Ion conducting membranes for aqueous flow battery systems

The Critical Analysis of Membranes toward Sustainable and Efficient Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFB) are a promising technology for large-scale storage of electrical energy, combining safety, high capacity, ease of scalability, and prolonged durability; features which have triggered their early commercial implementation. Furthering the deployment of VRFB technologies requires addressing challenges associated to a pivotal component: the membrane. Examples include vanadium crossover, insufficient conductivity, escalated costs, and sustainability concerns related to the widespread adoption of perfluoroalkyl-based membranes, e.g., perfluorosulfonic acid (PFSA). Herein, recent advances in high-performance and sustainable membranes for VRFB, offering insights into prospective research directions to overcome these challenges, are reviewed. The analysis reveals the disparities and trade-offs between performance advances enabled by PFSA membranes and composites, and the lack of sustainability in their final applications. The potential of PFSA-free membranes and present strategies to enhance their performance are discussed. This study delves into vital membrane parameters to enhance battery performance, suggesting protocols and design strategies to achieve high-performance and sustainable VRFB membranes.

Starch-mediated colloidal chemistry for highly reversible zinc-based polyiodide redox flow batteries

Aqueous Zn-I flow batteries utilizing low-cost porous membranes are promising candidates for high-power-density large-scale energy storage. However, capacity loss and low Coulombic efficiency resulting from polyiodide cross-over hinder the grid-level battery performance. Here, we develop colloidal chemistry for iodine-starch catholytes, endowing enlarged-sized active materials by strong chemisorption-induced colloidal aggregation. The size-sieving effect effectively suppresses polyiodide cross-over, enabling the utilization of porous membranes with high ionic conductivity. The developed flow battery achieves a high-power density of 42 mW cm-2 at 37.5 mA cm-2 with a Coulombic efficiency of over 98% and prolonged cycling for 200 cycles at 32.4 Ah L-1posolyte (50% state of charge), even at 50 °C. Furthermore, the scaled-up flow battery module integrating with photovoltaic packs demonstrates practical renewable energy storage capabilities. Cost analysis reveals a 14.3 times reduction in the installed cost due to the applicability of cheap porous membranes, indicating its potential competitiveness for grid energy storage.

Sulfonated Poly(2,6-dimethyl-1,4-phenylene ether)-Modified Mixed-Matrix Bifunctional Polyelectrolyte Membranes for Long-Run Anthrarufin-Based Redox Flow Batteries

The resurgence in designing polyelectrolyte membrane (PEM) materials has propound grid-scale electrochemical energy storage devices. Herein, we report on studies corroborating the synergistic influence of ionic domain microstructure modification and intercalation of telechelic bis-piperidinium-functionalized graphene oxide (GO) to fabricate stable bifunctional membranes from sulfonated poly(2,6-dimethyl-1,4-phenylene ether) (sPPE) for efficient anthrarufin-based alkaline redox flow batteries. A critically long-lasting quest on alkaline stability and -OH conductivity dilemma in hydrocarbon-based PEMs is meticulously resolved via a bifunctional ion-conducting matrix. Preferential studies on hydrophilic domain distribution in sPPE suggest that, with high microphase homogeneity, higher specific capacity retentions are achievable during galvanostatic charge-discharge (GCD) analysis. Moreover, the low-capacity issues were overcome by improving the redoxolyte-membrane interface affinities incorporating bis-piperidinium-bearing graphene oxide (bis-QGO). Consequently, at 1.0 and 2.0 wt % intercalation of bis-QGO, the bifunctional polyelectrolyte membranes (BFPMs) impart lowest overpotentials of 93 mV (for BFPM-1.0) and ∼100 mV (for BFPM-2.0) which are ∼43 and 40% lower than that of Nafion-117 (i.e., ∼164 mV). Furthermore, the efficiency of BFPMs, viz., the Coulombic, voltage, and energy efficiencies, was ∼95-98%, ∼85%, and ≥80% at 20 mA cm-2, respectively. In long-cycling operations, the GCD profile evidenced ∼99% efficiency retention over 450 cycles and illustrated reproducible rate capability. Finally, the polarization studies of BFPMs revealed ∼54% higher peak power density (87.5 mW cm-2) delivery than Nafion-117 (∼57 mW cm-2). We believe that this strategic designing approach could offer newer and simple avenues to avail high-performance BFPMs at low intercalation loads for alkaline electrochemical energy storage and related applications.

