The redox reactions inside a Vanadium Redox Flow Battery (VRFB) are catalyzed by a porous carbon felt electrode, through which the vanadium containing electrolyte is pumped. Thus, a high Electrochemically Active Surface Area (ECSA), corresponding to a high saturation of the porous electrode, is beneficial to the performance. Maintaining a low mass transport resistance inside the flow felt is important to reduce parasitic pumping losses, which decrease the overall efficiency of the battery system. The pressure drop was measured, as it is directly related to the pumping losses caused by the flow through the pores. X-ray tomography and X-ray radiography were used to visualize the distribution of the electrolyte inside the pores. The effect of thermal activation (400 °C, 25 h) and compression of the carbon electrode on the wetting behavior of the electrolyte was investigated, as it correlates with the ECSA and the oxygen content on the electrode surface¹. Furthermore, the changes in flow behavior of the redox active vanadium species with a different oxidation number are of interest, as it could imply separate optimization parameters for the anode side and the cathode side. The redox-active vanadium species in the electrolyte exist in four different states, V(II), V(III), V(IV), and V(V).

In this work, the X-ray based techniques were used in an ex situ injection setup². VRFB electrolytes (V(IV), V(V), V(II), V(III), all 0.1 M in 2M H₂SO₄) were injected into a commercially available porous carbon felt electrode ((GFA 6EA) from SIGRACELL® battery electrodes (SGL Carbon, Germany)) under controlled degrees of compression (20%, 50%, 70% of the uncompressed height). After an initial imbibition of the electrode, which was stopped immediately after the electrode wicked the electrolyte, a higher flow-rate was applied to simulate operating conditions. Synchrotron X-ray radiography shows the invasion and progression of the liquid electrolyte into the pores and indicates that capillary forces are initially dominant, but numerical simulation of the flow-through suggests that other forces come into play as well at higher flow velocities³. The study shows that the thermal activation plays the major role in the wetting behavior, increasing the saturation inside the electrode by more than 200% and decreasing the pressure drop by a factor of two when the electrolyte is injected into a thermally activated electrode, in agreement with Banerjee et al.³. The influence of the vanadium species in the electrolyte is relatively weak and it shows a slightly higher saturation for V(III), but this effect is outweighed by a stronger effect of compression. Synchrotron X-ray tomography highlights the occurrence of the Cassie-Baxter effect (the emergence of trapped air bubbles inside the electrode) and implies movement of these bubbles during continuous flow, in agreement with Greco et al.¹.

Figure 1: a) Saturation after injection of various electrolyte species into thermally activated carbon felt electrodes under different degrees of compression. b) Effect of compression on the pressure during the simulation of continuous flow-through conditions. The bar color corresponds to the color of the electrolyte solution in both figures.

1 Greco, K. V., Forner-Cuenca, A., Mularczyk, A., Eller, J. J. & Brushett, F. R. Elucidating the Nuanced Effects of Thermal Pretreatment on Carbon Paper Electrodes for Vanadium Redox Flow Batteries. ACS Applied Materials & Interfaces 10, 44430-44442, doi:10.1021/acsami.8b15793 (2018).

2 Bevilacqua, N., George, M. G., Galbiati, S., Bazylak, A. & Zeis, R. Phosphoric Acid Invasion in High Temperature PEM Fuel Cell Gas Diffsuion Layers. Electrochim. Acta 257, 89-98, doi:10.1016/j.electacta.2017.10.054 (2017).

3 Banerjee, R., Bevilacqua, N., Eifert, L. & Zeis, R. Characterization of carbon felt electrodes for vanadium redox flow batteries - A pore network modeling approach. Journal of Energy Storage 21, 163-171, doi:10.1016/j.est.2018.11.014 (2019).