Microstructure and Performance of Ni-Free Nano La0.75Sr0.25Cr0.5Mn0.5O3 - Gd0.2Ce0.8Ox Composite Anode

Abstract

Degradation of conventional solid oxide fuel cell (SOFC) anode is often associated with nickel coarsening and redox instability. Great efforts have been made to find alternative materials to replace the state-of-the-art nickel - yttria-stabilized zirconia (Ni-YSZ) anode cermet. In the previous study, La0.9Sr0.1Cr0.5Mn0.5O3-Gd0.1Ce0.9Ox (LSCM-GDC) composite anode was investigated as one of the possible alternatives for Ni-free SOFC (1). The LSCM-GDC composite was fabricate by mixing commercial powders in the designated proportion. It was shown that the surface area of GDC has to be increased and the grain size has to be decreased in order to compensate for the poor electrocatalytic properties.In this study, the microstructure and electrochemical performance of Ni-free anode fabricated with La0.75Sr0.25Cr0.5Mn0.5O3 : Gd0.2Ce0.8Ox = 50 : 50 w.% (LSCM-GDC) nano-powder are investigated. The LSCM-GDC nano-composite particles were prepared through colloidal-processing-based approach (2). The growth of homogenous nano-particles is induced by the heterogeneous nucleation of a LSCM precursor on the surface of GDC nanocrystals. The nucleation is further followed by heat treatment at 1000 oC to convert precursors into oxides. The resulting nano-powder consists of well dispersed LSCM and GDC nano-particles with the size of approximately 50 nm.The electrolyte-supported SOFC cells are fabricated by screen-printing LSCM-GDC anode and pure LSCM current collector on the surface of YSZ pellet. The cathode is fabricated on the opposite of the YSZ pellet by screen-printing GDC blocking layer and LSCF electrode. The effects of sintering temperature (1100oC and 1200oC) and LSCM-GDC anode thickness are investigated. The characterization of the cells is conducted by a series of electrochemical impedance spectroscopy (EIS) measurements at 800 – 500 oC. The high resolution focused ion beam - scanning electron microscopy (FIB-SEM) is used to evaluate 3-D microstructures of the anodes sintered at 1100 oC and at 1200 oC.The microstructures of samples are shown in Fig. 1. The well dispersed 50 nm LSCM and GDC particles moderate grain growth of the composite and result in fine microstructure. The GDC average intercept lengths are 80 and 134 nm for samples sintered at 1100 and 1200 oC, respectively.The polarization resistance strongly depends on the electrode thickness and sintering temperature. The higher sintering temperature induces grains coarsening and decreases the GDC surface area. Therefore, better performance is achieved for samples sintered at 1100 oC than at 1200 oC. The best performance of 0.14 Ω cm-2 (measured at 800 oC with H2 : H2O : N2 = 40 : 20 : 40 %) is achieved for the thinnest LSCM-GDC-1100oC electrode of 6 µm. Polarization resistance increased with the electrode thickness. This is attributed to the poor connectivity of the LSCM network. The connectivity of LSCM is lost in the range of 1.5 µm from the current collector layer for LSCM-GDC-1100oC, and the electron transport has to be maintained by GDC. Due to the relatively low electronic conductivity of GDC, the electrode thickness has to be limited.1. A. Sciazko, R. Yokoi, Y. Komatsu, T. Shimura, and N. Shikazono, ECS Trans., 91, 1711–1720 (2019).2. K. Sato, C. Iwata, N. Kannari, and H. Abe, J. Power Sources, 414, 502–508 (2019). Figure 1