Effect of hydrogen reduction on the microstructure and elastic properties of Ni-based anodes for SOFCs

One route for the synthesis of solid-oxide fuel cells incorporating Ni-based anodes involves co-sintering in air of the electrolyte with a NiO-YSZ precursor to a Ni-YSZ anode. Prior to the operation of the cell it is necessary to convert NiO into metallic Ni by hydrogen-reduction. The reduction of NiO into Ni is characterized by significant volumetric changes, changes in porosity and hence, elastic properties and strength. These changes in turn will modify the state of residual stresses in the cell and impact its reliability. In this study, the kinetics of hydrogen reduction (using a gas mixture of 4%H ₂-96%Ar) of 23vol% porous 75mol%NiO/YSZ anode materials was investigated by thermogravimetry between 600°C and 800°C. In addition, samples were reduced at 800°C for different periods of time to monitor the evolution of structural changes as a function of fraction of reduced NiO. The kinetics of reduction were found to exhibit two stages: At all temperatures the fraction of reduced NiO was found to increase linearly with time until nearly 70-80% of NiO was reduced, and the rate of reduction was found to increase with temperature according to an Arrhenius law with an activation energy of 25.2 kJ/mol. Optical and scanning electron microscopy, and electron microprobe chemical analysis indicate that the first stage of the reduction process is associated with the displacement of the reduction front across the thickness of the sample, whereas the second stage, which occurs at a much slower rate involves further reduction of NiO behind the reduction front. Young's and shear moduli of Ni-based anodes were determined by Resonant Ultrasound Spectroscopy and Impulse Excitation as a function of fraction of reduced NiO. It was found that elastic moduli decrease with extent of the reduction reaction predominately due to increase in porosity.