In-Situ Monitoring of Electrode Degradation in Vanadium Redox-Flow Batteries By Distribution of Relaxation Times Analysis
Authors: Schneider Jonathan , Tichter Tim , Khadke Prashant , Roth Christina 
Published: May 2020

Recent studies[1,2] suggest that ion crossover through the membrane of a vanadium redox-flow battery (VRFB) can not be considered the only source of irreversible capacity loss. Instead, degradation of the macroporous carbon felt electrodes has to be taken into account as well. Especially in the negative half-cell, any degradation-induced alteration of the fibres’ surface composition will significantly impede the kinetics of the V2+/V3+ redox-system. Since the resulting increase in kinetic overvoltage will limit the accessible capacity, a reasonable lifetime prediction of a VRFB is only possible if all aspects of capacity fade mechanisms, including electrode degradation are well understood. This raises the demand for a measurement technique, which allows for in-operando state-of-health-assessment of the electrodes in a non-invasive fashion. Employing electrochemical impedance spectroscopy (EIS), the characteristics of any electrochemical system can be easily probed by applying non-harmful, small-amplitude sinusoidal perturbations. Interpretation of EIS usually relies on fitting suitable equivalent circuit models to experimental data. However, validation of such models can be a cumbersome task due to the inherent ambiguity of EIS, meaning that different circuits may approximate the same data set equally well. Furthermore, a full cell spectrum will always yield a superposition of the respective half-cell impedances. Discriminating the contributions of the individual half-cell electrodes to the impedance of a VRFB has therefore been attempted by implementing reference electrodes. However, these are known to be a source of artefacts and distortion in the high frequency range.
In the present study we apply the distribution of relaxation times (DRT) analysis[3] to EIS data of a single cell VRFB. This innovative approach allows us to resolve up to three different time constants per frequency decade. It therefore enables processes that overlap in the classical Bode- and Nyquist representation to be displayed as distinct peaks. Consequently, the individual half-cells can be studied from full cell spectra, circumventing the necessity of finding appropriate equivalent circuit models and rendering the implementation of reference electrodes obsolete.
We outline our experimental strategy for unravelling the physical nature of the processes related to the individual signals in the DRT spectrum and finally demonstrate how these findings enable an online monitoring of electrode degradation in VRFB.

Keywords: electrochemical impedance spectroscopy, distribution of relaxation times, electrode degradation, vanadium redox-flow battery
References:
[1] I. Derr et al., Electrochim. Acta 2017, 224, 9-16.
[2] O. Nibel et al., J. Electrochem. Soc. 2017, 164, A1608-A1615.
[3] E. Ivers-Tiffée, A. Weber, J. Ceram. Soc. Japan 2017, 125, 193-201.

Figure 1:
DRT analysis of a single cell VRFB: complex-plane plot of measured impedance (top) and DRT representation (bottom).

Figure 1