In-situ Environmental Transmission Electron Microscopy Investigation of Perovskite Oxide Thin Films During Electrochemical Oxygen Evolution Reaction.

Abstract

Despite many theoretical and experimental efforts to find active catalysts in oxygen evolution reaction (OER), the mechanism for high activity and long-term OER stability remains elusive. Catalyst-electrolyte interface governs the OER activity and stability by forming catalytically active or inactive phase, cation leaching, surface reconstruction, decomposition, formation of the amorphous layer, and agglomeration1. Direct observation of atomic dynamics at catalyst-H2O interface under OER conditions may significantly advance the mechanistic understanding of the complex OER mechanism. This work utilizes an environmental transmission electron microscope (ETEM) technique to visualize real-time imaging of atomic dynamics, structural changes, and discrepancies in a lifetime at the perovskite catalyst-H2O interface. The ETEM approach is applied on (001) surface of La0.6Sr0.4MnO3 (LSMO), Pr(1-x)Ca(x)MnO3 (PCMO) (x= 0.1 and 0.33) and La0.6Sr0.4CoO3 (LSCO) perovskite oxides, respectively. Furthermore, Monte–Carlo-based least-squares optimization simulated images based on the multislice method is used to interprete the atomic contrast, in particular surface termination, and adatom occupancies at the catalyst surface. Here I report the atomic dynamics of highly mobile manganese (Mn) adatoms at single-crystalline Lanthanum (La)/ Strontium (Sr) terminated (001) surface of LSMO and irreversible Mn leaching at the mixed terminated (001) surface of PCMO (x=0.33) manganites2. Furthermore, I discuss the dynamic processes at the LSCO-H2O interface associated with the formation of a disordered surface layer that governs the OER stability. These results provide key findings of highly mobile transition metal Mn and Cobalt (Co) adatoms on (001) surfaces of LSMO and LSCO, respectively. In the case of LSMO, ETEM observations suggests the partially solvated Mn adatoms in H2O act as an active catalytic site due to flexible coordination of Mn. In contrast, partially solvated Co adatoms are highly mobile in the low partial pressure of H2O, whereas in high partial pressure of H2O, the LSCO surface shows a transformation to a disordered layer. This is in coincidence with deactivating OER activity in high current densities and potential in electrochemical experiments. ETEM investigations also suggest that the change of TM valence and TM-O covalency governs the change of OER activity and stability of PCMO for the x=0.1 and 0.33 systems. Besides a detailed investigation of the atomic dynamic, these results demonstrate a powerful strategy of combining real electrochemistry and ETEM to develop a fundamental understanding of OER catalysts.