Development of flow battery technologies using the principles of sustainable chemistry

Realizing decarbonization and sustainable energy supply by the integration of variable renewable energies has become an important direction for energy development. Flow batteries (FBs) are currently one of the most promising technologies for large-scale energy storage. This review aims to provide a comprehensive analysis of the state-of-the-art progress in FBs from the new perspectives of technological and environmental sustainability, thus guiding the future development of FB technologies. More importantly, we evaluate the current situation and future development of key materials with key aspects of green economy and decarbonization to promote sustainable development and improve the novel energy framework. Finally, we present an analysis of the current challenges and prospects on how to effectively construct low-carbon and sustainable FB materials in the future.

High Ion Conductive and Selective Membrane Achieved through Dual Ion Conducting Mechanisms

Conventional ion-selective membranes, that is ion-exchange and porous membranes, are unable to perform high conductivity and selectivity simultaneously due to the contradictions between their ion selecting and conducting mechanisms. In this work, a bifunctional ion-selective layer is developed via the combination of nanoporous boron nitride (PBN) and ion exchange groups from Nafion to achieve high ion conductivity through dual ion conducting mechanisms as well as high ion selectivity. A template-free method is adopted to synthesize flake-like PBN, which is further enmeshed with Nafion resin to form the bifunctional layer coated onto a porous polyetherimide membrane. The double-layer membrane exhibits excellent ion selectivity (1.49 × 10⁸ mS cm^-3  min), which is 22 times greater than that of the pristine porous polyetherimide membrane, with outstanding ion conductivity (64 mS cm^-1 ). In a vanadium flow battery, the double-layer membrane achieves a high Coulombic efficiency of 97% and outstanding energy efficiency of 91% at 40 mA cm^-2 with a stable cycling performance for over 700 cycles at 100 mA cm^-2 . PBN with ion exchange groups may therefore offer a potential solution to the limitation between ion selectivity and conductivity in ion-selective membranes.

Fabricating a Partially Fluorinated Hybrid Cation-Exchange Membrane for Long Durable Performance of Vanadium Redox Flow Batteries

The long-term durability of vanadium redox flow batteries (VRFBs) depends on the stability and performance of the membrane separator. We have architected a hybrid membrane by uniform dispersion of MIL-101(Cr) (Cr-MOF) in a partially fluorinated polymer grafted with sulfonic acid groups (PHP@AMPSCr-MOF(1.0)). The single cell VRFB performance of the PHP@AMPSCr-MOF(1.0) membrane was studied in comparison with the Cr-MOF incorporated Nafion membrane (NafionCr-MOF(1.0)) and showed an excellent result with 97.5% Coulombic efficiency (CE) at 150 mA/cm2 without any significant deterioration in the charge-discharge process for 1500 cycles (over 650 h). Meanwhile, the CE value of the NafionCr-MOF membrane (94.5%) deteriorated after 800 cycles (about 360 h) under similar conditions. The high VRFB performance of the PHP@AMPSCr-MOF(1.0) membrane has been attributed to the synergized properties and good interactions between Cr-MOF and partially fluorinated polymer matrix responsible for the creation of hydrophilic proton-conducting channels to achieve high selectivity. Furthermore, the cost-effective polymer and thus membranes may open new windows for practical applications in other energy devices such as fuel cells, electrolysis, and water treatment.

Application of Poly(ether sulfone)-Based Membranes in Clean Energy Technology

Poly(ether sulfone) (PES) is a kind of polymer materials with excellent electrical insulation and acid/alkali stability. PES can be operated at high temperature continuously for a long time and still maintain excellent property stability in the environments with rapidly changed temperature, namely, great thermostability. Moreover, PES has low molding shrinkage, good dimensional stability and excellent film-forming characteristics. Compared with inorganic membranes, PES-based membranes have lower cost, which have received more attention and wide recognition in the field of clean energy technologies in recent years, such as flow batteries, fuel cells, water treatment, and gas separation. Therefore, this review summarizes the research status and prospect of the utilization of PES-based membranes in clean energy fields, in order to further promote their development and application.

