All-vanadium Redox Flow Battery Research Articles

Nitrogen-Doped Tungsten Carbide-Modified Graphite Felt as a Bifunctional Electrocatalyst for an All-Vanadium Redox Flow Battery

Nitrogen-Doped Tungsten Carbide-Modified Graphite Felt as a Bifunctional Electrocatalyst for an All-Vanadium Redox Flow Battery

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 ΔEp 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.

Mass transfer enhancement in electrode and battery performance optimization of all-vanadium flow based on channel section reconstruction

The all-vanadium redox flow battery(VRFB) is one of the most commercially developed energy storage technologies due to the high efficiency, long cycle life, flexible design. The novel battery performance optimization based on the channel section reconstruction is proposed, which can enhance the electrolyte penetration and promote the species distribution. A 3-D numerical model for VRFB is built to investigate the mass transfer behavior and battery performance with different channel sections. The results show that the mass transfer behaviors in the porous electrode and battery performance are influenced by the channel section, which can influence the Reynolds number and hydromechanics. The VRFB with the semicircular channel section design needs the lowest charging voltage and exhibits highest discharge voltage, which is mainly attributed to the lower over-potential and the improved distribution uniformity of active species. The average concentration of semicircular design is 15.5 % higher than that of triangular channel at 3/4L cut plane. The uniformity factor of semicircular flow channel is 15.4 % higher than that of triangular channel. It is concluded that semicircular is the best design of channel section shapes, which can contribute to the application of the VRFBs.

Experimental Investigation on the Performance Characteristics of Flow Fields in Redox Flow Batteries Under Various Electrode Parameters

All-vanadium redox flow battery (VRFB) is a promising energy storage technique. Flow fields play a crucial role in distributing the electrolyte into the electrode uniformly, but their performance characteristics under different electrode parameters are still unclear. In this work, taking the total pressure drop and total overpotential as performance characterizations, the influence of electrode parameters and operating conditions on the performance of serpentine flow field (SFF) and interdigitated flow field (IFF) are experimentally investigated. It is found that the battery with IFF exhibits lower pressure drop than that with SFF because of the shunt effect of IFF on electrolyte. In terms of promoting the uniform distribution of the electrolyte into the electrode, the SFF outperforms IFF when the electrode porosity is higher than 0.810, but the performance of SFF and IFF could be reversed as the electrode porosity decreases to 0.714, indicating that there may be a performance reversal between SFF and IFF when the electrode porosity decreases from 0.810 to 0.714. Moreover, the increase of current density, the decrease of electrolyte input, and the decrease of electrode thickness strengthen the performance reversal at low electrode porosity. The results well explain the debate on the superiority of IFF and SFF and the discussion on the preference between flow fields and electrode thickness in literatures and provide guidance for the selection of optimal flow field in VRFBs.

Three-dimensional modeling study of all-vanadium redox flow batteries with the serpentine and interdigitated flow fields

• A comprehensive 3-D VRFB model with different flow fields is proposed. • The SFF with smaller channel and rib widths enhances the mass transfer in the carbon paper electrodes. • The performance of the carbon paper-based VRFBs is more sensitive to channel width than rib width. A comprehensive three-dimensional (3-D) model is developed to study the performance of all-vanadium redox flow batteries (VRFBs) with both serpentine flow field (SFF) and interdigitated flow field (IFF) by using 9 cm 2 carbon paper electrodes. Thanks to the induced stronger electrolyte convection in the electrodes, the batteries with SFF have better performance than IFF, though SFF may cause a higher pressure drop, especially at high flow rates. The effect of flow field geometry of SFF on the battery performance is further studied. Decreasing the channel depth of SFF enhances the mass transfer of electrolytes in the electrode, and the battery performance is more sensitive to the change of channel depth at high current densities. In addition, the battery performance declines with the increase of SFF channel width and rib width, and the influence of channel width is more pronounced than that of rib width. Reducing the rib width increases the electrolyte convection in the electrode more evidently for SFF with a wider channel while avoiding an excessive increase in pressure drop. Therefore, in the case of high current density or low electrolyte flow rate, the SFF with small flow channel depth and small rib width is conducive to improving the performance of VRFBs.

