Operation Of Vanadium Redox Flow Battery Research Articles

Optimizing of working conditions of vanadium redox flow battery based on artificial neural network and genetic algorithms

The integration of electrode compression in a vanadium redox flow battery (VRFB) with optimized operating conditions is essential for achieving the maximum net discharge power. This study presents the development of a 3D steady-state model for VRFB aimed at maximizing net discharge power (Pnet) by optimizing key working conditions, including electrode compression ratio (CR), applied current density (Iapp), and electrolyte inflow rate (Qin). The comprehensive impacts of CR, Iapp, and Qin on VRFB charging and discharging curves, energy efficiency (EE), and system efficiency (SE), and Pnet were investigated. Subsequently, the optimization of CR and Qin under a fixed applied current density range of 20–400 mA/cm2 was carried out employing a hybrid methodology integrating artificial neural network (ANN) and genetic algorithms (GA). Results reveals that increasing CR does not linearly improve system efficiency and net discharge power, with an optimal CR identified to maximize net discharge power. The optimal CR varies with operating conditions and is influenced by factors such as Qin and Iapp. Notably, both CR and Qin negatively impact net discharge power due to increased pump losses, while Iapp and state of charge (SOC) positively affect net discharge power through enhanced electron transfer and higher active substance concentration, respectively. The optimal CR is identified around 65 % for Iapp exceeding 60 mA/cm2. Furthermore, the optimal Qin correlates directly with increasing Iapp, as higher Iapp rates necessitate greater Qin to sustain active species supply. This study offers an optimization approach for addressing coupled operating conditions and provides insights for achieving high net discharge power in VRFB operation.

Proton conducting zeolite composite membrane boosts the performance of vanadium redox flow battery

In a vanadium redox flow battery (VRFB), the selectivity of protons over vanadium ions for an ion-conducting separator is crucial to achieving capacity retention in charge/discharge cycles. Conventionally used perfluorinated or hydrocarbon-based membranes are not able to resolve cross-contamination of vanadium during VRFB operation. Engineered inorganic materials like zeolites have inherent selectivity towards specific ions based on the size and charge of the ions. In this work, ZSM-5 zeolites with different Si/Al ratios were modified by a facile ion-exchange process to obtain proton rich zeolites, followed by successful infiltration into the SPEEK matrix. The composite membranes demonstrated excellent oxidative stability in 2 M H2SO4 containing 1.5 M VO2+ ions as well as favourable electrochemical properties. In a single-cell VRFB, the membrane with the highest proton conductivity i.e., SPEEK-ZSM 5 (25) 2H+ was able to achieve an average Coulombic efficiency of 97.0% and average energy efficiency of 78.0% over 300 charge/discharge cycles at 80 mA cm−2 current density. The polarization curve and self-discharge experiment results illustrated 21.7% higher peak power density and 2.5 times slower self-discharge compared to state-of-the-art Nafion-117 in identical operating conditions indicating zeolite-composite membranes as a potential candidate to enhance the durability and performance of VRFBs.

Evaluation of mitigation of capacity decay in vanadium redox flow batteries for cation- and anion-exchange membrane by validated mathematical modelling

Vanadium redox flow battery (VRFB) is a potential electrochemical energy storage solution for residential accumulation and grid stabilization. Long-term durability, non-flammability and high overall efficiency represent the main advantages of the technology. The ion-exchange membrane, an essential component of the battery stack, is largely responsible for the efficiency of the battery and capacity losses caused by asymmetric cross-over of vanadium ions and a solvent.To mitigate these losses, we developed a mathematical model of the VRFB single-cell for both cation-exchange membrane (CEM) and anion-exchange membrane (AEM) and validated it against our own experimental data. Our model simulates the charge-discharge cycling of a VRFB single-cell under selected sets of operating conditions differing in the following parameters: applied current density, initial volume and concentration of electrolytes, arrangement of storage tanks (hydraulic shunt) and option of periodic rebalancing of electrolytes. The model includes a description of vanadium ions permeation and osmotic flux across the membrane and kinetics of electrode reactions. The hydraulic connection of electrolyte tanks appears to be the most promising mitigating strategy, reducing capacity losses by 69 % over 150 cycles when compared to standard VRFB set-up, which we have also confirmed experimentally. Moreover, by combining the operation methods, our model shows that using AEM with the hydraulic electrolyte connection and periodic rebalancing, the overall battery utilization can be increased by 80 % compared to a standard operation of VRFB using CEM. The developed model offers useful optimization tool for the construction and operation of flow batteries and can be easily adapted for other chemistries.

