Vanadium Redox Flow Batteries: A Review Oriented to Fluid-Dynamic Optimization

Introducing an Experimental Route to Identify and Unify Lab-Scale Redox-Flow Battery Cell Performances via Molar Fluxes and Cell Constants

Redox flow batteries (RFBs) are a promising technology for grid energy storage based on their high potential for scalability, design flexibility, high efficiency, and long durability, hence great effort has been invested in this area of research. However, due to the large differences in lab-scale RFB cell design and construction as well their operational performance, fundamental studies on innovative RFB components (e.g., active materials, separators, additives) compare poorly due to the lack of standard setups, settings, and procedures. This work introduces an experimental calibration route for aqueous as well as nonaqueous RFBs based on a simple mass transport model using molar fluxes, enabling one to compare dissimilar lab-scale RFB cell setups by introducing several RFB parameters: First, K1, which summarizes the operating parameters of an RFB to identify the critical ratio (K1critical) needed for efficient charge-discharge cycling using a simple overvoltage and charge efficiency evaluation; second, the RFB cell constant ζ, quantifying the influence of a lab-scale RFB setup on its performance; and finally, K2, ultimately enabling full comparison of (idealized) K1critical operating parameters across RFB cell setups.

Direct Ink Writing of 3D-Structured All-Carbon Electrodes with High Electrical Conductivity for (Vanadium) Redox Flow Batteries

Redox flow batteries are attractive systems for large-scale energy storage due to their capability to uncouple energy and power but still need to make several improvements to reach full commercial scale. The need to search for better components, including electrode materials that allow the internal flow of electrolytes and have optimal electrochemical performance is a hot topic in the development of this kind of battery. The use of direct ink writing technology to engineer complex electrode materials both in the architecture and chemical composition opens a new field of research to optimize electrode performance. In this study, several formulations are prepared using graphite, multiwall carbon nanotubes, and two different Polyacrylonitrile (PAN)-based short carbon fibers. Furthermore, a graphitizable binder is added to the formulation to help consolidate the printed object into a highly conductive (3000-8000 Sm-1) and mechanically resistant carbon electrode after a moderate heat treatment (800 °C). The 3D electrodes are successfully tested in an all vanadium redox flow cell showing a competitive performance when compared to benchmark electrodes (graphite felts).

Investigating Bubble Formation and Evolution in Vanadium Redox Flow Batteries via Synchrotron X-Ray Imaging

The parasitic hydrogen evolution reaction (HER) hinders electrolyte transport. It reduces the effective electrochemical surface area in the negative half-cell of vanadium redox flow batteries (VRFBs), resulting in substantial efficiency losses. This study investigates the formation and evolution of hydrogen bubbles within VRFB electrodes through comprehensive experimental characterization and a detailed analysis of the resolved bubbles. The electrode is imaged using synchrotron X-ray tomography, and gas bubbles in the images are identified and characterized using a deep learning model combined with a morphological analysis tool. The HER intensity increases at more negative working electrode potentials, causing residual bubbles to grow and fuse in the electrode central region. In contrast, independent bubbles predominantly form at the electrode edges. Furthermore, bubble growth leads to the gradual development of irregular shapes. These observations provide insights into bubble formation and evolution rules, contributing to a better understanding of the system.

Structure and dynamics of aqueous VOSO4 solutions in conventional flow through cell design: a molecular dynamics simulation study

A theoretical model has been proposed to study the structure and dynamics of aqueous vanadyl sulfate (VOSO4) solution used in the conventional flow (CF) through cell design operating under varying thermodynamic conditions. Classical molecular dynamics simulations have been carried out for aqueous solutions of vanadyl sulfate (VOSO4) and sulfuric acid (H2SO4) at two different concentrations and temperatures considering the temperature dependent degree of dissociation of sulfuric acid. The MD trajectories are used to study the equilibrium structural, dynamical properties such as viscosity, diffusivity and surface tension of the aqueous solution of vanadyl sulfate (VOSO4). According to the new model, the cation-cation and cation-anion interaction should be low in order to have a good current density in the conventional flow through cell design and further explains the importance of considering mass transport when designing high energy density redox flow batteries. The model is further validated by calculating the viscosity of each system, individual diffusion coefficient of each ion and by comparing them with the experimental data wherever they are available.

