Thorn-like nanostructured NiCo2S4 arrays anchoring graphite paper as self-supported electrodes for ultrahigh rate flexible supercapacitors
Graphical abstract
Vertically stacked thorn-like nanostructured NiCo2S4 arrays are designed to in-situ grow on the electrochemical oxidized graphite paper, giving an extraordinarily high rate capability for supercapacitor electrodes with the specific capacitance of 1276 F g−1 at 1 A g−1, and even 1218 F g−1 at 20 A g−1.
Introduction
In recent years, environmental pollution and fossil fuel issues have inspired researchers to develop energy storage devices with higher power and energy density. The continuous increase in demand of flexible and portable wearable devices has significantly stimulated to design the corresponding flexible electrochemical energy conversion and energy storage devices [1]. In comparison to other conventional electrochemical energy conversion and energy storage devices, supercapacitors have the overall advantages of high-power density, high capacity, long cycle life and fast charging and discharging. Flexible electrodes are the key points to develop flexible all-solid-state supercapacitors that can possess high capacitance, outstanding mechanical and superior cycling stability [2]. At present, great efforts are devoted to develop the flexible electrodes, including electrode materials deposited on soft-supported substrates, free-standing carbon films (CNT, carbon fiber [3], graphene or Mxene-carbon[4]) and hydrogels electrodes, where any kinds of electrode materials can be designed to deposit on the soft-supported substrates. The soft-supported substrates are focused on the carbon paper, cloth, sponge and plastic. Graphite papers are regarded as the favorable candidates of soft-supported substrates due to their excellent electrical and thermal conductivity, lightweight, bendable, superior inertness and especially low cost [5].
The electrode materials need urgently to meet the requirements of high capacity and excellent characteristics of supercapacitors. The developments of carbon and carbon derived electrode materials are seriously limited by the bottlenecks of their theoretical capacity. Transition metal oxides and transition metal sulfides have been expected to the replacement of supercapacitor electrodes with high pseudo capacitance. In particular, the transition metals of Co and Ni and the corresponding metal oxides as supercapacitors electrode materials demonstrate high redox reactions and conductivity, which are generally beneficial to better electrochemical capacities, for example, NiO (708.52 F g−1 at 10 mA cm−2) [6], Co3O4/PSAC (94 F g−1 at 1 A g−1) [7], Co3O4/3D graphene (768 F g−1 at 10 A g−1) [8], NiCo2O4 (1254 F g−1 at 2 A g−1) [9], NiCo2O4@Co-FeLDH (1557 F g−1 at 1 A g−1) [10]. However, the rate performance of these transition metals of Co and Ni is still unsatisfactory. As reported previously [34], by the replacement of O atoms with S atoms, transition metal sulfides can not only provide more active sites for desirable reversible redox reactions, but also contains a smaller band gap and relatively higher conductivity with respect to the corresponding metal oxides. For example, NiS as supercapacitors electrode can provide a high specific capacity of 968.22 F g−1 at 1 A g−1 [6], while the rich valence states of Co atom allow porous Co3S4 single crystal nanosheet arrays with superior high capacity up to 1081 F g−1 at 1.61 A g−1 [11]. Concerned with binary metal sulfides, NiCo2S4 can provide more active sites than single metal sulfides due to the co-existence of Co and Ni atoms, as well as has higher conductivity and more categories of reversible redox reactions. Therefore, the bimetal sulfides of NiCo2S4 is expected to exhibit better electrochemical performance that the porous NiCo2S4 nanotubes synthesized by a sacrificial template method achieve the specific capacitance as high as 1093 and 933 F g−1 at the current density of 0.2 and 1 A g−1, respectively [12]. Another onion-like morphology of NiCo2S4 binary spine particles is successfully prepared using sequential ion exchange reactions [13], giving an outstanding specific capacity of 1016 F g−1 at 2 A g−1 together with high rate capacitance performance.
The traditional method to fabricate the supercapacitor electrode is that the powder active materials mixed with carbon black and a binder are homogeneously spread out on the current collector at the expense of the electrode conductivity and energy density. The alternative strategy is to fabricate the bind-free electrode. For example, a simple hydrothermal method followed by a vulcanization process is used to obtain uniformly distributed NiCo2S4 nanotubes anchored on a 3D ultra-thin nitrogen-doped graphene frame (NGF), where the three-dimensional conductive graphene scaffold (NGF) can not only ensure the uniform distribution of NiCo2S4, but also promote electron transfers. At the same time, hollow NiCo2S4 nanotubes can also shorten the diffusion pathways of electrolyte ions. The strong synergistic effect between NiCo2S4 nanotubes and NGF substrate can be conducive to obtaining satisfactory electrochemical performance that the specific capacitance of the prepared NiCo2S4/NGF composite is superior high up to 1240 F g−1 at 1 A g−1 [14].
