Vanadium Redox Flow Battery Cell Research Articles

Advanced Sulfonated Poly(Ether Ether Ketone)/Graphene-Oxide/Titanium Dioxide Nanoparticle Composited Membrane with Superior Cyclability for Vanadium Redox Flow Battery.

The purpose of this study was to improve the repulsion ability of sulfonated poly(ether ether ketone) (SPEEK) membrane for the vanadium ions crossover. For this purpose graphene oxide (GO) nanosheet and titanium dioxide (TiO₂) nanoparticles were employed into the polymer matrix to prepare SPEEK/GO/TiO₂ hybrid membrane via solution-casting method for vanadium redox flow battery (VRFB). The morphology, permeability of vanadium ions and device performance of asprepared membrane were investigated and discussed. It was observed that with the barrier block effect by the filler, the VRFB single cell with the optimized SPEEK/GO/TiO₂ hybrid membrane exhibited high coulombic efficiency (~99%), excellent energy efficiency (~85%) and vigorous cyclability (~97.2% capacity retention after 100 cycles). Moreover, the VRFB cell with this blend membrane showed lower vanadium ions permeability than Nafion 212 or pure SPEEK membranes. These results demonstrated that the comprehensive properties of hybrid membrane have been remarkably improved comparing to pristine SPEEK which suggested that the hybrid membrane was applicable for VRFB energy storage system.

Synergistic effects of niobium oxide–niobium carbide–reduced graphene oxide modified electrode for vanadium redox flow battery

A powerful, low-cost niobium oxide–niobium carbide–graphene oxide (NbO2/NbC/rGO) hybrid electrocatalyst is prepared by refluxing, hydrothermal method, followed by annealing process at the optimum temperature of 1050 °C for all vanadium redox flow battery (VRFB) application. The NbO2/NbC/rGO electrocatalyst shows the best electrochemical activity with the highest peak current densities and the lowest peak differences toward VO2+/VO2+ and V2+/V3+ redox reactions from the cyclic voltammetry compared to other samples. The rate performance of VRFB cell using NbO2/NbC/rGO modified graphite felts (NbO2/NbC/rGO/GF) shows the voltage efficiencies of 89.3%, 81.7%, 71.0%, and 59.3% by the charge-discharge current densities of 40 mA cm−2, 80 mA cm−2, 120 mA cm−2, and 160 mA cm−2, respectively, which these performances are superior to those using the pretreated GFs and the untreated GF. The charge-discharge stability test of the VRFB cell using NbO2/NbC/rGO/GF demonstrates no significant decay after 100 cycles by the current density of 80 mA cm−2. This observation is due to the synergistic effects related to the presence of excess oxygen vacancies on NbO2/NbC, the high content of oxygen functional groups, and the high electrical conductivity characteristics of the graphene sheet.

Cover Feature: Synchrotron X‐ray Radiography and Tomography of Vanadium Redox Flow Batteries—Cell Design, Electrolyte Flow Geometry, and Gas Bubble Formation (ChemSusChem 12/2020)

Cover Feature: Synchrotron X‐ray Radiography and Tomography of Vanadium Redox Flow Batteries—Cell Design, Electrolyte Flow Geometry, and Gas Bubble Formation (ChemSusChem 12/2020)

Effect of electrolyte convection velocity in the electrode on the performance of vanadium redox flow battery cells with serpentine flow fields

Sufficient convective velocity in the electrode is essential for the good performance of a flow battery. The present study is concerned with identifying how this condition is attainable for a given size of a vanadium redox flow battery cell. To this end, detailed experimental investigations of fluid dynamic and electrochemical characteristics with serpentine flow fields have been done in cells with active area of 416, 918 and 1495 cm2, and optimal operating conditions of reasonably high efficiency, discharge capacity and energy density have been identified for each size. Investigations using calibrated computational fluid dynamics simulations show that optimal performance condition for each cell size corresponds to the case where the local convective velocity in the electrode is the same and is of the order of Reynolds number ~2. Based on this condition, a model has been developed to determine the optimal electrolyte circulation rate for a given cell size and operating current density.

