| The increasing energy crisis puts forward a high demand for eco-friendly,safe and reliable new energy conversion systems with high energy densities.The metal-air batteries（MAB）are distinctively promising for its high theoretical energy density,low utilization cost and good safety performance.The zinc-air batteries（ZAB）benefit from the abundant reserves of zinc and have good application prospects.However,the slower kinetics of oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）in ZAB are still the main bottleneck restricting the improvement of ZAB performance.Although platinum,ruthenium and other precious metals base catalysts are the most well-known electrocatalysts for ORR and OER in ZAB systems,their high costs and poor stabilities hinder their practical application and commercialization.Therefore,the development of low-cost ORR-OER bifunctional oxygen catalysts based on transition metals has become a research focus in this field.In order to improve the dispersion and conductivity of the catalyst,non-noble metals and carbon have been combined.At present,there have been many studies on carbon-based non-precious metal ORR and OER catalysts.Cobalt-based catalysts have also attracted attention as one of the research hotspots.However,the currently synthesized Co-based bifunctional catalysts are mainly through high energy consumption routes,and it is difficult to simultaneously realize the nanometerization of metal/alloy particles.In addition,uneven dispersion of active sites in the catalyst,leaching of metallic cobalt and even deactivation are also problems.In view of the above problems,this paper designed and prepared a dual-element and bifunction catalyst materials based on metallic cobalt through a“one-step method”,and applied it to zinc-air battery.The research content mainly includes:（1）A series of CoFex@NC dual-element catalysts are obtained by adjusting the ratio of CoFe and then compounding with g-C3N4,which will be applied to zinc-air battery.The results show that:the best CoFex@NC catalyst Co/CoFe-NC@g-C3N4has ultra-high specific surface area,good electrical conductivity and excellent electrocatalytic performance.Among them,the Co/CoFe-NC@g-C3N4 catalyst shows the ORR activity with half-wave potential(E1/2≈0.87 V),the diffusion-limited current density of 5.90 m A cm-2,and the delivered current density of 10.0 m A cm-2(Ej=10)is at 1.64 V versus RHE.In addition,when it is applied as a positive catalyst for liquid zinc-air battery,the battery can last for 110 hours with an open-circuit potential of 1.51 V,and the peak power density of the battery is as high as 111 m W cm-2;when it is applied as a positive catalyst for all-solid zinc-air battery,the battery can last for 60 hours with an open-circuit potential of 1.43 V,and the peak power density of the battery is as high as 93 m W cm-2.（2）Removing the additional g-C3N4 prepared above,prepare CoCux@NC dual-element catalysts with a special morphology and structure of"doughnuts",and apply them to zinc-air battery.The results show that the amount of Cu doped can change the morphology of the material,thereby affecting the electrocatalytic performance.Moreover,the CoCu0.5-PVP@NC best among CoCux@NC materials has better electrocatalytic performance than the aforementioned CoFex@NC catalyst.CoCu0.5-PVP@NC shows the ORR activity with half-wave potential,the diffusion-limited current density of 5.55 m A cm-2,and the delivered current density of 10.0 m A cm-2is at 1.57 V.In addition,when it is applied as a positive catalyst for liquid zinc-air battery,the battery can last for 160 hours with an open-circuit potential of 1.55 V,and the peak power density of the battery is as high as 128 m W cm-2;when it is applied as a positive catalyst for all-solid zinc-air battery,the battery can last for 80 hours with an open-circuit potential of 1.44 V,and the peak power density of the battery is as high as 110 m W cm-2. |
