Functional group tuning of two-dimensional carbon nanosheets for boosting oxygen reduction electrocatalysis

The N-doped carbon materials have natural advantages as the potential catalyst of oxygen reduction reaction (ORR). However, the lack of practical synthesis techniques for enabling highly efficient ORR performance hinders the go-ahead of robust carbocatalysts. Here, we design and synthesize nitrogen-doped carbon nanosheets (N–CN-Fd) with abundant activate sites from Sterculia lychnophora via a brief freeze-drying technology. This universal route promotes the specific surface areas, strengthens the degree of graphitization and increases the atom ratio of electron-donating groups of N–CN-Fd. The obtained N–CN-Fd displays an onset potential of 0.91 V, a half-wave potential of 0.79 V, and a limited-diffusion current density of 3.6 mA/cm² for ORR under alkaline conditions, comparable to commercial Pt/C. Meanwhile, a four-electron ORR pathway is revealed and N–CN-Fd maintains higher stability than Pt/C in chronoamperometric response. Moreover, the advantage of the N–CN-Fd catalyst in increasing the discharge-charge capacity and improving the cycling stability of the Li–O₂ battery is demonstrated. The theoretical calculation result illustrates that the abundant electron-donating surface groups on the samples able to accomplish the activation of ortho-C atoms, which endow N–CN-Fd the excellent ORR activity. Our findings provide guidelines for designing various carbon-based electrocatalysts with multiple active sites through freeze-drying crafts.