Preparation Of Co/Fe Based Transition Metal Oxides And Investigations Of Their Electrocatalytic Properties

Rechargeable zinc-air batteries have the advantages of high energy-conversion efficiency,environmental friendliness and excellent recyclability,are widely used in electric vehicles,portable electronics and power plants.The performance of a rechargeable zinc-air battery strongly depends on the cathodic oxygen reduction and oxygen evolution kinetics.At present,the rechargeable zinc-air battery mainly uses noble metals such as Pt,Pd,Au and their alloys as oxygen reduction catalysts,and noble metal oxides such as RuO₂ and IrO₂ as oxygen evolution catalysts.These precious metal catalysts suffer from problems such as scarce resources,slow reaction kinetics,poor stability,and etc.,seriously hindering the development of zinc-air batteries.Exploitation of low-cost,high catalytic activity bifunctional catalysts are therefore of great significance to improve cathode oxygen reduction and oxygen evolution kinetics and promote the development of rechargeable zinc-air batteries.Transition metal oxides are considered as an alternative material for oxygen reduction and oxygen evolution catalysts due to their abundance in earth and catalytic activities towards oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?.However,most of transition metal oxide catalysts generally have problems such as low catalytic activity and poor stability.Therefore,to make them directly used in rechargeable zinc-air batteries,the catalytic activities of transition metal oxides toward ORR and OER have to be improved.With aims to develop low-cost,high-performance transition metal oxide-based catalysts and promote their applications in zinc-air batteries,the design of core-shell structured transition metal oxides and their catalytic applications for ORR and OER are investigated in this work.Specifically,the following of researches have been conducted:?1?ORR/OER catalytic activities of core-shell structured Fe₃O₄@CoO nanoparticles?NCs?.The synthesis of the Fe₃O₄@CoO NCs used iron acetylacetonate,cobalt acetylacetonate,oleylamine,oleic acid and tetrabutylammonium bromide as raw materials.Firstly,the Fe₃O₄NCs were synthesized,and the CoO shell was epitaxially grown on the surface of the Fe₃O₄NCs.The Fe₃O₄@CoO NCs are found to have high catalytic activities for ORR and OER.Specifically,the ORR activity of the Fe₃O₄@CoO NCs is comparable to Pt/C.The overpotential at 10 mA cm^-2 for the OER is lower than that of RuO₂/C?420 mV?.Additionally,the Fe₃O₄@CoO NCs exhibits a potential difference of 0.764 V in the OER current density at 10mA cm^-2 and the ORR current density at-3 mA cm^-2,which is lower than those of most state-of-the-art catalysts,indicating that Fe₃O₄@CoO NCs has excellent bifunctional catalytic activity.The zinc–air battery with the Fe₃O₄@CoO NCs shows higher current density and power density than the batteries with Pt/C–RuO₂/C.These results clearly demonstrate that the Fe₃O₄@CoO NCs is a durable and efficient bifunctional catalyst with great potential for practical applications.The excellent catalytic activity of Fe₃O₄@CoO NCs for oxygen reduction and oxygen evolution is demonstrated to arise from the specific interaction between the core and shell materials.The thickness of the CoO shell is found to have a great influence on the catalytic activities of the Fe₃O₄@CoO NCs.The results present here may be extendable to the design of other core-shell transition metal oxide electrocatalysts,opening up a new way for the design of bifunctional catalysts for ORR/OER with improved catalytic activities.?2?ORR/OER catalytic activities of CoO@Co₃O₄-NCNT and applications in zinc-air batteries.Carbon nanotubes,cobalt acetate,ethylenediamine,sodium nitrite was used as raw materials.The synthesis of CoO@Co₃O₄-NCNT is achieved by the growth of the CoO NPs on the surface-aminated CNTs by a hydrothermal method,followed by their heat-treating in air.The ORR onset potential of CoO@Co₃O₄-NCNT close to that of commercial Pt/C and the OER overpotential at a current density of 10 mA cm^-2 close to that of the RuO₂/C.In both the ORR and OER conditions,the CoO@Co₃O₄-NCNT exhibits excellent catalytic stabilities.In addition,the?E of the CoO@Co₃O₄-NCNT is 0.810 V,which is lower than most of the catalysts reported in the literatures,indicating that CoO@Co₃O₄-NCNT has excellent bifunctional catalytic activities.When used in zinc-air batteries,the CoO@Co₃O₄-NCNT can delivers an excellent stability,high current density and power density.This demonstrate that the CoO@Co₃O₄-NCNT is an excellent bifunctional catalyst with potential applications.In summary,the Fe₃O₄@CoO and CoO@Co₃O₄-NCNT as bifunctional electrocatalysts have been developed with aims to improve the long-cycle stabilities and reduce high costs of zinc-air batteries.Our work suggests that the core-shell structure can improve electrocatalytic activities of the Fe₃O₄@CoO.The CoO@Co₃O₄-NCNT obtained by the air heating can provide more reactive sites,reduce ORR\OER charge transfer resistance,and improve ORR and OER electrocatalytic activities.The work present here therefore provides new ideas for designing and preparing ORR/OER bifunctional electrocatalysts with high performance,which is of great significance to promote the commercialization of zinc-air batteries.