Redox-Active Nitroxide Radicals Grafted onto MXene: Boosting Energy Storage via Improved Charge Transfer and Surface Capacitance

Nitroxide radicals have fast and reversible redox reactions and high electron transfer rates, while the instability in electrolytes and low conductivity restrict their applications on electrodes. Here, we employ two-dimensional MXene Ti3C2Tx as a conductive film-forming agent for 4-amino-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl) to prepare a freestanding and flexible electrode. Pendant nitroxide radicals experience two-electron storage through reversible redox reactions and realize synergistically boosted charge storage with redox-active Ti3C2Tx nanosheets. A systematic electrochemical investigation demonstrates that nitroxide radicals greatly contribute to the surface capacitance of the hybrid electrode, which significantly improves its diffusion-capacitive dual-model energy storage and cycling stability. Further, the assembled symmetric supercapacitor achieves a high energy density of 60.3 Wh kg–1 and the all-in-one asymmetric supercapacitor shows a wide potential window of 1.8 V, demonstrating great promise in energy storage devices.