Magnesium oxide scaffolded preparation of N, O self-doped biochar with super-hydrophilic surface for aqueous supercapacitor with desired energy density

To assemble electrochemical supercapacitor with attractive energy density, heteroatoms self–doped biochar with hydrophilic surface and natural pore system is prepared by pyrolyzing Flammulina velutipes (FV) pickled with Mg(CH 3 COO) 2 for the first time. The resultant biochar FVB-2-900 displays a super hydrophilicity. It also shows specific surface area of 1174.2 m 2 ·g −1 and pore volume up to 1.57 cm 3 ·g −1 . Due to its N, O co-doped hydrophilic surface, the electrode based on it displays a specific capacitance of 470.5 F·g −1 at current density of 0.5 A·g −1 in three-electrode system. Its outstanding energy density of 18.0 Wh·kg −1 is calculated at power density of 0.7004 kW·kg −1 in the electrolyte 1.0 M H 2 SO 4 for the assembled symmetric super-capacitor. Furthermore, a higher energy density of 26.1 Wh·kg −1 at power density of 1.0 kW·kg −1 is recorded in the electrolyte 1.0 M Na 2 SO 4 . It also exhibits a good cyclic stability of 100 % retention rate after 10,000 cycles at 5.0 A·g −1 . The simple synthesis process, outstanding property, and excellent electrochemical super-capacitor performance make the biochar FVB-2-900 a competitive candidate in the green energy storage. • Flammulina velutipes filled with oxide is converted into hydrophilic biochar. • The biochar surface co-doped with O and N and displays natural pore system. • The biochar FVB-2-900 shows specific surface area of 1174.2 m 2 ·g −1 and pore volume of 1.57 cm 3 ·g −1 . • The biochar FVB-2-900 exhibits a high energy density of 18.0 Wh·kg −1 in electrolyte 1.0 M H 2 SO 4 . • The biochar FVB-2-900 displays a 100 % retention rate after 10,000 cycles at 5.0 A·g −1 .