Facile fabrication of molybdenum compounds (Mo₂C, MoP and MoS₂) nanoclusters supported on N-doped reduced graphene oxide for highly efficient hydrogen evolution reaction over broad pH range

Highlights

• Develop new molybdenum compounds (Mo₂C, MoP and MoS₂) nanoclusters supported on N-doped reduced graphene oxide.

• Prepared catalysts exhibit excellent and durable electrocatalytic performance over a broad pH range.

• Potential applications of ordered self-assembled composites for water splitting to produce hydrogen.

Abstract

Electrocatalytic water splitting for hydrogen production is highly desirable to replace oil energy. The catalytic performance is closely related to the conductivity, active sites and reaction Gibbs free energy of the catalyst. A general guideline for improving catalytic performance is to obtain porous carbon-based materials with a high surface area, plentiful defects and metal compound loading. Here, defect-rich nitrogen-doped reduced graphene oxide (RGO) with molybdenum (Mo)-based compound loading was designed through a two-step method. The peroxide-assisted step under low-temperature carbonization with an air atmosphere promotes the formation of defects, and the hydrothermal process improves the crosslinking degree to form a porous structure at a high carbonization temperature. The obtained catalysts exhibit excellent and durable electrocatalytic performance over a broad pH range. In addition, the transmission electron microscopy (TEM) and electron paramagnetic resonance (EPR) results clearly reveal the presence of defects. The theoretical analysis demonstrates that the RGO and Mo-based compounds have an efficient synergetic effect on the catalytic activity. This work provides clues for the development of new catalysts for water splitting to produce hydrogen.

Introduction

Hydrogen has attracted increasing attention due to its environmentally friendly advantages and it is considered to be the next generation of energy as a substitute for fossil fuels [1], [2], [3]. The zero emission of exhaust gases and the high heat associated with hydrogen energy contribute to the demand for hydrogen production. To develop a highly efficient method for hydrogen production, enormous efforts have been made to obtain a highly active catalyst. The traditional method is the catalytic cleavage of fossil fuels, which is associated with pollution exhaust, and fossil fuel sources are limited on earth. Electrocatalytic water splitting for hydrogen production has received considerable attention due to the easy availability of water resources [4], [5], [6], [7]. The traditional catalysts for hydrogen production are platinum and its alloys, which show a nearly zero Gibbs free energy (ΔG_H*) and low overpotential for the hydrogen evolution reaction (HER) [8], [9]. However, the rare sources and high cost of such materials have impeded widespread development. Therefore, catalysts with low cost and plentiful sources have been widely reported for the HER. Transition metal compounds with excellent conductive properties and a characteristic d-band structure present tremendous potential in the HER catalytic field [10], [11], [12], [13]. Such conductivity may be beneficial to electron transfer, and the d-band structure can change the electron cloud density distribution to reduce the adsorption ΔG_H* and desorption ΔG_H* [14], [15], [16].

