Sulfonated poly(ether ether ketone)/amine-functionalized graphene oxide hybrid membrane with various chain lengths for vanadium redox flow battery: A comparative study

Preparation of Sulfonated Poly(arylene ether)/SiO2 Composite Membranes with Enhanced Proton Selectivity for Vanadium Redox Flow Batteries

Proton exchange membranes (PEMs) are an important type of vanadium redox flow battery (VRFB) separator that play the key role of separating positive and negative electrolytes while transporting protons. In order to lower the vanadium ion permeability and improve the proton selectivity of PEMs for enhancing the Coulombic efficiency of VRFBs, herein, various amounts of nano-sized SiO2 particles were introduced into a previously optimized sulfonated poly(arylene ether) (SPAE) PEMs through the acid-catalyzed sol-gel reaction of tetraethyl orthosilicate (TEOS). The successful incorporation of SiO2 was confirmed by FT-IR spectra. The scanning electron microscopy (SEM) images revealed that the SiO2 particles were well distributed in the SPAE membrane. The ion exchange capacity, water uptake, and swelling ratio of the PEMs were decreased with the increasing amount of SiO2, while the mechanical properties and thermal stability were improved significantly. The proton conductivity was reduced gradually from 93.4 to 76.9 mS cm−1 at room temperature as the loading amount of SiO2 was increased from 0 to 16 wt.%; however, the VO2+ permeability was decreased dramatically after the incorporation of SiO2 and reached a minimum value of 2.57 × 10−12 m2 s−1 at 12 wt.% of SiO2. As a result, the H+/VO2+ selectivity achieved a maximum value of 51.82 S min cm−3 for the composite PEM containing 12 wt.% of SiO2. This study demonstrates that the properties of PEMs can be largely tuned by the introduction of SiO2 with low cost for VRFB applications.

Engineering Polymeric Nanofluidic Membranes for Efficient Ionic Transport: Biomimetic Design, Material Construction, and Advanced Functionalities

Design elements extracted from biological ion channels guide the engineering of artificial nanofluidic membranes for efficient ionic transport and spawn biomimetic devices with great potential in many cutting-edge areas. In this context, polymeric nanofluidic membranes can be especially attractive because of their inherent flexibility and benign processability, which facilitate massive fabrication and facile device integration for large-scale applications. Herein, the state-of-the-art achievements of polymeric nanofluidic membranes are systematically summarized. Theoretical fundamentals underlying both biological and synthetic ion channels are introduced. The advances of engineering polymeric nanofluidic membranes are then detailed from aspects of structural design, material construction, and chemical functionalization, emphasizing their broad chemical and reticular/topological variety as well as considerable property tunability. After that, this Review expands on examples of evolving these polymeric membranes into macroscopic devices and their potentials in addressing compelling issues in energy conversion and storage systems where efficient ion transport is highly desirable. Finally, a brief outlook on possible future developments in this field is provided.

Polymer Electrolyte Membranes Containing Functionalized Organic/Inorganic Composite for Polymer Electrolyte Membrane Fuel Cell Applications

To mitigate the dependence on fossil fuels and the associated global warming issues, numerous studies have focused on the development of eco-friendly energy conversion devices such as polymer electrolyte membrane fuel cells (PEMFCs) that directly convert chemical energy into electrical energy. As one of the key components in PEMFCs, polymer electrolyte membranes (PEMs) should have high proton conductivity and outstanding physicochemical stability during operation. Although the perfluorinated sulfonic acid (PFSA)-based PEMs and some of the hydrocarbon-based PEMs composed of rationally designed polymer structures are found to meet these criteria, there is an ongoing and pressing need to improve and fine-tune these further, to be useful in practical PEMFC operation. Incorporation of organic/inorganic fillers into the polymer matrix is one of the methods shown to be effective for controlling target PEM properties including thermal stability, mechanical properties, and physical stability, as well as proton conductivity. Functionalization of organic/inorganic fillers is critical to optimize the filler efficiency and dispersion, thus resulting in significant improvements to PEM properties. This review focused on the structural engineering of functionalized carbon and silica-based fillers and comparisons of the resulting PEM properties. Newly constructed composite membranes were compared to composite membrane containing non-functionalized fillers or pure polymer matrix membrane without fillers.

