Pyrolysis combined with KOH activation to turn waste apple pruning branches into high performance electrode materials with multiple energy storage functions

In this paper, porous activated carbon is successfully prepared from waste apple pruning branches (PGZ), which demonstrates an ultra-high specific capacity of 505 F g ⁻¹ at a current density of 1 A g ⁻¹ with excellent rate performance (215 F g ⁻¹ at 50 A g⁻¹). The assembled supercapacitor also exhibits excellent specific capacitance of 320 F g ⁻¹ (at 0.5 A g⁻¹) and 160 F g ⁻¹ (at 20 A g⁻¹), with a high energy density of 12.03 Wh kg ⁻¹ at a power density of 250.45 W kg ⁻¹ in 6 M KOH. In 1 M Na₂SO ₄ and 1 M Et₄ NBF₄/AC electrolytes, high energy densities of 18.8 Wh kg ⁻¹ and 44.1 Wh kg ⁻¹ could be achieved. It also exhibits high reversible lithium storage capacity of 636.2 mAh g ⁻¹ at 0.2 C and retains 390 mAh g ⁻¹ after 1000 cycles. Even at 0.8 C, the storage capacity is still as high as 327 mAh g ⁻¹, with 282 mAh g ⁻¹ retained after 1000 cycles. These excellent electrochemical properties are attributed to the hierarchical porous structure of the sample prepared under the optimum conditions. The large pores in the hierarchical structure will lead to high-speed capacitive properties due to their low ion transport resistance while the small mesopore and micropore provide a large electrode and electrolyte interface, which can facilitate charge transfer reaction. These outstanding performances highlights the first example of using waste apple pruning branches as a sustainable source of raw materials for the preparation of high value-added porous carbon materials with multiple energy storage functions.