Molecular Strain Accelerates Electron Transfer for Enhanced Oxygen Reduction

Abstract

Fe–N–C materials are emerging catalysts for replacing precious platinum in the oxygen reduction reaction (ORR) for renewable energy conversion. However, their potential is hindered by sluggish ORR kinetics, leading to a high overpotential and impeding efficient energy conversion. Using iron phthalocyanine (FePc) as a model catalyst, we elucidate how the local strain can enhance the ORR performance of Fe–N–Cs. We use density functional theory to predict the reaction mechanism for the four-electron reduction of oxygen to water. Several key differences between the reaction mechanisms for curved and flat FePc suggest that molecular strain accelerates the reductive desorption of *OH by decreasing the energy barrier by ∼60 meV. Our theoretical predictions are substantiated by experimental validation; we find that strained FePc on single-walled carbon nanotubes attains a half-wave potential (E1/2) of 0.952 V versus the reversible hydrogen electrode and a Tafel slope of 35.7 mV dec–1, which is competitive with the best-reported Fe–N–C values. We also observe a 70 mV change in E1/2 and dramatically different Tafel slopes for the flat and curved configurations, which agree well with the calculated energies. When integrated into a zinc–air battery, our device affords a maximum power density of 350.6 mW cm–2 and a mass activity of 810 mAh gZn–1 at 10 mA cm–2. Our results indicate that molecular strain provides a compelling tool for modulating the ORR activities of Fe–N–C materials.