Electrocatalytic Water Oxidation by a Water-Soluble
Nickel Porphyrin Complex at Neutral pH with Low Overpotential
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Abstract
The
water-soluble cationic nickel(II) complex of <i>meso</i>-tetrakis(4-<i>N</i>-methylpyridyl)porphyrin (<b>1</b>) can electrocatalyze water oxidation to O<sub>2</sub> in neutral
aqueous solution (pH 7.0) with the onset of the catalytic wave appearing
at ∼1.0 V (vs NHE). The homogeneous catalysis with <b>1</b> was verified. Catalyst <b>1</b> exhibited water oxidation
activity in a pH range 2.0–8.0 and had a strict linear dependence
of catalytic current on its concentration. After 10 h of constant
potential electrolysis at 1.32 V (vs NHE), a negligible difference
of the solution was observed by UV–vis. In addition, inspection
of the working electrode by electrochemistry, scanning electron microscope
(SEM), and energy dispersive X-ray spectroscopy (EDX) showed no sign
of deposition of NiO<sub><i>x</i></sub> films. These results
strongly argued that <b>1</b> is a real molecular electrocatalyst
for water oxidation. The turnover frequency (TOF) for this process
was 0.67 s<sup>–1</sup> at 20 °C. On the basis of results
from the kinetic isotope effect (KIE) and inhibition experiments,
electrochemical studies in various buffer solutions with different
anions and pHs, and DFT calculations, a catalytic cycle of <b>1</b> for water oxidation via a formally Ni(IV) species was proposed
