Ionomer Membranes Produced from Hexaarylbenzene-Based Partially Fluorinated Poly(arylene ether) Blends for Proton Exchange Membrane Fuel Cells

In this study, a series of high molecular weight ionomers of hexaarylbenzene- and fluorene-based poly(arylene ether)s were synthesized conveniently through condensation and post-sulfonation modification. The use a of blending method might increase the stacking density of chains and affect the formation both of interchain and intrachain proton transfer clusters. Multiscale phase separation caused by the dissolution and compatibility differences of blend ionomer in high-boiling-point solvents was examined through analysis and simulations. The blend membranes produced in this study exhibited a high proton conductivity of 206.4 mS cm−1 at 80 °C (increased from 182.6 mS cm−1 for precursor membranes), excellent thermal resistance (decomposition temperature > 200 °C), and suitable mechanical properties with a tensile strength of 73.8−77.4 MPa. As a proton exchange membrane for fuel cell applications, it exhibits an excellent power efficiency of approximately 1.3 W cm−2. Thus, the ionomer membranes have strong potential for use in proton exchange membrane fuel cells and other electrochemical applications.

The degradation effect on proton dissociation and transfer in perfluorosulfonic acid membranes

In the operation of proton exchange membrane fuel cells (PEMFCs), the ionomer-perfluorosulfonic acid (PSFA) membrane side chains are easily attacked by free radicals, resulting in membrane degradation. In this work, the chemical degradation effect of side chains in the PSFA membrane on proton dissociation and transfer behaviors is investigated by means of the quantum chemistry calculation. The rotation of the H atom in the acid group after the degradation is evaluated. The impact of the electrostatic potential (ESP) and electronegativity of the side chains is analyzed. The results demonstrate that the membrane degradation decreases the positive potential of the proton in the acid group, leading to the proton being less active so that more water molecules are required for the spontaneous proton dissociation. The rotation of the H atom in the acid group affects the proton dissociation mode owing to the change of the hydrogen bond network. It is found that the ESP of the acid group in two side chain fragments influences each other and the water molecules between two side chains can be shared to reduce the number of water molecules for the proton dissociation.

Single-Step Fabrication of Polymeric Composite Membrane via Centrifugal Colloidal Casting for Fuel Cell Applications

Recent interest in polymer electrolyte membranes (PEMs) for fuel cell systems has spurred the development of infiltration technology by which to insert ionomers into mechanically robust reinforcement structures by solution casting in order to produce a cost effective and highly efficient electrolyte. However, the results of the fabrication process often continue to present challenges related to the structural complexity and self-assembly dynamics between the hydrophobic and hydrophilic parts of the constituents which in turn, necessitates additional processing steps and increases production costs. Here, a single-step process is reported for highly compact polymeric composite membranes (PCMs), fabricated using a centrifugal colloidal casting (C3) method. Combined structural analyses as well as coarse-grained molecular dynamics simulations are employed to determine the micro-/macroscopic structural characteristics of the fabricated PCMs. These findings indicate that the C3 method is capable of forming highly dense ionomer matrix-reinforcement composites consisting of microphase-separated ionomer structures with tailored crystallinity and ionic cluster sizes. An outcome that is very unlikely with the single-step coating steps in conventional methods. These structural attributes ensure PCMs with better proton conductivity, greater strain stability, and lower gas crossover properties compared to commercial pristine membranes, expanding their possible range of applicability to PEMs.

Proton Transfer in Phosphoric Acid-Based Protic Ionic Liquids: Effects of the Base

Electronic structure calculations were performed to understand highly decoupled conductivities recently reported in protic ionic liquids (PILs). To develop a molecular-level understanding of the mechanisms of proton conductivity in PILs, minimum-energy structures of trimethylamine, imidazole, lidocaine, and creatinine (CRT) with the addition of one to three phosphoric acid (PA) molecules were determined at the B3LYP/6-311G** level of theory with the inclusion of an implicit solvation model (SMD with ε = 61). The proton affinity of the bases and zero-point energy corrected binding energies were computed at a similar level of theory. Proton dissociation from PA occurs in all systems, resulting in the formation of ion pairs due to the relatively strong basicity of the bases (proton acceptors) and the effect of the high dielectric constant solvent in stabilizing the charge separation. The second and third PA molecules preferentially form "ring-like" hydrogen bonds with one another instead of forming hydrogen bonds at the donor and acceptor sites of the bases. Potential energy scans reveal that the bases with stronger proton affinity exert greater influence on the energetics of proton transfer between the individual PA molecules. However, the effects are minimal when shifted into a single-well from a double-well potential. Barrierless proton transfer was observed to occur in the CRT system with several PA molecules present, implying that the CRT may be a promising PA-based PIL.

New Insights into Perfluorinated Sulfonic-Acid Ionomers

In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers' complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradation phenomena, and PFSA thin films are presented. Throughout, the impact of PFSA chemistry and side-chain is also discussed to present a broader perspective.