State of charge and power rating gains in industrial-scale vanadium redox flow batteries through thermal activation of electrodes

Vanadium redox flow batteries (VRFB) are typically envisaged for application in large-scale energy storage systems. High-power operation of VRFBs reduces the cost of such installations but also significantly affects the energy storage capacity owing to markdown in the operable range of the state of charge (SoC). This situation can be remedied to some extent by minimizing the area-specific resistance (ASR) of the VRFB cells. In the present study, the application of thermal activation of the electrodes was studied on an industrial-scale VRFB cell with an active electrode area of 936 cm2. The carbon felt electrode was heat treated at 350, 400, and 450 °C for 30 h. Following physical and electrochemical characterization of the heat-treated carbon felt, electrochemical testing of the full cell was carried out through charge-discharge cycling studies conducted at current densities of 60, 90, 120 and 150 mA/cm2. Comparative studies were also conducted with an unactivated carbon felt. Significant reduction in ASR was observed with the optimally heat-treated carbon felt, leading to nearly 20 % expansion in the operating SoC range at a current density of 120 mA/cm2 and 34 % enhancement in the rated power of the cell. Long-term durability studies over 200 cycles and post-test inspection and physical characterization confirmed the stability of thermal activation.

A Review of Electrolyte Additives in Vanadium Redox Flow Batteries.

Vanadium redox flow batteries (VRFBs) are promising candidates for large-scale energy storage, and the electrolyte plays a critical role in chemical-electrical energy conversion. However, the operating temperature of VRFBs is limited to 10-40 °C because of the stability of the electrolyte. To overcome this, various chemical species are added, but the progress and mechanism have not been summarized and discussed yet. This review summarizes research progress on electrolyte additives that are used for different purposes or systems in the operation of VRFBs, including stabilizing agents (SAs) and electrochemical mass transfer enhancers (EMTEs). Additives in vanadium electrolytes that exhibit microscopic stabilizing mechanisms and electrochemical enhancing mechanisms, including complexation, electrostatic repulsion, growth inhibition, and modifying electrodes, are also discussed, including inorganic, organic, and complex. In the end, the prospects and challenges associated with the side effects of additives in VRFBs are presented, aiming to provide a theoretical and comprehensive reference for researchers to design a higher-performance electrolyte for VRFBs.

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

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

In Situ Characterization of Kinetics, Mass Transfer, and Active Electrode Surface Area for Vanadium Redox Flow Batteries

Engineering the electrochemical reactor of a vanadium redox flow battery (VRFB) is critical to deliver sufficiently high power densities to achieve cost-effective, grid-scale energy storage. Understanding and ultimately alleviating the cell-level resistive losses in VRFBs fundamentally depend on the ability to accurately measure the electron and mass transfer rates as a function of applied potential and interpret the results in the context of VRFB operation. In this study, an in situ electroanalytical technique of electrochemical reaction in porous electrodes is proposed by a symmetrical cell design for VRFB. For both V2+/V3+ and VO2+/VO2 + redox couples, the polarization curves at different flow rates are acquired on the symmetrical flow cell. The high-frequency resistance is also obtained by electrochemical impedance spectroscopy at open circuit. The ohmic, kinetic, and mass transfer resistance are obtained by deconvoluting the total polarization curve. Corresponding key parameters (i.e., membrane conductivity, reaction rates, and mass transfer coefficients) are obtained along with the specific surface area of porous electrode. The full-cell simulations using extracted key parameters are in excellent agreement with experimental full-cell tests at different applied currents. This novel in situ electroanalytical technique provides an invaluable approach to characterize the performance of electrolyte and electrode in redox flow batteries.

