Proton-Conducting Poly-γ-glutamic Acid Nanofiber Embedded Sulfonated Poly(ether sulfone) for Proton Exchange Membranes

Construction of three-dimensional proton-conduction networks with functionalized PU@PAN/UiO-66 nanofibers for proton exchange membranes

Proton exchange membranes (PEMs) play an important role in fuel cells. For realizing a nanofiber (NF) structure design in PEMs, the material should have tunable pores and a high specific area. In this study, we attempt to design a novel NF with synergistic architecture doped MOF for constructing three-dimensional (3D) proton conduction networks in PEMs. In this framework, UiO-66-COOH serves as a platform for proton sites to synergistically promote proton conductivity via polyvinylpyrrolidone dissolution, hydrolyzation of polyacrylonitrile, and sulfamic acid functionalization of the shell-layer NF. Benefiting from enriched proton-transfer sites in NFs, the obtained composite membrane overcomes the trade-off among proton conductivity, methanol permeability, and mechanical stability. The composite membrane with 50 % fiber (Nafion/S@NF-50) exhibited a high proton conductivity of 0.212 S cm-1 at 80 °C and 100 % relative humidity, suppressed methanol permeability of 0.66 × 10-7 cm2 s-1, and the maximum power density of direct methanol fuel cell is 182.6 mW cm-2. Density functional theory was used to verify the important role of sulfamic acid in proton transfer, and the activation energy barriers under anhydrous and hydrous conditions are only 0.337 and 0.081 kcal, respectively. This study opens up new pathways for synthesizing NF composite PEMs.

Proton conductivity of the protein-based velvet worm slime

The properties of complex bodily fluids are linked to their biological functions through natural selection. Velvet worms capture their prey by ensnaring them with a proteinaceous fluid (slime). We examined the electrical conductivity of slime and found that dry slime is an insulator. However, its conductivity can increase by up to 106 times in its hydrated state, which can be further increased by an order in magnitude under acidic hydration (pH ≈ 2.3). The transient current measured using ion-blocking electrodes showed a continuous decay for up to 7 h, revealing slime's nature as a proton conducting material. Slime undergoes a spontaneous fibrilization process producing high aspect ratio ≈ 105 fibers that exhibit an average conductivity ≈2.4 ± 1.1 mS cm-1. These findings enhance our understanding of slime as a natural biopolymer and provide molecular-level guidelines to rationally design biomaterials that may be employed as hygroscopic conductors.

Construction of Successive Proton Conduction Channels to Accelerate the Proton Conduction Process in Flexible Proton Exchange Membranes

Successive proton conduction channels are constructed with the spin coating method in flexible proton exchange membranes (PEMs). In this research, phosphoric acid (PA) molecules are immobilized in the multilayered microstructure of Kevlar nanofibers and polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) polymer molecular chains. As a result, successive proton conduction channels can accelerate the proton conduction process in the prepared membrane with the multilayered microstructure. Additionally, the microstructure fractures of the composite membranes from the external force of folding and stretching operations are modified by the inner PA molecules. Notably, numerous PA molecules are further combined through formed intermolecular hydrogen bonding. The stretched membrane absorbs more PA molecules owing to the arrangement of PA molecules, Kevlar nanofibers, and SEBS molecular chains. The stretched membrane thus exhibits the enhanced proton conduction ability, such as the through-plane proton conductivity of 1.81 × 10-1 S cm-1 at 160 °C and that of 4.53 × 10-2 S cm-1 at 120 °C lasting for 600 h. Furthermore, the tensile stress of PA-doped stretched membranes reaches (3.91 ± 0.40)-(6.15 ± 0.43) MPa. A single proton exchange membrane fuel cell exhibits a peak power density of 483.3 mW cm-2 at 120 °C.

Morphology and Transport Study of Acid-Base Blend Proton Exchange Membranes by Molecular Simulations: Case of Chitosan/Nafion

Blending a basic polymer (e.g., chitosan) with Nafion can modify some membrane properties in direct methanol fuel cell applications, e.g., controlling methanol crossover, by regulating the morphology of hydrophilic channels. Unraveling the mechanisms by which the channel morphology is modified is essential to formulate design strategies for acid-base blend membrane development. Thus, we use molecular simulations to analyze the morphological features of a blend membrane (at 75/25 chitosan/Nafion wt %), i.e., (i) water/polymer phase organizations, (ii) number and size of water clusters, and (iii) quantitative morphological measures of hydrophilic channels, and compare them to the pure Nafion in a wide range of water contents. It is found that the affinity of water to different hydrophilic groups in the blend membrane can result in more distorted and dispersed hydrophilic phase and fewer bulk water-like features compared to pure Nafion. Also, the width of the hydrophilic network bottleneck, i.e., pore limiting diameter (PLD), is found to be almost five times smaller for the blend membrane compared to Nafion at their maximum water contents. Moreover, by changing the chitosan/Nafion weight ratio from 75/25 to 0/100, we show that as Nafion content increases, all channel morphological characteristics alter monotonically except PLD. This is mainly due to the strong acid-base interactions between Nafion and chitosan, which hinder the monotonic growth of PLD. Interestingly, water and methanol diffusion coefficients are strongly correlated with PLD, suggesting that PLD can be used as a single parameter for tailoring the blending ratio for achieving the desired diffusion properties of acid-base membranes.

