Copper single atom-modulated functionalization of iron clusters on a porous carbon nanosheet for the oxygen reduction reaction

Abstract

Engineering a catalyst with a large surface area and effective pore structure holds great significance in increasing the number of active sites, thereby enhancing the performance of the oxygen reduction reaction (ORR). In this study, a catalyst featuring iron clusters functionalized with CuN₂O single atoms anchored on a porous carbon nanosheet was synthesized using a dual nitrogen precursor (melamine and urea) strategy. FeCu-NC-0.5 with a melamine-to-urea molar ratio of 0.5 exhibited the largest surface area with a high content of atomically dispersed iron atoms (6.69 wt%) and Cu atoms (1.29 wt%). Compared to the two catalysts (Fe-NC-0.5 and Cu-NC-0.5) prepared using either of the single metal sources, the addition of Cu effectively prohibited the aggregation of iron and enhanced the micropore surface area, while Fe significantly provided active sites to assist the ORR of Cu-NC-0.5. Not only limited to metal sites, the functions of two nitrogen precursors in fabricating efficient pore structures favorable for the ORR were also focused on. Melamine was important in building up large pores as was urea in setting up small pores. These two were combined to construct developed pore channels with high level mesoporous networks and high specific surface areas that facilitated mass transfer for metal active sites. Based on the above factors, FeCu-NC-0.5 demonstrated the most positive half-wave potential of 0.883 V in 0.1 M KOH and 0.691 V in 0.05 M phosphate-buffered saline, both overmatching those of Pt/C (0.851 V and 0.643 V, respectively). Density functional theory calculation clearly unraveled the unique function of Cu single atoms in modification of the Fe₅ cluster. Compared with the supporting effect of FeN₄ and CuN₄, that of the single atom CuN₂O was more effective and evident, which reduced the d-band center of the Fe cluster, weakened oxygen adsorption, and thus enhanced ORR performance. When applied as a cathodic catalyst for Zn–air batteries (ZAB) and microbial fuel cells (MFCs), FeCu-NC-0.5 demonstrated excellent power densities of 287.4 mW cm⁻² and 2578 ± 41 mW m⁻², respectively. This study provides a new methodology for developing high metal-loaded electrocatalysts suitable for energy conversion devices.