Recent Advances in the Unconventional Design of Electrochemical Energy Storage and Conversion Devices

As the world works to move away from traditional energy sources, effective efficient energy storage devices have become a key factor for success. The emergence of unconventional electrochemical energy storage devices, including hybrid batteries, hybrid redox flow cells and bacterial batteries, is part of the solution. These alternative electrochemical cell configurations provide materials and operating condition flexibility while offering high-energy conversion efficiency and modularity of design-to-design devices. The power of these diverse devices ranges from a few milliwatts to several megawatts. Manufacturing durable electronic and point-of-care devices is possible due to the development of all-solid-state batteries with efficient electrodes for long cycling and high energy density. New batteries made of earth-abundant metal ions are approaching the capacity of lithium-ion batteries. Costs are being reduced with the advent of flow batteries with engineered redox molecules for high energy density and membrane-free power generating electrochemical cells, which utilize liquid dynamics and interfaces (solid, liquid, and gaseous) for electrolyte separation. These batteries support electrode regeneration strategies for chemical and bio-batteries reducing battery energy costs. Other batteries have different benefits, e.g., carbon-neutral Li-CO2 batteries consume CO2 and generate power, offering dual-purpose energy storage and carbon sequestration. This work considers the recent technological advances of energy storage devices. Their transition from conventional to unconventional battery designs is examined to identify operational flexibilities, overall energy storage/conversion efficiency and application compatibility. Finally, a list of facilities for large-scale deployment of major electrochemical energy storage routes is provided.

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Emerging chemistries and molecular designs for flow batteries

Redox flow batteries are a critical technology for large-scale energy storage, offering the promising characteristics of high scalability, design flexibility and decoupled energy and power. In recent years, they have attracted extensive research interest, with significant advances in relevant materials chemistry, performance metrics and characterization. The emerging concepts of hybrid battery design, redox-targeting strategy, photoelectrode integration and organic redox-active materials present new chemistries for cost-effective and sustainable energy storage systems. This Review summarizes the recent development of next-generation redox flow batteries, providing a critical overview of the emerging redox chemistries of active materials from inorganics to organics. We discuss electrochemical characterizations and critical performance assessment considering the intrinsic properties of the active materials and the mechanisms that lead to degradation of energy storage capacity. In particular, we highlight the importance of advanced spectroscopic analysis and computational studies in enabling understanding of relevant mechanisms. We also outline the technical requirements for rational design of innovative materials and electrolytes to stimulate more exciting research and present the prospect of this field from aspects of both fundamental science and practical applications.

Redox Targeting of Energy Materials for Energy Storage and Conversion

The redox-targeting (RT) process or redox-mediated process, which provides great operation flexibility in circumventing the constraints intrinsically posed by the conventional electrochemical systems, is intriguing for various energy storage and conversion applications. Implementation of the RT reactions in redox-flow cells, which involves a close-loop electrochemical-chemical cycle between an electrolyte-borne redox mediator and an energy storage or conversion material, not only boosts the energy density of flow battery system, but also offers a versatile research platform applied to a wide variety of chemistries for different applications. Here, the recent progress of RT-based energy storage and conversion systems is summarized and great versatility of RT processes for various energy-related applications is demonstrated, particularly for large-scale energy storage, spatially decoupled water electrolysis, electrolytic N₂ reduction, thermal-to-electrical conversion, spent battery material recycling, and more. The working principle, materials aspects, and factors dictating the operation are highlighted to reveal the critical roles of RT reactions for each application. In addition, the challenges lying ahead for deployment are stated and recommendations for addressing these constraints are provided. It is anticipated that the RT concept of energy materials will provide important implications and eventually offer a credible solution for advanced large-scale energy storage and conversion.

Technologies and perspectives for achieving carbon neutrality

Global development has been heavily reliant on the overexploitation of natural resources since the Industrial Revolution. With the extensive use of fossil fuels, deforestation, and other forms of land-use change, anthropogenic activities have contributed to the ever-increasing concentrations of greenhouse gases (GHGs) in the atmosphere, causing global climate change. In response to the worsening global climate change, achieving carbon neutrality by 2050 is the most pressing task on the planet. To this end, it is of utmost importance and a significant challenge to reform the current production systems to reduce GHG emissions and promote the capture of CO2 from the atmosphere. Herein, we review innovative technologies that offer solutions achieving carbon (C) neutrality and sustainable development, including those for renewable energy production, food system transformation, waste valorization, C sink conservation, and C-negative manufacturing. The wealth of knowledge disseminated in this review could inspire the global community and drive the further development of innovative technologies to mitigate climate change and sustainably support human activities.

