Kinetic studies at carbon felt electrodes for vanadium redox-flow batteries under controlled transfer current density conditions

In this work we present a combination of mathematical modeling and experimental kinetic characterization of carbon felt electrodes in positive electrolyte (PE) and negative electrolyte (NE) of vanadium redox-flow batteries. The mathematical model is applied to check for homogeneous transfer current density within a radially connected carbon electrode intensively flown through by the electrolyte. Transfer current homogeneity depends mainly on electrolyte conductivity, electrode thickness and charge transfer coefficient. The transfer current inhomogeneity of the investigated electrode samples is below 5%, when single layers of carbon electrodes (415 mm thickness at 89% porosity and 210 mm thickness at 75% porosity) are applied and high electrolyte conductivities (>800 mS.cm(-1)) are maintained by low vanadium concentrations (< 15 mM). Experimental results show charge transfer coefficients for high overpotentials of 0.26 +/- 0.03 for the reduction reaction in NE and 0.37 +/- 0.04 for the oxidation reaction in NE. The charge transfer coefficients of the PE are 0.13 +/- 0.02 for reduction and 0.30 +/- 0.04 for the oxidation reaction. Application of the Butler-Volmer equation to describe the polarization behavior shows adequate agreement with the experimental results at states of charge between 25% and 75%. The rate constants for the PE reaction are nearly two times higher than for the NE reaction. (C) 2017 Elsevier Ltd. All rights reserved.