Flow Battery Research Articles

Enhanced Vanadium Redox Flow Battery Performance with New Amphoteric Ion Exchange Membranes.

Vanadium redox flow batteries (VRFBs) depend on the separator membrane for their efficiency and cycle life. Herein, two amphoteric ion exchange membranes are synthesized, based on sulfonic acid group-grafted poly(p-terphenyl piperidinium), for VRFBs. Using ether-free poly(p-terphenyl piperidine) (PTP) as the polymer matrix, and sodium 2-bromoethanesulphonate (ES) and 1,4-butane sultone (BS) as grafting agents, We achieve quaternization of PTP through an environmentally friendly process without alkaline catalysts. PTP-ES and PTP-BS membranes exhibit low area resistance, high H+ permeability, and significantly reduced vanadium ion permeability, leading to exceptional ion selectivity, which is 3.06×106Smincm-3 and 4.34×106Smincm-3, respectively,three orders of magnitude higher than that of Nafion115 (0.27×104Smincm-3). The VRFB with PTP-BS achieves a self-discharge duration of 190h, compared to 86h for Nafion 115. Additionally, under current densities of 40-160mAcm-2, PTP-BS shows coulombic efficiencies of 98.1-99.1% and energy efficiencies of 92.0-82.1%, outperforming Nafion 115. The VRFB with PTP-BS also demonstrates excellent cycle stability and discharge capacity retention over 300 cycles at 100mAcm-2. Therefore, the amphoteric PTP-BS membrane shows remarkable performance, offering significant potential for VRFB applications.

Pd2+-coordinated polybenzimidazole membranes with fast and selective ion transport for alkaline aqueous organic redox flow battery

The advancement of aqueous organic redox-flow batteries (AORFBs) is impeded by the lack of efficient, low-resistance, and highly selective ion-conducting membranes (ICMs). Although polybenzimidazole (PBI) is one of the most promising low-cost non-fluorinated ion-conducting membranes, it remains challenging to design stable PBI membranes with fast and selective ion transport channels. Here, we engineer the chemical structure of PBI and regulate the ion transport channels to prepare highly conductive and selective Pd2+-coordinated membranes with grafting and crosslinking structures. In this design, the positively charged quaternary ammonium groups on the side chain improve the conductivity of OH−, while the continuous cross-linked network formed by ionic bonds between quaternary ammonium groups and deprotonated imidazole groups significantly enhances the membrane's mechanical properties. Furthermore, the coordination of Pd2+ with PBI and plasticization of PBI chain by trifluoroacetate anions expand the molecular free volume while inducing local contraction. Consequently, the QPBI-xPBI-Pd membrane enables fast charge carrier transport and suppresses the diffusion of active substances. The AORFBs assembled with the QPBI-10PBI-Pd membranes exhibit high coulombic efficiency (99.6 %) and energy efficiency (77.67 %) at 100 mA cm−2, maintaining excellent stability for 500 cycles. This study provides an innovative strategy to design high-performance PBI membranes for RFBs, thereby advancing the viability of RFBs in the realm of large-scale energy storage technologies.

Composite Modified Graphite Felt Anode for Iron–Chromium Redox Flow Battery

The iron–chromium redox flow battery (ICRFB) has a wide range of applications in the field of new energy storage due to its low cost and environmental protection. Graphite felt (GF) is often used as the electrode. However, the hydrophilicity and electrochemical activity of GF are poor, and its reaction reversibility to Cr3+/Cr2+ is worse than Fe2+/Fe3+, which leads to the hydrogen evolution side reaction of the negative electrode and affects the efficiency of the battery. In this study, the optimal composite modified GF (Bi-Bio-GF-O) electrode was prepared by using the optimal pomelo peel powder modified GF (Bio-GF-O) as the matrix and further introducing Bi3+. The electrochemical performance and material characterization of the modified electrode were analyzed. In addition, using Bio-GF-O as the positive electrode and Bi-Bio-GF-O as the negative electrode, the high efficiency of ICRFB is realized, and the capacity attenuation is minimal. When the current density is 100 mA·cm−2, after 100 cycles, the coulomb efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) were 97.83%, 85.21%, and 83.36%, respectively. In this paper, the use of pomelo peel powder and Bi3+ composite modified GF not only promotes the electrochemical performance and reaction reversibility of the negative electrode but also improves the performance of ICRFB. Moreover, the cost of the method is controllable, and the process is simple.

