Composite electrodes commonly consist of interconnected pore, electronic-conductor, and ionic-conductor phases that are responsible for the transport of gas phase species, electrons, and oxygen ions, respectively. Attempts to understand how this complicated structure influences electrochemical performance can be facilitated through the acquisition of quantitative microstructural data. In this work, nine different cathode compositions ranging from 30 wt% La0.8Sr0.2MnO3 (LSM) - 70 wt% Y2O3-stabilized ZrO2 (YSZ) to 70 wt% LSM - 30 wt% YSZ were screen printed symmetrically on both sides of YSZ-electrolytes. 3D reconstructions of the cathodes were achieved using focused ion beam (FIB) tomography techniques, and the connectivity of the phases was quantified to determine the electrochemically active triple-phase boundary (EA-TPB) densities. The composition dependence of the polarization resistance, calculated with an electrochemical model that employed the EATPB densities along with measured YSZ tortuosities, was compared with the measured resistances.