Design And Properties Of Cobalt-based Electrocatalysts For Bifunctional Oxygen Reduction And Evolution Reaction

Rechargeable metal-air batteries and renewable fuel cells are attracting more and more attention due to their high energy density and environmental friendliness.However,the kinetics of oxygen reaction in these two kinds of batteries is slow,and the noble metal catalysts which catalyze the oxygen reaction are facing problems of high cost,poor stability and limited resource on earth.It is therefore vital of importance to develop non-noble metal catalysts in order to fulfill their large-scale application.The composite materials made of transition metals/metal oxides and carbon have become one of the most promising substitutes of precious metal catalysts due to its unique properties and synergistic effect promoting the oxygen reaction.In this thesis,three types of composite materials based on transition metal and carbon have been synthesized.First,earth-abundant coal,which is an important family member of carbon,has been used to prepare Ndoped carbon by chemical vapor deposition method in the presence of ammonia.Different Co/CoO_x-carbon composites have been successfully prepared with the addition of cobalt salts to coal-derived carbon by means of microwave-assisted method.The prepared Co/CoO_x-carbon composites showed very good electrocatalytic performance towards oxygen evolution reaction in alkaline condition.The metal core-graphene shell structure was accidentally found in the composite.Nitrogen-doped graphene-encapsulated transition metal catalysts(e.g.Co,Mn,Ni)was subsequently specifically designed by using a molecular sieve as a template.Among them,the catalyst with cobalt as the core showed satisfied electrochemical performance towards not only oxygen evolution reaction,but also oxygen reduction reaction.In order to further improve the electrocatalytic bifunctionality of the catalyst and reduce the restriction of electrolyte diffusion on the catalyst,ZIF-67 was used as the precursor to synthesize a three-dimensional carbon network with wrapping cobalt in it through reasonable structure design.With the additional introduction of defects and the sulfur dopant,the electrocatalytic performance of the prepared catalysts towards both oxygen reduction reaction and oxygen evolution reaction has been drastically improved.The constructed rechargeable zinc-air battery,which consisted of a Zn-foil as anode and the catalyst loaded on carbon paper as the cathode,has shown excellent charging and discharging capability.This superior rechargeability unequivocally demonstrates its potential application in practice.