Ulrich P. Muecke

Solid oxide fuel cells: Systems and materials

A solid oxide fuel cell (SOFC) is a solid-state energy conversion system that converts chemical energy into electrical energy and heat at elevated temperatures. Its bipolar cells are electrochemical devices with an anode,electrolyte, and cathode that can be arranged in a planar or tubular design with separated gas chambers for fuel and oxidant. Single chamber setups have bipolar cells with reaction selective electrodes and no separation between anode and cathode compartments. A nickel/yttria-stabilized-zirconia (YSZ) cermet is the most investigated and currently most widespread anode material for the use with hydrogen as fuel. In recent years, however, doped ceria cermet anodes with nickel or copper and ceria as the ceramic phase have been introduced together with ceria as electrolyte material for the use with hydrocarbon fuels. The state-of-the-art electrolyte material is YSZ of high ionic and nearly no electronic conductivity at temperatures between 800-1000 °C. In order to reduce SOFC system costs, a reduction of operation temperatures to 600-800 °C is desirable and electrolytes with higher ionic conductivities than YSZ are aimed for such as bismuth oxide, lanthanum gallate or mixed conducting ceria and the use of thin electrolytes. Proton conducting perovskites are researched as alternatives to conventional oxygen conducting electrolyte materials. At the cathode, the reduction of molecular oxygen takes place predominantly on the surface. Todays state-of-the-art cathodes are La x Sr 1 - x MnO 3 - δ for SOFC operating at high temperature i.e. 800-1000 °C, or mixed conducting La x Sr 1 - x Co y Fe 1 - y O 3 - δ for intermediate temperature operation, i.e. 600-800 °C. Among the variety of alternative materials, Sm x Sr 1 - x CoO 3 - δ and Ba x Sr 1 - x Co x Fe 1 - x O 3 - δ are perovskites that show very good oxygen reduction properties. This paper reviews the materials that are used in solid oxide fuel cells and their properties as well as novel materials that are potentially applied in the near future. The possible designs of single bipolar cells are also reviewed.

Nanoporous Ni–Ce0.8Gd0.2O1.9−x thin film cermet SOFC anodes prepared by pulsed laser deposition

Nickel oxide-gadolinia-doped ceria thin films with a ceria composition of 80 at% Ce and 20 at% Gd were grown by pulsed laser deposition on sapphire and SiO2/Si wafers as well as on yttria stabilized zirconia polycrystalline substrates. Upon reduction of the NiO phase in a H2/N2 atmosphere at 600 degrees C, a stable three-phase, 3-D interconnecting microstructure was obtained of metallic Ni, ceramic, and pores. Coarsening and segregation of the Ni to the surface of the film was observed at higher temperatures. The kinetics of this process depend strongly on the microstructures that can be developed in situ during deposition or post-deposition heat treatments. In situ minimization of Ni-coarsening can be achieved at temperatures as low as 500 degrees C when the deposition pressure does not exceed 0.02 mbar. For films deposited at higher pressure and at temperatures below 800 degrees C, coarsening can be minimized post deposition by annealing in air at 1000 degrees C. The films showed very good metallic conductivity and stability upon thermal cycling in a reducing atmosphere. Redox cycles performed at 600 degrees C between air and H2 induced a loss of connectivity of the metallic phase and consequent degradation of the conductivity. After 16 cycles, corresponding to 65 hrs, the conductivity is reduced by one order of magnitude.

Fabrication of Micro Solid Oxide Fuel Cell by Thin Film Processing Hybridization: I. Multilayer Structure of Sputtered YSZ Thin Film Electrolyte and Ni-Based Anodes deposited by Spray Pyrolysis

Physical properties of sputtered YSZ thin film electrolytes on anode thin film by spray pyrolisis has been investigated to realize the porous electrode and dense electrolyte multilayer structure for micro solid oxide fuel cells. It is shown that for better crystallinity and density, YSZ need to be deposited at an elevated temperature. However, if pure NiO anode was used for high temperature deposition, massive defects such as spalling and delamination were induced due to high thermal expansion mismatch. By changing anode to NiO-CGO composite, defects were significantly reduced even at high deposition temperature. Further research on realization of full cells by processing hybridization and cell performance characterization will be performed in near future.

LSCF Thin Film Cathodes Deposited by Spray Pyrolysis for Micro-SOFC

Porous LSCF thin film cathodes suitable for application in Micro Solid Oxide Fuel Cells (μ-SOFCs) were fabricated by spray pyrolysis and pulsed laser deposition. During spray pyrolysis a precursor is sprayed onto a heated substrate, where an amorphous metal oxide film is obtained. Porosity forms during a subsequent annealing process depending on the maximum crystallization temperature. It was possible to fabricate these cathodes with a maximum processing temperature of 650°C, which is desirable for the processes and materials involved in μSOFCs fabrication. The films are thinner than 1 μm with nanometersized grains. The cathode performance was evaluated by measuring the area specific resistance. The microstructure proofed to have a strong influence on the cathode performance. Smaller grain sizes led to better performance and can be achieved by low annealing temperature. Furthermore, the microstructure achieved by spray pyrolysis led to better results, than the microstructure achieved by pulsed laser deposition. ..

Electrochemical Performance of Ni-CGO Nano-Grained Thin Film Anodes for Micro SOFCs

NiO-Ce0.8Gd0.2O1.9-x (CGO) thin film anodes with thicknesses around 400 nm were prepared by air blast spray pyrolysis. The film composition was 60/40 vol% Ni/CGO in the reduced state. The films were deposited on tape-cast YSZ electrolytes. The material was amorphous after deposition and was crystallized by sintering in air between 650 and 1200°C. The temperature treatment resulted in films with average grain sizes of the NiO and CGO grains between 5 and 250 nm. The area specific resistance of the thin film anodes was measured in a humidified 1:4 H2:N2 atmosphere as a function of grain size within the temperature interval of 400-600°C. The area specific resistance (ASR) was predominantly depending on the grain size of the films. At 550°C, an ASR of 0.5 Ωcm 2 was found for the 5 nm grain size anode. The value increased to 30 Ωcm 2 for the 250 nm grain size anode.