Self-graphenized biochar with huge pore volume prepared from pre-boiled Ulva lactuca for electrochemical supercapacitor with high energy density

To enhance energy density of water-based electrochemical supercapacitor , a highly self-graphenized bicohar with huge pore volume is pyrolyzed from the biomass Ulva lactuca (UL) pre-boiled in Mg(CH 3 COO) 2 solution. During the pyrolysis process , the MgO converted from the Mg(CH 3 COO) 2 re-crystallized in the biomass UL inhibits the pore shrinkage and helps to inherit the pristine pore system . The resultant biochar HUB-2.0 exhibits high specific surface area (SSA) of 2296.7 m 2  g −1 and huge pore volume of 4.59 m 3  g −1 . It is also featured with moderate self-graphenization and heteroatoms self-doped surface in the connected pore system inherited from the precursor. It achieves a specific capacitance of 445.0 F g −1 at 0.5 A g −1 in the three-electrode system with the electrolyte 1.0 M H 2 SO 4 . A superior energy density of 27.17 Wh kg −1 is also calculated at a power density of 222.8 W kg −1 in the two-electrode system with the neutral electrolyte Na 2 SO 4 . The work provides a fast and efficient method to prepare the biochar with balanced properties to achieve superior electrochemical supercapacitor performance, especially for high energy density.