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.

Ni migration in solid oxide cell electrodes: Review and revised hypothesis

“…Lei et al [59] developed a phase-field model to simulate Ni particle growth and redistribution through formation and diffusion of gaseous Ni(OH) 2 . Their basic assumption [61] was that Ni(OH) 2 diffusion would be in the direction of the pH 2 O gradient, and their conclusion was: Nakajo et al [60] tried to correlate variation in dihedral angle with potential and Ni particle migration with some success, but they leave a Ni migration mechanism via gas phase open.…”

Effect of Nanoscale Ce0.8Gd0.2O2−δ Infiltrant and Steam Content on Ni–(Y2O3)0.08(ZrO2)0.92 Fuel Electrode Degradation during High-Temperature Electrolysis

“…For Ni–YSZ, the R p values during the life test remain well above the expected values, often appearing to increase during the 200 h segments, and show an overall increase by a factor of five during the 1000 h test, suggesting that there was significant degradation. The R p value increase was particularly pronounced in the first 200 h; the high-steam conditions present in this segment have been noted to cause SOEC degradation associated with Ni migration away from the electrode–electrolyte interface. ,,− It has been proposed that this effect is driven by a gradient in the Ni contact angle with YSZ, which is associated with the large gradient in oxygen potential in the electrode active regions driven by the electrode overpotential. − Continued R p degradation at a lower rate was observed as the life test continued from 200–400 h, with f = 70%, and 400–600 h, with f = 50% (Figure ). This may represent a continuation of the same Ni migration and degradation mechanism, although it should be reduced at the lower steam concentration .…”

Ni-GDC and Ni-YSZ electrodes operated in solid oxide electrolysis and fuel cell modes

“…The increase of porosity and Ni depletion can be found in the vicinity of the electrolyte for both samples in SOEC mode. In addition, Ni grains coarsened in the vicinity of the electrolyte, which is often reported as a cause of fuel electrode degradation for Ni-YSZ electrodes in both SOFC and SOEC operations (Nakajo et al, 2020;Trini et al, 2020). Trini et.al.…”

“…This method has been widely and successfully used in the scope of SOFCs (Iwai et al, 2010;Shikazono et al, 2010;Wilson et al, 2006). On the other hand, the number of studies for SOEC is limited, and they are mostly focused on the conventional Ni-YSZ cermet (Nakajo et al, 2020;Shimura et al, 2017;Trini et al, 2020). The difficulty in investigating the degraded microstructures is due to the non-uniform migration phenomena, which can significantly differ depending on the position within the cell (Sciazko et al, 2020).…”