Ionic Conductivity of Nonaqueous Solvent-Swollen Ionomer Membranes Based on Fluorosulfonate, Fluorocarboxylate, and Sulfonate Fixed Ion Groups

Thermodynamic Interactions as a Descriptor of Cross-Over in Nonaqueous Redox Flow Battery Membranes

Grid-scale energy storage is increasingly needed as wind, solar, and other intermittent renewable energy sources become more prevalent. Redox flow batteries (RFBs) are well suited to this application because of the advantages in scalability and modularity over competing technologies. Commercial aqueous flow batteries often have low energy density, but nonaqueous RFBs can offer higher energy density. Nonaqueous RFBs have not been studied as extensively as aqueous RFBs, and the use of organic solvents and organic active materials in nonaqueous RFBs presents unique membrane separator challenges compared to aqueous systems. Specifically, organic active material cross-over, which degrades battery performance, may be affected by membrane/active material thermodynamic interactions in a fundamentally different way than ionic active material cross-over in aqueous RFB membranes. Hansen solubility parameters (HSPs) were used to quantify these interactions and explain differences in organic active material permeability properties. Probe molecules with a more unfavorable HSP-determined enthalpy of mixing with the membrane polymer exhibited lower permeability or cross-over properties. The HSP approach, which accounts for the uncharged polymer backbone and the charged side chain, revealed that interactions between the uncharged organic probe molecule and the hydrophobic polymer backbone were more important for determining permeability or cross-over properties than interactions between the probe molecule and the hydrophilic side chain. This result is significant for nonaqueous RFBs because it suggests a decoupling of ionic conduction expected to predominantly occur in charged polymer regions and cross-over of organic molecules via hydrophobic or uncharged polymer regions. Such decoupling is not expected in aqueous systems where active materials are often polar or ionic and both cross-over and conduction occur predominantly in charged polymer regions. For nonaqueous RFBs, or other membrane applications where selective organic molecule transport is important, HSP analysis can guide the co-design of the polymer separator materials and soluble organic molecules.

Single-Step Fabrication of Polymeric Composite Membrane via Centrifugal Colloidal Casting for Fuel Cell Applications

Recent interest in polymer electrolyte membranes (PEMs) for fuel cell systems has spurred the development of infiltration technology by which to insert ionomers into mechanically robust reinforcement structures by solution casting in order to produce a cost effective and highly efficient electrolyte. However, the results of the fabrication process often continue to present challenges related to the structural complexity and self-assembly dynamics between the hydrophobic and hydrophilic parts of the constituents which in turn, necessitates additional processing steps and increases production costs. Here, a single-step process is reported for highly compact polymeric composite membranes (PCMs), fabricated using a centrifugal colloidal casting (C3) method. Combined structural analyses as well as coarse-grained molecular dynamics simulations are employed to determine the micro-/macroscopic structural characteristics of the fabricated PCMs. These findings indicate that the C3 method is capable of forming highly dense ionomer matrix-reinforcement composites consisting of microphase-separated ionomer structures with tailored crystallinity and ionic cluster sizes. An outcome that is very unlikely with the single-step coating steps in conventional methods. These structural attributes ensure PCMs with better proton conductivity, greater strain stability, and lower gas crossover properties compared to commercial pristine membranes, expanding their possible range of applicability to PEMs.

Li-Nafion Membrane Plasticised with Ethylene Carbonate/Sulfolane: Influence of Mixing Temperature on the Physicochemical Properties

The use of dipolar aprotic solvents to swell lithiated Nafion ionomer membranes simultaneously serving as electrolyte and separator is of great interest for lithium battery applications. This work attempts to gain an insight into the physicochemical nature of a Li-Nafion ionomer material whose phase-separated nanostructure has been enhanced with a binary plasticiser comprising non-volatile high-boiling ethylene carbonate (EC) and sulfolane (SL). Gravimetric studies evaluating the influence both of mixing temperature (25 to 80 °C) and plasticiser composition (EC/SL ratio) on the solvent uptake of Li-Nafion revealed a hysteresis between heating and cooling modes. Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) revealed that the saturation of a Nafion membrane with such a plasticiser led to a re-organisation of its amorphous structure, with crystalline regions remaining practically unchanged. Regardless of mixing temperature, the preservation of crystallites upon swelling is critical due to ionomer crosslinking provided by crystalline regions, which ensures membrane integrity even at very high solvent uptake (≈200% at a mixing temperature of 80 °C). The physicochemical properties of a swollen membrane have much in common with those of a chemically crosslinked polymer gel. The conductivity of ≈10⁻⁴ S cm⁻¹ demonstrated by Li-Nafion membranes saturated with EC/SL at room temperature is promising for various practical applications.

Critical Advances in Ambient Air Operation of Nonaqueous Rechargeable Li-Air Batteries

Over the past few years, great attention has been given to nonaqueous lithium-air batteries owing to their ultrahigh theoretical energy density when compared with other energy storage systems. Most of the research interest, however, is dedicated to batteries operating in pure or dry oxygen atmospheres, while Li-air batteries that operate in ambient air still face big challenges. The biggest challenges are H₂ O and CO₂ that exist in ambient air, which can not only form byproducts with discharge products (Li₂ O₂ ), but also react with the electrolyte and the Li anode. To this end, recent progress in understanding the chemical and electrochemical reactions of Li-air batteries in ambient air is critical for the development and application of true Li-air batteries. Oxygen-selective membranes, multifunctional catalysts, and electrolyte alternatives for ambient air operational Li-air batteries are presented and discussed comprehensively. In addition, separator modification and Li anode protection are covered. Furthermore, the challenges and directions for the future development of Li-air batteries are presented.

