Nafion®/poly(vinyl alcohol) blends: effect of composition and annealing temperature on transport properties

Journal of Membrane Science, 282(1-2): pp. 217-224. http://www.elsevier.com/wps/find/journaldescription.cws_home/502692/description?navopenmenu=1In this study, the transport properties (proton conductivity and methanol permeability) of Nafion® 117, solution-cast Nafion®, poly(vinyl alcohol) (PVA), and Nafion®/PVA blend membranes were measured as a function of annealing temperature (60-250oC) and blend composition for application to the direct methanol fuel cell (DMFC). A Nafion®/PVA blend membrane at 5 wt% PVA (annealed at 230oC) resulted in similar proton conductivity, but 3 times lower methanol permeability compared to Nafion® 117. In addition, an unusual trend was observed in Nafion®/PVA (50 wt% PVA) blend membranes, where proton conductivity remained relatively constant, but methanol permeability decreased by approximately one order of magnitude with increasing annealing temperature. Infrared spectroscopy reveals a band shift in the hydroxyl peak to higher wavenumbers in Nafion®/PVA blends (25-90 wt% PVA) with increasing annealing temperature suggesting an increase in the interaction between the hydroxyl groups in PVA and the sulfonic acid groups in Nafion®. For Nafion® alone, proton and methanol transport rates increased and then decreased with increasing annealing temperature with a maximum at 210oC for both solution-cast and as-received (extruded) Nafion®. This trend coincides with two transition temperatures observed by other investigators using differential scanning calorimetry, suggesting that transport properties are affected by morphological changes in Nafion®

In Situ Molecular Level Measurements of Ion Dynamics in an Electrochemical Capacitor

Improving the energy storage capability of batteries and capacitors is inherently dependent on clarifying our understanding of ion dynamics of advanced electrolytes in a variety of materials. Herein we report a new attenuated total reflectance–surface-enhanced infrared absorption spectroscopy technique that can selectively and simultaneously measure both cation and anion transport of an ionic liquid (1-ethyl-3-methylimidazolium triflate (EMIm-Tf)) in a functioning electrochemical pseudocapacitor (actuator). This new capacitor–spectroscopy technique was utilized to probe the gold current collector/RuO₂ electrode interface during both square wave and cyclic voltammetry experiments. Results show that the cations and anions transport as aggregates and the cation dominates and dictates the direction of ion transport in these devices. Results also show that ion dynamics in pseudocapacitors is a diffusion-limited process

Nanofiber Cathode Catalyst Layer Model for a Proton Exchange Membrane Fuel Cell

The cathode catalyst layer in a proton exchange membrane fuel cell is now known to contain ionomer nanofibers and experiments have demonstrated a fuel cell performance increase of $10$7% greater for the case with 10% of the cathode catalyst layer ionomer in nanofiber form versus the same case without nanofibers. This difference is consistent with trends observed in previously published experimental results. These results are significant since they suggest alternative methods to reduce platinum loading in fuel cells and to optimize fuel cell performance

Prediction of Water Solubility in Glassy Polymers Using Nonequilibrium Thermodynamics

In this study, the sorption of water in poly­(methyl methacrylate) (PMMA) was measured at various water vapor activities (0–0.85) at 25, 35, and 45 °C using a quartz spring microbalance. Furthermore, the water sorption isotherms in PMMA were predicted using two nonequilibrium thermodynamic models: the nonequilibrium lattice fluid (NELF) model and nonequilibrium statistical associating fluid theory (NE-SAFT), where excellent agreement between the NE-SAFT prediction and experimental data was observed. In contrast, deviation between the NELF model prediction and water sorption isotherms in PMMA was observed above a water activity of ca. 0.50. In situ time-resolved Fourier transform infrared attenuated total reflectance spectroscopy confirmed the presence of self-associated water (i.e., water clusters) at elevated water activities, providing a rationale for deviation between the NELF model and experimental data, where unlike NE-SAFT, the NELF model does not account for these self-association interactions. The NE-SAFT model prediction was extended to five additional glassy polymers, including poly­(lactide), poly­(acrylonitrile), poly­(ethylene terephthalate), poly­(vinyl chloride), and poly­(styrene), where good agreement between the model predictions and water sorption isotherms was also observed. Additionally, a correlation between the polymer segment number and water solubility was observed

In Situ Molecular Level Measurements of Ion Dynamics in an Electrochemical Capacitor

Improving the energy storage capability of batteries and capacitors is inherently dependent on clarifying our understanding of ion dynamics of advanced electrolytes in a variety of materials. Herein we report a new attenuated total reflectance–surface-enhanced infrared absorption spectroscopy technique that can selectively and simultaneously measure both cation and anion transport of an ionic liquid (1-ethyl-3-methylimidazolium triflate (EMIm-Tf)) in a functioning electrochemical pseudocapacitor (actuator). This new capacitor–spectroscopy technique was utilized to probe the gold current collector/RuO₂ electrode interface during both square wave and cyclic voltammetry experiments. Results show that the cations and anions transport as aggregates and the cation dominates and dictates the direction of ion transport in these devices. Results also show that ion dynamics in pseudocapacitors is a diffusion-limited process

Alkaline Chemical Stability of Polymerized Ionic Liquids with Various Cations

The success of long-lasting low-cost (nonplatinum) alkaline fuel cells is dependent on the development of anion exchange membranes (electrolyte separator) with high alkaline chemical stability. In this study, a series of methacrylate-based polymerized ionic liquids (PILs) were synthesized with various covalently attached cations: butyl­imidazolium, butylmethyl­imidazolium, trimethyl­ammonium, pentamethyl­guanidinium, butylpyrrolidinium, and trimethylphosphonium. The alkaline chemical stability of these PILs was examined in tandem with their analogous ionic salts: 1-butyl-3-methyl­imidizolium chloride, 1-butyl-2,3-dimethyl­imidazolium chloride, tetramethyl­ammonium chloride, benzyltrimethyl­ammonium chloride, hexamethyl­guanidinium chloride, 1,1-butylmethyl­pyrrolidinium chloride, and tetramethyl­phosphonium chloride. The degradation mechanisms and extent of degradation were quantified using ¹H NMR spectroscopy at various pHs (in D₂O), and temperature. The PILs with imidazolium and pyrrolidinium cations showed enhanced chemical stability relative to the PILs with ammonium and phosphonium cations. Interestingly, direct correlations were not observed between the PILs and their analogous small molecule ionic salts; significant degradation was observed in imidazolium ionic salts, most notably at high temperature/high pH conditions, while the pyrrolidinium-, ammonium-, and phosphonium-based ionic salts showed no degradation under any of the conditions examined. Additionally, results on the imidazolium ionic salts showed that methyl substitution in the C2 position limited the ring-opening degradation reaction, whereas the PIL with the unsubstituted imidazolium actually showed higher chemical stability relative to its substituted PIL counterpart. Overall, the alkaline chemical stability of the PILs in this study showed no correlation to that of their analogous small molecule ionic salts, suggesting that alkaline chemical stability studies on small molecules may not provide a solid basis for evaluating alkaline stability in polymers, counter to the hypothesis in many previous studies