Hydration and Proton Transfer in Highly Sulfonated Poly(phenylene sulfone) Ionomers: An Ab Initio Study

Abstract

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.

<i>Ab Initio</i> Study of Proton Transfer and Hydration in Phosphorylated Nata de Coco

“…was then obtained from the various optimized conformance structure energy calculations. These calculations heavily influence subsequent results in this study [16].…”

“…The average and maximum barrier energies of phosphorylated nata de coco dimers was found to be approximately 5.3 and 12.5 J, respectively, far below the observed barrier energy of PFSA fragments and sPSO2. This shows that the backbone of pyranose rings are more flexible (less stiff) compared to PTFE backbones and sPSO2 [16,[21][22]. A possible explanation is that the majority of the hydrogen bonds formed in phosphorylated nata de coco are unaffected by rotation along central C-O bonds.…”

“…The binding strength (due to hydrogen bonding) was evaluated at the B3LYP/6-311G(d) level and subsequently corrected for zero-point energy (ZPE) and basis set superposition error (BSSE) as has been done by Wang and Paddison systems sulfonated poly phenylene sulfone. [16]. The calculation of energy changes (ΔE) using the formula of equation 1, the same principle used to calculated the enthalpy changes (ΔH) and free Gibbs energy change (ΔG).…”

“…The results of ab initio computation on sPSO2 ionomers show that, as a whole, proton transfer occurs in various structures within sPSO2, with meta-meta conformity generally preferred over orto-orto conformity. sPSO2 ionomers are more effective in producing moving protons in low-water conditions [16]. A study of proton transfer energetics in phosphoric acid clusters shows that the (second) energy barrier of proton transference and its endothermic properties fall with an increase in the size of phosphoric acid clusters [17].…”

Surface adsorption of sulfonated poly(phenylene sulfone)/C14TAB mixtures and its correlation with foam film stability

“…23 Dissipative particle dynamics (DPD) simulations have shown that the sPSO 2 -220's polyelectrolyte backbone is significantly stiffer as for example the polyethylene backbones of PAMPS or PSS. 24,25 Furthermore, due to the electron withdrawing sulfonyl group (-SO 2 -) the sulfonate group is especially acidic for sPSO 2 -220, which leads to high dissociation degrees of 54-61%, 26,27 in comparison to a dissociation degree of 33-38% for PSS or PAMPS. 28,29 C 14 TAB was chosen as surfactant, as it is the shortest surfactant in the C n TAB series that forms stable films on its own.…”