The mass transport and ABT 702 coefficients of the gases (hydrogen, carbon dioxide, and methane) in the CEM and FO membrane are stated in Table 1. The diffusion coefficient of hydrogen in the FO membrane was lower than in the CEM: 7.56 × 10−8 cm2/s (CEM) >6.74 × 10−8 cm2/s (FO membrane under dry condition) and 1.87 × 10−8 cm2/s (FO membrane under wet condition). The fast diffusion of hydrogen through the CEM, followed by carbon dioxide and methane, suggested that hydrogen produced in the cathode chamber can be lost into the anode chamber during operation. The high hydrogen diffusion coefficient of the CEM consequently leads to hydrogen recovery in MECs. Firstly, methanogens that can be grown in the presence of hydrogen at the anode chamber produce methane by utilizing carbon sources, which are fuels for electrochemically active bacteria to generate electrons (Chae et al., 2008a). Also, hydrogen at the anode chamber can be oxidized to generate current by anode respiring organisms, thus causing internal circulation of electrons and lowering final hydrogen production (Chae et al., 2008a, Lu et al., 2011 and Torres et al., 2008).