Application of in-Situ Radiography to Porous Carbon Felt Electrodes in Vanadium Redox Flow Batteries

Abstract

Porous carbon felts are routinely applied as electrodes in vanadium redox flow batteries. They offer several advantages, such as low cost, low weight, good electron conductivity, and chemical inertness. However, due to their comparatively low activity in catalyzing the vanadium reactions at the anode and cathode side of the battery, an activation treatment is required to enhance the kinetics. In the literature, a heat-treatment at 400 °C in air has been reported as sufficient to introduce more oxygen functional groups at the surface, also improving the wettability of the felt and its contact with the electrolyte significantly. In this context, it is however not clear, which mechanisms are responsible for the observed performance increase. In another approach, bismuth is deposited on the carbon felts to enhance the V2+/3+ reaction in the anolyte. While a better battery performance is indeed observed, the bismuth is assumed to be dissolved and re-deposited during the charge-discharge cycles.The aim of this work is to utilize in-situ X-ray radiography to study both the activation process and the catalytic enhancement effect due to bismuth decoration in more detail. We will highlight the potential of in-situ (synchrotron) radiography for the investigation of electrode/electrolyte interaction, pore filling and electrolyte flow behaviour complementing other more routine characterization techniques nicely [1]. To elucidate the effect of air activation, pristine and heat-treated carbon felts have been brought into contact with water, sulfuric acid and an acidic vanadium solution and ex-situ contact angles determined. This study was complemented by in-situ X-ray radiography, in which an in-house built flow cell served to monitor the electrolyte flow through the porous felt materials (pristine and heat-treated). The effect of the heat-treatment can be clearly discerned in water, see figure 1 (top: pristine felt, bottom: heat-treated felt). In contrast, when the commercial vandium electrolyte was being used, the differences between pristine and heat-treated felt vanished. Consequently, only a negligibly higher pore filling degree was observed for the treated felts, while all felts were flooded in less than 1 minute. [2]Apart from the phenomena described above, we will also highlight the potential of in-situ radiography with respect to Bi-decorated felts during operation. [3][1] L. Eifert, N. Bevilacqua, K. Köble, K. Fahy, L. Xiao, M. Li, K. Duan, A. Bazylak, P.-C. Sui, R. Zeis, ChemSusChem 13 (2020) 3154.[2] M. Gebhard, M. Schnucklake, A. Hilger, M. Röhe, M. Osenberg, U. Krewer, I. Manke, C. Roth, Energy Technol. 8 (2020) 1901214.[3] M. Gebhard, T. Tichter, J. Schneider, J. Mayer, A. Hilger, M. Osenberg, M. Rahn, I. Manke, C. Roth J. Power Sources 478 (2020) 228695. Figure 1