Electrochemical Characterization of Pr(A)MnO3 Perovskites As Oxygen Electrodes for Solid Oxide Fuel-Assisted Electrolysis Cells

Abstract

Long-term degradation remains the main issue for the viability of solid oxide electrolysis cell (SOEC) technology as a practical hydrogen production system. The major specific degradation mechanism in SOECs relates to delamination phenomena at or near electrolyte/anode interface. The principle of so-called fuel-assisted electrolysis is to supply the carbon-containing species which can react with oxygen at the anode side thus bringing down the oxygen chemical potential at the electrolyte/anode interface and improving its stability. The present work is aimed at the characterization of PrMnO3-based perovskites for potential application as anodes in solid oxide fuel-assisted electrolysis cells.Pr0.6-x A0.4MnO3 ±δ (A = Sr, Ca; x = 0 and 0.05) powders were synthesized by glycine-nitrate combustion technique. The characterization included XRD, SEM/EDS, XPS, dilatometry and thermogravimetry, measurements of electrical properties, and determination of oxygen nonstoichiometry by coulometric titration. XRD analysis confirmed the formation of solid solutions with orthorhombic perovskite-like structure. Pr0.6-x A0.4MnO3 ±δ exhibit negligible variations of oxygen content with temperature and oxygen partial pressure under oxidizing conditions, while reducing p(O2) below 10-4 atm results in a progressive oxygen losses from the lattice and reduction of variable-valence cations. The low-p(O2) stability boundary of the perovskite phase at 800°C corresponds to ~10-17-3×10-16 atm; the stability domain is wider for Ca-substituted compositions and narrows with introduction of A-site vacancies. Dilatometric studies confirmed a good thermomechanical compatibility with common solid electrolytes. The electrical conductivity of Pr0.6-x A0.4MnO3 ±δ ceramics is p-type electronic and decreases with reducing p(O2), but still exceeds 40-50 S/cm under oxygen chemical potentials anticipated for the oxygen electrode operation conditions at 800°C. The electrochemical performance of Pr0.6-x A0.4MnO3 ±δ porous electrode layers was evaluated in contact with yttria-stabilized zirconia solid electrolyte using symmetrical cell configuration as function of relevant parameters (electrode fabrication conditions, with and without ceria-based interface layers, modifications via impregnations by praseodymia or gadolinia-doped ceria).