The Impact of Electrode Aging on the Structural and Mechanical Properties of Carbon Felt Electrodes for Vanadium Redox Flow Batteries
Authors: Köble Kerstin , Jaugstetter Maximilian , Eifert László , Tschulik Kristina , Zeis Roswitha 
Published: December 2020

Vanadium Redox Flow Batteries (VRFBs) represents a promising approach to large-scale energy storage of intermittently available renewable energy. However, this technology still faces major problems in terms of lifetime, efficiency, and costs. One limitation is the degradation of the electrode during operation, which influences the (electro)chemical and mechanical stability of the electrode.1,2 Performance deteriorations during cycling are frequently reported and the harsh environment of the sulfuric acid itself leads to electroless oxidation of the carbon-based electrodes.3 Previously, we have already presented the effect of thermal treatment and accelerated stress tests on the electrochemical and compositional properties of commercial carbon felt electrodes and their affinity towards side reactions like hydrogen evolution and carbon corrosion.4,5 These studies focused on the chemical aging at high temperature in sulfuric acid and the electrochemical aging in V(V) electrolyte to imitate the conditions of a charged, positive VRFB half-cell. To deepen the knowledge about (electro)chemical aging of carbon electrodes, we present a study on the impact of degradation on the structural and mechanical properties of carbon electrodes.
Corresponding to our previous studies, different treatments were applied to mimic the effect of thermal activation, chemical, and electrochemical aging on the structure of carbon fiber electrodes. Thereby, the electrode material was either thermally treated at 400°C in air atmosphere, and immersed in sulfuric acid, or vanadium electrolyte under potential control. Structural changes were observed by scanning electron microscopy (SEM) and confirmed by topography images from atomic force microscopy (AFM) on single fibers. Furthermore, physisorption measurements were conducted to quantify the changes in porosity, surface area, and pore size distribution after each of the aforementioned treatments. Adhesion profiles were measured by AFM to analyze the wettability of the respective samples and to determine the modulus of elasticity.
The characterization methods presented in this study are beneficial to investigate the mechanical degradation of carbon felt electrodes during operation. The extracted values like pore size distribution, fiber thickness, or modulus of elasticity are important for theoretical simulations on compression or flow behavior. Also, accelerated stress test procedures are helpful to evaluate the stability of new materials or modified carbon electrodes since electrode stability is an important aspect to consider for the long-term operation of VRFBs.
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