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 (B_i) and phosphate (P_i) buffers, anions must be displaced to allow water to access the edge sites for an O-O bond coupling to occur. Anion exchange in P_i is slow, taking days to equilibrate at room temperature. Conversely, anion exchange in B_i is rapid (k_assoc = 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 B_i and P_i. The P_i 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 P_i on the OER current density; in contrast, B_i exchange is sufficiently facile such that B_i 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.