Unveiling the role of oxygen doping in activated carbon cathode for potassium-ion capacitors

Heteroatom-doped activated carbon that induces pseudocapacitive anion storage is a promising high-capacity cathode compatible with advanced anodes for potassium-ion capacitors (PICs). However, the mechanism of heteroatom-related anion storage in potassium-based electrolyte has rarely been explored and is not well understood. Herein, activated carbon fibers with oxygen doping (ACNFs-O) is conducted to explore the role of oxygen functional groups on the anion storage performance. The ACNFs-O cathode shows a high reversible capacity of 88.4 mAh g−1 at 0.1 A g−1 with a high-rate performance of 57.3 mA h g−1 at 6.0 A g−1, and a long-term stability, which is much higher than that of the non-oxygen-doped sample (ACNFs-H). The experimental characterizations and theoretical calculations reveal that the redox-active carbonyl (CO) group tunes the electrochemical properties of the carbon to provide a pseudocapacitive contribution for improved anion storage. As expected, the PICs with the ACNFs-O as the cathode materials exhibit a high energy density and power density (125.4 Wh kg−1 at 193 W kg−1), which performs 50% better than that of ACNFs-H. This work could provide insight into the design of low-cost and sustainable cathodes and an understanding of the anion storage mechanism of heteroatom-doped carbons for PICs.