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University of Sydney team advances rechargeable zinc-air batteries with bimetallic oxide–graphene hybrid electrocatalyst

Researchers at the University of Sydney, with colleagues at Nanyang Technological University, have found a solution for one of the biggest stumbling blocks hindering the commercialization of rechargeable zinc-air batteries.
Zinc-air batteries are powered by zinc metal and oxygen from the air. Cheaper to produce than lithium-ion batteries, they can also store more energy (theoretically five times more than that of lithium-ion batteries), are much safer, and are more environmentally friendly. However, while primary zinc-air batteries are currently used as an energy source in hearing aids and some film cameras and railway signal devices, their widespread use has been hindered by the fact that, up until now, recharging them (secondary batteries) has proved difficult. This is due to the lack of electrocatalysts that successfully reduce and generate oxygen during the discharging and charging of a battery. The research team outlines a new three-stage method to overcome this problem in a papaer published in Advanced Materials.
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 gZn-1 (corresponding to an energy density of 904 W h kgZn-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.
According to lead author Professor Yuan Chen, from the University of Sydney’s Faculty of Engineering and Information Technologies, the new method can be used to create bifunctional oxygen electrocatalysts for building rechargeable zinc-air batteries from scratch.
Up until now, rechargeable zinc-air batteries have been made with expensive precious metal catalysts, such as platinum and iridium oxide. In contrast, our method produces a family of new high-performance and low-cost catalysts.
These new catalysts are produced through the simultaneous control of the 1) composition, 2) size and 3) crystallinity of metal oxides of earth-abundant elements such as iron, cobalt and nickel. They can then be applied to build rechargeable zinc-air batteries.
Paper co-author Dr Li Wei, also from the University’s Faculty of Engineering and Information Technologies, said trials of zinc-air batteries developed with the new catalysts had demonstrated excellent rechargeability—including less than a 10% battery efficacy drop over 60 discharging/charging cycles of 120 hour