Tailoring Metalloporphyrin Frameworks for an Efficient Carbon Dioxide Electroreduction: Selectively Stabilizing Key...

The electrocatalytic reduction of carbon dioxide (CO₂ER) 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 CO₂ER at low overpotentials. This is because these competing reactions produce intermediates (adsorbed H and OH) with chemical bonds similar to those formed in CO₂ER (adsorbed COOH and OCHO). Here, we report the adsorption free energies of CO₂ER 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 CO₂ER intermediates rather than the HER. Inspired by molecular catalysts, we proposed and studied H-bonding interfaces that specifically stabilize the target intermediates of the CO₂ER. The selective H-bonding stabilization reduced the limiting potential for CO₂ER 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 CO₂ER selectivity by preventing the porphyrin from stacking and keeping H-bonding interfaces in close proximity to only CO₂ER intermediates. Improved selectivity to the desired CO₂ER 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.