A series of cationic
cobalt porphyrins was found to catalyze electrochemical water oxidation to O2 efficiently at room temperature in neutral aqueous
solution. Co-5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrin, with a highly electron-deficient meso-dimethylimidazolium
porphyrin, was the most effective catalyst. The O2 formation rate was 170 nmol · cm(-2) · min(-1) (k(obs) = 1.4 × 10(3) s(-1)) with a Faradaic efficiency near 90%. Mechanistic investigations indicate the generation of a Co(IV)-O
porphyrin cation radical as the reactive
oxidant, which has accumulated two oxidizing equivalents above the Co(III) resting state of the catalyst. The
buffer base in
solution was shown to play several critical roles during the catalysis by facilitating both redox-coupled
proton transfer processes leading to the reactive
oxidant and subsequent O-O bond formation. More basic
buffer anions led to lower catalytic onset potentials, extending below 1 V. This homogeneous
cobalt-
porphyrin system was shown to be robust under active catalytic conditions, showing negligible decomposition over hours of operation. Added
EDTA or
ion exchange resin caused no catalyst
poisoning, indicating that
cobalt ions were not released from the
porphyrin macrocycle during catalysis. Likewise, surface analysis by energy dispersive X-ray spectroscopy of the working
electrodes showed no deposition of heterogeneous
cobalt films. Taken together, the results indicate that Co-5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrin is an efficient, homogeneous, single-site water oxidation catalyst.