Turning Adversity into Advantage: Investigating the Capacity Decay Mode of Carboxylate Functionalized-Anthraquinone in Organic Redox Flow Batteries

Turning Adversity into Advantage: Investigating the Capacity Decay Mode of Carboxylate Functionalized-Anthraquinone in Organic Redox Flow Batteries

Cation Engineering Perovskite Cathodes for Fast and Stable Anion Redox Chemistry in Zinc‐Iodine Batteries

AbstractZinc‐halogen batteries are promising for sustainable energy storage, offering high redox capacities at economical price points. However, they are hindered by issues such as the irreversibility caused by the dissolution of intermediates and the sluggish charge transfer, which limit their widespread adoption. Addressing these issues, bismuth‐iodide perovskite cathodes are employed as a halogen element enriched model system. These perovskite cathodes demonstrate considerable anion redox capacities. In a combined simulation and experimental study, it is uncovered that the (BAD)BiI4 (BAD+ denotes benzamidinium) cathode, greatly improves interactions with iodine species and boosts charge transfer capability compared to its (BA)BiI4 (BA+ denotes benzylaminium) counterpart. These enhancements can be attributed to the synergistic effects arising from stronger Bi−I···I halogen bonds and C═N─H···I hydrogen bonds. The (BAD)BiI4 cathode attains a reversible I−/I0 redox chemistry with 92% capacity retention after 30 000 cycles at a current density of 10 A g−1I, outperforming previously reported anion redox batteries. Additionally, the defect‐tolerant property and the I−/I0/I+ conversion of the (BAD)BiI4 are elucidated. The I5− forms a notably stronger bond with (BAD)BiI4 in comparison to I3−, which effectively mitigates polyiodide shuttling. These advantageous characteristics highlight the promise and adaptability of the developed perovskite cathodes for high‐performance anion redox chemistry.

High durable SPEEK/TiO2 nanopaper composite membrane for vanadium redox flow battery

Sulfonated poly (ether ether ketone) (SPEEK) ion exchange membranes for VRFB are promising alternatives to Nafion, but require improved mechanical and chemical stability for long-term operation. Here, we have fabricated composite membranes using SPEEK as proton conductive medium and TiO2 nanopapers as reinforcing framework to improve the mechanical and chemical stabilities of SPEEK membranes. The SPEEK/TiO2 nanopaper composite membranes exhibited almost twice the tensile strength and only one-third the vanadium ion permeability compared to pristine SPEEK (DS=60 %). Due to the excellent cell performance such as high EE, slow capacity degradation and long-term lifetime, these high durable composite membranes could be found their potential use as ion exchange membranes for commercial VRFBs.

Quantifying concentration distributions in redox flow batteries with neutron radiography

The continued advancement of electrochemical technologies requires an increasingly detailed understanding of the microscopic processes that control their performance, inspiring the development of new multi-modal diagnostic techniques. Here, we introduce a neutron imaging approach to enable the quantification of spatial and temporal variations in species concentrations within an operating redox flow cell. Specifically, we leverage the high attenuation of redox-active organic materials (high hydrogen content) and supporting electrolytes (boron-containing) in solution and perform subtractive neutron imaging of active species and supporting electrolyte. To resolve the concentration profiles across the electrodes, we employ an in-plane imaging configuration and correlate the concentration profiles to cell performance with polarization experiments under different operating conditions. Finally, we use time-of-flight neutron imaging to deconvolute concentrations of active species and supporting electrolyte during operation. Using this approach, we evaluate the influence of cell polarity, voltage bias and flow rate on the concentration distribution within the flow cell and correlate these with the macroscopic performance, thus obtaining an unprecedented level of insight into reactive mass transport. Ultimately, this diagnostic technique can be applied to a range of (electro)chemical technologies and may accelerate the development of new materials and reactor designs.

