Flow Battery Research Articles

Design and Development of Flow Fields with Multiple Inlets or Outlets in Vanadium Redox Flow Batteries

In vanadium redox flow batteries, the flow field geometry plays a dramatic role on the distribution of the electrolyte and its design results from the trade-off between high battery performance and low pressure drops. In the literature, it was demonstrated that electrolyte permeation through the porous electrode is mainly regulated by pressure difference between adjacent channels, leading to the presence of under-the-rib fluxes. With the support of a 3D computational fluid dynamic model, this work presents two novel flow field geometries that are designed to tune the direction of the pressure gradients between channels in order to promote the under-the-rib fluxes mechanism. The first geometry is named Two Outlets and exploits the splitting of the electrolyte flow into two adjacent interdigitated layouts with the aim to give to the pressure gradient a more transverse direction with respect to the channels, raising the intensity of under-the-rib fluxes and making their distribution more uniform throughout the electrode area. The second geometry is named Four Inlets and presents four inlets located at the corners of the distributor, with an interdigitated-like layout radially oriented from each inlet to one single central outlet, with the concept of reducing the heterogeneity of the flow velocity within the electrode. Subsequently, flow fields performance is verified experimentally adopting a segmented hardware in symmetric cell configuration with positive electrolyte, which permits the measurement of local current distribution and local electrochemical impedance spectroscopy. Compared to a conventional interdigitated geometry, both the developed configurations permit a significant decrease in the pressure drops without any reduction in battery performance. In the Four Inlets flow field the pressure drop reduction is more evident (up to 50%) due to the lower electrolyte velocities in the feeding channels, while the Two Outlets configuration guarantees a more homogeneous current density distribution.

Highly ion-selective sulfonated poly (4,4′-diphenylether-5,5′-bibenzimidazole) membranes for vanadium redox flow battery

Renewable energy requires long-duration energy storage technologies, of which vanadium flow battery is regarded as the ­most suitable candidate for safe and long-duration energy storage ­applications. However, vanadium flow batteries still suffer from capacity decay and low power density, which arise from membrane degradation and high resistance. Herein, a sulfonated poly (4,4′-diphenylether-5,5′-bibenzimidazole) (SOPBI) membrane is reported for use in vanadium redox flow batteries. The prepared SOPBI membranes show an area resistance of 0.63 Ω cm2 at room temperature after doped with 3 M H2SO4. Moreover, membranes with 3 M H2SO4 doping exhibit tensile stress at break of 15.1 MPa. Benefiting from the SOPBI membrane, the all-vanadium redox flow battery delivers a superior energy efficiency of 77.7% at 140 mA cm−2 while operating stably at a current density of 120 mA cm−2 for 200 cycles.

In situ growth of CoO on MXene sheets for modification of all‑vanadium redox flow battery electrodes

In order to greatly improve the energy efficiency and electrolyte utilization efficiency of vanadium redox flow batteries (VRFBs) and to enable them to operate at high current densities, MXene/CoO composites were used for the first time in this research as electrocatalysts for the V3+/V2+ reaction. The catalytic effect of the materials was characterized by cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) tests. Then battery performance of MXene/CoO composite modified plasma-treated graphite felt (GF) negative electrodes (MXene/CoO/GF) were tested in a VRFB. The results show that MXene/CoO composites can improve the catalytic activity. There is a 22.93 % increase in energy efficiency and 124.94 % increase in electrolyte utilization efficiency when MXene/CoO/GF is applied as VRFB negative electrode at a current density of 200 mA cm−2 compared to GF. At a current density of 250 mA cm−2, the energy efficiency of MXene/CoO/GF is maintained at 75.55 %. The excellent performance is attributed to the addition of MXene/CoO, which increases the active sites for redox reactions and thus improves the kinetics of the V3+/V2+ reaction.

Measurement and modelling of liquid density (298.15 and 313.15 K) and vapour pressure osmometry (313.15 K) for binary aqueous solutions of organic salts

