| 研究実績の概要 |
Oxygen reduction in Fe-N-C type catalysts is not understood. The chemical structure is complicated, and the roles of Fe and N are unclear. Here, nitrogen-doped carbon foams (N-C) are synthesized to gain more insight.
The synthesis was changed from a solvothermal to a solution-based process, increasing reproducibility, scale (from 0.5 - 5 g), and surface area (from 500 - 2500 m2/g). The N content was controlled to optimize the electrochemical properties, which were investigated in acid and alkaline conditions. In acid, the N-Cs had reasonable activity, with mixed 2/4-electron mechanisms, but poor durability. In alkaline, the N-Cs match the activity of Pt, have a 4-electron mechanism, and excellent durability.
Fe-N-Cs were made by impregnating with iron acetate and heating at 900C. In acid, Fe-N-C out-performed a commercial alternative (NPC-2000, Pajarito Powder). In alkaline it out-performed Pt/C, and matched NPC-2000. The catalysts have excellent stability over 60,000 potential cycles, but with some leaching of Fe.Detailed materials characterization (e.g. XPS, XRD, SEM, HR-TEM, STEM, BET etc) was carried out before and after durability tests, confirming some acid leaching but also the existence of highly stable Fe nanoparticles encapsulated in carbon.
The above insights lead us to the conclusion that the high activity and durability of Fe-N-C catalysts can be attributed not only to porphyrin-like Fe-N sites, but also carbon-encapsulated Fe nanoparticles. Metal-free sites may also play a minor role in activity / stability.
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