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The modeling of molecular structure and ion transport in sulfonic acid based ionomer membranes

Pages: 197-207

S. J. Paddison

Computational Nanoscience Group, Motorola Inc. Los Alamos Research Park, 4200 W. Jemez Rd, Suite #300, Los Alamos, NM 87544, USA

Received July 10, 2001 ; receved in revised form September 17, 2001

Abstract: Reported here is a molecular modeling investigation into the proton dissociation of the hydrophilic components of hydrated Nafion and PEEKK membranes and its connection to proton transport within the membrane pores. Minimum energy conformations for trifluoromethane and para-toluene sulfonic acids with clusters of 1–6 water molecules were obtained using ab initio electronic structure calculations. These calculations revealed the influence of both the structure and strength of the conjugate base (sulfonate anion) on the dissociation and hydration of the acid. Although spontaneous dissociation was observed for both sulfonic acids after the addition of three water molecules, the proton (as a hydronium ion) is less bound for the perfluorinated system than with the aromatic system. This is due to the increased electron stabilization afforded by the electron withdrawing of the –CF₃ group. The molecular membrane specific information was used in combination with membrane morphology data to compute proton diffusion coefficients for both Nafion 117 and 65% sulfonated PEEKK membranes at hydration levels where the number of water molecules per sulfonic acid fixed site were: 6, 13, and 22.5; and 15, 23, and 30, respectively. The agreement with pulsed field gradient NMR diffusion measurements was very good for both membranes across the entire range of membrane hydration, attesting to the substantial predictive capability of the transport model.

Keywords: Nafion^®, PEEKK, electronic structure, proton transport, mechanism