Ab Initio Study of Hydration and Proton Dissociation in Ionomer Membranes
We present a comparative study of proton dissociation in various functional acidic units that are promising candidates as building blocks for polymeric electrolyte membranes. Minimum energy structures for four acidic moieties with clusters of 1-6 water molecules were determined using density functional theory at the B3LYP/6-311G** level starting from chemically rational initial configurations. The perfluoro sulfonyl imide acid group (CF3CF2SO2NHSO2CF3) was observed to be the strongest acid, due to the substantial electron withdrawing effect of both fluorocarbon groups. The hydrophilic functional group (CH3OC6H3OCH3C6H4SO3H) of sulfonated polyetherether ketone (SPEEK) membrane was found to be the strongest base with the acidic proton dissociation requiring the addition of six water molecules and the hydrated proton being more tightly bound to the conjugate base. Even though both perfluoro sulfonyl imides and sulfonic acids (hydrophilic functional groups for sulfonyl imide and Nafion ionomers respectively) required only three water molecules to exhibit spontaneous proton dissociation, the largest possible solvent-separated hydronium ion was attained only for the sulfonyl imide moiety. These results provide a scientific basis for understanding the improved conductivity of perfluorinated sulfonyl imide-based membranes relative to that of the widely-used Nafion membrane.
- Research Organization:
- Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 984212
- Report Number(s):
- PNNL-SA-71804; 40083; 24811; KC0302020
- Journal Information:
- Journal of Physical Chemistry A, 114(25):6904-6912, Journal Name: Journal of Physical Chemistry A, 114(25):6904-6912 Journal Issue: 25 Vol. 114; ISSN 1089-5639; ISSN 1520-5215
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
74 ATOMIC AND MOLECULAR PHYSICS
DISSOCIATION
ELECTROLYTES
ELECTRONS
Environmental Molecular Sciences Laboratory
FUNCTIONALS
HYDRATION
IMIDES
KETONES
MEMBRANES
OXONIUM IONS
PROTONS
SULFONIC ACIDS
WATER
acidity
density functional theory
polymer electrolyte membrane