Boundary Observation and Contrast Tuning of Ni/YSZ Anode by TEM and FIB-SEM

The most widely used anode material in SOFC is Ni/yttria stabilized zirconia (Ni/YSZ). Its microstructure depends on the fabrication techniques, and determines its performance. Electron microscopy is often adopted to characterize the microstructure in wide scales. In this report, we focus on the microstructure observation of conventional Ni/YSZ anodes in two open issues: atomic structure of boundary by transmission electron microscopy (TEM) and phase contrast tuning by focused ion beam-scanning electron microscopy (FIB-SEM).

It is well-known that Ni/YSZ interface and Ni/YSZ/pore triple-phase boundary (TPB) play important roles in the stability and reaction activity of Ni/YSZ anodes. However, most of the microstructure researches related to interface and TPB lie in micro- or nano-scales. In atomic scale, few reports on the interfaces of conventional Ni/YSZ anode can be found. Moreover, there is no experimental study on the real atomic structures of TPB. So, high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) were adopted to observe the atomic structures at the interfaces and TPBs. Before observation, the pores of the porous electrode were infiltrated by low viscosity epoxy resin under vacuum, so that they could be easily distinguished in a microscopy and the bulk cell could be readily handled. The specimen for TEM was lifted out in a focused ion beam-scanning electron microscopy (FIB-SEM). After observation, high-index contact surfaces (e.g. Ni(15 13 1)/YSZ(4 4 10)) were found between Ni and YSZ. At TPB, YSZ(200) was found to be the contact surface while Ni(111) shows an included angle of ~13° with YSZ(200).

3D reconstruction is becoming popular with the help of several techniques, such as FIB-SEM and TEM. In case of FIB-SEM, many of previous works showed indistinct contrast among different phases, which will make the analysis difficult and the results distorted in some extent. Herein, we used a high-resolution machine (HITACHI MI4000L) and tuned many conditions to improve the contrast. The related mechanism will be discussed. For Ni/YSZ anode, distinct contrast was obtained among Ni, YSZ and pore (actually resin) by using InLens detector. The three phases could be automatically segmented by Avizo software. Then the 3D microstructure is reconstructed.