Evolution of the Morphology Near Triple-Phase Boundaries in Ni–Yttria Stabilized Zirconia Electrodes Upon Cathodic Polarization

Abstract

Microstructural changes in Ni–yttria stabilized zirconia (YSZ) near the YSZ electrolyte were examined by three-dimensional (3D) electron microscopy after electrolysis and fuel cell operation up to 10,700 h and 15,000 h, respectively. The depletion of Ni and three-phase boundaries (TPBs) close to the electrolyte was detected upon cathodic polarization. It corresponded to spatial variations of dihedral angles (θ) at TPBs and Ni surface curvature along the direction perpendicular to the electrolyte, which comport with electrowetting and Zener pinning theory on several aspects. θNi decreased by up to 6 deg next to the electrolyte after electrolysis but remained uniform after fuel cell operation. This is in line with predictions from electrowetting theory with capacitances measured by electrochemical impedance spectroscopy and distribution of relaxation times. The decrease in θNi was concurrent to transition toward concave Ni/pore interfacial shapes and lower genus of the Ni phase, which suggests the pinch-off of Ni ligaments following surface diffusion-controlled Rayleigh instability. The increase in absolute mean curvature near the electrolyte interface is a driving force for outward transport of Ni. The decrease in θYSZ further suggests that TPB lines relocate on YSZ surface features that provide higher Zener pinning force. In contrast, few localized contact losses between Ni and YSZ that can also occur under high cathodic polarization and trigger Ni depletion were detected. The results are expected to advance the understanding of the driving forces that cause Ni depletion near the electrolyte in electrolysis for the design of improved solid oxide cell electrode microstructures.