Highly selective electrocatalysis for carbon dioxide reduction to formic acid by a Co(II) complex with an equatorial N4 ligand

Abstract

A Co complex, [CoII(bibpy)L] (1-L, L is an axial ligand) with an equatorial N4 ligand of bibpy2− (H2bibpy = 6,6’-bis-(1H-benzimidazol-2-yl)-2,2’-bipyridine) has been newly synthesized to study electrocatalytic CO2 reduction in a homogeneous DMF solution. Cyclic voltammogram (CV) of 1-L in DMF under Ar exhibited two reversible redox responses at -1.42 V (1st step) and -2.32 V (2nd step) vs. the ferrocene/ferrocenium (Fc/Fc+) redox potential, which could be assigned to 1-L/1− (with release of L) and 1−/12− pairs, respectively. The CV measurement with adding 4.0% water suggests that water replaces the axial L ligand on 1-L, but no longer interacts with 1− and 12− because of requiring no axial ligand. In CV of 1-L in the CO2-saturated DMF solution, a relatively weak reduction wave appeared at -1.70 V of the peak reduction potential (Ep1’) in the 1’st step, which is attributed to the reduction of 1-L with the concerted oxidative coordination of CO2 onto the axial site to form [1-CO2]−. CV of 1-L exhibited the significant catalytic current below the onset potential (Eon) of -1.9 V for CO2 reduction. The electrocatalytic CO2 reduction proceeded stably in the presence of 2.0% water as a proton source. The Eon value increased linearly from -1.98 to -1.89 V with the water content increase from 0 to 4.0%. The hypothetical [1-CO2]−/[1-CO2]2− reduction (2’nd step) involved in the catalytic CO2 reduction might be proton-coupled redox process of [1-CO2]−/[1-CO2H]−. The bulk electrolysis by 1-L in the 2.0% water-containing DMF solution at -2.32 V afforded H2 with 3.0% Faraday efficiency (FE) and HCO2H with 68% FE. The selectivity (RHCO2H) for HCO2H production was 96%, defined as the molar fraction of HCO2H in all the products. The overpotential (η) for CO2 reduction to HCO2H was 0.62 V (at -2.07 V vs Fc/Fc+). These performances of 1-L are among those of the hitherto-reported state-of-the-art Co-complexes for electrocatalytic CO2 reduction to HCO2H in homogenous solutions.