Hollow Spherical (Co, Zn)/N, S-Doped Carbons: Efficient Catalysts for Oxygen Reduction in Both Alkaline and Acidic Media

Platinum (Pt)-based catalysts are widely used for the oxygen reduction reaction (ORR) in many energy conversion and storage devices, such as fuel cells and metal-air batteries. However, Pt is among the least earth abundant and most expensive elements; as a result, these energy devices are currently difficult to scale up. This has motivated researchers to try to develop sustainable and alternative catalysts that can replace Pt as well as other noble metal-based catalysts. Herein, novel Co nanocrystals-loaded, Zn, N, and S codoped carbon materials, dubbed (Co, Zn)/NSC, which have hollow spherical structures and which can serve as efficient ORR catalysts, are developed via a facile synthetic method involving microemulsion and pyrolysis. The key in the synthesis of these nonprecious metal-containing heteroatom-doped nanostructured carbon catalysts is the preparation of nanosized, mixed metal-organic frameworks (MOFs) in solution using microemulsion, and the exploitation of their morphology to produce hollow spherical structures via pyrolysis. Elemental mapping shows the presence of Co, Zn, N, and S atoms throughout the structures of the materials. The material obtained at pyrolysis temperature of 800 °C, denoted (Co, Zn)/NSC-800, exhibits excellent electrocatalytic activity for ORR in alkaline and acidic media, with onset potentials of 1.000 and 0.802 V vs RHE, respectively. In addition, the catalyst is stable and tolerates the methanol crossover reaction in both cases. Thus, this material has the potential to replace Pt catalysts in fuel cells. Moreover, the synthetic approach used to make it can be extended to produce other robust, nonprecious transition metal-based catalysts for ORR.