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.…”

Air Plasma Modification of Graphite-Based Electrode for Improved Performance of Aqueous Redox Flow Batteries

“…11,12 This is typically achieved through thermal and/or acidic treatments, although air plasma and microwave 13 treatments have also been reported. [14][15][16][17][18][19][20][21] In the field of electrochemistry, it is well known that oxides on carbon play a significant role. 22,23 The Fe(CN) 6 required to catalyze the process, the specific chemical nature of these moieties (such as carbonyls, phenolics, lactones, ethers, or carboxylates) is not critical.…”

“…In addition to thermal annealing, plasma treatments offer an efficient and controlled means to modify the surface chemistry and structure of the electrode. Despite several reports of such treatments for VRFBs, [14][15][16][17][18][19][20][21] only one publication has explored surface modification using plasma for AORFBs, 31 focusing on acidic AORFB, which is not the most common medium for AORFBs. In this latter study, Permatasari et al demonstrated that the electron transfer rate of Tiron A was improved thanks to oxygen functional groups formed by O 2 plasma treatments.…”

Recent Progress in our Understanding of the Degradation of Carbon‐Based Electrodes in Vanadium Redox Flow Batteries – Current Status and Next Steps

“…In a further study, Greese et al investigated the impact of thermal and plasma treatment of GF on its (long-term) performance in VRFBs. [37] They found that a longer temperature treatment at 400 °C under air atmosphere results in a lower initial charge-transfer resistance (R CT ) and that its increase during prolonged stress is also lower. Scanning electron microscopy (SEM) and krypton adsorption were used to identify material and surface changes as a function of the thermal treatment time, whereby a roughening and an increased total surface area were observed.…”