Controlling crystal orientation in multilayered heterostructures toward high electro-catalytic activity for oxygen reduction reaction.

Nd 0.8 Sr 1.2 CoO 4±δ (NSC 214)/Nd 0.5 Sr 0.5 CoO 3-δ (NSC 113) multilayered heterostructured thin film cathodes of SOFCs exhibit ∼ 20 times difference in polarization resistance and catalytic activity toward oxygen reduction reaction (ORR), which are synthesized by pulsed laser deposition under different temperatures. The thin-films grown at 650 °C possess consistent crystal orientation, while those grown at 500 °C have different crystal orientations between NSC 113 and NSC 214 layers. As observed, they show ∼10² times and ∼10¹ times, respectively, higher ORR performance than a single-phase NSC 113 thin-film electrode, as characterized by constructing full cell operated at intermediate-temperature. The origin of such performance differences is analyzed on the basis of crystal orientation by HRXRD, HRTEM, HAADF-STEM, TOF-SIMS, and DFT calculation, and the results indicate that the increased crystal orientations can significantly weaken the positive effects of heterointerface. This observation is attributed to the formation of excessive grain boundaries, which leads to increased formation energy of oxygen vacancy and limited oxygen transportation. These results highlight that the precise control of crystal orientation can make full use of heterostructure to enhance the ORR performance of electrochemical devices. Figure Preparation and ORR performance enhancement of multilayer heterostructured epitaxial thin films (NSC 214/113) with consistent (S-NSC 214/113 , single crystal orientation, blue) or inconsistent (D-NSC 214/113 , double crystal orientations, red) crystal orientation. (The yellow layer or target in schematic is Nd 0.5 Sr 0.5 CoO 3-δ (NSC 113) and the green one is Nd 0.8 Sr 1.2 CoO 4±δ (NSC 214), the four different ORR enhancement values of each sample are calculated under various temperatures or oxygen pressures). Image 1 • Consistent crystal orientation strengthens the positive effects of heterostructure. • Heterostructures with single, double and multiple crystal orientations show 10², 10¹, and 10⁰ times of ORR enhancement. • Excessive grain boundaries lead to increased E vac and limited oxygen transportation. • Precise control of crystal orientation can make full use of heterostructure. [ABSTRACT FROM AUTHOR]