In situ TEM study of reduction and reoxidation of NiO/ceramic composites

Nickel/ceramic solid oxide fuel cell anodes exhibit a dimensional instability when experiencing a reduction-oxidation cycle. As fuel is supplied on the anode side, the as-sintered nickel oxide phase (NiO) reduces to metallic Ni and then remains in this state during operation. Yet, several factors may lead to an accidental reoxidation of the Ni, which may rupture parts of the cell, hence degrading its performance. The mechanisms behind the dimensional instability of Ni/yttria-stabilised zirconia (YSZ) anodes are investigated here through an innovative environmental transmission electron microscopy assessment. NiO particles and NiO/YSZ composites are reduced and reoxidised in the microscope in a few mbar of hydrogen and oxygen, respectively, up to 500-850 °C. Images, diffraction patterns, electron energy-loss spectra and energy-filtered micrographs are acquired, usually at constant temperature intervals during the reactions, to capture in situ the nanostructure, crystallography and chemistry. The reaction kinetics are retrieved from both the changes in shapes of the Ni L23 edges in energy-loss spectra and from energy-filtered images (with nm-resolution), analysed to provide quantitative data and correlated to the structure. Complementary data include post-exposure microscopy, in situ X-ray diffraction and density functional theory computations. While the surface nucleation of Ni domains, their growth and impingement control the reduction of NiO particles, the results reveal a modification of the mechanisms in the presence of yttria-stabilised zirconia, with the transfer of oxygen from NiO to the oxygen vacancies of the YSZ ceramic now triggering the reaction. Intragranular voids form in both cases as oxygen is removed. The final Ni structure at high temperature is then observed to coarsen as it minimises its surface energy, with the percolation of the Ni phase influenced by the symmetry of its grain boundaries. The reoxidation of Ni is controlled mainly by the outward diffusion of Ni ions through the grain boundaries of the growing NiO film. While some NiO inward growth occurs through the formation of oxide film cracks, the Ni2+ outward diffusion process remains unbalanced and voids form in the NiO phase. These internal voids are responsible for the dimensional instability of the composite along with Ni coarsening at high temperature. Several parameters for improved performance and redox tolerance are then identified based on these results.