Characteristics of the all-vanadium redox flow battery using ammonium metavanadate electrolyte

Characteristics of the all-vanadium redox flow battery using ammonium metavanadate electrolyte

Novel Anion-Exchange Blend Membranes Comprised of a Commercially Available & Water-Soluble Ionomer for All-Vanadium Redox Flow Batteries

The All-Vanadium Redox Flow Battery is a promising technology for solving the problem of cost-efficient and safe large-scale storage of renewable energies [1]. One key component of the batteries, which alters the performance and long-term stability of those systems, is the membrane. For this purpose, Nafion based membranes are often used. However, these membranes are high in cost and show a significant capacity fading and reduction of coulombic efficiency mainly due to crossover of vanadium species [2]. One way around this is to use commercially available, low-cost materials with positively charged ammonium groups, which can effectively repel positively charged vanadium species due to the Donnan exclusion effect [3, 4].In this work we present novel anion-exchange blend membranes comprised of a commercially available and water-soluble ionomer and a PBI blend matrix, which can be used in All-Vanadium Redox Flow batteries. For this approach the water-soluble ionomer was modified to achieve solubility in organic solvents and miscibility with the matrix polymer. The resulting membranes show good mechanical properties even at high IECs and can be modified in a wide range of properties by adjusting the amounts of ionomer and matrix polymer and/or by the addition of crosslinking agents.The above-mentioned membranes were characterised ex-situ by using EIS, NMR spectroscopy, IR spectroscopy, DMA, TGA, tensile tests and permeability tests. In addition, first promising in-situ characterisations were performed in a VRFB single-cell test station.Literature[1] Fan, X; Liu, B; Liu, J; Ding, J; Han, X; Deng, Y; Lv, X; Xie, Y; Chen, B; Hu, W; Zhong, C.: Battery Technologies for Grid-Level Large-Scale Electrical Energy Storage. Trans. Tianjin Univ. 2020, 92 – 103.[2] Düerkop, D; Widdecke, H; Schilde, C; Kunz, U; Schmiemann, A.: Polymer Membranes for All-Vanadium Redox Flow Batteries: A Review. Membranes 2021.[3] Xi, J; Wu, Z; Teng, X; Zhao, Y; Chen, L; Qiu, X.: Self-assembled polyelectrolyte multilayer modified Nafion membrane with suppressed vanadium ion crossover for vanadium redox flow batteries. J. Mater. Chem. 2008, 1232.[4] Mohammadi, T; Kazacos, M.: Modification of anion-exchange membranes for vanadium redox flow battery applications. Journal of Power Sources 1996, 179 – 186.

An Exploration of Nitrogen-Rich Fused Heteroaromatic Quinones for Redox Flow Battery Applications

