Structure-dependent electrode properties of hollow carbon micro-fibers derived from Platanus fruit and willow catkins for high-performance supercapacitors†
Normally, structural details of the tissue of bio-waste affect the final properties of carbon materials. In this study, we selected two types of bio-wastes, Platanus fruit and willow catkins, to prepare hollow carbon micro-fibers, where their size and microstructure are dependent on the Platanus fruit fibers and willow catkin fibers. The electrode properties of the Platanus-derived hollow micro carbon fibers are much higher than those of the willow-derived micro carbon fibers, although carbonization and activation processes are the same for the two types of materials. It is found that the content of the organic-related elements, C, N, and S, and the content of inorganic ions, K or Na, are different. The high content of N and S induced a high doping concentration of the hollow carbon micro-fibers, which endows the Platanus-derived carbon materials with high conductivity, and the high content of inorganic elements causes a self-activation effect during the carbonization process and results in a special porous microstructure of the Platanus-derived carbon. Therefore, compared with the willow-derived hollow carbon micro-fibers, after carbonization and KOH activation, the hollow carbon micro-fibers derived from Platanus seeds possessed much higher supercapacitor electrode properties. After carbonization and activation under optimized conditions, the specific capacitance of the Platanus- and willow-derived hollow carbon micro-fibers are 304.65 F g−1 and 276.13 F g−1, respectively, at the current density of 0.5 A g−1, with a good rate capability and 88.5% and 81.05% capacity retention at 10 A g−1, respectively. The coin-type symmetric device of these two samples with 6 M KOH electrolyte exhibited a high specific capacitance of 286.5 and 267.5 F g−1, respectively, at 0.25 A g−1 (PFs 900, WFs 800), with an excellent cycling stability and 97.03% and 91.12% capacity retention after 10 000 cycles, respectively. This work not only provided two types of promising supercapacitor carbon materials but also, most importantly, offered us clues for the design and synthesis of high-performance electrode materials using the knowledge gleaned from nature.