Elsevier

Journal of Power Sources

Volume 492, 30 April 2021, 229643
Journal of Power Sources

Sandwich-like honeycomb Co2SiO4/rGO/honeycomb Co2SiO4 structures with enhanced electrochemical properties for high-performance hybrid supercapacitor

https://doi.org/10.1016/j.jpowsour.2021.229643Get rights and content

Highlights

  • The sandwich-like honeycomb Co2SiO4/rGO/honeycomb Co2SiO4 structures were prepared.

  • The rGO in the middle of the sandwich-like s-rGO/Co2SiO4 can significantly improve the conductivity.

  • s-rGO/Co2SiO4 exhibits the specific capacitance of 429 F g−1 rather than Co2SiO4 and rGO + Co2SiO4.

  • The s-rGO/Co2SiO4 electrode shows 92% capacitance retention after 10,000 cycles.

  • The s-rGO/Co2SiO4//AC HSC delivers the areal capacitances of 651 mF cm−2 (103 F g−1, 912 mC cm−2).

Abstract

Recent studies demonstrate that cobalt silicate-based materials are considered as the potential electrode materials for supercapacitors (SCs). However, improving the conductivity of Co2SiO4 remains a pressing priority. Herein, the sandwich-like honeycomb Co2SiO4/rGO/honeycomb Co2SiO4 structures are developed to improve the electrochemical capabilities of Co2SiO4 by using the sandwich-like GO/SiO2 as template and with a moderate hydrothermal method. Co2SiO4 nanosheets on the surface of rGO are interconnected to form the 3D honeycomb-like structures. This strategy, “Two for one” construction, means that we put the two-phase (Co2SiO4 and rGO) into one-phase (Co2SiO4/rGO integrated composite) to avoid the low electronic transmission problem of the traditional two-phase hybrid method. The s-rGO/Co2SiO4 electrode exhibits the specific capacitance of 429 F g−1 at 0.5 A g−1, which is superior to the capacitance of Co2SiO4 and rGO + Co2SiO4. The s-rGO/Co2SiO4//AC HSC delivers the areal capacitances of 651 mF cm−2 at 1 mA cm−2, and powers the bulb more than 300 s, which is strongly related to the energized capacitance s-rGO/Co2SiO4. This work not only proves that the s-rGO/Co2SiO4 can improve the electrochemical properties of Co2SiO4, but also provides a strategy for the synthesis of metal silicates/rGO/metal silicates sandwich-like structures with enhanced electrochemical performances for energy storage and conversion.

Introduction

Energy is closely bound up our lives and the inquisition about new energy sources and various energy storage technologies has been ever-lasting. As a type of energy storage devices, supercapacitors (SCs) recently stand out among the crowded competition of batteries and conventional physical capacitors, and researchers pay a great of interest in SCs owing to their outstanding characteristics including high-power density, wide temperature range, long cycling life, free maintenance, green and environmentally friendly [[1], [2], [3], [4], [5], [6]]. Generally, SCs can be divided into two kinds on account of the energy storage mechanisms: electric double layer capacitors (EDLCs) and pseudo-capacitors (PCs) [7]. As is well known, EDLCs are originated from the surface of electrodes forming the double layers, and PCs store charge through redox reactions [7]. As for high-performance SCs, the energy density (E) is a crucial factor because the high energy density decides the long operation of the devices in the practical application. The E is calculated by the equation E = 1/2CV2, where C represents the specific capacitance and V means the working voltage. Obviously, the high energy density is decided by the specific capacitance and voltage range. Thus, it is essential to develop and design electrode materials with special structure showing high electrochemical properties [[8], [9], [10], [11], [12]].

