Characterization of Activated and Aged Carbon Felts for All-Vanadium Redox Flow Batteries
Authors: Eifert László , Banerjee Rupak , Zeis Roswitha 
Published: February 2020

The electrodes are a crucial part of the Vanadium Redox Flow Batteries (VRFB), since its chemical, physical and structural properties affect cell performance, efficiency, durability as well as acquisition and operating costs1. The most common electrode materials are carbonized or graphitized polyacrylamide (PAN) or Rayon (cellulose) fibres, and commercially available as paper or felts. Due to its relatively low activity towards vanadium and poor wettability, several approaches have been made to improve this surface related properties2. The conventional treatment is a thermal activation procedure, where the electrode material is oxidized in an air atmosphere for up to 30 h at 400°C3.
In this current study, the influence of thermal activation, chemical ageing and electrochemical ageing has been investigated on commercially available Rayon (GFA6 EA) and PAN based (GFD 4.6 EA, KFD 2.5 EA) carbon felts from SGL carbon (Meitingen, Germany). The activation process is conducted by heating all the samples for 25 h at 400°C4. The chemical ageing consists of soaking the previously activated samples in 4 M sulfuric acid at 40°C for 15 days. The electrochemical ageing is realized by applying 1.2 V vs. RHE to the felt for 5 days in a vanadium electrolyte (0.1 M V(V+) in 2 M sulfuric acid, prepared by electrolysis of adequate V(IV+) electrolyte). This electrolyte represents a nearly charged state of a VRFB and a typical potential exposure for the positive electrode during charging4. All samples were characterized by scanning electron microscopy (SEM), thermogravimetric analysis coupled with mass spectrometry (TGA-MS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and cyclic voltammetry (CV).
We found significant differences in how the surface composition, thermal stability and electrochemical behaviour is affected by the means of the different treatment methods. Thermal treatment was found to reduce the overall surface oxygen content5 for all felt types but increases the content of single bonded oxygen for graphitized carbon felts, only. Furthermore, for the carbonized felt type, thermal activation leads to a decreased thermal stability and highly reduces the electrochemical activity, presumably due to increasing C=O and C=C contents. Soaking increases the surface oxygen content of all felts due to the oxidizing nature of sulfuric acid and even further increases the electrochemical reversibility of Rayon based carbon felts, as shown in Figure 1 (a). This correlates to the high content of single bonded oxygen species and combined with an increased sp²/sp³ ratio (Figure 1 (b)). Figure 1 (c) and (d) show SEM images of an untreated and electrochemically aged felt, respectively. The yellow circle in the latter highlights a flaked off part of the carbon fibre. In general, soaking and electrochemical ageing both lead to a much lower thermal stability for all felt types. It is also noteworthy that electrochemically aged samples are more prone to side reactions.
This work contributes to a better understanding in how the precursor material and the processing affects the properties of electrode materials in a VRFB. Furthermore, it provides a comprehensive overview of commercially available carbon felts by SGL and general characterization techniques.

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Figure 1