Advanced Nafion hybrid membranes with fast proton transport channels toward high-performance vanadium redox flow battery

Proton Exchange Membranes from Sulfonated Lignin Nanocomposites for Redox Flow Battery Applications

Redox flow batteries (RFBs) are increasingly being considered for a wide range of energy storage applications, and such devices rely on proton exchange membranes (PEMs) to function. PEMs are high-cost, petroleum-derived polymers that often possess limited durability, variable electrochemical performance, and are linked to discharge of perfluorinated compounds. Alternative PEMs that utilize biobased materials, including lignin and sulfonated lignin (SL), low-cost byproducts of the wood pulping process, have struggled to balance electrochemical performance with dimensional stability. Herein, SL nanoparticles are demonstrated for use as a nature-derived, ion-conducting PEM material. SL nanoparticles (NanoSLs) can be synthesized for increased surface area, uniformity, and miscibility compared with macrosized lignin, improving proton conductivity. After addition of polyvinyl alcohol (PVOH) as a structural backbone, membranes with the highest NanoSL concentration demonstrated an ion exchange capacity of 1.26 meq g-1, above that of the commercial PEM Nafion 112 (0.98 meq g-1), along with a conductivity of 80.4 mS cm-1 in situ, above that of many biocomposite PEMs, and a coulombic efficiency (CE), energy efficiency (EE) and voltage efficiency (VE) of 91%, 68% and 78%, respectively at 20 mA cm-2. These nanocomposite PEMs demonstrate the potential for valorization of forest biomass waste streams for high value clean energy applications.

An Ultra-Low Self-Discharge Aqueous|Organic Membraneless Battery with Minimized Br2 Cross-Over

Batteries dissolving active materials in liquids possess safety and size advantages compared to solid-based batteries, yet the intrinsic liquid properties lead to material cross-over induced self-discharge both during cycling and idle when the electrolytes are in contact, thus highly efficient and cost-effective solutions to minimize cross-over are in high demand. An ultra-low self-discharge aqueous|organic membraneless battery using dichloromethane (CH2 Cl2 ) and tetrabutylammonium bromide (TBABr) added to a zinc bromide (ZnBr2 ) solution as the electrolyte is demonstrated. The polybromide is confined in the organic phase, and bromine (Br2 ) diffusion-induced self-discharge is minimized. At 90% state of charge (SOC), the membraneless ZnBr2 |TBABr (Z|T) battery shows an open circuit voltage (OCV) drop of only 42 mV after 120 days, 152 times longer than the ZnBr2  battery, and superior to 102 previous reports from all types of liquid active material batteries. The 120-day capacity retention of 95.5% is higher than commercial zinc-nickel (Zn-Ni) batteries and vanadium redox flow batteries (VRFB, electrolytes stored separately) and close to lithium-ion (Li-ion) batteries. Z|T achieves >500 cycles (2670 h, 0.5 m electrolyte, 250 folds of membraneless ZnBr2  battery) with ≈100% Coulombic efficiency (CE). The simple and cost-effective design of Z|T provides a conceptual inspiration to regulate material cross-over in liquid-based batteries to realize extended operation.

Two-Dimensional MFI-Type Zeolite Flow Battery Membranes

Vanadium flow battery (VFB) is one of the most reliable stationary electrochemical energy-storage technologies, and a membrane with high vanadium resistance and proton conductivity is essential for manufacturing high-performance VFBs. In this study, a two-dimensional (2D) MFI-type zeolite membrane was fabricated from zeolite nanosheet modules, which displayed excellent vanadium resistance (0.07 mmol L-1 h-1 ) and proton conductivity (0.16 S cm-1 ), yielding a coulombic efficiency of 93.9 %, a voltage efficiency of 87.6 %, and an energy efficiency of 82.3 % at 40 mA cm-2 . The self-discharge period of a VFB equipped with 2D MFI-type zeolite membrane increased up to 116.2 h, which was significantly longer than that of the commercial perfluorinated sulfonate membrane (45.9 h). Furthermore, the corresponding battery performance remained stable over 1000 cycles (>1500 h) at 80 mA cm-2 . These findings demonstrate that 2D MFI-type membranes are promising ion-conductive membranes applicable for stationary electrochemical energy-storage devices.

Polarization Processes in Nafion Composite Membranes Doped with Rare-Earth Metals

Dielectric spectroscopy (frequency range f = 100…107 Hz and temperatures T = 293…403 K (accuracy 0.5 K), measuring voltage applied to the sample was 1.0 V) was used to study composite materials based on perfluorosulfonic membranes with inclusions of rare-earth elements, in particular, europium (III) and terbium (III) chlorides. The dispersion of the permittivity and the presence of maxima, corresponding to losses, were revealed, which indicates that relaxation processes of various natures were present. The membrane layers under investigation are characterized by relaxation parameters that correspond to a symmetrical distribution of relaxers over relaxation times. The spectrum of relaxers changed when terbium and europium metal impurities were introduced into the polymer matrix. The investigation of these polymer systems demonstrated a power-law dependence of the specific conductivity on frequency. A decrease in the exponent with increasing temperature indicates the existence of a traditional hopping mechanism for charge transfer. The observed changes in the dielectric permittivity and specific conductivity are due to a change in the nature of polarization processes because of the strong interaction of metal (terbium and europium) ions with the polymer matrix of Nafion.