The growing need to transition from fossil fuels to renewable energy sources will require innovative development of grid-level electrochemical energy storage.1 Suitable grid-level storage must provide a high cycle number durability, long calendar life, high efficiency, low cost and fast response time.2–4 Redox flow batteries (RFBs) are one potential solution boasting a decoupled power and capacity scaling.1,4,5 However, the low energy density1,6,7 and high capital costs6,7 of current systems preclude wide-scale deployment of this technology. Increasing energy density can be achieved in various ways: expanding the voltage window or by minimising the mass and/or volume per electron transferred.8 These offer potential strategies for electrolyte exploration but the challenge is to minimise the cost while providing as much electrical energy storage as possible.Current electrolyte systems for RFBs rely on a variety of metal-based systems (vanadium,9 iron,10 chromium10)1,11 and a range of organic molecules (nitroxide radicals,12 phenazines,12–14 viologens,15 and quinones16–18).4,5 Quinones offer fast kinetics, high tunability and low cost.5 Of these, higher order quinones offer increased chemical and electrochemical stability.5 In this work, an exploration of nitrogen-rich fused heteroaromatic quinones was carried out to investigate new avenues for electrolyte development. The electrolytes were screened using electrochemical techniques and the most promising candidate was tested in a lab-scale flow battery as an anolyte under aqueous conditions. Sitting at -0.7 V(SHE), a capacity fade rate of 0.004%.cycle-1 was found in symmetric cycling. In situ UV-Vis, NMR19,20 and EPR19 spectroscopy were used to investigate the electrochemical stability and nature of the charged species involved during operation, complemented by density functional theory modelling. These studies indicate that fused systems of this type may be promising candidates for aqueous RFBs. Alotto, P., Guarnieri, M. & Moro, F. Redox flow batteries for the storage of renewable energy: A review. Renew. Sustain. Energy Rev. 29, 325–335 (2014).Weber, A. Z. et al. Redox flow batteries: A review. J. Appl. Electrochem. 41, 1137–1164 (2011).Gyuk, I. et al. Grid Energy Storage. (2013).Cao, J., Tian, J., Xu, J. & Wang, Y. Organic Flow Batteries: Recent Progress and Perspectives. Energy and Fuels 34, 13384–13411 (2020).Ding, Y., Zhang, C., Zhang, L., Zhou, Y. & Yu, G. Molecular engineering of organic electroactive materials for redox flow batteries. Chem. Soc. Rev. 47, 69–103 (2018).Potash, R. A., McKone, J. R., Conte, S. & Abruña, H. D. On the Benefits of a Symmetric Redox Flow Battery. J. Electrochem. Soc. 163, A338–A344 (2016).Wang, W. et al. Recent progress in redox flow battery research and development. Adv. Funct. Mater. 23, 970–986 (2013).Armand, M. & Tarascon, J. M. Building better batteries. Nature 451, 652–657 (2008).Ulaganathan, M. et al. Recent Advancements in All-Vanadium Redox Flow Batteries. Adv. Mater. Interfaces 3, (2016).Sun, C. & Zhang, H. A review of the development of the first‐generation redox flow battery : iron chromium system. ChemSusChem (2021). doi:10.1002/CSSC.202101798Noack, J., Roznyatovskaya, N., Herr, T. & Fischer, P. The Chemistry of Redox-Flow Batteries. Angew. Chemie - Int. Ed. 54, 9776–9809 (2015).Winsberg, J. et al. Aqueous 2,2,6,6-Tetramethylpiperidine-N-oxyl Catholytes for a High-Capacity and High Current Density Oxygen-Insensitive Hybrid-Flow Battery. ACS Energy Lett. 2, 411–416 (2017).Kwon, G. et al. Multi-redox Molecule for High-Energy Redox Flow Batteries. Joule 2, 1771–1782 (2018).Romadina, E. I., Komarov, D. S., Stevenson, K. J. & Troshin, P. A. New phenazine based anolyte material for high voltage organic redox flow batteries. Chem. Commun. 57, 2986–2989 (2021).Hu, S. et al. Phenylene-Bridged Bispyridinium with High Capacity and Stability for Aqueous Flow Batteries. Adv. Mater. 33, 2005839 (2021).Suo, L. et al. Advanced High-Voltage Aqueous Lithium-Ion Battery Enabled by “Water-in-Bisalt” Electrolyte. Angew. Chemie - Int. Ed. 55, 7136–7141 (2016).Shimizu, A. et al. Liquid Quinones for Solvent-Free Redox Flow Batteries. Adv. Mater. 29, 1606592 (2017).Yang, Z. et al. Alkaline Benzoquinone Aqueous Flow Battery for Large-Scale Storage of Electrical Energy. Adv. Energy Mater. 8, 1702056 (2018).Zhao, E. W. et al. Coupled in Situ NMR and EPR Studies Reveal the Electron Transfer Rate and Electrolyte Decomposition in Redox Flow Batteries. J. Am. Chem. Soc. 143, 1885–1895 (2021).Zhao, E. W. et al. In situ NMR metrology reveals reaction mechanisms in redox flow batteries. Nature 579, 224–228 (2020). Figure 1

Effect of Tungsten Oxide Structure on Enhancing the V(II)/(VIII) Reaction and Inhibiting H2 Evolution for All-Vanadium Redox Flow Battery

