Journal of Electroceramics, 13, 663–668, 2004 C  2004 Kluwer Academic Publishers. Manufactured in The Netherlands. Electrochemical Characterization of Thin Films for a Micro-Solid Oxide Fuel Cell J.L. HERTZ & H.L. TULLER Department of Materials Science and Technology, Massachusetts Institute of Technology, 77Massachusetts Avenue, Room 13-4010, Cambridge, MA 02169, USA Submitted February 12, 2003; Revised March 22, 2004; Accepted March 23, 2004 Abstract. One of the ﬁrst technological benchmarks towards the realization of a micro solid oxide fuel cell is the production of thin ﬁlm structures with adequate electrochemical properties. This paper describes the deposition of thin ﬁlm yttria-stabilized zirconia electrolytes and lithographically patterned platinum and gold electrodes. By using conventional, ultraviolet lithography, electrode patterns were produced with features sizes as ﬁne as 15 µm, enabling a more direct investigation into the role of the triple phase boundary. Impedance spectroscopy measurements show three arcs, ascribed to the grain, grain boundary and electrode processes, and an offset on the real axis due to the leads. The high frequency arc, ascribed to the ohmic resistance of the YSZ electrolyte, exhibited an activation energy of 1.0 eV, while the intermediate frequency arc, attributed to blocking grain boundaries, exhibited an activation energy of 0.69 eV. The low frequency, non-ohmic arc was found to be highly dependent upon the electrode material and exhibited activation energies of 0.91 eV for gold electrodes and 0.77 eV for platinum electrodes. The electrode impedances for different sample geometries were similar when normalized to the triple phase boundary length. Keywords: solid oxide fuel cell, impedance spectroscopy, microfabrication Introduction Battery technology has two signiﬁcant shortcomings, namely, relatively low energy density (60 W·hr/kg for nickel-metal hydride and 130 W·hr/kg for lithium-ion rechargeable batteries [1]) and long recharging times requiring connection to a supplementary power source. These limitations decrease the utility of batteries as portable power sources and thus new opportunities ex- ist for miniaturized power sources, including micro- fuel cells. Fuel cell devices have two distinct advan- tages over batteries. First, the energy storage medium (i.e. fuel) has a very high energy density (6,200 W·hr/kg for methanol and 33,000 W·hr/kg for pure hydrogen). Combined with the high efﬁciencies inherent in these technologies, ﬁnal energy densities much higher than batteries are foreseeable [2]. Secondly, unlike batteries wherein the electrode materials participate directly in the electrochemical reaction, the fuel is not an integral part of the device. This allows for rapid refueling and freedom from the “memory effects” that can reduce en- ergy density during battery cycling. Moreover, if a mi- crofabricated fuel cell could be integrated onto a single chip with other electronic circuits, this would enable extended, remote operation of electronic devices. The use of thin ﬁlms may decrease diffusional and ohmic polarizations to the extent that surface reaction kinetics control fuel cell performance. It has been found in conventional SOFCs that increasing the triple phase boundary [TPB] length increases power output. How- ever, a detailed analysis of the rate limiting processes at or near the TPB is complicated by the complex ge- ometry of composite structures normally utilized. In conventional SOFCs, the performance of the porous, often composite electrodes is found to improve with electrode thickness, up to the order of 20–50 microns [3]. This clearly presents a challenge to a thin ﬁlm elec- trode, and other methods to increase the TPB length, such as nano-scale composites or lithographic pattern- ing, must be examined. The work described in this paper details the con- struction and electrochemical testing of model systems to study the kinetics of oxygen transfer in thin ﬁlm sys- tems. Microfabricated electrode structures have been