6904
J. Phys. Chem. A 2010, 114, 6904–6912
Ab Initio Study of Hydration and Proton Dissociation in Ionomer Membranes Nagesh Idupulapati,* Ram Devanathan, and Michel Dupuis Chemical and Materials Science DiVision, Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed: March 25, 2010; ReVised Manuscript ReceiVed: May 10, 2010
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 rationale for the enhanced conductivity of perfluorinated sulfonyl imide-based membranes relative to that of the widely used Nafion membrane. 1. Introduction The design and development of reliable polymeric electrolyte membrane fuel cells (PEMFCs) represent a critical step toward a proposed hydrogen economy.1 PEMFCs have attracted considerable interest, as there is a pressing need for highefficiency energy conversion and because of their potential applications in stationary and portable devices.1,2 An important component of PEMFCs that strongly affects their performance is the polymeric electrolyte membrane (PEM) that facilitates proton conduction from anode to cathode.3 One of the best known and widely used membranes is the perfluorinated sulfonic acid polymer, Nafion, because of its good proton conduction as well as excellent mechanical and chemical stability.1,2 However, some potential drawbacks such as high methanol permeability, low operating temperature (