The key factors to optimize the electrodes with high electrochemical performance have been demonstrated, including their morphologies, sizes and compositions. The favorable morphological characteristics can tolerate volume expansions and maintain the structural stability during the repeated cycles. It is previously reported that the nano-sized/nano-structured NiCo2S4 can provide much shorter diffusion pathway and higher specific surface area, thereby exhibiting the satisfactory electrochemical performance in the practical supercapacitor's application [15]. Likewise, various morphologies for NiCo2S4 particles are found to be effectively constructed together with different electrochemical performances, such as hollow spheres (1516 F g−1 at 2 A g−1 and 1349 F g−1 at 16 A g−1) [16], nanoparticles (1379 F g−1 at 1 A g−1 and 1130 F g−1 at 50 A g−1) [17], nanocages (1635 F g−1 at 1 A g−1 and 817 F g−1 at 16 A g−1) [18]. However, these measured capacitances of NiCo2S4 are far lower than the theoretical capacity of 1055 C g−1 due to the incomplete utilization for the unfavorable structure. As reported previously [19], this shortcoming can be overcome by constructing vertically arranged NiCo2S4 nanorods grown on nickel foams by a one-step hydrothermal method, showing the capacitance as high as 1623 F g−1 at 10 A g−1 together with the excellent cycle stability. However, the rate performance of NiCo2S4 grown on nickel foams is unsatisfactory with the capacitance retention rate only 60 % at a current density ranging from 10 A g−1 to 25 A g−1.
In the previous studies, various morphologies of NiCo2S4 flexible electrodes are fabricated by two ways where one is the powders spread out the flexible substrates, and the other way is growing on the flexible substrates as self-supporting electrodes. However, NiCo2S4 arrays are still not found to grow on the low cost graphite papers for ultra-high rate flexible supercapacitors. Here, vertically stacked thorn-like nanostructures of NiCo2S4 arrays are designed to in-situ grow on the electrochemical oxidized graphite paper by a two-step hydrothermal method where the graphite paper acts as heterogeneous nuclear sites to rationally construct the morphology of NiCo2S4. This graphite paper supported NiCo2S4 as a self-supported electrode (NiCo2S4@EGP) shows an extraordinarily high rate capability with capacitance retention of 95 % (specific capacitance of 1276 F g−1 at 1 A g−1, and even 1218 F g−1 at 20 A g−1) and the excellent cycle stability with capacitance retention of 86% after 5000 cycles, respectively. When assembling into an all-solid-state electrochemical capacitor device with NiCo2S4 and activated carbon (AC) as the positive and negative electrodes, this device reaches a high-power density of 55.3 Wh kg−1 at a power density of 747.6 W kg−1, together with a good cycle stability (retained 95% in 5000 cycles).
Section snippets
Materials
Cobaltous nitrate hexahydrate (Co(NO3)2•6H2O), nickel nitrate hexahydrate (Ni(NO3)2•6H2O), sodium sulfide (Na2S•9H2O), urea, sulfuric acid (98%, H2SO4), sodium nitrate (NaNO3), permanganate (KMnO4), hydrogen peroxide (H2O2), N,N-dimethylformamide (DMF), acetone, polyvinyl alcohol (PVA) and NaOH were purchased from Sinopharm Chemical Reagent Co. Ltd and directly used without the further purification. Graphite paper (GP) purchased from Qingdao Huagao Graphene Co., Ltd (Qingdao, China) was cleaned
Results and discussion
A two-step hydrothermal method is used to fabricate vertically stacked thorn-like nanostructures of NiCo2S4 arrays anchoring on EGP substrate, as schematically illustrated in Scheme 1. After the first step of hydrothermal reactions for urea, cobalt salt and nickel salt, the surface containing oxygen groups or defects of EGP can provide heterogeneous nucleated sites, which can be tightly bounded to the Ni-Co precursor ) constructed by the six coordination of Co2+ and Ni2+
Conclusions
Thorn-like nanostructured NiCo2S4 arrays are firstly designed to anchor onto the low cost graphite paper by a facile method with two-step hydrothermal strategies. The surface functional groups of graphite paper produced by the electrochemical oxidized method can provide a lot of active sites which can be tightly bound with NiCo2S4 arrays. Likewise, graphite paper can also act as the heterogeneous nucleated sites to induce the formation of this unique thorn-like nanostructure where NiCo2S4
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 is financially supported by National Natural Science Foundation of China (grant no. 21674034) and Natural Science Foundation of Hunan province (2020JJ4167).
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