Use In Vanadium Redox Flow Batteries Research Articles

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.

Enhanced Proton Selectivity in Ionomer Nanocomposites Containing Hydrophobically Functionalized Silica Nanoparticles

Perfluorosulfonic acid (PFSA) ionomers are ubiquitous as proton-exchange membranes (PEMs) in vanadium redox flow batteries (VRFBs), as they provide high proton conductivity and robust chemical stability. However, traditional PFSA ionomers suffer from high vanadium ion crossover, i.e., low ion selectivity, which reduces the efficiency and lifetime of the battery. Herein, a novel method to fabricate PFSA nanocomposites containing fluorocarbon-decorated silica nanoparticles is presented. These composite ionomers exhibit drastically reduced vanadium ion permeability and an almost two orders of magnitude increase in proton selectivity when compared to the current benchmark commercial ionomer. Small-angle neutron scattering data suggest that the nanostructures of these nanocomposites are drastically different from their pristine counterpart, where the periodic spacing of the hydrophobic domains is significantly reduced, while changes to the ionic structure were seen to be minimal. This work suggests that composite PEMs containing a secondary phase that alters the hydrophobic, nonion-conducting phase of the ionomer may prove to be a fruitful fabrication route to produce ionomer membranes with enhanced performance for use in VRFBs.

X‐Ray‐Computed Radiography and Tomography Study of Electrolyte Invasion and Distribution inside Pristine and Heat‐Treated Carbon Felts for Redox Flow Batteries

Porous carbon felts (CFs) are widely used electrode materials for vanadium redox flow batteries (VRFBs). These materials differ in their precursor material, thickness, or graphitization degree and demonstrate broad differences in electrochemical performance. Prior to operation, an activation step, such as acid or heat treatment (HT), is commonly performed to improve their performance. A thermal treatment in air functionalizes the surface of the electrode and improves reaction kinetics as well as the wettability of the electrode. Herein, pristine and heat‐treated CFs are compared regarding their electrolyte wetting behavior for the use in VRFB. Contact angle (CA) measurements are conducted ex situ to investigate the effect of the HT. Furthermore, the porous CFs are examined in situ with an in‐house‐built flow cell regarding their invasion behavior with different types of electrolytes by X‐ray radiography. Additionally, the distribution of the electrolyte inside the felts is investigated by X‐ray tomography. The results demonstrate the effect of the HT and choice of electrolyte on the wetting behavior and electrolyte distribution.

Pore-Size-Tuned Graphene Oxide Frameworks as Ion-Selective and Protective Layers on Hydrocarbon Membranes for Vanadium Redox-Flow Batteries.

The laminated structure of graphene oxide (GO) membranes provides exceptional ion-separation properties due to the regular interlayer spacing ( d) between laminate layers. However, a larger effective pore size of the laminate immersed in water (∼11.1 Å) than the hydrated diameter of vanadium ions (>6.0 Å) prevents its use in vanadium redox-flow batteries (VRFB). In this work, we report an ion-selective graphene oxide framework (GOF) with a d tuned by cross-linking the GO nanosheets. Its effective pore size (∼5.9 Å) excludes vanadium ions by size but allows proton conduction. The GOF membrane is employed as a protective layer to address the poor chemical stability of sulfonated poly(arylene ether sulfone) (SPAES) membranes against VO2+ in VRFB. By effectively blocking vanadium ions, the GOF/SPAES membrane exhibits vanadium-ion permeability 4.2 times lower and a durability 5 times longer than that of the pristine SPAES membrane. Moreover, the VRFB with the GOF/SPAES membrane achieves an energy efficiency of 89% at 80 mA cm-2 and a capacity retention of 88% even after 400 cycles, far exceeding results for Nafion 115 and demonstrating its practical applicability for VRFB.

A gradient bi-functional graphene-based modified electrode for vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) are an ideal choice for large-scale energy storage because they have the advantages of long cycle life, flexible design and high safety. However, the poor electrocatalytic activity of carbon-based materials results in a large polarization resistance and energy loss during charge/discharge that greatly limits their commercial viability. Here we report a hybrid electrode with a gradient bi-functional oxygen-containing groups for VRFBs. It consists of a hybrid material of graphene oxide (GO), reduced graphene oxide (rGO) and graphene foam (GF) that combines materials with a high electrocatalytic activity (GO) and a high electrical conductivity (GF). One side is enriched with functional groups and provides preferential redox reversibility for VO2+/VO2+ and V3+/V2+ redox couples because of the electrocatalytic nature of the many oxygen functional groups. The side with the low concentration of functional groups has a high electrical conductivity and facilitates electron transfer. As a result, this VRFB electrode achieves a low polarization, high discharge capacity, high energy density, and high energy efficiency and has great promise for use in VRFBs.