Due to the eco-friendliness of organic salts, they have been gradually replacing other electrolytes in different fields of chemistry. For example, they provide biocompatible salting-out agents for liquid–liquid extractions and earth-abundant materials for flow batteries. Nevertheless, reliable liquid density and vapour pressure data, which are paramount for a proper parameterisation of thermodynamic models, are often hard to find for aqueous solutions of organic salts, delaying the development of these technologies.In this work, the liquid density (ρ) of five binary aqueous solutions of organic salts (disodium tartrate, sodium potassium tartrate, dipotassium tartrate, trisodium citrate and tripotassium citrate) were measured and correlated at 298.15 and 313.15 K and 0.1 MPa. In these assays, second-degree polynomials provided determination coefficients (R2) larger than 0.9982 in the correlation of liquid density with salt molality, for which a very accurate description of this property was accomplished. Moreover, vapour pressure osmometry (VPO) studies were carried out for these solutions at 313.15 K and 0.1 MPa. Then, the obtained osmotic coefficients (ϕ) were successfully modelled using the Extended Pitzer Model of Archer, yielding standard deviations (σϕ) lower than 8.61·10-3. Finally, the mean molal activity coefficients (γ±) and excess Gibbs free energies (GE/RT) of the binary aqueous solutions of organic salts were effectively calculated, with potassium- and tartrate-based salts attaining significantly higher values than the ones composed of sodium and citrate, hinting at a more ideal solution behaviour of the former.

Nonlinear observer for online concentration estimation in vanadium flow batteries based on half-cell voltage measurements

This paper presents a nonlinear observer to estimate the active species concentrations in vanadium flow batteries. To conduct the estimation, the observer relies only on current, flow rate and two half-cell voltage measurements. In contrast to previous works in the field, the proposed observer is capable to deal simultaneously with two significant and challenging conditions: (1) a not necessarily high flow rate, which results in different concentrations for tanks and cells, and (2) presence of crossover and oxidation side reactions, that result in imbalance between the electrolytes on the positive and negative sides of the system. The stability and convergence of the observer are formally demonstrated using a Lyapunov analysis and subsequently validated through comprehensive computer simulations. Finally, utilising the information provided by the observer, a strategy to independently regulate the flow rate of each electrolyte based on their individual state of charge is developed.

Digitization of flow battery experimental process research and development

Rising atmospheric CO2 concentrations urgently call for advanced sustainable energy storage solutions, underlining the pivotal role of renewable energies. This perspective delves into the capabilities of redox flow batteries as potential grid storage contenders, highlighting their benefits over traditional lithium-ion batteries. While all-vanadium flow batteries have established themselves, concerns about vanadium availability have steered interest toward Organic Flow Batteries. The multifaceted nature of organic materials calls for an integrated approach combining artificial intelligence, robotics, and material science to enhance battery efficacy. The union of artificial intelligence and robotics expedites the research and development trajectory, encompassing everything from data assimilation to continuous refinement. With the burgeoning metaverse, a groundbreaking avenue for collaborative research emerges, potentially revolutionizing flow battery research and catalyzing the progression towards sustainable energy resolutions.

A green and cost-effective zinc-biphenol hybrid flow battery with eutectic electrolyte

Eutectic electrolytes have been widely used in redox flow batteries (RFBs) due to their unique features such as abundant availability, biodegradability, and low cost, as well as their excellent dissolution ability for redox-active substances. Despite these advantages, eutectic electrolytes still have some limitations in important aspects such as reaction kinetics, mass transport and ionic conductivity due to their relatively high intrinsic viscosity. In this work, we explore a green and cost-effective zinc-based eutectic electrolyte containing small molecules additives, which not only has elevated ionic conductivity, but also can dissolve organic cathode materials 3,3′,5,5′-tetramethylaminemethylene-4,4′-biphenol (TABP) at high concentrations. In this zinc-based eutectic electrolyte, TABP exhibits a highly positive redox potential (1.50 V versus Zn/Zn2+), quasi-reversible redox reaction kinetics and good redox stability. When operated in a practical hybrid flow battery, the Zn-TABP cell based on this eutectic electrolyte exhibits excellent rate performance, high capacity utilization, and low capacity decay rate (0.119 % per cycle). Post-analysis for the TABP catholyte suggests that the Michael side reaction rate of TABP in electrochemical cycling appears to depend upon its concentration.

