Imidazole for Pyridine Substitution Leads to Enhanced Activity Under Milder Conditions in Cobalt Water Oxidation Electrocatalysis

Abstract

A previously reported cobalt complex featuring a tetraimidazolyl-substituted pyridine chelate is an active water oxidation electrocatalyst with moderate overpotential at pH 7. While this complex decomposes rapidly to a less-active species under electrocatalytic conditions, detailed electrochemical studies support the agency of an initial molecular catalyst. Cyclic voltammetry measurements confirm that the imidazolyl donors result in a more electron-rich Co center when compared with previous pyridine-based systems. The primary changes in electrocatalytic behavior of the present case are enhanced activity at lower pH and a marked dependence of catalytic activity on pH.

Electrocatalytic Hydrogen Production with a Molecular Cobalt Complex in Aqueous Solution

“…The redox 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 process was assigned to the electrochemically irreversible Co III j II redox couple. As expected, the redox potential of the Co III j II couple in 1 is lower than the one of B [9] and C [10] in water and D in MeOH/H 2 O. [12] In acetate-buffered MeCN/water solution at pH of 5.6, the complex exhibits an irreversible reduction process at E p,c = À 0.15 V vs. NHE and a very broad reoxidation feature (Figure 3, Figure S12).…”

“…I. Siewert Universität Göttingen,Institut für Anorganische Chemie Tammannstr. 4,37077 Göttingen,Germany [b] [ 8] B, [9] C, [10] and D [12] with N 5 pyridine, pyrazole, and imidazole donor ligands.…”

“…These reports prompted us to investigate B as catalyst in the hydrogen evolution reaction. B, [9] as well as C, [10] are capable to catalyze the electrochemical water oxidation reaction, which makes them potentially interesting as catalysts in the oxidative and reductive water splitting. [11] Initial investigation showed, that B is indeed active, however, the catalytic activity for the hydrogen evolution reaction (HER) was rather low at rather high overpotential ( Figure S8).…”

On the Homogeneity of a Cobalt-Based Water Oxidation Catalyst

“… 77 − 79 Later, the Anderson group reported very similar molecular complexes to be unstable during electrolysis, resulting in the formation of heterogeneous CoO x . 80 The complex [Co III (hydroxydi(pyridin-2-yl)methanolate) 2 ] + was reported by Zhao et al to be a light-driven WOC. 81 Under electrochemical conditions, the very same catalytic species was shown to form Co-containing structures on the electrode surface, according to studies by the group of Najafpour.…”

Crystal structure of bis(hydroxydi(pyridin-2-yl)methanolato-κ3 N,N′O)cobalt(III) 7,7,8,8-tetracyanoquinodimethane, C34H22CoN8O4

“…Rational design and construction of cobalt complexes has garnered remarkable attention in the field of molecular materials owing to their potential applications in biochemistry, catalysis, electrochemistry, hydrogen production and magnetism [3][4][5].…”