Carbon Cloth Electrodes for Vanadium Redox Flow Batteries
Authors: Schilling Monja , Scherer Simon , Vocht Marc P. , Debastiani Rafaela , Hermanutz Frank , Zeis Roswitha 
Published: August 2024

Renewable energy sources such as wind and solar are playing an increasingly important role as the decarbonization of the energy industry moves forward. However, wind and solar-based energy production fluctuates daily and seasonally. For a stable electrical grid, a large-scale energy storage system is required. One promising technology for this is the Vanadium Redox Flow Battery (VRFB), developed in the late 1980s by Maria Skyllas-Kazacos (1), which has been continuously researched and improved over the last decades.
VRFBs are already commercially available but must overcome significant lifetime and efficiency challenges. Polarization and pumping losses account for a large part of the efficiency losses due to the high flow-through resistance of the electrolyte in the electrode (2). Therefore, the electrode material must be optimized to achieve high catalytic activity and excellent flow properties to enhance the battery's performance.
In a previous study, we identified key factors such as the electrode's structure, wettability, and electrochemical performance. Therefore, we developed a suitable set of characterization techniques (3). This multimodal characterization approach has been applied to evaluate woven and knitted carbon cloths. We designed electrode materials with specific fiber structures and weaving and knitting patterns. The raw material for the fibers is cellulose from hemp. To successfully commercialize VRVBs, the manufacturing process of the material must be low-cost and easy to fabricate on an industrial scale. Here, the electrode materials are fabricated by weaving or knitting, a well-established technique in the textile industry for centuries, and are ideally suited for the large-scale industrial production of electrodes. After weaving or knitting, the cloths are subsequently carbonized.
To understand the influence of the fabrication pattern, the flow direction, and the 3D electrode structure on the flow behavior, we used Electrochemical Impedance Spectroscopy (EIS) coupled with the Distribution of Relaxation Times (DRT) analysis (4,5). This method allows simultaneously investigating the electrochemical performance, wettability, and structure in an application-oriented, in-house-designed setup. To develop a deeper understanding of the microscopic structure of the fiber and its influence on the key factors, Scanning Electron Microscopy (SEM) and nano-X-ray computed tomography (nano-CT) were utilized. These results were combined with Dynamic Vapor Sorption measurements (DVS), which are used to investigate the wettability of the material, and Cyclic Voltammetry (CV), which accesses the electrochemical performance. Additionally, we studied the effect of thermal activation on these parameters with EIS and DRT analysis, SEM, nano-CT, DVS, CV, and X-ray photoelectron spectroscopy.
The materials presented in this paper exhibited excellent electrode performance, demonstrating good wettability, high catalytic activity, and large electrochemically active surface area. These are promising examples of electrode engineering as a pathway to optimal designs of VRFBs.
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3. K. Köble, M. Jaugstetter, M. Schilling, M. Braig, T. Diemant, K. Tschulik and R. Zeis, Journal of Power Sources,
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5. M. Schilling and R. Zeis, submitted.

Figure 1