Renewable energy sources require high-efficiency storage systems to allow for its integration within the electrical grid and to supply that energy when needed. All-vanadium redox flow batteries (VRFBs) are very promising for this application because they can store high amounts of energy, are designed to work for long periods, and their solutions can be used indefinitely.1 VRFBs have various advantages when combined with renewable energy sources. The relatively high cost of 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. Different carbon materials are used as electrodes in VRFBs (carbon felt, paper and cloth), and each of these materials affects the performance of the battery differently either through sluggish kinetics or a high chance for parasitic reactions catalyzation.2 Modification of these electrodes by carbon-based3 or metal oxides2 nanomaterials can increase the power density of the battery and suppress the parasitic reactions. Some metal oxides like tungsten oxide4 can enhance vanadium reactions alone or when used as composites with nanocarbon materials.This work aims at enhancing the VRFBs negative half-cell reaction (V2+/V3+) kinetics and inhibiting the hydrogen evolution parasitic reaction. Carbon cloth electrodes were modified, using different tungsten oxide nanostructures such as nanowires, nanoflakes, and nanospheres, and tested at different loadings. The results suggest that the change in tungsten oxide structure would lead to a change in the electrode overall performance as a result of changing the electrical conductivity, the wettability of the electrodes, the coverage of the carbon cloth, and the number and nature of the active sites available to catalyze the V2+/V3+ reaction, with the tungsten oxide nanowires showing to be the best electrode modifier.

(Invited) Redox Flow Battery Innovation

Flow battery innovations should offer significant improvements in performance, without compromising the durability / lifetime, and be cost-effective and scalable. The presentation will review some of the progress that has been made to enhance flow battery performance, and will discuss a number of recent innovations that aim to deliver these characteristics. These will include: Magnetic flowable electrodes applied in a polysulfide-iodide flow battery.Using flow through the current feeder to enhance mass transport and enable dendrite free zinc deposition in the zinc-iodide flow battery.Graphene modified membrane for enhanced power density. Flowable electrodes have emerged as a novel concept for high energy density batteries. To date, in most cases the flowable solid phase includes a redox active energy storage material, for example in zinc-nickel, sodium-sulfur, and lithium-sulfur systems [1-3]. In contrast, we have demonstrated the use of a carbon – magnetite nanocomposite which acts as an electrocatalyst but is not redox active [4,5]. This nanomaterial can be dispersed in the electrolyte and circulated through the battery to enhance the performance of a conventional static electrode. The magnetic characteristics of the nanocomposite can also be exploited, by using a magnetic field to assemble a high surface area electrode comprising a percolating network of the nanomaterial on the current feeder. The electrode also can be removed by releasing the magnetic field at the current feeder, and after being washed out of the cell the nanocomposite can be separated in a magnetic field. This enables replacement of the active electrode without the need to dismantle the cell.Zinc-iodide flow batteries offer high energy density due to the high aqueous solubility of the ZnI2. However, the power density that can be achieved is limited by potential for the dendritic growth of zinc deposits, and as zinc metal builds up in the cell the areal capacity is limited. We have found that by drawing some of the electrolyte through the current feeder, improved performance can be obtained [6]. This enables operation at higher power density and the denser uniform deposit should enable increased areal capacity.We attempted to reduce crossover in the all-vanadium redox flow battery by using a graphene modified nafion membrane. However, we found that the addition of the graphene reduced the losses in the battery and enabling a significant increase in the power density and discharge capacity. Currently we are working to optimize and scale up the membrane modification process, and to explore the mechanism of performance enhancement. References G. Zhu et al. (2020) High-energy and high-power Zn–Ni flow batteries with semi-solid electrodes. Sustainable Energy Fuels, 4, 4076-4085.Yang et al. (2018) Sodium–Sulfur Flow Battery for Low-Cost Electrical Storage. Advanced Energy Materials, 11, 1711991.Suo et al. (2015) Carbon cage encapsulating nano-cluster Li2S by ionic liquid polymerization and pyrolysis for high performance Li–S batteries. Nano Energy, 13, 467-473.Rahimi, A.M. Dehkordi, E.P.L. Roberts (2021) Magnetic nanofluidic electrolyte for enhancing the performance of polysulfide/iodide redox flow batteries. Electrochimica Acta, 309, 137687.Rahimi, A.M. Dehkordi, H. Gharibi, E.P.L. Roberts (2021) Novel Magnetic Flowable Electrode for Redox Flow Batteries: A Polysulfide/Iodide Case Study. Ind. Eng. Chem. Res., 60, 824-841.F. ShakeriHosseinabad et al. (2021) Influence of Flow Field Design on Zinc Deposition and Performance in a Zinc-Iodide Flow Battery. ACS Applied Mat. & Interfa

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

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

(Keynote) Pro and Con of Bi-Decorated Carbon Felts in Vanadium Redox Flow Batteries

