In Situ TEM Observation on Redox Cycling of SOFC Anode

Abstract

Solid Oxide Fuel Cell (SOFC) technology is a promising energy conversion option which generates electricity with high efficiency. SOFCs have additional advantages including fuel flexibility (e. g. CO, H2 and CH4), distributed power generation, and reduction of pollutants such as NOx and SOx. The standard SOFC design is based on the use of a porous cermet composite anode made of yttria (Y2O3)-stabilized zirconia (YSZ) and Ni. NiO particles are sintered with YSZ and then reduced to metallic Ni before normal operation of the cell. Ni-based anode-supported cells display sufficient performance and steady state durability, however, the anode exhibits structural instability in successive redox cycles: although Ni is kept in its reduced state under normal operating conditions, the Ni may reoxidize due to factors such as seal leakage, shutdown in the absence of protecting gas, or high fuel utilization. It has been suggested that coalescence and sintering of Ni during reduction and the formation of porosity on reoxidation may account for the instability of Ni on redox cycling. However, there have been only a few reports on the reduction and oxidation processes of Ni-zirconia cermets observed directly with nanometer spatial resolution in real time. Environmental transmission electron microscopy (ETEM) can be used to provide dynamic in situ observations of chemical reactions at the nano- to micrometric scale. It allows gases at pressures above 10 mbar to be introduced into the electron microscope column with the sample at elevated temperature and without compromising other aspects of the performance of the microscope. In this study, in situ reduction and reoxidation for Ni and Sc2O3-stabilized zirconia (ScSZ) cermet anode were performed in a differentially pumped FEI TitanTM ETEM with the specimen mounted in a NanoExTM heating holder. This configuration allows in situ studies in the microscope in gaseous atmospheres up to 20 mbar and at temperatures up to 1000℃. The SOFC anode samples for the ETEM examination were prepared by a focused ion beam (FIB) lamella lift-out method on MEMS Chips for the NanoEX holder using a FEI Versa 3D. Prior to heating the sample, H2 or O2was introduced into the column, corresponding to measured pressures of 3-5 mbar at the sample. After that, scanning TEM (STEM) high-angle annular dark-field and bright-field (BF) images of the SOFC lamella were continuously taken by the BB FlashBack software. The temperature was kept at 300 ℃, 400 ℃, 500 ℃, 600 ℃, and 700 ℃ for 5 minutes during heating up to 800 ℃. In the range between 400 ℃ and 800 ℃, the specimen was heated at a ramp of 1 ℃/sec, although it was possible to heat and to cool it immediately in the holder. In the case of heating as-sintered NiO-ScSZ cermet in a H2 atmosphere of 3 mbar, nanovoids were observed at around 400 ℃ at the grain boundaries of NiO/ScSZ and NiO/NiO. Nano-pores were formed in the NiO grains to accommodate the volume shrinkage that resulted from the reduction of NiO to Ni. When the temperature was over 500 ℃, the NiO-free surface was reduced directly, and porosity formed uniformly across the NiO grains. At higher temperatures, coalescence of the porosity was observed to result from Ni reorganization. This sample was reoxidized with heating again up to 800 ℃ under an O2atmosphere of 3 mbar in the ETEM. When the Ni-ScSZ cermet samples were heated in the O2 atmosphere of 3-5 mbar, both the grain boundary of Ni/ScSZ and the Ni free surface were reduced at around 500 ℃. Electron energy loss spectroscopy revealed that Ni particles were completely oxidized to NiO after heating up to 800 ℃. From these ETEM studies, it was found that the formation of porous NiO and the coarsening of Ni during the redox cycling result in the reduction of triple-phase boundaries in the cells. This work is partly supported by the Japan Science and Technology Agency (JST) through its Center of Innovation (COI) Program. International Institute for Carbon-Neutral Energy Research (I2CNER) is supported by World Premier International Research Center Initiative (WPI), MEXT, Japan. Figure 1