Ni nanoparticles supported on graphene layers: An excellent 3D electrode for hydrogen evolution reaction in alkaline solution
Graphical abstract
Introduction
Hydrogen, with the advantages of high energy storage, no carbon emission and a valuable by-product of water which occupies about 70% of earth, is recognized as the most promising substitute for fossil fuel. It is a renewable and secure way to create hydrogen from water using electrolysis [1], [2]. However, the water electrolysis system consumes plenty of electrical energy because of its large overpotential for HER. So the issue of finding electrode materials with high electrocatalytic activity and stability is imminent.
Metal Ni, which acts as one of an inexpensive and lavish transition metals, has been generally considered as the most potential electrocatalyst material due to its relatively powerful activity [3], [4]. Studies show that noble metal decorated Ni-based electrodes have been demonstrated as an effective method to enhance the HER activity, for example, Pt-Ni [5], Pt-Ru-Ni [6], Ni3N-Pt [7] and so on. However, the high cost and limited supply of noble metal largely hinder their industrialized application. Hence it's necessary for us to exploit efficient catalysts of Ni-non-noble-metal alloys and Ni-based composites for HER, such as binary or ternary transition metal alloys: Ni-B [8], Ni-P [9], [10], [11], Ni-S [12], Ni-Se [13], Ni-Cu [14], Ni-Mo [15], Ni-Mo-N [16], Ni-Fe-S [17], Ni-Mn-S [18], and Ni-based composites: Ni-MnO2 [19], Ni-CeO2 [20], [21], Ni-polyaniline [22] and Ni-CNT [23], [24], etc. For example, McKone et al. have reported a method for commonly unsupported nanopowders of Ni-Mo, which can be suspended in general solvents and projected to random substrates with good stability under alkaline conditions [15]. On the basis of binary transition metal alloys, Zhang et al. have synthesized ternary transition metal alloys which prepared by N2 plasma treatment of Ni-Mo alloy films and the 3D hierarchical porous Ni-Mo-N on carbon cloth for HER efficiently [16]. For the case of Ni-modified carbon electrode, Chen et al. have recently prepared Ni-CNTs composite cathodes are used as noble metal-free catalyst with superior electrocatalytic activity for HER in alkaline solutions [23]. All above catalysts present better hydrogen evolution performance than pure Ni in alkaline solution. Therefore, we can come to the conclusion that the electrocatalytic activity of electrode materials can be enhanced by incorporating metals or composites into Ni base. It is due to Ni-alloys and Ni-based composites could not only provide a high specific surface area by refining Ni grain size during the crystal growth process, but also improve the intrinsic activity by fabricating a synergistic effect between Ni and embedded active metals or composites.
Nowadays the researchers are gradually focus on drawing graphene into Ni base to prepare Ni-graphene composite materials for HER [25], [26], [27], [28], [29]. As we know, graphene [30], [31] has already gained tremendous attention because of its outstanding chemical and physical properties like good conductivity, high mechanical strength, especially large surface area, and these remarkable advantages make it well suited for electrocatalysis. Recently, Huang and co-workers have developed Ni-graphene (Ni-G) composite which fabricated on the surface of stainless steel by using a simple and traditional electrodeposition technique for HER. Graphene acted as a communicating platform in facilitating the electron transfer and transport during the HER [25]. However, the aggregation of Ni particles and graphene sheets leads to negative activity for HER. Moreover, Wang group has taken advantages of Ni foam, with 3D structure and good conductivity, to synthesize a self-assembly 3D nickel foam-graphene (NF-G) cathode by facile hydrothermal approach for HER, and the electrocatalytic activity of the composite cathode was improved after coating graphene [26]. Their work suggests that NF-G electrode not only possesses large specific surface area but also provides effective mass transfer. But the surface of as-prepared NF-G cathode is relatively smooth, and the combining ability of Ni foam and graphene sheets is weak which is not good for synergistic action. These are the reasons why NF-G cathode did not present the high performance for HER. In addition, Ni-graphene composite also exhibits good performance for oxygen evolution reaction. Sun group has prepared Ni nanoparticles-graphene hybrid film on conductive substrate. The hybrid film was an efficient catalyst for oxygen evolution reaction with highly catalytic activity and good durability [32]. Based on the above discussions, we can conclude that Ni-rGO catalyst displays good performance for electrocatalysis, yet there is much space for improving its morphology, microstructure and electrochemical activity.
