Impact of Plasma and Thermal Treatment on the Long-term Performance of Vanadium Redox Flow Electrodes – Significance of Surface Structure vs Oxygen Functionalities

Abstract

Graphite felt (GF) electrodes of vanadium redox flow batteries show enhanced performance when thermally treated before their assembly. Thermal treatment works by simultaneously increasing electrode wettability, kinetic activity, and total surface area (TSA). This study examines these performance determining yet inseparable effects, especially considering the electrodes’ long-term operation. We exposed GF electrodes to 5 min plasma treatment, ensuring equal wettability, and thermally treated them in air at 400 °C for different durations. We then linked the resulting GF surface structure with the electrode performance, monitored with a high temporal resolution, and controlled electrolyte conditions. The performance, expressed in charge-transfer resistances and voltage efficiencies, correlated accurately with the thermal treatment times. According to XPS, against expectation, the thermal treatment decreased the number of surface oxygen functionalities. Instead, SEM and krypton adsorption revealed that the surface had become rougher, and the TSA increased. Upon corrosion, the surface presumably exposed more carbon edge sites being catalytically active, explaining the improved performance. Therefore, compared with the commonly suggested surface oxygen enrichment, increasing the GF surface roughness and TSA may be the more promising strategy to enhance and stabilize the long-term VRF electrode performance.

Assessing the Feasibility of Implementing Porous Carbon Foam Electrodes Derived from Coal in Redox Flow Batteries

“…This is to be expected, as improved mass transport and wettability following thermal treatment and/or the use of flow channels has also been reported in literature. [109][110][111][112] The as-received coal foam appeared to be very hydrophobic (Fig. 5a), which was thought to contribute to the high ohmic resistance seen during electrochemical measurements.…”

Review—Electrode Kinetics and Electrolyte Stability in Vanadium Flow Batteries

“…[166][167][168][169] Such strategies may also have the effect of removing impurities from electrode surfaces; for example, anodic removal of trace deposits of copper metal 170 from the negative would tend to decrease the rate of any hydrogen-evolution side reactions. Recently Greese et al 168,171 have proposed that adsorbed V 2+ also may play an important role.…”