Beyond a mere support: The multifaceted roles of carbon black in CO₂ electrolysis

Carbon black (CB) has long been considered an inert conductive additive and support in the electrochemical CO₂ reduction reaction (CO₂RR). A growing body of evidence, however, demonstrates that CB assumes considerably more complex and active functions. This review critically re-evaluates the multifaceted roles of CB in CO₂RR, moving beyond its conventional function as a mere substrate. We first summarize the structural and physicochemical properties of different CB types and their established performance as conductive supports that facilitate three-phase interfaces and stabilize catalytic sites. We then highlight recently uncovered contributions of CB, including (1) the modulation of the reaction microenvironment, such as local pH, ion enrichment, and hydrophobicity, (2) the reconstruction of catalyst via electrochemical, confinement, and surface-chemistry pathways, (3) the enhancement of mass transport and reaction kinetics, and (4) the direct participation of CB as an active component via crystal-phase control, heteroatom doping, and active-site construction. Furthermore, CB enables several emerging functionalities, including defect-driven catalysis, universal synthesis of single-atom catalysts, and surface organometallic chemistry (SOMC). Finally, we discuss future directions, emphasizing precise catalyst design, the seamless integration of in situ characterization with theoretical modelling, and device engineering toward industrial application. This review aims to reshape the perception of CB from an inert scaffold to a versatile functional material, thereby providing a foundation for the rational design of high-performance and cost-effective CO₂RR catalysts.