Carbon felts are commonly utilized electrodes in vanadium flow batteries due to their low cost, light weight, good electronic conductivity, and chemical inertness. In contrast, their activity in catalyzing the vanadium redox reactions is less remarkable. In particular at the negative side, where V3+ reduction competes with the parasitic hydrogen evolution reaction (HER) at low potentials. In the literature, the addition of Bi3+ into the electrolyte [1] or its deposition on the felts has been suggested as a remedy. However, it is not clear yet, whether the Bi decoration remains stable with time and cycling and which mechanisms are responsible for the observed performance increase.The aim of this work is therefore to unravel more details about the catalyzing effect of Bi and its stability on the cell level. To this effect, the electrode kinetics with and without Bi decoration were studied comparing results obtained using Friedl’s method [2] with cyclic voltammetry (CV) data analyzed by the Polarographica curve fitting routine [3]. This approach can also be generalized to obtain precise kinetic information with other decorating metals.We furthermore applied negative dynamic hydrogen bubble templating (DHBT) deposition as an interesting way to coat commercially available carbon felts with a structured metallic layer of high surface area. Using this strategy, a significant positive effect was observed. However, the drying parameters and post-processing of the felts are important parameters which severely affect the final performance.In order to study the stability of Bi deposits in realistic battery experiments, in-situ synchrotron radiography has been used to monitor the Bi-decorated felts during charge-discharge cycles [4]. The figure demonstrates the effect of different flow rates on the Bi deposition patterns. Dark spots show metal-decorated areas within the felts at flow rates of 5 mlmin-1 (left), 15 mlmin-1 (middle) and 25 mlmin-1 (right). It can be easily seen that the operation conditions influence the felt decoration significantly, while Bi deposits dissolve and deposit during the charge-discharge routine. Further studies will be necessary to find out a) whether different deposit positions affect the performance enhancement and b) if DHBT-Bi remains more stable under operating conditions.In our presentation, we will not only focus on the materials, but also highlight techniques especially suitable for the stuctural and electrochemical analysis of felt electrodes.[1] B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenkle, W. Wang, Bismuth Nanoparticle Decorating Graphite Felt as a High-Performance Electrode for an All-Vanadium Redox Flow Battery, Nano Letters 13(3) (2013) 1330-1335.[2] J. Friedl and U. Stimming, Determining Electron Transfer Kinetics at Porous Electrodes, Electrochim. Acta 227 (2017) 235–245.[3] T. Tichter, J. Schneider, D. Andrae, M. Gebhard, and C. Roth, Universal algorithm for simulating and evaluating cyclic voltammetry atmacroporous electrodes by considering random arrays of microelectrodes, ChemPhysChem 21 (2019) 428 –441.[4] M. Gebhard, T. Tichter, J. Schneider, J. Mayer, A. Hilger, M. Osenberg, M. Rahn, I. Manke, C. Roth J. Power Sources 478 (2020) 228695. Figure 1

An Advanced Composite Membrane for the All-Vanadium Redox Flow Battery

Redox flow batteries (RFBs) are a promising technology for grid scale stationary energy storage to complement renewable energy systems. These batteries have a relatively low energy density; however, they offer important advantages, including: long life-time; decoupled energy (arbitrarily large electrolyte volume) and power (electrode area); high round-trip efficiency; scalability and design flexibility; fast response; and low environmental impacts. These advantages make them superior to many energy storage technologies for stationary applications [1-4]. Among the various types of RFBs, vanadium RFBs (VRFBs) are an emerging technology for grid scale energy storage and the integration of renewable energy generation [5]. The membrane is a key component of a VRFB that separates the two half-cell electrolytes and prevents cross-mixing, while allowing the transport of ions during charge-discharge cycles [6]. The VRFB membrane should exhibit low vanadium ion permeability to minimize self-discharge, low cost, and long‐term chemical stability under normal operating conditions. A high proton conductivity and low vanadium ion crossover are known to improve the efficiency of VRFBs [6-7]. In this study, we present a novel composite Nafion based membrane that results in a significant increase in the VRFB performance. The composite membrane has been characterized for its chemical, structural, and thermal properties using appropriate analytical techniques. The battery performance was evaluated in a flow cell using a ‘zero-gap’ design with an electrode area of 5 cm2. The electrolytic solution, 1.6 M VOSO4 in 3 M H2SO4, was circulated through the cell. Thermally treated carbon papers were used as the cathode and anode electrodes. For charge-discharge experiments, a constant current density (10 to 80 mA cm−2) was applied with upper and lower voltage cut-offs of 1.65 and 0.8 V, respectively. The stability of the battery using the composite membrane was evaluated over 100 cycles. Figures 1 and 2 show the energy efficiency and capacity retention during 100 charge-discharge cycles. The results reveal that the energy efficiency was improved from 51% to 63% by using the composite membrane. In addition, the charge-discharge capacity and capacity retention improved by around 200% and 25%, respectively. This improvement can be attributed to a higher proton conductivity and lower vanadium permeability of the composite membrane.

