Understanding ageing phenomena of chemically modified graphite felt electrodes for vanadium redox-flow batteries Matthias Kogler 1,2 , Markus Valtiner 1,2 , Christian M. Pichler 1,2 1 Centre for Electrochemistry and Surface Technology GmbH, Wr. Neustadt/Austria, 2 Technical University of Vienna, Vienna/Austria Renewable energy sources are subject to natural volatility. In order to compensate for these fluctuations and thus optimize the use of renewable energy sources, suitable energy storage systems (ESS) are needed with fast response time and large storage capacity. Vanadium redox-flow batteries (VRFB) are therefore reliable and safe long-term energy storage systems that allow the balancing of the fluctuating nature of renewable energy sources. In VRFBs, ionic vanadium species in an acid based electrolyte serve as charge carrier. Via reversible redox processes, which take place on the surface of the graphite felt electrode, fast and efficient charge storage is achieved. Although the principle of VRFBs has been already proposed in the-1980s 1 , the exact nature of the interaction between the vanadium species and the carbon electrode is still unclear. There is ongoing debate regarding the mechanism of the aforementioned processes and the species involved. However, this knowledge is essential, not only for the overall evolution of the battery system, but especially for understanding and controlling the degradation of the electrode and thus the lifetime of the battery. For generating a more detailed insight into the underlying mechanism, we have developed a method for chemical modification of the graphite felt electrodes, which allows us to customize the surface of the electrode according to our demands and introduce defined functional groups. In order to extract the influence of the individual involved components and surface groups , a complete characterisation of the modified felts was carried out using a wide range of analytical methods, including electrochemical as well as various surface analytical techniques such as XPS, IR and Raman spectroscopy. As a result, we are able to provide a more accurate picture of the interactions between electrolyte and electrode surface. Furthermore, by precisely controlling the electrode surface the electrolyte interactions can be tuned, resulting in more efficient charge/discharge cycles and increased lifetimes. This represents an additional step towards the continued advancement of the all-vanadium redox flow system, which can contribute to the improvement of both performance and lifetime of the battery. References 1. Skyllas-Kazacos,M. et alJ. Electrochem. Soc. 1986 ,133,1057.
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