Abstract: Abstract: In this work, the effects of applied voltages on the characteristics of hydrogen production were investigated by the simultaneous saccharification and fermentation of rice straw in a fabricated microbial electrical cell (MEC) with 2 chambers, and the hydrogen production performance and conversion efficiency of cellulosic material were analyzed. The chambers of anode and cathode in this MEC were 2 concentric cylinders, and the inner one was anode chamber and the external one was cathode chamber. The gross volume of this reactor was 460 mL with 98 mL working volume in anode chamber and 260 mL working volume in cathode chamber. The carbon cloth fixed around the external wall of anode chamber was used as anodic electrode which was linked with cathode electrode by an external circuit with a 50 Ω resistance, and the carbon clothing sprinkled with platinum (Pt) was used as cathode electrode. The 2 chambers were separated by cation exchange membrane (Nafion TM 117, Dupont Co., US). Before testing, the rice straw powder with 60 meshes was mixed with 1% NaOH solution at the solid-liquid ratio of 1:10 for 24 h pretreatment, and then was rinsed with water to neutral and dried at 105 ℃ to constant weight. The 2.0 g pretreated cellulosic material was mixed with 80 mL distilled water and inoculated with the bacterial seed solution at a proportion of 30% (V/V) for compost and fermentation at room temperature for 3 d. The bacterial seed solution was obtained by 7 d domestication of municipal sewage sludge of Chongqing City. The MEC was started using the starting mode of microbial fuel cell (MFC), which was inoculated seed solution in anode chamber and 0.1 mol/L potassium ferricyanide was used as electron acceptor of cathode during the startup. After finishing the startup of MEC, the cellulosic material pretreated by compost and fermentation was mixed with 53.36 mg cellulase (Worthington, 115 U/mg) and 0.2 mL β-glucosidase (Novozyme 188, Sigma, ≥25 U/g) again and transported into the anode chamber of the MEC, the culture medium for bacteria growth was pumped into the anode chamber at a flow rate of 40 mL/h, and 10 mmol/L phosphate buffer (K2HPO4 2.28 g/L, KH2PO4·3H2O 1.53 g/L) was pumped into the cathode of the MEC. Then, the system was sealed tightly for hydrogen production through simultaneous saccharification and fermentation at 35 ℃ constant temperature. It was observed that the voltage of the MEC rose up obviously 30 h after startup, which indicated that the startup of the MEC succeeded using MFC mode. Then, the effects of applied voltages (0, 0.4, 0.6, 0.8, and 1.0 V) on hydrogen production, pH value change of fermented solution and production of intermediates in the MEC were investigated. It was found that hydrogen production rate, hydrogen yield, amount of substrate consumption and total energy yield in the MEC increased with the increase of the applied voltage. Conversely, the energy yield to consumed electric energy decreased. The maximal energy yield to consumed electric energy was 377.59% at 0.4 V applied voltage, while the maximal amount of hydrogen production was 44.8 mL and the maximal total energy yield was 2.84% at the applied voltage of 1.0 V. The pH value of anode chamber first increased, and then slightly decreased during the fermentation for hydrogen production. The testing results of organic acids during the simultaneous saccharification and fermentation reveal that the process of hydrogen production in the MEC is butyric acid type fermentation.