(Invited) Fullerene and Fullerene Nanocomposites for an Enhanced All-Vanadium Redox Flow Battery

The dependence on fossil fuels to meet the world energy demand caused several social, economical, and environmental problems. An immediate switch to sustainable, clean, and renewable sources of energy is a necessity. To achieve this switch, developing efficient technologies to convert renewable energy sources into electricity is a necessity but not enough due to the intermittent behavior of renewable sources. The incorporation of energy storage systems as regulators allows for the sustainable integration of renewable energy resources into the electrical grid. Vanadium redox flow batteries (VRFBs) are the most promising candidates of the currently available energy storage technologies, as they have an unlimited capacity (theoretically) and design flexibility. Enhancing the kinetics of the VRFBs electrochemical reactions will enhance their energy efficiency, and hence decrease the kWh cost of VRFBs. Currently, carbon-based electrodes are employed due to their suitable stability and conductivity, but with no treatment, they suffer from sluggish kinetics, hydrophobicity, and parasitic reactions catalyzation.1 Modifying these carbon-based electrodes with carbon nanostructures2 and metal oxides(MOx)3 was found to enhance the kinetics at the VRFB electrodes.Even though fullerenes have very unique structural and chemical properties they were rarely explored for the VRFBs application.4 In this work, fullerene C76 showed superior electrocatalytic activity towards VO2+/VO2 + redox reaction in G1 and G3 VRFBs and eliminated the necessity for thermal treatment of the carbon cloth support while inhibiting the chlorine evolution in G3 VRFB. A composite of C76 and hydrated WOx that reduces the fullerene content to half further inhibited the chance for chlorine evolution while maintaining the catalytic activity.

Exploring the Effectiveness of Carbon Cloth Electrodes for All-Vanadium Redox Flow Batteries

Redox flow batteries (RFBs) have proven to be a promising technology for the integration of intermittent renewable energy sources into the existing electrical grid due to their ability to decouple energy and power ratings, and the associated cost-effectiveness in long-duration energy storage [1]. Among various flow battery chemistries, vanadium redox flow battery (VRFB) is one of the most prominently studied flow battery system [2]. In comparison to other chemistries, four soluble oxidation states of vanadium are available and accessible. This characteristic of VRFBs eliminates cross-contamination between the positive and negative half-cells that would otherwise deteriorate the life and durability of the battery [3]. In a flow battery cell, porous electrode is the key component that determines the performance of the system by delivering liquid electrolytes, facilitating ion/charge transfer, and providing sites for electrochemical reactions [4]. Recently, it has been found that significant performance improvements can be achieved with fabric (carbon cloth) electrodes when coupled with a wetting electrolyte [5].In this talk, we will report our experimental approach to evaluate the short- and long-term effectiveness of various carbon cloth electrode samples for use in VRFBs. Results generated from symmetric flow cell cycling coupled with electrochemical impedance spectroscopy and polarization curve measurements will be discussed. The key physico-chemical properties and their evolution during cycling will be elucidated.

Comparison of Zinc Bromine and Zinc Iodine Flow Batteries: From Electrolde to Electrolyte

