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

- 2D potential and current density distribution within a porous electrode is modeled.
- Adapted experimental conditions limit transfer current inhomogeneity to less than 5%.
- Carbon paper and carbon felt are characterized for SoC from 5% to 95%.
- Charge transfer coefficients below 0.4 for cathodic and anodic reactions are determined.
- Butler-Volmer equation adequately describes polarization curve for SoC from 25 to 75%.

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 μm thickness at 89% porosity and 210 μm 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.

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