Graphene quantum dot/Co(OH)2 electrode on nanoporous Au–Ag alloy for superior hybrid micro-supercapacitors

Abstract

Recent years have witnessed a surge of research in using nanostructured hybrid composites as electrode materials for micro-supercapacitors (micro-SCs). Herein, we synthesized a graphene quantum dot (GQD)/Co(OH)₂ composite by one-step co-deposition on nanoporous Au–Ag alloy wires, focusing on the effect of the GQDs on the operating performance of supercapacitors. We observed the formation of an interconnected network structure with Co(OH)₂ nanosheets grown on Au–Ag alloy, and GQDs were uniformly distributed in the interior of Co(OH)₂. Electrochemical measurements showed an enhanced specific capacity of 273 mC cm⁻² at 1.0 mA cm⁻² for GQD/Co(OH)₂ as compared to 151 mC cm⁻² for pure Co(OH)₂. A wire-shaped, quasi-solid-state hybrid supercapacitor device was fabricated by using the GQD/Co(OH)₂ composite as the positive electrode and active carbon as the negative electrode, which exhibits an excellent capacitance retention rate of 99.5% after 10 000 cycles at a current density of 2 A cm⁻³ and can withstand bending tests nearly without degradation of the electrochemical performance. We further performed systematic first-principles calculations on GQD/Co(OH)₂ to explore its atomic electronic properties associated with the interface. Due to the addition of GQDs, we revealed increased electronic states that spread over the entire energy gap of Co(OH)₂, which are closely related to the electrical conductivity and device performance. We expect these findings to shed new light on using novel hybrid composites for future construction of high-performance and flexible energy-storage devices.