Solid oxide fuel cells (SOFC) are hydrogen energy conversion devices at high efficiencies, however, require high operation temperatures. Reducing the electrolyte thickness results in a significant decrease of ohmic resistance, thereby the use of nanoscale thin film electrolytes may be an attractive option to lower the SOFC operation temperature. However, it is necessary to develop techniques to grow high-quality pin-hole free ultra-thin electrolytes in order to enable the application of thin film oxides in SOFC devices, along with a deep understanding of ionic transport in thin membranes. In this presentation, we report on detailed structural (using transmission electron microscopy) and high temperature electrochemical studies of yttria doped zirconia (YDZ) thin films grown by ultraviolet (UV) oxidation of Zr-Y metal alloys. Among several deposition techniques for preparation of ultra-thin metal oxide thin films, UV oxidation is an unique approach since it can grow dense conformal ultra-thin crystalline oxide thin films with nearly atomically tailored stoichiometry at low temperatures. YDZ was chosen as a model material for this study as it is a widely used solid oxide electrolyte. Zr and Y metal were grown by magnetron co-sputtering on various substrates at room temperature (RT). The composition of Y/Zr was controlled by changing the growth parameters including sputtering gun power during the deposition. After about 30 Å of Zr-Y was grown on substrates, the samples were transferred to the load lock and exposed to UV in 100 % O₂ for 15-30 minutes in a custom-designed in-situ oxidation chamber. After UV oxidation, the samples were transferred back into the main chamber for further growth. This process was repeated until the film reaches desired thickness, followed by the post-deposition UV irradiation for 1 hour in air. To compare the results with films grown by other techniques, YDZ films were also grown by thermal oxidation of Zr-Y alloy and RF sputtering of YDZ ceramic targets. The as-grown Zr-Y metal film was nanocrystalline, which was oxidized into fully crystalline structure with 5-10 nm grains in the presence of UV light at RT. The crystal structure of oxidized alloy was cubic, indicating Y doping can effectively stabilize the cubic phase at RT in UV oxides. This is unlike pure zirconia where tetragonal phase has been observed to be stable at low temperatures due to size effects. Interestingly, the film underwent cubic-to-tetragonal transformation during annealing at 900°C for 1 hour in air. The grain size increased comparable to the film thickness upon annealing. The high temperature conductivity of UV oxides was found comparable to the films grown by other methods. (~ 0.1 S/cm at 885°C) Those results indicate that this is an exciting route to grow high quality ultra-thin oxide electrolytes at low temperatures. We anticipate these results to be of significance to developing novel film synthesis techniques for advanced micro-SOFC devices.

Exploring new materials is of high importance for the next generation of high performance cathodes in solid oxide fuel cells (SOFCs). The present cathodes are operating at high temperatures (800-1000°C) and have material compatibility challenges to overcome which give rise to high running costs. [1] A new mixed ionic and electronic conducting (MIEC) cathode material with promising property is the perovskite structured Ba0.5Sr0.5Co0.8Fe0.2O3-d [2]. The behaviour of this oxygen deficient perovskite inspired us to study a similar Co/Fe containing perovskite, the series Bi0.15Sr0.85Co1-xFexO3-d 0.0 < x < 1.0. Here, we present a study based on neutron powder diffraction (NPD), high resolution microscopy/electron diffraction (HREM/ED), thermogravimetric analysis (TGA) and magnetic susceptibility measurements.A complicated series of different perovskite structures appeared as the degree of Fe content increased. Weak diffraction peaks, resolved from NPD and HREM/ED for the as-prepared sample with x = 0.1 Fe content, resulted in a P4/mmm supercell, a = b ≈ ap and c ≈ 2ap; for x = 0.2-0.6 a disordered cubic unit cell P m3m a ≈ ap, and for x = 0.8-1.0 a pseudo-cubic P 4/mmm, a = b ≈ ap, c ≈ ap unit cell, with increased tetragonality with increasing Fe, were obtained. With increased Fe addition in this system, short range ordering was detectable by diffuse scattering, indicating the local ordering of oxygen vacancies. Oxygen annealing of all as-prepared samples resulted in contracted simple cubic unit cells and the diffuse scattering disappeared.Detailed magnetisation results revealed spin glass behaviour for the x = 0.1, 0.25 and 1.0 oxygen annealed samples, in contrast to the antiferromagnetically ordered ground state of the as-prepared materials. [3] Preliminary conductivity curves of the as-prepared samples show promising results for a good candidate for a (MIEC) solid oxide cathode.In an extension to this work we also performed structural and TGA/DSC studies on A-site substituted Sr0.85-xAxBi0.15CoO3-d perovskite, where A = Ba and Ca. Refined PXD data cell parameters are similar to the pure Sr0.85Bi0.15CoO3-d perovskite [4], P m3m with either a expanded or contracted cell consistent with the ionic size of the A-site dopant.[1] Z. Shao and S.M Halle, Nature 431 (2004) 170[2] S. McIntosh et al. Solid State Ionics, 177 (2006) 1737[3] A. K. Eriksson et al., submitted to J. Solid State Chem. 2007[4] Knee et al. Chem. Mater. 18 (2006) 1354

