Optimizing the oxygen reduction catalytic activity of a bipyridine-based polymer through tuning the molecular weight

Abstract

Conjugated polymer (CP) electrocatalysts with adjustable molecular structures have emerged as promising cathode materials for fuel cells. The molecular weight of the CP is a critical parameter determining the intrinsic electrical conductivity and charge transfer effect, which may have further effects on the active site density and catalytic activity. Here, three bipyridine-based linear conjugated polymers with controllable number-average molecular weights (M_n) (PBIPYT_L, M_n = 13.11 kDa; PBIPYT_M, M_n = 38.09 kDa; PBIPYT_H, M_n = 72.35 kDa) were designed and developed for enhancing the charge transport and electrochemical reactivity. The effect of the M_n of PBIPYT on the electrical, morphological and electrocatalytic properties was systematically studied. In particular, increasing the M_n of the PBIPYT polymers induced a redshifted maximum absorption peak and a more effective conjugation length of the backbones. The electrochemical data show that the high M_n of PBIPYT_H enhanced the electron transfer property and catalytic activity. Further density functional theory (DFT) calculations reveal that PBIPYT_H with delocalized molecular orbitals over the polymer backbone endows a slightly larger dipole moment. Therefore, the PBIPYT_H-based material exhibits superior ORR performance compared to its counterparts. We thus propose that the molecular weight is a core factor to be considered for designing efficient CP electrocatalysts.