Co,N-codoped graphene as efficient electrocatalyst for hydrogen evolution reaction: Insight into the active centre
Graphical abstract
Introduction
Hydrogen, an abundant and renewable clean fuels, has been expected a promising candidate to replace traditional fossil fuels in the future [1]. Electrochemical water splitting via hydrogen evolution reaction (HER) is one of the most ideal pathway for sustainable hydrogen production with high purity in large quantities [2]. HER is intrinsically quite difficult, thus requires catalyst to make the reaction more energy-efficiency [3]. Pt is the typical choice as a catalyst, but its high cost and rarity are big hurdles for practical commercialization [4]. Enormous efforts have been committed to developing noble metal-free catalyst. In this regards, a variety of transitional metal (Co, Ni, Fe, Mo, Mn, W) derivatives (i.e., carbide, nitride, oxide, phosphide, borides, and chalcogenide) [5], [6], [7], [8], [9], [10], [11], [12] have been proposed. Unfortunately, most of them normally suffer from the drawbacks of low electric conductivity and short lifetime [12].
Carbon materials, including porous carbon and carbon nanotube (CNT) as well as graphene, feature the merits of excellent electric conductivity and strong tolerance to acidic or alkaline medium. Recent advances in carbon composites have presented their intriguing potentials in energy-related electrocatalytic reaction such as oxygen reduction reaction (ORR) [13], oxygen evolution reaction (OER) [14], and HER [15]. Theoretically, pristine carbon materials show limited electrocatalytic activities because they are chemically inertial. Doping carbon with heteroatoms is a shortcut to enhance electrocatalytic activity [16]. For example, N- [17], B- [18], P- [19], and S- [20] doped carbon materials have been extensively studied as alternative electrocatalysts for ORR, OER, or HER. Unfortunately, these single heteroatom-doped carbon materials still can not compete with noble metal-based catalysts yet in performance. To further improve electrocatalytic activities, dual heteroatom-codoped carbon materials such as B-N [21], Fe-N [22], S-N [23], P-N [24], and Co-N [25] codoped carbon materials, have been proposed. Among them, B-N [21], Fe-N [22], and S-N [23] codoped carbons are outstanding ORR electrocatalysts, and P-N codoped carbons are remarkable OER electrocatalyst [24], while Co-N co-doped carbons are typical HER electrocatalyst [25]. However, when two heteroatoms are jointly doped into carbon matrix, it is fundamentally essential to determine which one is electrocatalytically active centres so that critical insights and scientific basis can be gained for rational design of more efficient electrocatalysts accordingly. Thus, the corresponding synthesis strategy is required to be investigated to maximize electrocatalytic activity by doping either simultaneously or sequentially. For example, B-N codoped carbon materials prepared by sequential doping presents much higher ORR activity [21], whereas Fe-N codoped carbon materials prepared by simultaneous doping delivers better ORR performance [26]. This implies that efficacious electrocatalyst systems with relevant synthesis strategy cannot be simply generated with a set of simple heuristics. Although Co and N codoped carbon materials have been extensively studied and demonstrated to be excellent electrocatalyst for HER as alternative of precious metals, the favorable doping strategy, together with their electrocatalytic centre, still remains elusive. Therefore, it is desirable to study the proper doping strategies for the fabrication of Co,N-codoped carbon materials and unravel their active centres for HER.
Moreover, despite many fascinating properties of graphene, it always suffers from the problems of serious restacking due to its high surface energy. Assembling two dimensional (2D) planar graphene into 3D architecture is generally an effective way to overcome restacking [27], [28], [29]. Unfortunately, the conventional 3D graphene prepared by frozen-dry technique has poor mechanical stability. Recently, we have proposed a 3D graphene crosslinked by chemical bond, which possesses robust 3D hierarchically porous architecture [30]. Herein, two types of Co and N codoped 3D robust graphene (Co,N/3DG) materials were engineered to compare their HER performance. One was fabricated by simultaneous doping of Co and N (Co,N/3DG-1), while the other was fabricated by sequential doping of them (Co,N/3DG-2). The structure characterization elucidates that the Co,N/3DG-2 features the more moderate Co-N coordinated structure than Co,N/3DG-2. Electrocatalytic tests proved that the latter material has overall superior HER performance to the former one. In combination with the result of theoretical calculations, CoC2N2 structure is believed to be the most active site in Co and N codoped carbon material.
Section snippets
Reagents and apparatus
Natural flake graphite (325 meshes) was purchased from Qingdao Graphite Co., Ltd. (Qingdao, China). Cobalt nitrate, formaldehyde aqueous solution (35 wt%), melamine, vitriol, phosphoric acid, and potassium permanganate were obtained from Aladdin Chemicals Reagent Co., Ltd. (Shanghai, China). All other reagents not mentioned were of analytical grade and used as received. The water used in the electrochemical experiments was freshly prepared by a Milipore ultrapure water system (Milipore-RO Plus)
Results and discussion
The fabricating process of the two types of Co,N-codoped 3D robust graphene are schematically illustrated in Fig. 1. For the preparation of Co,N/3DG-1, a one-step codoping route was performed to dope N and Co simultaneously. Typically, the mixture of GO suspension, formaldehyde, melamine, and cobalt nitrate was hydrothermally treated in a Teflon-lined autoclave. The as-obtained hydrogel was then dried in air, and the solid was calcined under Ar atmosphere. After leached by acid and base to
Conclusions
In summary, we have designed two types of Co and N codoped 3D graphene materials which are fabricated by one-step of simultaneous doping strategy (Co,N/3DG-1) and two-step of sequential doping strategy (Co,N/3DG-2), respectively. It has been found that Co,N/3DG-2 presents the more moderated Co-N coordinated structure than Co,N/3DG-1. When serving as the electrocatalysts for HER, Co,N/3DG-2 exhibits significantly better HER performance than Co,N/3DG-1. The DFT calculations unraveled that the
Acknowledgements
Shumin Wang and Lei Zhang contributed equally to this work.This work is financially supported by the Natural Science Foundation of Jiangsu Province, China (BK20161191), the National Natural Science Foundation of China (2147603), the Advanced Catalysis and Green Manufacturing Collaborative Innovation Center of Jiangsu Province. The computational work used Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number TG-DMR140083,
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