The Dependence of Mass Transfer Coefficient on the Electrolyte Velocity in Carbon Felt Electrodes: Determination and Validation

Abstract

In a flow battery, the salient impact of the electrolyte velocity on the mass transfer coefficient in carbon felt electrodes is demonstrated and quantified. A lab-scale flow battery, fed with identical electrolyte solutions containing Fe2+/Fe3+ as active substances in both the anode and the cathode, is used to realize stable tests free from side reactions in a broad range of current densities. The electrolyte velocities ranging from 2.5 to 15 mm s−1 are selected in this work, which are typical in flow through electrodes in most flow batteries. By measuring limiting currents at various flow rates, a correlation between the mass transfer coefficient and the velocity in dimensionless form is obtained as Sh = 1.68 Re0.9. Meanwhile, a 2-D numerical model incorporating this correlation and the experimentally measured electrolyte conductivity is proposed. Voltage losses of the battery fed with adequate reactants at different velocities are both experimentally measured and numerically simulated. The agreement between simulated results and experimental data verifies the applicability of this correlation under normal operating conditions below limiting currents.