In-situ electron microscopy investigation of reduction-induced microstructural changes in NiO

The structural characteristics of NiO–YSZ (YSZ: Yttria-stabilized zirconia) composites and the redox stability of NiO have been explored by in-situ and high-resolution transmission electron microscopy (TEM). NiO has been reduced on alumina support while doing in-situ microscopy, and the observations on redox instability and microstructural changes have been extrapolated to understand the behavior of NiO/Ni–YSZ system. In the composite, the NiO and YSZ grains are strain free and no reaction product is observed at the grain boundaries after long term annealing. Under a low partial pressure of oxygen, NiO may begin to reduce to metallic Ni at temperatures as low as ~200 °C. The volume change associated with such a reduction process is ~40.3%. Although metallic Ni nucleates in NiO coherently, such a huge volume contraction may lead to porosity generation and mechanical instability. Long term exposure of metallic Ni to high temperatures leads to coarsening, which reduces the available surface area. This in turn has a direct, negative consequence on the catalytic properties of the metallic Ni particles. The coarsened particles may even undergo faceting, and the facets may not be the most active catalytically. Synthesis of finely dispersed and percolatively connected Ni–YSZ anodes for SOFCs and core–shell or compositionally graded electrodes for batteries may be viable solutions to enhance the lifetime and structural integrity of such electrodes.