Research in flow batteries and their application in large scale energy storage has received a growing amount of attention and promise over the past two decades. Although the energy density of flow batteries is low relative to the Li-ion battery, their comparatively lower costs, preferred safety, and ease of scalability has made flow batteries some of the most promising contenders for large-scale stationary energy storage, and are currently commercially available for this purpose. The zinc-bromine flow battery (ZBFB), despite being one of the first proposed flow batteries in the 1980s, has only recently gained enough traction to compete with the well established all-vanadium redox flow batteries. This is largely due to the high solubility of the bromine redox species in aqueous electrolytes, which has allowed the ZBFB is achieve double the energy density of the all-vanadium technology. Recently, an analogue to the zinc-bromine flow battery was introduced: the zinc-iodine flow battery (ZIFB). Similar to the ZBFB, the main advantages of this technology arose from the high solubility of the electroactive species in the electrolyte (iodine/tri-iodide). The solubility of the iodine redox species is even higher than that of analogous bromine electrolytes, and accordingly, the highest energy densities of all aqueous flow batteries to date has been for the ZIFB.Despite the similarities between the two technologies, they are held back by different issues, and so different approaches have been taken to improving the performances of the ZBFB and ZIFB. The ZBFB primarily suffers from a low power-density due to the sluggish kinetics of the bromine redox couple. Therefore, a majority of research on the ZBFB has focused on identifying new, low cost electrode materials that minimize kinetic losses at the bromine half-cell. In contrast, a majority of research on the ZIFB has been on improvements to the electrolyte composition. The ZIFB is plagued by issues of a thick, high impedance iodine film that forms at the positive electrode on charge. Due to the strong Lewis acid nature of the iodine species, a variety of charge-transfer complexes can be formed in the electrolyte, having a variety of effects on the battery performance.This presentation provides an overview on the similarities and differences between the ZBFB and ZIFB technologies. We performed a variety of half-cell and flow battery tests varying the electrode and electrolyte compositions. A number of low cost carbon materials are used as electrode materials, along with a variety of modifications to the bromine and iodine electrolytes. Through the use of high-surface area carbon blacks, the exchange current of the bromine redox couple is able to be increased by two orders to magnitude in comparison to glassy carbon. Additions of MSA or other acids to the ZBFB increases the oxidation kinetics greatly, and accordingly the overall energy efficiency of the ZBFB. For the ZIFB, the presence of high surface area catalysts have little to no effect on the overall performance. We found that in aqueous electrolytes, the iodine electrode is largely held back by the iodine film that forms on charge. Therefore, by adding ions to the electrolyte such as Br-, Cl-, and SO4 2-, we were able to increase the solubility of the iodine film and the reversibility of the battery, and accordingly its efficiency. Although the ZIFB initially performs better than the ZBFB, after making systematic adjustments to both the electrode and electrolyte compositions, the discrepancy between their performances is largely minimized, demonstrating both can be viable for the future of large-scale energy storage. Figure 1

(Digital Presentation) The Influence of Post-Processing on Dynamic Hydrogen Bubble Templated Bi Modified Graphite Felt in Vanadium Redox Flow Batteries

Graphite felts (GFs) are a widely used electrode material in vanadium redox flow batteries (VRFB) by virtue of their high porosity, chemical stability, low cost and good electrical conductivity. However, the hydrophobicity and the low electrochemical activity are the main obstacles for the use of pristine GF in VFRB applications. To tackle such problems, bismuth has been reported to be a promising catalyst for V2+/ V3+ redox reactions [1]. Post-processing parameters, such as air/vacuum drying, exposure time to water during rinsing step, etc. can impact the performance of the Bi modified GFs, which will correspondingly affect morphology, purity and distribution of the catalyst leading to different kinetics. These parameters are rarely studied and reported in the literature.In this work, an effective synthesis method, the so-called dynamic hydrogen bubble template (DHBT) electrodeposition, has been employed for the deposition of high surface area Bi on GFs [2]. This method involves galvanostatic deposition of the metal at high current densities which is accompanied by hydrogen evolution, and the hydrogen bubbles serve as dynamic template for metal deposition. Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) were carried out in a half-cell setup. The kinetics of Bi modified GFs could be obtained qualitatively by using Friedl’s method [3], in which the charge transfer resistance and double layer capacitance from impedance measurements are correlated, and quantitatively from the CV fitting routine in Polarographica developed by Tichter et al. [4]. The kinetic trends derived from both methods are consistent, with Polarographica exhibiting specific quantitative values as compared to Friedl’s method.It has been found that the post processing and drying conditions after Bi electrodeposition affect the performance of the GFs significantly. Therefore, the impacting parameters including electrochemically active surface area, Bi distribution, impurity, etc. are further studied by excluding certain parameters under different conditions. By the two previously mentioned methods of obtaining kinetics and the characterization of SEM and XRD, it is demonstrated that long time exposure to water during the rinsing step combined with vacuum drying after synthesis will promote a relatively higher kinetics than the other post-processing treatments on the Bi modified GFs.Full-cell tests with a VRFB test bench have been carried out. According to the polarization curves and charge-discharge curves at 25 mA/cm2 and 50 mA/cm2, the electrodes with Bi show pronounced advantages over pristine GF, namely, lower overpotential, lower charge voltage, higher discharge voltage and higher electrolyte utilization. Long term cycling will be compared and discussed in detail as well.[1] B. Li et al., “Bismuth Nanoparticle Decorating Graphite Felt as a High-Performance Electrode for an All-Vanadium Redox Flow Battery,” Nano Lett., vol. 13, no. 3, pp. 1330–1335, Mar. 2013, doi: 10.1021/nl400223v.[2] M. Yang, “Fern-shaped bismuth dendrites electrodeposited at hydrogen evolution potentials,” J. Mater. Chem., vol. 21, no. 9, p. 3119, 2011, doi: 10.1039/c0jm03213a.[3] J. Friedl and U. Stimming, “Determining Electron Transfer Kinetics at Porous Electrodes,” Electrochimica Acta, vol. 227, pp. 235–245, Feb. 2017, doi: 10.1016/j.electacta.2017.01.010.[4] T. Tichter et al., “Real-space simulation of cyclic voltammetry in carbon felt electrodes by combining micro X-ray CT data, digital simulation and convolutive modeling,” Electrochimica Acta, vol. 353, p. 136487, Sep. 2020, doi: 10.1016/j.electacta.2020.136487.

