Comparative Analysis of Solar-to-Fuel Conversion Efficiency: A Direct, One-Step Electrochemical CO₂ Reduction Reactor versus a Two-Step, Cascade Electrochemical CO₂ Reduction Reactor

Electrochemical and photoelectrochemical (PEC) CO_2 reduction (CO_2R) have the potential to produce sustainable, zero greenhouse gas emission fuels and chemicals. One of the key components in a PEC CO2 reduction device is the electrocatalyst materials for the CO_2R reaction. While significant research advances have been made in the development of CO_2 reduction catalysts and in the understanding of the reaction mechanisms, selective, active, and stable catalyst materials have yet to be identified to directly convert CO_2 into higher reduction products, such as ethanol and ethylene. In contrast, several electrocatalyst systems have exhibited promising selectivity and activity for the first two-electron, two-proton process, such as CO_2R to CO or formate. For example, nanostructured silver electrodes, metal dichalcogenides, and single metal atoms in graphene nanosheets exhibited high Faradaic efficiency (FE) and high reaction rates for CO_2R to CO. A Pd/C nanoparticle-based catalyst incorporated in a 10% efficient solar-to-formate conversion device also exhibited near-unity FE at 10s of mA cm^(–2) for CO_2R to formate. Hence, one alternative strategy is to leverage the efficient first two-electron, two-proton reaction by using a two-step, cascade CO_2 reactor, in which the first catalytic reactor converts CO_2 into CO or formate and the second catalytic reactor converts CO or formate into higher-order reduction products such as ethanol or ethylene. Herein, the solar-to-fuel (STF) conversion efficiencies in a direct, one-step CO2reduction reactor and a two-step, cascade CO_2 reduction reactor were analyzed and compared for two distinctive device configurations.