Crosslinked anion exchange membranes with primary diamine-based crosslinkers for vanadium redox flow battery application

A series of polysulfone-based crosslinked anion exchange membranes (AEMs) with primary diamine-based crosslinkers has been prepared via simple a crosslinking process as low-cost and durable membranes for vanadium redox flow batteries (VRFBs). Chloromethylated polysulfone is used as a precursor polymer for crosslinked AEMs (CAPSU-x) with different degrees of crosslinking. Among the developed AEMs, CAPSU-2.5 shows outstanding dimensional stability and anion (Cl−, SO42−, and OH−) conductivity. Moreover, CAPSU-2.5 exhibits much lower vanadium ion permeability (2.72 × 10−8 cm2 min−1) than Nafion 115 (2.88 × 10−6 cm2 min−1), which results in an excellent coulombic efficiency of 100%. The chemical and operational stabilities of the membranes have been investigated via ex situ soaking tests in 0.1 M VO2+ solution and in situ operation tests for 100 cycles, respectively. The excellent chemical, physical, and electrochemical properties of the CAPSU-2.5 membrane make it suitable for use in VRFBs.

Disulfonated Copolymer Membranes with Improved Selectivity and Lifetime for Vanadium Redox Flow Batteries

One of the critical needs to increase the widespread implementation of vanadium redox flow batteries (VRFBs) is identifying reliable and cost-effective membranes with improved lifetime. Among ion exchange membranes, Nafion® has been the most thoroughly studied and widely used material in VRFB applications due to its good proton conductivity, chemical stability and durability. However, the low vanadium selectivity of Nafion®, which leads to significant crossover and related capacity fade with cycling, hinders the use of this type of membrane for commercial VRFB applications [1]. In order to solve this problem, several post-sulfonated hydrocarbon polymers such as poly(sulfone) [2] and poly(benzimidazole) [3] have been studied to explore their effectiveness for use in VRFBs. Although these post-sulfonated membranes have been shown to provide improved proton conductivity and vanadium selectivity, one major limitation of the post-sulfonation method is that the amount of sulfonation and related ion exchange capacity (IEC) cannot be controlled precisely [4]. Especially, the sulfonation of polysulfone with strong acids (e.g. sulfuric and chlorosulfuric) often leads to undesirable side reactions (e.g. cross-linking) and chain cleavage, making the control of sulfonation even more difficult, thereby degrading the mechanical integrity of the final membrane [4]. One potential approach to precisely control the degree of sulfonation of an ion exchange membrane is using direct sulfonation by copolymerization [5]. With this technique, the sulfonic acid groups can be positioned on the more stable and more acidic deactivated sulfone unit, which results in more precise control of ionic group concentration in the final product. In this talk, we will discuss the performance evaluation of disulfonated copolymer membranes for use in a VRFB application. Basic membrane properties and the results of charge/discharge cycling analysis with respect to various degrees of sulfonation and fluorination will be reported and then compared with Nafion® 212. The study will be presented with emphasis on tailoring the membrane properties of directly disulfonated copolymers and establishing the structure-property-performance relation for these membranes.

Sulfonated polyimide/s-MoS2 composite membrane with high proton selectivity and good stability for vanadium redox flow battery

We report the fabrication and properties of high performance and inexpensive composite membranes of sulfonated polyimide (SPI) and sulfonated molybdenum disulfide (s-MoS2) for the vanadium redox flow battery (VRFB) application. Scanning electron microscopy (SEM) images reveal the uniformity of SPI/s-MoS2 membrane. Both EDS and XRD results verify the existence of s-MoS2 in SPI/s-MoS2 membrane and show the interaction between SPI and s-MoS2. The physico-chemical properties of as-prepared SPI/s-MoS2 membrane, including water uptake and ion exchange capacity, etc. are evaluated and compared to those of SPI, SPI/MoS2 and Nafion 117 membranes, respectively. The SPI/s-MoS2 membrane possesses increased proton conductivity and reduced vanadium ion permeability than pure SPI membrane, and it has the highest proton selectivity (2.24×105Smincm−3) among all membranes. The VRFB with SPI/s-MoS2 membrane presents a higher coulombic efficiency (CE) and energy efficiency (EE) compared with Nafion 117 at the current density ranging from 20 to 80mAcm−2. The SPI/s-MoS2 membrane shows a longer discharge time (193h above 0.8V) than Nafion 117 (72h). Moreover, the SPI/s-MoS2 membrane exhibits stable operation performance up to 500 cycles with no significant decline in CE and EE. All experimental results confirm that the SPI/s-MoS2 membrane is suitable for use in VRFB.

