Metal-free sites with multidimensional structure modifications for selective electrochemical CO2 reduction

Electrochemical CO2 reduction (ECR) to fuels and chemicals driven by renewable energy sources represents a promising solution to current energy, resource, and environmental issues. Carbon-based metal-free electrocatalysts exhibit great prospects for ECR based on their multi-dimensionally tunable structures from atom/molecule level to nano/microscale and the resulting controllable performance. However, ECR on metal-free sites (MFS) still suffers from low control over activity and selectivity, resulting from a limited mechanism understanding of complicated interactions between MFS and intermediates. This review presents that most optimizations refer to multidimensional structure modifications of MFS involving inner p-orbital electron structure, surface structure and/or outer environment, which vary intermediate adsorption. Absolute and relative changes of intermediate adsorption result in the lower-energy-barrier pathway to the products of CO, HCOOH, CH3OH, CH3CH2OH, etc. How engineering atomic dopant/defect/surface curvature, molecule modifier, nano/microscale pore structure, and external electrolyte/potential can alter the adsorption strength/density/configuration of ECR intermediates, is discussed. Adsorbed intermediates are detected by in situ techniques, and their variations for selectively ECR through different reaction pathways are described by Gibbs free energy calculations. Finally, challenges to rationally develop in situ techniques and theoretical simulation methods, and strategies for optimizing intermediate adsorption and reactivity toward desired ECR products are presented.