Solvent environment engineering to synthesize Fe–N–C nanocubes with densely Fe-N_x sites as oxygen reduction catalysts for Zn-air battery

Zeolitic imidazole framework (ZIF)-derived iron–nitrogen-carbon (Fe–N–C) materials are expected to be high-efficiency catalysts for oxygen reduction reaction (ORR). However, increasing the density of active sites while avoiding metal accumulation still faces significant challenges. Herein, solvent environment engineering is used to synthesize the Fe–N–C containing dense Fe-N_x moieties by adjusting the solvent during the ZIF precursor synthesis process. Compared with methanol and water/methanol, the aqueous media can provide a more moderate Fe content for the ZIF precursor, which facilitates the construction of high-density Fe-N_x sites and prevent the appearance of iron-based nanoparticles during pyrolysis. Therefore, the Fe–N–C(C) nanocubes synthesized in an aqueous media have the highest single atom Fe loading (0.6 at%) among the prepared samples, which presents excellent oxygen reduction properties and durability under alkaline and acidic conditions. The advantage of Fe–N–C(C) is proven in Zn-air batteries, with outstanding performance and long-term stability.