Hyunguk Kwon

Suppressing cation segregation on lanthanum-based perovskite oxides to enhance the stability of solid oxide fuel cell cathodes

The slow rate of oxygen reduction reaction (ORR) at the cathode side has been considered as a scientific challenge to improve the overall performance of solid oxide fuel cell (SOFC). Unfortunately, dopant cation enrichment on the surface of perovskite-type cathode materials often leads directly to the reduction in activity and stability of ORR. For this reason, we quantitatively assessed the main driving force of dopant segregation on LaBO3(001) surfaces (B = Cr0.50Mn0.50, Mn, Fe, Co0.25Fe0.75, Co, and Ni) using density functional theory (DFT) calculations. Based on our findings, the minimization of elastic energy, which is closely related to the size of both A-site and B-site cations, plays an important role on the A-site dopant segregation. The degree of dopant segregation can be controlled by the proper choice of cations contained in LaBO3-type perovskite oxides. We therefore suggest a valuable principle that can be applied to design high performance cathode materials by suppressing the dopant cation enrichment at the surface.

2-(N-Methylbenzyl)pyridine: A Potential Liquid Organic Hydrogen Carrier with Fast H2 Release and Stable Activity in Consecutive Cycles

The liquid organic hydrogen carrier (LOHC) 2-(N-methylbenzyl)pyridine (MBP) shows good potential for H2 storage based on reversible hydrogenation and dehydrogenation, with an H2 storage density of 6.15 wt %. This material and the corresponding perhydro product (H12 -MBP) are liquids at room temperature. Remarkably, H2 release is much faster from H12 -MBP over Pd/C than from the benchmark perhydro benzyltoluene over Pt/C at lower temperatures than 270 °C, owing to the addition of N atom into the benzene ring. Since this positive effect is unfavorable to the hydrogenation reaction, more Ru/Al2 O3 catalyst or prolonged reaction time must be applied for complete H2 storage. Experiments with repeated hydrogenation-dehydrogenation cycles reveal that reversible H2 storage and release are possible without degradation of the MBP/H12 -MBP pair. The prepared MBP satisfies the requirements for chemical stability, handling properties, and cytotoxicity testing.

Enhanced oxygen exchange of perovskite oxide surfaces through strain-driven chemical stabilization

Surface cation segregation and phase separation, of strontium in particular, have been suggested to be the key reason behind the chemical instability of perovskite oxide surfaces and the corresponding performance degradation of solid oxide electrochemical cell electrodes. However, there is no well-established solution for effectively suppressing Sr-related surface instabilities. Here, we control the degree of Sr-excess at the surface of SrTi0.5Fe0.5O3−δ thin films, a model mixed conducting perovskite O2-electrode, through lattice strain, which significantly improves the electrode surface reactivity. Combined theoretical and experimental analyses reveal that Sr cations are intrinsically under a compressive state in the SrTi0.5Fe0.5O3−δ lattice and that the Sr–O bonds are weakened by the local pressure around the Sr cation, which is the key origin of surface Sr enrichment. Based on these findings, we successfully demonstrate that when a large-sized isovalent dopant is added, Sr-excess can be remarkably alleviated, improving the chemical stability of the resulting perovskite O2-electrodes.

Suppression of Cation Segregation in (La,Sr)CoO3−δ by Elastic Energy Minimization

Strontium segregation at perovskite surfaces deteriorates the oxygen reduction reaction kinetics of cathodes and therefore the long-term stability of solid oxide fuel cells (SOFCs). For the systematic and quantitative assessment of the elastic energy in perovskite oxides, which is known to be one of the main origins for dopant segregation, we report the fractional free volume as a new descriptor for the elastic energy in the perovskite oxide system. To verify the fractional free volume model, three samples were prepared with different A-site dopants: La0.6Sr0.4CoO3-δ, La0.6Sr0.2Ca0.2CoO3-δ, and La0.6Ca0.4CoO3-δ. A combination of the theoretical calculations of the segregation energy and oxide formation energy and experimental measurements of the structural, chemical, and electrochemical degradation substantiated the validity of using the fractional free volume to predict the dopant segregation. Furthermore, the dopant segregation could be significantly suppressed by increasing the fractional free volume in the perovskite oxides with dopant substitution. Our results provide insight into dopant segregation from the elastic energy perspective and offer a design guideline for SOFC cathodes with enhanced stability at elevated temperatures.