In-situ TEM reduction of a solid oxide cell with NiO/YSZ fuel electrode

Ni/YSZ fuel cell electrodes of high-temperature solid oxide cells are commonly reduced from NiO/YSZ under hydrogen atmosphere at high temperatures, prior to operation. Ni provides the required electronic conductivity and electrocatalytic activity and thus enables operability of the cell. The reduction results in a significant change in microstructure which can be observed by in-situ transmission electron microscopy. As specific microstructural properties are crucial to achieve high performance and durability of the cell, a comprehensive understanding of the reduction process is required. In this contribution, an in-situ real-time observation of microstructural changes during reduction of a NiO/YSZ-compound as well as pure NiO-samples is performed and compared to an ex-situ analysis of a corresponding cell after the reduction. For the first time, we reduce NiO/YSZ with in-situ TEM at H$_2$ pressures up to 1 atm and different temperatures up to 850 °C, relevant to the actual reduction conditions of the fuel electrodes. With three different reduction schemes, we show a drastic influence of the reduction conditions on Ni microstructure and Ni/YSZ interface. ... mehrAt low temperatures, grain boundaries and triple junctions between NiO and YSZ are determined as the starting points of the reduction process. This process reduces the connection between Ni and YSZ. Novel in-situ TEM results from this work provide a clear evidence at the nanoscale that high initial reduction temperatures lead to the different microstructure of Ni, better connection between Ni grains and superior quality of Ni/YSZ interface. This is crucial to achieve a larger number of electrochemically active triple phase boundaries between Ni/YSZ and gas.