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

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• Proton exchange membrane fuel cells (PEMFCs) require ionomers stable under demanding operating conditions.
• Sulfonated poly(phenylene) sulfone (sPSO(2)) ionomers are being investigated as alternatives to traditional perfluorosulfonic acid (PFSA) ionomers.
• Understanding ionomer structure-property relationships is crucial for improving fuel cell performance.

Purpose of the Study:

• To investigate the hydration and proton transfer energetics in sulfonated poly(phenylene) sulfone (sPSO(2)) ionomers.
• To compare the conformational flexibility and proton transfer barriers of sPSO(2) with existing PFSA ionomers.
• To elucidate the influence of sulfonic acid group interactions and water molecules on proton mobility.

Main Methods:

• First-principles electronic structure calculations were employed.
• Oligomeric fragments of two sPSO(2) ionomers were modeled.
• Conformational analysis and energy barrier calculations for proton transfer were performed.
• Rotational energy surfaces of polymer backbones were computed.

Main Results:

• The interaction between adjacent sulfonic acid groups influences fragment conformation and stability.
• Proton transfer in the first hydration shell requires three water molecules for meta- and four for ortho-conformations.
• sPSO(2) ionomers exhibit significantly stiffer backbones compared to PFSA ionomers.
• Proton transfer energy barriers are lower in sPSO(2) (0.7–1.9 kcal/mol) than in PFSA systems.

Conclusions:

• sPSO(2) ionomers demonstrate favorable proton transfer characteristics, particularly under conditions involving multiple sulfonate groups.
• The increased backbone rigidity of sPSO(2) may contribute to enhanced mechanical stability in fuel cell environments.
• These findings support the potential of sPSO(2) ionomers for high-performance fuel cells operating under challenging conditions.