Linking Composition, Structure and Thickness of CoOOH Layers to Oxygen Evolution Reaction Activity by Correlative Microscopy.

The role of beta-CoOOH crystallographic orientations in catalytic activity for the oxygen evolution reaction (OER) remains elusive. We combine correlative electron backscatter diffraction/scanning electrochemical cell microscopy with X-ray photoelectron spectroscopy, transmission electron microscopy, and atom probe tomography to establish the structure-activity relationships of various faceted beta-CoOOH formed on a Co microelectrode under OER conditions. We reveal that approximate to 6 nm beta-CoOOH(01 1? ${\bar{1}}$ 0), grown on [ 1?21? ${\bar{1}2\bar{1}}$ 0]-oriented Co, exhibits higher OER activity than approximate to 3 nm beta-CoOOH(10 1? ${\bar{1}}$ 3) or approximate to 6 nm beta-CoOOH(0006) formed on [02 2?1] ${\bar{2}1]}$ - and [0001]-oriented Co, respectively. This arises from higher amounts of incorporated hydroxyl ions and more easily reducible Co-III-O sites present in beta-CoOOH(01 1? ${\bar{1}}$ 0) than those in the latter two oxyhydroxide facets. Our correlative multimodal approach shows great promise in linking local activity with atomic-scale details of structure, thickness and composition of active species, which opens opportunities to design pre-catalysts with preferred defects that promote the formation of the most active OER species.