Hollow-Particles Quasi-Solid-State Electrolytes with Biomimetic Ion Channels for High-Performance Lithium-Metal Batteries

Solid-state electrolytes (SSEs) are the core material of solid-state lithium metal batteries (SLMBs), which are being researched urgently owing to their high energy and safety. Both high ionic conductivity and excellent cycling stability remain the primary goal of solid-state electrolytes. Herein, inspired by K⁺ /Na⁺ ion channels in cell membrane of eukaryotes, a novel hollow UiO-66 with biomimetic ion channels based on quasi-solid-state electrolytes (QSSEs) is designed. The hollow UiO-66 spheres containing biomimetic ion channels can spontaneously combine anions and incorporate more lithium ions, creating improved ionic conductivity (1.15 × 10^-3 S cm^-1 ) and lithium-ion transference number (0.70) at room temperature. The long-term cycling of symmetric batteries and COMSOL simulations demonstrate that this biomimetic strategy enables uniform ion flux to suppress Li dendrites. Furthermore, the Li metal full cells paired with LiFePO₄ cathode exhibit excellent cycling stability and rate performance. Consequently, the strategy of designing biomimetic QSSEs opens up a new path for developing high-performance electrolytes for SLMBs.

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.

Fabricating a Partially Fluorinated Hybrid Cation-Exchange Membrane for Long Durable Performance of Vanadium Redox Flow Batteries

The long-term durability of vanadium redox flow batteries (VRFBs) depends on the stability and performance of the membrane separator. We have architected a hybrid membrane by uniform dispersion of MIL-101(Cr) (Cr-MOF) in a partially fluorinated polymer grafted with sulfonic acid groups (PHP@AMPSCr-MOF(1.0)). The single cell VRFB performance of the PHP@AMPSCr-MOF(1.0) membrane was studied in comparison with the Cr-MOF incorporated Nafion membrane (NafionCr-MOF(1.0)) and showed an excellent result with 97.5% Coulombic efficiency (CE) at 150 mA/cm2 without any significant deterioration in the charge-discharge process for 1500 cycles (over 650 h). Meanwhile, the CE value of the NafionCr-MOF membrane (94.5%) deteriorated after 800 cycles (about 360 h) under similar conditions. The high VRFB performance of the PHP@AMPSCr-MOF(1.0) membrane has been attributed to the synergized properties and good interactions between Cr-MOF and partially fluorinated polymer matrix responsible for the creation of hydrophilic proton-conducting channels to achieve high selectivity. Furthermore, the cost-effective polymer and thus membranes may open new windows for practical applications in other energy devices such as fuel cells, electrolysis, and water treatment.

Supramolecular Anchoring of Polyoxometalate Amphiphiles into Nafion Nanophases for Enhanced Proton Conduction

Advanced proton exchange membranes (PEMs) are highly desirable in emerging sustainable energy technology. However, the further improvement of commercial perfluorosulfonic acid PEMs represented by Nafion is hindered by the lack of precise modification strategy due to their chemical inertness and low compatibility. Here, we report the robust assembly of polyethylene glycol grafted polyoxometalate amphiphile (GSiW₁₁) into the ionic nanophases of Nafion, which largely enhances the comprehensive performance of Nafion. GSiW₁₁ can coassemble with Nafion through multiple supramolecular interactions and realize a stable immobilization. The incorporation of GSiW₁₁ can increase the whole proton content in the system and induce the hydrated ionic nanophase to form a wide channel for proton transport; meanwhile, GSiW₁₁ can reinforce the Nafion ionic nanophase by noncovalent cross-linking. Based on these synergistic effects, the hybrid PEMs show multiple enhancements in proton conductivity, tensile strength, and fuel cell power density, which are all superior to the pristine Nafion. This work demonstrates the intriguing advantage of molecular nanoclusters as supramolecular enhancers to develop high-performance electrolyte materials.

