Hydrophilic porous poly(sulfone) membranes modified by UV-initiated polymerization for vanadium flow battery application

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.

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.

The Application of a Modified Polyacrylonitrile Porous Membrane in Vanadium Flow Battery

Vanadium flow battery (VFB) is one of the most promising candidates for large-scale energy storage. A modified polyacrylonitrile (PAN) porous membrane is successfully applied in VFB. Herein, a simple solvent post-processing method is presented to modify PAN porous membranes prepared by the traditional nonsolvent induced phase separation (NIPS) method. In the design, polymer PAN is chosen as the membrane material owing to its low cost and high stability. The large-size pores from NIPS method are well optimized by the solvent swelling and shrinking during the solvent post-processing. Meanwhile, the interconnectivity of pores is maintained well. As a result, the ion selectivity of PAN porous membranes is dramatically improved, and the CE of a VFB with PAN porous membranes rises from 68% to 93% after the solvent post-processing process. A VFB with the modified PAN porous membranes is capable of delivering a limiting current density of 900 mA cm⁻², and a high peak power density of 650 mW cm⁻², which is very competitive among the various flow batteries.

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.

Ion conducting membranes for aqueous flow battery systems

Flow batteries, aqueous flow batteries in particular, are the most promising candidates for stationary energy storage to realize the wide utilization of renewable energy sources. To meet the requirement of large-scale energy storage, there has been a growing interest in aqueous flow batteries, especially in novel redox couples and flow-type systems. However, the development of aqueous flow battery technologies is at an early stage and their performance can be further improved. As a key component of a flow battery, the membrane has a significant effect on battery performance. Currently, the membranes used in aqueous flow battery technologies are very limited. In this feature article, we first cover the application of porous membranes in vanadium flow battery technology, and then the membranes in most recently reported aqueous flow battery systems. Meanwhile, we hope that this feature article will inspire more efforts to design and prepare membranes with outstanding performance and stability, and then accelerate the development of flow batteries for large scale energy storage applications.

Porous membranes in secondary battery technologies

Secondary batteries have received huge attention due to their attractive features in applications of large-scale energy storage and portable electronic devices, as well as electrical vehicles. In a secondary battery, a membrane plays the role of separating the anode and cathode to prevent the occurrence of a short circuit, while allowing the transport of charge carriers to achieve a complete circuit. The properties of a membrane will largely determine the performance of a battery. In this article, we review the research and development progress of porous membranes in secondary battery technologies, such as lithium-based batteries together with flow batteries. The preparation methods as well as the required properties of porous membranes in different secondary battery technologies will be elucidated thoroughly and deeply. Most importantly, this review will mainly focus on the optimization and modification of porous membranes in different secondary battery systems. And various modifications on commercial porous membranes along with novel membrane materials are widely discussed and summarized. This review will help to optimize the membrane material for different secondary batteries, and favor the understanding of the preparation-structure-performance relationship of porous membranes in different secondary batteries. Therefore, this review will provide an extensive, comprehensive and professional reference to design and construct high-performance porous membranes.

The next generation vanadium flow batteries with high power density - a perspective

Vanadium flow batteries (VFBs) have received increasing attention due to their attractive features for large-scale energy storage applications. However, the relatively high cost and severe polarization of VFB energy storage systems at high current densities restrict their utilization in practical industrial applications. Optimization of the performance of key VFB materials, including electrodes, electrolytes and membranes, can realize simultaneous minimization of polarization and capacity decay. The power density and energy density of VFBs are thus simultaneously enhanced. Moreover, relevant theoretical mechanisms and foundations based on virtual investigations of VFB models and simulations can guide these optimizations. The improved power density and energy density can reduce the cost of VFB energy storage systems, accelerating their successful industrialization. In this perspective, modification methods to optimize the performance of key VFB materials and investigations of models and simulations of VFBs will be discussed. Therefore, the available ideas and approaches will be provided to direct further improvements in the power density and energy density of VFB systems.

