A review of polymer–nanocomposite electrolyte membranes for fuel cell application

Role of Surface Chemistry on Nanoparticle Dispersion and Vanadium Ion Crossover in Nafion Nanocomposite Membranes

While the introduction of nanoparticles into Nafion membranes has proven to be a viable method to tune the ion selectivity in energy storage technologies such as the vanadium redox flow battery, there still remains a limited understanding of the fundamental mechanism by which the nanoparticles selectively restrict ion crossover. Herein, the surface chemistry and loading of SiO₂ nanoparticles (SiNPs) were systematically varied to elucidate the relationship between nanoparticle dispersion (or dispersion state) and vanadium ion permeability in Nafion nanocomposite membranes. Specifically, nanoparticle surface functionalization was altered to achieve both attractive (amine-functionalized) and repulsive (unfunctionalized and sulfonic acid-functionalized) electrostatic interactions between the SiNPs and the ionic groups of Nafion. At a nanoparticle loading of 5 wt %, membranes containing unfunctionalized and amine-functionalized SiNPs demonstrated ∼25% reduction in vanadium ion permeability as compared to unmodified Nafion. Drastically different dispersion states were observed in the electron microscopy images of each nanocomposite membrane, where most notably, aggregates on the order of 500 nm were observed for membranes containing amine-functionalized SiNPs (at all nanoparticle loadings). Results from this work indicate that both dispersion state and surface chemistry of the SiNPs play a critical role in governing the vanadium ion transport in these ionomer nanocomposite membranes.

Meniscus Shape around Nanoparticles Embedded in Molecularly Thin Liquid Films

Individual nanoparticles embedded in molecularly thin films at planar substrates and the resulting film surface distortion (meniscus) adjacent to the nanoparticles are investigated by conventional optical reflection microscopy. Even for nanoparticles much smaller than the Rayleigh diffraction limit, the meniscus creates such a pronounced optical footprint that the location of the nanoparticles can be identified. This is because the decay length (lateral extension) of the meniscus exceeds the size of the nanoparticle by orders of magnitude. Therefore, for the first time, the exact shape of the meniscus of the liquid adjacent to a nanosize object could be measured and analyzed. The meniscus has a zero curvature shape (cosine hyperbolic). The liquid in the meniscus is in pressure equilibrium with the far-field planar film. The decay length decreases with the decreasing nanoparticle size. However, it is independent of the far-field film thickness. Supposedly, the decay length is determined by van der Waals interactions although it is unknown what determines its (unexpectedly large) absolute value. The presented technical approach may be used to investigate biological systems (for instance, surface distortions in supported membranes caused by proteins or protein aggregates).

Polymer Nanocomposite Membranes

Based on the results of research works reflected in the scientific literature, the main examples, methods and approaches to the development of polymer inorganic nanocomposite materials for target membranes are considered. The focus is on membranes for critical technologies with improved mechanical, thermal properties that have the necessary capabilities to solve the problems of a selective pervaporation. For the purpose of directional changes in the parameters of membranes, effects on their properties of the type, amount and conditions of nanoparticle incorporation into the polymer matrix were analyzed. An influence of nanoparticles on the structural and morphological characteristics of the nanocomposite film is considered, as well as possibilities of forming transport channels for separated liquids are analyzed. Particular attention is paid to a correlation of nanocomposite structure-transport properties of membranes, whose separation characteristics are usually considered within the framework of the diffusion-sorption mechanism.

Enhanced Performance of a Sulfonated Poly(arylene ether ketone) Block Copolymer Bearing Pendant Sulfonic Acid Groups for Polymer Electrolyte Membrane Fuel Cells Operating at 80% Relative Humidity

The series of sulfonated poly(arylene ether ketone) (SPAEK) block copolymers with controlled F-oligomer length bearing pendant diphenyl unit were synthesized via a polycondensation reaction. Sulfonation was verified by ¹H NMR analysis to introduce sulfonic acid group selectively and intensively on the pendant diphenyl unit of polymer backbones. The SPAEK membranes fabricated by the solution casting approach were very transparent and flexible with the thickness of ∼50 μm. These membranes with different F-oligomer lengths were investigated to the physiochemical properties such as water absorption, dimensional stability, ion exchange capacity, and proton conductivity. As a result, the SPAEK membranes (X4.8Y8.8, X7.5Y8.8, and X9.1Y8.8) in accordance to increasing the length of hydrophilic oligomer showed excellent proton conductivity in range of 131-154 mS cm^-1 compared to Nafion-115 (131 mS cm^-1) at 90 °C under 100% relative humidity (RH). Among the SPAEK membranes, proton conductivity of SPAEK X9.1Y8.8 (140.7 mS cm^-1) is higher than that of Nafion-115 (102 mS cm^-1) at 90 °C under 80% RH. The atomic force microscopy image demonstrated that number of ion transport channels is increased with increase in the length of hydrophilic oligomer in the main chains, and the morphology is proved to be related to the proton conductivity. The synthesized SPAEK membrane exhibited a maximum power density of 324 mW cm^-2, which is higher than that of Nafion-115 (291 mW cm^-2) at 60 °C under 100% RH.

