Controllable ZnFe2O4/reduced graphene oxide hybrid for high-performance supercapacitor electrode
Introduction
With the growing demand for clean energy, supercapacitors (SCs), as a promising energy storage technology, have attracted considerable interest due to their high power density, fast recharge capability, and long cycling life [[1], [2], [3]]. Supercapacitors can be classified into carbon-based electrochemical double layer capacitors (EDLCs) and redox-based pseudocapacitors based on the charge storage mechanisms [[4], [5], [6], [7], [8]]. EDLCs store charges electrostatically and offer a high power density but relatively low energy density. Then much work focused on pseudocapacitive electrode materials such as transition metal oxides (RuO2, MnO2, Co3O4, NiO) and conducting polymers (polyaniline, polypyrrole and polythiophene), which can increase capacitance via reversible redox reactions [[9], [10], [11], [12], [13], [14], [15], [16]]. Among them, spinel transition metal oxide (AB2O4) is emerging due to the unique electronic structures and the utilization of two metal elements [[17], [18], [19]].
ZnFe2O4 (ZFO) is one of the most promising spinel transition metal oxide for SCs electrode because of its much better electrical conductivity and higher electrochemical activity [[20], [21], [22]]. However, its electrical conductivity is still not high enough to be kinetically favorable, thereby limiting their use. In recent years, the development of incorporating carbon materials with ZnFe2O4 has been considered as an effective approach for achieving high-performance. Vadiyar et al. have investigated ZnFe2O4 nanoflake@ZnFe2O4/C nanoparticle heterostructure electrode, in which the conductivity is greatly improved by carbon nanoparticles [17]. Graphene with atom-thick layer features, has attracted much attention recently for conductive substrate/scaffold due to its high specific surface area, fast electron transfer ability, and good mechanical properties. The unique characteristics of graphene inspired us to design and synthesize graphene-supported ZFO hybrid. Li et al. have prepared the ZnFe2O4/NRG composite, and the ZnFe2O4/NRG composite as supercapacitor electrode exhibits a favorable specific capacitance of 244 F g−1 at 0.5 A/g [23]. Although some achievements have been made in ZFO/graphene, there is no study focused on the ZFO mass ratio and the ZFO particle size control in the composites and their influence on SCs performances. Hence, the development of ZFO/graphene hybrid system with high ZFO mass ratio and controllable ZFO particle size in the composites and investigation of its SCs performances is of fundamental and practical significance.
In this work, we report a ZFO/reduced graphene oxide (ZFO/rGO) hybrid with high ZFO mass ratio (82.3%) and average ZFO diameters of ∼15 nm dispersed on the graphene surfaces via a one-step hydrothermal method. Small ZFO nanoparticles can be fully utilized and facilitated fast ion transport during electrochemical reactions, whereas the high mass ratio of ZFO can improve the specific capacitance of the composite electrode. Therefore, the specific capacitances of the ZFO/rGO electrode at current densities of 1 A g−1 can reach 352.9 F g−1. Enhanced rate capability and excellently cycling stability (92.3% retention after 10000 cycles) were also observed.
Section snippets
Preparation of ZFO/rGO hybrid
All chemicals were of analytical grade in this study and used as received without further purification. Graphite oxide (GO) was synthesized using natural graphite according to the reported method [24]. The ZFO/reduced graphene oxide (ZFO/rGO) hybrid was fabricated by a facile hydrothermal process. In a typical synthesis, FeCl3·6H2O and ZnCl2 in the molar ratio 2:1 was dissolved into 30 mL of ethylene glycol by stirring at room temperature. Then 0.54 g of NaAc, 0.5 g of polyethylene glycol
Results and discussion Discussion
The ZFO/rGO hybrid fabrication with or without PEG-4000 is schematically illustrated in Fig. 1. It is noteworthy that a smaller ZFO nanoparticles is observed in the ZFO/rGO hybrid with use of PEG-4000 compared to the ZFO/rGO hybrid without use of PEG-4000 (L-ZFO/rGO). By controlling the size of ZFO nanoparticles with a smaller size, improved performance can be achieved. The smaller ZFO nanoparticles in the ZFO/rGO hybrid offer large specific surface area for easy access of electrolyte,
Conclusions
In summary, ZFO/rGO hybrid with a high ZFO mass ratio (82.3%) and a controllable ZFO particle size is systematically prepared. The structure of the resulting ZFO/rGO hybrid electrode demonstrates an efficient way to get high electrochemical performance. This unique structure combines the advantages of small ZFO nanoparticles (∼15 nm), exceptional conductivity of rGO, and high ZFO loading mass (82.3%), leading to effective utilization of ZFO, fast ion/electron transport, and increased specific
Acknowledgments
This work is supported by NSFC (51702123, 51472110), Shandong Provincial Natural Science Foundation (ZR2016EMB05, ZR2017ZB0315), University of Jinan Science Foundation (No. XKY1630). S.Y. thanks the start-up research funding and B.C. thanks the Taishan Scholar Professorship both from University of Jinan.
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