Competition between Gas and Liquid Solar Fuel Product Formation from the CO2 Reduction Reaction on Two-Dimensional Metal-Organic Framework

Abstract

Rising CO2 emissions in the atmosphere continue to be among the world’s most preeminent environmental challenges. Indeed, the solution of carbon sequestration via geologic storage has proved insufficient to combat this problem. The alternative to sequestration is the electrochemical reduction of carbon dioxide to liquid products, a common step to combat net warming effects. However, the electrochemical reduction of carbon dioxide remains limited because of several competing pathways and potential-dependent kinetics at catalytic surfaces. Herein, we consider Cu-phthalocyanine Metal–Organic Framework (MOF) as a candidate electrocatalyst to reduce CO2 to a single hydrocarbon product. To achieve this, we develop a first-principles Grand Canonical Potential Kinetics (GCP-K) framework for quantifying the activity and product selectivity of said MOF catalyst. Our model is derived directly from constant-potential DFT calculations, from which reaction free energies and activation barriers are extracted across the relevant electrochemical manifold. Potential dependence is incorporated through explicit charge-transfer symmetry factors, enabling the systematic construction of potential-resolved rate constants. On this basis, we assemble a microkinetic reaction network that encompasses the competing pathways leading to C1 and C2 liquid products. We calculate Faradaic efficiency maps across multiple selectivity windows from branch points in different microenvironments. We find that charge-transfer processes shift the adsorption energy of any individual intermediate by only a few millivolts per 0.1 V of applied bias, but the cumulative effect across multistep PCET sequences displaces the selectivity windows by tens to hundreds of millivolts. We calculate selectivity-limiting steps and the various kinetic rates associated with them as a function of potential and surface coverage. In addition to mechanistic investigations of each reaction framework in question, we elucidate the stabilization of various oxygenated intermediates and their role in creating different C1 and C2 products. Taken as a whole, these results show a variety of catalyst-level design levers that could increase the liquid product window and offer a mechanistic foundation for selecting the operating potentials that maximize desired product selectivity at functional current densities.