Graphene quantum dot induced tunable growth of nanostructured MnCo2O4.5 composites for high-performance supercapacitors

Abstract

Achieving an optimal surface structure and adequate carbon distribution is an important criterion for transition metal oxide/carbon composites targeted for supercapacitor applications. In this study, a one-step hydrothermal reaction is developed to incorporate graphene quantum dots (GQDs) and MnCo₂O_4.5 with GQDs serving as both a conductive filling material and structure inducing agent. The process is fine-tuned based on GQD quantities to generate various morphologies including nanospheres, nanoneedles and nanoarrays. And a mechanism is proposed to explain the roles of GQDs in triggering the structural transformation. Among the different structures, the nanoneedle composites (denoted as MCO-40 GQDs) exhibit an apparent porous structure and even GQD distribution, which establishes a conductive network that provides excellent charge transfer capability for optimal electrochemical performances. As a result, the MCO-40 GQD nanoneedle electrode delivers the largest capacitance of 1625 F g⁻¹ at 1 A g⁻¹, which is four times higher than the capacitance of a pure MnCo₂O_4.5 nanosphere electrode (368 F g⁻¹ at 1 A g⁻¹). Moreover, the asymmetric supercapacitor fabricated with MCO-40 GQDs and reduced graphene oxide exhibits long cycle stability and a high energy density of 46 Wh kg⁻¹ at a power density of 66 W kg⁻¹, surpassing all previously reported capacitive devices based on MnCo₂O_4.5. Overall, this study illustrates the exciting roles of GQDs in material synthesis as a structure inducing agent and provides for the first time a reference for constructing MnCo₂O_4.5 based advanced nanomaterials with various shapes for energy storage applications.