Sulfonated polysulfone proton exchange membrane influenced by a varied sulfonation degree for vanadium redox flow battery

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.

A High-Performance Polyimide Composite Membrane with Flexible Polyphosphazene Derivatives for Vanadium Redox Flow Battery

A sulfonated polyimide, S-F-abSPI, with alkyl sulfonic acid side chains, and a polyphosphonitrile derivative, poly[4-methoxyphenoxy (4-fluorophenoxy) phosphazene] (PFMPP), were designed and synthesized. Composite modification of the S-F-abSPI membrane was carried out using PFMPP, resulting in the preparation of composite membranes with different composite ratios, which were then subjected to performance testing and characterization. Experimental results revealed a significant enhancement in the proton conductivity of the S-F-abSPI membrane, reaching 0.116 S cm-1, slightly higher than that of the N212 membrane. The S-F-abSPI/1% PFMPP composite membrane exhibited the optimal comprehensive performance, with a surface resistance as low as 0.54 Ω cm2, comparable to that of the N212 membrane. At a high current density of 200 mA cm-2 during charge-discharge, the composite membrane achieved a voltage efficiency (VE) of 83.12% and an energy efficiency (EE) of 81.95%. Cycling tests over 200 cycles demonstrated the composite membrane's excellent long-term cycling stability. The alkyl sulfonic acid side chains enhanced the proton conductivity of the membrane, while electrostatic potential distribution calculations indicated strong interactions between PFMPP and the base membrane, enhancing the membrane's mechanical strength, reducing vanadium ion permeability, and improving chemical stability and vanadium ion selectivity. This composite membrane holds promise for high-performance VRFB applications.

Optimized Sulfonated Poly(Ether Ether Ketone) Membranes for In-House Produced Small-Sized Vanadium Redox Flow Battery Set-Up

The ionic exchange membranes represent a core component of redox flow batteries. Their features strongly affect the performance, durability, cost, and efficiency of these energy systems. Herein, the operating conditions of a lab-scale single-cell vanadium flow battery (VRFB) were optimized in terms of membrane physicochemical features and electrolyte composition, as a way to translate such conditions into a large-scale five-cell VRFB stack system. The effects of the sulfonation degree (SD) and the presence of a filler on the performances of sulfonated poly(ether ether ketone) (SPEEK) ion-selective membranes were investigated, using the commercial perfluorosulfonic-acid Nafion 115 membrane as a reference. Furthermore, the effect of a chloride-based electrolyte was evaluated by comparing it to the commonly used standard sulfuric acid electrolyte. Among the investigated membranes, the readily available SPEEK50-0 (SD = 50%; filler = 0%) resulted in it being permeable and selective to vanadium. Improved coulombic efficiency (93.4%) compared to that of Nafion 115 (88.9%) was achieved when SPEEK50-0, in combination with an optimized chloride-based electrolyte, was employed in a single-cell VRFB at a current density of 20 mA·cm-2. The optimized conditions were successfully applied for the construction of a five-cell VRFB stack system, exhibiting a satisfactory coulombic efficiency of 94.5%.

Block Copolymer-Based Membranes for Vanadium Redox Flow Batteries: Synthesis, Characterization, and Performance

Nonfluorinated polymers have been widely proposed to replace Nafion as raw materials for redox flow battery ion-exchange membranes. Hereby, block copolymers based on polysulfone (PSU) and polyphenylsulfone (PPSU) are synthesized and employed as precursors of membranes for vanadium redox flow batteries. A series of copolymers with varying molar proportions of PSU (75/25, 60/40, 50/50 mol %) were prepared. The 60/40 and 75/25 copolymers exhibit concentrated sulfonic groups predominantly in the PSU unit, favoring the formation of hydrophobic and hydrophilic domains. The 50/50 copolymer presents a balanced degree of sulfonation between the two units, leading to a homogeneous distribution of sulfonic groups. An ex situ study of these materials comprising vanadium ion permeability and chemical and mechanical stability was performed. The best performance is achieved with 50/50 membranes, which exhibited performance comparable to commercial Nafion membranes. These results signify a promising breakthrough in the pursuit of high-performance, sustainable membranes for next-generation VRFBs.

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.

High-Reversibility Sulfur Anode for Advanced Aqueous Battery

Despite being extensively explored as cathodes in batteries, sulfur (S) can function as a low-potential anode by changing charge carriers in electrolytes. Here, a highly reversible S anode that fully converts from S8 0 to S2- in static aqueous S-I2 batteries by using Na+ as the charge carrier is reported. This S anode exhibits a low potential of -0.5 V (vs standard hydrogen electrode) and a near-to-theoretical capacity of 1404 mA h g-1 . Importantly, it shows significant advantages over the widely used Zn anode in aqueous media by obviating dendrite formation and H2 evolution. To suppress "shuttle effects" faced by both S and I2 electrodes, a scalable sulfonated polysulfone (SPSF) membrane is proposed, which is superior to commercial Nafion in cost (US$1.82 m-2  vs $3500 m-2 ) and environmental benignity. Because of its ultra-high selectivity in blocking polysulfides/iodides, the battery with SPSF displays excellent cycling stability. Even under 100% depth of discharge, the battery demonstrates high capacity retention of 87.6% over 500 cycles, outperforming Zn-I2 batteries with 3.1% capacity under the same conditions. These findings broaden anode options beyond metals for high-energy, low-cost, and fast-chargeable batteries.

