Fabrication and Characterization of Ni/ScSZ Cermet Anodes for Intermediate Temperature SOFCs
Creators: name: Somalu, Mahendra Rao
Publisher: name: Imperial College London
Year: 2012

description: Solid oxide fuel cells (SOFCs) are of increasing interest as low emission, high efficiency, energy conversion devices for the production of electricity, and in some cases heat, from a wide range of fuels. In general, a porous cermet of nickel/ytrria-stabilized-zirconia (Ni/YSZ) is used as an anode with a dense electrolyte of ytrria stabilized zirconia (YSZ), enabling operation at temperatures above around 750 oC. Nevertheless, operating at high temperature leads to various problems such as metal corrosion, electrode sintering, and unwanted interfacial diffusion in the cell. In this regard, cermet anodes such as nickel/samarium-doped-ceria (Ni/SDC) and nickel/gadolinium-doped-ceria (Ni/CGO) have been proposed as alternative anodes to be operated with SDC and CGO electrolytes at intermediate temperatures (&lt; 700 oC), respectively. However, less attention has been given to nickel/scandia-stabilized-zirconia (Ni/ScSZ) cermet anodes as alternatives for intermediate temperature SOFCs, despite the fact that the ScSZ electrolyte exhibits a higher ionic conductivity compared to other zirconia electrolytes, which may offer some advantages, especially at lower temperature. Furthermore, Ni/ScSZ anodes have shown improved tolerance towards carbon deposition and sulphur poisoning in addition to improved durability when compared to Ni/YSZ anodes. To date there have only been limited studies into the relationship between the materials used, the processing conditions, and the properties and performance of Ni/ScSZ anodes. This thesis is therefore aimed at (i) studying the optimum fabrication conditions and properties of Ni/ScSZ anodes, (ii) investigating the effect of ingredients such as binder content, solvent type and solid content in the ink formulation on the rheological properties of NiO/ScSZ inks and their applicability for screen-printing and (iii) relating the rheological properties of NiO/ScSZ inks to the performance and properties of the resultant anode films. A large part of the work is focussed on the fabrication and rheological properties of NiO/ScSZ screen-printing inks. The properties are linked to the particle network strength within the formulated inks and relate to the microstructure, mechanical strength, electrical performance and electrochemical performance of the resultant anode films. Overall, Ni/ScSZ anodes having 40 vol% Ni were found to be optimum in terms of both electronic conductivity and electrode polarization resistance. The anode exhibited improved tolerance towards carbon deposition compared to Ni/YSZ at intermediate temperature (700 oC). The effects of binder and solid content on the rheological properties of NiO/ScSZ screen-printing inks were studied by evaluating the thixotropic properties, yield stress and viscoelastic properties of the inks. The study indicated improved thixotropic properties, yield stress and particle network strength within the inks as the binder and solid content increased. These improved properties can be related to better particle bridging within the inks. A percentage ink recovery of 40 to 65 % was determined sufficient for the production of quality films with minimum defects. From the study, inks having 26 vol% solid with 3 wt% binder or 28-30 vol% solid with 2 wt% binder were determined as optimum for screen-printing using squeegee load, squeegee length, printing speed, snap-off and screen type of 6 kg, 5 in, 0.02 m/s, 2 mm and 325, respectively. Furthermore, films fabricated using these inks revealed improved particle connectivity, higher electronic conductivity, lower electrode polarization resistance and improved mechanical hardness. The solvent type was determined to have only a small impact compared to the binder and solid contents in the inks.