Preparation And Properties Of Electrocatalysts For Hydrogen Peroxide Synthesis Based On 2e^- Oxygen Reduction Reaction

Hydrogen peroxide(H₂O₂),as a recognized environmentally friendly oxidant,has been widely used in chemical synthesis,medical disinfection,sewage treatment,semiconductor cleaning,pulp bleaching,the most beneficial aspect of H₂O₂ is that it breaks down into water and oxygen after reaction.Moreover,H₂O₂ could also be utilized as an ideal oxidant in fuel cells and sustainable energy carrier alternative since it possesses the advantages of high oxidation potential,easy storage and safe operation.The synthesis of H₂O₂ based on anthraquinone method has high energy consumption,large pollution and complicated steps,and the high concentration products are not conducive to transportation and storage.The direct syhthesis of H₂O₂ from the mixture of hydrogen and oxygen is potentially explosive.The synthesis of H₂O₂ based on two-electron(2e^-)electrocatalytic oxygen reduction reaction(ORR)pathway is an economical,safe and environmentally friendly strategy.However,the four electron(4e^-)transfer passway is a strong competition to the 2e^-ORR,limiting the efficiency for the electrochemical synthesis of H₂O₂.Therefore,the design and development of highly selective and active electrocatalysts to drive the industrialization of H₂O₂ synthesis based on 2e^-ORR is the biggest challenge in this field.This dissertation aims to improve the electrochemical activity and selectivity of 2e^-ORR electrocatalysts,and focuses on carbon-based materials and their composites with high electrical conductivity and large specific surface area.Considering that the microstructure,electronic structure and proton transfer efficiency of the catalysts have a great influence on their catalytic performance,a series of electrocatalysts for H₂O₂synthesis based on electrochemical ORR were designed and prepared by means of heteroatom doping,defect introduction,in situ reconstruction and in situ transformation,and the catalytic mechanism and regulation rule were explored in combination with experiments and theories.The main research contents and conclusions are summarized as follows:(1)Nitrogen doped hollow carbon nanospheres(NHCSs)with high activity and selectivity were prepared by sacrificial template method and in-situ polymerization combined with high-temperature pyrolysis.In ORR route,NHCSs not only favor full contact between electrolyte and O₂ molecules and promote mass transfer and diffusion of the reactants but ensure effective entrapping of O₂ gas to further facilitate the binding/interaction of catalyst with O₂ molecules.Electrochemical test results indicate that such NHCSs possess a H₂O₂ selectivity of 96.6%,a yield rate of 7.32 mol·g^-1_cat.·h^-1,a Faradaic efficiency of 96.7%and good electrochemical stability at 0.5 V vs.RHE in H-Cell system.Density functional theory calculation reveals that nitrogen doping can tune the electronic structure of carbon atoms and the adsorption strength of OOH^*intermediates,the unique synergistic effect between the pyrrolic N species and COOH groups in carbon skeleton is the key factor to promote the 2e^-ORR activity and H₂O₂selectivity of NHCSs.(2)Core-shell structured amorphous TiO₂coated titanium carbide(a-TiO_2-x/TiC)composite was prepared by surface oxidation strategy.TiC acts as a conductive species,which can promote the transfer of electrons on the surface of a-TiO_2-x/TiC during the reaction process;a-TiO_2-x acts as an active catalytic layer,which is beneficial to improving the H₂O₂ selectivity.The resulting a-TiO_2-x/TiC exhibits a low overpotential and high H₂O₂ selectivity of 91.8%at 0.5 V vs.RHE in 0.1 mol L^-1 KOH electrolyte,and it also possesses robust stability and a remarkable productivity of 7.19 mol·g^-1_cat.·h^-1 with an excellent FE of 93.6%at 0.3 V vs.RHE.The electrocatalytic mechanism of a-TiO_2-x/TiC is further revealed by density functional theory calculations.Oxygen vacancies and low-coordinated Ti5 sites in a-TiO_2-x/TiC,as well as other abundant Tiactive sites can combine well with OOH^*,are important active sites for the efficient reduction of O₂ to H₂O₂.(3)Two-dimensional Pt Se₂ nanosheet with 2e^-ORR performance was successfully synthesized by in-situ selenization of commercial Pt/C nanoparticles with 4e^-ORR performance.The existence of carbon support can inhibit the agglomeration of Pt Se₂nanosheets during selenization.This catalyst is capable of achieving O₂ reduction to H₂O₂with a superior H₂O₂ selectivity of 90.9%at 0.3 V vs.RHE in 0.1 mol·L^-1 phosphate buffer solution(PBS).Impressively,such electrocatalyst delivers a maximum H₂O₂ yield rate of 2.88 mol·g^-1_cat.·h^-1 and a Faradaic efficiency of 93.3%at 0.3 V vs.RHE with a robust stability during bulk electrolysis in H-type device.In addition,Pt Se₂/C also possesses a H₂O₂ selectivity of 85.1%at 0.2 V vs.RHE in 0.1 mol·L^-1 HCl O₄.In situ attenuated total reflection infrared spectroscopy reveals that ORR occurred on Pt Se₂/C through 2e^-transfer pathway,and the OOH^*intermediates are hydrogenated and converted to HOOH_ad on Pt Se₂/C.