An Advance Control of Grid Integrated Wind Turbine Driven DFIG-Battery System with Grid Power Shaping Under Gust Wind Variation

There has been an increase in wind power penetration into the utility grid. The wind speed variations exhibit undesirable oscillations that influence power quality and system reliability. Therefore, the grid operation is challenging in this scenario. Battery energy storage system (BESS) smooths out wind power production and reduces fluctuations. However BESSs low power density and high capacity requirements to compensate for fluctuation limit its implementation for high-power applications. This paper introduces a wind energy conversion system (WECS) based on a doubly fed induction generator (DFIG) incorporating a vanadium redox flow battery (VRFB) in the DC-link, which supports grid power smoothing and power balance even under varying wind speed. Real-site wind data was used to develop a technique for designing the VRFB. The rotor side converter (RSC) employs an off-maximum power point tracking to keep the VRFB operation within the limit and deliver consistent power to the grid. An improved phase lock loop control is designed for DFIG that suppresses DC offset at the input and allows RSC and grid side converter (GSC) to perform appropriately even under grid disturbances. The GSC control is designed with an improved DC-link voltage control by adopting an anti-windup fractional-order proportional-integral (AWFOPI) controller, which can improve transient response. A sparrow search algorithm is adopted to tune the AWFOPI controller. Matlab Simulink is used to simulate the system. Test results show that the system performs well under various operating conditions and is satisfactory. It has also been tested on the OPAL-RT real-time simulator test bench.

Operando UV/Vis spectra deconvolution for comprehensive electrolytes analysis of vanadium redox flow battery

• 0.5 and 1 M vanadium electrolytes were successfully analyzed using the method. • The analysis provides reliable data regardless of supporting electrolyte composition. • V 2 O 3 3+ complex of posolyte was obtained using the proposed method. • Unknown residual absorbance of negolyte was registered using the deconvolution. • The method provides comprehensive data during VRFB cycling. Monitoring the state of charge (SoC) is one of the most important challenges of vanadium redox flow battery (VRFB) technology to solve. Among other methods, optical spectroscopy seems promising, however, existing approaches have their problems, primarily because of limited applicability for high vanadium concentrations, uncertainty in accounting for electrolyte imbalance during VRFB operation, and applicability limitations for mixed acid electrolytes. In this work, a method for determining SoC of VRFB based on the deconvolution of electrolyte absorption spectra is proposed. This method was successfully implemented for 0.5 M and 1 M vanadium electrolytes with 0.05 M phosphoric acid additive and demonstrated good accuracy for determining vanadium concentration and SoC in both half-cells, including analysis during VRFB galvanostatic cycling. Besides, this method allows us to obtain complex spectra: V 2 O 3 3+ in posolyte and a previously never demonstrated complex in negolyte that presumably corresponds to the V 3+ complex.

