Nanoscale thin film ceramic fuel cells

The ceramic fuel cell (CFC) refers to fuel cells employing solid state ceramic electrolytes, including two types of fuel cells: the solid oxide fuel cell (SOFC) and the proton-conducting oxide fuel cell (PCOFC). The goal of this study is to minimize Ohmic losses and activation losses at the electrolyte and electrode-electrolyte interface respectively by engineering CFC components to run fuel cells at reduced temperatures.;Successful synthesis of nano-scale oxide ion-conducting yttria-stabilized zirconia (YSZ) with an optimal Y:Zr composition of 8 molar % yttria was demonstrated using atomic layer deposition (ALD). Structural analyses of ALD YSZ films grown on amorphous Si3N4 showed a polycrystalline morphology with volume expansion confirmed by x-ray diffraction (XRD), x-ray reflection (XRR) and transmission electron microscopy (TEM) techniques. Fine spherical grains were observed in atomic force microscopy (AFM) images. Fuel cell performance data between 265∼450°C showed maximum power densities of 28∼587mW/cm 2, respectively. Performance enhancement originated from an increase in exchange current density at the electrode-electrolyte interface. Our hypothesis is that the polycrystalline surface of the membrane with grains in several tens of nanometers is responsible for the high exchange current density through the introduction of many grains and grain boundaries. This speculation is based on the surface morphology of the ALD YSZ or RF sputtered YSZ consisting of 20∼30nm grains measured by AFM. In contrast, the size of grains in bulk YSZ is on the order of several microns.;To investigate relationship between surface morphology and fuel cell performance, we traced the surface incorporation and diffusion of oxygen ions using 18O isotopes and the secondary ion mass spectrometry (SIMS) technique. ALD films were grown on top of Si3N4-buffered Si(100) wafers and single crystal YSZ(100) substrates. 100mum thick polycrystalline YSZ samples were used to investigate the role of grain and grain boundaries in cathodic reactions. Diffusivity and surface exchange rate were calculated by fitting SIMS depth profiles. Diffusivities of all of tested samples including single crystal YSZ(100) were aligned well on a single line in the Arrhenius plot indicating that there is no enhancement in bulk diffusion by the ALD YSZ surface coating. However, the surface exchange rate on the ALD YSZ coated YSZ(100) increased by a factor of 2.4∼3.0 in comparison with the one from bare YSZ(100). AFM morphology analysis suggested that the ALD YSZ coating formed granular surface and we speculated that this granular surface enhanced the exchange rate by introducing additional gas-ion contact sites.;We studied yttrium-doped barium zirconate (BYZ), a proton conducting perovskite, which attracts attention as a new electrolyte material for future ceramic fuel cells due to its high ionic conductivity and chemical stability. The ionic conductivity of nano-scale BYZ films (60-670nm) epitaxially grown on single crystalline MgO(100) by pulsed laser deposition (PLD) was investigated in relation to the crystal and grain structures. Crystal structure and surface morphology changes were identified by XRD and AFM analyses, respectively. Ionic conductivity was measured as a function of temperature and film thickness. Measured conductivities and the resulting activation energies were in good agreement with the reference value for bulk BYZ. Enhanced conductivity was observed in thin samples oriented in (100) texture, while the ionic conductivity of thick polycrystalline films showed no thickness dependence. The difference is likely due to morphological changes as the film grows thicker. This was confirmed by AFM and XRD analyses of thicker films, which feature granular structure in the vicinity of the surface. Fuel cells with 130nm thick PLD BYZ electrolytes were successfully fabricated. The best performing PLD BYZ cells exhibited a maximum power density of 11∼120mW/cm2 at 300∼450°C. Fuel cells with PLD BYZ electrolytes performed inferior to conventional polymer fuel cells due to their low exchange current density. As an alternative to PLD BYZ, a 110 nm thick ALD BYZ electrolyte was tested to enhance the overall performance. We found that the ALD BYZ exhibits superior surface properties showing a higher exchange current density. The best performing ALD BYZ cells showed a maximum power density of 15∼136mW/cm2 at 300∼400°C. (Abstract shortened by UMI.)...