Ageing and Degradation Effects Observed in Carbon Felts Used As Electrodes in Vanadium Redox Flow Batteries
Authors: Roth Christina , Derr Igor , Fetyan Abdulmonem , Schutjajew Konstantin , Schweer Jakob , Schneider Jonathan , Schnucklake Maike 
Published: February 2020

With dwindling fossil resources and growing environmental concerns, clean and efficient energy conversion and storage technologies are getting more and more attention from the public. Lithium ion batteries, redox flow batteries as well as fuel cells hold the promise and burden to sustain our energy-demanding society. In recent years, several efforts have been addressed to vanadium redox flow batteries (VRFB) which show great promise for the economical storage of electrical energy intermittently available from wind and solar. The excess energy is stored in liquid electrolytes using the four oxidation states of Vanadium, V2+/3+ at the negative, V4+/5+at the positive side. The liquid electrolytes flow through porous carbon felt electrodes, which provide a heterogeneous reaction surface. The main reasons to use carbon felts are their corrosion resistance in acidic electrolyte, high overpotential towards the unwanted hydrogen evolution reaction (HER) and low material costs. In addition, their activity towards the positive and negative reaction, respectively, can be modified using thermal oxidation in air atmosphere, acid treatment, heteroatom doping, and others.
With respect to stationary applications with projected system runtimes of 10˙000 hours and more, not only activity, but also durability of the electrode materials moves into the focus of current research. It has been reported [1] that the carbon felts undergo significant degradation when cycled for a certain number of times. However, already the electro-less contact with the highly acidic electrolytes seems to affect the felts properties. In this study, static electro-less ageing and dynamic degradation of heat-treated carbon felt electrodes were investigated by combining different electrochemical and analytical methods. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to monitor the performance loss of the carbon-based electrodes, whereas scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were applied to study cause and effects of material’s degradation. In this respect, oxygen functional groups, electron conductivity, wettability, accessible surface area and other characteristics play an important role.
In general, the HER is a parasitic reaction, which should be hindered in favor of the V3+ reduction. It has been reported that bismuth additions to the electrolyte enhance the activity of the felt for the negative reaction without enhancing the HER [2]. However, the competition between HER and V3+reduction can also be used as an indicator of the felt’s state of health when probed at different temperatures and in different electrolyte concentrations. We will discuss this for differently degraded felts. Furthermore, we use our battery tester with implemented reference electrodes to compare the individual half-cell potentials with full cell potentials and to study the effect of different cut-off voltages on the projected electrode degradation. Higher cut-off potentials might prove effective for the system’s operation strategy, if they do not lead to the expected increased degradation phenomena.
[1] I. Derr, M. Bruns, J. Langner, A. Fetyan, J. Melke, C. Roth „Degradation of all-vanadium redox flow batteries (VRFB) investigated by electrochemical impedance and X-ray photoelectron spectroscopy: Part 2 electrochemical degradation, Journal of Power Sources 325 (2016) 351-359.
[2] B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenkle, W. Wang Nano Letters. 13 (2013) 1330-1335.