Oxygen reduction reaction (ORR) electrocatalysts are an integral component of many next-generation energy storage and conversion technologies, including fuel cells and metal-air batteries. Metal and nitrogen co-doped carbons (M,N-C), synthesized by the pyrolysis of metal and nitrogen containing precursors, are very promising alternatives to conventional and expensive Pt/C electrocatalysts for ORR. However, the ORR performance of most M,N-C materials reported to date are inferior compared to commercial Pt/C electrocatalysts.
Prof. Tierui Zhang, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China, found that the inferior ORR performance of M,N-C electrocatalysts is attributable to irreversible fusion and aggregation of electrocatalyst nanoparticles during the high temperature pyrolysis step used in M,N-C manufacture. Zhang’s group developed a new synthetic method for preparing M,N-C electrocatalysts, which involved surface coating of a Zn,Co-ZIF-8 metal organic framework (MOF) with mesoporous silica (mSiO2) followed by high temperature treatment and finally etching of the mSiO2 shell.
This mesoporous-silica protection strategy prevented the typical fusion and aggregation of Co,N-CNF nanoparticles during the pyrolysis stage, yielding high surface area Co,N-CNF electrocatalysts with outstanding ORR activity, superior to Pt/C in alkaline media and comparable to Pt/C in acidic media. Importantly, the Co,N-CNF electrocatalysts prepared by this new method displayed excellent durability and resistance to deactivation by methanol cross over, confirming their potential as very promising low cost ORR catalysts. The mSiO2-protection strategy adopted here is versatile, and could be used in the development and performance optimization of M,N-C electrocatalysts from other precursors.