Predictive power fluctuation mitigation in grid-connected PV systems with rapid response to EV charging stations

Currently, renewable energies and electric vehicle charging stations are essential for energy sustainability. However, the variable generation from renewable sources, such as photovoltaic systems, can lead to power peaks that impact the stability of the grid. This challenge is exacerbated by the increasing demand in fast-charging stations. Addressing these demand peaks is crucial to ensure the stability of the electrical grid. This paper introduces the predictive-flex smoother, an innovative method designed to mitigate power fluctuations in grid-connected photovoltaic systems while optimizing energy management in electric vehicle charging stations. The predictive-flex smoother method incorporates a hybrid energy storage system comprising supercapacitors and vanadium redox flow batteries to respond rapidly to electric vehicle charging stations demands, enhance grid electricity purchase optimization, and improve energy quality delivery. The proposed method integrates two control strategies: photovoltaic fluctuation reduction strategy and peak demand reduction strategy for electric vehicle charging stations. By leveraging prediction algorithms and machine learning techniques, the predictive-flex smoother method achieves precise power fluctuation forecasts, allowing efficient utilization of supercapacitors and vanadium redox flow batteries to smooth photovoltaic power fluctuations and reduce electrical vehicles peak demand. Comprehensive experimental investigations and simulations validate the method's performance under various operational conditions. The results demonstrate the effectiveness of the predictive-flex smoother method, significantly improving the quality of power delivered to the grid while reducing costs. The experimental platform, validates the real-time response of the proposed method, with response times under 500 ms. The experimental results further confirm the efficiency of the method in power smoothing and charging strategies with varying electrical vehicles models and connection coefficients.

Partial fluorinated silica incorporated with tuneable alkyl cross-linker based multi-cationic alkaline membrane for vanadium redox flow battery

Herein, we report partial fluorinated polymer (poly(vinylidene fluoride-co-hexafluoropropylene); PVDF-co-HFP)-based multi-cationic (quaternary ammonium, and imidazolium groups) cross-linked (tuneable carbon chain cross-linker) anion exchange membrane (AEM) incorporated with functionalized silica to avoid any structural deformations under harassed environment. Prepared membranes are cross-linked using different cross-linking agents, to study the effect of cross-linker chain length on the membrane performance. Incorporation of functionalized silica by acid-catalyzed sol-gel process improves membrane properties and stabilities. Reported PVIm-C10-APS AEM shows high conductivity (7.20 × 10−2 S/cm), and ion exchange capacity (1.48 meq/g). Well optimized positively charged PVIm-C10-APS AEM exhibits extremely low permeability of vanadium cations because of “Donnan exclusion” (9.80 × 10−7 cm2/s). During VRFB operation, PVIm-C10-APS AEM exhibits 98.10% coulombic efficiency (CE) and 77.60% voltage efficiency at 100 mA/cm2 without any significant deterioration up to 300 charge-discharge cycles. Further, PVIm-C10-APS membrane also shows >1.50 V OCV and 345 mW/cm2 peak power density at 400 mA/cm2. Reported AEM has been bench-marked with other membranes reported in the literature, and assessed as a promising material for VRFB or membrane separator for energy devices.

Improving the Sustainability of Electrochemical Direct Air Capture in a 3D Printed Redox Flow Cell

Improving the Sustainability of Electrochemical Direct Air Capture in a 3D Printed Redox Flow Cell

Carbon black-coated SPEEK membrane for efficient vanadium flow batteries

Ion exchange membrane is a key component of vanadium flow batteries (VFB). Development of low-cost and high ion selectivity membrane is essential to promote practical application of VFB. Here we report a kind of carbon-coated membrane by spraying coating Super P/polyvinylidene fluoride slurries on the surface of sulfonated poly(ether ether ketone) membrane (SPEEK/SP@PVDF). The SP attached to the membrane surface or infiltrated into the membrane matrix can synergistically promote the transport of protons while inhibiting the penetration of vanadium ions. In addition, the surface SP@PVDF layer can effectively shield the strong oxidizing VO2+ in the posolyte. As a result, the VFB with the optimized SPEEK/SP@PVDF membrane demonstrates a stable energy efficiency of >80 % at 200 mA cm−2 and a peak power density of 1073.79 mW cm−2 at 1300 mA cm−2, indicating the great potential of this carbon black-coated membrane for potential application in VFB.

High-performance SPEEK composite membrane with ultrahigh selectivity enabled by sulfonated PANI for vanadium flow battery

The sulfonated poly(ether ether ketone) (SPEEK) composite membranes (S/SPANI) are prepared by blending sulfonated polyaniline (SPANI), which was synthesized through a post-sulfonation method. With the SPANI contents varying from 1 to 4 wt%, the S/SPANI membranes exhibit excellent physicochemical properties and cell performance, and at loading 3 wt% SPANI, S/SPANI-3 presents a higher proton conductivity (43.3 mS cm−1) and superior ion selectivity (49 × 103 S min cm−3) against SPEEK and Nafion 212. Moreover, its outstanding coulombic efficiency (95.9–98.4%) and energy efficiency (EE: 80.4–70.2%) at the current density of 100–200 mA cm−2 demonstrate an optimized tradeoff of proton conductivity and vanadium ion permeability. The performance optimization guarantees enhanced durability for 600 cycles and stable EE (76%) at 150 mA cm−2, which originated from the acid-base interaction, synergistic proton network, and interconnected chain structure. So, this provides a facile method to develop high-performance membranes by introducing desirable controllable topological molecular chains to construct synergistic proton transportation networks.

