CO₂ Electrolysis in Aqueous Media with Almost 100% Selectivity for CO Production by Water-Soluble Cobalt Dimethyl-Bipyridine Complex

We developed a water-soluble Co complex with dimethyl-bipyridine ligands ([Co(dmbpy)₃]³⁺, dmbpy: 4,4’-dimethyl-2,2’-bipyridine), which electrochemically reduces CO₂ to CO with almost 100% selectivity in an aqueous medium without an organic solvent. The reaction overpotential was 270 mV, and the Co complex (hereafter, [Co-dmbpy]) facilitated carbon dioxide reduction reaction (CO₂RR) to produce carbon monoxide (CO) with an almost 100% selectivity (against competitive H₂ production) at a low potential of −0.80 V vs. NHE (pH 6.8). We discuss a possible CO formation mechanism based on static and <i>operando</i> analyses combined with DFT calculations.[1]<br/>A cyclic voltammetry (CV) of [Co-dmbpy] (0.3 mmol L⁻¹) measured in an aqueous NaHCO₃ (0.1 mol L⁻¹) solution bubbled with CO₂ exhibited a higher reaction current than that in Ar bubbling (adjusted to pH 6.8) within the range of -0.4 to -1.6 V (vs. NHE). The DFT calculations suggested that a ligand-decoordinated species of [Co(dmbpy)₂]²⁺ is formed as an active monomer catalyst by the decoordination of one dmbpy ligand, which is consistent with an observed -1.5 eV shift in <i>operando</i> X-ray absorption spectroscopy (XANES) spectra obtained under an electrical bias in the aqueous NaHCO₃ (0.1 mol L⁻¹) solution bubbled with CO₂. Therefore, it is strongly suggested that a one-dmbpy-decoordinated species is the active catalyst, and a rate-determining step is a CO₂ coordination to the Co center of one-electron injected species [Co(dmbpy)₂]⁺.<br/>We have also reported artificial photosynthetic systems consisting of metal complexes and two coexisting semiconductors, which produce formic acid or CO under visible light irradiation by the Z-scheme (2-step photoexcitation) mechanism like photosynthesis in plants in a particulate dispersion setup. [2-3] The use of the aqueous dispersion of particulate photocatalysts for a membrane-free and one-compartment reactor is a simple and scalable approach to producing valuable chemicals. In particular, a system containing [Co(dmbpy)₃]³⁺ generated CO with a product selectivity of 98% (and H₂ of 2%). The reaction accompanied O₂ production under visible light irradiation, which suggested this photocatalytic system utilized H₂O as an electron donor for the CO formation. The ability of electrochemical CO₂RR over the [Co-dmbpy] with a nearly 100% CO selectivity contributed to the extraordinarily high CO selectivity of 98 % in the Z-scheme photocatalytic system by receiving photoexcited electrons in the conduction band of a particulate semiconductor in an aqueous media.<br/>CO is a desirable target product made from CO₂ and H₂O because it can be converted into various hydrocarbons via Fischer–Tropsch synthesis. Gaseous CO exhibits low solubility in aqueous media, making its collection easier than liquid products such as formic acid. Therefore, photocatalytic and (photo)electrochemical CO production using H₂O is valuable.<br/><br/><b>References</b><br/>[1] T. M. Suzuki, A. Kudo, T. Morikawa, <i>Chem. Commun.</i> (2023) in press, DOI https://doi.org/10.1039/D3CC03940D.<br/>[2] T. M. Suzuki, A. Kudo, T. Morikawa, et al., <i>Chem. Commun.,</i> 54 (2018) 10199-10202.<br/>[3] T. M. Suzuki, A. Kudo, T. Morikawa, et al., <i>Appl. Catal. B.,</i> 316 (2022) 121600.