Apple-pomace-based porous biochar as electrode materials for supercapacitors

The apple-pomace-based porous biochar (APPC) was prepared as electrode materials for supercapacitors utilizing hydrothermally carbonized apple pomace as carbon source and K 2 FeO 4 as activator and modifier with the mass ratio of 1:1 (K 2 FeO 4 to hydrothermally carbonized apple pomace) under the nitrogen atmosphere for carbonization at 800 °C for 2 h. The Brunauer-Emmett-Teller (BET) analysis showed that the as-prepared biochar, designated as APPC 1–2-800 , possessed a hierarchically porous framework owning a relatively higher specific surface area of 2010.16 m 2 g −1 , XRD and Raman spectroscopy demonstrated some extents of graphitization occurred in APPC 1–2-800 . Additionally, X-ray photoelectron spectroscopy (XPS) analysis manifested that heteroatoms of oxygen have been successfully doped into APPC after K 2 FeO 4 activation and modification. Electrochemical tests show that the electrochemically active specific surface area ( ECSA ) of the APPC 1–2-800 is calculated as 1804 cm 2 per square centimeter of the electrode and the APPC 1–2-800 electrode has good capacitance performance in a three-electrode system, with a specific capacitance of 360.1 F g −1 at a current density of 0.5 A g −1 . The symmetric supercapacitor assembled using the 2 identical APPC 1–2-800 electrodes, denoted as SSC-APPC 1–2-800 , exhibits a high energy density of 16.53 W h kg −1 at an assigned power density of 250 W kg −1 and 13.17 W h kg −1 at 10000 W kg −1 . The Coulombic efficiency is noted to range from 96 % to 100 % after 5000 cycles. In addition, the specific capacitance decreased by only 4.3 % after 5000 cycles at a current density of 5 A g −1 , indicating preeminent cyclic stability. • An apple-pomace-based hierarchical porous biochar (APPC) was prepared. • The APPC 1–2-800 possesses a good hierarchical porous structure. • The APPC 1–2-800 electrode exhibited excellent capacitance performance. • The SSC assembled with 2 identical APPC 1–2-800 electrodes has good cyclic stability.