Interaction, structure and tensile property of swollen Nafion® membranes

Effect of Electro-Sprayed Porous Electrodes on the Performance and Stability of Water Electrolysis

The efficiency of proton exchange membrane water electrolysis (PEMWE) is a critical issue in realizing the production of green hydrogen. The coexistence of three phases in the catalyst layer of PEMWE causes mass transport limitation at the interfaces between them. In particular, the vigorous production of gaseous hydrogen and oxygen derived from liquid water is generated in the form of bubbles that seriously deactivate the membrane electrode assembly (MEA). In this study, we investigated the effect of porous structure in the electrode on the efficiency of hydrogen production at a high current density, which is highly related to the mass transport limitation. A widely used commercial catalyst (IrO2) was directly coated on the membrane by the electro-spray method. Porous electrodes on the membrane were formed by the charged catalyst particles that repulsed each other due to their electrostatic forces. Our membrane electrode assembly (MEA) exhibited outstanding electrolysis performances such as 5.3 A cm-2 and 3.2 A cm-2 at 2.0 and 1.8 V, respectively, which are the highest values compared with the results published in the current studies. In addition to porosity, it was confirmed that optimum binder contents positively affect the hydrophobicity and contact resistance of MEA. Through a simple porosity-controlled technique, the performance of PEMWE, in which three phases coexist, can be improved by more than 60 %. Accordingly, we expect that our systematic study on the role of porosity in the electrodes opens a new era to efficiently produce green hydrogen.

Influence of Graphene Oxide on Mechanical and Morphological Properties of Nafion® Membranes

This study explored the influence of graphene oxide (GO) on morphological and mechanical properties of Nafion® 115 membranes with the objective of enhancing the mechanical properties of the most widely employed membrane in Proton Exchange Membrane Water Electrolyzers (PEMWE) applications. The membrane surface was modified by ultrasonically spraying a GO solution and different annealing temperatures were tested. Scanning Electron Microscopy (SEM) cross-sectional images revealed that annealing the composite membranes was sufficient to favor an interaction between the graphene oxide and the surface of the Nafion® membranes. The GO covering only 35% of the membrane surface increased the composite's wettability from hydrophobic (105.2°) to a highly hydrophilic angle (84.4°) while slightly reducing membrane swelling. Tensile tests depicted an increase in both the strain levels and tensile loads before breaking. The samples with GO presented remarkable mechanical properties when the annealing time and temperature increased; while the Nafion® control samples failed at elongations of 95% and 98%, their counterparts with GO on the surface achieved elongations of 248% and 191% when annealed at 80 °C and 110 °C respectively, demonstrating that the presence of GO mechanically stabilizes the membranes under tension. In exchange, the presence of GO altered the smoothness of the membrane surface going from an average 1.4 nm before the printing to values ranging from 8.4 to 10.2 nm depending on the annealing conditions which could affect the quality of the subsequent catalyst layer printing. Overall, the polymer's electrical insulation was unaffected, making the Nafion®-GO blend a more robust material than those traditionally used.

Efficient Pervaporation for Ethanol Dehydration: Ultrasonic Spraying Preparation of Polyvinyl Alcohol (PVA)/Ti3C2Tx Nanosheet Mixed Matrix Membranes

Polyvinyl alcohol (PVA) pervaporation (PV) membranes have been extensively studied in the field of ethanol dehydration. The incorporation of two-dimensional (2D) nanomaterials into the PVA matrix can greatly improve the hydrophilicity of the PVA polymer matrix, thereby enhancing its PV performance. In this work, self-made MXene (Ti₃C₂T_x-based) nanosheets were dispersed in the PVA polymer matrix, and the composite membranes were fabricated by homemade ultrasonic spraying equipment with poly(tetrafluoroethylene) (PTFE) electrospun nanofibrous membrane as support. Due to the gentle coating of ultrasonic spraying and following continuous steps of drying and thermal crosslinking, a thin (~1.5 μm), homogenous and defect-free PVA-based separation layer was fabricated on the PTFE support. The prepared rolls of the PVA composite membranes were investigated systematically. The PV performance of the membrane was significantly improved by increasing the solubility and diffusion rate of the membranes to the water molecules through the hydrophilic channels constructed by the MXene nanosheets in the membrane matrix. The water flux and separation factor of the PVA/MXene mixed matrix membrane (MMM) were dramatically increased to 1.21 kg·m^-2·h^-1 and 1126.8, respectively. With high mechanical strength and structural stability, the prepared PGM-0 membrane suffered 300 h of the PV test without any performance degradation. Considering the promising results, it is likely that the membrane would improve the efficiency of the PV process and reduce energy consumption in the ethanol dehydration.

SAXS Investigation of the Effect of Freeze/Thaw Cycles on the Nanostructure of Nafion® Membranes

In this study, we performed small-angle X-ray scattering (SAXS) to investigate the structure of Nafion^® membranes. The effect of freeze/thaw (F/T) cycles (from ambient temperature down to −40 °C) on the membrane nanostructure was considered for the first time. The SAXS measurements were taken for different samples: a commercial Nafion^® 212 membrane swollen in water and methanol solution, and a water-swollen silica-modified membrane. The membrane structure parameters were obtained from the measured SAXS profiles using a model-dependent approach. It is shown that the average radius of water channels (R_wc) decreases during F/T cycles due to changes in the membrane structure as a result of ice formation in the pore volume after freezing. The use of water-methanol solution (methanol content of 20 vol.%) for the membrane soaking prevents changes in the membrane structure during F/T cycles compared to the water-swollen membrane. Modification of the membrane surface with silica (SiO₂ content of 20 wt.%) led to a redistribution of water in the membrane volume and resulted in a decrease in R_wc. However, R_wc for the modified membrane did not decrease with the increasing number of F/T cycles due to the involvement of SiO₂ in the sorption of membrane water and, therefore, the prevention of ice formation.

Perspective: Morphology and ion transport in ion-containing polymers from multiscale modeling and simulations

Ion-containing polymers are soft materials composed of polymeric chains and mobile ions. Over the past several decades they have been the focus of considerable research and development for their use as the electrolyte in energy conversion and storage devices. Recent and significant results obtained from multiscale simulations and modeling for proton exchange membranes (PEMs), anion exchange membranes (AEMs), and polymerized ionic liquids (polyILs) are reviewed. The interplay of morphology and ion transport is emphasized. We discuss the influences of polymer architecture, tethered ionic groups, rigidity of the backbone, solvents, and additives on both morphology and ion transport in terms of specific interactions. Novel design strategies are highlighted including precisely controlling molecular conformations to design highly ordered morphologies; tuning the solvation structure of hydronium or hydroxide ions in hydrated ion exchange membranes; turning negative ion-ion correlations to positive correlations to improve ionic conductivity in polyILs; and balancing the strength of noncovalent interactions. The design of single-ion conductors, well-defined supramolecular architectures with enhanced one-dimensional ion transport, and the understanding of the hierarchy of the specific interactions continue as challenges but promising goals for future research.