Vanadium Redox Flow Battery Cell Research Articles

A low-cost design approach to vanadium redox flow test cell

Vanadium redox flow batteries (VRFB) will be essential in maximizing the large-scale use of renewable energy after achieving technical maturity. But as the test cells needed to conduct VRFB research are very expensive, the cost factor can prevent participation of developing countries in critical research. To address such issue, this work presents a novel low-cost design for laboratory VRFB cell yielding good performances. This design relies on sturdy ultra-high molecular weight polyethylene (UHMWPE) end plates with flow channels to provide the strength needed to seal other components while properly distributing electrolyte. This simpler design uses a reduced number of components compared to traditional cells, thus resulting in easier manufacturing and assembly while keeping costs low. This novel design approach achieved performances that are comparable to a commercial VRFB test cell in various testing conditions, but at roughly a third of the cost. Performances have been further improved by 3D printing end plates with optimized flow channels, with the added benefit of avoiding the complexity and costs of computer numerical control (CNC) milling. This novel design approach can thus be useful to many researchers in developing countries for their VRFB test.

Sulfonated Polystyrene/Polybenzimidazole Bilayer Membranes for Vanadium Redox Flow Batteries

AbstractPolybenzimidazole has been widely examined as a separator for vanadium redox flow batteries (VRFBs) due to its low vanadium permeability. Its low conductivity can be tackled by combining 1–4 µm thin dense PBI layers with highly conductive mechanically supporting layers, either by lamination or loose stacking. While gel‐PBI is very soft and conductive, the shadow effect of the non‐conductive pore walls of porous supports adds resistance. In this work, these issues are addressed by coating a 25 µm thick highly conductive sulfonated polystyrene layer (S) with a 1 µm thin selective PBI layer (P) to block vanadium crossover. To reduce the number of potential defects, two bilayer membranes can be stacked. A 52 µm thick stack of two membranes (PS–SP, PBI faces the electrodes) shows an area‐specific resistance of 144.8 mΩ cm2 in VO2+‐containing electrolyte and a permeability of 6.85 × 10−14 m2 s−1, both lower than the values for Nafion 212. A VRFB cell test over 3500 charging cycles (1660 h) with an energy efficiency of up to 88.5% at 100 mA cm−2 is shown. Performance losses are reversed by electrolyte rebalancing. With material costs of 1.84 USD m−2, the PSSP(1‐25‐25‐1) membrane promises high performance at low costs.

Two-in-one strategy for optimizing chemical and structural properties of carbon felt electrodes for vanadium redox flow batteries

ABSTRACT Vanadium redox flow batteries (VRFBs) have received significant attention for use in large-scale energy storage systems (ESSs) because of their long cycle life, flexible capacity, power design, and safety. However, the poor electrochemical activity of the conventionally used carbon felt electrode results in low energy efficiency of the VRFBs and consequently impedes their commercialization. In this study, a carbon felt (CF) electrode with numerous nanopores and robust oxygen-containing functional groups at its edge sites is designed to improve the electrochemical activity of a carbon felt electrode. To achieve this, Ni metal nanoparticles were initially precipitated on the surface of the CF electrode, followed by etching of the precipitated Ni nanoparticles on the CF electrode using sulfuric acid. The resulting CF electrode had a specific surface area eight times larger than that of the pristine CF electrode. In addition, the oxygen-containing functional groups anchored at the graphite edge sites of the nanopores can act as robust electrocatalysts for VO2+/VO2 + and V2+/V3+ redox reactions. Consequently, the VRFB cell with the resulting carbon felt electrode can deliver a high energy efficiency of 86.2% at the current density of 60 mA cm−2, which is 20% higher than that of the VRFB cell with the conventionally heat-treated CF electrode. Furthermore, the VRFB cell with the resultant carbon felt electrodes showed stable cycling performance with no considerable energy efficiency loss over 200 charge-discharge cycles. In addition, even at a high current density of 160 mA cm−2 , the developed carbon felt electrode can achieve an energy efficiency of 70.1%.

