Nitrogen-carbon-encapsulated Fe3C nanoparticles as highly efficient earth-abundant oxygen reduction electrocatalysts

The design and synthesis of the highly active metal-organic framework (MOF)-based catalysts open new avenues to facilitate the kinetically unfavorable oxygen reduction reaction (ORR). In this paper, we elucidate the design and fabrication of an efficient electrocatalyst with a novel structure for the enhancement of the ORR performance by decorating the surface of the ZIF-8 precursor with ferrocene formic acid, followed by a two-step carbonization process, which is critical for the encapsulation of pyrolytic Fe3C nanoparticles (NPs) into carbon nanotubes (CNTs) and the isolation of Fe single atoms onto an N-doped carbon (NC) matrix. Moreover, the relative Fe content is vital to optimize the ORR performance of the catalysts. The resulting Fe3C@CNT/NC-M catalyst has an optimized structure. It shows great long-term stability and excellent electrocatalytic ORR performance in alkaline solution, with the half-wave potential and limiting current reaching 0.941 V and 6.31 mA cm−2, respectively. Furthermore, the electrocatalyst has a strong tolerance to and good stability in a methanol solution. The Fe3C@CNT/NC-M zinc-air battery delivers a large open-circuit potential of 1.525 V, a peak power density of 348 mW cm−2 at 420 mA cm−2, and a maximum capacity of 843 mA h gZn−1 at 10 mA cm−2. Thus, this synthetic strategy provides a promising pathway toward constructing MOF-based electrocatalytic materials with effective and stable ORR performance.