In situ X-ray Rietveld analysis of Ni–YSZ solid oxide fuel cell anodes during NiO reduction in H2

Abstract

A synthesis and characterization of solid oxide fuel cell (SOFC) anodes of nickel with 8%mol yttrium stabilized zirconia (Ni–YSZ) is presented. Attention was focused on the kinetics and phase composition associated with the transformation of NiO–YSZ to Ni–YSZ. The anodes were prepared with an alternative synthesis method that includes the use of nickel acetylacetonate as an inorganic precursor to obtain a highly porous material after sintering at 1400°C and oxide reduction (NiO–YSZ → Ni–YSZ) at 800°C for 8 h in a tubular reactor furnace using 10% H2/N2. The obtained material was compressed by unidirectional axial pressing into 1 cm-diameter discs with 15–66 wt% Ni and calcinated from room temperature to 800°C. A heating rate of 1°C min−1 showed the best results to avoid any anode cracking. Their structural and chemical characterization during the isothermal reduction were carried out by in situ time-resolved X-ray diffraction, refined with the Rietveld method (which allowed knowing the kinetic process of the reduction), scanning electron microscopy and X-ray energy dispersive spectroscopy. The results showed the formation of tetragonal YSZ 8%mol in the presence of nickel, a decrement in the unit cell volume of Ni and an increment of Ni in the Ni–YSZ anodes during the temperature reduction. The analysis indicated that the Johnson–Mehl–Avrami equation is unable to provide a good fit to the kinetics of the phase transformation. Instead, an alternative equation is presented.