Nano Ce2O2S with Highly Enriched Oxygen-Deficient Ce3+ Sites Supported by N and S Dual-Doped Carbon as an Active Oxygen-Supply Catalyst for the Oxygen Reduction Reaction.

The design of rare-earth-metal oxide/oxysulfide catalysts with high activity and durability for the oxygen reduction reaction (ORR) is still a grand challenge at present. In this study, Ce-species (Ce2O2S/CeO2)/N, S dual-doped carbon (Ce-species/NSC) catalysts with promising oxygen storage/release capacities are prepared at different temperatures (800-1000 °C) to enhance the ORR efficiency. Mechanisms for the effects of temperature on crystalline phase transition between CeO2 and Ce2O2S and structure evolution of Ce-species/NSCs are inferred to better understand their catalytic activity. Porous Ce2O2S/NSC (950 °C) catalyst as the air-breathing cathode exhibits a maximum power density of 1087.2 mW m-2, which is higher than those of other Ce-species/NSCs and commercial Pt/C (989.13 mW m-2) in microbial fuel cells. The decline of the power density of Ce2O2S/NSC (950 °C) cathode is 8.7% after 80 days of operation, which is far lower than that of Pt/C (36.7%). Ce2O2S/NSC (950 °C) has a four-electron selectivity toward the ORR and a low charge-transfer resistance (5.49 Ω), contributing to high ORR activity and durability. The promising ORR catalytic activity of Ce2O2S/NSC (950 °C) is attributed to its high specific surface area (338.9 m2 g-1), varied active sites, high electrical conductivity, and sufficient oxygen vacancies in the Ce2O2S skeleton. The high content of Ce3+ in Ce2O2S/NSC (950 °C) facilitates the formation of more oxygen-deficient Ce3+ sites that generate more oxygen vacancies to release/store more oxygen to stabilize the available oxygen for the ORR. Thus, this study provides a new perspective for preparation and application of this new type of the ORR catalyst.