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

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