No abstract

The morphology of dry and hydrated perfluorosulfonic acid (PFSA) ionomers at cryo and room temperature is examined using TEM/STEM with EELS capability. Z-contrast imaging was utilized to identify the micro-phase separation of the hydrophilic side chains containing water and the hydrophobic polytetrafluoroethylene (PTFE) backbones. The results compare very favourably with hydrated morphologies obtained through mesoscale dissipative particle dynamics (DPD) simulations. The cryo-STEM images of plunge-frozen samples was also found to agree with morphologies based on SAXS experiments.

The hydrated morphologies of sulfonated poly(phenylene) sulfone (sPSO2) ionomers as a function of equivalent weight (EW), molecular weight (MW), and water content were investigated by using mesoscale dissipative particle dynamics (DPD) simulations. The morphological changes were characterized by analyzing the water distribution and plotting the radial distribution functions for the water particles. The results were compared to typical PFSA ionomers (i.e., Nafion and Aquivion) to evaluate the effects of backbone and side chain chemistry. Our results show that water is more likely to be equally distributed within the hydrophilic domains of the sPSO2 ionomers particularly at low water content, which is in contrast to strong phase separation observed in PFSA ionomers at the same level of hydration. As the degree of sulfonation is increased (i.e., decreasing the EW), well-connected water clusters develop in the sPSO2 ionomers even at low water content which are less affected by changes in the MW than observed for PFSA ionomers. The size of the water clusters is estimated to be from 1.2 to 1.5 nm (compared to ∼ 3.5 nm in Nafion) at a water content of 7H2O/SO3H, which is consistent with results determined from previous experiments. This suggests that the high proton conductivity observed in the sPSO2 ionomers is due to the well-connected hydrophilic pathways.

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.

Ionomers with protogenic groups such as phosphonic acid have recently been proposed as suitable polymer electrolyte membranes for fuel cells operating with little humidification due to their extraordinary amphotericity. The hydrogen bonding and energetics associated with proton transfer of substituted phosphonic acid molecules and their self-condensation products (anhydride + H(2)O) are examined through ab initio electronic structure calculations. The global minimum energy structures of methyl-, phenyl-, benzyl-, trifluoromethyl- and phenyldifluoro-phosphonic acids determined at the B3LYP/6-311G** level indicate that the fluorinated molecules exhibit slightly stronger binding to water than the non-fluorinated molecules. The potential energy profiles for transfer of a proton within the condensation products were obtained at the B3LYP/6-311G** level for each of the acids, and revealed that the trifluoromethyl system has the lowest endothermicity (5.3 kcal mol(-1)) in contrast to the methyl system which has the greatest endothermicity (7.1 kcal mol(-1)) and a barrier of 7.6 kcal mol(-1). When no constraints are imposed on the system, proton transfer from the anhydride to a water molecule (primary) is accompanied by a secondary proton transfer (from protonated water molecule back to the anhydride) in all cases except the trifluoromethyl anhydride where charge separation and formation of a hydronium ion occurs. It appears that the secondary proton transfer would substantially determine the emergence of transition states and the correlated endothermicities.