FIB Characterization of the Cathode Microstructure in a Single Solid Oxide Fuel Cell (SOFC)

Abstract

The relationship between the cathode microstructure and electrochemical performance of lanthanum strontium manganite (LSM) /yttria stabilized zirconia (YSZ) SOFCs was studied for the isochronal sintering at temperatures between 950°C and 1400°C for 1 hour. Previous studies have shown that the Triple-Phase-Boundary (TPB) length at the cathode/electrolyte interface contributes to charge transfer resistance. However, it is unclear the role of the tertiary phase formation. This study investigated the effects of 1) TPB length and 2) tertiary phase formation on the charge transfer resistance. Techniques were developed using a dual-beam FIB system to automatically slice serial-sections of the cathode. These images were assembled to create a 3-D image of the microstructure. Also, cross-section TEM samples were made using the Omniprobe manipulator. The homogeneity of the sample was analyzed using a disector probe. High-resolution TEM-EDS was used to characterize tertiary phase formation at the cathode/electrolyte interface.It was found that TPB length was reduced from 1.41±0.07 µm/µm 2 to 0.21±0.01 µm/µm 2 as the sintering temperature increased from 950°C to 1400°C. Formation of a low conductivity tertiary phase was observed above 1200°C, which appears to contribute to the increase in charge transfer resistance.

Sintering of porous strontium doped lanthanum manganite-yttria stabilized zirconia composite in controlled oxygen atmosphere at 1400 °C

“…Several studies have indicated that La 2 Zr 2 O 7 forms for x !0.0 and SrZrO 3 along with the La 2 Zr 2 O 7 starts to form for x !0.3 [6,24,26], while other studies reported that both zirconates can form at x !0.17, which is also consistent with thermodynamic calculations [19,23]. Formation of SrZrO 3 at 1:1 LSM and YSZ ratio and La 2 Zr 2 O 7 at 1:3 ratio was reported during annealing at 1000 C in N 2 (PO 2 w 10 À5 atm) atmosphere [27]. However, SrZrO 3 formation was not observed when LSM and YSZ ratio !1.0 [19].…”

“…Mn ions diffuse readily in YSZ due to its smaller ionic radii and the highest diffusivity among the constituent lanthanum and strontium ions [6,27]. The Mn in solid solution is a mixture of Mn 3þ and Mn 2þ [45].…”

“…For instance, La 2 Zr 2 O 7 forms at 1200 C but SrZrO 3 forms at 1350 C for La 0.6 Sr 0.4 MnO 3 eYSZ composites sintered in air for 120 h [24]. The amount of the zirconate phases increases with increase in sintering time [24,25,27]. Interdiffusion of respective cations across the LSMeYSZ boundary during the sintering of LSMeYSZ has been reported to be responsible for zirconate phases formation [6,9,17,24,25,29].…”

Impact of catalyst support morphology on 3D electrode structure and polymer electrolyte membrane fuel cell performance

“…It has been shown previously that the 3D architecture of electrodes in electrochemical energy devices can be investigated in detail by the focused ion beam (FIB)-scanning electron microscopy (SEM) slice&view technique and subsequent 3D reconstruction. [12,13,14,15] This technique has for instance been used to study degradation phenomena in PEM fuel cell electrodes, [16,17] as well as the impact of various electrode fabrication processes on the resulting 3D electrode structure. [18] Early work by Ettingshausen et al [19] and Litster et al [2] already indicated the important role of the support material's shape for the formation of the porous structure of the catalytic layer in fuel cells.…”