Synergistic Effect Of Co-C₃N₄/Ti₃C₂T_x Derivatives In Electrocatalic Oxygen Reduction Reaction

Oxygen reduction reaction（ORR）with sluggish kinetics has become a hot topic in fuel cells and metal-air batteries.As the replacers of noble metal catalysts,transitional metal（M）（such as Fe,Co,and Ni）can combine with nitrogen to form highly efficient electrocatalytic active sites for ORR,but the instabilities of catalytic performances limit their practical applications.It is shown that C₃N₄ with graphene-like structure（g-C₃N₄）is a stable and green nitrogen-containing compound with abundant in-plane nitrogen dopants and defects,as well as the unique electronic structure,which trap a large number of transition metals to form potential active sites.Adiditionally,Ti₃C₂T_x,a two-dimensional nanosheet,has unique physical and chemical properties such as ultra high conductivity,large specific surface area and excellent thermal stability,and is expected to be used as the support of transition metals.In this connection,the dissertation provide the preparations of Co-N catalysts supported on Ti₃C₂T_x,their catalytic activity and stability in ORR,and the mechanism of synergistic catalysis among Co,C₃N₄ and Ti₃C₂T_x.The main contents include:1.The supports of g-C₃N₄ and Ti₃C₂T_x were respectively synthesized by the modified methods,and their morphology,composition and structure were represented.After the metal nitrides were directly obtained by the coordination of g-C₃N₄to cobalt,the Co-C₃N₄complexes was were loaded on Ti₃C₂T_x by electrostatic self-assembly,and finally the electrocatalysts was were harvested by calcination at different temperatures.The as-prepared Co-C₃N₄/Ti₃C₂T_x showed the high catalytic activity towards ORR,with the onset and half-wave potentials respectively at 0.97 V and 0.79 V,and importantly the catalyst displays good stability.The systematic analysis revealed the synergistic catalysis among Co,C₃N₄ and Ti₃C₂T_x:A large number of metal nitrides provided the active sites for catalysis.Besides the conductivity,Ti₃C₂T_x can fully disperse the metal nitrides to the high catalytic activity.The good electrochemical stability profit from the moderate excess cobalt to inhibit the dissociation of metal nitride.2.After the dicyandiamide,Co（NO₃）₂·6H₂O and Ti₃C₂T_x were uniformly mixed by ball milling,the mixture was calcined at different tempertures.The active sites of metal nitride were generated by the coordination of transition metal with C₃N₄ through in situ synthesis.The studies showed that C₃N₄ was formed by the calcination of dicyandiamide with the occurrence of Ti-N and Co-N bonds in the catalysts.In the electrocatalyzing ORR,the onset potential reached 0.87 V and the half-wave potential reached 0.73 V,despite of poor stability.Although the active sites of metal nitrides were formed under the conditions,the systematic analysis confirmed the physical grinding cannot control the pyrolysis products and their distribution,a part of dicyandiamide was pyrolyzed itself and does not combine with Co metal.Hence,the fewer active sites lead to the low catalytic activity,and to deactivation in ORR.3.From the control experiments of two-components,it is shown that the transition metal nitrides are difficult to catalyze the reaction,and the transition metal or carbonitride supported on Ti₃C₂T_x has no catalytic activity in ORR.ORR can be effectively catalyzed only when the active sites of metal nitride are formed and loaded on Ti₃C₂T_x.These facts further confirm the above research results and conclusions.