Revealing the Effects of Impurities among Phosphoric Acid Electrolytes for the Stability of All-Vanadium Redox Flow Batteries

Revealing the Effects of Impurities among Phosphoric Acid Electrolytes for the Stability of All-Vanadium Redox Flow Batteries

A comparative study of chemically oxidized carbon cloth and thermally treated carbon paper electrodes applied on aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs) are emerging as promising candidates for efficient energy storage in grid-scale applications. In this study, we present a comprehensive analysis comparing the performance of thermally treated carbon paper and chemically oxidized carbon cloth electrodes, which have been extensively examined for their potential suitability in AORFBs. Results indicate that carbon cloth electrodes exhibit a higher concentration of oxygenated functional groups, contributing to enhanced electrochemical kinetics and improved electrochemical stability. These attributes translate to an AORFB with 62 times higher power density when compared to paper-based electrodes used. Through repeated charge-discharge cycles utilizing the BQDS║AQS model as a representative AORFB prototype, superior performance in the carbon cloth electrodes is consistently observed. Our findings even suggest the potential for optimizing the flow rate of the electrolyte to further enhance performance.

Multi-porous electrode derived from zeolitic imidazolate framework-8 coating through continuous two-stage heat treatment for vanadium redox flow batteries

Developing high-performance electrodes is imperative for improving the energy efficiency and power density of vanadium redox flow battery (VRFB). This study developed a multi-porous graphite felt (GF) electrode, comprising micropores, mesopores, and macropores. This was achieved through a continuous two-stage heat treatment based on zeolitic imidazolate framework-8 coating. Cyclic voltammetry and electrochemical impedance spectroscopy results confirmed the effect of the pore size on the electrochemical properties by comparing the single-scale pore structure and multi-porous structure. The electrochemical activities were enhanced both by micropores, which significantly increased the number of active sites, and by mesopores and macropores, which improved the accessibility of vanadium ions to the active sites. Consequently, the multi-porous electrode showed the highest electrochemical activity for vanadium ion redox reactions owing to the synergistic advantages conferred by the presence of different pore sizes. The VRFB with the multi-porous electrode exhibited the highest energy efficiency (70.75 %) at a current density of 200 mA cm−2, which was 12.07 % higher than that of Bare GF. Furthermore, it exhibited a specific discharge capacity of 13.51 Ah L−1 at a current density of 250 mA cm−2. The VRFB with multi-porous electrodes showed stable operation for 3300 cycles without a significant decrease in efficiency or modifications to the pore structure. The proposed method can be used to effectively fabricate high-performance multi-porous electrodes applicable to VRFBs and other energy storage systems.

Regulated adsorption capability by Interface–Electric–Field enabling promoted electrochemical kinetics of Zinc–Bromine flow batteries

Zinc–bromine flow batteries (ZBFB) are gaining significant attention for large–scale energy storage due to the high energy density and affordable cost. Nevertheless, the diffusion of Br2 and the relatively low activity of Br2/Br− impede their broader application. Herein, a bi–phasic rutile/anatase–TiO2 embedded in carbon composite with hollow sphere structure (R/A–TiO2@HCS) is proposed as the catalyst for the Br2/Br− electrode. The internal electric field (IEF) formed between rutile–TiO2 and anatase–TiO2 regulates the adsorption capability for active bromine species. The appropriate adsorption capability can rapidly transfer the generated Br2 complex (QBrn) to the adsorbent component R/A–TiO2 during charging, liberating the carbon active sites, and release QBrn back to the carbon reaction sites timely during discharging, satisfying the both adsorption and desorption requirements of QBrn in different processes. The kinetics for oxidation and reduction are simultaneously enhanced. Moreover, the hollow construction offers storage space for QBrn, relieving its penetration problem. Under the synergistical cooperation, the ZBFB assembled with R/A–TiO2@HCS demonstrates the energy efficiency (EE) of 83.6 % at 80 mA cm−2, without significant decay over 500 cycles. And the EE still maintains 70.0 % even under 160 mA cm−2. This work highlights the significance of tailored electrocatalyst for ZBFB.

