Recent Development in Composite Membranes for Flow Batteries

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.

Rechargeable Aqueous Zinc-Halogen Batteries: Fundamental Mechanisms, Research Issues, and Future Perspectives

Aqueous zinc-halogen batteries (AZHBs) have emerged as promising candidates for energy storage applications due to their high security features and low cost. However, several challenges including natural subliming, sluggish reaction kinetics, and shuttle effect of halogens, as well as dendrite growth of the zinc (Zn) anode, have hindered their large-scale commercialization. In this review, first the fundamental mechanisms and scientific issues associated with AZHBs are summarized. Then the research issues and progresses related to the cathode, separator, anode, and electrolyte are discussed. Additionally, emerging research opportunities in this field is explored. Finally, ideas and prospects for the future development of AZHBs are presented. The objective of this review is to stimulate further exploration, foster the advancement of AZHBs, and contribute to the diversified development of electrochemical energy storage.

Ionic-Nanophase Hybridization of Nafion by Supramolecular Patching for Enhanced Proton Selectivity in Redox Flow Batteries

Nafion, as the mostly used proton exchange membrane material in vanadium redox flow batteries (VRFBs), encounters serious vanadium permeation problems due to the large size difference between its anionic nanophase (3-5 nm) and cationic vanadium ions (∼0.6 nm). Bulk hybridization usually suppresses the vanadium permeation at the expense of proton conductivity since conventional additives tend to randomly agglomerate and damage the nanophase continuity from unsuitable sizes and intrinsic incompatibility. Here, we report the ionic-nanophase hybridization strategy of Nafion membranes by using fluorinated block copolymers (FBCs) and polyoxometalates (POMs) as supramolecular patching additives. The cooperative noncovalent interactions among Nafion, interfacial-active FBCs, and POMs can construct a 1 nm-shrunk ionic nanophase with abundant proton transport sites, preserved continuity, and efficient vanadium screeners, which leads to a comprehensive enhancement in proton conductivity, selectivity, and VRFB performance. These results demonstrate the intriguing potential of the supramolecular patching strategy in precisely tuning nanostructured electrolyte membranes for improved performance.

Review of the Development of First-Generation Redox Flow Batteries: Iron-Chromium System

The iron-chromium redox flow battery (ICRFB) is considered the first true RFB and utilizes low-cost, abundant iron and chromium chlorides as redox-active materials, making it one of the most cost-effective energy storage systems. ICRFBs were pioneered and studied extensively by NASA and Mitsui in Japan in the 1970-1980s, and extensive studies on ICRFBs have been carried out over the past few decades. In addition, ICRFB is considered to be one of the most promising directions for cost-effective and large-scale energy storage applications, as its cost can theoretically be lower than that of zinc-bromine and all-vanadium RFBs, giving it the potential for large-scale promotion. With the resolution of problems such as hydrogen evolution and electrolyte intermixing, the ICRFB technology is moving out of the laboratory and striving for greater power and more stable industrialization requirements. This Review summarizes the history, development, and research status of key components (carbon-based electrode, electrolyte, and membranes) in the ICRFB system, aiming to give a brief guide to researchers who are involved in the related subject.

Synthesis and Characterization of Lithium-Conducting Composite Polymer-Ceramic Membranes for Use in Nonaqueous Redox Flow Batteries

Redox flow batteries (RFBs) are a burgeoning electrochemical platform for long-duration energy storage, but present embodiments are too expensive for broad adoption. Nonaqueous redox flow batteries (NAqRFBs) seek to reduce system costs by leveraging the large electrochemical stability window of organic solvents (>3 V) to operate at high cell voltages and to facilitate the use of redox couples that are incompatible with aqueous electrolytes. However, a key challenge for emerging nonaqueous chemistries is the lack of membranes/separators with suitable combinations of selectivity, conductivity, and stability. Single-ion conducting ceramics, integrated into a flexible polymer matrix, may offer a pathway to attain performance attributes needed for enabling competitive nonaqueous systems. Here, we explore composite polymer-inorganic binder-filler membranes for lithium-based NAqRFBs, investigating two different ceramic compounds with NASICON-type (NASICON: sodium (Na) superionic conductor) crystal structure, Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) and Li_1.4Al_0.4Ge_0.2Ti_1.4(PO₄)₃ (LAGTP), each blended with a polyvinylidene fluoride (PVDF) polymeric matrix. We characterize the physicochemical and electrochemical properties of the synthesized membranes as a function of processing conditions and formulation using a range of microscopic and electrochemical techniques. Importantly, the electrochemical stability window of the as-prepared membranes lies between 2.2-4.5 V vs Li/Li⁺. We then integrate select composite membranes into a single electrolyte flow cell configuration and perform polarization measurements with different redox electrolyte compositions. We find that mechanically robust, chemically stable LATP/PVDF composites can support >40 mA cm^-2 at 400 mV cell overpotential, but further improvements are needed in selectivity. Overall, the insights gained through this work begin to establish the foundational knowledge needed to advance composite polymer-inorganic membranes/separators for NAqRFBs.

Thermodynamic Interactions as a Descriptor of Cross-Over in Nonaqueous Redox Flow Battery Membranes

Grid-scale energy storage is increasingly needed as wind, solar, and other intermittent renewable energy sources become more prevalent. Redox flow batteries (RFBs) are well suited to this application because of the advantages in scalability and modularity over competing technologies. Commercial aqueous flow batteries often have low energy density, but nonaqueous RFBs can offer higher energy density. Nonaqueous RFBs have not been studied as extensively as aqueous RFBs, and the use of organic solvents and organic active materials in nonaqueous RFBs presents unique membrane separator challenges compared to aqueous systems. Specifically, organic active material cross-over, which degrades battery performance, may be affected by membrane/active material thermodynamic interactions in a fundamentally different way than ionic active material cross-over in aqueous RFB membranes. Hansen solubility parameters (HSPs) were used to quantify these interactions and explain differences in organic active material permeability properties. Probe molecules with a more unfavorable HSP-determined enthalpy of mixing with the membrane polymer exhibited lower permeability or cross-over properties. The HSP approach, which accounts for the uncharged polymer backbone and the charged side chain, revealed that interactions between the uncharged organic probe molecule and the hydrophobic polymer backbone were more important for determining permeability or cross-over properties than interactions between the probe molecule and the hydrophilic side chain. This result is significant for nonaqueous RFBs because it suggests a decoupling of ionic conduction expected to predominantly occur in charged polymer regions and cross-over of organic molecules via hydrophobic or uncharged polymer regions. Such decoupling is not expected in aqueous systems where active materials are often polar or ionic and both cross-over and conduction occur predominantly in charged polymer regions. For nonaqueous RFBs, or other membrane applications where selective organic molecule transport is important, HSP analysis can guide the co-design of the polymer separator materials and soluble organic molecules.