Comprehensive Impedance Analysis of Carbon Electrode Materials in Vanadium Redox Flow Batteries

Vanadium redox flow battery (VRFB) electrodes have been the subject of numerous studies, as they play a key role determining kinetic and mass transport losses within an operational battery¹. Many studies have investigated modification of the electrodes through chemical and thermal treatments. These studies are typically confined to one type of electrode material. Enhanced performance is generally attributed to improved electrode kinetics or enhanced wettability.

Quantitative analysis of kinetic, ohmic, and mass transport losses is possible with electrochemical impedance spectroscopy (EIS), as Sun and coworkers have recently shown². With this technique, the effects of electrode treatments on kinetics and mass transport can be resolved. In this work, a method by which to obtain noise-free low-frequency impedance data at flow rates much higher than the 1.5 mL min^-1 by Sun and coworkers will be demonstrated. Fig. 1 shows the impedance-resolved losses of the negative electrode at a flow rate of 30 mL min^-1.

The kinetics and mass transport overvoltages in carbon paper and carbon felt electrodes typically used in VRFBs will also be shown. Furthermore, the effects of chemical modification on both carbon paper and carbon felt will be shown; specifically resolving the effects of the treatments on both electrode kinetics and mass transport processes. A comparison of the ways in which specific treatments affect paper vs. felt will be discussed.

References

1.   A.Z. Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick, Q. Liu, J. Appl. Electrochem., 41, 1137, (2011).

2.   C.-N. Sun, F.M. Delnick, D.S. Aaron, A.B. Papandrew, M.M. Mench, T.A. Zawodzinski, J. Electrochem. Soc., 161, A981, (2014).

Figure 1. Impedance resolved losses on negative electrode in VRFB.