Degradation phenomena in the anode side of solid oxide fuel cell (SOFC) stacks

Solid Oxide Fuel Cells are envisioned to offer an attractive alternative for the combined generation of electricity and heat, especially for stationary applications. Although commercial systems using these fuel cells are available in the market, the disadvantages associated with their high costs and limited lifetimes continue to hinder their widespread usage. In this thesis, the latter issue is tackled by addressing specific degradation mechanisms that occur in these fuel cells. Focus is on single components from the anode side of the fuel cell stack assembly. The components from the anode side are subjected to conditions that would normally be prevalent during typical operational loads. Components from the anode side of the stack include a Ni-mesh as a current collector, Crofer 22 H as an interconnect and the NiO-8YSZ anode substrate with an anode functional layer as the electrode. Parameters such as temperature, current density and humidity in fuel gas atmosphere are varied from 700-800 °C, 0-0.7 A.cm-2 and 3-80% H2O in H2 respectively. The resistance contributions from the components and their interfaces are measured using experimental methods such as 4 probe DC resistivity and electrochemical impedance spectroscopy. Long term tests up to 1000 hours and short term tests ranging from 0-300 hours are carried out along with thermal/redox cycles to monitor these resistances. In this way, using both experimental techniques, the ohmic and polarization resistance contributions are measured, and the influence of changing operating parameters on degradation are evaluated.It is observed that the ohmic resistance offered by the Ni-mesh – interconnect interface and the interconnect-anode substrate interface is very low and suffer insignificant degradation over the tested periods. Regardless of change in temperature and the presence of high humidity in the fuel gas, the resistance contribution remains low. Even the thermal cycles, interdiffusion of elements, nickel oxidation and the buildup of oxide scales did not aggravate the resistances. Post test microscopic analysis revealed several metallic contact points at these interfaces offering a parallel path for the low resistance conduction of current.On the other hand, impedance measurements using symmetric anode cells helped in isolating the ohmic and polarization resistances as a function of operating parameters. Both types of resistances offered by the anode-electrolyte interface are found to increase with time. In addition, thermal cycling is also found to be a source for substantial degradation for this interface. However, the presence of artefacts and inductive effects possibly due to the positioning of the reference electrode lead to distortion and ‘scaling’ of the spectra from the individual electrodes. Hence the exact magnitudes of the resistances obtained from the impedance measurements are prone to errors and therefore provide qualitative information. Finally, areas for future investigations on degradation are identified and corrective measures for obtaining reliable and reproducible measurements are recommended