(Invited) Optimized Operation of the Vanadium Redox Flow Batteries

The Redox Flow Battery (RFB) systems are unique chemically, mechanically, and electrically compared to other kinds of batteries. Among RFBs, the Vanadium Redox Flow Batteries (VRFBs) are the most commercialized type. VRFBs are a suitable option for large-scale energy storage with exceptional advantages like the decoupled power and energy design (scalability), long life-time, safe battery chemistry (non-toxic, and non-flammable), etc. The power and energy designs in VRFBs are decoupled, known as the scalability benefit of the VRFBs. This implies more output energy from the battery is possible only by adding more electrolytes to the reservoir tanks.There are only a few research studies in the literature about the optimized operation of the Vanadium redox flow batteries. Most of these papers are not applicable to develop in real practice. The introduced optimized operation algorithms of VRFBs in this lecture can help the participants learn how to improve the performance of the battery and operate the battery more efficiently. Different objective functions are considered in this lecture to be optimized, e.g. minimizing the time duration of battery charging (for fast charging), energy loss, voltage loss, and capacity loss of the battery. The participants can find the results of this useful in optimized operation and implementation of the VRFBs.High charging current density results in faster charging and reduces the capacity fading in Vanadium Redox Flow Batteries (VRFB). On the other hand, it leads to the reduced energy efficiency of the battery. Also, the lower electrolyte flow rate in VRFBs results in less energy consumption by the pumps leading to the higher energy efficiency of the VRFBs. However, higher flow rates have the benefit of reducing voltage loss of VRFBs. To address these trade-offs, closed-loop charge control and flow management in VRFBs are necessary. In this lecture, a multi-objective optimization is proposed in first section to optimize the charging duration and flow management of the VRFB simultaneously during its charging. An innovative method is proposed for modeling pump consumption based on affinity laws for centrifugal pumps, which leads to new electrolyte flow management. Further, three case studies are defined in charging mode on a nine-cell VRFB unit laboratory prototype to validate the proposed optimization's performance, involving the duration of charging, flow management, and energy efficiency of the VRFB. The method is compared with previously published research studies on the optimal operation of VRFBs, which shows the uniqueness and consistency of the proposed optimization method for simultaneous controlling VRFB's charging duration and flow management.Capacity fade in Vanadium Redox Flow Batteries (VRFB) relies on the loss of electrolyte volume in each of charge and discharge cycles. The loss of volume in each cycle, also known as the bulk electrolyte osmosis, is due to Vanadium ions' diffusion from the membrane. The lower electrolyte flow rate in VRFB can reduce capacity fade as the electrolyte's velocity across the membrane decreases. However, the lower electrolyte flow increases the battery’s voltage loss. A new electrolyte flow management is introduced in second section of the lecture to address this trade-off, which considers the decrease of both capacity fade and voltage loss in VRFBs simultaneously. The proposed multi-objective flow management shows a significant reduction of both capacity and voltage losses in VRFBs.Moreover, typically complex electrochemical models and equations are needed to model capacity fade in VRFBs, which are not straightforward to model. The capacity fade modeling can lead to the estimation of available capacity and the battery’s State of Health (SoH). Therefore, a new mathematical model is proposed for the VRFB’s available capacity based on the electrochemical-based capacity fade model results. The model further is developed to estimate the State of Charge (SoC) and the SoH of VRFBs per cycle of charge and discharge.

Thermally cross-linked sulfonated poly(ether ether ketone) membranes containing a basic polymer-grafted graphene oxide for vanadium redox flow battery application

Sulfonated poly(ether ether ketone) (SPEEK) with a very high degree of sulfonation (DS) and poly(2,5-benzimidazole)-grafted graphene oxide (ABPBI-GO) are prepared and used as a polymer matrix and a filler, respectively, to prepare thermally cross-linked membranes for vanadium redox flow battery (VRFB) application. The thermally cross-linked SPEEK membranes containing ABPBI-GO exhibit significantly enhanced physicochemical stabilities due to the formation of the well-developed cross-linked structures by the chemical bonds formed through the solvothermal process and the acid–base interactions between the benzimidazole groups in filler and sulfonic acid groups in polymer chain. In addition, ABPBI-GO in the polymer matrix is found to be effective in decreasing the vanadium ion permeability while retaining high proton conductivity producing a high-performance VRFB membrane. The ionic selectivity of the thermally cross-linked SPEEK membrane containing 0.5 wt% of ABPBI-GO (TC-SPEEK/0.5) is 13.15 × 107 mS min cm−3, which is approximately 8 times larger than that of Nafion 212 (1.57 × 107 mS min cm−3). Accordingly, the VRFB cell fabricated using TC-SPEEK/0.5 membrane exhibits higher coulombic efficiencies (CE: 98.1–99.4%), voltage efficiencies (VE: 92.5–85.2%), and energy efficiencies (EE: 90.7–84.7%) than that using Nafion 212 (CE: 89.2–93.3%, VE: 90.7–81.2%, EE: 80.9–75.7%) at current density of 40–100 mA cm−2. Furthermore, the long-term stability of the VRFB cell using TC-SPEEK/0.5 membrane evaluated by the cycle test under realistic VRFB operation conditions is also found to be much better than that using Nafion 212.

