Microstructural Degradation of Ni/YSZ Electrodes in Solid Oxide Electrolysis Cells under High Current

Summary In this work, formation of ZrO 2 nano-particles on Ni grains wasobserved in the active Ni/YSZ electrode, only in the region close tothe cathode-electrolyte interface, i.e. where one will find the most re-ducing conditions in the Ni/YSZ electrode during electrolysis testing.The ZrO 2 nano-particles are observed in cells tested at current den-sity equal to or above −1 A/cm 2 , but not in non-tested cells or cellstested at lower current density. The harsher the conditions (in terms ofcathode polarization, temperature, and testing period), the larger theabundance of the ZrO 2 particles.It is emphasized that Zr reduction may occur at a much higher p (O 2 ) in the presence of Ni than when in pure ZrO 2 (orYSZ)form.The threshold value is around 3.2 ×10 −29 ± 3 bar at 850 ◦ C, assumingan error range of 50 kJ/mol for G of Reaction 1. The observation ofa clear correlation with cathode polarization and the observation of avariety of Zr/Y ratios from the bulk YSZ to the ZrO 2 nano-particlesformed at the Ni-YSZ or Ni-pore interface support qualitatively theformulatedhypothesisfortheformationmechanismbeingareduction,followed by solid diffusion and re-precipitation at sites with higher