While activated carbons derived from biomass resources have led to a notable enhancement in the performance of electrochemical energy storage systems, the presence of limited active sites resulting from randomly developed pores and relatively low electrical conductivity remains to be addressed. Herein we introduce a simple and cost-effective approach to generate a graphene-nanoplatelet-based structure with a large specific surface area, with prominent development of ultra-nanopores smaller than 3 nm. The electrochemical characteristics of the nanoplatelet structure were evaluated as active materials in an electrochemical double layer capacitor. To create a monolithic structure primarily composed of cellulose, we subjected balsa wood to delignification. The resulting cellulose-based monolith was subsequently subjected to carbonization and activation at various temperatures by using a chemical agent (potassium hydroxide). Structural analyses of the prepared materials revealed a high density of micro/nanopores within nanoplatelet-shaped two-dimensional particles. Especially, the Act. 900 sample (900 °C activation temperature) exhibited a large specific surface area of 1384 m2/g and a pore volume of 0.602 cm3/g with a large number of ultra-nanopores (<2 nm). Furthermore, the Act. 900-based electrode exhibited significantly enhanced capacitance (209.2 F/g), a capacitance retention of 96% at a scan rate of 300 mV/s, and cycling stability of 98% without discernible fading or decaying in capacitance after 10,000 testing cycles. This improvement in electrochemical performance can be attributed to ultra-nanopore formation in the graphene nanoplatelets and diffusion length optimization. These factors enable faster ion access and a greater number of electron pathways, thus enhancing performance. Our approach has potential applications in sustainable energy storage systems, making it feasible for practical implementation.