Energy storage through the electrocatalytic generation of chemical fuels such as hydrogen is an attractive pathway for storing intermittent renewable energies, and perovskite oxides are among the most attractive candidate materials to catalyze the kinetically limiting half reaction, the oxygen evolution reaction (OER).  OER activity is typically correlated to electronic and atomic structure parameters. But the catalyst surface – i.e. where the reaction happens – changes during the reaction. To design next-generation electrocatalysts, a detailed operando understanding of the relationships between catalytic activity, stability and atomic-level surface properties during the reaction is required.

In my talk I will address two essential ingredients to achieve this next-level understanding: controllable atomic-level surface properties and the development of surface-sensitive operando characterization routes.  

Epitaxial thin films are a direct route for single crystalline model electrocatalysts that can be fabricated with atomically-tailored surface composition. These offer the ideal platform to derive structure-property-function relationships, track the evolution of the surface properties with applied potential and enable direct comparison to the surfaces investigated in density functional theory. I will summarize our recent findings for the role of surface termination, surface contamination, point defects and transformation pathways during the reaction and the role of solid/solid interfaces in proximity of the solid/liquid interface.

Next, I will summarize recent advances in surface-sensitive operando characterization in a liquid medium and with specific attention to studies with atomically-defined thin film model electrode surfaces. A focus will be the current development of meniscus photoelectron spectroscopy, a discussion about utmost surface- or interface-sensitivity using a standing-wave approach^1,2 and a pathway to extract surface-sensitive information from nominally bulk-sensitive techniques such as UV-Vis spectroscopy.^1,3

The key example throughout the talk will be LaNiO₃ thin films, which are atomically flat both before and after application as electrocatalysts for the OER during water electrolysis. We selectively tuned the surface cationic composition in epitaxial growth. The Ni-termination is approximately twice as active for the OER as the La-termination. Using a suite of ex situ, in situ and operando spectroscopy tools, we found that the Ni-rich surface undergoes a surface transformation towards a catalytically active Ni hydroxide-type surface.¹ If LaNiO₃ surfaces are exposed to the ambient, however, surface carbonate groups form, which prohibit the formation of the active phase, leading to an activity decrease compared to the clean surfaces.⁴

Our work thus demonstrates tunability of surface transformation pathways by modifying a single atomic layer at the surface and it shows that active surface phases only develop for select as-synthesized surface terminations, highlighting the instructional value of epitaxial model electrocatalysts. It also highlights the need of and summarizes pathways for the exploration of the three-step relationship between as-prepared surface, transformation under applied potential, and electrocatalytic activity.