The present study investigated the independent roles of biofilm and external power supply for volatile fatty acids degradation and methane production in the microbial electrolysis cell. Reactors were operated in fed-batch mode in the presence of graphite felt (GF) or titanium rod (Ti) as electrodes, in open circuit (OC, without applied voltage) or closed circuit (CC) conditions, i.e. R1 (Ti + CC), R2 (GF + CC) and R3 (GF + OC). The first-order kinetic analysis of acetate degradation indicated that the presence of GF biofilm and applied voltage in R2 improved the degradation rate of acetate by 23% as compared to R1, while it was only a 7% increment in R3 with GF biofilm. The degradation of butyrate was accelerated by 12% in the first 24 h, whereas there was no enhancement of the propionic acid digestion. Generally, methane yields from the three reactors followed the sequence: R2 > R1 > R3, indicating the positive effect of external power supply on methane generation. High-throughput sequencing revealed that Geobacter sp. could be enriched on conductive GF even without electric stimulation. The clustered Geobacter and Methanosarcina in R2 presented the potential to promote interspecies electron transfer and accelerate substrate utilization and methane production.