Impact of Power Electronic Converter Ripples on Vanadium Redox Flow Batteries

The worldwide effort to decarbonize the electricity grid has made battery energy storage system (BESS) an essential technology. A promising technology for long duration storage on the grid is Vanadium Redox Flow Batteries (VRFBs), which can support the grid by regulating peak load, reducing intermittency of large-scale renewables, and as emergency power supply. There are two primary methods of integrating a BESS to the grid. Frist, the BESS is directly connected to the DC-link of a DC-AC converter that interfaces with the grid. Second, the BESS is connected to a DC-DC converter that in turn interfaces with the DC-link of a DC-AC converter. These converters all have a characteristic ripple voltage and current during operation. The magnitude and frequency of the ripples depend on topology and switching frequency of a converter, controller design and protocol, type of connection to the grid, voltage magnitude and load characteristics. Currently, studies on the impact of ripples have been limited to Lithium-ion and Lead-Acid batteries. The impact of ripple current and voltage has not been studied in VRFBs. In a VRFB, potential impact includes excessive gassing of hydrogen and carbon corrosion of the bipolar plate and porous electrode and cell heating. To better integrate and operate VRFBs to the electricity grid, the impact of ripples needs to be studied in detail.Using a novel experimental platform, where both magnitude and frequency of ripples will be incorporated, we will emulate the performance of different power electronic converter configurations. It will comprise standard 5cm2 VRFB cells with a carbon felt electrode and Nafion ion exchange membrane. The various converter ripples in an electricity grid setting will be emulated by superimposing the signals from a potentiostat (constant current) and a high frequency transformer (modulated ripples), which will be connected to the cathode of the cells.The experimental testbed will be used to cycle the VRFB cells at various states of charge with ripples that emulate the performance of power electronic converters during grid operations and disturbances. Varied parameters will include ripple amplitude, ripple frequency, electrolyte flow rate, applied current density and state of charge. Measurement data from monitoring voltage of each electrode, cell voltage and current, gas generation, electrolyte crossover and electrolyte loss will be analyzed to determine the impact of ripples on VRFB cells. Finally, the cells will be monitored for signs of gas generation and postmortem signs of carbon corrosion.Our analysis will determine the impact of various ripple magnitude and frequencies on VRFBs in a simulated grid integration environment. This work will facilitate the development of design standards for power electronic converters that are intended for battery applications, while also informing future efforts to optimize VRFB cell and stack performance.

Computational Modeling of a 2D Vanadium Redox Flow Battery Cell

These days, the implementation of sustainable power generation has led to a difference in propensity in the energy creation and capacity frameworks, compelling them to conquer the hardships that it addresses. Considering the entirety among the suggested technologies, vanadium redox flow batteries (VRFB) stand out as a wonderful choice regarding cyclability and versatility. The point of this study is to break down electrochemical performance of a vanadium redox flow battery cell in two dimensions. To accomplish this, a two-dimensional model comprising an ion exchange membrane, electrode and flow channel was created. A set of electrode compression and flow rates was tested to envision the impact on the velocity field, species concentration and potential and current distributions. As a result of compression, velocity profiles and reaction rates are both increased, by 12.7% and 9.2%, respectively, when applying 50% compression. Higher reaction rates and more stable concentration gradients were induced by higher electrolyte rates. Additionally, overpotential was reduced by 1.5% with the lowest flow rate.

