| Solid Oxide Fuel Cell (SOFC) power densities typically drop rapidly as the operating temperature is decreased, due to electrolyte ohmic losses and/or electrode overpotentials. In this dissertation, I describe SOFCs utilizing {dollar}<{dollar}10 {dollar}mu{dollar}m thick yttria-stabilized zirconia (YSZ) electrolytes and YSZ-YDC (Yttria Doped Ceria) bi-layer electrolytes to provide low ohmic losses. The two key factors in achieving the fully-dense films were porous substrate preparation and the use of substrate bias during electrolyte film depositions. Increasing the negative DC substrate bias {dollar}rm Vsb{lcub}S{rcub}{dollar} resulted in increasing film density and film stress. {dollar}rm Vsb{lcub}S{rcub}=75V{dollar} was chosen to yield high density without excessive film compressive stresses. Electrodes containing electronic and ionic conductors, namely Ni-YSZ anodes and (La,Sr)MnO{dollar}sb3{dollar}-YSZ cathodes, were prepared for low interfacial resistances at low operating temperatures. Their composition and structure effects on the electrode performance were investigated. The Ni-YSZ anode films--deposited by DC reactive magnetron sputtering of Ni-Zr-Y targets in Ar-O{dollar}sb2{dollar} mixtures--were porous, two-phase, and exhibited an equiaxed structure with grain sizes of {dollar}approx{dollar}35 nm. With optimal deposition conditions, very low interfacial resistances, 0.15 to 0.35 {dollar}Omega{dollar}-cm{dollar}sp2{dollar} were measured at 750{dollar}spcirc{dollar}C in {dollar}rm97%Hsb2+3%Hsb2O.{dollar} The interfacial resistance of (La,Sr)MnO{dollar}sb3{dollar}-YSZ cathode/substrates--prepared by ceramic processing--decreased with increasing the YSZ volume fraction in (La,Sr)MnO{dollar}sb3{dollar}-YSZ from 0 to 60%. Adding thin porous YDC layers on either side of the YSZ yielded much-reduced interfacial resistances at both the (La,Sr)MnO{dollar}sb3{dollar} cathodes and Ni-YSZ anodes. On the anode side, YDC promoted the charge transfer process due to its mixed conductivity. On the cathode side, the high oxygen surface exchange coefficient of YDC is believed to be the reason for the enhanced cathode performance. These cells provide higher power densities than previously reported below 750{dollar}spcirc{dollar}C, e.g. 300 and 760 mW/cm{dollar}sp2{dollar} at 600 and 750{dollar}spcirc{dollar}C, respectively (measured in 97% H{dollar}sb2{dollar} + 3% H{dollar}sb2{dollar}O and air). |
