Supercapacitors with carbonaceous electrode materials represent an efficient energy storage system with a long lifetime and high power density. In this work, we elaborated a route to prepare hard carbons with specified surface properties and tailored pore structures using pyrolysis of cellulose and cellulose acetate microspheres, which are environmentally friendly and sustainable materials in comparison to other carbon sources. In order to enhance the specific surface area of carbons, a facile activation with various amounts of KOH was carried out. Carbonized cellulose acetate without activation shows specific surface areas of about 440 m2 g–1 and after activation up to 1200 m2 g–1. Activated carbon electrodes provide a specific capacitance of 186 F g–1 at a scan rate of 10 mV s–1 in symmetric supercapacitors with an aqueous 4 M KOH electrolyte, providing a high energy density of 40 Wh kg–1 at a power density of 15 kW kg–1. While using neutral electrolytes (1 M Li2SO4 and 1 M Na2SO4), a capacitance of 120 F g–1 was achieved with a maximal power density of 21 kW kg–1. Since many parameters influence the electrochemical performance of carbons, a correlation between synthesis conditions, structural, and electrochemical properties was elaborated. Overall, it was shown that carbons, based on spherical cellulose and cellulose acetate microparticles, represent promising electrode materials for aqueous supercapacitors in terms of their simple and reliable preparation procedure and electrochemical performance.