Intermittent renewable energy sources like wind and solar require energy storage to be fully exploited. The H-2-Br-2 flow battery has been identified as a great candidate for this application because of its high energy conversion efficiency and power density capability. To evaluate the performance of this system a fuel cell with a membrane electrode assembly made of a Nafion 212 membrane and electrodes with 0.55 mg Pt/cm(2) loading was tested at 22 degrees C under the H-2-H-2, H-2-O-2, H-2-Br-2 modes to compare the performance between these systems. The exchange current density of the Br-/Br-2 reactions on platinum was found to be about a quarter of that of the hydrogen reactions on the same substrate (0.3 mA/cm(2) Pt versus 1.2 mA/cm(2) Pt). Peak power density during discharge was 0.30 W/cm(2) at a voltage efficiency of 70% for the H-2-Br-2 mode versus 0.17 W/cm(2) at 40% for the H-2-O-2 mode under similar conditions. The performance of the H-2-Br-2 fuel cell is determined mainly by the ohmic and mass transport resistances in the cell, which could be improved by using higher reactant concentrations, higher operating temperatures, more conductive membranes, and electrode and cell designs that enhance transport. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.086207jes] All rights reserved.