Identification of performance limiting electrode using asymmetric cell configuration in vanadium redox flow batteries

In this study, the performance of a vanadium redox flow battery (VRFB) is investigated using asymmetric electrode configurations with raw and functionalized (i.e., acid-treated and heat-treated) electrodes. The use of heat-treated electrodes in both half-cells is chosen as the baseline case for comparison, as this configuration shows the best performance. When the positive electrode in the baseline case is replaced with a raw or acid-treated electrode, the voltage efficiency is found to be comparable to that of the baseline case. However, in the case where the negative electrode in the baseline case is replaced with a raw or acid-treated electrode, a significantly lower efficiency is observed, suggesting that the negative half-cell reactions limit the performance of a VRFB. To further investigate this observation, an additional analysis is performed using cyclic voltammetry. The reaction kinetics data suggests that the poor performance of the negative half-cell is not due to the slow kinetics, but rather stems from the fact that the reduction reaction in the negative half-cell occurs at a potential that is very close to the onset of hydrogen evolution. The formation of hydrogen gas bubbles blocks the reaction sites and suppresses the favorable effects of functionalization in the negative half-cell. (C) 2012 Elsevier B.V. All rights reserved.