High-Performance Battery-Type Supercapacitors Based on Self-Oriented Growth of Nanorods/Nanospheres Composite...

MnO-based materials have limited capacity and poor conductivity over various voltages, hampering their potential for energy storage applications. This work proposes a novel approach to address these challenges. A self-oriented multiple-electronic structure of a 1D-MnO₂-nanorod/2D-Mn₂O₃-nanosphere composite was assembled on 2D-graphene oxide nanosheet/1D-carbon nanofiber (GO/CNF) hybrids. Aided by K⁺ ions, the MnO₂ nanorods were partially converted to Mn₂O₃ nanospheres, while the GO nanosheets were combined with CNF through hydrogen bonds resulting in a unique double binary 1D-2D mixed morphology of MnO₂/Mn₂O₃-GO/CNF hybrid, having a novel mechanism of multiple Mn ion redox reactions facilitated by the interconnected 3D network. The morphology of the MnO₂ nanorods was controlled by regulating the potassium ion content through a rinsing strategy. Interestingly, pure MnO₂ nanorods undergo air-annealing to form a mixture of nanorods and nanospheres (MnO₂/Mn₂O₃) with a distinct morphology indicating pseudocapacitive surface redox reactions involving Mn²⁺, Mn³⁺, and Mn⁴⁺. In the presence of the GO/CNF framework, the charge storage properties of the MnO₂/Mn₂O₃-GO/CNF composite electrode show dominant battery-type behavior because of the unique mesoporous structure with a crumpled morphology that provides relatively large voids and cavities with smaller diffusion paths to facilitate the accumulation/intercalation of charges at the inner electroactive sites for the diffusion-controlled process. The corresponding specific capacity of 800 C g^-1 or 222.2 mAh g^-1 at 1 A g^-1 and remarkable cycling stability (95%) over 5000 cycles at 3 A g^-1 were considerably higher than those of the reported electrodes of similar materials. Moreover, a hybrid supercapacitor device is assembled using MnO₂/Mn₂O₃-GO/CNF as the positive electrode and activated carbon as the negative electrode, which exhibits a superior maximum energy density (∼25 Wh kg^-1) and maximum power density (∼4.0 kW kg^-1). Therefore, the as-synthesized composite highlights the development of highly active low-cost materials for next-generation energy storage applications.