Facile synthesis of Fe3C-dominated Fe/Fe3C/FeN0.0324 multiphase nanocrystals embedded in nitrogen-modified graphitized carbon as efficient pH-universal catalyst for oxygen reduction reaction and zinc-air battery

Iron-based nitrogen-doped carbonaceous materials are currently the most promising alternative towards oxygen reduction reaction (ORR) electrocatalysts due to the highly efficient active sites of single-atom Fe-NX coordination. However, the fact that iron-containing nanocrystals are easier to form during pyrolysis without additional tuning while providing activity comparable to single-atom sites cannot be ignored. Herein, we propose a facile and efficient strategy to synthesize iron nanocrystals sites with multiphase embedded in porous nitrogen-doped graphitized carbon (Fe/Fe3C/FeN0.0324@N-GC-X, X = 700, 850, and 1000). Based on highly active Fe3C@N-GC sites and the synergistic effect of Fe3C-dominated multiple iron-based, Fe/Fe3C/FeN0.0324@N-GC-850 exhibits excellent electrocatalytic activity towards ORR in pH-universal media. Specifically, it exhibits satisfactory onset potential (Eonset), half-wave potential (E1/2) and stability, and thus surpassing the benchmark Pt/C in both alkaline and neutral media as well as approaching Pt/C in acidic media. When employed as an air cathode in zinc-air batteries (ZABs), it also presents higher open-circuit voltage (OCV), discharge voltage plateaus, capacity, and peak power density compared with Pt/C. Density functional theory (DFT) calculations demonstrate that Fe3C (2 2 0)/N-GC has lower activation energy during ORR process and the overpotential generated by Fe3C/N-GC (0.7 V) is obviously less than one of Fe/N-GC (1.37 V) and FeN0.0324/N-GC (1.69 V).