Composite Cathodes Assisted Low Temperature Micro- SOFCs

摘要

One of the important questions in micro solid oxide fuel cells (SOFCs) that are operated at low temperatures concerns the required triple line boundary (TBL) density for obtaining high current densities. We investigated this issue by means of micromachined cells with a cathode electrode having a defined TBL value. The cell design allows for single side electrical characterization on a probe stage. The electrolyte consisted of a 150 nm thick gadolinium doped ceria (CGO) layer (anode side) and a 500 nm thick yttrium stabilized zirconia (YSZ) layer. The first is obtained by reactive RF sputtering from an alloy target, the latter by RF sputtering from a ceramic target. The inner side of the 200 μm diameter membrane is uncovered up to a diameter of 160 μm. Given the CGO can be considered as a mixed conductor when exposed to reducing Ar: H2 fuel, it is the cathode limited to a circular TBL. This allows for measuring the current density per TBL length since the Pt layers stayed unchanged as dense films at these low temperatures. For the currents at maximal power output, we obtained 0.9 mA/m at 450 °C. Extrapolating these values to a chess board like porous microstructure of a cathode layer covering the whole electrolyte uniformly, each element having a sidelength of 50 nm, and assuming no resistive losses, current densities of roughly 1.0 A/cm~2 corresponding power densities of 1 W/cm~2 are calculated for 450 °C. The OCV reaches 0.93 V and stabilizes at an operating temperature of 450 o C. From these studies, the porous Pt/YSZ composite thin films were deposited by co-sputtering in order to further optimize the TBL value. Peak power densities improved 10 times even though Pt dewetted in the composite making phase segregation. Our results provide fundamental insight to pathways of high performance micro-SOFC membranes.