Pristine and Modified Porous Membranes for Zinc Slurry-Air Flow Battery

The membrane is a crucial component of Zn slurry–air flow battery since it provides ionic conductivity between the electrodes while avoiding the mixing of the two compartments. Herein, six commercial membranes (Cellophane™ 350PØØ, Zirfon^®, Fumatech^® PBI, Celgard^® 3501, 3401 and 5550) were first characterized in terms of electrolyte uptake, ion conductivity and zincate ion crossover, and tested in Zn slurry–air flow battery. The peak power density of the battery employing the membranes was found to depend on the in-situ cell resistance. Among them, the cell using Celgard^® 3501 membrane, with in-situ area resistance of 2 Ω cm² at room temperature displayed the highest peak power density (90 mW cm⁻²). However, due to the porous nature of most of these membranes, a significant crossover of zincate ions was observed. To address this issue, an ion-selective ionomer containing modified poly(phenylene oxide) (PPO) and N-spirocyclic quaternary ammonium monomer was coated on a Celgard^® 3501 membrane and crosslinked via UV irradiation (PPO-3.45 + 3501). Moreover, commercial FAA-3 solutions (FAA, Fumatech) were coated for comparison purpose. The successful impregnation of the membrane with the anion-exchange polymers was confirmed by SEM, FTIR and Hg porosimetry. The PPO-3.45 + 3501 membrane exhibited 18 times lower zincate ions crossover compared to that of the pristine membrane (5.2 × 10⁻¹³ vs. 9.2 × 10⁻¹² m² s⁻¹). With low zincate ions crossover and a peak power density of 66 mW cm⁻², the prepared membrane is a suitable candidate for rechargeable Zn slurry–air flow batteries.

Trust is good, control is better: a review on monitoring and characterization techniques for flow battery electrolytes

Flow batteries (FBs) currently are one of the most promising large-scale energy storage technologies for energy grids with a large share of renewable electricity generation. Among the main technological challenges for the economic operation of a large-scale battery technology is its calendar lifetime, which ideally has to cover a few decades without significant loss of performance. This requirement can only be met if the key parameters representing the performance losses of the system are continuously monitored and optimized during the operation. Nearly all performance parameters of a FB are related to the two electrolytes as the electrochemical storage media and we therefore focus on them in this review. We first survey the literature on the available characterization methods for the key FB electrolyte parameters. Based on these, we comprehensively review the currently available approaches for assessing the most important electrolyte state variables: the state-of-charge (SOC) and the state-of-health (SOH). We furthermore discuss how monitoring and operation strategies are commonly implemented as online tools to optimize the electrolyte performance and recover lost battery capacity as well as how their automation is realized via battery management systems (BMSs). Our key findings on the current state of this research field are finally highlighted and the potential for further progress is identified.

A Chemistry and Microstructure Perspective on Ion-Conducting Membranes for Redox Flow Batteries

Redox flow batteries (RFBs) are among the most promising grid-scale energy storage technologies. However, the development of RFBs with high round-trip efficiency, high rate capability, and long cycle life for practical applications is highly restricted by the lack of appropriate ion-conducting membranes. Promising RFB membranes should separate positive and negative species completely and conduct balancing ions smoothly. Specific systems must meet additional requirements, such as high chemical stability in corrosive electrolytes, good resistance to organic solvents in nonaqueous systems, and excellent mechanical strength and flexibility. These rigorous requirements put high demands on the membrane design, essentially the chemistry and microstructure associated with ion transport channels. In this Review, we summarize the design rationale of recently reported RFB membranes at the molecular level, with an emphasis on new chemistry, novel microstructures, and innovative fabrication strategies. Future challenges and potential research opportunities within this field are also discussed.

