One of the main performance challenges in microbial electrochemical cells (MXCs) is the low voltage efficiency in comparison to other fuel and electrolysis cells. In this study, we aimed to improve the design and operation of microbial electrolysis cells (MECs) to achieve current densities >10 A m(-2) with reduced applied voltages, using a thorough analytical framework involving electrochemical techniques such as chronoamperometry, voltammetry and electrochemical impedance spectroscopy. We developed a design that allows high surface area for the anode using carbon fibers, but without creating a large distance between the anode and the cathode (<0.5 cm) to reduce Ohmic overpotential. We determined that Ohmic overpotential, at current densities >10 A m(-2) remained <0.1 V even when using an anion exchange membrane to separate the anode and the cathode. We observed the largest overpotential from cathode related phenomena. The increase in pH in the cathode chamber, often to similar to 13, results in >0.3 V of Nernstian concentration overpotential. We showed how by adding CO2 to the cathode, this overpotential could be reduced to negligible. We also tested two different cathode materials - stainless steel and nickel - to compare the cathode activation overpotentials. Overall, through our design and operation improvements, we were able to reduce the applied voltages from 1.1 to 0.85 V, at 10 A m(2). Our results also provide important guidelines for further optimizations of MXCs. (C) 2015 Elsevier B.V. All rights reserved.