Optimized Sulfonated Poly(Ether Ether Ketone) Membranes for In-House Produced Small-Sized Vanadium Redox Flow Battery Set-Up

The ionic exchange membranes represent a core component of redox flow batteries. Their features strongly affect the performance, durability, cost, and efficiency of these energy systems. Herein, the operating conditions of a lab-scale single-cell vanadium flow battery (VRFB) were optimized in terms of membrane physicochemical features and electrolyte composition, as a way to translate such conditions into a large-scale five-cell VRFB stack system. The effects of the sulfonation degree (SD) and the presence of a filler on the performances of sulfonated poly(ether ether ketone) (SPEEK) ion-selective membranes were investigated, using the commercial perfluorosulfonic-acid Nafion 115 membrane as a reference. Furthermore, the effect of a chloride-based electrolyte was evaluated by comparing it to the commonly used standard sulfuric acid electrolyte. Among the investigated membranes, the readily available SPEEK50-0 (SD = 50%; filler = 0%) resulted in it being permeable and selective to vanadium. Improved coulombic efficiency (93.4%) compared to that of Nafion 115 (88.9%) was achieved when SPEEK50-0, in combination with an optimized chloride-based electrolyte, was employed in a single-cell VRFB at a current density of 20 mA·cm-2. The optimized conditions were successfully applied for the construction of a five-cell VRFB stack system, exhibiting a satisfactory coulombic efficiency of 94.5%.

Analysis of the effect of thermal treatment and catalyst introduction on electrode performance in vanadium redox flow battery

All-vanadium redox flow batteries (VRFB) have the advantages of high safety and long life, and have broad application prospects in the field of large-scale power energy storage. Low energy density is the main factor restricting its development. In this study, the carbon felt used as the electrode was pretreated in various ways to improve the performance of the vanadium redox flow battery. The pretreatment conditions of carbon felt were compared to the performance of carbon felt after treatment at different temperatures and different times. The properties of the pretreated carbon felt were investigated and their effect on cell performance was tested.Next, by introducing a noble metal catalyst into the carbon felt, the characteristics of the carbon felt were studied and the effect on the performance of the vanadium redox flow battery was investigated. It was found that Carbon felt thermal-treated at 500 °C for 2 h showed the best characteristics and had the longest charge/discharge time and the lowest resistance. The results also show that Carbon felt with catalyst introduced without PTFE(Polytetrafluoroethylene) binder showed larger BET(Brunauer-Emmett-Teller) surface area and electrical conductivity compared to PTFE mixed, and cell performance was also excellent.

Voltage prediction of vanadium redox flow batteries from first principles

Global energy demand has been increasing for decades, which has created a necessity for large scale energy storage solutions for renewable energy sources. We studied the voltage of vanadium redox flow batteries (VRFBs) with density functional theory (DFT) and a newly developed technique usingab initiomolecular dynamics (AIMD). DFT was used to create cluster models to calculate the voltage of VRFBs. However, DFT is not suited for capturing the dynamics and interactions in a liquid electrolyte, leading to the need for AIMD, which is capable of accurately modeling such things. The molarities and densities of all systems were carefully considered to match experimental conditions. With the use of AIMD, we calculated a voltage of 1.23 V, which compares well with the experimental value of 1.26 V. The techniques developed using AIMD for voltage calculations will be useful for the investigation of potential future battery technologies or as a screening process for additives to make improvements to currently available batteries.