Click-chemistry synergic MXene-functionalized flexible skeleton membranes for accurate recognition and separation

Membrane-based technology with accurate-recognition and specific-transmission has been regarded as one of the most promising strategies in environmental protection and energy conservation. However, membrane technique still faces challenges of "trade-off effect" between high selectivity and permeation flux within organic-aqueous mixed matrix. Here, well-intergrown click-chemistry synergic MXene-functionalized flexible skeleton membranes has been prepared in this strategy, enabling size-exclusion&structure selectivity by uniform location array imprinting unit and transport performance towards specific medicinal molecules of artemisinin (Ars). The well-assembled ultrathin cascade-type MXene layer guarantees the narrow interlayer nanochannels and the flexible skeleton modified mesoporous SiO2 nanoparticles provide active reaction platform for the construction of selective recognition space. The resulting membranes demonstrated outstanding selective separation performance with permeability factor that artesunate (Aru) /Ars and dihydro-artemisinin (d-Ars) / Ars of 3.17 and 2.89 and permeation flux of 1173.25 L·m-2·h-1·bar-1. Besides, combined with antibacterial durability, recycling performance, high separation performance in mobile phase stability of CMFMs, it is anticipated that this work hopefully opens a new avenue for efficient chiral separation to medicinal molecules, exhibiting broad potential for practical application.

Prediction of the phenol removal capacity from water by adsorption on activated carbon

High-performance sulfonated polysulfone (SPSf) mixed-matrix membranes (MMMs) were fabricated via a nonsolvent-induced phase separation (NIPS) method using zeolitic imidazolate frameworks-67 (ZIF-67) as a crosslinker. Acid-base crosslinking occurred between the sulfonic acid groups of SPSf and the tertiary amine groups of the embedded ZIF-67, which improved the dispersion of ZIF-67 and simultaneously improved the membrane strzcture and permselectivity. The dispersion of ZIF-67 in the MMMs and the acid-base crosslinking reaction were verified by energy-dispersive X-ray spectroscopy (EDX), X-ray diffractometry (XRD), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The pore structure analysis of MMMs indicated that filling ZIF-67 into SPSf enhanced the average surface pore sizes, surface porosities and more micropore in cross-sections. The crossflow filtrations showed the MMMs have higher pure water fluxes (57 to 111 L m^-2 h^-1) than the SPSf membrane (55 L m^-2 h^-1) but also higher bovine serum albumin (BSA) rejection rate of 93.9-95.8%, a model protein foulant. The MMMs showed a higher water contact angle than the SPSf membrane due to the addition of hydrophobic ZIF-67 and acid-base crosslinking, and also maintained high thermal stability evidenced by the thermogravimetric analysis (TGA) results. At the optimal ZIF-67 concentration of 0.3 wt%, the water flux of the SPSf-Z67-0.3 membrane was 82 L m^-2 h^-1 with a high BSA rejection rate of 95.3% at 0.1 MPa and better antifouling performance (FRR = 70%).

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.

Amphoteric Ion Exchange Membranes Prepared by Preirradiation-Induced Emulsion Graft Copolymerization for Vanadium Redox Flow Battery

A series of poly(vinylidene difluoride)-based amphoteric ion exchange membranes (AIEMs) were prepared by preirradiation-induced graft copolymerization of styrene and dimethylaminoethyl methacrylate in an aqueous emulsion media followed by solution casting, sulfonation, and protonation. The effects of absorbed dose and comonomer concentration on grafting yield (GY) were investigated. The highest GY of 44.5% at a low comonomer concentration of 0.9 M could be achieved. FTIR, TGA, and X-ray photoelectron spectroscopy (XPS) confirmed the successful grafting and sulfonation of the as-prepared AIEMs. Properties of the AIEMs such as water uptake, ion exchange capacity (IEC), ionic conductivity, and crossover behavior of VO²⁺ ions prepared by this novel technique were systematically investigated and compared with those of the commercial Nafion 115 membrane. It was found that at a GY of 28.4%, the AIEMs showed higher IEC and conductivity, lower permeability of VO²⁺ ions, and a longer time to maintain open circuit voltage than Nafion 115, which was attributed to their high GY and elaborate amphoteric structure. Consequently, this work has paved the way for the development of green and low-cost AIEMs with good performance for vanadium redox flow battery applications.