Composite Nafion-based membranes with nanosized tungsten oxides prepared in supercritical carbon dioxide

A novel method for introduction of tungsten oxide nanoparticles into a Nafion membrane following the infiltration concept is presented. Non-polar supercritical carbon dioxide is used as a solvent for impregnation of tungsten metalorganic precursor into phase-separated polymer structure. Uniformly distributed 2–4 nm particles of tungsten oxides are obtained. This size coincides with the typical diameter of hydrophilic channels in Nafion. Larger crystallites and aggregates appear also but they are easily washed out of the membrane by subsequent treatment with hydrogen peroxide. The resulting composite membrane favourably combines both increased ionic selectivity and improved proton conductivity, which makes it promising for use in vanadium redox flow batteries. Tests of the composite membranes in operating vanadium redox flow battery cells show up to 5% energy efficiency increase over wide range of current densities as compared to pristine Nafion membranes.

Multiple parameter identification using genetic algorithm in vanadium redox flow batteries

We propose a multiple parameter identification method for vanadium redox flow batteries (VRFBs) to estimate the model parameter in a VRFB model. The proposed method consists of an evaluation of identifiability based on the Fisher Information Matrix (FIM) to determine the best subset of model parameters to be identified, a numerical modeling of semi-two-dimensional steady-state VRFB model, and a genetic algorithm to estimate optimal model parameters. In the optimization, we introduce a fitness function involving the mean square errors of the voltage between available experimental data and results of the VRFB model. We validate the proposed method by calculating confidence intervals of identifying parameters in the subset based on the FIM from the state of charge-voltage data obtained from a small VRFB cell experiment; we compare the curves of the identified-parameter model with those obtained experimentally. Further, we demonstrate the robustness of the proposed method through its application to a kW-scale VRFB stack utilizing advanced mixed electrolytes. The capacity-voltage curves predicted by the identified-parameter model show good agreement with those obtained experimentally under various operating conditions, with mean relative errors of less than 1.9%.

A numerical study of electrode thickness and porosity effects in all vanadium redox flow batteries

Vanadium redox flow battery (VRFB) is one of the promising technologies suitable for large-scale energy storage in power grids due to high design flexibility, low maintenance cost and long-life cycle. Vanadium redox flow cell consists of two porous electrodes with serpentine flow channels and electrolyte solutions which is separated by an ion-exchange membrane. The temperature has been set to 298 K for the electrolytes which is composed of 1500 mol/m³ initial vanadium concentration with 4000 mol/m³ initial H2SO4 concentration. We developed a three-dimensional model to scrutinize the complexities of fluid dynamics and electrochemical reactions when considering different electrode thickness sizes, electrode porosity and electrolyte flow rates. In this study, a three-dimensional numerical simulation have been performed in order to investigate the effect of electrode thickness and electrode porosity on the performance of VRFB. The impact of electrolyte solution flow rate on the VRFB electrical characteristics and efficiencies are also numerically investigated. The results show that the cell voltage increases with increasing the electrolyte flow rate and electrode porosity during discharging process of VRFB. Increasing the initial vanadium concentration, the VRFB cell voltage is significantly increased due to reduced overpotential in the porous electrodes. The maximum power-based efficiency of 96.8% is calculated with the electrode thickness of 1 mm at 10 ml/min, while the power-based efficiency of 96.4% is calculated with the electrode thickness of 4 mm at 50 ml/min. This work gives comprehensive insights on electrode configurations for VRFBs.

Enhancing the vanadium redox flow battery efficiency by adjusting the electrode configuration

The vanadium redox flow battery (VRFB) is being investigated as one of the promising candidates for large-scale energy storage systems. In the present work, the role of electrode shape on a single VRFB cell performance has been studied through a 3D numerical model. The model accounts for the coupling between electrochemistry and fluid mechanic physics. Seven cases of electrode configurations at the same volume size (10 cm3) have been simulated to investigate the overall battery performance. Results from the simulation show that the configuration (case 7) has the best battery efficiency, while the worse one is (case 4) under the same operating conditions, e.g., 71.72% and 63.82%, respectively. Also, the ratio of output to input power for (case 7) is 0.72, while 0.63 for (case 4). In addition, there is an optimal flow rate for each case to get maximum battery efficiency, which means a balance between electrochemical reactions and pumping losses is required. Overall, adopting a thicker electrode (cases 2, 3 and 4) results in lower battery performance due to the poor voltage responses. One feature of the introduced model is its capability to predict the performance of most flow batteries cell configurations.