Owing to the low costs, nontoxic, easier to structure adjustment and stability, the transition metal silicates (TMSs) with high theoretical capacity have been received great attention in the field of energy storage devices [[13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]]. For examples, Wang et al. [22] demonstrated that mesoporous metal (Mn, Co, Ni) silicates with hollow spheres exhibited good electrochemical properties as the electrode materials for SCs. Typically, the mention of silicate-based electrode is followed by cobalt silicate (Co2SiO4) based electrode owing to its high theoretical capacity and stability [29]. Recent studies demonstrate that cobalt silicate-based materials are considered as the potential electrode materials for SCs [[29], [30], [31], [32], [33], [34]]. For example, Cheng et al. synthesized coupled Co2SiO4 nanobelt-on-nanobelt with rGO using Co2SiO4 nanobelt as the templates and this composite exhibited improved supercapacitive properties [32]. All the previous works illustrated that the study on the synthesis and electrochemical properties of Co2SiO4 has achieved great progress. However, it still exists some insufficiency due to the experimental capacitance being lower than its theoretical value caused by the poor conductivity of Co2SiO4 [15]. In order to gain the high performance of Co2SiO4 for SCs, a scientific and reasonable strategy for the improvement of the conductivity Co2SiO4 by designing novel structures is extremely important among many constraints [29,32]. At present, the mainstreams to improve the conductivity of Co2SiO4 with novel structures are as follows: (1) Preparation of bimetallic silicate by virtue of the difference of electrical conductivity between bimetallic silicate [33]; (2) Carbon/silicon in biomass was used as the skeleton to grow Co2SiO4 for high conductivity [[35], [36], [37]]; (3) The conductive material (CNTs, GO, carbon paper and carbon cloth, etc.) is introduced to improve the conductivity of the Co2SiO4 composite electrode [32,34]. Especially, it should be pointed out that the third method that mixing conductive material with TMSs to prepare composite electrode is inefficient [38]. The properties of the two-phase hybrid can't provide a rapid path for electrons transmission, and the conductivity problem is seemingly solvable, which is extremely negligible [[39], [40], [41]]. The concept of the growth of Co2SiO4 on the conductive material (e.g.: rGO) may be a good tool to improve the electrochemical performance of Co2SiO4 [42]. This concept “Two for one” means that we put the two-phase (Co2SiO4 and rGO) into one-phase (Co2SiO4/rGO integrated composite) [43]. Based on the above assumption, we try to construct the Co2SiO4/rGO/Co2SiO4 sandwich-like structures using the sandwich-like GO/SiO2 as the template to improve the electrochemical properties of Co2SiO4. This structure may enhance the electrochemical performance of Co2SiO4 on the basis of the following two aspects: (1) The rGO in the middle of the sandwich-like structures can boost the conductivity of the electrode materials. That's to say, once the electrons flow to the rGO, they can quickly flow to outside Co2SiO4 [44]. (2) The integrated structure can be conducive to the electron/mass transfer kinetics [14].

As expected, herein, we successfully developed a strategy for the preparation of the sandwich-like honeycomb Co2SiO4/rGO/honeycomb Co2SiO4 structures (denoted as s-rGO/Co2SiO4) using the sandwich-like GO/SiO2 as the template by a moderate hydrothermal method. Co2SiO4 nanosheets on the surface of rGO are interconnected to form the 3D honeycomb-like structures. This novel material exhibited improved electrochemical properties compare to Co2SiO4 and the results are corresponding with our original aim. This work provides an effective method for the synthesis of metal silicates/rGO/metal silicates sandwich-like structures with enhanced electrochemical performances for energy storage applications and conversions.

Section snippets

Synthesis

The raw materials and the synthesis of GO and rGO are represented in Supplementary data. The sandwich-like honeycomb Co2SiO4/rGO/honeycomb Co2SiO4 structures (denoted as s-rGO/Co2SiO4) were prepared using sandwich-like GO/SiO2 as the template by a hydrothermal method (Fig. 1), which mainly contains three steps. Firstly, the GO sheets were synthesized by a modified Hummer's method (see in Supplementary data). Secondly, GO/SiO2 was prepared by a reformative StÖber technology [43]. In detail,

Structure and composition of s-rGO/Co2SiO4

Fig. 1 describes the formation process of s-rGO/Co2SiO4 as well as the mechanism procedure of GO/SiO2. As shown in Fig. 1, the formation process of s-rGO/Co2SiO4 mainly included two stages of the generation of GO/SiO2 template and the hydrothermal process of s-rGO/Co2SiO4. Firstly, the sandwich-like GO/SiO2 template (Figure S1) is prepared by a reformative StÖber technology [43], and the process is further decomposed into two stages [22]: (1) the in-situ hydrolysis process of Si(OC2H5)4 and (2)

Conclusion

In conclusion, s-rGO/Co2SiO4 was prepared using sandwich-like GO/SiO2 as the template by a hydrothermal method. The rGO is in the middle of this sandwich structure and Co2SiO4 nanosheets are interconnected to form the 3D honeycomb-like structures on the surface of rGO. This novel structure can improve the conductivity of s-rGO/Co2SiO4 to facilitate the electron transport and enhance the interaction between Co2SiO4 and rGO surface to promote the charge transfer. Thus, the s-rGO/Co2SiO4 exhibits

Author contributions

Yifu Zhang conceived the idea and directed the experiments. Xueying Dong and Yuting Yu fabricated all the samples. Xueying Dong and Yuting Yu performed the characterizations. Xueying Dong, Yuting Yu, Xuyang Jing, Hanmei Jiang, Tao Hu, Yifu Zhang, Changgong Meng and Chi Huang discussed the results and commented on the manuscript. Xueying Dong, Yuting Yu and Yifu Zhang analysed the data and drafted and revised the manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (Grant No. 21771030) and Natural Science Foundation of Liaoning Province (2020-MS-113).

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