Elsevier

Materials Letters

Volume 183, 15 November 2016, Pages 290-295
Materials Letters

Carbon supported Co9S8 hollow spheres assembled from ultrathin nanosheets for high-performance supercapacitors

https://doi.org/10.1016/j.matlet.2016.07.106Get rights and content

Highlights

  • The spheres are rationally designed and synthesized via a facile solution-based method.

  • The as-prepared spheres exhibit high surface area and excellent structural stability.

  • The spheres demonstrate significantly enhanced electrochemical performance for supercapacitors.

Abstract

UltrathinCo9S8nanosheets were successfully assembled into a carbon supported nanostructure of C@Co9S8core-shell hollow spheres by a facile solution-based method consisting of template-engaged growth of a precursor shell, followed by simultaneous sulfidation and removal of template. The prepared C@Co9S8hollow spheres exhibit enhanced electrochemical performance in supercapacitors due to their high surface area and excellent structural stability.

Graphical abstract

Carbon supported Co9S8hollow spheres (C@Co9S8) assembled from ultrathin nanosheets are prepared via a facile solution-based approach including the template-engaged growth of precursor shell followed by simultaneous sulfidation and template removal. The as-fabricated hierarchical hollow spheres possesses a high specific surface area (266 m2 g−1) with good structural stability, and exhibit high specific capacitance with excellent cycling performance as electrode materials for supercapacitors.

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Introduction

The global climate change and greenhouse effect have placed urgent demand on sustainable and renewable energy sources in recent years, such as solar and wind energy [1], [2]. At the meantime, reliable energy storage technologies are of equal importance to make full utilization of the above-mentioned renewable energy. Electrochemical energy storage devices such as lithium-ion batteries and supercapacitors offer the feasibility to reversibly store electric energy in low cost and high efficient manner and serve as promising power sources for various applications [3], [4], [5], [6], [7]. In particular, supercapacitors, which are also known as electrochemical capacitors, could deliver higher power density and longer cycle life compared to lithium-ion batteries, and higher energy density compared to traditional capacitors [5], [6], [7], [8]. Hence, they play an indispensable role for upcoming large-scale applications such as electric vehicles and stationary energy storage.

Exploration of novel electrode materials has been considered as one essential way to develop high-performance supercapacitors. Active carbon materials and conducting polymers are commonly used electrode materials for supercapacitor due to their low cost, ease of processability, and controllable porosity [9], [10]. In recent years, transition metal oxides/sulfides based materials have received extensive research attention and are considered as rising stars for the next generation high-performance supercapacitors, because of their pseudocapacitive property which could generate much higher specific capacitance compared to the double-layer based capacitors [11], [12], [13], [14], [15]. For example, a maximum specific capacitance of 401 F g−1 has been achieved for NiO nanoflakes in a very recent report [16]. In another work by Du et al. the Co3O4 hollow boxes obtained from a re-crystallization process exhibited a reversible specific capacitance as high as 278 F g−1[17]. Wang et al. reported the fabrication of Co3S4/graphene composites as advanced electrode materials for supercapacitors [18].

An efficient strategy to improve the electrochemical activity of metal oxide/sulfides is forming hollow nanostructures, which significantly increases the electrochemical active sites by providing large surface area and improving the electrode/electrolyte contact [19], [20], [21]. Meanwhile, it has been well demonstrated that nanocomposites based on metal oxides/sulfides and carbon supports usually exhibit improved electrochemical performance due to the improved electrical conductivity and mechanical stability [18], [22], [23], [24], [25], [26]. However, despite the recent advances in preparing hollow structures and nanocomposites, preparation of carbon-supported metal sulfide hollow structures that simultaneously realize the two above-mentioned strategies via facile synthetic routes still remains challenging.

In this work, we report the rational design and synthesis of carbon supported Co9S8 (denoted as C@Co9S8) hollow spheres via a facile solution-based method. The synthetic strategy involves the template-engaged growth of a precursor shell, followed by simultaneous sulfidation and removal of template. The as-prepared C@Co9S8 hollow spheres are assembled from ultrathin nanosheets and supported by an inner carbonaceous layer, which exhibits high surface area and excellent structural stability. Owing to these beneficial features, the C@Co9S8 hollow spheres demonstrate significantly enhanced electrochemical performance when evaluated as electrode materials for supercapacitors.

