Materials and chemicals

Graphite powders was provided by The Six Element (Changzhou) Materials Technology Co., Ltd. Na2MoO4·2H2O, Ni(NO3)2·6H2O, C6H12N4 (hexamethylenetetramine, HMT), C2H6O (ethanol), and S powders were purchased from Sinopharm Chemical Reagent Co., Ltd. 5 wt.% Nafion ethanol solution was afforded by Yi Er Sheng (Kunshan) International Trade Co. Ltd. Melamine sponge was purchased from Outlook Company (Chengdu).

Synthesis of MoO2-Ni(OH)2/NGF

NGF was firstly prepared through a facile and fast method as reported in our previous work [

Preparation and morphology

The NGF was firstly synthesized using a template-assisted strategy by burning GO-absorbed melamine sponge (ca. 1 cm × 1 cm × 0.5 cm, containing C, H, N, and O elements), which is clearly illustrated in Fig. 1. Scanning electron microscopy (SEM) images of the melamine sponge in Fig. 2a–b show the 3D interconnected macroscopic porous structure constructed of solid melamine fibers (with diameters of ˜5 μm). Such unique network structure is expected to be beneficial to the fast diffusion of

Conclusions

In summary, we have prepared bimetallic sulfides nanoparticles anchored 3D nitrogen-doped graphene foam (MoS2-NiS2/NGF) as a bifunctional electrocatalyst in strong alkaline solution for overall water splitting. Benefiting from the synergistic effect of bimetallic sulfides and NGF and the unique 3D interconnected tubular structure, MoS2-NiS2/NGF affords not only abundant active sites, but also diversified pathways for fast and sufficient transport of mass and electrons. MoS2-NiS2/NGF delivers a

Acknowledgements

This work was partially supported by National Natural Science Foundation of China (U1705251, 21433007, and 51772234), National Key Research and Development Program of China (2018YFB1502001), Innovative Research Funds of SKLWUT (2017-ZD-4), and National Postdoctoral Program for Innovative Talents (BX20180231).

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