) for each Rext were calculated, and power density and polarization curves were generated (Figure ). The maximum power density of P ̅ max =546 mW m -2 was obtained with an external resistor of Rext=10 k applied, whereas, in separate measurements on a membraneless MFC with 6 mm electrode spacing, the maximum power density was only P ̅ max =376 mW m -2 with Rext=25 k. The maximum current density of I ̅ max =3.54 A m -2 was obtained at QT=0.1 mL h -1 with an external resistor Rext=4 kΩ for membrane MFCs which is higher than 0.91 A m -2 obtained from membraneless microfluidic MFCs at QT=1.0 mL h -1 with Rext=10 kΩ. As a measure of the internal resistance, 10 kΩ is indeed low compared to the other microfluidic MFC used in this study (25 kΩ). We transformed maximum current densities from Figure into maximum currents based on electrode areas and combined them with the corresponding flow rates. Then using Eqn. 6, we calculated the acetate conversion efficiency for the two microfluidic MFCs used in this work.