Tailoring Metalloporphyrin Frameworks for an Efficient Carbon Dioxide Electroreduction: Selectively Stabilizing Key Intermediates with H-Bonding Pockets

Abstract

The electrocatalytic reduction of carbon dioxide (CO 2 ER) is a great challenge within the field of energy and environmental research. Competing reactions, including hydrogen evolution reactions (HER) and surface oxidation, limit the conversion of CO 2 ER at low overpotentials. This is because these competing reactions produce intermediates (adsorbed H and OH) with chemical bonds similar to those formed in CO 2 ER (adsorbed COOH and OCHO). Here, we report the adsorption free energies of CO 2 ER and competitive intermediates within H-bonding functionalized metalloporphyrin frameworks using first-principles calculations. The functionalized frameworks shift the scaling relation of adsorption free energies to favor the CO 2 ER intermediates rather than the HER. Inspired by molecular catalysts, we proposed and studied H-bonding interfaces that specifically stabilize the target intermediates of the CO 2 ER. The selective H-bonding stabilization reduced the limiting potential for CO 2 ER by up to 0.2–0.3 V. Our results agree with previous experiments that found that cobalt- and iron-based metalloporphyrins exhibited the most promising catalytic activity in CO 2 -to-CO reduction, with small potential barriers for the adsorbed COOH intermediate. In addition, embedding the functionalized metalloporphyrin moieties in a rigid framework structure acted to enhance the CO 2 ER selectivity by preventing the porphyrin from stacking and keeping H-bonding interfaces in close proximity to only CO 2 ER intermediates. Improved selectivity to the desired CO 2 ER was achieved through three steps: first by systematically screening for metal centers, second by creating an ideal H-bonding environment, and finally by using a rigid macrocycle ring structure.