Graphitic Carbon Materials with Various Nanostructures Decorated with Fe-N-C Catalytically Active Sites for Air Electrodes

Active and stable bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are required for the air electrode of rechargeable metal-air batteries. A promising catalyst could be a carbonaceous material containing transition metal ions coordinated by nitrogen and embedded in the carbon surface (metal-N–C site) as the catalytically active site, although sufficient activity and stability in such materials have not yet been attained. In this study, a systematic investigation of the effects of nanostructures on bifunctional catalytic activity was carried out to gain insight into the activity enhancement. A series of graphitic carbon materials with various concave and convex nanostructures were used as a substrate, which is advantageous for forming stable catalysts. The graphitic carbon surface was covered with a carbonaceous thin film containing the Fe–N-C site, which was prepared by iron phthalocyanine sublimation, deposition on the substrate, and decomposition. Distinct dependence of the ORR and OER activities on the nanostructures was observed. The dependence was explained by the surface Fe concentration and specific surface area, and the correlation between the OER and the concave structure was also suggested. The charge–discharge and cycling performances of a zinc-air battery with an air electrode prepared from the Fe–N-C-decorated graphitic carbon material were also tested to confirm the bifunctionality. Our study provides a useful guideline for developing active and stable bifunctional catalysts from the viewpoint of the catalyst nanostructure. Graphical abstract