We investigate two types of polysulfone-based membranes (quaternary ammonium-functionalized anion exchange membrane and sulfonated proton exchange membrane) using molecular dynamics simulations to compare their nanophase-segregated structures and transport properties. Although the distribution of ionic groups on the polymer backbone is similar for both types, the quaternary ammonium groups and hydroxide ions in the anion exchange membrane are more solvated by water compared to the sulfonate groups and hydronium ions in the proton exchange membrane. Correspondingly, such better solvation of the ammonium groups and hydroxide ions leads the internal structure to a less matured hydrogen bonding network in the water phase, especially at low water content conditions. Through analysis of the nanophase segregation of the membranes, it is found that the characteristic correlation length has a similar value for both membranes, whereas the concentration contrast between the polymer domain and water phase is more distinct in the anion exchange membrane relative to the proton exchange membrane. Within such nanophase-segregated structures, it is found that the diffusion of hydroxide is ∼6% and ∼11% of that of hydronium at 10 and 20 wt % of water content, respectively, which might be due to the strong correlation at ∼4 Å among the hydroxide in the anion exchange membrane.

Proton-conducting superacidic polymer membranes with different fluoroalkyl sulfonate pendants attached to aromatic polymer backbones were synthesized via C−H functionalization and Suzuki coupling reactions. Variation in the chemical structures of the pendant acidic sulfonate moieties and their effects on membrane properties including water uptake, ion exchange capacity, morphology, and proton conductivity were systemically investigated. Membranes containing the short −OCF 2 SO 3 H pendant (PSU-S 5 ) showed a smaller hydrophilic domain size and lower proton conductivity than those containing the longer pendants −OCF 2 CF 2 SO 3 H (PSU-S 1 ) and −SCF 2 CF 2 SO 3 H (PSU-S 4 ), likely due to the short chain's less favorable aggregation and lower acidity. Polymer electrolyte membranes with unique branched fluoroalkyl sulfonate pendants (PSU-S 6 ) gave larger ionic domain sizes, more uniform hydrophilic channels, and higher proton conductivity than samples with analogous linear pendant chains (PSU-S 1 ), indicating that branched sulfonate structures may be a key future direction in the field of fuel cell membrane. ■ INTRODUCTIONIncreasing concerns about the environmental impact of our overdependence on fossil fuels have motivated research on alternative clean energy technologies. Proton exchange membrane (PEM) fuel cells, which are composed of a cathode, an anode, and a PEM, generate electricity cleanly via electrochemical reactions of hydrogen and oxygen to yield water and heat as the only byproducts. 1−5 The development of perfluorosulfonic acid ionomers, such as Nafion, has greatly contributed to fuel cell technologies, and these materials are still widely used as the benchmark membrane in fuel cells. Because of its perfluorinated structure and superacidic pendant side chain, Nafion possesses high proton conductivity as well as good chemical stability. However, Nafion is still not an ideal PEM material, and its drawbacks (e.g., high cost, low operation temperature, and high methanol crossover) require development of alternative PEMs for successful adoption of fuel cells as reliable and inexpensive energy conversion devices. 6−9 Over the past decades, extensive efforts have been devoted to the development of hydrocarbon-based PEMs, and many aryl and alkyl sulfonated polymers have been described. 10−16 In general, these sulfonated aromatic polymer PEMs with high ion exchange capacity and high conductivity swell excessively under high hydration conditions and give much lower proton conductivity than Nafion when the relative humidity (RH) or water content of the membrane is reduced. PEMs with good proton conductivity at high temperature (above 100°C) and low RH can bring many advantages to the fuel cell system, such as fast electrode reaction kinetics, good tolerance toward carbon monoxide and other fuel and air impurities, and simplified water management strategies. 8,13,17−19 To achieve highly conductive materials under low hydration conditions, creation of well-connected hydrophilic channels within the membrane throu...

The deformation of hydrated Nafion 117 was implemented using full-atomistic molecular dynamics simulation method to elucidate how the mechanical deformation affects the structure and transport of hydrated Nafion membrane. First, Nafion 117 membrane was equilibrated with 20 wt. % water content through an annealing procedure. The simulated characteristic correlation length and the diffusion coefficient of water and hydronium ions were analyzed for comparison with those observed in experiments. Then, the equilibrated Nafion membrane was deformed uniaxially up to 300 % of strain with a constant strain rate. The change in nanophase-segregation of hydrated Nafion during the deformation process was characterized using a directional structure factor as well as the pair correlation function in order to achieve fundamental understanding of the relationship of such structural change as a function of strain with the proton transport. It was found from the pair correlation analysis that the sulfonate distribution and sulfonate-hydronium correlation became stronger through the deformation while the hydronium ion solvation and the internal structure of water phase were not dependent on the deformation. From the directional structure factor profile, it was found that the long range correlation was developed in the perpendicular direction to the extension. The diffusions of water and hydronium ions were enhanced by 30 and 2 %, respectively, after the deformation. From this study, we suggested that it is desirable to investigate the proton transport using simulation methods covering larger dimensions with a long time scale.

Infrared spectra of Nafion, Aquivion, and the 3M membrane were acquired during total dehydration of 18 fully hydrated samples. Fully hydrated exchange sites are in a sulfonate form with a C 3V local 19 symmetry. The mechanical coupling of the exchange site to a side chain ether link gives rise to 20 vibrational group modes that are classified as C 3V modes. These mode intensities diminish 21 concertedly with dehydration. When totally dehydrated, the sulfonic acid form of the exchange site is

Multiblock sulfonated poly(arylene ether sulfone) membranes with various block lengths are investigated using molecular dynamics simulation methods. Three different polymers are modeled to have unique block lengths. The resulting blocks include (X5Y5)4, (X10Y10)2 and (X20Y20)1, where five, ten and twenty monomeric units, respectively, are consecutively connected to form a block. Each block is then alternatingly connected to another block type. All other molecular variables such as molecular weight, equivalent weight, and number of water molecules are controlled to have the same value among the three polymers. Despite the variation of the block length, the equilibrated structures of the membranes appear very similar. Through pair correlation function analysis, every aspect of the detailed local structures is nearly identical, indicating no effect of block length. Accordingly, the transport of both water and hydronium is also similar regardless of block length variation. The similarities between the membranes, regardless of block length, are attributed to the presence of hydrophilic keto and sulfone groups in the hydrophobic blocks. The even distribution of hydrophilic groups throughout the hydrophobic phase leads to more dispersed water molecules and a poorly developed water phase. Therefore, more contrast in hydrophobicity/hydrophilicity between the blocks is suggested to induce more nanophase-segregation.