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Two-dimensional Ti 3 C 2 MXene with high bulk density can act as prospective electrode material to construct high volumetric energy density supercapacitors. Herein, a series of nitrogen-doped Ti 3 C 2 (N-Ti 3 C 2) films are successfully synthesized by adjusting the quantity of urea using a facile solvothermal method. The incorporation of nitrogen atoms can enlarge interlayer spacing and diminish -F terminal groups of Ti 3 C 2 to promote the diffusion and intercalation of electrolyte ions, and bring about defect sites to provide more electrochemical reactive sites, thus improving the specific capacitance of N-Ti 3 C 2. As a result, the 2.0 N-Ti 3 C 2 film shows the highest volumetric capacitance of 2898.5 F cm−3 at scan rate of 2 mV s−1 in 3 M H 2 SO 4 solution, which is mainly attributed to its high pyridinic N content and low -F terminal groups. Density functional theory (DFT) calculations indicate that the doped N atoms have a higher adsorption energy for protons to enhance the pseudocapacitance of 2.0 N-Ti 3 C 2 electrode, and the increased density of states demonstrate the improvement of its electric conductivity after N doping, which promotes the rate performance of 2.0 N-Ti 3 C 2 electrode. The assembled symmetric supercapacitor based on 2.0 N-Ti 3 C 2 film delivers a high volumetric energy density of 40.8 Wh L−1 and good cycling stability. Our work provides an effective and alternative strategy to improve capacitance of other MXenes for high-performance supercapacitors. [Display omitted] • Flexible nitrogen-doped Ti 3 C 2 films are synthesized by a solvothermal method. • Nitrogen atoms can enlarge interlayer spacing and generate defect sites in Ti 3 C 2. • High pyridinic N can act as electron-donors to absorb more electrolyte ions of H+. • Low -F terminal groups are beneficial to enhance pseudocapacitance of N-Ti 3 C 2. • Optimal 2.0 N-Ti 3 C 2 shows a volumetric capacitance of 2898.5 F cm−3 at 2 mV s−1. [ABSTRACT FROM AUTHOR]

• We first explained the interface style of the Co atoms anchored on the carbon substrate derived from biochar systematically. • The self-doped phosphorus inherited from manila grass play an important role of supplying extra capacitance. • The novel hive-like 3D hierarchical carbon skeleton was low cost. • The ASC based on this electrode delivers excellent rate abilities and cycle performance. Hive-like Co 3 O 4 @C composites with well performance were fabricated using Manila grass as a novel carbonous precursor with self-doped phosphorus. [Display omitted] A novel hive-like Co 3 O 4 @C composite is synthesized solvothermally and calcinated using Manila grass as an interesting precursor of porous carbonous skeleton with self-doped phosphorus and oxygen, anchored by Co 3 O 4 nanocryclusters through the sites rich in doped atoms. The capacitance of the as-fabricated materials is 767.6 F·g−1 (1 A·g−1) with long life of 96.2% retention after 12,000 cycles, according with the phosphorus content in composites. Moreover, the supercapacitor is assembled by KC-600 (+) and active-carbon () delivers an energy density of 81.42 Wh·kg 1 at 875 W·kg 1 and satisfactory cycling stabilities (9.17% lost after 24,000 cycles), owing to its unique microstructure and the self-doped phosphorus of Manila grass. Therefore, the porous hive-like Co 3 O 4 @C composite can better achieve as an electrode material through an easy and eco-friendly process based on biocarbon for the energy storage devices in the future. [ABSTRACT FROM AUTHOR]