Characterization and Modeling of Free Volume and Ionic Conduction in Multiblock Copolymer Proton Exchange Membranes

Free volume plays a key role on transport in proton exchange membranes (PEMs), including ionic conduction, species permeation, and diffusion. Positron annihilation lifetime spectroscopy and electrochemical impedance spectroscopy are used to characterize the pore size distribution and ionic conductivity of synthesized PEMs from polysulfone/polyphenylsulfone multiblock copolymers with different degrees of sulfonation (SPES). The experimental data are combined with a bundle-of-tubes model at the cluster-network scale to examine water uptake and proton conduction. The results show that the free pore size changes little with temperature in agreement with the good thermo-mechanical properties of SPES. However, the free volume is significantly lower than that of Nafion®, leading to lower ionic conductivity. This is explained by the reduction of the bulk space available for proton transfer where the activation free energy is lower, as well as an increase in the tortuosity of the ionic network.

Branched Sulfonimide-Based Proton Exchange Polymer Membranes from Poly(Phenylenebenzopheneone)s for Fuel Cell Applications

Improved proton conductivity and high durability are now a high concern for proton exchange membranes (PEMs). Therefore, highly proton conductive PEMs have been synthesized from branched sulfonimide-based poly(phenylenebenzophenone) (SI-branched PPBP) with excellent thermal and chemical stability. The branched polyphenylene-based carbon-carbon backbones of the SI-branched PPBP membranes were attained from the 1,4-dichloro-2,5-diphenylenebenzophenone (PBP) monomer using 1,3,5-trichlorobenzene as a branching agent (0.1%) via the Ni-Zn catalyzed C-C coupling reaction. The as-synthesized SI-branched PPBP membranes showed 1.00~1.86 meq./g ion exchange capacity (IEC) with unique dimensional stability. The sulfonimide groups of the SI-branched PPBP membranes had improved proton conductivity (75.9-121.88 mS/cm) compared to Nafion 117 (84.74 mS/cm). Oxidation stability by thermogravimetric analysis (TGA) and Fenton's test study confirmed the significant properties of the SI-branched PPBP membranes. Additionally, a very distinct microphase separation between the hydrophobic and hydrophilic moieties was observed using atomic force microscopic (AFM) analysis. The properties of the synthesized SI-branched PPBP membranes demonstrate their viability as an alternative PEM material.

Diffusivity and free anion concentration of ionic liquid composite polybenzimidazole membranes

PBI composite membranes containing 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-NTf₂) at 1, 5, 10, 20 and 50 wt% have been prepared and the conductivity has been analyzed by electrochemical impedance spectroscopy.

Sulfonated Copper Phthalocyanine/Sulfonated Polysulfone Composite Membrane for Ionic Polymer Actuators with High Power Density and Fast Response Time

Ionic polymer composite membranes based on sulfonated poly(arylene ether sulfone) (SPAES) and copper(II) phthalocyanine tetrasulfonic acid (CuPCSA) are assembled into bending ionic polymer actuators. CuPCSA is an organic filler with very high sulfonation degree (IEC = 4.5 mmol H⁺/g) that can be homogeneously dispersed on the molecular scale into the SPAES membrane, probably due to its good dispersibility in SPAES-containing solutions. SPAES/CuPCSA actuators exhibit larger ion conductivity (102 mS cm^-1), tensile modulus (208 MPa), strength (101 MPa), and strain (1.21%), exceptionally faster response to electrical stimuli, and larger mechanical power density (3028 W m^-3) than ever reported for ion-conducting polymer actuators. This outstanding actuation performance of SPAES/CuPCSA composite membrane actuators makes them attractive for next-generation transducers with high power density, which are currently developed, e.g., for underwater propulsion and endoscopic surgery.

