The Controllable Preparation Of Monolayer Nickel Cobalt Hydroxyl Carbonate For All-solid-state Asymmetric Supercapacitors

Supercapacitors,as one of the most promising energy storage devices between the traditional capacitor and power sources,are of great interest owing to their high power/energy density,superior rate capacity,exceptionally long cycle life,and economic environmental characteristics.They have already become the hot spot of oversea and domestic researchers.The emergence of atomically-thick nanolayer materials,which features a short ion diffusion channel and provide more exposed atoms in the electrochemical reactions,offers a promising occasion to optimize the performance of supercapacitors on the atomic level.Firstly,by controlling the amount of L-ascorbic acid(LAA),Ni-Co compounds with different components and morphology were synthesized by a simple one-step hydrothermal method.It can be found that LAA can control the constituent and morphology of the nickel-cobalt composite.With 120 mg LAA,Ni-Co hydroxyl carbonate(Ni,Co-HC)was prepared featured 2D uniform nanolayers.From the electrochemical tests,the as prepared samples with different amount of LAA preceded over the Ni-Co hydroxide with no LAA,indicating the addition of LAA have a positive effect on the capacity performance.Furthermore,Ni-Co composite exhibited the best electrochemical performance with the LAA amount of 120 mg.Then,through characterization analysis,it was found that sample Ni,Co-HC featured monolayered structure with an average thickness of 1.07 nm.This unique structure makes the material entirely exposed to the electrolyte which can provide almost 100% exposure of electroactive atoms,and thus obtaining better electrochemical performance.The electrochemical performance results showed the specific capacitance up to 2266 F g-1 at the current density of 0.5 A g-1.Besides,samples at different hydrothermal reaction time were prepared to figure out the time-dependent morphology evolution process of the products.By individual controlling the hydrothermal reaction time and the atomic mass ratio of nickel-cobalt,we studied the electrochemical properties of the as-prepared electrode materials.In order to certify the application potential of the Ni,Co-CH forenergy storage,an all-solid-state asymmetric supercapacitor(denoted as Ni,Co-HC//HPC)was assembled with Ni,Co-HC as positive electrode,a previously reported hierarchical porous carbon(HPC)as negative electrode,and gel polymer PVA/KOH as solid electrolyte.A high SC of 140 F g-1was achieved at current density of 0.1 A g-1.Moreover,it performed remarkable cycling stability(no capacitance fade after 19,000 cycles)together with promising energy density of 30 Wh kg-1(137.4 ?Wh cm-2)even at the power density of 8.69 kW kg-1(35.1 mW cm-2).Such energy density,power density and cycling stability are highly competitive compared with the previously reported ASSCs.