posted on 2024-01-10, 22:06authored byNobuaki Kikkawa, Masayuki Kimura
Mesoporous carbon is often used as a support for platinum
catalysts
in polymer electrolyte fuel-cell catalyst layers. Mesopores in the
carbon support improve the performance of fuel cells by inhibiting
the adsorption of ionomer onto the catalyst particles. However, the
mesopores may impair mass transport. Hence, understanding molecular
behaviors in the pores is essential to optimizing the mesopore structures.
Specifically, it is crucial to understand the oxygen transport in
the high-current region. In this study, the diffusion coefficients
of oxygen molecules in carbon mesopores were calculated for various
pore lengths, pore diameters, filling rates, and water contents in
the ionomer via molecular dynamics simulations. The results show that
oxygen diffusion slows by 2 orders of magnitude because of pore occlusion,
and it slows down by an additional 1 or 2 orders of magnitude if ionomers
are present in the pores. The occlusion can be theoretically predicted
by considering the surface free energy. This theory provides some
insight into mesoporous carbon designs; for instance, the theory suggests
that narrow pores should be shortened to prevent occlusion. Slow diffusion
in the presence of ionomers was attributed to the localization of
oxygen at the dense ionomer–carbon interface. Thus, to improve
oxygen transport properties, carbon surfaces and ionomer structures
may be designed in such a manner as to prevent densification at the
interface.