Advanced electrode decorated with peanut-shell-derived carbon for vanadium redox flow battery

Vanadium redox flow batteries (VRFBs) have been regarded as one of the most promising energy storage systems at grid-scale owing to their advantages of safe operation, flexible design, and long cycle life. However, the low electrochemical activities and poor hydrophilicity of conventional carbon-based electrodes (e.g., carbon felt, CF) greatly restrict the further improvements of cell performance and commercial applications. Therefore, fabricating advanced electrodes is crucial to the progress and practical applications of VRFBs. In this work, the waste biomass of peanut shells (PS) is used as the raw material to produce carbon particles that coat on the surface of CF (PS/CF) via adsorption and carbonization procedures. The redox reactions of VO2+/VO2+ are remarkably promoted on the as-prepared PS/CF electrode. When PS/CF is employed as the positive electrode, the resulting VRFB cell shows energy efficiency of 87% over 500 cycles under 100 mA cm−2, which is significantly higher than the pristine CF-based VRFB cell (79%). Additionally, the cycling stability of PS/CF-based cell has been remarkably enhanced. This study demonstrates that the cost-effective and sustainable peanut-shell-derived carbon modified CF holds a broad application prospects in the practical applications of advanced VRFBs as energy storage systems.

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.

Tunable surface chemistry of carbon electrodes and the role of surface functionalities towards vanadium redox reactions

The surface chemistry of carbon electrodes plays a vital role towards the kinetics of vanadium redox reactions of vanadium redox flow batteries (VRFB). In this study, O2, CO2, N2, and NH3 plasmas have been employed to modify the surface chemistry of graphite felt electrodes, and the effect of various surface functional groups on negative and positive electrode reactions of VRFB has been explored. Various analytical techniques, electrochemical characterizations and asymmetric charge–discharge experiments reveal that the surface functional groups have a specific role towards vanadium redox reactions on each side of a VRFB cell. The C=O and O-C=O groups show catalytic effects while the C-O group has an inhibiting effect on the kinetics of negative and positive electrode reactions. Pyrrolic-N shows catalytic effects on the positive electrode reactions whereas oxidized-N has catalytic effects on the negative electrode reactions. Amine groups introduced by NH3 plasma worsen the sluggish kinetics of the negative electrode reactions by boosting hydrogen evolution reaction. Based on the specific behavior, an optimal arrangement of each plasma-treated felt has also been proposed by either utilizing it as negative electrode, positive electrode or both in a VRFB cell, leading to an overall improved performance.

BiVO4-Decorated Graphite Felt as Highly Efficient Negative Electrode for All-Vanadium Redox Flow Batteries

Recently, discovering high-performance electrocatalytic materials for vanadium redox flow batteries (VRFBs) has been one of the most crucial tasks. This paper details the successful fabrication of a low-cost platelike bismuth vanadate (BiVO4) material through a simple one-step hydrothermal route, employed as an electrocatalyst to adorn graphite felt (GF) for use as the negative electrode in VRFBs. The experimental results show that BiVO4-3h exhibits the optimal electrocatalytic activity and reversibility for the vanadium redox couples among all samples. The energy efficiency of the VRFB cell assembled with BiVO4-decorated GF as the negative electrode is found to be 75.42% at 100 mA cm–2, which is about 10.24% more efficient than that of the cell assembled with a heat-treated graphite felt (HT-GF) electrode. The possible reasons for the activity enhancement can be ascribed to the existence of oxygen vacancies in the BiVO4 lattice structure and the relatively high surface area of BiVO4, which provide more active sites for facilitating the vanadium redox reactions. Furthermore, the BiVO4-GF electrode obstructs the competitive irreversible hydrogen evolution reaction on the negative side of the cell, and it also has better wettability. Impressively, BiVO4-GF as the negative electrode shows good stability over 100 cycles.

High-performance ultrathin perfluorinated sulfonic acid membranes with thermo-morphology control for a vanadium redox flow battery

We demonstrate the morphology control of pre-aligned hydrophilic channels of ultrathin (∼30 nm) perfluorinated sulfonic acid membranes leading to enhancement of ion-selectivity and vanadium redox flow battery cell performance.

