Space-confined iron nanoparticles in a 3D nitrogen-doped rGO-CNT framework as efficient bifunctional electrocatalysts for rechargeable Zinc–Air batteries

Transitional metal nitrogen-doped carbon catalysts are increasingly attractive due to their high bifunctional activities in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of rechargeable zinc-air batteries. In the present work, space-confined iron nanoparticles in a 3D nitrogen-doped reduced graphene oxide-carbon nanotube (rGO-CNT) framework (Fe-NCNTs/NG) were synthesized by pyrolysis of a compound comprising urea, anhydrous ferric chloride (FeCl3) and ammonium chloride (NH4Cl) in the presence of graphene oxide (GO). Carbon nanotubes (CNTs) were grown from urea under the catalysis of iron before the pyrolysis to obtain Fe-NCNTs/NG. Nitrogen was introduced into the rGO-CNT framework from urea and NH4Cl in the pyrolysis process at a temperature of 700/800/900/1000 °C. XRD, SEM, HRTEM and XPS techniques were employed to characterize Fe-NCNTs/NG and showed iron and/or Fe3O4 nanoparticles are confined in the 3D nitrogen-doped rGO-CNT framework. The electrochemical test results demonstrate that as-prepared Fe-NCNTs/NG can conduct better catalytic performances than Pt/C and IrO2 in the aspects of a higher half-wave potential and a lower overpotential. Specifically, the best catalytic performance of Fe-NCNTs/NG was achieved when it was prepared by a 30:1 mass ratio of urea to FeCl3 and pyrolyzed at a temperature of 900 °C. Furthermore, a home-made zinc-air battery loading a Fe-NCNTs/NG electrode showed better charge-discharge stability and higher power density than a Pt/C loaded zinc-air battery. The present work demonstrates that the Fe-NCNTs/NG framework is promising to be an alternative to Pt/C and/or IrO2 in the rechargeable zinc-air battery industry.