Research article

Hydrothermal synthesis of zirconia doped with naturally mixed rare earths oxides and their electrochemical properties for possible applications in solid oxide fuel cells

¹ National R&D Institute for Nonferrous and Rare Metals-IMNR, Laboratory for Advanced and Nanostructured Materials and HighPTMET Center, 102 Biruintei Blvd, Pantelimon, 077145 Ilfov, Romania
² National Research Center for Micro and Nanomaterials, University Politehnica from Bucharest, 060042 Bucharest, Romania
³ Faculty of Materials Science and Engineering, University Politehnica of Bucharest, 313 Splaiul Independentei, 011061 Bucharest, Romania

^* e-mail: slobozeanu.anca@yahoo.com; lidia.licu@imnr.ro; rpiticescu@imnr.ro

Received: 6 September 2023
Accepted: 8 October 2023

Abstract

Solid oxide fuel cells (SOFC) are electrochemical conversion devices that produces electricity directly from oxidizing a fuel and their development became of high importance to drastically reduce the greenhouse emission. Rare earth elements (REEs) are widely used as materials and dopants in controlling the ionic conductivity of solid electrolytes for SOFCs. Their criticality and high costs for separation to individual REEs lead to first studies aiming to search possible use of mixed REEs with natural occurrence as extracted from concentrates. This paper focused on obtaining sintered pellets based on zirconia doped with natural mixture of REEs extracted from monazite and study their microstructure, impedance spectra and dielectric properties vs. operating temperatures to assess their potential applications as solid electrolyte. ZrO₂ doped powders with 8% natural mixture of REEs (8ZrMZ) were synthesized by hydrothermal process. ZrO₂ doped with 4% Y₂O₃ (4ZrY) and 8%Y₂O₃ (8ZrY) were also obtained by the same route and used as standard materials already used in commercial SOFCs. All powders were uniaxially pressed and sintered in air, with highest densities obtained for 1400 °C. The Niquist diagrams for 8ZrMZ samples show significantly lower ionic conductivity compared to standards 4ZrY and 8 ZrY. This may be attributed to the presence of detrimental Fe and Si impurities following the mixed REE after Th and U removal from monazite concentrates and the ratio of REEs in the dopant composition affecting the ionic conductivity due to possible association of structural defects. Research works are further needed to improve the receipt for using naturally mixed REEs and asses their possible use as a competitive dopant for solid electrolytes.