Probing Edge Site Reactivity of Oxidic Cobalt Water Oxidation Catalysts

Differential
electrochemical mass spectrometry (DEMS) analysis of the oxygen isotopologues
produced by 18O-labeled Co-OEC in H216O reveals that water splitting catalysis proceeds by a mechanism
that involves direct coupling between oxygens bound to dicobalt edge
sites of Co-OEC. The edge site chemistry of Co-OEC has been probed
by using a dinuclear cobalt complex. 17O NMR spectroscopy
shows that ligand exchange of OH/OH2 at Co­(III) edge sites
is slow, which is also confirmed by DEMS experiments of Co-OEC. In
borate (Bi) and phosphate (Pi) buffers, anions
must be displaced to allow water to access the edge sites for an O–O
bond coupling to occur. Anion exchange in Pi is slow, taking
days to equilibrate at room temperature. Conversely, anion exchange
in Bi is rapid (kassoc = 13.1
± 0.4 M–1 s–1 at 25 °C),
enabled by facile changes in boron coordination. These results are
consistent with the OER activity of Co-OEC in Bi and Pi. The Pi binding kinetics are too slow to establish
a pre-equilibrium sufficiently fast to influence the oxygen evolution
reaction (OER), consistent with the zero-order dependence of Pi on the OER current density; in contrast, Bi exchange
is sufficiently facile such that Bi has an inhibitory effect
on OER. These complementary studies on Co-OEC and the dicobalt edge
site mimic allow for a direct connection, at a molecular level, to
be made between the mechanisms of heterogeneous and homogeneous OER.