Highly functionalized nanoporous thin carbon paper electrodes for high energy density of zero-gap vanadium redox flow battery

Low energy density of a vanadium redox flow battery (VRFB) due to limited solubility and stability of vanadium ions constrains its wide spread applications and this issue becomes more critical by low active surface areas of electrodes and mass transport limitations of active species on the electrodes that lead to low electrolyte utilization. In this study the issue of low energy density is addressed by improving the electrode performance through modifying the surface properties and morphology of thin (0.19 mm thick) carbon paper electrodes instead of commonly used several millimeters thick carbon felts. Surface functionalization and pore formation of carbon paper are carried out using a catalytic etching method at high temperaturewhere the diameters of nanopores are controlled by tuning the etching conditions. The synergistic effects of thin, nanoporous and functionalized carbon paper result in more effective electrode/electrolyte interaction and the less mass transport resistance. Therefore, the zero-gap VRFB cell employing the nanoporous electrodes displays remarkable performance improvement in terms of electrolyte utilization by 110%, discharge energy density by 155% and energy efficiency by 29% as compared to the one using pristine electrodes at a current density of 50 mA cm−2. The results imply that the more energy can be harvested by employing nanoporous and functionalized carbon paper electrodes having larger active surface areas.

Effect of electrode compression and operating parameters on the performance of large vanadium redox flow battery cells

Studies in small cells of vanadium redox flow battery have shown electrode compression to have significant effect on the cell performance. However, the results are conflicting and the data have been obtained for very high electrolyte circulation rates. In the present study, experimental investigations have been conducted in vanadium redox flow battery cells of active area of 426 cm2 and 936 cm2 over a wide range of operating conditions typical of large cells and for electrode compressions of 20, 35, 50 and 70%. The influence of operating current density, electrolyte circulation rate and state of charge has been specifically studied for both cell sizes. Cell pressure drop studies have also been conducted. Results show that the beneficial effects of electrode compression are mostly nullified by the loss of permeability at current densities in the range of 30–60 mA cm−2. At higher current densities, intermediate compressions of 35 and 50% are found to be the most optimal. Low state of charge tolerance is found to be better for the larger cell than the smaller cell, presumably due to better convective transport of electrolyte through the electrode arising from higher pressure drop.

Three-dimensional detached serpentine flow field design for redox flow batteries

In the present study, we propose a novel three-dimensional detached serpentine flow field. The flow field applies two staggered serpentine channels to the bipolar plate and electrode (near the membrane side), respectively. The detached inflow and outflow channels aim to enhance convective mass transfer of active species in both in-plane and through-plane directions of the electrode, thereby, leading to enhanced mass transfer, complete electrolyte penetration into the electrode, and reduced pressure loss. The combined effects via the detached flow field lead to the increases of approximately 4.2% and 3.2% in the voltage and energy (pump consumption included) efficiencies of a vanadium redox flow battery cell, respectively, when compared with those of the interdigitated flow field (under the current density corresponding to 100 mA cm−2 and flow rate corresponding to 20 mL min−1). The experimental results also reveal that the performance of the proposed design is more sensitive to the felt compression and electrolyte flow rate compared to the conventional interdigitated and serpentine flow fields. This is potentially due to the presence of the significant pressure gradient in the through-plane direction of the electrode by using the proposed detached serpentine flow field.

In-Situ Electrode Potential Measurements in Vanadium Redox Flow Batteries

Vanadium redox flow battery (VRFB) cells typically consist of a proton-conducting membrane sandwiched by two porous carbon-fiber based electrodes. The V2+/V3+ and V4+/V5+ redox couples, dissolved in acidic electrolytes, flow through the negative and positive electrodes, respectively. And, the primary reactions during charge and discharge are the redox reactions of these two vanadium couples. The round-trip efficiency (RTE) of a VRFB cell is a complex combination of multiple factors, including the electrodes, membrane, electrolytes, and cell-design configuration, as well as the operating conditions, i.e. charge/discharge current, range of the electrolyte state-of-charge (SOC) and electrolyte flow rates.1,2 The ohmic losses in the membrane and overpotentials on the two electrodes are two major factors that contribute to the cell’s efficiency losses. Undesired side reactions, including hydrogen evolution on the negative electrode and carbon corrosion on the positive electrode, may also occur, especially when the cell is overcharged, which may lead to a change in the electrolyte average vanadium oxidation state and degradation of the carbon electrodes and bipolar plates.3-5 Since both the vanadium redox reactions and these side reactions are strongly potential driven, it is useful to have a reliable reference electrode integrated into the VRFB system so that the electrode potentials vs. reversible hydrogen electrode (RHE) can be measured under in-situ conditions. The use of reference electrodes, such as the dynamic hydrogen electrode, have been reported in VRFB cells to measure in-situ electrode potentials.6,7 However, a significant challenge is the vanadium ion contamination that shifts the RHE potential higher, up to ~ 0.3V by 1.6M vanadium electrolyte. Here we describe a specially designed reference-electrode apparatus (REA), which is depicted in Fig.1 and consists of a small VRFB cell, a RHE, and a continuously acid-washed region between the VRFB cell and the RHE to prevent vanadium contamination of the RHE. This unique REA enables a reliable RHE for a VRFB cell operating under various operating conditions. For example, the REA can be used to measure both the overpotentials and electrochemical potentials (vs. RHE) of the working electrodes in a VRFB cell during a complete charge and discharge cycle, as shown in Fig. 2. The REA also provides real-time measurement of the electrolyte OCV, which can be used to determine the SOC of the posolyte and negolyte individually. The REA can also be used to provide insights into VRFB-electrolyte rebalancing options. Therefore, the REA that will be introduced here can be a useful tool to guide the selection of both electrode materials and the optimization of VRFB operating protocols, and examples of both will be presented. Acknowledgements This work was made possible by financial supported from Vionx Energy.

