Effect of SrZrO3 Formation at Lscf-Cathode/GDC-Interlayer Interfaces on the Electrochemical Properties of Solid Oxide Fuel Cells

Abstract

Formation of oxide scales such as SrZrO3 (SZO) at the gadolinia-doped ceria (GDC)-interlayer/yttria-stabilized zirconia (YSZ) electrolyte interface during fabrication and operation negatively affects the electrochemical properties and long-term stability of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode/GDC/YSZ based solid oxide fuel cells (SOFCs). In this work, we investigated the effect of SZO formation at LSCF-cathode/GDC-interlayer interfaces on the electrochemical properties of LSCF/GDC/YSZ using scanning electron microscopy and electrochemical impedance spectroscopy. The GDC interlayers were grown on YSZ by pulsed laser deposition and fired using a multi-step heat treatment in air at 1400ºC for 1.5 h followed by 1000ºC for 10 h. The obtained GDC interlayer is dense with minimal cracks and pores avoiding the complexity arising from the porosity of the interlayer and cation diffusion via gas phase. The porous LSCF cathode layer with 12.0 µm-thickness was then screen-printed on top of GDC surface and fired at 1080ºC for 3.5 h in air. Cells consisting of LSCF cathode/GDC interlayer/YSZ electrolyte/Pt electrode was tested under open-circuit voltage and -0.3A cathodic polarization conditions at 800ºC for 300 h in air. It has been found that both cells showed an initial formation of nanosized SrZrO3 (SZO) particles along the LSCF/GDC due to the fast Zr transport in PLD-grown GDC interlayers after the tests. However, the size and distribution of SZO differ between the OCV and cathodically polarized interface. The SZO particles on the LSCF/GDC interface are substantially smaller and are highly distributed across the GDC surface in OCV as compared to cathodically polarized interface characterized by larger localized SZO grains. From this result, we infer that the SZO grains may have impaired the O2 reduction and oxidation reactions at the O2/LSCF/GDC triple-phase boundary (TPB) areas responsible for the observed increase in polarization resistances as a function of time. However, the formation and subsequent growth of larger localized SZO grains on the cathodically polarized LSCF/GDC interface may have led to oxygen permeable TPB areas corresponding to no apparent cell performance degradation. Our results show that the morphology, size, and distribution of SZO along the LSCF/GDC interface influenced the electrocatalytic activity of LSCF cathode affecting electrochemical cell performance.