Self-Standing Covalent Organic Polymer Membrane with High Stability and Enhanced Ion-Sieving Effect for Flow Battery.

Fabrication of ion-conducting membranes with continuous sub-nanometer channels holds fundamental importance for flow batteries in achieving safe integration of renewable energy into grids. Self-standing covalent organic polymer (COP) membranes provide feasibility due to their rapid and selective ion transport. However, the development of a scale-up possible, mechanically robust and chemically stable membranes remains a significant challenge. Herein, using irreversible strong secondary amine linkage, we propose a self-standing COP membrane with sub-nanometer pores ranging from 4.5 to 6.4 Å, by a simple and efficient in-situ polymerization approach. This membrane exhibits enhanced selectivity for proton and vanadium ions, especially excellent electrochemical stability, delivering an energy efficiency of over 80% at the current density of 200 mA cm-2 over 1000 cycles for an all-vanadium redox flow battery (VFB). This study provides novel insights for COP-based ion-sieving membranes in sustainable energy fields.

Effect of Hydrogen Pressure on The Mass Transfer Characteristics of Hydrogen-Bromine Flow Battery Electrodes

The mass transfer characteristics of porous carbon electrodes in the liquid side of a hydrogen bromine redox flow battery (H2-Br2 RFB) were investigated under compressive deformation caused by operation at elevated hydrogen pressure. Here, flow cell measurements of permeability and micro-particle image velocimetry (μPIV), alongside electrochemical measurements of capacitance and battery discharge were used to characterize changes in the liquid side electrode compression, in-plane liquid flow, accessible surface area, polarization, and mass transfer scaling brought by hydrogen pressure. We studied two electrode types with different structures, carbon paper and carbon cloth, in untreated well as heat-treated forms in the pressure range 0–8 bar H2. It was found that pressure-induced compression of the liquid side electrode increases the accessible area of untreated electrodes, with little effect on heat-treated electrodes, but decreases the electrochemical performance of the battery in all cases by increasing the ohmic resistance of the cell and decreasing the mass transfer coefficient of the porous electrode. Overall, heat treatment is shown to affect the rigidity, saturation behavior, and generalized mass transfer of paper electrodes but not of cloth electrodes. Our findings will guide the selection of electrode materials and operation parameters for the H2-Br2 RFB.

Maximizing flow battery membrane performance via pseudo-nanophase separation enhanced by polymer supramolecular sidechain

Pseudo-nanophase separation, enabled by noncovalently grafted sidechains, offers a promising approach for constructing high-performance membranes, featuring rapid ion transport, robust chemical stability, and simplified manufacturing. However, striking a balance between ionic conductivity and mechanical/chemical stability proves challenging since excessive hydrophilic grafting leads to overswelling and compromised integrity of the membranes, rendering them unsuitable for demanding applications like vanadium redox flow batteries (VRFBs). In this study, we describe a new approach for achieving high-performance VRFB membranes via employing polymer as supramolecular sidechains, rather than small molecules. This strategy achieves remarkable pseudo-nanophase separation while minimizing the utilization of functional (hydrophilic) sites. As a result, the resulting membranes exhibit exceptional robustness and proton conductivity, with an extraordinarily low area resistance of merely 0.11 Ω cm2, thus circumventing the prevailing trade-off between ionic conductivity and mechanical/chemical stability. Ultimately, VRFBs integrated with these membranes achieve energy efficiencies up to 80 % even at high current densities of 240 mA cm−2, accompanied by a remarkably low capacity decay rate of 0.064 % per cycle during long-cycle tests. This work not only achieves ultra-high conductivity with minimal functional groups, but also advances pseudo-nanophase separation strategies and provides valuable insights into optimized utilization of limited functional groups in membrane design.

