Preparation of Freeze-Casted Tubular SOFC with a Modified Anode Functional Layer

Abstract

Methods that result in randomly orientated porosity are considered conventional for creating micro-porosity within ceramic and cermet structures and coatings that make-up porous electrodes in solid-state, open-system electrochemical devices such as the Solid Oxide Fuel Cell (SOFC). The porosity is necessary to allow sufficient transport of gas species to and away from active sites at the electrode-electrolyte interfaces. Alignment of micro-porosity in porous electrode structures is considered ideal when considering mass transport and tortuosity of gas species diffusing in and out of the active sites. Aligned, micro-porosity can be achieved with the freeze-cast process during which ice-templating is used as the primary agent for pore formation and structural manipulation. In this research, we present a refined method to manufacture and electrochemically evaluate tubular SOFCs (T-SOFCs) whereabout the supporting electrode structure is formed by using the freeze-casting process. All additional layers of the electrochemical cell are achieved via a conventional dip-coating technique. The casting temperature is selected such that ice-templating results in platelet-like structuring of the particles within the ceramic tube, which in this case is designated as the anode of the SOFC, and with alignment of porosity in the radial direction. The anode materials consist of nickel metal and yttrium stabilized zirconium oxide particles homogenously dispersed throughout the bulk anode structure. We show that the total porosity of the tube, under selected experimental conditions, is approximately 50% thus requiring a more conductive anode functional layer connecting the bulk, anode tube to the solid-state electrolyte coating. The anode functional layer is also critical as an intermediate support layer to allow for a dense electrolyte coating to be applied onto such a porous support that has more porosity than conventional anodes which typically have < 30% total bulk porosity. The tubular anode support with the anode functional layer and electrolyte is imaged with scanning electron microscopy to reveal the plate-like microstructure of ceramic particles orientated within the porous network. The solid-state electrolyte is composed of scandium stabilized zirconium oxide while the anode functional layer is composed of nickel metal and scandium stabilized zirconium oxide particles homogenously mixed together within the coating. Complete, T-SOFCs are fabricated and evaluated by additionally coating a ceria-based barrier layer underneath that of a cathode outer layer composed of a standard lanthanum strontium cobalt ferrite perovskite material, which results in a full, electrochemical cell. Polarization curves show a maximum power density of 500mW per cm2 at 800 oC under low fuel utilization conditions using dry hydrogen at a feed pressure of about 250 Pascals above atmospheric pressure and air provided in abundance with respect to the stoichiometric requirement around the outside of the cathode in the testing furnace. Two-wire, electrical impedance spectroscopy with scanning frequencies from 100kHz to 0.05Hz at open circuit potential (OCV), varying load conditions, and with varying perturbation AC signals corresponding to the different load conditions reveal the contributions of overpotentials at both high and low fuel utilization conditions in the temperature range of 700 oC to 800 oC. Figure 1