Low-cost, scalable, and abundant biomass-derived carbon materials have attracted considerable attention. Herein, we report on novel self-doped (nitrogen) porous carbon nanosheets by carbonization/activation of acorn shells as a biomass precursor for use in high-performance supercapacitors (SCs), which contain SiO2 nanoparticles embedded in carbon nanosheets. This oaknut shell-based activated carbon (AOC) generates an ultrahigh surface area of 3757 m2 g-1, with micropore-mesopore distribution and formation of moderately graphitized sp2 carbon structures. The AOC electrode delivers an impressive specific capacitance of 505 F g-1 at 1.6 A g-1 in three-electrode systems, which is higher than the reported biomass-derived electrodes so far. Further, the symmetric SC displays a remarkable specific energy of 12.7 W h kg-1 at a specific power of 200 W kg-1 in neutral electrolytes, exhibiting high rate capability and retaining 96% of capacitance compared to commercial activated carbon. The outstanding electrochemical performance of the electrode can be ascribed to the large surface area with optimum pore tortuosity for ion transport, high electrical conductivity, and presence of self-doped heteroatoms. Overall, we believe that this high-specific energy self-doped tunable porous carbon can be utilized for industrial SCs. © 2020 American Chemical Society.