Asymmetric porous polybenzimidazole membrane with tunable morphology for vanadium flow battery (VFB)

Polybenzimidazole (PBI) membranes have attracted much attention for flow batteries due to their good chemical stability. However, the high area resistance (AR) of dense PBI limits its further development in VFBs. Herein, we design a sintering method to remove the phthalates with different molecular structures in the ternary system (PBI as polymer, N-methylpyrrolidone as solvent, and phthalates as non-solvent). Asymmetric PBI membranes with adjustable and disconnected pores, as well as ultrathin-dense layers, are prepared. This series of porous membranes shows excellent overall performance in VFBs. Among them, the porous NPBI-DBP-200 membrane (0.139 Ω cm2) with DBP as the non-solvent exhibits a lower AR compared to the dense NPBI (0.489 Ω cm2). At a current density of 200 mA cm−2, the energy efficiency (EE) of a single cell assembled with the NPBI-DBP-200 membrane (EE = 78.26 %) has improved by 26.37 % compared to the dense NPBI membrane (EE = 61.93 %). In addition, the porous NPBI-DBP-200 membrane has excellent in-situ stability, and the single cell can operate stably for 1800 cycles at the current density of 120 mA cm−2. This work presents a novel, easy-to-use technique for preparing high-performance porous PBI membranes with adjustable pore morphology.

Redox Activity Modulation in Extended Fluorenone-Based Flow Battery Electrolytes with π-π Stacking Effect

Redox flow battery shows promise for grid-scale energy storage. Aqueous organic redox flow batteries are particularly popular due to their potentially low material cost and safe water-based electrolyte. Commonly, redox active molecules used in this field feature aromatic rings, and increasing π-aromatic conjugation has been a popular strategy to achieve high energy density, high power density, and reduced crossover in new material design. However, this approach can inadvertently hinder redox activity depending on redox mechanism. This study reveals the underlying π-π stacking effect in extended aromatic redox active compounds, where aromatic radical intermediates are involved in the redox process. We report a molecular design strategy to mitigate the negative effect of π-π stacking by altering solvation dynamics and introducing molecular steric hindrance.

Mitigation of Dendrite Growth in Zinc-iodide Flow Battery with Tröger’s Base Anion Exchange Membrane

Tröger’s base anion exchange membrane (TB-AEM) was readily prepared by condensation polymerization of biphenyl diamine and dimethoxymethane in the presence of trifluoroacetic acid followed by quaternization with methyl iodide. The film cast from N-Methyl-2-Pyrrolidone (NMP) solvent displayed good mechanical strength, a tensile modulus of 1.18 GPa with elongation at break of 17%, and a glass transition temperature (Tg) at 248 °C. It exhibited OH− ion conductivity of 108 mS cm−1 by impedance measurement at 80 °C in 1M KOH. The membrane exhibited good affinity toward I2, resulting in the formation of I2Br− ions in the membrane matrix. Over 300 charge/discharge cycles at a 50 mA cm−2 current density, the battery exhibited 95.5% Coulombic efficiency (CE), 76.4% voltage efficiency (VE), and 74.0% energy efficiency (EE) and delivered a capacity of 24.8 Ah L−1. Over a span of 60 h, the open-circuit voltage (OCV) of the cell remained constant at 1.2 V. Collectively, our findings suggest that the anion exchange membrane's charge and porosity tuning are key factors in the design of new generation separators for zinc-iodide flow batteries.

Scanning Electrochemical Microscopy Meets Optical Microscopy: Probing the Local Paths of Charge Transfer Operando in Booster-Microparticles for Flow Batteries.

Understanding the oxidation/reduction dynamics of secondary microparticles formed from agglomerated nanoscale primary particles is crucial for advancing electrochemical energy storage technologies. In this study, the behavior of individual copper hexacyanoferrate (CuHCF) microparticles is explored at both global and local scales combining scanning electrochemical microscopy (SECM), for electrochemical interrogation of a single, but global-scale microparticle, and optical microscopy monitoring to obtain a higher resolution dynamic image of the local electrochemistry within the same particle. Chronoamperometric experiments unveil a multistep oxidation/reduction process with varying dynamics. On the one hand, the global SECM analysis enables quantifying the charge transfer as well as its dynamics at the single microparticle level during the oxidation/reduction cycles by a redox mediator in solution. These conditions allow mimicking the charge storage processes in these particles when they are used as solid boosters in redox flow batteries. On the other hand, optical imaging with sub-particle resolution allows the mapping of local conversion rates and state-of-charge within individual CuHCF particles. These maps reveal that regions of different material loadings exhibit varying charge storage capacities and conversion rates. The findings highlight the significance of porous nanostructures and provide valuable insights for designing more efficient energy storage materials.