Polymer Membranes for All-Vanadium Redox Flow Batteries: A Review

Redox flow batteries such as the all-vanadium redox flow battery (VRFB) are a technical solution for storing fluctuating renewable energies on a large scale. The optimization of cells regarding performance, cycle stability as well as cost reduction are the main areas of research which aim to enable more environmentally friendly energy conversion, especially for stationary applications. As a critical component of the electrochemical cell, the membrane influences battery performance, cycle stability, initial investment and maintenance costs. This review provides an overview about flow-battery targeted membranes in the past years (1995-2020). More than 200 membrane samples are sorted into fluoro-carbons, hydro-carbons or N-heterocycles according to the basic polymer used. Furthermore, the common description in membrane technology regarding the membrane structure is applied, whereby the samples are categorized as dense homogeneous, dense heterogeneous, symmetrical or asymmetrically porous. Moreover, these properties as well as the efficiencies achieved from VRFB cycling tests are discussed, e.g., membrane samples of fluoro-carbons, hydro-carbons and N-heterocycles as a function of current density. Membrane properties taken into consideration include membrane thickness, ion-exchange capacity, water uptake and vanadium-ion diffusion. The data on cycle stability and costs of commercial membranes, as well as membrane developments, are compared. Overall, this investigation shows that dense anion-exchange membranes (AEM) and N-heterocycle-based membranes, especially poly(benzimidazole) (PBI) membranes, are suitable for VRFB requiring low self-discharge. Symmetric and asymmetric porous membranes, as well as cation-exchange membranes (CEM) enable VRFB operation at high current densities. Amphoteric ion-exchange membranes (AIEM) and dense heterogeneous CEM are the choice for operation mode with the highest energy efficiency.

Effect of Electrolyte Additives on the Water Transfer Behavior for Alkaline Zinc-Iron Flow Batteries

Alkaline zinc-iron flow batteries (AZIFBs) are a very promising candidate for electrochemical energy storage. The electrolyte plays an important role in determining the energy density and reliability of a battery. The substantial water migration through a membrane during cycling is one of the critical issues that affect the reliability and performance of an AZIFB. In this work, it has been proven that the reason for water migration in AZIFBs is the synergetic effect of concentration gradient, different ionic strengths of negolyte and posolyte, and the electric field. To address the issue of water migration in AZIFBs, a series of additives are employed and the effects of additives on the water transfer behavior and electrochemical performance of AZIFBs are investigated in detail. The results indicate that all investigated additives can suppress water migration through a polybenzimidazole membrane because of the shrunken gap of osmotic pressure and ionic strength between negolyte and posolyte. Moreover, organic additives such as glucose can decrease battery performance because of the increased polarizability of the electrode, whereas inorganic additives such as Na₂SO₄ demonstrate no distinct effect on battery performance. Specifically, an AZIFB that employs Na₂SO₄ as an additive in the negative electrolyte can afford a Coulombic efficiency of ∼99% and a voltage efficiency of ∼88% for 120 cycles at 80 mA cm^-2, together with a good effect for inhibiting water migration behavior. This paper presents an effective way to suppress water migration and increase the reliability of AZIFBs.

Porous Membrane with High Selectivity for Alkaline Quinone-Based Flow Batteries

Aqueous organic-based flow batteries are increasingly receiving attention owing to their appealing traits of high safety and low cost. An economic and high-performance membrane is always regarded as the heart of the batteries. Here, we introduce a cost-effective, homemade porous membrane with high performance for alkaline quinone-based flow batteries. The membrane is constituted by highly stable poly(ether sulfone) (PES) and sulfonated poly(ether ether ketone) (SPEEK) that serves dual functions of (1) adjusting the membrane microstructure and (2) endowing the membrane with a charge trait. Benefiting from the well-tuned structure and charge property of the membrane, a high ion selectivity and transport of OH^- with much higher mobility serving as the primary charge-balancing ion can be realized. By employing alkaline alizarin red (ARS)/ferro-ferricyanide flow battery as the platform, a battery delivers a coulombic efficiency (CE) of 98.28% and an energy efficiency (EE) of 85.81% at 40 mA cm^-2, which is higher than that of the battery with a Nafion 212 membrane (CE ∼ 99.19%, EE ∼ 84.60%), however, with much lower cost. The successful application of homemade porous membrane may provide a new strategy to engineer and fabricate membranes with high efficiency for alkaline quinone-based flow batteries and further decrease the batteries' cost.

