In
this study, hydrated Nafion film in the catalyst layer of the
cathode for a polymer electrolyte membrane fuel cell is investigated
using the molecular dynamics simulation method, exhibiting different
structural characteristics on Pt and carbon surfaces. First, it is
found that water molecules, hydronium ions, and sulfonate groups are
highly concentrated at the interfacial region between the Nafion phase
and the Pt surface, whereas Nafion backbone chains are present in
a high concentration at the interface between the Nafion phase and
the carbon surface. Second, it is also found from pair correlation
function analysis that the water molecules and sulfonate groups in
the hydrated Nafion phase are more associated with the Pt surface
compared to the carbon surface, which is due to their strong attractive
interactions with the Pt surface that makes the dimension of the hydrated
Nafion phase 4–7% thinner on the Pt surface. Third, it is observed
from water-occupied volume analysis that water molecules on the carbon
surface can form large-size water phase between the Nafion phase and
the carbon surface because the Nafion–carbon interface is not
tightly integrated due to their weak interaction. In these structural
characteristics, it is demonstrated that the water diffusion and proton
vehicular diffusion are suppressed in the interfacial region of the
Pt surface due to the highly packed structures in the water phase
as well as the polymer phase in addition to the strong molecular interaction
with the Pt surface, whereas the proton hopping diffusion is enhanced
due to the well-developed organized water phase via the hydrogen bonding
network.