An overview on the progress and development of modified sulfonated polyether ether ketone membranes for vanadium redox flow battery applications

Vanadium redox flow batteries (VRFB) are among the most promising approaches to efficiently store renewable energies. In such battery type, Nafion is commonly used as membrane material but suffers from high vanadium crossover and cost. These drawbacks negatively influence the widespread commercial application of VRFBs. Alternative membrane materials with high performance and low cost are thus being developed to address these shortfalls. Among those, possible materials for the VRFB membrane is sulfonated polyether ether ketone (SPEEK), which recently attracted considerable attention due to its low cost, combined with mechanical and chemical stability, and ease of preparation. This review summarizes the research activities related to the development of SPEEK-based membranes for VRFB applications and gives an overview of the properties of PEEK and its sulfonated form. A critical analysis on the challenges of SPEEK-based membranes is also discussed.

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.

Thin-film nanocomposite NF membrane with GO on macroporous hollow fiber ceramic substrate for efficient heavy metals removal

The ceramic membrane has been widely used in the wastewater treatment based on the chemical resistance and superior separation performance. A robust and defect-free thin-film nanocomposite (TFN) nanofiltration (NF) membrane on the macroporous hollow fiber ceramic (HFC) substrate was novelly developed for heavy metals removal. Before interfacial polymerization (IP), the aqueous solution of graphene oxide (GO) grafted with ethylenediamine (EDA) was deposited on the HFC substrate by vacuum filtration. Then, a thin polyamide (PA) film was fabricated by EDA and 1,3,5-trimesoyl chloride (TMC), followed by heat treatment. The effects of GO content and EDA concentration on the performance of the NF membrane have been systematically investigated. The results showed that when the GO content was 0.015 mg·mL^-1 and the EDA concentration was 0.75 wt.%, the as-prepared eGO₃/PA-HFC membrane had a rejection rate of 94.12% for MgCl₂ and a pure water flux of 18.03 L·m^-2·h^-1. Additionally, the removal ability of eGO₃/PA-HFC membranes for heavy metal ions was satisfactory (93.33%, 92.73%, 90.45% and 88.35% for Zn²⁺, Cu²⁺, Ni²⁺ and Pb²⁺, respectively). The study explored further that it was efficient and stable for heavy metal ions removal during 30 h in the simulated tap water and mining wastewater, which indicated that the eGO/PA-HFC membrane has great application potential in wastewater treatment.

Oriented Proton-Conductive Nanochannels Boosting a Highly Conductive Proton-Exchange Membrane for a Vanadium Redox Flow Battery

In this work, we propose a sulfonated poly (ether ether ketone) (SPEEK) composite proton-conductive membrane based on a 3-(1-hydro-imidazolium-3-yl)-propane-1-sulfonate (Him-pS) additive to break through the trade-off between conductivity and selectivity of a vanadium redox flow battery (VRFB). Specifically, Him-pS enables an oriented distribution of the SPEEK matrix to construct highly conductive proton nanochannels throughout the membrane arising from the noncovalent interaction. Moreover, the "acid-base pair" effect from an imidazolium group and a sulfonic group further facilitates the proton transport through the nanochannels. Meanwhile, the structure of the acid-base pair is further confirmed based on density functional theory calculations. Material and electrochemical characterizations indicate that the nanochannels with a size of 16.5 nm are vertically distributed across the membrane, which not only accelerate proton conductivity (31.54 mS cm^-1) but also enhance the vanadium-ion selectivity (39.9 × 10³ S min cm^-3). Benefiting from such oriented proton-conductive nanochannels in the membrane, the cell delivers an excellent Coulombic efficiency (CE, ≈ 98.8%) and energy efficiency (EE, ≈ 78.5%) at 300 mA cm^-2. More significantly, the cell maintains a stable energy efficiency over 600 charge-discharge cycles with only a 5.18% decay. Accordingly, this work provides a promising fabrication strategy for a high-performance membrane of VRFB.