Ion selective redox active anion exchange membrane: Improved performance of vanadium redox flow battery

In-sight understanding of synergetic and trade-off behaviour between chemical and structural properties of emerging redox active membrane separator for batteries is necessary for high performance and widespread adoption of technologies. Our aim is to provide sustainable solution such as stable membrane with fast ionic-transport (low resistivity), and impervious nature for redox active species that leads to deleterious capacity fade and materials underutilization. We report architecture of redox active anion exchange membranes (AEMs) by free radical polymerization using tert-butylmethaacrylate (tBuMA), vinylimidazole (VIm) and vinyl ferrocene (VFc). The VFc acts as redox responsive species (ferrocenium ion) due to protonation under operating conditions. These ion selective redox active membranes offer chemical modularity, high conductivity, stability, and prevents VO2+ cross-over, necessary for improved vanadium redox flow battery (VRFB) performance. The fundamental electrochemistry of redox active membranes is fascinating, due to redox peaks (Fc/Fc+) in voltammetry. Suitable redox-active membrane separator exhibits 98.0% current efficiency corresponding to 85.0% voltage efficiency at 60 mA cm−2 in VRFB operation. Therefore, AEMs with redox active functional moieties can open a new avenue for efficient VRFB and various electrochemical applications.

Inspired by “quenching-cracking” strategy: Structure-based design of sulfur-doped graphite felts for ultrahigh-rate vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) are perceived as promising candidates for grid-scale energy storage systems. However, limited improvements in electrode structures restrict the operation of VRFBs at high current densities. Herein, finite element simulations are used to guide the construction direction of the electrode structure. Afterwards, a quenching-cracking strategy is ingeniously employed to successfully construct parallel-aligned micron flow channels on electrode fibers in high agreement with the model, and the consistency of the flow channel structure is verified via deep learning technique. The well-constructed flow channels achieve high specific surface areas of electrodes while enabling the smooth flow of electrolyte over the fiber surfaces. Subsequent graphitization and sulfur-doping processes yield hierarchical fibers with highly conductive cores and well-active surfaces. Benefiting from fine structural modulation, the battery equipped with the as-prepared electrodes delivers an energy efficiency of 80.44 % at an ultra-high current density of 500 mA cm−2 and achieves a peak power density of 1.68 W cm−2. Additionally, the battery is consistently cycled for 1000 cycles at 500 mA cm−2 and the average energy efficiency decay is only 0.01032 % per cycle. Notably, finite element simulations are applied to investigate the velocity distribution of electrolyte in the flow channels, and first-principle calculations are employed to reveal the cause for energy efficiency decay of the battery after long-term cycling. Most importantly, the establishment of structure-activity relationships highlights the significance of comprehensive modulation of electrode fiber structures towards enhancing the performance of VRFBs.

The Impact of Electrode Aging on the Structural and Mechanical Properties of Carbon Felt Electrodes for Vanadium Redox Flow Batteries

