A non-ionic membrane with high performance for alkaline zinc-iron flow battery

Redox flow batteries as energy storage systems: materials, viability, and industrial applications

The rapid development and implementation of large-scale energy storage systems represents a critical response to the increasing integration of intermittent renewable energy sources, such as solar and wind, into the global energy grid. Redox flow batteries (RFBs) have emerged as a promising solution for large-scale energy storage due to their inherent advantages, including modularity, scalability, and the decoupling of energy capacity from power output. These attributes make RFBs particularly well-suited for addressing the challenges of fluctuating renewable energy sources. Several redox couples have been investigated for use in RFBs, some of which have already achieved commercialization. However, advancement in RFBs technology faces significant hurdles spanning scientific, engineering, and economic domains. Key challenges include limited energy density, high overall costs, electrolyte instability, and issues related to solvent migration across cation exchange membranes, leading to cross-contamination between anolyte and catholyte. Additionally, anion exchange membranes introduce reverse flow complications, and graphite felt used in the catholyte compartment is susceptible to corrosion. These issues necessitate ongoing research to develop viable solutions. This comprehensive review provides an in-depth analysis of recent progress in electrolyte technologies, highlighting improvements in electrochemical performance, stability, and durability, as well as strategies to enhance the energy and power densities of RFBs. Moreover, it classifies various three-dimensional (3D) electrode materials, including foam, biomass, and electrospun fibers, and examines how their structural and compositional modifications can facilitate improved mass transport and increase active sites for redox reactions in vanadium redox flow batteries (VRFBs). By exploring innovative electrode designs and functional enhancements, this review seeks to advance the conceptualization and practical application of 3D electrodes to optimize RFB performance for large-scale energy storage solutions.

Sulfonated Styrene-Grafted Polyvinylidene Fluoride Copolymers for Proton Exchange Membranes for AQDS/Bromine Redox Flow Batteries

This study presents the preparation and electrochemical testing of sulfonated styrene-grafted poly(vinylidene fluoride) (pVDF) copolymers as proton exchange membranes (PEMs) for semi-organic redox flow batteries (RFBs) based on 9,10-anthraquinone-2,7-disulfonic acid (AQDS)/bromine. The copolymers are synthesized via a two-step procedure, involving i) atom transfer radical polymerization of styrene (Sty) for the grafting to the pVDF backbone and ii) the sulfonation of the polystyrene grafted side chains. Copolymers with different amounts of sulfonated styrene (SSty) in the side chains (i.e., degree of sulfonation (DS)) are obtained and used for the preparation of PEMs by solution casting. The PEMs are characterized to assess their thermal, mechanical, water absorption, and ion exchange properties, to evaluate the effect of DS on membrane properties, and to select the membrane with the best overall performance for application in single cell tests. Electrochemical testing reveals that the pVDF-g-(Sty26-co-SSty14) membrane exhibits low crossover of redox species, favorable ohmic resistance, and energy efficiency. Results from single cell tests, as compared with commercial benchmarks such as Nafion 115 and Aquivion E87-12s, highlight the potential of such copolymers as alternative membranes for RFBs.

Next-Generation Ultrathin Lightweight Electrode for pH-Universal Aqueous Flow Batteries

Aqueous flow batteries (AFBs) are promising long-duration energy storage system owing to intrinsic safety, inherent scalability, and ultralong cycle life. However, due to the thicker (3-5 mm) and heavier (300-600 g m-2) nature, the current used graphite felt (GF) electrodes still limit the volume/weight power density of AFBs. Herein, a lightweight (≈50 g m-2) and ultrathin (≈0.3 mm) carbon microtube electrode (CME) is proposed derived from a scalable one-step carbonization of commercial cotton cloth. The unique loose woven structure composed of carbon microtube endows CME with excellent conductivity, abundant active sites, and enhanced electrolyte transport performance, thereby significantly reducing polarization in working AFBs. As a consequence, CME demonstrates excellent cycling performance in pH-universal AFBs, including acidic vanadium flow battery (maximum power density of 632.2 mW cm-2), neutral Zn-I2 flow battery (750 cycles with average Coulombic efficiency of 99.6%), and alkaline Zn-Fe flow battery (energy efficiency over 70% at 200 mA cm-2). More importantly, the estimated price of CME is only 5% of GF (≈3 vs ≈60 $ m-2). Therefore, it is reasonably anticipated that the lightweight and ultrathin CME may emerge as the next generation electrode for AFBs.