The synthetic route of the C@Co9S8 hollow spheres is schematically presented in Scheme 1. Firstly, silica nanocolloids (400 nm) are coated with a layer of carbon-rich polysaccharide to form SiO2@C nanospheres (Fig. S1, see the ESI) [27]. The SiO2@C nanospheres are then used as templates for the uniform growth of cobalt precursor (denoted as CoP) nanosheets, leading to the formation of SiO2@C@CoP core-shell structures. Finally, the CoP nanosheets are hydrothermally converted into Co9S8 nanosheets in the presence of Na2S without destroy the integrity of the shell structure. Meanwhile, the silica cores are removed by the alkaline environment created by the hydrolysis of Na2S, finally forming the C@Co9S8 hollow spheres.

The uniform growth of CoP nanosheets on SiO2@C templates is clearly shown in the field-emission scanning electron microscopy (FESEM) images as given in Fig. 1a and b. The nanospheres are uniform in size and no obvious aggregation is observed. The magnified image reveals that the surface of the nanospheres is composed of sheet-like subunits. Transmission electron microscopy (TEM) images (Fig. 1c and d) further confirm the SiO2@C@CoP core-shell structures and hierarchical shells that are composed of ultrafine nanosheets. It is worth noting that a distinct gap is observed between the silica core and the shell, which implies the engagement of silica template during the growth of CoP shell as discussed shortly. Powder X-ray diffraction (XRD) is applied to determine the crystallographic structure of CoP shell as shown in Fig. 1e. The broad diffraction peaks in the pattern can be assigned to Co3(Si2O5)2(OH)2, which is consistent with a previous report [28]. The distinct gap which can be observed in the TEM results (Fig. 1c and d) could result from the formation of the Co-P layer. In the hydrothermal process, the silica surface was etched by urea and the released silicate ions may travel outwards and reacted with the cobalt ions to form CoP in the carbon layer. So the observed gap may be due to the both the consumption of silica core and the carbonaceous layer that was deposited. The chemical composition of the product is confirmed by energy dispersive X-ray spectroscopy (EDX) analysis (Fig. 1f) with evident existence of Si, C, O and Co elements (Pt is sputtered on the surface of sample to conduct SEM observation, and Cu is from the substrate). Thus, the silica cores participate in the formation of CoP nanosheets by being slightly etched during the reaction. Such template-engaged reaction would possibly result in more uniform growth of the product shell compared with conventional hard templating method. Meanwhile, it is also found that the pre-deposited carbonaceous layer on the silica template is important to achieve uniform growth of CoP nanosheets (Fig. S2, see the ESI). The carbon layer which was coated onto silica spheres is hydrophilic because the products were not annealed at a higher temperature. In this case, hydroxyl functional groups are still retained in the carbon structures, leading to a hydrophilic carbonaceous layer for the subsequent growth of CoP. Such hydrophilic carbonaceous layer is expected to serves as a stable support that favors the firmly anchoring of CoP as the silica core is gradually etched.

Fig. 2a gives a typical FESEM image of the product after the hydrothermal sulfidation process. It can be seen that the morphology of the hierarchical spheres is well preserved. The crystallographic phase of the as-prepared product is examined by XRD (Fig. 2b). All the diffraction peaks can be assigned to Co9S8 phase (JCPDS NO. 86-2273), indicating the complete conversion from Co3(Si2O5)2(OH)2 to Co9S8. EDX analysis (Fig. S3, see the ESI) reveals the presence of Co, S and C, whereas the absence of Si suggests the removal of silica template during the sulfidation process. This is further confirmed by TEM images shown in Fig. 2c and d. The interior of the hierarchical spheres becomes completely hollow, while the outer layer consisting of carbon supported Co9S8 nanosheets remains intact. No deformation or collapse of particles is observed in the field of view, showing the good structural stability and integrity of these hollow structures. A magnified TEM image (Fig. 2e) on the shell elucidates the ultrathin characteristic of the nanosheets and the porous framework formed by the interconnected nanosheets. Fig. 2f gives the high resolution (HR) TEM image taken from a flat lying ultrathin nanosheet indicated by the black rectangle in Fig. 2e. The spacings of the observed lattice planes are 0.29 and 0.303 nm, which are consistent with that of (222) and (311) planes of Co9S8, respectively [29]. Although the inner carbon support could not be clearly identified from the image, its presence is supported by the EDX analysis as discussed above. Meanwhile, Co9S8 nanorods are also synthesized and used for comparison, which were prepared via a similar approach without silica template (Fig. S4, see the ESI). Owing to the hierarchical shell and large voids of these hollow spheres, high Brunauer-Emmett-Teller (BET) specific surface area of ca. 266 m2 g−1 is achieved, which is significantly higher than that of Co9S8 nanorods with only a BET specific surface area of ca. 15.5 m2 g−1 (N2 sorption isotherms are given in Fig. S5, see the ESI). Apparently, such hierarchical hollow structures are expected to provide more active sites to facilitate the electrochemical reactions, and the void structure could also serve as reservoir for electrolyte, which would endow fast and efficient ion diffusion at the interface between the active materials and electrolyte [30].