A solid oxide fuel cell (SOFC) is an all solid device that converts the chemical energy of gaseous fuels such as hydrogen and simple hydrocarbons into electricity through electrochemical processes. Lowering the operating temperature of SOFCs is one of the main goals in current SOFC research. A reduction in operating temperature would reduce performance degradation, lessen sealing problems, and enable replacement of ceramic interconnects by cheaper metallic materials. However, at reduced operating temperature, the thermally-activated electrode reactions will become slower, resulting in lowered fuel cell performance. Therefore, the electrode material requires particular attention in the development and optimization of low temperature SOFCs. We are exploring the use of atomic layer deposition (ALD) for the fabrication of SOFC electrode components, including an ultra-thin Pt film for use as the electrocatalyst, and a Pt mesh structure for a current collector.We have successfully carried out the Pt ALD process using (methylcyclopentadienyl) trimethylplatinum and oxygen as precursors and nitrogen as a carrier and purging gas with an optimal growth rate of 0.4 Å/cycle. Under the optimal ALD conditions, we can achieve deposition of Pt thin films with low resistivity and high purity on an yttria stabilized zirconia electrolyte material. In addition to the blanket deposition of Pt, we have used area selective ALD, with a combined ALD and microcontact printing method, for fabrication of spatially patterned Pt to be used as a current collector grid for SOFCs. Working SOFC fuel cells were fabricated with ALD-deposited Pt, and their performance was characterized by a potentiostat-impedance system. The reference fuel cell sample is of the following structure: RF sputtered Pt /RF sputtered YSZ /RF sputtered Pt (anode/electrolyte/cathode). To investigate the dependence of the fuel cell performance on the thickness of the ALD deposited Pt anodes, we have fabricated functional fuel cells with the anode material replaced with ALD Pt. I-V measurements were performed for different Pt thin film thicknesses and operating temperatures (300oC~450oC). The measured performance illustrates that comparable peak power densities were achieved with ALD-deposited Pt anodes with only one-eighth of the Pt loading relative to the DC-sputtered Pt anodes. Consequently, the ALD technique may be used to significantly reduce the Pt load in SOFCs and, in turn, reduce material cost. In addition to the continuous electrocatalyst layer, we fabricated a micropatterned Pt structure via the technique of area selective ALD, and used it as a current collector grid for the fuel cells. An improvement of the fuel cell performance by a factor of 10 was observed by using the Pt current collector grids integrated onto cathodic material La0.6Sr0.4Co0.2Fe0.8O3-δ .

Submicron electrolyte films are a key component of intermediate temperature solid oxide fuel cells (IT-SOFCs). Several groups have reported size-dependent ionic conductivity in thin films of yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC), in either in-plane or through-plane configurations. Here we report measurements in both configurations of dense YSZ films varying in thickness from 20 to 200 nm. In-plane measurements were performed on YSZ films grown on silicon wafers coated with SiO2 or Si3N4. Detailed structural studies were performed on the YSZ films to investigate crystallinity and uniformity. Micro-fabricated strips (of several device dimensions) with Pt electrodes were used to obtain conductivity as a function of temperature from 200 – 600 C in a custom-designed micro-probe station. These films have activation energies in the range of 0.77 to 0.93 eV and lower conductivity compared with other reports, and show little or no thickness or substrate dependence. Through-plane and fuel cell measurements were performed by depositing YSZ on a nitrided silicon wafer, then etching holes in the wafer and depositing platinum electrodes on both sides. We will discuss the electrochemical conduction studies in detail along with fuel cell performance and correlation with electrode microstructure. The results have potential relevance to fabrication of micro-fuel cells for environmentally friendly portable energy generation.