Structural regulation of porous MnO2 nanosheets and their electrocapacitive behavior in aqueous electrolytes

Abstract

Both nanosheet structure and porous feature are two important factors to improve the performance of electrode materials. In this work, two types of MnO2 nanosheets (MnO2-NSs), namely MnO2-NS-K and MnO2-NS-N, are synthesized respectively from the starting systems containing potassium hydroxide and ammonia. Ultrathin MnO2-NS-N nanosheets have a poor layered structure and a large surface area of 116.0 m2 g−1. The capacitance of MnO2-NS-N is about 200 F g−1 at 0.1 A g−1 in aqueous Na2SO4 electrolyte while 329 F g−1 at 0.1 A g−1 in MgSO4 solution. Thick MnO2-NS-K nanosheets with a surface area of 46.4 m2 g−1 show the capacitances of around 190 and 217 F g−1 at 0.1 A g−1 in Na2SO4 and MgSO4 electrolytes, respectively. The capacitances of MnO2-NS in MgSO4 electrolyte decrease quickly with the increase of current density, possibly due to the sluggish dynamics of intercalation and deintercalation of Mg2+ ions compared with Na+ ions. It is found that the surface capacitance plays an important role in the capacitive behavior. Both of MnO2-NSs show a better cycle stability in aqueous Na2SO4 electrolyte. Based on the experimental data and features of the MnO2-NSs, the structure-property relationship has been discussed.