Rational Design of Catalytically Active Sites in Metal-Free Carbon Materials for Electrocatalytic CO2 Reduction: A Review

Electrocatalytic conversion of carbon dioxide (CO2) into value-added chemicals is a promising avenue for reducing greenhouse gas emissions while storing renewable energy in its chemical form. Metal-free carbon-based materials have attracted growing interest as electrocatalysts for CO2 reduction due to their abundance, low cost, stability, and tunable electronic structures. However, pristine carbon materials, such as graphene, lack catalytic activity due to the weak physisorption of CO2, necessitating the creation of active sites to boost electrocatalytic performance. In this review, we highlight recent advances in tailoring carbon frameworks through two key strategies to enhance the performance of electrocatalytic CO2 reduction. The first strategy, heteroatom doping-including nitrogen, phosphorus, boron, and fluorine-creates localized electronic states that direct reaction pathways toward specific products. The second strategy involves engineering defects, such as vacancies or pentagonal/octagonal ring structures, which significantly boost local electron density and adsorbate binding affinity, thereby lowering CO2 activation barriers. By creating these active sites, the electrocatalytic performance of carbon materials can be enhanced by over 2 orders of magnitude compared with inert carbon. Additionally, mechanistic insights from both experimental and computational studies are discussed, illustrating how electronic reconfiguration, spin density, and local coordination environments govern catalytic activity and selectivity. Finally, we outline challenges and future research directions for achieving sustainable CO2 electroreduction.