Recently, transition metal phosphides, sulfides and carbides have attracted more interest for the HER and have been widely investigated [17], [18], [19], [20]. Heteroatoms (P, S and C) can induce the d-band electron distribution of adjacent metal atoms, which can accelerate the adsorption and desorption of intermediate H*. Phosphorus atoms are negative in transition metal phosphides, thus they can acquire positive protons for the HER. Liu et al. reported a facile method to obtain reduced graphene oxide (RGO) with bimetallic Co-Ni-P hollow nanosphere loading, which exhibited ideal catalytic performance for the HER [21]. The nanostructure provided a stable environment for catalysts in the HER process. In addition, the transition metal dichalcogenides have also become effective candidates for catalysis of the hydrogen production process, which is attributed to their structural electron distribution, low band-gap energy, and adjustable phase interface. Because MoS₂ has a similar ΔG_H* to that of Pt and a 2D structure, it has been a research hotspot for catalytic HER. The 2D layer structure can benefit charge transport from the electrode surface to active sites, and the edge sites provide a low adsorption ΔG_H* for the HER. Yuan et al. had reported a series of works about the formation of MoS₂ supported on different materials [22], [23]. They constructed a unique core–shell structure to improve the catalytic performance and stability of the composite, which revealed a facile method that the morphologies of core–shell structure could expose abundant active sites to facilitate the catalytic process. Deng et al. revealed a new method to improve the catalytic performance of MoS₂ catalysts [24]. They adopted N doping with PO₄³⁺ intercalation with a synergistic effect to efficiently convert 2H-MoS₂ into 1 T-MoS₂ (up to 41%). The low band gap, adductive d-band center, and nearly zero ΔG_H* value facilitate the reaction kinetics and improved the electrocatalytic properties. To date, transition-metal carbides (TMCs) have also drawn increasing attention as desirable substitute catalysts for hydrogen production. Among the aforementioned TMC catalysts, molybdenum carbides are a promising representative owing to the synergistic effect between transition metals and carbon atoms. Specifically, the band between the Mo3d and C2p orbitals in Mo-C can localize the C2p orbitals to reduce the ΔG_H* barrier, facilitating the adsorption/desorption of intermediate H* for the HER. Dong and coworkers reported that the excellent electrode conductivity resulting from the doping of Mo atoms accelerated charge transfer to form H₂ in the HER process [25]. Some strategies have been adopted to improve the catalytic performance, such as synergistic electron transfer and mass transfer phenomena. Yang et al. prepared heterogeneous electrocatalysts with a unique interfacial structure (O-Mo-C interfaces) to facilitate charge transfer [26]. The interfacial structure acted as active sites to adsorb/desorb intermediate H*, forming H₂.

Mo-based catalysts, such as Mo₂C, MoP and MoS₂, have been widely reported as candidates to replace commercial Pt/C catalysts for the HER. Due to the effective cost advantage and similar Pt-like electrocatalytic performance of Mo-based catalysts, it has attracted great attention in the development of large-scale applications of HER catalysts. 2D and 3D nanocarbon materials have been adopted to alleviate the problems of poor conductivity, limited exposed active sites and low surface area. Among the nanocarbon support matrices used, RGO has been considered an ideal matrix due to its specific electron structure, plentiful defects, outstanding electrical conductivity and excellent ductility. Catalysts supported on graphene were regarded as great synergistic multi-composite catalysts for the HER. Ghasemi et al. prepared a Pd-graphene nanocomposite for HER catalysis, which showed an obvious improvement caused by the high dispersity of Pd particles on the graphene surface and fast electron transfer [27]. Zhang et al. reported a facile and novel method to obtain transition metal phosphide (FeP) nanoparticles supported on graphene [28]. The good catalytic performance was attributed to the plentiful exposed active sites caused by high dispersity, intimate interactions and electronic coupling between the FeP nanoparticles and graphene.

It is well known that previous reports on transition metal compounds (TMC) supported on graphene mainly focused on the dispersion of TMC nanoparticles on the surface of traditional RGO. However, traditional RGO is prone to agglomeration, causing a decrease in the number of exposed active sites. In this work, 1,3,5-trisbenzoic acid (TBA) and 2-amino-benzoic acid (ABA) were polymerized to form carbon-based materials. Polyvinylpyrrolidone (PVP) acts as a free radical initiator to promote polymerization in an air atmosphere, while ammonium chloride (NH₄Cl) provides a shielding gas atmosphere. The first low-temperature carbonization step provided an O₂ peroxide-assisted process to form defective edges. In addition, hydrothermal treatment of the added Mo-based compound increased the crosslinking degree, which was favorable for porous structure formation. Moreover, NH₃ from polymer decomposition could chemically reduce GO at high temperature to obtain RGO. Combining the electrochemical catalytic effects of defective porous RGO and Mo-based compounds, the as-prepared catalysts showed a low overpotential over a broad pH range with very good stability. This strategy paves the way for the design of defect-rich porous carbon-based catalysts.