High-Performance and Recyclable Al-Air Coin Cells Based on Eco-friendly Chitosan Hydrogel Membranes

Aluminum-air batteries are a promising power supply for electronics due to their low cost and high energy density. However, portable coin-type Al-air batteries operating under ambient air condition for small electronic appliances have rarely been reported. Herein, coin cell-type Al-air batteries using cost-effective and eco-friendly chitosan hydrogel membranes modified by SiO₂, SnO₂, and ZnO have been prepared and assembled. The Al-air coin cell employing chitosan hydrogel membrane containing 10 wt % SiO₂ as a separator exhibits better discharge performance with a higher flat voltage plateau, longer discharge duration, and higher power density than the cells using a chitosan hydrogel membrane containing SnO₂ or ZnO. Moreover, we also demonstrate that the presented Al-air coin cell can be recycled by a series of eco-friendly procedures using food-grade ingredients, resulting in recycled products that are environmentally safe and ready for reuse. The Al-air coin cell adopting a recycled cathode from a fully discharged Al-air coin cell using the above-mentioned procedure has shown comparable performance to cells assembled with a new cathode. With these merits of enhanced electrochemical performance and recyclability, this new Al-air coin cell with modified chitosan hydrogel membrane can find wide applications for powering portable and small-size electronics.

Sulfonated Poly(Arylene Ether Sulfone) and Perfluorosulfonic Acid Composite Membranes Containing Perfluoropolyether Grafted Graphene Oxide for Polymer Electrolyte Membrane Fuel Cell Applications

Sulfonated poly(arylene ether sulfone) (SPAES) and perfluorosulfonic acid (PFSA) composite membranes were prepared using perfluoropolyether grafted graphene oxide (PFPE-GO) as a reinforcing filler for polymer electrolyte membrane fuel cell (PEMFC) applications. PFPE-GO was obtained by grafting poly(hexafluoropropylene oxide) having a carboxylic acid end group onto the surface of GO via ring opening reaction between the carboxylic acid group in poly(hexafluoropropylene oxide) and the epoxide groups in GO, using 4-dimethylaminopyridine as a base catalyst. Both SPAES and PFSA composite membranes containing PFPE-GO showed much improved mechanical strength and dimensional stability, compared to each linear SPAES and PFSA membrane, respectively. The enhanced mechanical strength and dimensional stability of composite membranes can be ascribed to the homogeneous dispersion of rigid conjugated carbon units in GO through the increased interfacial interactions between PFPE-GO and SPAES/PFSA matrices.

Janus or homogeneous nanoparticle mediated self-assembly of polymer electrolyte fuel cell membranes

The functionality of polymer electrolyte fuel cell membranes depends on the self-assembled structure of the graft polymer. To control self-assembly, nanoparticles (NPs) are often used as catalysts. Hence, we investigate the effect of hydrophilic (HI), hydrophobic (HO), and Janus nanoparticles (JNPs) for the self-assembly of graft polymers using dissipative particle dynamics (DPD) simulations. We found that the differences that appeared among the self-assembled structures of water depended on the concentration of PEFC. We also calculated the diffusion constant of water (D(H2O)) from the slopes of the time-averaged mean square displacement (MSD) curves. HI NPs had the largest effect in suppressing the diffusion of water because the HI NPs incorporated into the water particles. It was also seen that D(H2O) with various NPs gradually decreased as the number of NPs increased for three PEFC concentrations (70%, 80%, and 90%). Thus, a close correlation between the position and chemical composition of NPs in polymer electrolyte fuel cell (PEFC) membrane systems has been found. Moreover, the mean square radius of gyration 〈R g〉 and the mean square end-to-end distance 〈R〉 was calculated to analyse the self-assembled structures of PEFC. The 〈R g〉 and 〈R〉 increased as the concentration of PEFC was increased, with and without various NPs.

