Scalable mechanochemical coupling of homogeneous Co3O4 nanocrystals onto in-situ exfoliated graphene sheets for asymmetric supercapacitors

Abstract

Graphene/Co3O4 composites have been considered as promising electrode materials for high-performance supercapacitors; however, their practical applications have been limited by the complex synthesis technologies and narrow potential window of around 0.5 V. Herein we report a facile and economical approach to the massive production of graphene/Co3O4 composites through the one-pot ball-milling of graphite, (NH4)2CO3, and Co(CH3COO)2. During the process, graphite was intercalated by (NH4)2CO3 and synchronously exfoliated into few-layer graphene sheets containing functional groups. Co3O4 nanocrystals were then in-situ grown on the surface of graphene sheets via mechanochemical reactions between (NH4)2CO3 and Co(CH3COO)2. The as-prepared graphene/Co3O4 composites exhibit a high specific capacitance of 570 F g−1 at 1 A g−1, and retain 93% of initial capacitance when the current density is increased by 20 times. The excellent electrochemical performance can be attributed to the synergistic effect between the highly-conductive graphene and nanostructured Co3O4 capable of accelerating the electron conduction and ion transport for increasing the capacitive contribution. Asymmetric supercapacitors were also assembled using graphene and graphene/Co3O4 as negative and positive electrodes, respectively. The asymmetric devices demonstrate a wide operating potential window up to 1.6 V, thus achieving a high specific capacitance of 190 F g−1 at 1 A g−1. Accordingly, the energy density is as high as 67.5 W h kg−1 at the power density of 0.8 kW kg−1. This work provides an affordable and scalable approach to prepare graphene-based composites with metal oxides for high-performance electrode materials for electrochemical energy storage.