Active site switching of Fe-N-C as a chloride-poisoning resistant catalyst for efficient oxygen reduction in seawater-based electrolyte

Abstract

It remains a great challenge to develop highly efficient catalysts of oxygen reduction reaction (ORR) applied in seawater electrolyte for aqueous metal-air batteries, because of the extremely low reaction kinetics together with the strong blocking effect of Cl− adsorption. Herein, Fe-Nx atomically dispersed on N-doped activated carbon (Fe-Nx/NAC) is prepared via a facile adsorption strategy. Fe-Nx/NAC shows high catalytic activity in NaCl electrolyte with a diffusion-limited current density of 3.8 mA cm−2 and a half-wave potential (E1/2) of 0.589 V, surpassing commercial Pt/C. The highly efficient ORR activity is attributed to the numerous atomically dispersed Fe-Nx moieties, a large surface area of activated carbon support and, most importantly, the poisoning immunity towards chloride ions. Density functional theory calculations reveal that Fe-Nx/NAC is free from anions adsorption such as Cl− and OH− at a low potential and the active site gradually switches from Fe center to the C atoms surrounding the Fe-N4 sites at a high potential, despite the Cl− adsorption on the Fe-Nx centers. Furthermore, the good performance in custom-made batteries indicates the promising application of Fe-Nx/NAC as a practical ORR catalyst in 3.5 wt% NaCl solution. The study will provide a new insight to the mechanism of chloride-poisoning resistance of a catalyst for the applications in seawater-based electrolyte.