Development of Membranes and Separators to Inhibit Cross-Shuttling of Sulfur in Polysulfide-Based Redox Flow Batteries: A Review

The global rapid transition from fossil fuels to renewable energy resources necessitates the implementation of long-duration energy storage technologies owing to the intermittent nature of renewable energy sources. Therefore, the deployment of grid-scale energy storage systems is inevitable. Sulfur-based batteries can be exploited as excellent energy storage devices owing to their intrinsic safety, low cost of raw materials, low risk of environmental hazards, and highest theoretical capacities (gravimetric: 2600 Wh/kg and volumetric: 2800 Wh/L). However, sulfur-based batteries exhibit certain scientific limitations, such as polysulfide crossover, which causes rapid capacity decay and low Coulombic efficiency, thereby hindering their implementation at a commercial scale. In this review article, we focus on the latest research developments between 2012-2023 to improve the separators/membranes and overcome the shuttle effect associated with them. Various categories of ion exchange membranes (IEMs) used in redox batteries, particularly polysulfide redox flow batteries and lithium-sulfur batteries, are discussed in detail. Furthermore, advances in IEM constituents are summarized to gain insights into different fundamental strategies for attaining targeted characteristics, and a critical analysis is proposed to highlight their efficiency in mitigating sulfur cross-shuttling issues. Finally, future prospects and recommendations are suggested for future research toward the fabrication of more effective membranes with desired properties.

A Critical Review of Electrolytes for Advanced Low- and High-Temperature Polymer Electrolyte Membrane Fuel Cells

In the 21st century, proton exchange membrane fuel cells (PEMFCs) represent a promising source of power generation due to their high efficiency compared with coal combustion engines and eco-friendly design. Proton exchange membranes (PEMs), being the critical component of PEMFCs, determine their overall performance. Perfluorosulfonic acid (PFSA) based Nafion and nonfluorinated-based polybenzimidazole (PBI) membranes are commonly used for low- and high-temperature PEMFCs, respectively. However, these membranes have some drawbacks such as high cost, fuel crossover, and reduction in proton conductivity at high temperatures for commercialization. Here, we report the requirements of functional properties of PEMs for PEMFCs, the proton conduction mechanism, and the challenges which hinder their commercial adaptation. Recent research efforts have been focused on the modifications of PEMs by composite materials to overcome their drawbacks such as stability and proton conductivity. We discuss some current developments in membranes for PEMFCs with special emphasis on hybrid membranes based on Nafion, PBI, and other nonfluorinated proton conducting membranes prepared through the incorporation of different inorganic, organic, and hybrid fillers.

Significantly Increased Stability of Donor-Acceptor Molecular Complexes under Heterogeneous Conditions: Synthesis, Characterization, and Photosensitizing Activity of a Nanostructured Porphyrin-Lewis Acid Adduct

While the BF₃ complexes of meso-tetra(aryl)porphyrins are readily decomposed into their components under aqueous conditions, immobilization of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (H₂TMPyP) on a nanosized polymer (sodium salt of Amberlyst 15, nanoAmbSO₃Na) formed a water-stable BF₃ complex applicable for efficient aerobic photooxidation of 1,5-dihydroxylnaphthalene and sulfides under green conditions. NanoAmbSO₃@H₂TMPyP(BF₃)₂ was characterized by diffuse reflectance UV-vis spectroscopy, dynamic light scattering, thermal gravimetric analysis, Brunauer-Emmett-Teller analysis, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy techniques. The catalyst was successfully used for 10 consecutive reactions with no detectable degradation of the complex and decrease in the catalyst activity. NanoAmbSO₃@H₂TMPyP(BF₃)₂ was also completely stable toward dissociation to its components under different light conditions in acetonitrile. The singlet oxygen quantum yields φ_Δ of H₂TMPyP, nanoAmbSO₃@H₂TMPyP, and their molecular complexes with BF₃, determined chemically by using 1,3-diphenylisobenzofuran, revealed substantially higher values in the case of the heterogenized porphyrin and molecular complex.

