Increased Water Retention in Polymer Electrolyte Membranes at Elevated Temperatures Assisted by Capillary Condensation

Abstract

We establish a new systematic methodology for controlling the water retention of polymer electrolyte membranes. Block copolymer membranes comprising hydrophilic phases with widths ranging from 2 to 5 nm become wetter as the temperature of the surrounding air is increased at constant relative humidity. The widths of the moist hydrophilic phases were measured by cryogenic electron microscopy experiments performed on humid membranes. Simple calculations suggest that capillary condensation is important at these length scales. The correlation between moisture content and proton conductivity of the membranes is demonstrated.

Ion transport in sulfonated polymers

“…It has been widely reported that the phase-separated hydrophobic phases can provide effective mechanical support while the well-arranged nanometer-scale hydrophilic domains facilitate ion transport by means of confinement effects (at the same ion exchange capacity (IEC) value, ions confined within microphase-separated hydrophilic phases are in close proximity, compared to their random analogues). 8,10,28,29 Examples of such polymers include sulfonated poly(styrene-b-isobutylene-b-styrene), 30,31 sulfonated poly(styrene-b-methylbutylene), 28,32 sulfonated poly(ether ether ketone)-b-poly(ether sulfone), 33,34 and sulfonated poly([vinylidene difluoride-chlorotrifluoro ethylene]-gstyrene). 19,35,36 Note in passing that although much of the research on the sulfonated polymers indicated that morphology is important in determining transport properties, the achievement of well-defined structures with long-range order is lacking and it has been the subject of the current studies.…”

Enhanced proton transport in nanostructured polymer electrolyte/ionic liquid membranes under water-free conditions

“…Among the many kinds of ion-conducting polymers, proton-conducting polymers have been the subject of extensive study for their possible use in PEFCs, which offer the prospect of supplying clean energy for automotives. Significant efforts have been devoted to improving the transport properties of PEFCs so they can be operated at relatively lower temperatures below 90 °C [8][9][10][11][12][13][14] . However, the development of a commercial PEFCpowered vehicle that replaces a gasoline-powered vehicle still seems distant.…”

Embedding of Hollow Polymer Microspheres with Hydrophilic Shell in Nafion Matrix as Proton and Water Micro-Reservoir

“…Hence, equipping Nafion membrane with a strong moisture-retention capability becomes necessary for maintaining the required proton conductivity through this temperature range. Incorporation of inorganic particles (e.g., silica, zeolites) into Nafion membrane matrix to enhance hygroscopy of membrane is representative of earlier efforts [4][5][6][7][8][9][10]. In the resulting composite membrane, hydrogen bonding between the surface hydroxyl group of inorganic filler and the pendant sulfonic acid group (-SO 3 H) of Nafion assists the dispersion of filler particles in the host matrix [11,12].…”

“…In the resulting composite membrane, hydrogen bonding between the surface hydroxyl group of inorganic filler and the pendant sulfonic acid group (-SO 3 H) of Nafion assists the dispersion of filler particles in the host matrix [11,12]. Grafting hydrophilic oligomeric chains to inorganic particles [3][4][5][6][7][8][9][10][11][12][13][14][15] has furthered the preparation technique of Nafion nanocomposite membranes. With the aim of achieving the forestated goal, use of hydrophilic hollow polymer spheres (HPS) as filler is apparently more attractive [16][17][18][19][20][21] because this particulate structure possesses larger interior space for storing water and the porous hydrophilic polymeric shell prevents quick evaporation of water while the membrane is subjected to dehydration [22,23].…”