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

• A catalyst of atomic Fe on N-doped activated carbon (Fe-N x /NAC) was prepared. • Fe-N x /NAC shows high ORR activity with Cl-resistance in NaCl electrolyte. • Fe-N x /NAC is free from Cl − adsorption at a low potential. • Nearby C atoms act as active sites after Fe center is poisoned at a high potential. 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-N x atomically dispersed on N-doped activated carbon (Fe-N x /NAC) is prepared via a facile adsorption strategy. Fe-N x /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 (E 1/2 ) of 0.589 V, surpassing commercial Pt/C. The highly efficient ORR activity is attributed to the numerous atomically dispersed Fe-N x 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-N x /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-N 4 sites at a high potential, despite the Cl − adsorption on the Fe-N x centers. Furthermore, the good performance in custom-made batteries indicates the promising application of Fe-N x /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.