Amorphous Bimetallic Oxide-Graphene Hybrids as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries

Metal oxides of earth-abundant elements are promising electrocatalysts to overcome the sluggish oxygen evolution and oxygen reduction reaction (OER/ORR) in many electrochemical energy-conversion devices. However, it is difficult to control their catalytic activity precisely. Here, a general three-stage synthesis strategy is described to produce a family of hybrid materials comprising amorphous bimetallic oxide nanoparticles anchored on N-doped reduced graphene oxide with simultaneous control of nanoparticle elemental composition, size, and crystallinity. Amorphous Fe_0.5 Co_0.5 O_x is obtained from Prussian blue analog nanocrystals, showing excellent OER activity with a Tafel slope of 30.1 mV dec^-1 and an overpotential of 257 mV for 10 mA cm^-2 and superior ORR activity with a large limiting current density of -5.25 mA cm^-2 at 0.6 V. A fabricated Zn-air battery delivers a specific capacity of 756 mA h g_Zn^-1 (corresponding to an energy density of 904 W h kg_Zn^-1 ), a peak power density of 86 mW cm^-2 and can be cycled over 120 h at 10 mA cm^-2 . Other two amorphous bimetallic, Ni_0.4 Fe_0.6 O_x and Ni_0.33 Co_0.67 O_x , are also produced to demonstrate the general applicability of this method for synthesizing binary metal oxides with controllable structures as electrocatalysts for energy conversion.