All-solid state asymmetric supercapacitor based on NiCoAl layered double hydroxide nanopetals on robust 3D graphene and modified mesoporous carbon
Graphical abstract
Hierarchical NiCoAl–LDH nanopetals were successfully deposited onto conductive three-dimensional graphene structure to prepare electrode material with superior electrochemical performance by combination with dept dip-coating method and hydrothermal reaction. To ensure the asymmetric supercapacitor owning high power and energy density simultaneously, modified mesoporous carbon derived form SBA-15 with high power density is selected as negative electrode. Finally, the as-assembled all solid-state supercapacitor exhibits exceptional electrochemical properties, including high specific capacitance, high power density as well as excellent cyclic performance.
Introduction
In recent years, the ever-decreasing demand of fossil fuels and deteriorating environmental problems have evoked a huge response for the development of green and reversible energy storage devices [1], [2], [3]. Supercapacitors, also called ultra-capacitors, have been gradually applied in many fields due to their excellent power output performance, high rate capability and cyclic efficiency [4]. However, their relatively low energy density, restricts their utilization for the next-generation of electrical vehicles.
Asymmetric supercapacitors, a combination of pseudocapacitive and electrical double layered capacitors, have attracted intense research interest due to their superior electrochemical performance. Nevertheless, asymmetric supercapacitors still suffer from low energy density compared with batteries, which greatly impedes their utilization in practical energy storage. As described in Eqs. (2) and (3), the relatively high voltage range and specific capacitance give rise to advances in energy and power densities. Obviously, the higher the performance of the electrode materials, the better the overall electrochemical performance of the device can be obtained.
Currently, the development of advanced positive electrode materials for asymmetric supercapacitors is mainly focused on pseudocapacitive materials with higher energy density, these include transition metal oxides, transition metal hydroxides and conductive polymers. In this respect, layered double hydroxides (LDHs) are regarded as emerging high energy electrode materials in view of their relatively low cost, high redox activity and environmental affinity [5]. Regardless of the preeminent specific capacitance of LDH materials, the limited rate capability and poor cyclic stability adversely affect their performance in supercapacitors. In contrast, graphene-based carbonaceous materials offer inherently high conductivity and rate capability, but suffer from low energy density. Hence, hybridizing LDHs with graphene can contribute to overcome the respective defects from these two materials [6], [7]. Nevertheless, it is challenging to fabricate LDH/RGO composite with beneficial architecture owing to the agglomeration of the materials.
An appropriate design of three-dimensional reduced graphene oxide (3D RGO) structure offers a potential solution to minimize agglomeration of graphene nanosheets, thus exposing a much more active surface for facilitating electron transportation during the electrical reaction process [8], [9]. In addition, RGO is an ideal substrate to hybridize LDH material, owing to a strong coupling between metal ions and the oxygen-containing groups on the surface through electrostatic attraction and to the superior electrical double layer capacitance of graphene based material. However, the ultrahigh operating temperature and complex reacting conditions will increase the overall cost of the traditional graphene-based electrode and greatly hinder their practical application [10], [11]. As reported in our previous work, a facile dip-coating method provides an appropriate way to construct three-dimensional RGO architecture. Whereas the graphene nanosheets are easily peeled off from nickel foam, which markedly degrades the cycling performance of electrodes [12]. A major challenge remains, therefore, on how to develop a low-cost and robust 3D graphene structure which is environmentally friendly.
Carbon materials with porous structure that contribute to ultrahigh specific capacitance and high conductivity, have been widely used as negative electrode material for supercapacitors. However, in comparison with pseudocapacitive materials, the relative low specific capacitance of carbonaceous species greatly hampers their practical application. To address this issue, an appropriate design of carbon material with optimized pore architecture and high specific capacitance, is required. To date, molecular sieve SBA-15 with numerious mesoporous pores, has been widely applied in the catalytic domain [13], [14], [15]. SBA-15 possesses an ordered pore distribution and high specific capacitance, which is expected to be suitable for electrolyte impregnation. Thus, SBA-15, selected as a template to construct carbon material, could contribute significantly to electrochemical performance, the mesoporous channels acting as towering scaffolds for transporting electrons at high speed. However, the hydrophobicity of the mesoporous carbon associated with high temperature carbonization, considerably impedes the electrolyte penetrating into the carbon structure.
Herein, we propose a unique design strategy to fabricate NiCoAl-LDH nanopetals onto 3D graphene substrate via a dept dip-coating method combined with alkaline reduction followed by hydrothermal reaction [16]. The integrated positive composite electrode can effectively decrease the dead surface caused by the traditional slurry-derived method and facilitate efficient charge transportation [17]. NiCoAl-LDH with ultrathin nanopetal morphology directly anchors onto the graphene structure, exhibiting enhanced electrochemical properties, such as high specific capacitance, rate capability and good cyclic stability. In addition, modified mesoporous carbon (MMC) is successfully fabricated through a simple template sacrifice method using sucrose as a carbon source, which is subsequently modified with HNO3. When selected as negative supercapacitor electrode, modified mesoporous carbon (MMC), delivers a maximum specific capacitance as high as 289 F g−1, which is comparable to previous scientific reports [18], [19], [20]. We also assemble an all solid state supercapacitor based on NiCoAl-LDH/3D RGO hybrid as positive electrode and MMC as negative electrode, which operates in the high potential range of 0–1.7 V in KOH/PVA electrolyte. The device also displays high energy density (57.1 Wh kg−1), high power density (8615 W kg−1) and superior flexible stability, providing an effective solution for diverse energy storage applications in the future.
Section snippets
Preparation of NiCoAl-LDH/3D RGO
Graphite oxide (GO) used here is prepared by modified Hummers method, as previously reported [21]. The synthesized GO is dispersed in water under ultrasonic conditions at 400 W for 1 h and finally centrifuged at 7000 rpm for 30 min to remove the un-exfoliated aggregates, forming a homogenous solution with mass concentration of 2g L−1. With the aim of enhancing adhesion of GO nanosheets onto the substrate, PTFE emulsion is added to the GO dispersion with a total mass ratio of 1%. Nickel foam is used
Positive electrode materials
A facile dip-coating method combined with alkaline reduction of 3D RGO structure followed by hydrothermal reaction is carried out to fabricate NiCoAl-LDH nanopetal-like structure. The overall process, illustrated in Scheme 1, is cost-effective and environmental affinity. Briefly, RGO nanosheets are coated onto nickel foam via a dept dip-coating method with the aid of PTFE, followed by treatment with alkaline solution. An ultrathin film of RGO formed on the surface of the nickel foam. The PTFE
Conclusion
In this report, nanopetal-like NiCoAl-LDH/3D RGO is successfully fabricated with enhanced electrochemical capacitance, high specific capacitance, high rate capability as well as superior cyclic stability. Moreover, an all-solid state asymmetric supercapacitor is assembled, based on NiCoAl-LDH/3D RGO and modified mesoporous carbon. The assembled asymmetric device exhibits high electrochemical performance in terms of its high energy density of 57.1 Wh kg−1, maximum power density of 8650 W kg−1 and
Acknowledgements
This work was supported by National Natural Science Foundation of China (NSFC 51402065, NSFC 51301050), Fundamental Research Funds of the Central University (HEUCFZ), Natural Science Foundation of Heilongjiang Province (B201404, B2015021), International Science & Technology Cooperation Program of China (2015DFR50050, 2015DFA50050) and the Major Project of Science and Technology of Heilongjiang Province (GA14A101).
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