Vanadium Redox Flow Batteries (VRFBs) represents a promising approach to large-scale energy storage of intermittently available renewable energy. However, this technology still faces major problems in terms of lifetime, efficiency, and costs. One limitation is the degradation of the electrode during operation, which influences the (electro)chemical and mechanical stability of the electrode.1,2 Performance deteriorations during cycling are frequently reported and the harsh environment of the sulfuric acid itself leads to electroless oxidation of the carbon-based electrodes.3 Previously, we have already presented the effect of thermal treatment and accelerated stress tests on the electrochemical and compositional properties of commercial carbon felt electrodes and their affinity towards side reactions like hydrogen evolution and carbon corrosion.4,5 These studies focused on the chemical aging at high temperature in sulfuric acid and the electrochemical aging in V(V) electrolyte to imitate the conditions of a charged, positive VRFB half-cell. To deepen the knowledge about (electro)chemical aging of carbon electrodes, we present a study on the impact of degradation on the structural and mechanical properties of carbon electrodes.Corresponding to our previous studies, different treatments were applied to mimic the effect of thermal activation, chemical, and electrochemical aging on the structure of carbon fiber electrodes. Thereby, the electrode material was either thermally treated at 400°C in air atmosphere, and immersed in sulfuric acid, or vanadium electrolyte under potential control. Structural changes were observed by scanning electron microscopy (SEM) and confirmed by topography images from atomic force microscopy (AFM) on single fibers. Furthermore, physisorption measurements were conducted to quantify the changes in porosity, surface area, and pore size distribution after each of the aforementioned treatments. Adhesion profiles were measured by AFM to analyze the wettability of the respective samples and to determine the modulus of elasticity.The characterization methods presented in this study are beneficial to investigate the mechanical degradation of carbon felt electrodes during operation. The extracted values like pore size distribution, fiber thickness, or modulus of elasticity are important for theoretical simulations on compression or flow behavior. Also, accelerated stress test procedures are helpful to evaluate the stability of new materials or modified carbon electrodes since electrode stability is an important aspect to consider for the long-term operation of VRFBs. References O. Nibel et al., J. Electrochem. Soc., 164, A1608–A1615 (2017) https://iopscience.iop.org/article/10.1149/2.1081707jes.I. Derr et al., J. Power Sources, 325, 351–359 (2016) http://dx.doi.org/10.1016/j.jpowsour.2016.06.040.I. Derr et al., Electrochim. Acta, 246, 783–793 (2017) http://dx.doi.org/10.1016/j.electacta.2017.06.050.L. Eifert, R. Banerjee, Z. Jusys, and R. Zeis, J. Electrochem. Soc., 165, A2577–A2586 (2018) https://iopscience.iop.org/article/10.1149/2.0531811jes.L. Eifert, Z. Jusys, R. J. Behm, and R. Zeis, Carbon N. Y., 158, 580–587 (2020) https://doi.org/10.1016/j.carbon.2019.11.029. Figure 1

A combined in situ monitoring approach for half cell state of charge and state of health of vanadium redox flow batteries

Efficient vanadium redox flow battery operation requires reliable half cell specific state of charge and capacity information at each point in time. This study uses long term correlations of electrolyte densities with negative half cell capacities and half cell potentials in Nernst equations for a joint in situ measurement approach. In principle, no ex situ reference data are needed for the calibration since a combination of Coulomb counting and half cell electrolyte potentials enables a self consistent calibration during battery operation cycles. By this way state of charge results of a negative half cell feasibility study are confirmed for long term data and extended to the positive half cell. The explicit consideration of H+ concentration and acid dissociation for the positive half cell leads to consistent half cell measurements with lower errors. Experiments have been carried out with two different cation exchange membranes achieving comparable measurement errors in the few per cent range.

Surface Modification of Sulfonated Poly(phenylene oxide) Membrane for Vanadium Redox Flow Batteries.

Sulfonated poly(phenylene) oxide (sPPO) polymer is coated in a dopamine hydrochloride solution to prepare a highly durable, low-price polymer membrane for vanadium redox flow batteries (VRFBs). The polydopamine (PDA) coating on the sPPO membrane is confirmed using SEM and EDX analysis. sPPO coated with PDA exhibits decreased proton conductivity due to high resistance. However, VO+₂ reducibility tests shows that the chemical stability is improved due to the introduction of the PDA coating layer on the sPPO membrane, which has a chemical structure with poor durability in VO+₂ solution under the operating conditions of a VRFB. These results show that this polymer electrolyte membrane based on PDA-coated sPPO is a candidate for application in the long-term operation of VRFBs.

