Long-term experiments have been carried out to investigate the impact of Nickel (Ni) coarsening on the performance of Solid Oxide Cell. Durability tests have been performed with H2 electrode supported cells at 850 °C and 750 °C in fuel cell and electrolysis modes. Microstructural changes in the composite electrode of Nickel and Yttria Stabilized Zirconia (YSZ) have been characterized by synchrotron X-ray nanotomography. Analysis of the reconstructions have revealed that Ni coarsening induces a significant decrease of both the density of Triple Phase Boundary lengths (TPBls) and the Ni/gas specific surface area. However, the contact surface between Ni and YSZ is not changed upon operation, meaning the Ni sintering is inhibited by the YSZ backbone. Moreover, the Ni coarsening rate is independent of the electrode polarization. The evolution of TPBls in operation has been fitted by a phenomenological law implemented in an electrochemical model. Simulations have shown that microstructural changes in the H2 electrode explain 30% of the total degradation in fuel cell mode and 25% in electrolysis mode at 850 °C after 1000-2000 h. Moreover, it has been highlighted that the temperature at which the degradation is estimated after the durability experiment plays a major role on the result.

Two types of solid oxide cells with different Ni-YSZ cermet microstructures have been aged in electrolysis and fuel cell modes for operating times ranging from 1000 to 15000 hours. The pristine and aged cermets have been reconstructed by synchrotron X-ray holotomography. Nickel agglomeration has been observed in the bulk of the operated samples inducing a significant loss of triple phase boundary lengths. The inspection of the microstructural properties has confirmed the stabilizing role of YSZ on Ni coarsening. Furthermore, the gradients of properties quantified at the electrolyte interface have revealed a depletion of Ni only in the electrochemically active region of the electrode. The process is strongly promoted for a coarse cermet microstructure when operated under electrolysis current. The evolution of the microstructural properties has been implemented in an in-house multiscale model. The simulations have shown that the loss of performance is dominated by the depletion of Ni in case of a coarse microstructure. Thanks to the computations, it has been shown that the Ni depletion is controlled by the cathodic overpotential. To explain this dependency, it has been proposed that the accumulation of oxygen vacancies in the double layer could deteriorate the Ni/YSZ interface and trigger the Ni depletion.

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A solid oxide short stack composed of 6 Ni-YSZ supported cells, YSZ electrolyte and GDC-LSC oxygen electrode has been tested for 10,700 hours in steam electrolysis. Initial degradation was followed by a global stabilization of the performance after lowering the current density, with a degradation rate below 0.5% kh -1 .Post-test analysis has been conducted on two repeating units (RUs) to highlight the most significant microstructure alterations. Nickel depletion was observed in the hydrogen electrode close to the interface with the electrolyte. Formation of small pores in the electrolyte was detected along the grain boundaries. A consequent detachment related to this phenomenon was observed in proximity of the GDC compatibility layer. At the oxygen electrode side, the formation of ã 1 mm dense mixed layer of GDC and YSZ was observed. Strontium from the LSC electrode migrated through GDC pores and reacted with YSZ, forming evident SrZrO 3 inclusions. Distinct accumulation of silicon at the Ni/YSZ interface and chromium on the GDC barrier layer have been observed in both RUs. Despite this range of alterations observed, the stack degradation remained limited, testified from the fact that performance decay between 4,000 and 10,000 hours of operation was virtually nil.