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Two corncob-derived carbon electrode materials mainly composed of micropores (activated carbon, AC) and mesopores/macropores (corncob carbon, CC) were prepared and studied after the anodic electrodeposition of MnO₂. The capacity of the MnO₂/activated carbon composite (MnO₂@AC) electrode did not noticeably increase after MnO₂ electrodeposition, while that of the MnO₂/corncob carbon composite (MnO₂@CC) electrode increased up to 9 times reaching 4475 mF cm^−2. An asymmetric all-solid-state supercapacitor (ASC) was fabricated using AC as the anode, MnO₂@CC as the cathode, and polyvinyl alcohol (PVA)/LiCl gel as the electrolyte. An ultrahigh specific capacitance of 3455.6 mF cm^−2 at 1 mA cm^−2, a maximum energy density of 1.56 mW h cm^−2, and a long lifetime of 10,000 cycles can be achieved. This work provides insights in understanding the function of MnO₂ in biomass-derived electrode materials, and a green path to prepare an ASC from waste biomass with excellent electrochemical performance.