In this paper, we introduce supergravity field [33], [34] into electrodeposition process to obtain the excellent Ni-rGO composite electrode for HER. Compared with traditional electrodeposition, on the one hand, supergravity electrodeposition could promote the specific surface area of composite catalyst by refining Ni particles and preventing Ni nanoparticles and rGO sheets from aggregating because of its fast micro mixing and mass transfer. On the other hand, the interfacial adhesion strength increases significantly for composites of Ni nanoparticles and rGO sheets which results from the higher potential during supergravity electrodepositon than normal electrodepositon. This is helpful to improve intrinsic activity and electrical conductivity of Ni-rGO electrode. Comprehensive the above two points, the interface density (the area of contact interphase within unit quality) between Ni particles and rGO sheets greatly increases under the supergravity electrodeposition. Furthermore, it has verified that the interfacial activity between Ni and rGO is much higher than pure Ni and graphene for HER by DFT calculations in this paper. Therefore, we fabricate an efficient 3D electrode for HER with rGO sheets and Ni nanoparticles co-deposited on Ni foam substrate by one-step supergravity electrodeposition. And the impacts of supergravity field speciality, carbon (C) content existed in coating and 3D structure of Ni foam on morphology, microstructure and electrochemical activity of the composite cathodes were studied elaborately.
Section snippets
Preparation of GO colloidal solution
In a classic experiment, GO was prepared by modified Hummers method [35]. 1 g graphite flakes (325 mesh) was added into a mixed solution of 1.5 g KNO3 and 50 mL concentrated H2SO4. The mixture was stirred evenly in the ice bath and 8 g KMnO4 was added very slowly. Then transferring the mixture to a 40 °C water bath for 6 h accompanied uninterruptedly stirring. After that the mixture was putted into 200 mL deionized water drop by drop and then warmed in 90 °C water bath for 30 min. Another
Morphology analysis
As mechanism diagram illustrated in Scheme 1, the 3D Ni-rGO/Ni foam electrode was constructed via a one-step approach of supergravity eletrodeposition, which converted Ni2+ and GO into Ni nanoparticles and rGO layers. rGO layers play an important role in facilitating charge transfer and supporting Ni nanoparticles more uniformly. H3O+ gains an electron from Ni which absorbed on the surface of rGO layers into H*, then two H* collide into H2. The SEM images of pure Ni foam are presented in Fig. S2
DFT calculation
For the better description of the performance of hydrogen evolution, we calculated adsorption free energy of H* (ΔGH*) used density functional theory (DFT). The optimal value of ΔGH* should be zero, which indicates that chemical adsorption of H* on the surface of catalyst is not too strong or too weak. As presented in Fig. 5, among the three catalysts researched, Ni-rGO displays the smallest |ΔGH*| value of 0.31 eV, which is more closer to zero than that of pure Ni (|ΔGH*| = 0.73 eV) and
Conclusion
In this work, we have used a one-step approach to design rGO sheets and Ni nanoparticles co-grown on Ni foam substrate to fabricate Ni-rGO/Ni foam composite catalyst which presents the excellent performance for HER in alkaline solution. Compared with former reported Ni-rGO composite catalysts, the Ni-rGO/Ni foam catalyst described here exhibits outstanding hydrogen evolution with a lower Tafel slope, a higher exchange current density, smaller overpotentials and better stability under the
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No. 51674221).
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