The Influence of Free Acid in Vanadium Redox‐Flow Battery Electrolyte on “Power Drop” Effect and Thermally Induced Degradation

A series of vanadium redox‐flow battery (VRFB) electrolytes at 1.55 m vanadium and 4.5 m total sulfate concentration are prepared from vanadyl sulfate solution and tested under conditions of appearance of “power drop” effect (discharge at high current density from high state‐of‐charge). A correlation between the initial electrolyte composition, the thermal stability of catholyte, and the susceptibility of VRFB to exhibit a “power drop” effect is derived. The increase in total acidity to 3 m, expressed as concentration of sulfuric acid in precursor vanadyl sulfate solution, enables “power drop”‐free operation of VRFB at least at 75 mA cm−2. Thermally‐induced degradation of electrolyte is evaluated based on decrease in vanadium concentration in the electrolyte series after exposure to the temperature of 45 °C and based on characterization of catholytes series using 51V, 17O, and 1H nuclear magnetic resonance spectroscopy.

Investigation of poly(phthalazinone ether ketone) amphoteric ion exchange membranes in vanadium redox flow batteries

Amphoteric ion exchange membranes (Q-x/S) for vanadium redox flow battery (VRFB) are prepared from brominated poly(phthalazinone ether ketone) with the degree of substitution (DS) in the range of 60–95% and sulfonated poly(phthalazinone ether ketone) through blending and amination reaction, and x refers to the DS. Q-x/S membranes contain quaternary ammonium and sulfonic groups, making it easy to adjust the permeability of different vanadium ions and net water transport. When the DS increases from 60 to 95%, the discrepancy of the permeability for VO2+/VO2+ and V2+/V3+ enhances. The permeability of VO2+, VO2+, V3+ and V2+ for Q-x/S membranes was 98.7–99.9%, 98.6–99.9%, 99.5–99.9% and 99.2–99.9% less than those for Nafion115. Compared to Nafion115, Q-88/S membrane shows 81.3 and 84.8% decrease in static and cycling water transport volume. The energy efficiency (EE) of Q-88/S and Q-95/S membranes reaches 90% at the current density of 40 mA cm−2, higher than that of Nafion115 (86.5%). Compared with Nafion115, Q-x/S membranes exhibit stable efficiencies and higher capacity retention in long-term VRFB operation. Low vanadium permeability and net water transport volume make Q-x/S membranes promising to be used in commercial VRFB.

Optimal operational strategy for a vanadium redox flow battery

The vanadium redox flow battery (VRFB) is regarded as a potential technology for large-scale and stationary electricity storage. Efficient operation of the VRFB to maximize the system efficiency and realize safe operation is necessary. In this work, an optimal operational strategy for improving the system efficiency is proposed. The optimization problem by minimizing the charging energy and maximizing the discharge energy under operational constraints during VRFB operation is formulated. In the proposed strategy, the electrolyte flow rate is determined and adjusted according to the variation in the active vanadium species during the charge–discharge process to ensure high system efficiency. Because operation of the VRFB under an electrolyte imbalance is considered, this new control strategy can extend the operating window in which VRFBs can operate with high discharge capacity and system efficiency.

Deconvolution of electrochemical impedance data for the monitoring of electrode degradation in VRFB

Understanding degradation phenomena occurring during the operation of vanadium redox-flow batteries (VRFB) requires a measurement technique which allows for differentiating the overall performance losses into individual performance losses of the cell components. For this purpose, electrochemical impedance spectroscopy (EIS) is a valuable and well established tool. However, the discrimination of processes taking place at similar time scales is challenging since they overlap in the commonly used Nyquist or Bode representation. Distribution of relaxation times (DRT) analysis tackles this issue by deconvoluting EIS data with respect to the time constants of the individual processes. It hence circumvents the necessity of finding a suitable equivalent circuit model and thus allows for data evaluation without any a-priori knowledge of the system under study. For the first time, we herein present the application of DRT transform to EIS data of a VRFB. By varying experimental conditions and employing full cell as well as double half cell operational modes, we are able to identify the faradaic process of the negative half cell. This enables us to visualize the negative half cell’s contribution to the overall impedance of a VRFB even in a full cell EIS measurement. By an accelerated degradation experiment we finally demonstrate the great potential of DRT analysis for future application in the monitoring of electrode degradation in VRFB.