Selective ion transport for a vanadium redox flow battery (VRFB) in nano-crack regulated proton exchange membranes

To develop new cost-effective and high performance ion exchange membranes (IEMs) for redox flow battery applications, poly(arylene ether sulfone) random copolymer membranes (BPSHs) with surface nano-crack coatings (P-BPSH) were prepared using a hydrophobic atmospheric plasma treatment. The effect of the nano-cracks on the membranes' ion transport properties and the performance of the vanadium redox flow batteries (VRFBs) were evaluated to better understand the feasibility of using this technique in membrane ionic selectivity. The ion-selective nano-crack surface significantly improved the proton selectivity, from 32.95 to 74.20, over vanadium (VO2+) ions. Consequently, the VRFB cell assembled using a P-BPSH-60 membrane showed higher coulombic and energy efficiencies compared to a VRFB with a pristine BPSH-60 membrane. The energy efficiency of the P-BPSH-60 membrane (85.37%) is comparable to that of a Nafion® 117 membrane (85.11%). The improved battery performance demonstrated that the surface nano-crack coating layer effectively blocked the transport of vanadium ions through the IEM without distinct reduction of the proton conductivity. The results suggest a promising strategy to enhance membrane proton selectivity over vanadium ions.

Operando studies on through-plane cell voltage losses in vanadium redox flow battery

Through-plane potential and overpotential distributions in a vanadium redox flow battery (VRFB) is investigated in-situ using saturated Ag/AgCl reference electrodes attached to each side of the membrane with edge-type configuration. Combined with electrochemical impedance spectroscopy (EIS), this operando approach enables detailed quantification of the voltage losses contributed from the individual VRFB cell components with the magnitudes of ohmic and charge transfer losses at practical charge-discharge conditions. It is found that the overpotentials for vanadium redox reactions on graphite felt electrodes at the positive and negative half-cells are one order magnitude larger than the voltage loss through the typical cation-exchange membrane (Nafion®117) for charging-discharging at current densities from 50 to 120 mA cm−2. Within the overall half-cell overpotentials, the ohmic voltage loss accounts for about 40%. The analysis reveals that the voltage loss by the anode charge transfer reaction is about three folds larger than that by the cathode reaction, suggesting that sluggish V2+/V3+ redox kinetics in the negative half-cell is the major factor limiting the overall VRFB cell performance. This operando methodology is effective for the in-depth analysis of VRFB performance in practical conditions as well as the development of high-performance cell components and efficient total cell design by a collective and quantitative approach.

Flow field design pathways from lab-scale toward large-scale flow batteries

Current demonstration projects show that the power capacity of redox flow batteries can span a large range from kW- to MW-scale. The large-scale, especially MW-scale, flow battery system can usually benefit from cell's large active area, due to that a large cell can reduce the required number of cells and thus assembling difficulties. However, the lack of practical pathways for scaling-up lab-scale toward large-scale flow field designs has been one of the barriers to the commercialization of flow batteries. The present study investigates the interdigitated flow field design for a large-scale (900 cm2 active area) vanadium redox flow battery cell, based on a three-dimensional, multi-physical model. Four pathways for scaling up the flow field are investigated, including (i) geometric similarity, (ii) channel length extension, (iii) same pressure drop, and (v) split-interdigitated flow field. The relation between the width and length of the channel and the concentration overpotential is formulated. The results show that the split-interdigitated flow field outperforms the other scaling-up methods in terms of the overall energy efficiency, while at the cost of the increased pressure drop. To alleviate the high pressure drop, the design can be improved by widening the main channels or adding one extra flow inlet.

