Influence of nickel exsolution on the electrochemical performance and rate-determining stages of hydrogen oxidation on Sr1.95Fe1.4Ni0.1Mo0.5O6-δ promising electrode for solid state electrochemical devices

Optimizing Reversible Exsolution and Phase Transformation in Double Perovskite Sr2Fe1.5-xCoxMo0.5O6-δ Electrodes for High-Performance Symmetric Solid Oxide Cells

Double perovskite (DP) oxides are promising electrode materials for symmetric solid oxide cells (SSOCs) due to their excellent electrochemical activity and stability. B-site cation doping in DP oxides affects the reversibility of phase transformation and exsolution, which plays a crucial role in the catalyst recovery. Yet, few studies have been conducted on this topic. In this study, the Sr2Fe1.5-xCoxMo0.5O6-δ (CSFM, x = 0, 0.1, 0.3, 0.5) DP system demonstrates modulated exsolution and phase transformation reversibility by manipulating the oxygen vacancy concentration. The correlation between Co-doping level and oxygen vacancy concentration is investigated to optimize the exsolution and phase transformation properties. Sr2Fe1.2Co0.3Mo0.5O6-δ (3CSFM) exhibits reversible transformation between DP and Ruddlesden-Popper phases with a high density of exsolved CoFe nanoparticles under redox atmospheres. The quasi-symmetric cell with 3CSFM shows a peak power density of 1.27 W cm-2 at 850 °C in H2 fuel cell mode and a current density of 2.33 A cm-2 at 1.6 V and 800 °C in H2O electrolysis mode. The 3CSFM electrode exhibits robust stability during continuous operation for ≈700 h. These results demonstrate the significant role of B-site doping in designing DP materials capable of dynamic phase transformation in diverse environments.

Ni-Doped SFM Double-Perovskite Electrocatalyst for High-Performance Symmetrical Direct-Ammonia-Fed Solid Oxide Fuel Cells

Ammonia has emerged as a promising fuel for solid oxide fuel cells (SOFCs) owing to its high energy density, high hydrogen content, and carbon-free nature. Herein, the electrocatalytic potential of a novel Ni-doped SFM double-perovskite (Sr1.9Fe0.4Ni0.1Mo0.5O6-δ) is studied, for the first time, as an alternative anode material for symmetrical direct-ammonia SOFCs. Scanning and transmission electron microscopy characterization has revealed the exsolution of Ni-Fe nanoparticles (NPs) from the parent Sr2Fe1.5Mo0.5O6 under anode conditions, and X-ray diffraction has identified the FeNi3 phase after exposure to ammonia at 800 °C. The active-exsolved NPs contribute to achieving a maximal ammonia conversion rate of 97.9% within the cell's operating temperatures (550-800 °C). Utilizing 3D-printed symmetrical cells with SFNM-GDC electrodes, the study demonstrates comparable polarization resistances and peak power densities of 430 and 416 mW cm-2 for H2 and NH3 fuels, respectively, with long-term stability and a negligible voltage loss of 0.48% per 100 h during ammonia-fed extended galvanostatic operation. Finally, the ammonia consumption mechanism is elucidated as a multistep process involving ammonia decomposition, followed by hydrogen oxidation. This study provides a promising avenue for improving the performance and stability of ammonia-based SOFCs for potential applications in clean energy conversion technologies.

Correlation between structure, surface defect chemistry and 18O/16O exchange for La2Mo2O9 and La2(MoO4)3

The La₂Mo₂O₉ and La₂(MoO₄)₃ powders were synthesized using a solid-state reaction method and used to prepare dense ceramics. X-ray photoelectron spectroscopy was used to study the chemical composition and charge numbers of the elements in the subsurface area of dense ceramics of lanthanum molybdates. The spectra were measured under an ultra-high vacuum of 7 × 10^-11 atm at 30 °C and 600 °C, and under an oxygen atmosphere at 2 × 10^-3 atm at 600 °C and 825 °C. High resolution spectra for La 3d, Mo 3d and O 1s states were obtained and analyzed. The kinetics of oxygen exchange were considered in the framework of a two-step model including the consecutive steps of dissociative adsorption and the incorporation of oxygen. The oxygen adsorption (r_a) and incorporation (r_i) rates were calculated. Correlations between the oxide surface defect chemistry and the rates of individual oxygen-exchange steps were discussed.