Degradation of Carbon Electrodes in the All‐Vanadium Redox Flow Battery

Abstract

The chargeÀ discharge operation of the vanadium redox flow battery degrades the electrodes over time and results in a performance and efficiency decay. The impact of extended chargeÀ discharge cycling operation on carbon electrodes is investigated using carbon paper as a model electrode. Electrode degradation along with 70 % degradation of chargeÀ discharge capacity was observed after 100 chargeÀ discharge cycles of a single cell vanadium redox flow battery operating at a current density of 80 mA cm À 2 at room temperature (23°C). Raman mapping of the electrodes shows a decrease in structural defects in the negative electrode, and an increase in defects in the positive electrode, indicating differences in the degradation mechanism at each electrode. Electrochemical investigation reveals an increase in the activation overpotential at both the positive and negative electrodes. However, the negative electrode showed a higher activation overpotential indicating a higher impact of electrode degradation on the negative side. Xray photoelectron spectroscopy shows around an eightfold increase in surface oxygen functional groups after degradation in both positive and negative electrodes. The composition of oxygen functional groups was also observed to change significantly after degradation from dominantly carbonyl-based to a combination of carbonyl-and carboxyl-based groups. This study provides insight into the electrode degradation mechanism and highlights the differences in the mechanism for the positive and negative electrodes.

Exfoliated Graphene Composite Membrane for the All-Vanadium Redox Flow Battery

“…Raman spectroscopy was performed to study the structural characteristics of graphene and graphite materials. The Raman D band (1350 cm –1 ) is due to the breathing mode vibrations of the sp 2 carbon atoms and is activated by the presence of defects in the structure, such as functional groups, edge sites, and structural disorder. , The Raman spectra obtained for EEG samples deposited on Si/SiO 2 wafers (Figure S4) show the typical spectra for exfoliated graphene, including the D, G, and 2D bands of the graphitic carbon structure. The observed spectrum is consistent with a reduced graphene oxide material and is similar to the Raman spectra obtained in previous studies of EEG prepared using similar methods. , The low-intensity ratio of the D and G peaks suggests that the EEG has a relatively low defect density.…”

“…This is consistent with previous studies and is due to decreasing electrochemical activity of the electrodes, most likely due to changes in the functional groups on the surface of the carbon paper. 8 The enhancement in the VE and EE of the VRFB due to the EEG coating remains present after the 100th cycle, demonstrating the stability of the membrane coating.…”

“…In all experiments, the negative and positive electrolyte tanks each contained 20 mL of 1.6 M vanadium electrolyte in 3 M H 2 SO 4 . The electrolytes were prepared using vanadium oxysulfate (VOSO 4 ), and an initial charging procedure described in previous studies was followed. , The electrolyte solutions were circulated through the cell at a flow rate of 20 ml min –1 using a dual-head peristaltic pump (Masterflex, Cole Parmer). Humidified nitrogen was continuously bubbled through the electrolyte solution 30 min before and during the charge–discharge experiment to remove dissolved oxygen.…”

“…6,7 Due to their long lifetime, independent power and energy, high round trip efficiency, scalability, and design flexibility, RFBs are a good candidate for large-scale energy storage. 2,8 The vanadium redox flow battery (VRFB) is currently the state-of-the-art RFB technology due to its use of a single element (vanadium) with different oxidation states on both sides of the battery, avoiding problems with cross-contamination in the system. 5 The membrane is a crucial component of VRFBs that allows transport of ions during cycling while separating the two halfcell electrolytes.…”

“…Although fossil fuel energy is still the largest energy source utilized globally, renewable energy use is growing rapidly and is expected to reach the same level by 2050 . Energy storage systems (ESSs) for intermittent renewable energy are becoming more important and urgent to fulfill the future demand. − Redox flow batteries (RFBs) can store electrical energy at a large scale for these applications. , Due to their long lifetime, independent power and energy, high round trip efficiency, scalability, and design flexibility, RFBs are a good candidate for large-scale energy storage. , The vanadium redox flow battery (VRFB) is currently the state-of-the-art RFB technology due to its use of a single element (vanadium) with different oxidation states on both sides of the battery, avoiding problems with cross-contamination in the system . The membrane is a crucial component of VRFBs that allows transport of ions during cycling while separating the two half-cell electrolytes.…”