A Universal Coulombic Efficiency Compensation Strategy for Zinc-Based Flow Batteries

Alkaline zinc-iron flow batteries (AZIFBs) are well suited for energy storage because of their good safety, high cell voltage, and low cost. However, the occurrence of irreversible anodic parasitic reactions results in a diminished coulombic efficiency (CE), unbalanced charge state of catholyte/anolyte and subsequently, a poor cycling performance. Here, a universal CE compensation strategy centered around the oxygen evolution reaction (OER) on the cathodic side, is reported. This strategy aims to equalize the charge state of the [Fe(CN)6]3-/4--based catholyte and counteract pH fluctuations. The OER process can be implemented either directly on the electrode through electrochemical reaction or in an external catalytic reactor column via a redox-mediated process. This innovative approach effectively mitigates the gradual accumulation of [Fe(CN)6]3- in discharged catholyte and [Zn(OH)4]2- in charged anolyte by consuming the extra OH- during a continuous cycling process. As a result, AZIFBs demonstrate exceptional cycling performance with an extremely low capacity fading rate of 0.0128%/day (or 0.0005%/cycle) over 600 cycles at 80% state of charge (SOC). The proposed CE compensation strategy not only provides an effective way to address the CE loss issue for AZIFBs, but also can be applied to diverse battery technologies encountering CE loss caused by water/oxygen-induced parasitic reactions.

Porous Ceramic Metal-Based Flow Battery Composite Membrane

In metal-based flow battery, membranes significantly impact energy conversion efficiency and security. Unfortunately, damages to the membrane occur due to gradual accumulation of metal dendrites, causing short circuits and shortening cycle life. Herein, we developed a rigid hierarchical porous ceramic flow battery composite membrane with a sub-10-nm-thick polyelectrolyte coating to achieve high ion selectivity and conductivity, to restrain dendrite, and to realize long cycle life and high areal capacity. An aqueous zinc-iron flow battery prepared using this membrane achieved an outstanding energy efficiency of >80%, exhibiting excellent long-term stability (over 1000 h) and extremely high areal capacity (260 mAh cm-2). Low-field nuclear magnetic resonance (NMR) spectroscopy, small-angle X-ray scattering, in situ infrared spectroscopy, solid-state NMR analysis, and nano-computed tomography revealed that the rigid hierarchical pore structures and numerous hydrogen bonding networks in the membrane contributed to the stable operation and superior battery performance. This study contributes to the development of next-generation metal-based flow battery membranes for energy and power generation.

Low-cost Zinc-Iron Flow Batteries for Long-Term and Large-Scale Energy Storage

Aqueous flow batteries are considered very suitable for large-scale energy storage due to their high safety, long cycle life, and independent design of power and capacity. Especially, zinc-iron flow batteries have significant advantages such as low price, non-toxicity, and stability compared with other aqueous flow batteries. Significant technological progress has been made in zinc-iron flow batteries in recent years. Numerous energy storage power stations have been built worldwide using zinc-iron flow battery technology. This review first introduces the developing history. Then, we summarize the critical problems and the recent development of zinc-iron flow batteries from electrode materials and structures, membranes manufacture, electrolyte modification, and stack and system application. Finally, we forecast the development direction of the zinc-iron flow battery technology for large-scale energy storage.

Life cycle thinking and safe-and-sustainable-by-design approaches for the battery innovation landscape

Developments in battery technology are essential for the energy transition and need to follow the framework for safe-and-sustainable-by-design (SSbD) materials, chemicals, products, and processes as set by the EU. SSbD is a broad approach that ensures that chemicals/advanced materials/products/services are produced and used in a way to avoid harm to humans and the environment. Technical and policy-related literature was surveyed for battery technologies and recommendations were provided for a broad SSbD approach that remains firmly grounded in Life Cycle Thinking principles. The approach integrates functional performance and sustainability (safety, social, environmental, and economic) aspects throughout the life cycle of materials, products, and processes, and evaluates how their interactions reflect on SSbD parameters. 22 different types of batteries were analyzed in a life cycle thinking approach for criticality, toxicity/safety, environmental and social impact, circularity, functionality, and cost to ensure battery innovation has a green and sustainable purpose to avoid unintended consequences.

Towards Integration of Two-Dimensional Hexagonal Boron Nitride (2D h-BN) in Energy Conversion and Storage Devices

The prominence of two-dimensional hexagonal boron nitride (2D h-BN) nanomaterials in the energy industry has recently grown rapidly due to their broad applications in newly developed energy systems. This was necessitated as a response to the demand for mechanically and chemically stable platforms with superior thermal conductivity for incorporation in next-generation energy devices. Conventionally, the electrical insulation and surface inertness of 2D h-BN limited their large integration in the energy industry. However, progress on surface modification, doping, tailoring the edge chemistry, and hybridization with other nanomaterials paved the way to go beyond those conventional characteristics. The current application range, from various energy conversion methods (e.g., thermoelectrics) to energy storage (e.g., batteries), demonstrates the versatility of 2D h-BN nanomaterials for the future energy industry. In this review, the most recent research breakthroughs on 2D h-BN nanomaterials used in energy-based applications are discussed, and future opportunities and challenges are assessed.

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.