Impact of Nickel agglomeration on Solid Oxide Cell operated in fuel cell and electrolysis modes

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 H-2 electrode supported cells at 850 degrees C and 750 degrees 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 H-2 electrode explain similar to 30% of the total degradation in fuel cell mode and similar to 25% in electrolysis mode at 850 degrees 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.