Manganese dioxide nanosheet assemblies as electrode materials for electrocapacitive storage of magnesium ions

Flower-like nanosheet assemblies (NSAs) of MnO2 have been synthesized with adjustable thickness and crystallinity. MnO2 NSAs in the size of about 80 nm have poor crystallinity, but display a high capacitance of 341 F g(-1) at the current density of 0.2 A g(-1) in aqueous MgSO4 electrolyte, while 162 F g(-1) and 174 F g(-1) in Al-2(SO4)(3) and Na2SO4 solution at 0.2 A g(-1), respectively. However, there is a decrease in capacitance for 120 nm MnO2 NSAs down to 270 F g(-1), 130 F g(-1) and 170 F g(-1) in MgSO4, Al-2(SO4)(3) and Na2SO4 electrolytes at 0.2 A g(-1), respectively. The capacitance of MnO2 NSAs in Al-2(SO4)(3) solution decreases more quickly with the increase of current density than that in other two electrolytes. The origin and variation of electrocapacitive behavior have been suggested by the combination of diffusion-controlled insertion and surface redox reaction of metal ions in aqueous electrolytes, which is related to the charge nature, hydrated radius and movement of metal ions. As the hydrated Mg2+ ion is moderate in both charge and size, the diffusion-controlled insertion gradually dominates in MgSO4 electrolyte, combined with the poor crystallinity of small NSAs, thus leading to the highest capacitance among the used electrolytes. Based on the experimental data, the phenomena resulted from the difference of electrolyte solutions and the intrinsic structure-property relationship of MnO2 NSAs have been discussed and analyzed. (C) 2019 Elsevier Ltd. All rights reserved.