Low-Voltage Gaseous HCl Electrolysis with an Iron Redox-Mediated Cathode for Chlorine Regeneration

Gaseous HCl as a by-product is often produced from chlorination processes using Cl₂ gas. Onsite Cl₂ regeneration from HCl is highly desirable as it eliminates the need to buy new Cl₂ and dispose HCl waste. A gaseous HCl electrolysis with Fe³⁺ /Fe²⁺ redox-mediated cathode is demonstrated for Cl₂ regeneration. HCl is oxidized to generate Cl₂ and protons in the anode while Fe³⁺ is reduced to Fe²⁺ in the cathode. Simultaneously Fe³⁺ is regenerated by chemical oxidation of Fe²⁺ by oxygen (air) that also produces water. A low operational voltage and high coulombic efficiency are achieved by using a novel composite porous membrane and hydrophobic anode. Specifically, a cell voltage of only 0.64 V is needed at the typical current density of 4 kA m^-2 , leading to a low energy consumption of 483 kWh per ton of Cl₂ (124 kJ molCl2 ^-1 ) which is about 50-55 % of state-of-the-art HCl electrolysis processes.

Hydrogen-Treated Rutile TiO2 Shell in Graphite-Core Structure as a Negative Electrode for High-Performance Vanadium Redox Flow Batteries

Hydrogen-treated TiO₂ as an electrocatalyst has shown to boost the capacity of high-performance all-vanadium redox flow batteries (VRFBs) as a simple and eco-friendly strategy. The graphite felt-based GF@TiO₂ :H electrode is able to inhibit the hydrogen evolution reaction (HER), which is a critical barrier for operating at high rate for long-term cycling in VRFBs. Significant improvements in charge/discharge and electron-transfer processes for the V³⁺ /V²⁺ reaction on the surface of reduced TiO₂ were achieved as a consequence of the formation of oxygen functional groups and oxygen vacancies in the lattice structure. Key performance indicators of VRFB have been improved, such as high capability rates and electrolyte-utilization ratios (82 % at 200 mA cm^-2 ). Additionally, high coulombic efficiencies (ca. 100 % up to the 96th cycle, afterwards >97 %) were obtained, demonstrating the feasibility of achieving long-term stability.

Decoration of Silica Nanoparticles on Polypropylene Separator for Lithium-Sulfur Batteries

A SiO₂ nanoparticle decorated polypropylene (PP) separator (PP-SiO₂) has been prepared by simply immersing the PP separator in the hydrolysis solution of tetraethyl orthosilicate (TEOS) with the assistance of Tween-80. After decoration, the thermal stability and the electrolyte wettability of the PP-SiO₂ separator are obviously improved. When the PP-SiO₂ separator is used for lithium-sulfur (Li-S) batteries, the cyclic stability and rate capability of the batteries are greatly enhanced. The capacity retention ratio of the Li-S battery configured with the PP-SiO₂ separator is 64% after 200 cycles at 0.2 C, which is much higher than that configured with the PP separator (45%). Moreover, the rate capacity of the Li-S batteries using the PP-SiO₂ separator reaches 956.3, 691.5, 621, and 567.6 mAh g^-1 at the current density of 0.2, 0.5, 1, and 2 C, respectively. The reason could be ascribed to that the polar silica coating not only alleviates the shuttle effect but also facilitates Li-ion migration.

Nanoporous polysulfones with in situ PEGylated surfaces by a simple swelling strategy using paired solvents

Strong and ductile nanoporous polymers are obtained from inexpensive PSF-b-PEG block copolymers using a facile and nondestructive swelling method.

Highly symmetric spongy porous poly(ether sulfone) membranes with selective open-cells for vanadium flow battery application

Poly(ether sulfone)/sulfonated poly(ether ether ketone) (PES/SPEEK) symmetric spongy porous membranes are prepared for vanadium flow battery (VFB) application.

Facile and Nonradiation Pretreated Membrane as a High Conductive Separator for Li-Ion Batteries

Polyolefin membranes are widely used as separators in commercialized Li-ion batteries. They have less polarized surfaces compared with polarized molecules of electrolyte, leading to a poor wetting state for separators. Radiation pretreatments are often adopted to solve such a problem. Unfortunately, they can only activate several nanometers deep from the surface, which limits the performance improvement. Here we report a facile and scalable method to polarize polyolefin membranes via a chemical oxidation route. On the surfaces of pretreated membrane, layers of poly(ethylene oxide) and poly(acrylic acid) can easily be coated, thus resulting in a high Li-ion conductivity of the membrane. Assembled with this decorated separator in button cells, both high-voltage (Li1.2Mn0.54Co0.13Ni0.13O2) and moderate-voltage (LiFePO4) cathode materials show better electrochemical performances than those assembled with pristine polyolefin separators.