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First principles modeling of sulfonic acid based ionomer membranes

Fuel Cell Technology and Applications

Polymer electrolyte membrane fuel cells and systems (PEMFC)

Membrane materials

  1. S. J. Paddison

Published Online: 15 DEC 2010

DOI: 10.1002/9780470974001.f303035

Handbook of Fuel Cells

Handbook of Fuel Cells

How to Cite

Paddison, S. J. 2010. First principles modeling of sulfonic acid based ionomer membranes. Handbook of Fuel Cells. .

Author Information

  1. Motorola Inc., Los Alamos, NM, USA

Publication History

  1. Published Online: 15 DEC 2010

Abstract

Proton dissociation in the sulfonic acid groups of hydrated Nafion® and polyetherketone membranes along with its connection to proton transport within the membrane pores is discussed. 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 [BOND]CF3 group. This molecular membrane specific information was used in combination with membrane morphology data to compute proton diffusion coefficients for both Nafion 117 and 65% sulfonated polyetherketone 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:

  • diffusion;
  • direct methanol fuel cell (DMFC);
  • dissociation coefficient;
  • drag coefficient;
  • electrolyte;
  • electroosmosis;
  • membrane hydration;
  • molecular orbitals (HOMO and LUMO);
  • Nafion® (membranes);
  • polymer electrolyte fuel cell (PEFC);
  • proton exchange membrane (PEM) fuel cell;
  • perfluorinated membrane;
  • permittivity of water;
  • polymer electrolyte;
  • proton conductivity;
  • Stokes law;
  • water diffusion;
  • water electroosmotic drag