Polymer electrolytes for metal-ion batteries

The results of studies on polymer electrolytes for metal-ion batteries are analyzed and generalized. Progress in this field of research is driven by the need for solid-state batteries characterized by safety and stable operation. At present, a number of polymer electrolytes with a conductivity of at least 10−4 S cm−1 at 25 °C were synthesized. Main types of polymer electrolytes are described, viz., polymer/salt electrolytes, composite polymer electrolytes containing inorganic particles and anion acceptors, and polymer electrolytes based on cation-exchange membranes. Ion transport mechanisms and various methods for increasing the ionic conductivity in these systems are discussed. Prospects of application of polymer electrolytes in lithium- and sodium-ion batteries are outlined.

The bibliography includes 349 references.

Polymer Electrolytes for LIBs Based on Perfluorinated Sulfocationic Nepem-117 Membrane and Aprotic Solvents

Polymer electrolytes have been obtained by using Nepem-117 membranes in a Li⁺ form intercalated by polar aprotic solvents, such as dimethylformamide, dimethyl sulfoxide (DMSO), and dimethylacetamide (DMA), and solvent mixtures, such as ethylene carbonate-propylene carbonate (EC-PC), EC-DMA, EC-PC-DMA, and EC-PC-DMA-tetrahydrofuran. The obtained electrolytes have been characterized by IR impedance and ⁷Li pulsed field gradient NMR spectroscopy. Ion mobility was observed to increase with higher degrees of solvation of the membranes. A method is demonstrated to determine the solvent uptake corresponding to the percolation threshold. With comparable solvent uptake, materials containing a solvent with a higher permittivity and a lower viscosity have higher values of ionic conductivity. The membranes containing the three-component mixture of EC-PC-DMA show the highest ionic conductivity values (8.1 and 2.1 mS/cm at 25 and -20 °C, respectively). Such values exceed the conductivity of electrolytes on the basis of the Nafion membranes solvated with aprotic solvents.

Highly Proton Conducting Polyelectrolyte Membranes with Unusual Water Swelling Behavior Based on Triptycene-containing Poly(arylene ether sulfone) Multiblock Copolymers

Multiblock poly(arylene ether sulfone) copolymers are attractive for polyelectrolyte membrane fuel cell applications due to their reportedly improved proton conductivity under partially hydrated conditions and better mechanical/thermal stability compared to Nafion. However, the long hydrophilic sequences required to achieve high conductivity usually lead to excessive water uptake and swelling, which degrade membrane dimensional stability. Herein, we report a fundamentally new approach to address this grand challenge by introducing shape-persistent triptycene units into the hydrophobic sequences of multiblock copolymers, which induce strong supramolecular chain-threading and interlocking interactions that effectively suppress water swelling. Consequently, unlike previously reported multiblock copolymer systems, the water swelling of the triptycene-containing multiblock copolymers did not increase proportionally with water uptake. This combination of high water uptake and low swelling behavior of these copolymers resulted in excellent proton conductivity and membrane dimensional stability under fully hydrated conditions. In particular, the triptycene-containing multiblock copolymer film with the longest hydrophilic block length (i.e., BPSH100-TRP0-15k-15k) had a water uptake of 105%, an excellent proton conductivity of 0.150 S/cm, and a volume swelling ratio of just 29% (more than 42% reduction compared to Nafion 212).

Tuning the Perfluorosulfonic Acid Membrane Morphology for Vanadium Redox-Flow Batteries

The microstructure of perfluorinated sulfonic acid proton-exchange membranes such as Nafion significantly affects their transport properties and performance in a vanadium redox-flow battery (VRB). In this work, Nafion membranes with various equivalent weights ranging from 1000 to 1500 are prepared and the morphology-property-performance relationship is investigated. NMR and small-angle X-ray scattering studies revealed their composition and morphology variances, which lead to major differences in key transport properties related to proton conduction and vanadium-ion permeation. Their performances are further characterized as VRB membranes. On the basis of this understanding, a new perfluorosulfonic acid membrane is designed with optimal pore geometry and thickness, leading to higher ion selectivity and lower cost compared with the widely used Nafion 115. Excellent VRB single-cell performance (89.3% energy efficiency at 50 mA·cm^-2) was achieved along with a stable cyclical capacity over prolonged cycling.

Moving Beyond Mass-Based Parameters for Conductivity Analysis of Sulfonated Polymers

The proton conductivity of polymer electrolytes is critical for fuel cells and has therefore been studied in significant detail. The conductivity of sulfonated polymers has been linked to material characteristics to elucidate trends. Mass-based measurements based on water uptake and ion exchange capacity are two of the most common material characteristics used to make comparisons between polymer electrolytes, but they have significant limitations when correlated to proton conductivity. These limitations arise in part because different polymers can have significantly different densities and because conduction occurs over length scales more appropriately represented by volume measurements rather than mass. Herein we establish and review volume-related parameters that can be used to compare the proton conductivity of different polymer electrolytes. Morphological effects on proton conductivity are also considered. Finally, the impact of these phenomena on designing next-generation sulfonated polymers for polymer electrolyte membrane fuel cells is discussed.