Dramatic Improvement of Homogeneous Carbon Dioxide and Bicarbonate Electroreduction Using a Tetracationic Water-Soluble Cobalt Phthalocyanine.

Electrochemical conversion of carbon dioxide (CO) offers the opportunity to transform a greenhouse gas into valuable starting materials, chemicals, or fuels. Since many CO 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 CO-to-CO conversion in aqueous solutions between pH 4.5 and 10.5. Likewise, the production of CO directly from 2 M KHCO 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 10 s are possible at catalyst concentrations of down to 250 nM in CO-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 CO-bound reduced intermediates in the catalytic cycle. The homogeneous, aqueous CO reduction that these molecules perform opens new frontiers for further development of the CoPc platform and sets a greatly improved baseline for CoPc-mediated CO upconversion. Ultimately, this discovery uncovers a strategy for the generation of platforms for practical CO reduction catalysts in alkaline solutions.