Cellulose Nanocrystal-Polyelectrolyte Hybrids for Bentonite Water-Based Drilling Fluids

Cellulose nanocrystals (CNCs), with their rodlike shape and nanoscale dimensions, greatly improve the filtration performance of bentonite-containing, water-based drilling fluids (BT-WDFs) through interactions with the BT platelets. When these WDFs are exposed to high salt concentrations, though, their fluid retention properties are greatly diminished due to reduced CNC-BT interaction and BT aggregation/flocculation. Consequently, we reduce BT-BT interaction at high salt by grafting polyelectrolytes (PE) to CNC particles (CNC-PE) to enhance CNC-BT interactions when incorporating these hybrid particles with BT-WDFs. The particles sterically and electrostatically screen BT platelets from associating, thus improving fluid filtration performance at high salt. Three types of CNC modifications were carried out: grafting from direct surface initiation, modification with vinyl-terminated glycidyl methacrylate (GMA) before grafting, and physical mixing of CNC with a polymer. These modifications were performed using three polyelectrolyte materials: anionic polystyrene sulfonate (PSS), cationic polyacrylamide (PAM), and a random copolymer of PSS and PAM (PSS-co-PAM). Formulations containing CNC-PEs prepared by covalent grafting exhibited superior filtration properties compared to those in which CNCs and PEs were physically mixed. The higher graft loading achieved with the GMA method resulted in poorer filtration results compared to the direct grafting method due to CNC-PE interparticle cross-linking. PSS-modified CNC-PEs appeared to attach to BT edges, while PAM-modified CNC-PEs attached to the BT faces. These interactions disrupted BT aggregation, with the PSS-co-PAM CNC hybrid displaying the most desired filtration properties. The results highlight the importance of steric and charge stabilization of the BT particle edges and faces to achieve high-performance WDFs for well excavation.

The Multilevel Structure of Sulfonated Syndiotactic-Polystyrene Model Polyelectrolyte Membranes Resolved by Extended Q-Range Contrast Variation SANS

Membranes based on sulfonated synditoactic polystyrene (s-sPS) were thoroughly characterized by contrast variation small-angle neutron scattering (SANS) over a wide Q-range in dry and hydrated states. Following special sulfonation and treatment procedures, s-sPS is an attractive material for fuel cells and energy storage applications. The film samples were prepared by solid-state sulfonation, resulting in uniform sulfonation of only the amorphous phase while preserving the crystallinity of the membrane. Fullerenes, which improve the resistance to oxidation decomposition, were incorporated in the membranes. The fullerenes seem to be chiefly located in the amorphous regions of the samples, and do not influence the formation and evolution of the morphologies in the polymer films, as no significant differences were observed in the SANS patterns compared to the fullerenes-free s-sPS membranes, which were investigated in a previous study. The use of uniaxially deformed film samples, and neutron contrast variation allowed for the identification and characterization of different structural levels with sizes between nm and μm, which form and evolve in both the dry and hydrated states. The scattering length density of the crystalline regions was varied using the guest exchange procedure between different toluene isotopologues incorporated into the sPS lattice, while the variation of the scattering properties of the hydrated amorphous regions was achieved using different H2O/D2O mixtures. Due to the deformation of the films, the scattering characteristics of different structures can be distinguished on specific detection sectors and at different detection distances after the sample, depending on their size and orientation.

Sulfonated Binaphthyl-Containing Poly(arylene ether ketone)s with Rigid Backbone and Excellent Film-Forming Capability for Proton Exchange Membranes

