Preparation Of Co-based Composite Catalyst Derived From Glucose And Investigations Of Oxygen Electrode Reactions

The renewable energy systems such as proton exchange membrane fuel cell and rechargeable metal-air batteries have attracted intense research interests due to their high theoretical energy densities,sufficient energy conversion efficiency and low to zero pollutant emission.However,the sluggish kinetics of oxygen electrode reactions including oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）restrict the overall efficiency of the battery.Hence,numerous attention have been paid on developing efficient electrocatalyst for improving their performance.Platinum（Pt）and Ruthenium oxides（Ru O₂）are widely accepted as benchmark electrocatalysts for ORR and OER,respectively.However,the low abundance,expensive and poor stability of these catalysts limited the large scale application of these renewable energy systems.Therefore,the development of inexpensive non-PGM electrocatalyst is extremely important.Transition metal cobalt has an unfilled 3d orbital,which is easy to adsorb oxygen and its reaction intermediates,promoting the decomposition of oxygen molecules.Meanwhile,cobalt is abundant reserve on the earth and cost-effective,making it an important substitute for PGM catalyst.Inspired by the configuration of MOFs and the growing interest on green chemistry,in this thesis,biomass materials such as glucose and urea were used as precursors to prepare cobalt based electrocatalyst based on the Schiff-base reaction that carbonyl groups from the dehydrated glucose react with the amino from urea,constructing a nitrogen-doped carbonaceous skeleton.Moreover,the rich oxygen and nitrogen groups in the skeleton are available to coordinate to cobalt ions,forming the precursor with homogeneous dispersing components.On this basis,two kinds of cobalt-based composite catalysts have been obtained:nitrogen-doped porous carbon-supported cobalt oxides particles（Co₃O₄/NPC）and carbon nanotube enthangled with nitrogen-doped carbon encased cobalt nanoparticles（Co@NC-CNTs）composite catalyst.In addition,the reaction temperature,reagent dosage as well as pyrolysis temperature and time during synthesis are also systematically investigated to reveal the formation mechanism of these electrocatalysts.Based on researches above,the following results have been obtained:1.The Co₃O₄/NPC bifunctional catalyst was in situ synthesized by hydrothermal-calcination method with glucose and urea as precursors.A large number of oxygen-containing functional groups in glucose promise for thesuccessfully preparation of Co₃O₄/NPC at800℃.Moreover,resulting from the low decomposition temperature of urea-derived polymer,the catalyst possesses hierarchical porous structure,which accelerates the transport of reactants and exposes sufficient active sites,improving the reaction kinetics.In addition,the catalyst prepared by in-situ method is beneficial for enhancing the interaction between Co₃O₄ and NPC,which enhances the electron transfer ability to adsorbed oxygen and improves oxygen electrode activity.Therefore,the Co₃O₄/NPC showed high ORR and OER catalytic activity in the alkaline electrolyte.The half-wave potential of the composite catalyst for ORR was 0.82 V（vs.RHE）,and tis overpotential for OER was 390 m V with the potential difference between these two reactions of 0.8 V（vs.RHE）,much less than that of Ru O₂（0.96 V）and Pt/C（0.94 V）.2.The composition and structure influence the active sites density of the catalysts and thus affect their catalytic activity.In this part,the effect of anion type and ammonium content on the structure of carbonaceous support and the dispersion of Co₃O₄ nanoparticles were investigated.The investigation on anions type showed that the acetate affect the distribution of Co₃O₄nanoparticles significantly due to the difference in chelation effect between cobalt ions and anions.On this basis,ammonium was adopted to modulate the composition and structure of composite catalyst obtained using cobalt acetate as cobalt source.It was found that the distribution of Co₃O₄ nanoparticles in the carbon support becomes uniform and the particle diameter decrease gradually with the increase of ammonium content.When 2 m L ammonium was added,the particle size was the smallest（21.1 nm）and the mesoporous and macropore ratio of the catalyst increased obviously.Further chemical state analysis indicated that the oxygen vacancy concentration of cobalt oxide as well as the six-membered nitrogen（pyridinic N and graphitic N）is the highest at this point,resulting in the best oxygen reduction activity among the four catalysts.3.carbon nanotube enthangled with nitrogen-doped carbon encased cobalt nanoparticles（Co@NC-CNTs）composite catalyst was prepared using urea,dicyandiamide and melamine as nitrogen sources respectively through a two-step pyrolysis method.The obtained Co@NC-CNTs possesses the nitrogen content as high as 6.9 at.%due to the high nitrogen content of the intermediate products g-C₃N₄ derived from nitrogen source.Besides,the in-situ grown carbon nanotubes from the precursor construct a three-dimensional electrocatalyst,providing high surface areas and large numbers of pores for the sufficient contact of the reactant with active sites and mass transfer.As a result,the obtained electrocatalyst exhibits superior catalytic performance with the half-wave potential of 0.85 V（vs.RHE）,which was comparable to Pt/C.