Facile synthesis of manganese carbonate quantum dots/Ni(HCO3)2-MnCO3 composites as advanced cathode materials for high energy density asymmetric supercapacitors

Abstract

We have developed a high performance supercapacitor cathode electrode composed of well dispersed MnCO3 quantum dots (QDs, similar to 1.2 nm) decorated on nickel hydrogen carbonate-manganese carbonate (Ni(HCO3)(2)-MnCO3) hedgehog-like shell@needle (MnCO3 QDs/NiH-Mn-CO3) composites directly grown onto a 3D macro-porous nickel foam as a binder-free supercapacitor electrode by a facile and scalable hydrothermal method. The MnCO3 QDs/NiH-Mn-CO3 composite electrode exhibited a remarkable maximum specific capacitance of 2641.3 F g(-1) at 3 A g(-1) and 1493.3 F g(-1) at 15 A g(-1). Moreover, the asymmetric supercapacitor with MnCO3 QDs/NiH-Mn-CO3 composites as the positive electrode and graphene as the negative electrode showed an energy density of 58.1 W h kg(-1) at a power density of 900 W kg(-1) as well as excellent cycling stability with 91.3% retention after 10 000 cycles, which exceeded the energy densities of most previously reported nickel or manganese oxide/hydroxide-based asymmetric supercapacitors. The ultrahigh capacitive performance is attributed to the presence of the high surface area core-shell nanostructure, the well dispersed MnCO3 quantum dots, and the high conductivity of MnCO3 quantum dots as well as the synergetic effect between multiple transition metal ions. The superior supercapacitive performance of the MnCO3 QDs/NiH-Mn-CO3 composites makes them promising cathode materials for high energy density asymmetric supercapacitors.