Phosphoric acid pre-treatment to tailor polybenzimidazole membranes for vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) use ion-selective membranes for transporting ionic species while separating the positive and negative electrolytes. In this paper, we report phosphoric acid doped polybenzimidazole (PBI) membranes that yield high ionic selectivity and conductivity in VRFBs. The phosphoric acid pre-treatment swells the PBI matrix irreversibly and increases its sulfuric acid doping in VRFB electrolytes. The pre-treated membranes show comparable area resistance with Nafion-212 with better selectivity towards vanadium ions. A low resistance was obtained with a reduced membrane thickness, which can reduce the overall cost of materials. The VRFB using an optimized phosphoric acid pre-treated PBI membrane demonstrates coulombic, voltage, and energy efficiencies of 99.7, 90.3, and 90.0%, respectively, at 40 mA cm−2 while achieving ∼40% higher discharge capacity compared to Nafion-212. Furthermore, the VRFB cell shows excellent cycling stability, i.e., only 6.8% or 0.068% per cycle capacity decay for 100 cycles while maintaining coulombic efficiency at >99.9% (98.5% coulombic efficiency for Nafion-212 with a capacity decay of 0.42% per cycle) proving the effectiveness of pre-treatment of phosphoric acid on PBI membranes.

Calibration-free coulometric sensors for operando electrolytes imbalance monitoring of vanadium redox flow battery

It is necessary for vanadium redox flow battery (VRFB) to become more cost-effective due to long-term stable operation with minimal life-cycle maintenance for its further development. Despite the absence of self-discharge in idle mode, the available capacity of VRFB gradually decreases during the battery cycling. The capacity decrease is attributed to side reactions appearance, which increase the average oxidation state (AOS) of vanadium electrolyte. This negative effect can be minimized by periodic electrolyte capacity recovery. However, it is crucial to have reliable information on the electrolytes composition to conduct rebalancing efficiently and timely. Various methods for vanadium electrolyte assessment have been previously proposed, however, none of them can be used coupled with industrial VRFBs. In this work, we present an operando coulometric analysis of an electrolyte sample in a VRFB cell and its subsequent return to the bulk solution. The proposed approach outperforms other methods due to sensitivity of the signal to the overall vanadium content and AOS and stable performance under imbalance conditions. The analysis of AOS was verified by three methods – coulomb-counting, ex-situ spectrophotometry, and coulometry. Therefore, this method can be successfully used not only in lab-scale VRFB studies, but can also be coupled with industrial VRFB stacks.

Polybenzimidazole Membrane for Vanadium Flow Battery Applications

The vanadium redox flow battery (VRFB) is one of the most promising secondary batteries as a large-capacity energy storage device for storing renewable energy.1 Perfluorinated-based ion exchange membranes such as Nafion are the most widely used membranes in VRFB due to their high proton conductivity. However, the extremely high cost and low ion selectivity of Nafion have limited their further application in VRFB. Among the aromatic hydrocarbon-based membranes that combine both high ion selectivity and high proton conductivity, polybenzimidazole- based membranes have served as one of the most promising alternatives to Nafion due to their excellent chemical and mechanical stability and low cost.2 In the present work m-PBI membranes of varying thickness and modified PBI were prepared, and their properties and VRFB performances were compared. The voltage efficiency (VE) increased with decreasing membrane thickness because of the decreasing ohmic resistance. The excellent current efficiency (CE) of VRFBs using meta-PBI based membranes is attributed to the negligible vanadium crossover observed in ex-situ permeability tests.The stability of the membranes after long term cycling is one of the most challenging problems in large-scale VRB applications. The durability of PBI membrane in VRFB cell testing and the capacity decay was measured by operating the cells for 100 cycles under a high current density of 200 mA cm−2. Fig 1: Flow battery efficiencies using electrodes: 2.5 mm SGL, CR = 20%, membrane: PBI 7 µm at current density of 200 mA/cm2, the electrolyte solution V(IV):V(III) (50:50) in 2.0 M H2SO4 and 0.05 M H3PO4 . The chemical stability of PBI membranes was analyzed after immersion of the PBI samples in various highly oxidizing and reducing sulfuric acid based vanadium and cerium solutions. The membranes stayed unchanged for more than 4 months. This result suggests that the PBI membranes demonstrate excellent ex-situ chemical stability in the harsh acid, oxidizing and reducing conditions.

