Porphyrin-containing conjugated microporous polymers with gradient asymmetric design for efficient oxygen reduction

Asymmetric charge regulation of metal-free electrocatalysts can promote the oxygen reduction reaction (ORR) process. However, there are few research reports on the degree of asymmetry in catalytic activity. In this study, the charge redistribution of active centers is asymmetrically modulated by gradually introducing thiophene units (PPA-BT) and boron nitrogen covalent bonds (PPA-BN) on model fluorene units (PPA-BB). As a result, the asymmetric PPA-BT and PPA-BN exhibit higher catalytic activity than the symmetric PPA-BB without thiophene and boron nitrogen covalent bonds. Notably, the highly asymmetric polymer PPA-BN caused by uneven-local-charge directional design with reduced graphene oxide (rGO) results in remarkable oxygen reduction activity and good electrochemical stability. Density functional theory calculations confirm that the highly efficient performances of the asymmetric polymer-based catalysts originate from heterocycle-S atoms (site-5), which are activated by neighboring C-N (N) groups. Importantly, the modulation of the asymmetry degree can also increase dipole moments, adjust local charge redistribution, and improve catalytic kinetics. This work provides insight into the development of promising metal-free polymer electrocatalysts by using the regulation of asymmetry degree. Gradient asymmetry is employed to design metal-free polymer electrocatalysts. The superior activity of the highly asymmetric PPA-BN is attributed to the abundant active sites and adequate oxygen intermediate adsorption by local charge distribution.