A Cost-effective Nafion Composite Membrane as an Effective Vanadium-Ion Barrier for Vanadium Redox Flow Batteries

Ion exchange membranes play a key role in all vanadium redox flow batteries (VRFBs). The mostly available commercial membrane for VRFBs is Nafion. However, its disadvantages, such as high cost and severe vanadium-ion permeation, become obstacles for large-scale energy storage. It is thus crucial to develop an efficient membrane with low permeability of vanadium ions and low cost to promote commercial applications of VRFBs. In this study, graphene oxide (GO) has been employed as an additive to the Nafion 212 matrix and a composite membrane named rN212/GO obtained. The thickness of rN212/GO has been reduced to only 41 μm (compared with 50 μm Nafion 212), which indicates directly lower cost. Meanwhile, rN212/GO shows lower permeability of vanadium ions and area-specific resistance compared to the Nafion 212 membrane due to the abundant oxygen-containing functional groups of GO additives. The VRFB cells with the rN212/GO membrane show higher Coulombic efficiencies and lower capacity decay than those of VRFB cells with the Nafion 212 membrane. Therefore, the cost-effective rN212/GO composite membrane is a promising alternative to suppress migration of vanadium ions across the membrane to set up VRFB cells with better performances.

A Comparative Review of Electrolytes for Organic-Material-Based Energy-Storage Devices Employing Solid Electrodes and Redox Fluids

Electrolyte chemistry is critical for any energy-storage device. Low-cost and sustainable rechargeable batteries based on organic redox-active materials are of great interest to tackle resource and performance limitations of current batteries with metal-based active materials. Organic active materials can be used not only as solid electrodes in the classic lithium-ion battery (LIB) setup, but also as redox fluids in redox-flow batteries (RFBs). Accordingly, they have suitability for mobile and stationary applications, respectively. Herein, different types of electrolytes, recent advances for designing better performing electrolytes, and remaining scientific challenges are discussed and summarized. Due to different configurations and requirements between LIBs and RFBs, the similarities and differences for choosing suitable electrolytes are discussed. Both general and specific strategies for promoting the utilization of organic active materials are covered.

Pore engineering of graphene aerogels for vanadium redox flow batteries

The all-vanadium redox flow battery is considered to be a dispersive and non-perennial energy source due to its grid reliability, high efficiency, standalone modular design, and excellent cycling stability. However, the large vanadium ionic size and relatively high viscosity lead to poor compatibility with most carbon-based microporous electrodes, resulting in sluggish mass diffusion and unsatisfied capacitance retention. Herein, a novel cross-coupled porous graphene aerogel is proposed via the NaNO3-template pore engineering strategy. The microscopic observations and N2 adsorption-desorption isotherms validate the successful regulation of the surface area and porous structure, with the addition of different porogen contents (6.25-25 g L-1). The vanadium redox flow battery delivers a specific capacity of 163.4 mA h g-1 (5.6 A h L-1) at a current density of 25 mA cm-2, surpassing most previously reported batteries with a similar reactor volume. This method holds great promise for the better design and preparation of porous electrodes, and potential suitable applications.

Stable and Highly Ion-Selective Membrane Made from Cellulose Nanocrystals for Aqueous Redox Flow Batteries

The design of chemically stable ion-exchange membranes with high selectivity for applications in an aqueous redox flow battery (RFB) at high acid concentrations remains a significant challenge. Herein, this study designed a stable and highly ion-selective membrane by utilizing proton conductive cellulose nanocrystals (CNCs) incorporated in a semicrystalline hydrophobic poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) matrix. The high hydrophobicity of the PVDF-HFP matrix mitigates crossover of the electrolytes, whereas the abundant and low-cost CNCs derived from wood provide high proton conductivity. The fundamental contributors for CNCs' excellent proton conductivity are the hydroxyl (-OH) functional groups, highly acidic sulfonate (-SO₃H) functional groups, and the extensive intramolecular hydrogen bonding network. In addition, CNCs exhibit a mechanically and chemically stable structure in the harsh acidic electrolyte attributed to the high crystallinity (crystalline index of ∼86%). Therefore, because of the high proton conductivity, excellent ion selectivity, high chemical stability, and structural robustness, the vanadium redox flow battery (VRFB) assembled with the homogeneous CNCs and PVDF-HFP (CNC/PVDF-HFP) membrane achieved a Coulombic efficiency (CE) of 98.2%, energy efficiency (EE) of 88.2%, and a stable cycling performance for more than 650 cycles at a current density of 100 mA cm^-2. The obtained membrane possesses excellent flexibility, high mechanical tensile strength, and superior selectivity. Meanwhile, the applied casting method is scalable for large-scale manufacturing.