Design of A Two-Stage Control Strategy of Vanadium Redox Flow Battery Energy Storage Systems for Grid Application

The low energy conversion efficiency of the vanadium redox flow battery (VRB) system poses a challenge to its practical applications in grid systems. The low efficiency is mainly due to the considerable overpotentials and parasitic losses in the VRB cells when supplying highly dynamic charging and discharging power for grid regulation. Apart from material and structural advancements, improvements in operating strategies are equally essential for achieving the expected high-performance VRB system, although an optimized solution has not been fully exploited in the existing studies. In this paper, a two-stage control strategy is thus developed based on a proposed and experimental validated multi-physics multi-time-scale electro-thermo-hydraulic VRB model. Specifically, in the first stage, the optimal flow rate of the VRB is obtained based on online optimization to reduce parasitic loss and enhance instantaneous system efficiency, and the result serves as the set point of a feedback flow rate controller. In the second stage, dual time scales are specifically considered. And the current and flow rate controllers are designed to meet the highly varying power demands for grid-connected applications. The effectiveness of the proposed control strategy is verified under a scenario to smooth wind power generation. Comparative studies show that compared to the prevailing approaches, higher efficiency can be achieved in tracking the theoretical optimal power profiles for online battery control.

Parameter identification of electrochemical model of vanadium redox battery by metaheuristic algorithms

AbstractAn accurate pre‐setting of the constant coefficients and parameters of the electrochemical model of the battery is essential for the accuracy of the model. The experimental methods are not precisely determined these parameters. In vanadium redox flow batteries (VRFB), like other types of batteries, the electrochemical model's coefficients vary by each battery cell design, different kinds of membranes, etc. Moreover, a VRFB cell's electrochemical model is highly nonlinear; thus, excellent optimization approach is needed to figure out these coefficients' optimal value. Some metaheuristic optimizers, such as particle swarm optimizer (PSO), grey wolf optimizer (GWO), genetic algorithm (GA), and the hybrid PSOGWO algorithms, are used in this study to identify these coefficients. An optimization framework is characterized to recognize the coefficients of the model by minimizing the mean square error between the measured values and the model‐based terminal voltage. The low RMS error of the modeled terminal voltage by the metaheuristic‐based electrochemical model demonstrates the accuracy of the proposed parameter identification approach for VRFBs. Further, the improved electrochemical model using the optimal coefficients is employed to estimate VRFB internal parameters, for example, the available battery capacity, the state of health, and the state of charge more accurately.

The Influence of Non-Charged Sidechains on the Performance of Meta-Polybenzimidazole Membranes in Vanadium Redox Flow Batteries