In-Plane Channel-Structured Catalyst Layer for Polymer Electrolyte Membrane Fuel Cells

In this study, we present a novel catalyst layer (CL) with in-plane flow channels to enhance the mass transports in polymer electrolyte membrane fuel cells. The CL with in-plane channels on its surface is fabricated by coating a CL slurry onto a surface-treated substrate with the inverse line pattern and transferring the dried CL from the substrate to a membrane. The membrane electrode assembly with the in-plane channel-patterned CL has superior power performances in high current densities compared with an unpatterned, flat CL, demonstrating a significant enhancement of the mass-transport property by the in-plane channels carved in the CL. The performance gain is more pronounced when the channel direction is perpendicular to the flow field direction, indicating that the in-plane channels increase the utilization of the CL under the rib area. An oxygen-transport resistance analysis shows that both molecular and Knudsen diffusion can be facilitated with the introduction of the in-plane channels. The direct CL patterning technique provides a platform for the fabrication of advanced CL structures with a high structural fidelity and design flexibility and a rational guideline for designing high-performance CLs.

Understanding methods of preparation and characterization of pore-filling polymer composites for proton exchange membranes: a beginner’s guide

Composite membranes based on porous support membranes filled with a proton-conducting polymer appear to be a promising approach to develop novel proton exchange membranes (PEMs). It allows optimization of the properties of the filler and the matrix separately, e.g. for maximal conductivity of the former and maximal physical strength of the latter. In addition, the confinement itself can alter the properties of the filling ionomer, e.g. toward higher conductivity and selectivity due to alignment and restricted swelling. This article reviews the literature on PEMs prepared by filling of submicron and nanometric size pores with Nafion and other proton-conductive polymers. PEMs based on alternating perfluorinated and non-perfluorinated polymer systems and incorporation of fillers are briefly discussed too, as they share some structure/transport relationships with the pore-filling PEMs. We also review here the background knowledge on structural and transport properties of Nafion and proton-conducting polymers in general, as well as experimental methods concerned with preparation and characterization of pore-filling membranes. Such information will be useful for preparing next-generation composite membranes, which will allow maximal utilization of beneficial characteristics of polymeric proton conductors and understanding the complicated structure/transport relationships in the pore-filling composite PEMs.

Facile construction of crosslinked anion exchange membranes based on fluorenyl-containing polysulfone via click chemistry

Crosslinked fluorenyl-containing polysulfone based anion exchange membranes have been successfully synthesized via click chemistry with improved properties by controlling the crosslinking degree and micro-phase structure.

Interfacial phenomena at a surface of individual and complex fumed nanooxides

Investigations of interfacial and temperature behaviors of nonpolar and polar adsorbates interacting with individual and complex fumed metal or metalloid oxides (FMO), initial and subjected to various treatments or chemical functionalization and compared to such porous adsorbents as silica gels, precipitated silica, mesoporous ordered silicas, filled polymeric composites, were analyzed. Complex nanooxides include core-shell nanoparticles, CSNP (50-200nm in size) with titania or alumina cores and silica or alumina shells in contrast to simple and smaller nanoparticles of individual FMO. CSNP could be destroyed under high-pressure cryogelation (HPCG) or mechanochemical activation (MCA). These treatments affect the structure of aggregates of nanoparticles and agglomerates of aggregates, resulting in their becoming more compacted. The analysis shows that complex FMO could be more sensitive to external actions than simple nanooxides such as fumed silica. Any treatment of 'soft' FMO affects the interfacial and temperature behaviors of polar and nonpolar adsorbates. Rearrangement of secondary particles and surface functionalization affects the freezing-melting point depression of adsorbates. For some adsorbates, open hysteresis loops became readily apparent in adsorption-desorption isotherms. Clustering of adsorbates bound in textural pores in aggregates of nanoparticles (i.e., voids between nanoparticles in secondary structures) causes reduced changes in enthalpy during phase transitions (freezing, fusion, evaporation). Freezing point depression and melting point elevation cause significant hysteresis freezing-melting effects for adsorbates bound to FMO in the textural pores. Relaxation phenomena for both low- and high-molecular weight adsorbates or filled polymeric composites are affected by the morphology of primary particles, structural organization of secondary particles of differently treated or functionalized FMO, content of adsorbates, co-adsorption order, and temperature.

Fourfold action of surfactants with superacid head groups: polyoxometalate–silicone nanocomposites as promising candidates for proton-conducting materials

Surfactants with a polyoxometalate (POM) [PW₁₁O₃₉]³⁻ head and protons as counter-ions are used for the emulsion polymerization of hybrid polydimethylsiloxane (PDMS) networks.