We are fabricating all-thin-film solid oxide fuel cell (SOFC) structures in order to probe fundamental transport properties of SOFC electrolytes in an out-of-plane measurement geometry using epitaxial films with ideal model interfaces. Epitaxial multilayer thin-film structures consisting of a bottom electrode (SuRuO3 (SRO) or Ba0.93La0.07SnO3 (BLSO)) and an electrolyte Sm0.2Ce0.8O2-δ (SDC20) (600 nm - 1000 nm) on SrTiO3 (100) single crystal substrates were fabricated using pulsed laser deposition. The heteroepitaxy of the multi-layer structures was confirmed by X-ray diffraction and high-resolution scanning transmission electron microscopy, and atomically sharp and structurally coherent interfaces were observed in the electrolyte/electrode bilayers. Electrochemical impedance spectroscopy (EIS) measurements of the devices at temperature range of 623 K to 823 K in air reveal electrochemical properties of SDC20 which are quantitatively consistent with known bulk transport properties of SDC20. This work demonstrates the utility of out-of-plane all-thin-film heteroepitaxial electrochemical devices as a model platform for directly investigating intrinsic transport properties of electrochemical materials in single-crystal-like heterostructures with well-defined sharp interfaces. We are also fabricating various combinatorial libraries where the composition of the cathode or the electrolyte layer is continuously varied across the library chips. Systematic measurements of microdot arrays on the libraries allow identification of optimum compositions as well as delineation of the rate determining step in the reaction. We will also discuss our progress in fabrication of epitaxial Li solid state battery structures. This work is carried out in collaboration with Yangang Liang, Xiaohang Zhang, Chris J. Kucharczyk, Huairuo Zhang, Leonid A. Bendersky, and Sossina M. Haile, and is funded by DOE, NSF, and NIST.