Aqueous dispersions of highly conductive carbon nanomaterials: Unusual aggregation mechanism

Determination of the optimal conductive composition and operational flow conditions are of profound significance for aqueous–based semisolid flow battery and supercapacitor applications. We systematically investigated the development of equilibrium microstructure and flow–induced structural transitions in aqueous dispersions of carbon black (CB) in 1 M LiNO3 at 25 °C. Simultaneous measurements of the impedance spectra and optical texture distinguished two unique percolated dispersions with distinct microstructure and aggregation mechanisms around a critical CB content: CCB = 4 wt%. A tenuous network of weakly–linked aggregates with rapid diffusion–limited aggregation mechanism (DLA) is formed at CCB < 4 wt%. A compact network of strongly fused aggregates with slow reaction–limited aggregation mechanism (RLA) dominates the dispersion microstructure above the critical CCB. The percolated dispersions commonly exhibited a three–regime (two thinning separated by thickening regime) flow curve consistent with structural breaking up (conductivity loss) and alignment of flowing units (conductivity recovery) at a critical shear rate that pertain to CCB.

Relating Solvent Parameters to Electrochemical Properties to Predict the Electrochemical Performance of Vanadium Acetylacetonate for Non-Aqueous Redox Flow Batteries

Non-aqueous redox flow batteries have shown promise for applications in grid energy storage. Increasing the efficiency of these batteries by developing the electrolyte chemistries is needed. Herein, we investigate the correlation between solvent properties and the electrochemical parameters of vanadium acetylacetonate V(acac)3. Using cyclic voltammetry (CV) and rotating disk electrode experiments (RDE), we show that trends in the performance of the V(acac)3 kinetics are directly related to solvent properties. We found strong relationships between the solvents polarity, viscosity, and donor number with the electrochemical behavior of V(acac)3 in terms of the electrochemical working widow, electron kinetics and stability towards cycling. Based on these finding, we also demonstrate how solvent selection can be improved with limited a priori knowledge.

Quantifying effect of faradaic imbalance and crossover on capacity fade of vanadium redox flow battery

In this work, we aimed to reveal two main contributors to the capacity fade of a vanadium redox flow battery (VRFB). These contributors are the oxidative imbalance caused by the hydrogen evolution reaction (HER) competing with V3+ reduction during charging, and crossover, particularly the net transfer of vanadium ions from negolyte to posolyte. To investigate this, we performed VRFB cycling under various operation conditions with sequential monitoring of the electrolytes composition. We discovered that shortly after cycling starts, the crossover makes the negolyte capacity-limiting. This leads to high polarization of the negative electrode inducing HER and increasing average oxidation state (AOS) of the electrolyte. Our examination shows that the magnitude of the oxidative imbalance correlates with that of the crossover, both being dependent on the state-of-charge (SoC). Therefore, by changing operation conditions, we can slightly influence the crossover, while dramatically affect the oxidative imbalance. We also found that the resulting magnitude of the capacity fade is a trade-off between these two side-processes. This necessitates careful optimization of the battery and electrolyte composition, along with its cycling regime. From the data obtained, we conclude that the only proper approach to achieve lower capacity fade is by ensuring that the negolyte is capacity-limiting over long-term cycling. We hope that the data on the capacity fade mechanisms presented here will facilitate development of more stable and cost-effective VRFBs.

Recent Progress in Modular Electrochemical Synthesis of Hydrogen and High-Value-added Chemicals based on Solid Redox Mediator.

Electrochemical synthesis of H2 and high-value-added chemicals is an efficient and cost-effective approach that can be powered using renewable electricity. Compared to a conventional electrochemical production system, the modular electrochemical production system (MEPS) based on a solid redox mediator (SRM) can separate the anodic and cathodic reactions in time and space. The MEPS can avoid the use of membranes and formation of useless products, as well as eliminate the mutual dependence of production rates at anode and cathode. The SRM can temporarily store or release electrons and ions to pair with cathodic and anodic reactions, respectively, in MEPS. Designing of SRMs with large charge capacity and good cyclability is of great significance for constructing a high-performance MEPS. This work summarizes the design principles, recent advances in MEPS based on SRM, and application in redox flow cells. Moreover, structure design strategies as well as in situ characterization techniques and theoretical calculations for SRM is also proposed. It is expected to promote the vigorous development of MEPS based on SRM. Finally, the challenges and perspectives of MEPS based on SRM are discussed.