Three-dimensional mesoporous graphene-modified carbon felt for high-performance vanadium redox flow batteries

Vanadium Redox Flow Battery: Review and Perspective of 3D Electrodes

Vanadium redox flow battery (VRFB) has garnered significant attention due to its potential for facilitating the cost-effective utilization of renewable energy and large-scale power storage. However, the limited electrochemical activity of the electrode in vanadium redox reactions poses a challenge in achieving a high-performance VRFB. Consequently, there is a pressing need to assess advancements in electrodes to inspire innovative approaches for enhancing electrode structure and composition. This work categorizes three-dimensional (3D) electrodes derived from materials such as foam, biomass, and electrospun fibers. By employing a flexible electrode design and compositional functionalization, high-speed mass transfer channels and abundant active sites for vanadium redox reactions can be created. Furthermore, the incorporation of 3D electrocatalysts into the electrodes is discussed, including metal-based, carbon-based, and composite materials. The strong interaction and ordered arrangement of these nanocomposites have an influence on the uniformity and stability of the surface charge distribution, thereby enhancing the electrochemical performance of the composite electrodes. Finally, the challenges and perspectives of VRFB are explored through advancements in 3D electrodes, 3D electrocatalysts, and mechanisms. It is hoped that this review will inspire the development of methodology and concept of 3D electrodes in VRFB, so as to promote the future development of scientific energy storage and conversion technology.

Comparison of the Influence of Oxygen Groups Introduced by Graphene Oxide on the Activity of Carbon Felt in Vanadium and Anthraquinone Flow Batteries

An increasing number of studies focus on organic flow batteries (OFBs) as possible substitutes for the vanadium flow battery (VFB), featuring anthraquinone derivatives, such as anthraquinone-2,7-disulfonic acid (2,7-AQDS). VFBs have been postulated as a promising energy storage technology. However, the fluctuating cost of vanadium minerals and risky supply chains have hampered their implementation, while OFBs could be prepared from renewable raw materials. A critical component of flow batteries is the electrode material, which can determine the power density and energy efficiency. Yet, and in contrast to VFBs, studies on electrodes tailored for OFBs are scarce. Hence, in this work, we propose the modification of commercial carbon felts with reduced graphene oxide (rGO) and poly(ethylene glycol) for the 2,7-AQDS redox couple and to preliminarily assess its effects on the efficiency of a 2,7-AQDS/ferrocyanide flow battery. Results are compared to those of a VFB to evaluate if the benefits of the modification are transferable to OFBs. The modification of carbon felts with surface oxygen groups introduced by the presence of rGO enhanced both its hydrophilicity and surface area, favoring the catalytic activity toward VFB and OFB reactions. The results are promising, given the improved behavior of the modified electrodes. Parallels are established between the electrodes of both FB technologies.

Insights into the Modification of Carbonous Felt as an Electrode for Vanadium Redox Flow Batteries

The vanadium redox flow battery (VRFB) has been regarded as one of the best potential stationary electrochemical storage systems for its design flexibility, long cycle life, high efficiency, and high safety; it is usually utilized to resolve the fluctuations and intermittent nature of renewable energy sources. As one of the critical components of VRFBs to provide the reaction sites for redox couples, an ideal electrode should possess excellent chemical and electrochemical stability, conductivity, and a low price, as well as good reaction kinetics, hydrophilicity, and electrochemical activity, in order to satisfy the requirements for high-performance VRFBs. However, the most commonly used electrode material, a carbonous felt electrode, such as graphite felt (GF) or carbon felt (CF), suffers from relatively inferior kinetic reversibility and poor catalytic activity toward the V²⁺/V³⁺ and VO²⁺/VO₂⁺ redox couples, limiting the operation of VRFBs at low current density. Therefore, modified carbon substrates have been extensively investigated to improve vanadium redox reactions. Here, we give a brief review of recent progress in the modification methods of carbonous felt electrodes, such as surface treatment, the deposition of low-cost metal oxides, the doping of nonmetal elements, and complexation with nanostructured carbon materials. Thus, we give new insights into the relationships between the structure and the electrochemical performance, and provide some perspectives for the future development of VRFBs. Through a comprehensive analysis, it is found that the increase in the surface area and active sites are two decisive factors that enhance the performance of carbonous felt electrodes. Based on the varied structural and electrochemical characterizations, the relationship between the surface nature and electrochemical activity, as well as the mechanism of the modified carbon felt electrodes, is also discussed.

Compressed composite carbon felt as a negative electrode for a zinc-iron flow battery

Flow batteries possess several attractive features including long cycle life, flexible design, ease of scaling up, and high safety. They are considered an excellent choice for large-scale energy storage. Carbon felt (CF) electrodes are commonly used as porous electrodes in flow batteries. In vanadium flow batteries, both active materials and discharge products are in a liquid phase, thus leaving no trace on the electrode surface. However, zinc-based flow batteries involve zinc deposition/dissolution, structure and configuration of the electrode significantly determine stability and performance of the battery. Herein, fabrication of a compressed composite using CF with polyvinylidene fluoride (PVDF) is investigated in a Zn-Fe flow battery (ZFB). Graphene (G) is successfully introduced in order to improve its electrochemical activity towards zinc reactions on the negative side of the ZFB. A compressed composite CF electrode offers more uniform electric field and lower nucleation overpotential (NOP) of zinc than a pristine CF, resulting in higher zinc plating/stripping efficiency. Batteries with modified electrodes are seen to provide lower overpotential. Particularly, the G-PVDF-CF electrode demonstrates maximum discharge capacity of 39.6 mAh cm^-2 with coulombic efficiency and energy efficiency over 96% and 61%, respectively. Finally, results lead to increased efficiency and cycling stability for flow batteries.

Tungsten oxide/fullerene-based nanocomposites as electrocatalysts and parasitic reactions inhibitors for VO2+/VO2+ in mixed-acids

The relatively high cost of all-vanadium redox flow batteries (VRFBs) limits their widespread deployment. Enhancing the kinetics of the electrochemical reactions is needed to increase the power density and energy efficiency of the VRFB, and hence decrease the kWh cost of VRFBs. In this work, hydrothermally synthesized hydrated tungsten oxide (HWO) nanoparticles, C₇₆, and C₇₆/HWO were deposited on carbon cloth electrodes and tested as electrocatalysts for the VO²⁺/VO₂⁺ redox reactions. Field Emission Scanning Electron Microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscope (HR-TEM,), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and contact angle measurements were used to characterize the electrodes’ material. The addition of the C₇₆ fullerene to HWO was found to boost the electrode kinetics towards the VO²⁺/VO₂⁺ redox reaction, by enhancing the conductivity and providing oxygenated functional groups at its surface. A composite of HWO/C₇₆ (50 wt% C₇₆) was found to be the optimum for the VO²⁺/VO₂⁺ reaction, showing a ΔE_p of 176 mV, compared to 365 mV in the case of untreated carbon cloth (UCC). Besides, HWO/C₇₆ composites showed a significant inhibition effect for the parasitic chlorine evolution reaction due to the W-OH functional groups.