Facile synthesis of novel and highly efficient CoNi2S4-Ni(OH)2 nanosheet arrays as pseudocapacitive-type electrode material for high-performance electrochemical supercapacitors
Graphical abstract
Scheme 1. Schematic illustration of the synthesis procedure of binder-free CoNi2S4-Ni(OH)2 NSAs grown on Ni foam: (a) Current collector; (b) CoNi2S4 NFAs; and (c) CoNi2S4-Ni(OH)2 NSAs. 
Introduction
In recent periods, supercapacitors (SCs) are one of the crucial energy storage applications, which having high potential performances in power origin, ultra-fast charging capabilities, remarkable cycling spam, and acceptable for green environment [1,2]. Nonetheless, SCs have been suffering from lower capacitance/capacity results and ultra-low energy density, which prevents the SCs in the ever-growing digital world, when we compared to Li-ion applications [3,4]. This limitation mainly arises from the electroactive electrodes attribute, which systemize the execution of the SCs. Thereby, it was needed to design and progress novel electrodes and approaches with higher-capacitive electroactive-based materials for SCs supplications.
SCs are mainly classified into two portions, which are carbon-based materials, pseudocapacitors (RuO2 and MnO2) and/or battery-kind (Ni2S4, Co2O4, Nis, NiO, Ni(OH)2, etc.) applications, which were widely suggested for usages in SCs because of their excellent progresses in Faradaic reaction activities and existence remarkable theoretical capacitive performances [5], [6], [7], [8], [9], [10], [11]. In addition, these type of electro-active materials delivers outstanding energy storage performances when we compared to carbon-based sources, because of their Faradaic redox reactions having multiple phases electrons-involving with ions of electrolyte [12,13]. Among them, Ni(OH)2 was well-familiar for morphological diversities including as nanowires, nanocube's, nanorods and nanoflowers depending on the fabrication approaches and aspects of precursors [14]. Moreover, Ni(OH)2-based metal oxide can offer excellent electrochemical performance and high-electrical conductivity with binder-free-based material than other metal oxides. Regrettably, SCs electrode with Ni(OH)2-based materials have often gained low-specific capacitance/capacities, slow-rate stability, and kind of lower energy density cause of the inadequate contact of working electrodes with the material and also insufficient electrical conductivity [15]. For that limitations, it is compulsion to develop and construction of a new-era battery-kind electroactive materials to recover the SCs performance contain of the energy capabilities.
To overcome these hindrance, many scientists have been experimentation on mixed/binary metal sulfide-based electrodes (ZnCo2S4, NiCo2S4, CuCo2S4, MnCo2S4, etc.), which delivered better raised in electrochemical energy storage performance when we compared to the simple and individual metal-oxide based electroactive electrodes [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Even though CoS4 and NiS4 electrode materials have been showed to deliver excellent energy storage performance, the literatures on mixed Co-Ni with sulfide are very limited. Thus, the synthesis of CoNi2S4 as an efficient electrode prospect candidate will have beneficial/advantageous source because of their merits of low-cost, nontoxicity, binder-free and superior electrochemical specific capacitances/specific capacities from the synergistic effect of Co and Ni ions [26], [27], [28], [29]. In this consideration, very few researchers think about employing CoNi2S4-based nano-architectures and simple metal oxides/hydroxides capacitive materials to assemble beneficial structures to overcome any restrictions and develop high-capability SCs, and they have accomplished staged favorable outcome. These kinds of integrations which are simple metal hydroxide (Ni(OH)2) with binary metal sulfide (CoNi2S4) would be provides excellent electrochemical performance activities and higher capacity results than simple metal hydroxides and binary metal sulfides due to their Faradaic redox reactions having multiple phases electrons-involving with ions of electrolyte and benefits of synergistic effect. Moreover, there are no reports on the selective combination of CoNi2S4 and Ni(OH)2 as a novel and highly efficient electrode material for SCs-based applications. To further improve the employ of CoNi2S4-Ni(OH)2 material, novel approach has been directly grown on current collectors to provide ultra-high surface areas, fast ion/electron transportations pathways between as-developed/constructed composite electrode interface and electrolyte (which were needed for better capacity), and stupendous cycling stability with rate capability.
Influenced by the above importance, for the first time, we took beneficial of unique nature of SCs-based CoNi2S4-Ni(OH)2 NSAs composite electrode through a simplistic and cost-effective based hydrothermal strategy. By this strategy, the CoNi2S4 which employ as the scaffold would give a boost to ion diffusion and electron transportations. By the introduction of Ni(OH)2 has been beneficially in electrochemical activities (specific capacity) and support to improve the enormous surface areas, which is given plenty of electroactive sites. As a beneficial of above consequence, the as-developed composite CoNi2S4-Ni(OH)2 NSAs electrode maintained an appreciable specific capacity of 251.3 mA h g−1 at a 1 A g−1. Furthermore, the SCs electrode of CoNi2S4-Ni(OH)2 NSAs show advanceable cycling spam and rate capabilities, specifying that our composite will have a highly efficient candidate for SCs applications.
Section snippets
Chemicals/Reagents
cobalt nitrate hexahydrate (Co(NO3)2•6H2O), Nickel nitrate hexahydrate (Ni(NO3)2•6H2O), sodium sulfide nonahydrate (Na2S•9H2O), ammonium hydroxide (NH4OH), urea (CH4N2O), potassium hydroxide (KOH) and were pick up from Sigma Aldrich and used without further purification. All the chemicals/reagents are analytical grade and have been utilized directly without further purification.
Composite electrode material of CoNi2S4-Ni(OH)2 NSAs growing on Ni foam substrate
At first, 0.05 M Co(NO3)2•6H2O, 0.1 M Ni(NO3)2•6H2O were mixed with 0.05 M Na2S•9H2O in 60 mL of DI water under
Investigations of structural and morphological characterization for CoNi2S4NFAs and CoNi2S4-Ni(OH)4 NSAs
Fig. 1 depicts the synthesis procedure and electrode binder-free nanoarchitecture of CoNi2S4-Ni(OH)2 NSAs developed on Ni foam current collector substrate through a two-step strategy. As allocated in Fig. 1a, first-class conductive and cheaper-cost Ni foam has been employ as a current collector/substrate; the resulting Ni foam can be assist as an highly efficient source for high-performance SCs. In the initial step, CoNi2S4 NFAs have been develop on the Ni foam substrate surface through a
Conclusion
In summary part, the hierarchical architecture profile of CoNi2S4-Ni(OH)2 NSAs composite material has been effectively constructed on Ni foam mesh through simplistic hydrothermal technique procedure. The as-developed CoNi2S4 NFAs and CoNi2S4-Ni(OH)2 NSAs electrode materials of physical and electrochemical properties were effectively researched. Utilization of SCs electrode material, the composite of CoNi2S4-Ni(OH)2 NSAs yielded up a best specific capacity up to 251.3 mA h g−1 at 1 A g−1 and
Notes
The authors declare no conflict of interest.
Acknowledgement
This work was financially supported by BK 21 PLUS, Creative Human Resource Development Program for IT Convergence (NRF-2015R1A4A1041584), Pusan National University, Busan, South Korea.
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