Metal-Free Covalent Organic Frameworks for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is the key reaction in metal air and fuel cells. Among the catalysts that promote ORR, carbon-based metal-free catalysts are getting more attention because of their maximum atom utilization, effective active sites and satisfactory catalytic activity and stability. However, the pyrolysis synthesis of these carbons resulted in disordered porosities and uncontrolled catalytic sites, which hindered us in realizing the catalysts' design, the optimization of catalyst performance and the elucidation of structure-property relationship at the molecular level. Covalent organic frameworks (COFs) constructed with designable building blocks have been employed as metal-free electrocatalysts for the ORR due to their controlled skeletons, tailored pores size and environments, as well as well-defined location and kinds of catalytic sites. In this Concept article, the development of metal-free COFs for the ORR is summarized, and different strategies including skeletons regulation, linkages engineering and edge-sites modulation to improve the catalytic selectivity and activity are discussed. Furthermore, this Concept provides prospectives for designing and constructing powerful electrocatalysts based on the catalytic COFs.