The electroreduction of carbon dioxide by macrocyclic cobalt complexes chemically modified on a glassy carbon electrode

Various macrocyclic cobalt complexes were chemically bonded to a glassy carbon (GC) electrode, on which CO₂ electroreduction was carried out to give CO in aqueous phosphate buffer solution of pH 6.3. The macrocyclic cobalt complexes were naphthalocyanato cobalt(II), phthalocyanato cobalt(II), dibromo(11-hydroxyimino-4,10-dipropyl-5,9-diazatrideca-4,9-dien-3-one oximato) cobalt(III) (denoted as CoDO), two kinds of hydrophobic vitamin B₁₂s (heptamethyl cobyrinate perchlorate and heptapropyl cobyrinate perchlorate), 5,10,15,20-tetraphenylporphyrinato cobalt(II), and 5,10,15,20-tetrakis (4-methoxyphenyl)porphyrinato cobalt(II). Their redox potentials of Co(I)/Co(II) are in the above order from positive to negative potentials in a 0.05 M TBAP DMSO solution, being between -0.23 and -1.0 V (SCE). These complexes were chemically bonded to GC through -CONH-pyridine which locates perpendicularly to a planar or semi-planar complex structure, where the N of the pyridine forms a coordinate bond with the Co atom of the above complexes as a fifth ligand. The catalytic activity for hydrogen evolution in aqueous solution was observed to be high on Co naphthalocyanine and Co phthalocyanine modified GC electrodes; the latter gave H₂ evolution at the most positive potentials among the Co complexes employed. When the lower potential limit in cyclic voltammetry became less than the hydrogen evolution potential, in the reverse positive-going sweep, an anodic hump current was observed at -0.35 ∼ -0.78 V, which is assigned to a cobalt hydride oxidation process; the hydride is suggested to form when hydrogen evolution takes place, and the hump disappeared after the introduction of CO₂ into the solution. It was observed that the Co complex chemically bonded on GC can give CO from CO₂ only at relatively low overvoltages, except for CoDO which was not able to reduce CO₂; phthalocyanato cobalt(II) gave CO at E = -1.0 V (0.26 V as overvoltage) at 20% current efficiency. The highest CO current efficiency was observed in the case of tetraphenyl-porphyrinato cobalt(II) chemically modified GC.