Controlled-synthesis Of MOF-derived Transition-metal Based Catalysts And Their Application In Zinc-air Batteries

In rencent years,zinc-air batteris have drawn much attentions for their advantages of high theretical energy density,high safety,environment friendliness and low-in-price.However,the sluggish of oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）on air cathode hinders its development.Therefore,exploring a new-type,effient and low-in-price catalyst is of great scientific significance and practical value.Researches imply that transition metal（TM,e.g.,Co,Ni,Fe）catalysts present high electrocatalytic activity,bifunctionality and natural advantages of low cost,high abundance.Nevertheless,it is crucial to ratinal design the structure to improve the catalytic activity of TM materials for its performance can not meet the requirement of people.In this paper,we synthesized a series of TM-based catalysts by the pyrolysis of metal-organic-frameworks at high temperature.In the first work,we fabricated the cobalt metal with different size from nanoparticles to atom-clusters and single-atom on N-doped carbon materials（Co-NPs@NC,Co-ACs@NC,Co-SAs@NC）and investigate their size effect in the electrocatalytic field.The results showed that the electrocatalytic activity of the materials increases with the reduction of metal size.Among them,the cobalt single atom catalyst with the minium size exhibits the highst electrocatalytic activity with onset potential of 0.96 V,half-wave potential of 0.82 V,limiting current density of 4.96 m A cm^-2,approximately to those of commercial Pt/C(0.98 V,0.82 V and 4.98 m A cm^-2).Furthermore,the single atom catalyst as-obtained has high stability.The maximum atom efficiency and unsaturated coordination environment of single atom structure accounts for the excellent performance of Co-SAs@NC.As the cathode of zinc-air batteries,the assembled battery with Co-SAs@NC has a high power density(105.3 m W cm^-2 at 158 m A cm^-2),capacity(897.1 m A h g^-1),cycling stability（250 cycles with negligibly voltage change）and flexibility.In the second work,we fabricated a self-assembled electrode（Ni₃Fe₂@NC/CC）of binary metal Ni/Fe and nitrogen-doped cabon on carbon cloth homogeneously by the pyrolysis of Ni/Fe MOFs growed on carbon cloth straightly in fabrication.Compared to powder catalysts,self-assembled electrodes can be used directly rather than prepared in advance.Compared with the traditional coated electrode,the catalytic active materials tend to uniformly load on self-assembled electrode,which has the advantage of structure,and thus has higher catalytic activity.Ni₃Fe₂@NC/CC reaches the current density of 10 m A cm^-2with an overpotential of 238 m V,much lower than that of the traditional coated electrode Ni₃Fe₂@NC（340 m V）.Moreover,compared with single cobalt metal on N-doped carbon composite materials,Ni₃Fe₂@NC/CC exhibites more excellent bifunctional catalytic activity.