Impedance spectroscopy has been used to optimize the three-phase-boundaries of porous electrodes in solid oxide fuel cells. The three-phase-boundaries (TPB`s) are the sites where the electrode reaction occurs and their nature is determined by the microstructure of the electrode. The TPB-length (TPBL) of the cathode can be optimized by using small powder particles and a high loading of the particles on the electrolyte surface. The TPBL of the Ni/8YSZ-anode can be optimized by using small Ni-particles and a high 8YSZ/Ni-particle size ratio. 5 figs., 4 tabs., 8 refs.