Determination of the influence of the role of the Hf dopant concentration variation on the stability to degradation of ZrO2 ceramics considered as materials for solid oxide fuel cells

Abstract

The key objective of this study is to determine the effect of hafnium stabilization of ZrO₂ ceramics and to identify its role in hardening and enhancement of resistance to degradation processes caused by the use of these ceramics as anode materials in solid oxide fuel cells. According to the X-ray structural analysis and Raman spectroscopy data, it was found that with an elevation in the Hf dopant concentration, the HfO₂ phase is formed in the composition of the ceramics, which has a similar structural motif to the monoclinic phase of ZrO₂, resulting in the formation of a continuous substitution solid solution of ZrO₂/HfO₂, and a change in the crystal lattice parameters of the monoclinic phase of ZrO₂ indicates the substitution of Zr⁴⁺ ions by Hf⁴⁺ ions, leading to a growth in the crystal lattice parameters, and consequently, the formation of oxygen vacancies. According to the assessment of the mechanical and strength properties of Zr(Hf)O₂ ceramics, it was found that the formation of inclusions in the ceramics in the form of HfO₂, which form a continuous solid solution at low concentrations, leads to an increase in resistance to external influences within 35 – 40 % compared to unstabilized ZrO₂ ceramics, but a rise in the density of oxygen vacancies above 4.4 × 10¹⁸, alongside the formation of T-defects in the structure leads to a decrease in resistance to external influences, but the strength parameters exceed the values for unstabilized ZrO₂ ceramics. Evaluation of the efficiency of using Zr(Hf)O₂ ceramics as anode materials revealed that at low dopant concentrations, not only an increase in specific power is observed, but also the stability of electrochemical parameters is maintained in the case of prolonged high-temperature exposure, which has a negative impact on maintenance of the performance of solid oxide fuel cells.