Biomass-derived activated carbon (Bio-AC) was prepared by catalytic activation of corn stover followed by milling and surface modification. The prepared Bio-AC was coated onto a fibrous material to fabricate the biomass-derived moisture diffusion energy harvester (Bio-MDEH), which generates electricity from the flow of ions in moisture that diffuse along the interface of conductive carbon particles. The performance of this device was evaluated by measuring open-circuit voltage (Voc) and short-circuit current (Isc). The maximum performance of 926.2 mV and 37.75 μA, which is 4.6-fold of carbon black MDEH, was achieved using KOH-catalyzed activated carbon. Development of mesopore structure, introduction of hydrophilicity, and improvement of electrical conductivity induced from catalytic activation contributed to the improvement of MDEH performance. Several Bio-MDEHs connected in series or parallel reliably generated 1.8 V and 214 μA of electricity and successfully lit a single light-emitting diode. A Bio-MDEH conduction model has also been proposed. Activated carbons have relatively large particle sizes due to their abundant mesoporous structures, which leads to high performance but low cross-connection between activated carbon particles. Thus, they need to be interconnected via carbon black, which acts as a cross-linking conductor. This paper showed that high energy harvesting efficiency of water diffusion energy harvesters can be achieved through the use of biomass-derived carbon materials. We hope that this study will contribute to the development of water diffusion energy harvesters into practical applications as sustainable solutions to future energy crises.