Graphene nanosheet-supported ultrafine RuO₂ quantum dots as electrochemical energy materials

Ruthenium oxide (RuO₂) shows great promise as an electrode material for supercapacitors, which are energy storage devices that have large power densities. However, RuO₂ materials suffer low surface area, poor electrical conductivity and a tendency to aggregate after reactions, limiting their rate capability and cycle stability and can lead to inadequate redox reactions with electrolytes. To address these problems, this work synthesized homogeneous ultrafine-RuO₂ quantum dots (QDs) materials with a size of around 2 nm on the surface of graphene nanosheets (GNS) via a facile hydrothermal method. The small size and uniform distribution of RuO₂ QDs in the composite electrode material allowed for a high concentration of active adsorption sites for ions in the electrolyte while also facilitating efficient electrochemical redox reactions. The resulting RuO₂/GNS composite exhibited a large specific area of 177.34 m²g^-1 and demonstrated an excellent capacitance performance of 1138.5 Fg^-1 at a current density of 0.5 Ag^-1. Additionally, a retention rate of 91.4 % when applying current densities from 0.5 to 10 Ag^-1 and cycling stability with 94.5 % retention after 20000 cycles, demonstrates the occurrence of a rapid and reversible faradaic redox reaction on the electrode surface. The symmetrical supercapacitor configuration with two identical RuO₂/GNS electrodes was fabricated and demonstrated a high capacity (309.16 Fg^-1 at a current density of 0.5 Ag^-1) and an outstanding retention rate of 91.2 % at a current density of 10 A g⁻¹). A device operated within a 1.8 V potential window showed high energy density from 126 Whkg^-1 to 139.2 Whkg^-1 and a power density from 440.1 Wkg^-1 to 8494.4 Wkg^-1 at current densities ranging from 0.5 Ag^-1 to 10 Ag^-1, respectively. This ultrafine-RuO₂ quantum dots composite is a potentially promising advanced functional electrode material for electrochemical applications beyond supercapacitors and may be suitable for a broader range of energy storage and conversion applications.