Insights into the Influence of Different Pre-Treatments on Physicochemical Properties of Nafion XL Membrane and Fuel Cell Performance

Perfluorosulfonic acid (PFSA) polymers such as Nafion are the most frequently used Proton Exchange Membrane (PEM) in PEM fuel cells. Nafion XL is one of the most recently developed membranes designed to enhance performance by employing a mechanically reinforced layer in the architecture and a chemical stabilizer. The influence of the water and acid pre-treatment process on the physicochemical properties of Nafion XL membrane and Membrane Electrode Assembly (MEA) was investigated. The obtained results indicate that the pre-treated membranes have higher water uptake and dimensional swelling ratios, i.e., higher hydrophilicity, while the untreated membrane demonstrated a higher ionic exchange capacity. Furthermore, the conductivity of the acid pre-treated Nafion XL membrane was ~ 9.7% higher compared to the untreated membrane. Additionally, the maximum power densities obtained at 80 °C using acid pre-treatment were ~ 0.8 and 0.93 W/cm² for re-cast Nafion and Nafion XL, respectively. However, the maximum generated powers for untreated membranes at the same condition were 0.36 and 0.66 W/cm² for re-cast Nafion and Nafion XL, respectively. The overall results indicated that the PEM’s pre-treatment process is essential to enhance performance.

Development of WO3-Nafion Based Membranes for Enabling Higher Water Retention at Low Humidity and Enhancing PEMFC Performance at Intermediate Temperature Operation

The proton exchange membrane (PEM) represents a pivotal material and a key challenge in developing fuel cell science and hydrogen technology. Nafion is the most promising polymer which will lead to its commercialisation. Hybrid membranes of nanosized tungsten oxide (WO₃) and Nafion were fabricated, characterised, and tested in a single cell. The incorporation of 10 wt% WO₃ resulted in 21% higher water uptake, 11.7% lower swelling ratio, almost doubling the hydration degree, and 13% higher mechanical stability of the hybrid membrane compared to the Nafion XL. Compared to commercial Nafion XL, the rNF-WO-10 hybrid membrane showed an 8.8% and 20% increase in current density of the cell at 0.4 V operating at 80 and 95 °C with 1.89 and 2.29 A/cm², respectively. The maximum power density has increased by 9% (0.76 W/cm²) and 19.9% (0.922 W/cm²) when operating at the same temperatures compared to the commercial Nafion XL membrane. Generally, considering the particular structure of Nafion XL, our Nafion-based membrane with 10 wt% WO₃ (rNF-WO-10) is a suitable PEM with a comparable performance at different operating conditions.

A State-of-Art on the Development of Nafion-Based Membrane for Performance Improvement in Direct Methanol Fuel Cells

Nafion, a perfluorosulfonic acid proton exchange membrane (PEM), has been widely used in direct methanol fuel cells (DMFCs) to serve as a proton carrier, methanol barrier, and separator for the anode and cathode. A significant drawback of Nafion in DMFC applications is the high anode-to-cathode methanol fuel permeability that results in over 40% fuel waste. Therefore, the development of a new membrane with lower permeability while retaining the high proton conductivity and other inherent properties of Nafion is greatly desired. In light of these considerations, this paper discusses the research findings on developing Nafion-based membranes for DMFC. Several aspects of the DMFC membrane are also presented, including functional requirements, transport mechanisms, and preparation strategies. More importantly, the effect of the various modification approaches on the performance of the Nafion membrane is highlighted. These include the incorporation of inorganic fillers, carbon nanomaterials, ionic liquids, polymers, or other techniques. The feasibility of these membranes for DMFC applications is discussed critically in terms of transport phenomena-related characteristics such as proton conductivity and methanol permeability. Moreover, the current challenges and future prospects of Nafion-based membranes for DMFC are presented. This paper will serve as a resource for the DMFC research community, with the goal of improving the cost-effectiveness and performance of DMFC membranes.

Composite Anion-Exchange Membrane Fabricated by UV Cross-Linking Vinyl Imidazolium Poly(Phenylene Oxide) with Polyacrylamides and Their Testing for Use in Redox Flow Batteries

Composite anion-exchange membranes (AEMs) consisting of a porous substrate and a vinyl imidazolium poly(phenylene oxide) (VIMPPO)/acrylamide copolymer layer were fabricated in a straightforward process, for use in redox flow batteries. The porous substrate was coated with a mixture of VIMPPO and acrylamide monomers, then subsequently exposed to UV irradiation, in order to obtain a radically cured ion-exchange coating. Combining VIMPPO with low-value reagents allowed to significantly reduce the amount of synthesized ionomer used to fabricate the mem- brane down to 15%. Varying the VIMPPO content also allowed tuning the ionic transport properties of the resulting AEM. A series of membranes with different VIMPPO/acrylamides ratios were prepared to assess the optimal composition by studying the changes of membranes properties-water uptake, area resistivity, permeability, and chemical stability. Characterization of the membranes was followed by cycling experiments in a vanadium RFB (VRFB) cell. Among three composite membranes, the one with VIMPPO 15% w/w-reached the highest energy efficiency (75.1%) matching the performance of commercial ion-exchange membranes (IEMs) used in VRFBs (Nafion® N 115: 75.0% and Fumasep® FAP 450: 73.0%). These results showed that the proposed composite AEM, fabricated in an industrially oriented process, could be considered to be a lower-cost alternative to the benchmarked IEMs.