Modifying Carbon Felt Electrodes Using High-Temperature Ammonia Treatment to Improve the Performance of All-Vanadium Redox Flow Batteries

Abstract

High temperature ammonia treatments were performed on carbon felt electrodes in this study. Their physical and electrochemical properties were investigated.Carbon felt electrodes are often used in vanadium redox flow batteries (VRFBs). Without intervention, carbon felt has poor wettability which frustrates electrochemical activity. This material must be modified to make it more effective, which has been historically accomplished with thermal treatments to promote oxygen functional groups. [1] Many attempts have been made to improve performance of carbon felt in VRFBs, but some of the more successful modifications have resulted in an increase in the specific surface area. [2-4] Preliminary experiments on carbon felt performed by this lab with ammonia at high temperatures have shown promising results. [5]For our experiments, commercially available carbon felts (SIGRACELL® GFD3 by SGL Carbon, Meitingen, Germany) were modified by exposure to flowing ammonia gas through a furnace at 900°C for varying lengths of time. The samples were tested in a single-cell flow battery using cyclic voltammetry (CV), polarization curves, and electrochemical impedance spectroscopy (EIS). This treatment results in a significant increase in electrochemical surface area and performance.The physical properties were characterized using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and Raman spectroscopy. A pitting effect was observed in SEM images, preceded by surface blistering at shorter lengths of exposure.Greatly increased active surface area, along with a substantial increase in cell performance, was achieved with the ammonia-modified carbon felts described in this work. Sun, B. and M. Skyllas-Kazacos, Modification of graphite electrode materials for vanadium redox flow battery application—I. Thermal treatment. Electrochimica Acta, 1992. 37(7): p. 1253-1260.Lu, W., et al., High-performance porous uncharged membranes for vanadium flow battery applications created by tuning cohesive and swelling forces. Energy & Environmental Science, 2016. 9(7): p. 2319-2325.Zhou, X.L., et al., A high-performance dual-scale porous electrode for vanadium redox flow batteries. Journal of Power Sources, 2016. 325: p. 329-336.Wei, L., et al., Highly catalytic hollow Ti3C2Tx MXene spheres decorated graphite felt electrode for vanadium redox flow batteries. Energy Storage Materials, 2020. 25: p. 885-892.Pezeshki, A.M., "Impedance-Resolved Performance and Durability in Redox Flow Batteries. " PhD diss., University of Tennessee, 2016. Figure 1