Degradation of Electrode Materials in Vanadium Redox Flow Batteries

Recent works in vanadium redox flow batteries (VRFBs) have been successful in improving the energy density by increasing vanadium solubility in the electrolyte1 and achieving high power density through electrode modification, membrane selection, and superior cell design 2,3. These accomplishments lead to significant reduction of the system cost and therefore enhance the economic attractiveness of VRFB technology.To achieve market implementation, the large-scale energy storage system requires a balance between performance, cost, and durability. While power and energy densities have been recently improved, the investigation of component durability, which is equally important, has yet to be reported in significant detail.In this work, we demonstrate an approach to study the electrode degradation in single-cell VRFBs. Ex-situ electrolyte soaking experiments were used to determine conditions under which the electrodes degrade, and the extent of the degradation. Several techniques were adopted to characterize property changes in the degraded materials. Various carbon materials were investigated to determine viable candidates and characteristics required for long-term use in VRFB systems.Additionally, in-situ cycling tests of various carbon materials have been carried out to illustrate the relationship between material degradation and cell performance decay. Declines in performance (Fig. 1) are monitored and correlated to changes in the material properties. Modification of the cycling parameters reveals operating conditions leading to the most severe decay in the carbon material, so that accelerated degradation protocols can be developed for evaluation of additional materials.

Membrane Development for Vanadium Redox Flow Batteries

Large-scale energy storage has become the main bottleneck for increasing the percentage of renewable energy in our electricity grids. Redox flow batteries are considered to be among the best options for electricity storage in the megawatt range and large demonstration systems have already been installed. Although the full technological potential of these systems has not been reached yet, currently the main problem hindering more widespread commercialization is the high cost of redox flow batteries. Nafion, as the preferred membrane material, is responsible for about 11% of the overall cost of a 1 MW/8 MWh system. Therefore, in recent years two main membrane related research threads have emerged: 1) chemical and physical modification of Nafion membranes to optimize their properties with regard to vanadium redox flow battery (VRFB) application; and 2) replacement of the Nafion membranes with different, less expensive materials. This review summarizes the underlying basic scientific issues associated with membrane use in VRFBs and presents an overview of membrane-related research approaches aimed at improving the efficiency of VRFBs and making the technology cost-competitive. Promising research strategies and materials are identified and suggestions are provided on how materials issues could be overcome.

Long-Term Stability of Nafion Hybrid Membranes for Use in Vanadium Redox-Flow Batteries

The membrane is the key component of a redox-flow system. It should possess high proton conductivity and should avoid cross contamination due to the diffusion of ions of the active material through membrane pores. A membrane with drastically reduced cross contamination ability would overcome the main problem of several redox couples and in consequence it would allow redoxflow systems with higher energy density and lower material costs. In previous works cross contamination of Nafion membranes could have been reduces significantly by several modification methods with SiO2 nano particles. This work deals with the long-term stability of this improvement that was investigated by cycling modified membranes for several weeks and analyzing the electrical performance in dependency of time. Nafion membrane modified with TEOS/DEDMS/NH3 showed a very good long-term stability.

Possible use of vanadium redox-flow batteries for energy storage in small grids and stand-alone photovoltaic systems

The all-vanadium redox-flow battery is a promising candidate for load leveling and seasonal energy storage in small grids and stand-alone photovoltaic systems. The reversible cell voltage of 1.3 to 1.4 V in the charged state allows the use of inexpensive active and structural materials. In this work, studies on the performance of inexpensive active materials for use in vanadium redox-flow batteries are reported. Additionally, a cost analysis for a load leveling and a seasonal energy storage system is given based on a flow battery technology well established in Zn-flow batteries.