Recent SOFC electrode investigations indicate that the Triple Phase Boundary reaction and the Internal Diffusion reaction should be regarded as occurring over different zones of the electrolyte surface. Under these conditions, an optimization consists of giving the faster process a reaction zone as large as possible. A duplex electrode microstructure composed of an ultra-thin layer of a mixed conductor covered by a porous current-collecting layer made of coarse grains is analyzed in terms of in-plane ohmic drop and diffusion overpotential. Relationships can be defined to optimize this microstructure. With currently available materials, excellent performances can be projected for the medium-temperature SOFC electrodes. A critical parameter for the mixed conducting layer is its electronic conductivity which should be higher than 1 S/cm. Solutions to improve a possibly slow surface oxygen exchange process are also examined.