Geophysical Properties of Precambrian Igneous Rocks in the Gwanin Vanadiferous Titanomagnetite Deposit, Korea

Precambrian igneous rocks (851–873 Ma) occur in Pocheon City, Korea. These rocks—crystallized during magmatic differentiation—formed vanadiferous titanomagnetite (VTM) deposit. Vanadium is a crucial element in vanadium redox flow batteries that are most appropriate for large-scale energy storage systems. We investigated the VTM deposit to evaluate its size and the possible presence of a hidden orebody. We demonstrated laboratory experiments of density, susceptibility, resistivity, and chargeability of the Precambrian igneous rocks to enhance the interpretation accuracy of geophysical surveys. The rocks consisting of underground ore (UO), discovered ore (DO), gabbro (GA), monzodiorite (MD), and quartz monzodiorite (QMD) were sampled from drilling cores and outcrops. The average density values were UO: 4.57 g/cm3, DO: 3.63 g/cm3, GA: 3.26 g/cm3, MD: 3.18 g/cm3, and QMD: 2.85 g/cm3. The average susceptibility values were UO: 0.8175 SI, DO: 0.2317 SI, GA: 0.0780, MD: 0.0126 SI, and QMD: 0.0007. The average resistivity values were UO: 2 Ωm, DO: 36 Ωm, GA: 257 Ωm, MD: 4571Ωm, and QMD: 7801 Ωm. The chargeability values were UO: 143 mV/V, DO: 108 mV/V, GA: 79 mV/V, MD: 42 mV/V, and QMD: 9 mV/V. We found that the properties of the mineralized rocks are considerably different from those of the surrounding rocks. This result may facilitate the mineral exploration of VTM deposits.

Characteristics of Graphite Felt Electrodes Treated by Atmospheric Pressure Plasma Jets for an All-Vanadium Redox Flow Battery

In an all-vanadium redox flow battery (VRFB), redox reaction occurs on the fiber surface of the graphite felts. Therefore, the VRFB performance highly depends on the characteristics of the graphite felts. Although atmospheric pressure plasma jets (APPJs) have been applied for surface modification of graphite felt electrode in VRFBs for the enhancement of electrochemical reactivity, the influence of APPJ plasma reactivity and working temperature (by changing the flow rate) on the VRFB performance is still unknown. In this work, the performance of the graphite felts with different APPJ plasma reactivity and working temperatures, changed by varying the flow rates (the conditions are denoted as APPJ temperatures hereafter), was analyzed and compared with those treated with sulfuric acid. X-ray photoelectron spectroscopy (XPS) indicated that the APPJ treatment led to an increase in O-/N-containing functional groups on the GF surface to ~21.0% as compared to ~15.0% for untreated GF and 18.0% for H₂SO₄-treated GF. Scanning electron microscopy (SEM) indicated that the surface morphology of graphite felt electrodes was still smooth, and no visible changes were detected after oxidation in the sulfuric acid or after APPJ treatment. The polarization measurements indicated that the APPJ treatment increased the limiting current densities from 0.56 A·cm^-2 for the GFs treated by H₂SO₄ to 0.64, 0.68, and 0.64 A·cm^-2, respectively, for the GFs APPJ-treated at 450, 550, and 650 °C, as well as reduced the activation overpotential when compared with the H₂SO₄-treated electrode. The electrochemical charge/discharge measurements showed that the APPJ treatment temperature of 550 °C gave the highest energy efficiency of 83.5% as compared to 72.0% with the H₂SO₄ treatment.

Flow field designs developed by comprehensive CFD model decrease system costs of vanadium redox-flow batteries

Different flow field designs are known for vanadium redox-flow batteries (VFB). The best possible design to fulfil a variety of target parameters depends on the boundary conditions. Starting from an exemplary interdigitated flow field design, its channel and land dimensions are varied to investigate the impact on pressure drop, channel volume, flow uniformity and limiting current density. To find a desirable compromise between these several partly contrary requirements, the total costs of the VFB system are evaluated in dependence of the flow field’s dimensions. The total costs are composed of the electrolyte, production and component costs. For those, the production technique (injection moulding or milling), the pump and nominal power density as well as depth of discharge are determined. Finally, flow field designs are achieved, which lead to significantly reduced costs. The presented method is applicable for the design process of other flow fields and types of flow batteries.

Graphical abstract