Free-standing activated flax fabrics with tunable meso/micropore ratio for high-rate capacitance

Free-standing and flexible activated flax fabrics (AFFs) with hierarchical meso/microporous structures have been prepared through a novel one-step synthetic strategy of rapid carbonization/activation of flax fabrics in CO₂. The fast heating and the high partial pressure of CO₂ inhibit the decomposition of flax fabrics during heating up, and thus keep more char materials for gasification at high temperature. It is found that such a process retains a considerable amount of oxygen groups and creates relatively large pores, bringing a one-step process to carbonize and activate flax fabrics at the same time and offering the freedom of tuning the mesopore volume/total pore volume (V_meso/V_total) ratio. Notably, the V_meso/V_total ratio is significantly increased from 27.6% to 67.0%. As a result, the flexible electrodes show excellent electrochemical performance in aqueous electrolyte, exhibiting a large specific capacitance (205 F g⁻¹ or 140 F cm⁻³ at current density of 0.1 A g⁻¹), a good rate capability (139 F g⁻¹ or 95 F cm⁻³ at 35 A g⁻¹), and an excellent cycling stability (∼96.6% retention after 3000 cycles). The excellent rate performance can be attributed to the improved ion transport (due to large pore size) and the good wettability (due to the oxygen group) of electrolyte.