Page 120 - Introduction to Transfer Phenomena in PEM Fuel Cells

Mass Transfer Phenomena 109
The way the atomic interaction forces (or the potentials from which these
forces derive) are determined characterizes the simulation type. If the
potentials are calculated precisely from the principles of quantum
mechanics, we refer to molecular dynamics (ab initio).
On the contrary, if the forces derive from a predetermined potential, fixed
empirically or determined by the calculation of an electronic structure, we
then refer to molecular dynamics (a priori). This method has been used
mainly to study the transport of protons in Nafion.

According to [BRA 07, CUI 07, DIN 98, EIK 02, ELL 07, LI 01,
PAD 01, PAD 03, WES 06], with this method, the motions of the atoms can
be seen as vibrations around an energy minimum. Their determination, for
each molecule, makes it possible to obtain fundamental information on the
local structure of the membrane, the proton diffusion, the aggregation of
ionic groups (through the formation of hydrogen bonds), or even the
dissociation of protons in the sulfonate groups.

Din and Michaelides [DIN 98], for example, conducted a study in
surface-filled cylindrical pores, in which flowed an electrolytic solution
(water + protons).

The pore radius is around one nanometer, and the surface charge density
–2
(κ) in the pore is constant and equal to (–0.1) or (–0.2) [C.m ].

Overall, the authors find that a high concentration of non-hydrated
protons (low volume) is adsorbed near the charged boundary, while a second
layer of hydrated (and therefore larger) protons is observed within a few
angstroms of the surface. The higher the surface charge density (κ), the more
the number of ions attracted to the boundary increases.

Finally, when the water content in the pore is high, the proportion of
hydrated protons is greater.

The information obtained by molecular dynamics simulations are
numerous and rich. In general, the influence of parameters such as the
temperature, the water content, the ion exchange capacity or the physical and
chemical characteristics of the main and side chains has been studied in the
literature [COM 02, DOK 06, SEE 05, SPO 02].