Sterically hindered (S)-1,1'-binaphthyl-2,2'-diol had been successfully copolymerized with 4,4'-sulfonyldiphenol and 4,4'-difluorobenzophenone to yield fibrous poly(arylene ether ketone)s (PAEKs) containing various amounts of binaphthyl unit, which was then selectively and efficiently sulfonated using ClSO₃H to yield sulfonated poly(arylene ether ketone)s (SPAEKs) with ion exchange capacities (IECs) ranging from 1.40 to 1.89 mmol·g-1. The chemical structures of the polymers were confirmed by 2D ¹H⁻¹H COSY NMR and FT-IR. The thermal properties, water uptake, swelling ratio, proton conductivity, oxidative stability and mechanical properties of SPAEKs were investigated in detail. It was found that the conjugated but non-coplanar structure of binaphthyl unit endorsed excellent solubility and film-forming capability to SPAEKs. The SPAEK-50 with an IEC of 1.89 mmol·g-1 exhibited a proton conductivity of 102 mS·cm-1 at 30 °C, much higher than that of the state-of-the-art Nafion N212 membrane and those of many previously reported aromatic analogs, which may be attributed to the likely large intrinsic free volume of SPAEKs created by the highly twisted chain structures and the desirable microscopic morphology. Along with the remarkable water affinity, thermal stabilities and mechanical properties, the SPAEKs were demonstrated to be promising proton exchange membrane (PEM) candidates for potential membrane separations.

Proton-conducting poly(ether sulfone ketone)s containing a high density of pendant sulfonic groups by a convenient and mild post-sulfonation

A class of new poly(ether sulfone ketone)s containing a high density of pendant sulfonic groups on the sulfonated structural units were designed and the resulting membranes exhibited overall good performance at low IEC levels.

Hollow and Microporous Organic Polymers Bearing Sulfonic Acids: Antifouling Seed Materials for Polyketone Synthesis

The reactor fouling is a notorious obstacle in managing continuous synthetic processes of polyketones. The fouling raises synthetic costs and reduces the bulk density of polymers. Thus, efficient seed materials are required for antifouling performance. Hollow microporous organic polymers (HMOPs) were prepared using ZIF-8 nanoparticles as templating materials. Sulfonic groups were incorporated into HMOP via a post synthetic approach. The resultant HMOP-SO₃H (20-90 μg/mL in methanol) showed successful catalyst activation and antifouling performance in polyketone synthesis.

Imaging the Phase Separation Between PEDOT and Polyelectrolytes During Processing of Highly Conductive PEDOT:PSS Films

Treating

PEDOT

PSS (Clevios) with certain additives, such as ethylene glycol (EG), dimethyl sulfoxide (DMSO) and sorbitol, has been shown to increase the conductivity of this material from roughly 1 to nearly 1000 S/cm. Using a slow drying method, we show that the additive induced a separation between free PSS and reorganized

PEDOT

PSS complexes in the highly conductive

PEDOT

PSS films. Additives (DMSO, DEG, and PEG 400) were included in

PEDOT

PSS aqueous dispersions at large volume fractions. The mixtures were slowly dried under room conditions. During drying, the evaporation of water resulted in an additive-rich solvent mixture from which the reorganized

PEDOT

PSS complexes aggregated into a dense film while free PSS remained in the solution. Upon complete drying, PSS formed a transparent rim film around the conducting PEDOT film. The chemical compositions of the two phases were studied using an infrared microscope. This removal of PSS resulted in more compact packing of PEDOT molecules, as confirmed by X-ray diffraction measurements. X-ray photoelectron spectroscopy and atomic force microscope measurements suggested the enrichment of PEDOT on the film surface after PSS separation. Through a simple drying process in an additive-containing dispersion, the conductivity of PEDOT films increased from 0.1 to 200-400 S/cm. Through this method, we confirmed the existence of two phases in additive-treated and highly conductive

PEDOT

PSS films. The proper separation between PSS and PEDOT will be of relevance in designing strategies to process high-performance plastic electrodes.

An environmentally friendly process for the synthesis of an fGO modified anion exchange membrane for electro-membrane applications

We report the synthesis of an anion exchange membrane (AEM) based on chemically covalently modified graphene oxide (GO) for electrodialysis and fuel cell applications.

Crosslinked tri-side-chain-type sulfonated poly(arylene ether ketones) with enhanced proton conductivity by a Friedel–Crafts acylation reaction

Cross-linked poly(arylene ether ketones) containing multi-pendent sulfonic groups were prepared by the Friedel–Crafts acylation reaction in order to solve the problem of high methanol crossover and maintain other high performances at the same time.