The widely investigated
oxygen reduction reaction (ORR) is well-known
to proceed via two competing routes, involving two or four electrons,
and yielding different reaction products, respectively. Both pathways
are believed to share a common, elusive intermediate, namely, the
hydroperoxyl radical. By exploiting a cobalt single-atom biomimetic
model catalyst, based on a self-assembled monolayer of Co-porphyrins grown on an almost free-standing
graphene sheet, we identify, in situ at room temperature in O2+H2O
atmosphere, a hydroperoxyl-water cluster that is stabilized at the
Co single-metal atom catalytic site. We show that the interplay between
charge transfer, dipole and H-bonding, and water solvation behavior
actually determines the hydroperoxyl-water complex stability, the
Co-OOH bonding geometry, and, prospectively, opens to the engineered
control of the selectivity of ORR pathways.