The approach using polyoxometalate (POM) as biomass oxidation catalysts and charge carriers in a flow fuel cell (FFC) is of great interest as it can directly convert biomass to electricity. However, the fundamentals, such as POM charge and discharge characteristics, electrode overpotentials, the regeneration of POMs in both cathode and anode cells, have not been understood. Studies focused on these fundamentals are critically important for improving the performance of these fuel cells. Herein, we constructed a POM-mediated biomass redox flow fuel cell (BRFFC) that generated power density as high as 42 mW cm(-2) directly from various high-moisture contented raw biomass fuels, such as wheat straw, sugarcane, corn cob, green corn stover, and peanut shells. Different from previous studies, a new design of the flow fuel cell was constructed by inserting a Ag/AgCl reference electrode. This new design is unique because the discharging process, electrode overpotential, and electrochemical kinetics can be investigated in situ, which provides a deeper fundamental understanding of the biomass flow fuel cell. The results indicate that the POM discharging at the cathode electrode rather than the anode plays a key role in the entire flow fuel cell performance. The energy analysis shows that the biomass flow fuel cell used in this study can generate maximum net energy 2.33 MJ (647 Wh) by consuming 1 kg of 59.5% moistured wheat straw.