Insights on the Electrochemical Activity of Porous Carbonaceous Electrodes in Non-Aqueous Vanadium Redox Flow Batteries

Abstract

© 2017 The Electrochemical Society. All rights reserved. Porous carbonaceous electrodes have the potential utility as electrodes for non-aqueous VRFBs due to being inexpensive, chemically stable, and electrochemically active. In this work, we provide insights on the electrochemical kinetics of [V(acac)3]0/[V(acac)3]+ and [V(acac)3]0/[V(acac)3]- redox reactions at freestanding sheets of multiwalled carbon nanotubes (MWCNTs), or buckypapers (BPs), and graphitized carbon fiber-based nonwovens papers (CPs). The use of freestanding sheets eliminates the need for a metal substrate; thus, the true electrochemical response at these electrodes is measured. Even though porous carbonaceous electrodes provide high surface area for the electrochemical reaction, their porous nature poses a fundamental challenge caused by thin-film-like diffusion process; leading to a marked ohmic resistance within the pores of the electrodes. Higher current densities are obtained with the BP electrode due to their larger surface area in comparison to the CP electrode; however, the BP electrode show less efficient kinetic facility for the investigated redox couples as evident by electrochemical impedance spectroscopy and polarization curves results. Charge-discharge cycling demonstrates that porous carbonaceous electrodes may require various treatments to modify the surface chemistries to obtain improved cycling efficiencies.