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

The electrocatalytic reduction of carbon dioxide (CO2 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 CO2 ER at low overpotentials. This is because these competing reactions produce intermediates (adsorbed H and OH) with chemical bonds similar to those formed in CO2 ER (adsorbed COOH and OCHO). Here, we report the adsorption free energies of CO2 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 CO2 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 CO2 ER. The selective H-bonding stabilization reduced the limiting potential for CO2 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 CO2 -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 CO2 ER selectivity by preventing the porphyrin from stacking and keeping H-bonding interfaces in close proximity to only CO2 ER intermediates. Improved selectivity to the desired CO2 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.