Spohr, Eckhard; Commer, P.; Kornyshev, Alexei A.:

Enhancing Proton Mobility in Polymer Electrolyte Membranes: Lessons from Molecular Dynamics Simulations

In: Journal of Physical Chemistry, B, Jg. 106 (2002) ; Nr. 41, S. 10560-10569
ISSN: 0045-6470
Zeitschriftenaufsatz / Fach: Chemie
Fakultät für Chemie
Typical proton-conducting polymer electrolyte membranes (PEM) for fuel cell applications consist of a
perfluorinated polymeric backbone and side chains with SO3H groups. The latter dissociate upon sufficient
water uptake into SO3
- groups on the chains and protons in the aqueous subphase, which percolates through
the membrane. We report here systematic molecular dynamics simulations of proton transport through the
aqueous subphase of wet PEMs. The simulations utilize a recently developed simplified version (Walbran,
A.; Kornyshev, A. A. J. Chem. Phys. 2001, 114, 10039) of an empirical valence bond (EVB) model, which
is designed to describe the structural diffusion during proton transfer in a multiproton environment. The
polymer subphase is described as an excluded volume for water, in which pores of a fixed slab-shaped geometry
are considered. We study the effects on proton mobility of the charge delocalization inside the SO3
- groups,
of the headgroup density (PEM “equivalent weight”), and of the motion of headgroups and side chains. We
analyze the correlation between the proton mobility and the degree of proton confinement in proton-carrying
clusters near SO3
- parent groups. We have found and rationalized the following factors that facilitate the
proton transfer: (i) charge delocalization within the SO3
- groups, (ii) fluctuational motions of the headgroups
and side chains, and (iii) water content.