Polyelectrolyte membranes based on phosphorylated-PVA/cellulose acetate for direct methanol fuel cell applications: synthesis, instrumental characterization, and performance testing

Designing and synthesis of cost-effective and improved methanol permeable and proton conductive membranes are the main challenges for preparation of polymeric electrolyte membrane (PEM). Herein, a cost-effective PEM membrane based on phosphorylated polyvinyl alcohol (PVA)-grafted-cellulose acetate (CA) was prepared by a solution-casting technique. Water and methanol uptakes of phosphorylated PVA/CA membranes were characterized as function with the molar ratio of CA. Additionally, structure and morphology of phosphorylated PVA/CA (Ph-PVA/CA) membranes were verified by FT-IR analysis, SEM investigation. Furthermore, ion exchange capacity (IEC), proton conductivity and methanol permeation of Ph-PVA/CA membranes were examined based on the concentration of OPA basically. The results manifested a perceptible improvement in proton conductivity from 0.035 to 0.05 S/cm at 25 and 70 °C, respectively using 600 μL of OPA, and IEC of 2.1 meq/g using 400 μL of OPA at ambient temperature. On the other hand, methanol permeability (P = 1.08 × 10-10 cm2/s) was lower than Nafion 117 admirably. The optimum OPA concentration was 200 μL according to conductivity measurements (at 10% PVA, 150 μL GA, and CA 7%). Finally, prepared Ph-PVA/CA membranes exhibited enhancement in critical natures such as proton conductivity and IEC combined with its low-cost materials, which make them excellent candidate as PEM for DMFCs application.

Construction of Proton Transport Highways Induced by Polarity-Driving in Proton Exchange Membranes to Enhance the Performance of Fuel Cells

The approach to constructing proton transport channels via direct adjustments, including hydrophilia and analytical acid concentration in hydrophilic domains, has been proved to be circumscribed when encouraging the flatter hydrophilic-hydrophobic microphase separation structures and reducing conductivity activation energy. Here, we propose a constructive solution by regulating the polarity of hydrophobic domains, which indirectly varies the aggregation and connection of hydrophilic ion clusters during membrane formation, enabling orderly self-assembly and homogeneously distributed microphase structures. Accordingly, a series of comb-shaped polymers were synthesized with diversified optimization, and more uniformly distributed ion cluster lattices were subsequently observed using high-resolution transmission electron microscopy. Simultaneously, combining with density functional theory calculations, we analyzed the mechanism of membrane degradations caused by hydroxyl radical attacks. Experimental results demonstrated that, facilitated by proper molecule polarity, beneficial changes of bond dissociation energy could extend the membrane lifetime more than the protection from side chains near ether bonds, which were deemed to reduce the probability of attacks by the steric effect. With the optimal strategy chosen among various trials, the maximum power density of direct methanol fuel cell and H₂/air proton exchange membrane fuel cell was enhanced to 95 and 485 mW cm^-2, respectively.

De Novo Ion-Exchange Membranes Based on Nanofibers

The unique functions of nanofibers (NFs) are based on their nanoscale cross-section, high specific surface area, and high molecular orientation, and/or their confined polymer chains inside the fibers. The introduction of ion-exchange (IEX) groups on the surface and/or inside the NFs provides de novo ion-exchangers. In particular, the combination of large surface areas and ionizable groups in the IEX-NFs improves their performance through indices such as extremely rapid ion-exchange kinetics and high ion-exchange capacities. In reality, the membranes based on ion-exchange NFs exhibit superior properties such as high catalytic efficiency, high ion-exchange and adsorption capacities, and high ionic conductivities. The present review highlights the fundamental aspects of IEX-NFs (i.e., their unique size-dependent properties), scalable production methods, and the recent advancements in their applications in catalysis, separation/adsorption processes, and fuel cells, as well as the future perspectives and endeavors of NF-based IEMs.

Recent progress and challenges in solution blow spinning

In the past 30 years, researchers have worked towards reducing the size of ordinary three-dimensional (3D) materials into 1D or 2D materials in order to obtain new properties and applications of these low-dimensional systems. Among them, functional nanofibers with large surface area and high porosity have been widely studied and paid attention to. Because of the interesting properties of nanofibers, they find extensive application in filtration, wound dressings, composites, sensors, capacitors, nanogenerators, etc. Recently, a variety of nanofiber preparation methods such as melt blowing, electrospinning (e-spinning), centrifugal spinning and solution blow spinning (SBS) have been proposed. This paper includes a brief review of the fundamental principles of the preparation of nanofibers for solution jet spinning, the influence of experimental parameters, and the properties and potential applications of the solution-blown fibers. And the industrialization and challenges of SBS are also included.