Dramatic Improvement of Homogeneous Carbon Dioxide and Bicarbonate Electroreduction Using a Tetracationic...

Electrochemical conversion of carbon dioxide (CO₂) offers the opportunity to transform a greenhouse gas into valuable starting materials, chemicals, or fuels. Since many CO₂ capture strategies employ aqueous alkaline solutions, there is interest in catalyst systems that can act directly on such capture solutions. Herein, we demonstrate new catalyst designs where the electroactive molecules readily mediate the CO₂-to-CO conversion in aqueous solutions between pH 4.5 and 10.5. Likewise, the production of CO directly from 2 M KHCO₃ solutions (pH 8.2) is possible. The improved molecular architectures are based on cobalt(II) phthalocyanine and contain four cationic trimethylammonium groups that confer water solubility and contribute to the stabilization of activated intermediates via a concentrated positive charge density around the active core. Turnover frequencies larger than 10³ s^-1 are possible at catalyst concentrations of down to 250 nM in CO₂-saturated solutions. The observed rates are substantially larger than the related cobalt phthalocyanine-containing catalysts. Density functional theory calculations support the idea that the excellent catalytic properties are attributed to the ability of the cationic groups to stabilize CO₂-bound reduced intermediates in the catalytic cycle. The homogeneous, aqueous CO₂ reduction that these molecules perform opens new frontiers for further development of the CoPc platform and sets a greatly improved baseline for CoPc-mediated CO₂ upconversion. Ultimately, this discovery uncovers a strategy for the generation of platforms for practical CO₂ reduction catalysts in alkaline solutions.