(334c) Combining Electrochemistry and Surface Science to Identify Electrocatalytic Structure-Property Relationships

The oxygen evolution reaction (OER) is the anodic process associated with the electrochemical CO₂ reduction reaction (CO₂RR). State-of-the-art OER electrocatalysts contain precious metals, such as Iridium, which introduce substantial cost-barriers to large-scale CO₂RR deployment. Current efforts have focused on identifying inexpensive, earth-abundant transition metals with satisfactory OER activity. Nickel, cobalt, iron and bimetallic CoFe and NiFe electrocatalysts have shown remarkable OER activity in basic conditions, but atomic-level understanding of how catalyst shape, size, and composition are still lacking in the literature.

Here, we present an approach that couples surface science and electrochemistry to synthesize, characterize and evaluate well-defined transition metal electrocatalysts. We demonstrate this technique with well-defined Fe₂O₃ nanocatalysts that show outstanding, structure-dependent mass activity with values exceeding 10⁵ A/g­_metal. Fe₂O₃ nanocatalysts were grown on Au(111) supports in ultra-high vacuum (UHV) and characterized with scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). Our synthetic approach provided extremely precise structural control and we varied catalyst structures from near monolayer coverage to discrete, isolated nanoparticles. High-resolution STM imaging quantified the nanocatalyst morphology, including size, thickness, total surface area, and perimeter-to-surface-area ratios. Samples were translated from UHV and tested for electrochemical OER in 0.1M KOH solutions. Post-reaction STM and XPS characterization revealed structural stability during OER and we identified a relationship between particle size, density of perimeter Fe-oxide(hydroxide) sites, and OER activity. This represents some of the clearest identification of reactive site location in any electrocatalyst system to date, and the results confirm long-held hypothesis that undercoordinated or “edge” sites are responsible for catalytic activity. Ongoing computational efforts within Density Functional Theory (DFT) will provide atomic-level details about the reaction energetics and mechanisms at various active sites to explain experimentally observed activity trends. Future efforts will be directed towards more complex OER systems to probe how catalyst structure, composition, and active site location impact electrocatalytic activity.