The global goal to combat climate change is growing year by year, as for example indicated by the latest climate conference by the United Nations, COP26. Hereby, one of the main goals concerns the transition from fossil fuels to more renewable energy means. In doing so, the importance of highly efficient energy storage technologies, such as supercapacitors, Li-ion batteries and/or redox flow batteries that both store and release energy on a short-, mid- and long-term scale, grows alongside it1. Among storage technologies, redox flow batteries (RFBs) are considered as promising candidates for mid-term storage (daily discharge for 2-12 h) due to their unique characteristic of a decoupled energy storage and power output2. To enable the incorporation of these systems into society further and thus smoothen the transition towards renewable energy technologies, highly efficient and low cost membranes in terms of efficiency and durability are needed, that will simultaneously improve the efficiency and reduce the cost of energy storage.One of the membrane types for vanadium redox flow battery (VRFB) applications that gained the interest of industry and the scientific community are of the polybenzimidazole (PBI) type. Although these materials possess excellent chemical stability and vanadium barrier properties, they suffer from a poor proton conductivity compared to commercial standards such as Nafion® 212. Several strategies were proposed to tackle this issue, ranging from the preparation of composite membranes with a thin PBI skin layer3, alkaline pretreatments that improve the swelling characteristics of the PBI film4 and chemical modifications to attach ionic conducting groups on the polymer backbone, thus directly improving the conductivity5. This latter approach showed that a highly conductive PBI membrane could be obtained by the addition of sidechains containing charged moieties such as sulfonic acid5. However, this improved conductivity comes at a price, specifically, a decrease in its vanadium barrier properties due to a significant crossover of vanadium ions, thus losing one of the main benefits of PBI type membranes5.The work presented herein describes the preparation of multiple PBI derivatives containing small and readily available non-charged sidechains, such as alkylated or benzylated PBI. Herein, it was envisaged that the addition of non-charged sidechains to the PBI backbone could increase the free volume of the membrane and thus enable the incorporation of electrolyte into the film without significantly affecting its repelling effect for vanadium ions. With this in mind, the influence of these non-charged sidechains on the ex-situ properties under VRFB conditions was studied in terms of dimensional swelling, acid uptake, vanadium barrier properties and mechanical strength. Hereby, key differences in the behavior of the membranes were observed between each individual type of functionalization, such as swelling, uptake and composition in acidic media. Subsequently, the correlation between the ex-situ properties of the PBI derivatives and in-situ VRFB cycling performance was studied to further understand their significance. Through this process, valuable knowledge was obtained that showed a clear correlation between the ex-situ properties and the observed in-situ area specific resistance, energy efficiency and capacity fading of the various materials, with the best material outperforming Nafion® 212 in terms of energy efficiency by 1% at a current density of 200 mA∙cm2, a current density regime that is normally dominated by cation exchange membranes due to their low resistance. Thereby proving that even a small modification of the PBI backbone can have a significant influence on the performance in a VRFB cell. Yuan, Z.; Duan, Y.; Zhang, H.; Li, X.; Zhang, H.; Vankelecom, I., Advanced porous membranes with ultra-high selectivity and stability for vanadium flow batteries. Energy & Environmental Science 2016, 9 (2), 441-447. Chae, I. S.; Luo, T.; Moon, G. H.; Ogieglo, W.; Kang, Y. S.; Wessling, M., Ultra-High Proton/Vanadium Selectivity for Hydrophobic Polymer Membranes with Intrinsic Nanopores for Redox Flow Battery. Advanced Energy Materials 2016, 6 (16), 1600517. Gubler, L.; Vonlanthen, D.; Schneider, A.; Oldenburg, F. J., Composite Membranes Containing a Porous Separator and a Polybenzimidazole Thin Film for Vanadium Redox Flow Batteries. Journal of The Electrochemical Society 2020, 167 (10), 100502. Noh, C.; Serhiichuk, D.; Malikah, N.; Kwon, Y.; Henkensmeier, D., Optimizing the performance of meta-polybenzimidazole membranes in vanadium redox flow batteries by adding an alkaline pre-swelling step. Chemical Engineering Journal 2020, 126574. Yan, X.; Dong, Z.; Di, M.; Hu, L.; Zhang, C.; Pan, Y.; Zhang, N.; Jiang, X.; Wu, X.; Wang, J.; He, G., A highly proton-conductive and vanadium-rejected long-side-chain sulfonated polybenzimidazole membrane for redox flow battery. Journal of Membrane Science 2020, 596, 117616.

Investigating the Role of Temperature on the Performance of Vanadium Redox Flow Batteries Using a Steady Validated Unit-Cell Model

