Quantifying Electron Transfer Kinetics on Porous Carbon Electrodes for Redox Flow Batteries

Despite the critical role of electron transfer kinetics in determining the energy efficiency and the power density of a redox flow battery (RFB), there has not been much progress quantifying the kinetics on commercial porous carbon electrodes, the most common RFB electrodes with no immediate planar-electrode counterparts for cyclic voltammetry (CV) analyses. Here, we tackle this challenge by encapsulating porous carbon electrodes in epoxy, whose cross-sections are used for CV analyses. These electrodes display behaviors of micro-electrodes with length scales characteristic of fibers in the original porous structures. We thereby characterize the kinetics of ferricyanide reduction and show that its rate constants on carbon paper and pre-treated carbon felt are at the order of 10(-4) cm s(-1), near the low end of values reported on glassy carbon. On pre-treated carbon paper, the rate is ten times slower, attributed to the low electrocatalytic activity of the new surface accessible only after the heat treatment. We corroborate these findings with symmetric RFB tests. The method developed here provides a much-needed assessment of kinetics on porous carbon electrodes free from the impacts of mass transports in the pores, enabling the identification of performance-determining factors of RFB electrodes.