Advanced Materials for Zinc-Based Flow Battery: Development and Challenge

Zinc-based flow batteries (ZFBs) are well suitable for stationary energy storage applications because of their high energy density and low-cost advantages. Nevertheless, their wide application is still confronted with challenges, which are mainly from advanced materials. Therefore, research on advanced materials for ZFBs in terms of electrodes, membranes, and electrolytes as well as their chemistries are of the utmost importance. Herein, the focus is on the scientific understandings of the fundamental design of these advanced materials and their chemistries in relation to the battery performance. The principles of using different materials in different ZFB technologies, the functions and structure of the materials, and further material improvements are discussed in detail. Finally, the challenges and prospects of ZFBs are summarized as well. This review provides valuable instruction on how to design and develop new materials as well as new chemistries for ZFBs.

Amphoteric-Side-Chain-Functionalized "Ether-Free" Poly(arylene piperidinium) Membrane for Advanced Redox Flow Battery

To solve the stability issue of cost-effective nonfluorinated membranes, an ether-free poly(arylene piperidinium) (PBPip)-based membrane is first applied in redox flow batteries (RFBs). For improved efficiencies of RFB, amphoteric side chains are introduced onto the PBPip. Without an ether bond in the polymer backbone, the membrane shows a good stability in a strong oxidation environment. The Fourier transform infrared (FTIR) spectra exhibit no obvious changes over 30 days of oxidation test. Different from traditional blended amphoteric membranes, the amphoteric side chain allows both cation- and anion-exchange capacities to increase with grafting degree, which leads to a very high total ion-exchange capacity (IEC) (4.19 mmol g^-1). Outstanding ion-conduction ability (area resistance: 0.22 Ω cm²) comparable to Nafion 212 (0.24 Ω cm²) is consequently achieved. Ionic cross-linking structure between cationic and anionic groups results in a low swelling rate (13.9%). Combined with the repelling effect of positively charged piperidinium, a low VO²⁺ permeability (1.31 × 10^-8 cm² s^-1) is accomplished. On the basis of these good properties, the membrane exhibits excellent vanadium battery performances, especially at high current densities. The VE and EE both exceed 80% even at 200 mA cm^-2. The battery performances have no obvious reductions after 500 cycles. These results indicate that this work provides a new orientation to design the membrane for RFB.

Revisiting the positive roles of liquid polysulfides in alkali metal-sulfur electrochemistry: from electrolyte additives to active catholyte

Polysulfide dissolution and shuttling in liquid organic electrolytes are considered as the most challenging detrimental effects of an Li-S cell, which is one of the most promising next-generation high-energy-density batteries. Therefore, considerable efforts have been devoted to confining solid sulfur or sulfide so as to avoid the formation and diffusion of dissolved polysulfides. However, the positive roles played by the liquid polysulfides in Li-S electrochemistry should not be overlooked. Polysulfide dissolution can promote the cell kinetics and sulfur utilization; as electrolyte additives, polysulfides can help stabilize the Li metal anode, redistribute the active mass in the cathode and act as extra back-up active sulfur sources. After being applied directly as active catholytes, a novel Li-polysulfide redox flow battery (Li-PS RFB) and an Li-polysulfide battery (Li-PS battery) have been developed. This review revisited these beneficial effects of polysulfides and provided a summary of the recent progress on Li-PS RFB and Li-PS batteries, especially with a more comprehensive emphasis on the latter. Furthermore, dissolved polysulfides applied as active catholytes in Na-S and K-S systems and as catholytes or anolytes in aqueous batteries were also briefly discussed. Hopefully, the Li-S electrochemistry can be better understood so as to overcome challenging issues in the way of the practical commercialization of the Li-S batteries.

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

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