Yuji Saito

High-Performance Ultrathin Solid Oxide Fuel Cells for Low-Temperature Operation

Thin-film solid oxide fuel cell (SOFC) structures containing electrolyte membranes 50-150 nm thick were fabricated with the help of sputtering, lithography, and etching. The submicrometer SOFCs were made of yttria-stabilized zirconia (YSZ) or YSZ/ gadolinium-doped ceria composites electrolyte and 80 nm porous Pt as cathode and anode. The peak power densities were 200 and 400 mW/cm 2 at 350 and 400°C, respectively. The high power densities achieved are not only due to the reduction of electrolyte thickness but also to the high charge-transfer reaction rates at the interfaces between the nanoporous electrodes (cathode and/or anode) and the nanocrystalline thin electrolyte.

Design and fabrication of a micro fuel cell array with “flip-flop” interconnection

Abstract A design configuration is presented for integrated series connection of polymer electrolyte fuel cells in a planar array. The design is particularly favorable for miniature fuel cells and has been prototyped using a variety of etch and deposition techniques adopted from microfabrication. The series path is oriented in a “flip-flop” configuration, presenting the unique advantage of a fully continuous electrolyte requiring absolutely no interconnecting bridges across or around the membrane. Electrical interconnections are made by thin-film metal layers that coat etched flow channels patterned on an insulating substrate. Both two-cell and four-cell prototypes have successfully demonstrated the expected additive performance of the integrated series, and peak power in a four-cell silicon assembly with hydrogen and oxygen has exceeded 40xa0mW/cm 2 . Factorial experimentation has been applied to investigate the adequacy of metal film conduction over etched topology, and results conclude that film thickness dominates over other design parameters. The design effort and subsequent testing has uncovered new topics for extended study, including the possibility of lateral ionic conduction within the membrane as well as the effects of non-uniform reactant distribution.

Development of portable fuel cell arrays with printed-circuit technology

Abstract Portable hydrogen/oxygen fuel cell power sources were constructed using printed-circuit board (PCB) technology. Multiple iterations of miniature planar fuel cell devices were prototyped, demonstrating fast cycle innovation and dramatic power density improvements in 200xa0W. The lightweight laminate PCB technology allows the best prototypes to achieve >700xa0mW/cm 2 area power density and >400xa0mW/cm 3 volumetric power density. PCB technology offers an intriguing platform for portable fuel cell development below 1xa0kW. Possibilities for on board diagnostics/control and further power density improvements are envisioned.

High ionic conductivity in ultrathin nanocrystalline gadolinia-doped ceria films

High ionic conductivities were observed in ultrathin nanocrystalline gadolinia-doped ceria (GDC) films with thicknesses comparable to the grain size (20–50nm). Conductivities were determined to be effectively three to four orders of magnitude higher relative to those of thicker films (>500nm). The distinct properties in ultrathin GDC films were attributed to the reduction of cross grain boundary resistance and the segregation of the Gd dopants in the vicinity of grain boundaries.

Thin-Film Solid Oxide Fuel Cells on Porous Nickel Substrates with Multistage Nanohole Array

A novel support/electrode/catalyst structure for a low-temperature thin-film solid oxide-fuel cell SOFC is fabricated using a two-step replication process. This so-called multistage nanoporous nickel substrate has channels connecting both sides of the substrate. The channel diameter gradually changes from about 20 nm to about 200 nm through the thickness. During fabrication, an anodic aluminum oxide (AAO) with a multistage nanopore structure is used as a template. The multistage pore structure is then filled with PMMA syrup to obtain the negative geometry. The filled PMMA syrup is UV cured, and the subsequent removal of AAO in a basic solution completes the negative structure. The final structure is obtained by nickel electroplating on the negative structure followed by the removal of the negative structure in an organic solvent. A thin-film SOFC with a 200 nm thick yttria stabilized zirconia (YSZ) is fabricated on the nanoporous substrate and the cell is operated at a low temperature range, between 370-550°C. The maximum output power density of 7 mW/cm 2 is obtained at 400°C.

Geometric Scale Effect of Flow Channels on Performance of Fuel Cells

Department of Mechanical and Aerospace Engineering, San Jose State University,San Jose, California 95192-0087, USAThis paper studies the effect of flow channel scaling on fuel cell performance. In particular, the impact of dimensional scale on theorder of 100 micrometers and below has been investigated. A model based on three-dimensional computational flow dynamics hasbeen developed which predicts that very small channels result in significantly higher peak power densities compared to their largercounterparts. For experimental verification, microchannel flow structures fabricated with varying sizes in SU-8 photoepoxy havebeen tested with polymer electrolyte membrane electrode assemblies. The experimental results confirm the predicted outcome atrelatively large scales. At especially small scales ~,100 mm!, the model ~which does not consider two-phase flow! disagrees withthe measured data. Liquid water flooding at the small channel scale is hypothesized as a primary cause for this discrepancy.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1799471# All rights reserved.Manuscript submitted October 14, 2003; revised manuscript received April 6, 2004. Available electronically October 8, 2004.

Thin-Film SOFCs Using Gastight YSZ Thin Films on Nanoporous Substrates

We fabricated yttria stabilized zirconia (YSZ) thin films having submicrometer thickness without gas leakage to be incorporated in low-temperature solid oxide fuel cells (SOFCs). We obtained 30-300-nm-thick YSZ films by oxidizing Y-Zr alloy thin films deposited onto anodic nanoporous alumina substrates having pore diameter of 20 and 200 nm using dc-magnetron sputtering at room temperature. During the thermal oxidation, the alloy films were successfully transformed to defect-free oxide thin films. Volume expansion induced from the oxidation of the alloy resulted in dense oxide thin films that are free from hydrogen permeation. Conductivity of YSZ thin films at room temperature was measured and compared with the reported conductivity of YSZ ceramics. And low-temperature operation testing was performed using the fabricated fuel cell.

Ion Conductivity Enhancement Effect by Introduction of Dislocations in Yttria-Stabilized Zirconia

High dislocation density in zirconia systems may significantly affect oxygen ion conductivity. Several processes for introduction of high dislocation density were investigated including ion irradiation. YSZ single crystals were irradiated with Ar+ ions and dislocations were observed by TEM. After heat treatment, high dislocation density was observed in a certain range of depth, and no dislocations were observed in the vicinity of the surface. Conductivity measurements by AC impedance spectroscopy indicated that conductivity improved by 55%.

Ion Irradiation Effects in Solid Oxide Fuel Cell Electrolytes

Single crystal Ytrria-stabilized Zirconia was irradiated with Xe 2+ and Xe 3+ ions at 320 and 450 keV over a range of doses from 10 13 to 10 16 ions/cm 2 . Damage appears as a 150 nm surface layer with a dense dislocation network. The X-ray diffraction pattern shows an increasing lattice expansion with increasing dose that reaches a saturation point. Ion irradiation increases the surface conductance of the material; this effect is removed with certain post-treatments. Preliminary isotope depth profiling indicates enhanced ion diffusion in the damaged layer.