Modifying Carbon Felt Electrodes By Poly(o-toluidine) to Enhance the Performance of All-Vanadium Redox Flow Batteries
Authors: Eifert László , Köble Kerstin , Zeis Roswitha 
Published: February 2020

The overall cell performance of Vanadium Redox Flow Batteries (VRFB) is highly influenced by the physical and chemical properties of their carbon fibre electrodes1. Available as carbonized or graphitized carbon felts or carbon paper, they show relatively low electrochemical activity and poor wettability, which lead to many approaches to improve the surface properties, either by thermal treatment or surface modifications2–4. Previously, we studied the effect of several treatment methods on carbon felts5,6, as well as their impact on side reactions7. Sivakumar et al. utilized poly(o-toluidine) (POT) to enhance the electrochemical capacitance and conductivity of electrochemical capacitors by electrochemical deposition on carbon fabrics8.
In this current study, we modified pristine and pre-treated, commercially available carbon felts (SIGRACELL® GFA6EA by SGL Carbon, Meitingen, Germany) by electrochemical polymerization and deposition of o-toluidine to increase the surface area and conductivity. The deposition of POT was carried out in 1 M H2SO4, containing 20 mM o-toluidine and controlled by a three-electrode setup with a carbon felt as working electrode, a Pt plate as counter electrode and an RHE reference electrode (Gaskatel Hydroflex). By varying the amount of cycles and the potential window, the amount of electrochemically polymerized and deposited POT can be controlled. The electrochemical activity towards vanadium was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The physical properties of the modified carbon felts were characterized via scanning electron microscopy, X-ray photoelectron spectroscopy and BET. The electrochemically most active, modified carbon felts were employed in a full redox flow cell (Scribner 857 redox flow test stand) and charge-discharge cycles at several current densities were performed.
The electrochemical activity of thermally pre-treated carbon felts decreases, if the amount of deposited POT is too high and we found that the optimum deposition condition was one potentiodynamic cycle with 1 mV/s to 1.0 V vs. RHE in 20 mM o-toluidine in 1 M H2SO4. The cyclic voltammograms in Fig. 1(a) show the electrochemical activity towards the V4+/V5+ redox reaction of the felts in 0.1 M VOSO4 in 2 M H2SO4 at a scan rate of 5 mV/s. While the deposition of o-toluidine with a potentiodynamic cycling to 1.5 V vs. RHE show a reduced activity, one cycle with a limit of 1.0 V reduces the vanadium peak separation and increases the peak currents, which indicates an increased activity towards the V4+/V5+ redox reaction, compared to pristine samples. Fig. 1(b)-(d) show SEM images of the different degrees of POT deposition. After 10 cycles to 1.5 V vs. RHE (Fig. 1(c)), a thick coating of POT is visible on the fibres, whereas a potential limit of 1.0 V vs. RHE and 10 cycles (Fig. 1(d)) or 1 cycle (Fig. 1(e)) show a thin coating or scattered coated spots, respectively.
A surface modification of carbon felts by electrochemically deposited o-toluidine was performed in this study. A slight coating of POT increases, whereas higher coatings reduce the electrochemical activity towards the V4+/V5+ redox reaction. The carbon felts were characterized by several techniques, as well as half cell and full cell cycling tests. In general, an increase in overall cell performance can be achieved by POT modified carbon felt electrodes.

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Figure 1