Redox Flow Batteries (RFBs) are an evolved electrochemical energy storage technology crucial for the transition into a renewable energy future. Mainstream adoption of RFBs is subject to reduction of the capital and operation costs. In that sense, of particular importance is the optimization of the electrochemical stack, which affects the overall efficiency of the battery. Several studies have investigated the importance of thermal effects on the performance of vanadium redox flow batteries through transient non-isothermal models [1-2]. However, due to the simplified assumptions made by these authors some important temperature-dependent features are not included. Therefore, reliable and validated continuum models are also crucial to study the impact of the system temperature on all the fundamental physics in the electrochemical cells as well as to find the key aspects to optimize the stack.In this work, we present a 2D stationary model of a vanadium redox flow battery cell with a comprehensive and updated multiphysics description. The model was validated at a constant temperature of 25 ºC by polarization, conductivity, and open circuit voltage measurements. In a second step, the temperature was also included as a key operating variable in the model. Electrolyte properties, electrochemical rate constants, and H2SO4 dissociation equilibrium were described as a function of temperature as well as the state of charge (SoC) and the total vanadium concentration. The model is currently being validated through a second experimental campaign (see Fig. 1) conducted inside a climatic chamber imitating different environmental scenarios. The model can be used to explore the relevance of each phenomenon or element in the electrochemical stack and the influence of the operating conditions on them, e.g. temperature, state of charge and volumetric flow rate. By using the proposed model, we can elucidate the best temperature strategy to increase the performance of vanadium redox flow batteries in diverse operating scenarios.Figure 1. Open circuit voltage – SoC relation at different operating temperatures. References Al-Fetlawi, A.A. Shah, F.C. Walsh, Electrochimica Acta, 55, 78-89 (2009).Tang, S. Ting, J. Bao, M. Skyllas-Kazacos, Journal of Power Sources, 203, 165-176 (2012). Acknowledgments This work has been partially funded by the Agencia Estatal de Investigación (PID2019-106740RB-I00 and RTC-2017-5955-3/AEI/10.13039/501100011033). Figure 1

A coupled-layer ion-conducting membrane using composite ionomer and porous substrate for application to vanadium redox flow battery

A thin ionomer layer composed of perfluorosulfonic acid (PFSA) polymer in composite with functionalized alkoxysilane (FAS) is prepared and coated onto one side of a porous polyethylene (PE) substrate for vanadium redox flow battery (VRFB) to produce a coupled-layer membrane. The approaches are: (1) suppression of permeation through membrane using additive component, (2) the use of thin ionomer layer for lowering the amount of costly PFSA, and (3) strengthening the thin ionomer layer by coating onto an inexpensive PE substrate. The membrane is easily prepared using automatic control coater. The observation of morphology and infrared spectroscopy at both sides of the coupled-layer membrane confirm the thin ionomer layer on only one side of the PE substrate, plus the addition of FAS helps achieve lower diffusion coefficient. After the unit cell cycling, the coupled-layer membrane shows the good cycling performance closing to Nafion 212 membrane level. Additionally, the morphology after cycling of the coupled layers between PFSA ionomer and PE substrate can remain the same as in initial state, indicating a stable combination from both layers. Based on the easy production and the good performance, the developed membrane can be readily adapted by membrane manufacturer for the wide application in VRFB. • A thin ionomer layer is coated on one side of porous polyethylene substrate. • The thin layer reduces amount of costly perfluorinated sulfonic acid ionomer. • VRFB cell performance was good by closing to that of Nafion 212 membrane. • Unchanged morphology between two layer was found after VRFB unit cell operation.

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.

An all-extruded tubular vanadium redox flow cell - Characterization and model-based evaluation

The vanadium redox flow battery (VRFB) as one of the most promising electrochemical storage systems for stationary applications still needs further cost reductions. Tubular cell designs might reduce production costs by extrusion production of cell components and small sealing lengths. Based on a first study of the authors [1], this work demonstrates the feasibility of extruded tubular VRFB cells with high power density in the flow-by electrode configuration. Extruded cell components are the perfluorosulfonic acid cation exchange membrane with a diameter of 5.0 mm and carbon composite current collectors. The cell performance is experimentally characterized by polarization curve, ohmic resistance and galvanostatic cycling measurements. A maximum volumetric power density of 407 kW/m 3 and a maximum current density of 500 mA/cm 2 can be achieved. A non linear E cell / i -model is used to evaluate exchange and limiting current densities while in-situ half cell SoC monitoring is applied to evaluate the extruded membrane.