  We formally derive an effective catalyst layer model comprising the reduction
of oxygen for periodically distributed pore geometries. By assumption, the
pores are completely filled with water and the surrounding walls consist of
catalyst particles which are attached to an electron conducting microstructure.
The macroscopic transport equations are established by a multi-scale approach,
based on microscopic phenomena at the pore level, and serve as a first step
toward future optimization of catalyst layer designs.