Ab initio molecular dynamics simulations of aqueous triflic acid confined in carbon nanotubes

Ab initio molecular dynamics simulations were performed to investigate the effects of nanoscale confinement on the structural and dynamical properties of aqueous triflic acid (CF3SO3H). Single-walled carbon nanotubes (CNTs) with diameters ranging from ∼11 to 14 Å were used as confinement vessels, and the inner surface of the CNT were either left bare or fluorinated to probe the influence of the confined environment on structural and dynamical properties of the water and triflic acidic. The systems were simulated at hydration levels of n = 1-3 H2O/CF3SO3H. Proton dissociation expectedly increased with increasing hydration. Along with the level of hydration, hydrogen bond connectivity between the triflic acid molecules, both directly and via a single water molecule, played a role on proton dissociation. Direct hydrogen bonding between the CF3SO3H molecules, most commonly found in the larger bare CNT, also promoted interactions between water molecules allowing for greater separation of the dissociated protons from the CF3SO3(-) as the hydration level was increased. However, this also resulted in a decrease in the overall proportion of dissociated protons. The confinement dimensions altered both the hydrogen bond network and the distribution of water molecules where the H2O in the fluorinated CNTs tended to form small clusters with less proton dissociation at n = 1 and 2 but the highest at n = 3. In the absence of nearby hydrogen bond accepting sites from H2O or triflic acid SO3H groups, the water molecules formed weak hydrogen bonds with the fluorine atoms. In the bare CNT systems, these involved the CF3 groups of triflic acid and were more frequently observed when direct hydrogen bonding between CF3SO3H hindered potential hydrogen bonding sites. In the fluorinated tubes, interactions with the covalently bound fluorine atoms of the CNT wall dominated which appear to stabilize the hydrogen bond network. Increasing the hydration level increased the frequency of the OH···F (CNT) hydrogen bonding which was highly pronounced in the smaller fluorinated CNT indicating an influence on the confinement dimensions on these interactions.

Hydration and proton transfer in highly sulfonated poly(phenylene sulfone) ionomers: an ab initio study

The need to operate proton exchange membrane fuel cells under hot and dry conditions has driven the synthesis and testing of sulfonated poly(phenylene) sulfone (sPSO(2)) ionomers. The primary hydration and energetics associated with the transfer of protons in oligomeric fragments of two sPSO(2)ionomers were evaluated through first-principles electronic structures calculations. Our results indicate that the interaction between neighboring sulfonic acid groups affect both theconformation and stability of the fragments. The number of water molecules required to affect the transfer of a proton in the first hydration shell was observed to be a function of the hydrogen bonding in proximity of the sulfonic acid groups: three H(2)O for the meta- and four H(2)O for the ortho-conformations. Calculations of the rotational energy surfaces indicate that the aromatic backbones of sPSO(2) are much stiffer than the polytetrafluoroethylene (PTFE) backbones in perfluorosulfonic acid (PFSA) ionomers: the largest energy penalty for rotating phenylene rings (i.e., 15.5 kcal/mol for ortho-ortho-sPSO(2)) is nearly twice that computed for the rotation of a CF(2) unit in a PTFE backbone. The energetics for the transfer of various protons in proximity to one or two sulfonate groups (-SO(3)(-)) was also determined. The computed energy barrier for proton transfer when only one sulfonic acid group is present is approximately 1.9 kcal/mol, which is 2.1 kcal/mol lower than similar calculations for PFSA systems. When two sulfonic acid groups are bridged by water molecules, a symmetric bidirectional transfer occurs, which gives a substantially small energy barrier of only 0.7 kcal/mol.

Interplay between structure and relaxations in perfluorosulfonic acid proton conducting membranes

This study focuses on changes in the structure of ionomer membranes, provided by the 3M Fuel Cells Component Group, as a function of the equivalent weight (EW) and the relationship between the structure and the properties of the membrane. Wide-angle X-ray diffraction results showed evidence of both non-crystalline and crystalline ordered hydrophobic regions in all the EW membranes except the 700 EW membrane. The spectral changes evident in the vibrational spectra of the 3M membranes can be associated with two major phenomena: (1) dissociation of the proton from the sulfonic acid groups even in the presence of small amounts of water; and (2) changes in the conformation or the degree of crystallinity of the poly(tetrafluoroethylene) hydrophobic domains both as a function of EW and membrane water content. All the membranes, regardless of EW, are thermally stable up to 360 °C. The wet membranes have conductivities between 7 and 20 mS/cm at 125 °C. In this condition, the conductivity values follow VTF behavior, which suggests that the proton migration occurs via proton exchange processes between delocalization bodies (DBs) that are facilitated by the dynamics of the host polymer. The conductivity along the interface between the hydrophobic and hydrophilic domains makes a larger contribution in the smaller EW membranes likely due to the existence of a greater number of interfaces in the membrane. The larger crystalline domains present in the higher EW membranes provide percolation pathways for charge migration between DBs, which reduces the probability of charge transfer along the interface. Therefore, at higher EWs although there is charge migration along the interface within the hydrophobic-hydrophilic domains, the exchange of protons between different DBs is likely the rate-limiting step of the overall conduction process.