Vanadium redox flow batteries (VRFBs) are considered as promising electrochemical energy storage systems due to their efficiency, flexibility and scalability to meet our needs in renewable energy applications. Unfortunately, the low electrochemical performance of the available carbon-based electrodes hinders their commercial viability. Herein, novel free-standing electrospun nanofibrous carbon-loaded composites with textile-like characteristics have been constructed and employed as efficient electrodes for VRFBs. In this work, polyacrylonitrile-based electrospun nanofibers loaded with different types of carbon black (CB) were electrospun providing a robust free-standing network. Incorporation of CBs (14% and 50% weight ratio) resulted in fibers with rough surface and increased mean diameter. It provided higher BET surface area of 83.8 m² g⁻¹ for as-spun and 356.7 m² g⁻¹ for carbonized fibers compared to the commercial carbon felt (0.6 m² g⁻¹). These loaded CB-fibers also had better thermal stability and showed higher electrochemical activity for VRFBs than a commercial felt electrode.

The performance of all-V redox flow batteries (VRFB) will decrease when they are exposed to dynamic electrochemical cycling, but also when they are in prolonged contact with the acidic electrolyte. These phenomena are especially severe at the negative side, where the parasitic hydrogen evolution reaction (HER) will be increasingly favored over the reduction of V(III) with ongoing degradation of the carbon felt electrode. Bismuth, either added to the electrolyte or deposited onto the felt, has been reported to suppress the HER and therefore to enhance the kinetics of the V(II)/V(III) redox reaction. This study is the first to investigate degradation effects on bismuth-modified electrodes in the negative half-cell of a VRFB. By means of a simple impregnation method, a commercially available carbon felt was decorated with Bi ${}_2$ O ${}_3$ , which is supposedly present as Bi(0) under the working conditions at the negative side. Modified and unmodified felts were characterized electrochemically using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a three-electrode setup. Surface morphology of the electrodes and composition of the negative half-cell electrolyte were probed using scanning electron microscopy (SEM) and X-ray fluorescence spectroscopy (TXRF), respectively. This was done before and after the electrodes were subjected to 50 charge-discharge cycles in a battery test bench. Our results suggest that not only the bismuth catalyst is dissolved from the electrode during battery operation, but also that the presence of bismuth in the system has a strong accelerating effect on electrode degradation.

Highly porous carbon–carbon composite electrodes for the implementation in redox flow battery systems have been synthesized by a novel soft-templating approach. A PAN-based carbon felt was embedded into a solution containing a phenolic resin, a nitrogen source (pyrrole-2-carboxaldehyde) and a sulfur source (2-thiophenecarboxaldehyde), as well as a triblock copolymer (Pluronic^® F-127) acting as the structure-directing agent. By this strategy, highly porous carbon phase co-doped with nitrogen and sulfur was obtained inside the macroporous carbon felt. For the investigation of electrode structure and porosity X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and nitrogen sorption (BET) were used. The electrochemical performance of the carbon felts was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The N- and S-doped carbon electrodes show promising activity for the positive side reaction and could be seen as a significant advance in the design of carbon felt electrodes for use in redox flow batteries.