Theoretical Study On The Stability And Electrocatalytic Activity Of Cobalt-based Catalyst Supported On Activated Carbon

With the advent of the energy crisis and the worsening of environmental pollution,the development of an environment-friendly,resource-saving clean energy system has become vital to the development of human society.The preparation of clean and high-quality liquid fuels and high-value-added chemicals through the synthesis gas reaction is a hot topic in our current research.For example,high-carbon alcohols prepared from synthesis gas provide cheap and sufficient raw materials for the fine chemical industry.The Co-based catalyst（Co/AC）supported on activated carbon showed excellent stability in the synthesis of high-carbon alcohols by the synthesis of syngas.Previous results showed that metallic Co can be easily converted into Co₂C during the Fischer-Tropsch reaction.With the synergistic effect of CO,the surface of the carrier may have atomic Co monomers or combining with a certain number of CO forming metal carbonyl compounds（Co（CO）_x）.However,under typical reaction conditions for the high-carbon alcohols synthesis,it is difficult to directly characterize the dynamic species formed by experimental characterization.It is important for us to prepare the low-cost and efficient electrocatalysts.Development of trifunctional catalysts for oxygen reduction reaction（ORR）,oxygen evolution reaction（OER）,and hydrogen evolution reaction（HER）is pivotal but still challenging for rechargeable Zn-air batteries and overall water splitting.Precious metal catalysts（for example,Pt,Ru）have excellent electrocatalytic performance,but their high price and relative scarcity of resources limit their commercial application prospects.N-doped graphitic carbon has unique structure and electronic properties,and is the most promising alternative to the noble metal materials,but its electrocatalytic performance is still unsatisfactory compared with the noble metal catalysts.Our collaborators found that N-doped graphitized carbon-coated iron-cobalt alloy nanocatalysts（Co Fe@NC/NCHNSs）showed high power density in zinc-air batteries,and a series of characterizations proved that Co Fe/graphitic NC had the best electrocatalytic performance.However,the synergistic effect of Fe-Co alloy and different types of N-doped graphitic carbon structures on the adsorption energy of oxygen intermediates and the mechanism of catalytic oxygen reduction reaction of the catalyst were still unclear.For the background outlined above,we explored the following two aspects via density functional theory in this thesis:1.At first,we studied the stability of Co atom and cobalt carbonyls adsorbed on various active sites of activated carbon and compared the formation Gibbs free energy of different adsorption configurations to obtain the stable phase of adsorbed Co species.The carrier activated carbon is composed of six-member ring network fragments of indefinite size,with abundant pore structure and high specific surface area.The amorphous hexagonal network segment structure of activated carbon makes it rich in vacancies and defects such as boundaries.At the same time,the segment boundaries contain a variety of oxygen-containing groups（hydroxyl,carboxyl,carbonyl,substituted O atoms,etc.）Our results revealed that the stability of CO species at different sites on the activated carbon surface was different.Formation of cobalt carbonyls can significantly enhanced the stabilization of Co on activated carbon surface and cobalt carbonyls prefered lower temperature and higher pressure.Under typical syngas conversion reaction to higher alcohols,formation of cobalt carbonyls was thermodynamically favorable on carbon vacancy,strip edge and oxygen-containing group.2.Experimental results demonstrated that the N-doped graphitized carbon coated alloy catalyst at 700℃had highly efficient and stable three functional electrocatalytic activities of ORR,OER and HER(half-wave potential(E_1/2)of 0.92 V for ORR,and low overpotentials of 285 m V and 120 m V at 10 m A cm^-2 for OER and HER,respectively).We used density functional theory to calculate the binding energy and reaction free energy of different intermediates（O*,OH*,OOH*）on graphitic N-C,pyridinic N-C,pyrrolic N-C,Co Fe/graphitic N-C,Co Fe/pyridinic N-C,and Co Fe/pyrrolic N-C under alkaline conditions.Co Fe@NC/NCHNSs can effectively reduce the energy barrier for O₂ absorption and improve the inherent activity of the reaction.The superior trifunctional performance can accredit the synergetic effect between the rich Co Fe alloy sites and the high-content graphitic N dopants which greatly enhanced the adsorption of O*,OH*and OOH*,as demonstrated by both theoretical calculations and experiments.Also,we found that the introduction of Co species in the synthesis of Co Fe@NC/NCHNSs can help anchor N in the carbon matrix and Fe species contributed to the generation of graphitic N species,which together promoted the formation and retention of high content of graphitic-N in the final product and led to an enhanced electrocatalytic activity.The synergistic effect between Fe-Co alloy substrate and N-doped graphitic carbon was beneficial to the ORR reaction.