We demonstrate a vanadium redox flow battery with a peak power density of 557 mW cm(-2) at a state of charge of 60%. This power density, the highest reported to date, was obtained with a zero-gap flow field cell architecture and non-wetproofed carbon paper electrodes. The electrodes were comprised of stacked sheets of carbon paper and optimized through systematic variation of the total electrode thickness. We anticipate significant reductions in the ultimate system cost of redox flow battery systems based on this design. (C) 2011 Elsevier B.V. All rights reserved.