| The increasingly serious energy crisis and environmental problems have been huge hidden trouble for the development of human society. As a kind of clean and green terminal use form of energy,electric energy is more suitable for human to use in the long run. Supercapacitors with both high energy density and power density have excellent performance in terms of energy storage/conversion ability and have attracted extensive attention. As a core component of supercapacitors, electrode materials have been a very hot research topic. Especially, nanostructure transition metal oxide materials have been an ideal choice of high-performance electrode materials because of their practical advantages, like high theoretical specific capacity, easily available raw materials, low cost, etc. In addition, graphene as the ideal supporting structure with good conductivity can be combined with transition metal oxide nanomaterials to share their advantages and further promote the performance of the products. But owing to the drawbacks like complex operation, low production efficiency, high equipment demanding, environmental pollution and so on, traditional nanomaterial preparation methods are limited a lot. In view of the above points, this paper has developed a facile one-step interface approach to fabricate MnO-NiO-r GO nanocomposites. Moreover the product performance and the reaction mechanism have been analyzed and explored at a certain extent.First, we selected manganese salts and nickel salts as raw materials because they are low cost, resource-rich, non-poisonous and environment friendly. DMF was used owing to its peculiar properties and a new DMF-water double solvent interface reaction system was successfully established. The excellent dissolving capacity of DMF allows metal inorganic salts to directly dissolve into organic phase and act as reagents instead of the traditional organics. Thus the available reactant categories and application scope of interface synthesis method were extended and none addition agent and organic reactant participated in the reaction process, which fundamentally reduced the internal resistance of the products. As far as we speculated, during the interface reaction process, a banded micellar system reaction zone was naturally formed between the two phases. That made the reactants contacted and reacted in the form of micelles in microcosmic category, which effectively narrowed the local spacial range of the reactions,hindered the large-scale agglomeration phenomenon and benefited the production of nanoscale products. Moreover, from macroscopic perspective, the banded zone replaced the traditional two-dimensional phase interface, slowed down the diffusion rate of reagents and the overall reaction rate, developed a larger macro reaction space and improved the synthetic efficiency of the products. We have successfully manufactured MnO-NiO hybrid nanoparticles with the improved interface method. The micro morphology of the obtained products was a netlike porous structure formed by plenty of spherical particles with 20~30 nm diameter. This material showed obvious psedocapacitance characteristic and superior specific capacitance and rate capability. At the current density of 1 A g-1, the capacitance was 408 F g-1, and at the high current density of 4 A g-1, it can still maintain 311 F g-1.On the base of MnO-NiO hybrid nanoparticles prepared by the improved interface method, graphene was introduced as the reinforcing phase of the composite and MnO-NiO-r GO nanocomposite was successfully prepared. The micro morphology showed many MnO-NiO nanoparticles uniformly and firmly attached on the graphene nanosheets. Here, graphene played an obvious structure-supporting role. The large number of functional groups on the surface of graphene nanosheets acting as anchor sites controlled the growth kinetics of MnO-NiO nanoparticles well, so that the site-specific in-situ growth of the nanoparticles was achieved and the agglomeration problem was effectively avoided. This composite material showed porous characteristic and large specific surface area, 179.47 m2 g-1. Its average pore width was 12.86 nm. This ensured the sufficient contact between active materials and electrolytes and also was of benefit to the fast transfer of charge particles and the process of pseudocapacitance redox reactions. As for the electrochemical performances, the test results under a three-electrode system indicated MnO-NiO-r GO nanocomposite possesses high specific capacitance up to 750 F g-1 at current density of 1 A g-1, and good rate capability with good retention rate 80.56% of specific capacitance at 4 A g-1. Moreover, the excellent cycling stability showed that after 2000 cycles at 4 A g-1, about 90.1% of the original capacitance can be remained. The electrochemical test under a two-electrode system was carried out by symmetric supercapacitor devices. The results demonstrated that the variable relationship between the power density and the energy density can well satisfy the performance requirements for practical applications of supercapacitors. And at 1 A g-1, the specific capacitance was 174 F g-1 while after 2000 cycles it stabilized at about 138 F g-1, but the coulombic efficiency has been always maintained at the level of 99%. |
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