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 Fe0.5Co0.5Ox 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, Ni0.4Fe0.6Ox and Ni0.33Co0.67Ox, are also produced to demonstrate the general applicability of this method for synthesizing binary metal oxides with controllable structures as electrocatalysts for energy conversion.