Ethanol-mediated dense and N/O/P tri-doped graphene xerogel for ultrahigh volumetric capacitive energy storage

An elaborate balance between porosity and density of carbon materials is a prerequisite for achieving high volumetric energy storage in carbon-based supercapacitors. Three-dimensional graphene aerogels have inter-connected and continuous pores, but suffer from unsatisfactory volumetric performances for their low densities. Herein, we report an ethanol-mediated capillary evaporation technique to prepare a dense and N/O/P-doped graphene xerogel (Ethyl-NPGX) for supercapacitor electrodes. The Ethyl-NPGX is featured with high compac-tion density of 1.53 g cm-3, moderate specific surface area of 203.6 m2 g-1, average mesopore diameter of 6.02 nm, and abundant heteroatoms, including 2.1 at% of N, 14.4 at% of O, and 0.2 at% of P. With simultaneously achieved high density and efficient pore utilization brought by profitable surface functionalities, the Ethyl-NPGX exhibits an ultrahigh volumetric capacitance of 413 F cm-3 in a three-electrode system using 6 M KOH elec-trolyte, which is significantly higher than those of reported graphene-based carbons. Moreover, as-assembled symmetric two-electrode cell delivers an exceptional high volumetric energy density of 25.7 Wh L-1 at 306 W L-1 and an excellent cycling stability with 84.2% capacitance retention after 20000 cycles in aqueous elec-trolyte. Our work provides a promising strategy to densify porous graphene towards compact energy storage.