Graphene oxide based nanohybrid proton exchange membranes for fuel cell applications: An overview

In the context of many applications, such as polymer composites, energy-related materials, sensors, 'paper'-like materials, field-effect transistors (FET), and biomedical applications, chemically modified graphene was broadly studied during the last decade, due to its excellent electrical, mechanical, and thermal properties. The presence of reactive oxygen functional groups in the grapheme oxide (GO) responsible for chemical functionalization makes it a good candidate for diversified applications. The main objectives for developing a GO based nanohybrid proton exchange membrane (PEM) include: improved self-humidification (water retention ability), reduced fuel crossover (electro-osmotic drag), improved stabilities (mechanical, thermal, and chemical), enhanced proton conductivity, and processability for the preparation of membrane-electrode assembly. Research carried on this topic may be divided into protocols for covalent grafting of functional groups on GO matrix, preparation of free-standing PEM or choice of suitable polymer matrix, covalent or hydrogen bonding between GO and polymer matrix etc. Herein, we present a brief literature survey on GO based nano-hybrid PEM for fuel cell applications. Different protocols were adopted to produce functionalized GO based materials and prepare their free-standing film or disperse these materials in various polymer matrices with suitable interactions. This review article critically discussed the suitability of these PEMs for fuel cell applications in terms of the dependency of the intrinsic properties of nanohybrid PEMs. Potential applications of these nanohybrid PEMs, and current challenges are also provided along with future guidelines for developing GO based nanohybrid PEMs as promising materials for fuel cell applications.

Proton conducting electrospun sulfonated polyether ether ketone graphene oxide composite membranes

A series of novel composite membranes, based on sulfonated poly(ether ketone) (SPEEK) with a graphene oxide (GO) layer, were prepared.

Facile synthesis of porous metal oxide nanotubes and modified nafion composite membranes for polymer electrolyte fuel cells operated under low relative humidity

We describe a facile route to fabricate mesoporous metal oxide (TiO2, CeO2 and ZrO1.95) nanotubes for efficient water retention and migration in a Nafion membrane operated in polymer electrolyte fuel cell under low relative humidity (RH). Porous TiO2 nanotubes (TNT), CeO2 nanotubes (CeNT), and ZrO1.95 (ZrNT) were synthesized by calcining electrospun polyacrylonitrile nanofibers embedded with metal precursors. The nanofibers were prepared using a conventional single spinneret electrospinning technique under an ambient atmosphere. Their porous tubular morphology was observed by SEM and TEM analyses. HR-TEM results revealed a porous metal oxide wall composed of small particles joined together. The mesoporous structure of the samples was analyzed using BET. The tubular morphology and outstanding water absorption ability of the TNT, CeNT, and ZrNT fillers resulted in the effective enhancement of proton conductivity of Nafion composite membranes under both fully humid and dry conditions. Compared to a commercial membrane (Nafion, NRE-212) operated under 100% RH at 80 °C, the Nafion-TNT composite membrane delivered approximately 1.29 times higher current density at 0.6 V. Compared to the Nafion-TiO2 nanoparticles membrane, the Nafion-TNT membrane also generated higher current density at 0.6 V. Additionally, compared to a NRE-212 membrane operated under 50% RH at 80 °C, the Nafion-TNT composite membrane exhibited 3.48 times higher current density at 0.6 V. Under dry conditions (18% RH at 80 °C), the Nafion-TNT, Nafion-CeNT, and Nafion-ZrNT composite membranes exhibited 3.4, 2.4, and 2.9 times higher maximum power density, respectively, than the NRE-212 membrane. The remarkably high performance of the Nafion composite membrane was mainly attributed to the reduction of ohmic resistance by the mesoporous hygroscopic metal oxide nanotubes, which can retain water and effectively enhance water diffusion through the membrane.

Oriented MOF-polymer composite nanofiber membranes for high proton conductivity at high temperature and anhydrous condition

The novel oriented electrospun nanofiber membrane composed of MOFs and SPPESK has been synthesized for proton exchange membrane fuel cell operating at high temperature and anhydrous conditions. It is clear that the oriented nanofiber membrane displays the higher proton conductivity than that of the disordered nanofiber membrane or the membrane prepared by conventional solvent-casting method (without nanofibers). Nanofibers within the membranes are significantly oriented. The proton conductivity of the oriented nanofiber membrane can reach up to (8.2 ± 0.16) × 10⁻² S cm⁻¹ at 160°C under anhydrous condition for the highly orientation of nanofibers. Moreover, the oxidative stability and resistance of methanol permeability of the nanofibers membrane are obviously improved with an increase in orientation of nanofibers. The observed methanol permeability of 0.707 × 10⁻⁷ cm² s⁻¹ is about 6% of Nafion-115. Consequently, orientated nanofibers membrane is proved to be a promising material as the proton exchange membrane for potential application in direct methanol fuel cells.