Preparation And Electrocatalytic Performances Of Cobalt-Based Oxygen (Sulfur)/Phosphide Composite Catalysts

Water electrolysis is an efficient and sustainable technology for producing clean hydrogen energy.However,the anodic oxygen evolution reaction involves multiple electron transfers,and a variety of intermediates are generated during the reaction,and slow kinetic rates,which greatly limits the efficiency of water electrolysis.Therefore,the development of non-precious metal catalysts with cost-effective,excellent catalytic activity and good stability is an effective way to accelerate the kinetic rate and improve the efficiency of water electrolysis.First transition metals,especially cobalt-based compounds,such as Co3O4,MnCo2O4,Co9S8 and Co2P,have high OER activity and are cheap,abundant and stable,however,these single catalysts are still far from meeting the needs of industrial production.By synergistic interaction between different components or by compounding catalysts with carbon materials,the electron density of the active center can be effectively adjusted,thus optimizing catalytic performance.Furthermore,carbon materials can also protect the active component from oxidation and enhance the conductivity of the catalyst during the electrocatalytic process.Based on the above considerations,in this thesis,composite electrocatalysts of Co-based oxides,sulfides and phosphides were designed and prepared for the catalysis of HER,OER and electrocatalytic water splitting.The activity of the composites was significantly enhanced compared to that of the single ones.The study consisted of three parts as follows:1.Using carbon spheres as templates,the Co²⁺and Mo⁶⁺are adsorbed on the surface of the spheres by hydrothermal methods and further calcined in a muffle furnace.The optimized catalyst Co3O4/CoMoO4-0.1-FO showed excellent OER catalytic activity with overpotentials of 217 and 342 m V at current densities of 10 and 100 m A/cm² in 1.0 M KOH,respectively,and with good stability.The results show that the hollow porous structure facilitates the penetration of the electrolyte,fully exposing the internal active sites and effectively enhancing the charge and mass transfer rates.Theoretical calculations indicate that the construction of the heterogeneous session full can effectively regulate the adsorption strength of the reaction intermediates,further reduce the reaction energy barrier of the decisive step and improve the catalytic activity.2.The three-dimensional chitosan carbon aerogel was used as a substrate,and Mn²⁺and Co²⁺ions were adsorbed on the surface of the carbon aerogel by hydrothermal method,oxidized by a tube furnace,and S-doped MnCo2O4/N-doped carbon composite catalysts were further obtained by S-doping.The optimized catalyst 2S-MnCo2O4/NC exhibited bifunctional catalytic activity with an overpotential of 180 and 258 m V for catalytic HER and OER in 1.0 M KOH at a current density of 10 m A/cm²,respectively,the over water splitting voltage was 1.57 V,and the stability of the catalyst was maintained for 28 h.The excellent electrocatalytic activity is due to the large specific surface area、good electrical conductivity and hydrophilicity provided by the carbon aerogel as well as the change in electronic structure caused by S doping.Theoretical calculations show that an appropriate amount of S doping optimizes the adsorption strength of the reaction intermediate and reduces the reaction energy barrier of the decisive step.3.Co9S8/Co2P nanoparticles embedded in carbon aerogel（Co9S8/Co2P@CA）were prepared by adding cobalt chloride to chitosan hydrogel with the simultaneous addition of melamine phosphate and thiourea followed by high temperature carbonization.Benefiting from large specific surface and good electrical conductivity provided by carbon aerogel and the synergistic effect of heterogeneous structure,the catalytic HER and OER shows an overpotentials of 174 and 262 m V at current density of 10 m A/cm²under alkaline conditions,respectively,and the over water splitting voltage was 1.56 V while maintaining excellent durability.The above experimental results show that the construction of heterogeneous structures or composites with carbon materials can complement each other,which is an important strategy to effectively improve the activity and stability of catalysts.