(Digital Presentation) High-Temperature Ammonia Treatment for Carbon Felt Electrodes in All-Vanadium Redox Flow Batteries

Abstract

Ammonia gas treatments of varying temperature 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 processing, carbon felt has poor wettability which frustrates electrochemical activity. This material must be modified to improve performance, which has been historically accomplished with thermal treatments ostensibly to promote oxygen functional groups, although this has been the subject of debate as morphological impacts have also been observed. [1-3] 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. [4-6] Previous research at the University of Tennessee Knoxville suggests edge sites play a role in improving performance. Preliminary experiments on carbon felt performed by this lab with ammonia at high temperatures have shown promising results. Additional experiments were performed at 900°C by varying length of treatment time, finding four hours to be optimal. [3] New experiments have been done to investigate the kinetic effects of temperature.For these experiments, commercially available carbon felts (SIGRACELL® GFD3 by SGL Carbon, Meitingen, Germany) were modified by exposure to flowing ammonia gas through a furnace at (500°C, 700°C, 900°C, and 1100°C) varying temperatures for four hours. 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. This was done to study the mechanism and kinetic effects of ammonia on the carbon felt material as temperature increases during the treatment itself. Samples treated at the highest temperatures exhibited a notable loss of mass, which is observed in the SEM images in Figure 1. This resulted in a significant increase in edge sites and active surface area, along with a substantial increase in cell performance, which was achieved with the ammonia-modified carbon felts described in this work, with over three times the current density at 80% voltage efficiency compared to untreated felt. 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.Pezeshki, A.M., "Impedance-Resolved Performance and Durability in Redox Flow Batteries. " PhD diss., University of Tennessee, 2016.Gass, K., High Performance Vanadium Redox Flow Battery Electrodes. [manuscript under submission], 2021.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. Figure 1. SEM images at 30,000X magnification of carbon felt electrodes: a) untreated, b) 4 hours NH3 treated at 500°C, c) 4 hours NH3 treated at 700°C, d) 4 hours NH3 treated at 1100°C. Scale was identical for all micrographs Figure 1