Improved electrochemical performance of graphene oxide via copper ion cross-linking and plasma functionalization

The electrochemical performance of graphene oxide (GO) is restricted by pi-pi stacking and other intermolecular interactions, hindering electrolyte access to the densely packed GO layers. Herein, the stacking of GO sheets is effectively prevented, and a nearly three-dimensional (3D) structure is constructed through the cross-linking of Cu2+ with GO. This cross-linking, via Cu2+ interactions with epoxy and carboxyl groups, results in a higher specific surface area after freeze-drying, achieving enhanced electrochemical properties. Furthermore, characterization results demonstrate that oxygen plasma processing can efficiently tailor the oxygen-containing groups of GO without damaging its structure, thereby creating more cross-linking sites. Subsequently, to restore the conductivity, GO is reduced via argon (Ar) plasma treatment while preserving most of the electrochemically active oxygen-containing groups. As a result, electrochemical performance optimization for Cu-GO is attained by synergistically utilizing Cu2+ cross-linking and plasma technology, and the entire process is very simple, facilitating scalable production. Cu-GO can deliver a specific capacitance of 235 F g-1 at 1.5 A g-1, representing a 197% increase compared to that of pure GO, excellent rate performance with a capacitance retention of 93% at 15 A g-1 and long-term cycling stability with no capacitance decay after 60 000 cycles. The assembled all-solid-state symmetric supercapacitor demonstrates an outstanding energy density of 8.1 W h kg-1 and impressive cycling stability with no capacitance decay after 10 000 cycles, showing promising application potential.