A magnetic chitosan for efficient adsorption of vanadium (V) from aqueous solution.

The all-vanadium redox flow battery (VRFB) is becoming a promising technology for large-scale energy storage due to its advantages such as scalability and flexibility. In recent years, the VRFB has been successfully developed and put into use in many countries. It is expected that the abandoned VRFB will generate a large amount of vanadium waste. To our knowledge, there are few reports on the disposal of spent VRFBs. Herein, chitosan-coated nano-zero-valent iron (CS-Fe0) is proposed for the first time as adsorbents for the treatment of spent VRFBs. It can provide a new approach to deal with the upcoming large number of spent VRFBs. The calculated maximum adsorption capacity for V(V) of chitosan and CS-Fe0 reached 209.5 and 511.3mg/g at 288K, respectively. CS-Fe0 showed better adsorption performance than chitosan under different pH conditions and is easy to be separated from the liquid phase. The Freundlich isotherm was suitable for the adsorption process of chitosan, and CS-Fe0 was more consistent with the Langmuir isotherm. Ionic strength (0.05-0.5M) had a positive effect on the adsorption capacity of CS-Fe0, and the influence of coexisting anions on CS-Fe0 could be negligible. FTIR and XPS analyses revealed that the primary mechanisms were the electrostatic attraction of chitosan and redox of Fe0. The present study confirmed that CS-Fe0 could be a potential material to efficiently trap V(V) from the VRFB electrolyte.

Comprehensive Analysis of Critical Issues in All-Vanadium Redox Flow Battery

Comprehensive Analysis of Critical Issues in All-Vanadium Redox Flow Battery

Ion-exchange membrane impact on preferential water transfer in all-vanadium redox flow battery

Preferential water transfer (PWT) is an important issue in vanadium flow batteries, which affects the cycling time. In this work, we propose a new way to control PWT by membrane modification. To assess PWT quantitatively, we developed an accurate method of water volume determination by optical monitoring of an anolyte. As result, we show that PWT for cation-, anion-exchange, and amphoteric benchmark membranes have opposite directions. Thus, we propose control of PWT by balancing concentrations of cation and anion groups on the polymer membrane surface. We modify Nafion 115 membrane by layer-by-layer modification. We use ATR-FTIR spectroscopy, AFM, and Kelvin probe, to track charged layer formation on the polymer membrane surface. The results of the observation show vanadium permeability drop seven times and proton conductivity decreased to 30 mS cm-1 with three times PWT suppression. We demonstrate a low capacity fade (70% vs 46% after 100 cycles for modified and non-modified membranes), and high energy efficiency (91%) for the two bilayer modified membranes. Thus, surface functionalization of ion-exchange membranes is one of the new ways of PWT control.