As an electroactive material, Co9S8 is a promising candidate for high-performance supercapacitors [31]. For the electrochemical evaluation, both C@Co9S8 hollow spheres and Co9S8 nanorods are examined. Fig. 3a presents the cyclic voltammetry (CV) of the C@Co9S8 hollow spheres, showing a pseudo-capacitive characteristic that differs from the typical electric double-layer capacitance with a rectangular CV shape [32]. A pair of anodic peaks can be seen within the voltage range of 0.3–0.5 V (vs. SCE), while a broad cathodic peak between 0.2 and 0.3 V (vs. SCE) is observed, corresponding to the oxidation and reduction process of cobalt sulfide materials, respectively [15]. Based on the different CV scan rates at 1, 2, 5, and 10 mV s−1, high specific capacitances of 800, 732, 643 and 493 F g−1 can be calculated, respectively, as given in Fig. 3b. The poorly defined plateaus observed in the galvanostatic charge-discharge curves (Fig. 3c) further manifest the pseudo-capacitive processes, which is consistent with the CV analysis. The galvanostatic charge-discharge (GCCD) measurements show that specific capacitance of 654, 617, 583, and 491 F g−1 can be delivered at current densities of 2, 3, 4, and 8 A g−1, respectively (Fig. 3d). While at the same current densities, lower capacitance of 325, 287, 257, and 228 F g−1 are obtained for the Co9S8 nanorods (Fig. S4, see the ESI). The investigation of cycling performance further verifies the superiority of the as-prepared C@Co9S8 hollow structures (Fig. 3e). At a current density of 5.8 A g−1, a high initial capacitance of 515 F g−1 is delivered by C@Co9S8 hollow nanospheres and 88.5% of the capacitance can be retained after cycling for 3000 times. Meanwhile, much lower initial capacitance (298 F g−1) and poorer retention (41.6%) are obtained for the Co9S8 nanorods. These enhanced specific capacitance and excellent cycling performance of C@Co9S8 hollow structures could be attributed to the unique hierarchical shell composed of ultrathin nanosheets and large specific surface area [33]. Furthermore, these Co9S8 nanosheets are anchored on the inner carbonaceous layer, which would serve as a mechanical support to enhance the structural stability and integrity, and improve the cyclic capability [34]. In order to verify the above claim, the C@Co9S8 electrode materials after the cycling test are examined by SEM as shown in Fig. S6 (see the ESI). Most of the particles are generally intact and remain the spherical shape without obvious cracking after 3000 cycles, confirming their good mechanical strength and robustness during repeated charge storage and release.

In summary, we report the fabrication of carbon supported Co9S8 hollow nanospheres through a facile solution-based approach. A layer of cobalt precursor composed of ultrathin nanosheets is firstly assembled on the surface of carbon-coated silica spheres via a template-engaged method. The cobalt precursor nanosheets are then hydrothermally converted into Co9S8 nanosheets, while the silica templates are simultaneously dissolved due to the alkaline environment. The as-prepared carbon-supported Co9S8 (C@Co9S8) hierarchical hollow nanospheres exhibit well-defined hollow structure with high uniformity, large surface area and excellent structural stability. In virtue of these advantageous features, these C@Co9S8 hierarchical hollow nanospheres manifest high specific capacitance with excellent cycling stability (88.5% retention after 3000 cycles), demonstrating their great potential in high-performance supercapacitors.

Section snippets

Synthesis of SiO2@C

The carbon coating onto silica nanocolloids is achieved via a modified method based on a previous report [27], and the procedures are as follows: 0.2 g of silica nanocolloids (400 nm) was dispersed into 20 mL of ethanol by ultrasonication for 20 min, marked as solution A. At the same time, 0.5 g of Polyvinylpyrrolidone (PVP, Mw ~29,000) and 4 g of glucose were dispersed into 20 mL of deionized (DI) water and stirred for 10 min to form a clear solution, which is marked as solution B. After that, 5 mL of

Acknowledgment

We acknowledge the financial support from the Chenguang Project of Wuhan Science and Technology Bureau (Grant No. 2015070404010212), 2014 Young Talents Development Plan of Hubei Province, the National Natural Science Foundation of China (Grant No. 21303045), Natural Science Foundation of Hubei Province of China (Grant No. 2014CFA527).

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