A High Current Density and Long Cycle Life Iron Chromium Redox Flow Battery Electrolyte

A High Current Density and Long Cycle Life Iron Chromium Redox Flow Battery Electrolyte

A High Discharge Power Density Single Cell of Hydrogen–Vanadium Flow Battery

A High Discharge Power Density Single Cell of Hydrogen–Vanadium Flow Battery

Numerical and experimental study on fractal flow field for improving the performance of non-aqueous redox flow battery

The flow field design of the non-aqueous redox flow battery (RFB) decides the electrolyte flow and mass transfer of active species inside porous electrode, which has a significant influence on the operation of RFB. In this work, a fractal tree-like flow channel is proposed to enhance the electrolyte convection and optimize the concentration distribution of active species inside the graphite felt electrode, thus boosting the overall battery performance. Using the finite element simulation method, a three-dimensional numerical model of single cell is established to comparatively study the steady-state discharge process characteristics of deep eutectic solvent (DES) electrolyte-based vanadium‑iron RFBs with fractal and serpentine flow fields, respectively. Moreover, by assembling the single cell of this RFB and building the full cell test system, an experimental study is conducted to compare the discharging output voltage and power of the RFBs with serpentine flow field and fractal flow fields of different fractal dimensions. The numerical results show that the fractal tree-like flow field can improve the discharging power of RFB as well as reduce the pumping loss compared with the conventional serpentine flow field. That is mainly attributed to that fractal flow field is conducive to reducing the polarization loss and strengthening the convection in porous electrode simultaneously. What's more, the role of fractal dimension of flow field is also investigated numerically, thus knowing that large fractal dimension promotes lower pumping loss while small fractal dimension is in favor of higher output voltage. In addition, the experimental results also demonstrate that the fractal flow field could improve the output voltage and maximum discharging power of RFB compared to serpentine flow field. This work provides an optimization method for the flow field design of DES electrolyte-based vanadium‑iron RFB, and is expected to be employed in the other non-aqueous RFBs.

Techno-economic analysis of a novel solar-based polygeneration system integrated with vanadium redox flow battery and thermal energy storage considering robust source-load response

In this study, a novel solar-based polygeneration system incorporated with a partially covered parabolic trough photovoltaic thermal (PCPVPVT) collector, vanadium redox flow battery (VRFB), thermal energy storage, and absorption chiller/heat pump is proposed. A robust energy management strategy is developed to alter the operating states of the system to respond to the inevitable intermittent source inputs and varying demands throughout the year. Furthermore, a comprehensive comparison of the techno-economic performance differences between the novel system and the Power to Hydrogen to Power (P2H2P) combined cooling, heating, and power (CCHP) system is carried out. In addition, the sensitivity analysis of the key technical and external economic parameters as well as a multi-objective optimization are investigated. The results illustrate that: the charging, discharging, and round-trip efficiencies of the VRFB unit reach 88.99%, 88.23%, and 78.52%, respectively, and the current of the VRFB has significant effects on the termination voltage, charge and discharge time, efficiencies, and stored and output energy of the VRFB. The PV coverage ratio of the PCPVPVT and the capacity of the VRFB unit considerably affect the system's thermodynamic and economic performance. The exergy efficiency of the proposed system is always greater than that of the P2H2P CCHP system because the efficiency of the VRFB with 78.52% is significantly higher than that of the P2H2P electricity storage with 41.73%, and the maximum difference reaches 4.25%. The levelized cost of energy of the present system is 0.0503 $/kWh, which is less than that of the P2H2P CCHP system by approximately 6.85%, indicating superior economic strengths.

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.

High‐stable all‐iron redox flow battery with innovative anolyte based on steric hindrance regulation

High‐stable all‐iron redox flow battery with innovative anolyte based on steric hindrance regulation