Composite electrodes such as LSM/ScSZ or LSM/YSZ are widely used as a cathode material of SOFC. In these electrodes, both triple phase boundary (TPB) consisting of LSM, ScSZ, and gas phases and double phase boundary (DPB) between LSM and gas phases are considered to work as an active site for oxygen reduction.1 Length of TPB, surface area of DPB, and spatial distribution of those have a great impact on electrochemical performance of cathode. Such kind of geometrical information can be investigated by focused-ion beam scanning electron microscope (FIB-SEM) while almost no information about active reaction site distribution or oxide ion path in composite cathode has been obtained so far. Atomic labeling using isotope oxygen is a powerful tool to investigate oxide ion flow in SOFC materials, and has been mainly applied to pattern electrode experiments to investigate reaction paths at anode or cathode.2 Here, we first conduct particle-scaled visualization of active site in composite LSM/ScSZ cathode by oxygen isotope labeling.
Methods, Results, and Discussions
A YSZ electrolyte-supported cell with Ni/YSZ anode and LSM/ScSZ cathode was fabricated for the experiment. Wight ratio of LSM and ScSZ is 1:1. In order to quench a reaction in a SOFC cell within a short time during operation at high temperature, a power generation equipment with a nozzle for a helium impinging jet, which is covered by a water cooling jacket, was prepared. At first, the cell was operated in 50% H2-50% Ar for the anode side and in pure 18O2 for the cathode side at 1073 K for 3 minutes. After that, the cell was cooled to below 423 K within 1.5 sec. by the helium impinging jet. The 18O distribution in cross section of the cathode was obtained by secondary ion mass spectroscopy (SIMS) with a spatial resolution of 50 nm (NanoSIMS 50L, CAMECA).
In this work, 18O concentration mappings of an interface between LSM/ScSZ cathode and YSZ electrolyte under OCV and 0.11 A/cm2 were obtained. The mappings of LSM (red) and ScSZ or YSZ (green) and of 18O concentration denoted by the ratio of 18O-/(16O- + 18O-) under 0.11 A/cm2 are shown in Figures 1(a) and 1(b), respectively. At cathode part, 18O diffuses to only ScSZ phase uniformly under OCV condition, which is driven by the isotopic oxygen concentration difference. On the other hand, relatively high 18O intensity (0.2-0.25) is observed at LSM particles near the cathode/electrolyte interface under 0.11 A/cm2 as shown in Fig. 1(b). This means that 18O diffuses into LSM particles only near the cathode/electrolyte interface. Isotopic exchange experiment of LSM-patterned cathode on YSZ was conducted by Horita et al.2 Their work disclosed that diffusion of oxide ion into LSM bulk under the higher cathodic polarization is promoted than that under the lower. This is because under the higher cathodic polarization, charge transfer currents increase and active sites are expanded from TPB to DPB. From Fig. 1(b), more charge transfer currents are generated at LSM particles in the region within about 3µm from the cathode/electrolyte interface than at other regions. This result gives the first direct evidence that reaction sites near the cathode/electrolyte interface are more electrochemically active than those at other regions, which has been predicted by electrochemical measurements and numerical simulations.3
Acknowledgements
SIMS analysis was conducted at Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by "Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
References
1. M. Gong, R.S. Gemmen, and X. Liu, J. Power Sources 201, 204-218 (2012).
2. T. Horita, K. Yamaji, N. Sakai, H. Yokokawa, T. Kawada, and T. Kato, Solid State Ionics 127, 55-65 (2000).
3. K. Miyoshi, T. Miyamae, H. Iwai, M. Saito, M. Kishimoto, and H. Yoshida, J. Power Sources 315, 63-69 (2016).