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 (<i>E</i>_1/2) 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 <i>E</i>_1/2 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 g_Zn^-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.