Hydrous ruthenium oxide quantum dots anchored on carbon nanocages for Zn-ion hybrid capacitors

Zn-ion hybrid capacitors (ZICs) are a novel and promising electrochemical energy storage system, while exploring high-performance cathode materials is essential for the development of ZICs. Herein, we propose hydrous ruthenium oxide quantum dots (RuO2 QDs) anchored on porous carbon nanocages (PCNCs) as a new cathode material for ZICs. For the RuO2 QDs@PCNCs material, hydrous RuO2 QDs with a small size of 2–3 nm uniformly disperse on the PCNC support, providing large electrode–electrolyte interface for electrochemical reactions and abundant active sites for ion storage. Consequently, the RuO2 QDs@PCNCs cathode material displays desirable electrochemical performance in ZICs, including a large capacity of 224 mAh g−1 at 0.4–1.8 V, superior rate capability (showing 156 mAh g−1 capacity even at a large current of 20 A g−1) and good stability during a long-term cycling test over 20,000 cycles, significantly superior to currently-reported cathode materials for ZICs. An ultrahigh energy density of 180 Wh/kg is achieved for the RuO2 QDs@PCNCs cathode-based ZICs. Mechanism investigations point out the capacitive behavior-dominated and Zn2+/H+ co-participated charge storage process with fast electrochemical kinetics for the RuO2 QDs@PCNCs cathode in ZICs. This work not only provides a new choice for high-performance cathodes but also enriches the electrochemical theory of ZICs.