Functional combination of methylene blue and porous carbon mutually promotes to deliver ultrahigh rate capacitive and energy storage performance.

The functional combination of methylene blue and activated carbon successfully introduced a significant redox reaction in the activated carbon, increasing the specific capacitance of the material and significantly improving the rate performance. The methylene blue adsorbed on the surface of activated carbon enables fast proton-coupled electron transfer. [Display omitted] • Anthracite can be activated at low temperature and show large specific surface area. • Oxygen groups could effectively contribute the pseudo-capacitance to AC. • The π-π binding endows AC/MB excellent electrochemical properties. • The optimized AC/MB as positive can deliver ultrahigh energy and power density. • Mathematical simulation can optimize the ratio of positive to negative electrodes. Supercapacitors are desired to deliver high power and energy density concurrently, and the introduction of a redox reaction is an effective strategy to improve their capacitive performance. Herein, methylene blue (MB) with redox was employed to functionalize coal-based activated carbons (ACs) as electrode materials for energy storage. The optimized AC shows a high specific capacitance of 408.1 F g−1 in 6 M KOH, deriving from the contribution of the electrical double-layer capacitance of pores and pseudocapacitance of oxygen groups. Interestingly, pure MB shows no capacitive performance mainly due to its low conductivity, while the optimized composite of AC adsorbed MB (AC/MB) surpasses the pure MB and optimized AC in any capacitive performance, displaying a relationship of mutual promotion based on functional combination. The best AC/MB sample exhibits a high specific capacitance of 431.8 F g−1 at 1 A g−1, and 356.1 F g−1 (82.5%) at an ultrahigh current density of 180 A g−1, implying an excellent rate performance. Aiming to deliver maximum energy density, an accurate universal method is proposed to match the mass ratio of negative to positive. The optimized asymmetric supercapacitor AC/MB(+) || AC(-) delivers an energy density of 11.0 Wh kg−1, a power density up to 15.3 kW kg−1 and a high capacity retention rate of 96.3% after 10,000 cycles, displaying good chemical stability. This work proposes a promising strategy for ultrafast store/release energy at high energy density, and also provides an application direction for AC and MB. [ABSTRACT FROM AUTHOR]