Studies on dynamic responses and impedance of the vanadium redox flow battery

This paper studies the feasibility of using the vanadium redox flow battery (VRB) for power quality control applications. This work investigates the dynamic voltage and current responses of the VRB to load changes over a range of frequencies (up to 5 kHz), through experimental studies on a laboratory scale testing system. Experiments were carried out under different operating conditions to examine the effects of system SOC, discharging current and temperature. The analysis shows that the magnitude of battery impedance is higher at low frequencies but lower at high frequencies. These results suggest that the VRB has the ability to handle charging-discharging power fluctuations in a frequency range up to a kHz level. By using the concept of fractional order systems, the transient behaviour of the VRB cell was modelled as an equivalent circuit that utilises a constant phase element to represent the electrochemical double layer and a Warburg element to describe the effect of concentration polarisation. This equivalent circuit model is useful for electrical interface design and power flow control applications.

Ion Selectivity and Stability Enhancement of SPEEK/Lignin Membrane for Vanadium Redox Flow Battery: The Degree of Sulfonation Effect

A membrane of high ion selectivity, high stability, and low cost is desirable for vanadium redox flow battery (VRB). In this study, a composite membrane is formed by blending the sulfonated poly (ether ether ketone) with lignin (SPEEK/lignin), and optimized by tailoring the degree of sulfonation. The incorporation of lignin into the SPEEK matrix provides more proton transport pathway and meanwhile adjusts the water channel to repulse vanadium ions. The VRB cells assembled with the composite membranes exhibit high coulombic efficiency (~99.27%) and impressive energy efficiency (~82.75%). The cells maintain a discharge capacity of ~95% after 100 cycles and ~85% after 200 cycles at 120 mA cm−2, much higher than the commercial Nafion 212. The SPEEK/lignin composite membranes are promising for application in VRB system.

The improved electrochemical performance of vanadium redox flow battery by decorating functionalized mesoporous carbon catalyst with three-dimensional interconnected network

Mesoporous carbon with an interconnected pore network containing oxygen functional groups (MC-Oxy) is proposed as a promising electrocatalyst for vanadium redox flow batteries (VRFBs). Abundant oxygen functional groups in introduced through an acidic treatment with a mixed solution of HNO3 and H2SO4 (1:3 v/v ratio). The electrochemical reactivity of MC-Oxy toward the VO2+/VO2+ redox reaction is effectively improved. Consequently, VRFB cell assembled with carbon felt containing MC-Oxy exhibits an outstanding energy efficiency compared to the bare carbon felt. This improvement is attributed mainly to the action of the MC-Oxy as an electrocatalyst toward the redox reaction of VO2+/VO2+ redox couples.

Detrimental role of hydrogen evolution and its temperature-dependent impact on the performance of vanadium redox flow batteries

Abstract This paper addresses the damaging role of the parasitic hydrogen evolution reaction (HER) in the negative half-cell of a vanadium redox flow battery (VRFB) on state-of-the-art carbon felt electrodes at different temperatures. It was found that increasing the temperature resulted in a better catalytic performance for both the positive and negative half-cell reactions. In addition, increasing the temperature significantly enhanced the undesired HER at the negative side. Operating the VRFB cell at higher temperature led to a decrease in the coulombic efficiency attributed to the higher hydrogen production. More pronounced hydrogen production caused an oxidation on the surface of the carbon fibers and a degradation of the electrode as indicated from scanning electron microscopy and X-ray photoelectron spectroscopy measurements. This observed degradation results in fading of the overall performance of the vanadium redox flow battery over time.

Zero dimensional dynamic model of vanadium redox flow battery cell incorporating all modes of vanadium ions crossover

A 0-D dynamic mathematical model for a single Vanadium Redox Flow Battery (VRFB) cell is proposed. The model is based on the conservation principles of charge and mass transfer focusing on the precise simulation of crossover with diffusion, migration and convection. The influence of these phenomena on the capacity decay was systematically analyzed, revealing considerable impact of convection component, which dominates under diffusion and migration and mainly responsible for observed capacity loss. The model allows to simulate main characteristics of VRFB systems (such as battery voltage, state of charge, charge/discharge time and capacity decay due to crossover) with high accuracy. The model was validated with experimental data in the wide range of current densities (40–100 mA cm−2), and the results demonstrated good agreement with experiments having an average error within 5% range. In addition, the model requires a modest computational time and power, and, therefore, it can be suitable for application in advanced control-monitoring tools, which are necessary for a long-service life and sustainable operation of VRFB systems.