| Oxygen reduction reaction(ORR)is a very important semi-reaction in fuel cells and metal-air batteries.Noble metals,carbon materials and metal oxides are commonly used in ORR catalysts.In this paper,the structure of catalysts was designed,and several noble metal/metal oxide/carbon nanocomposite catalysts were prepared.The catalysts were qualitatively analyzed by X-ray diffraction(XRD),morphology was characterized by scanning electron microscopy(SEM),valence state,bonding and electron transfer were characterized by X-ray photoelectron spectroscopy.Finally,the rotating ring disk electrode device was used.The ORR activity of the catalyst was tested.Characterization shows that carbon carriers bind to metals and metal oxides through metal-oxygen-carbonization bonds,and there are a lot of electron transfer.At the same time,precious metals and metal oxides can catalyze different ORR electron transfer pathways,and play a synergistic role to promote oxygen to be more inclined to four-electron reaction pathway in the reduction process,or to quickly recover hydrogen peroxide ions from intermediate products to oxygen through disproportionation,thus achieving a pseudo-four-electron pathway including catalytic regeneration process,which can significantly reduce the excess of intermediate products.The yield of hydrogen oxide and the improvement of its stability are of great significance to the practical application of catalysts.Two silver-based catalysts were prepared,including silver/lanthanum carbonate/carbon black prepared by urea hydrolysis and silver/cerium dioxide/carbon black prepared by calcination,which showed better activity than Silver-Carbon catalysts.Pt-based catalysts are the most commonly used noble metal catalysts for oxygen reduction.As a powerful substitute element for Pt,Ag has a lower price.However,the intrinsic activity of Ag nanoparticles is limited because of its larger size.The Co-loading of Ag and oxide on Ag/LaO2CO3/C and Ag/CeO2/C composite catalysts is beneficial to the fine and uniform dispersion of Ag,which can expose more active sites.Lanthanum and cerium compounds belong to rare earth oxides.Their unique electronic configuration and variable valence state contribute to the catalysis of ORR,and can significantly reduce the yield of hydrogen peroxide.The electrochemical characterization also showed that the yield of hydrogen peroxide was very low,which maintained at about 2.5%.XPS confirmed that Ag and metal oxides bond with carbon carrier in the form of metal-oxygen-carbon,and there is a certain process of electron transfer.The C atoms adjacent to Ag and metal oxides in carbon carrier lose electrons,while the electronegativity is enhanced,the adsorption of oxygen is strengthened,which is conducive to the reduction process of oxygen.The disadvantage of Ag-based catalysts is that their intrinsic activity is not high.In order to further improve the activity of the composite catalysts,Pt is chosen to replace Ag.Several carbon-supported Pt-based rare earth oxides composite catalysts,including carbon-supported cerium dioxide(CeO2),praseodymium oxide(PrxOy)and their three-phase composite catalysts with platinum,were prepared.On the basis of their excellent catalytic activity,the yield of hydrogen peroxide was further reduced.Carbon-loaded CeO2 was prepared by calcination,and the optimum loading rate of Ce02 was 20%.Carbon-supported praseodymium trioxide(Pr2O3)is prepared by precipitation hydrothermal method.It is rod-shaped and has a diameter of about ten nanometers.The optimum preparation conditions are loading rate of 10%,concentration of precipitator NaOH solution of 0.25 M and precursor concentration of 2 mM.After the carbon-supported oxide materials were obtained,the uniform and fine Pt particles were successfully loaded by the method of ethylene glycol reduction.After ORR test,Pt/CeO2/C and Pt/Pr2O3/C have excellent catalytic activity for oxygen reduction.The half-wave potential exceeds about 30 mV of commercial Pt/C,and the yield of hydrogen peroxide is lower,the lowest is 1%.In addition,the pure phase Pr6011 blended with commercial Pt/C prepared by heat treatment also showed excellent catalytic activity,which was comparable to commercial Pt/C on the basis of reducing the platinum consumption by 40%.The advantages of Pr6011 blended with commercial Pt/C were its excellent conductivity and the presence of Pr3+/4+ion pairs,indicating that the variable valence state had obvious advantages in electrochemical catalysis.Pt/V2O5/C,a three-phase composite catalyst of vanadium oxide vanadium pentoxide(V2O5)and platinum and carbon black,was prepared based on the advantages of multivalent vanadium.After experimenting with hydrothermal-oil bath two-step method and one-step hydrothermal method and one-step oil bath preparation method,Pt/V2O5/C samples were prepared by one-step oil bath method.The ORR half-wave potential reached 0.89V,and the yield of hydrogen peroxide was maintained at less than 1%.At the same time,due to the chemical properties of V2O5 amphoteric oxides,they also play a good catalytic activity in acidic system,with half-wave potential reaching 0.86 V.According to the characterization and analysis of the mechanism of Pt-based composite catalyst materials,it is known that Pt and oxide are both electronic donors,and the binding energy increases,so the adsorption capacity of active oxygen is enhanced,which is conducive to the realization of catalytic activity.Moreover,Pt and metal oxides can catalyze different reaction paths of oxygen reduction and promote pseudo-four electron transfer paths through synergistic action.At the same time,hydrogen peroxide can be transferred to the surface of negative charged C carrier by means of electron transfer between carbon carrier and metal and metal oxides,which can quickly detach and effectively reduce the yield of hydrogen peroxide,thus having excellent stability.According to the comparison of silver-based and platinum-based catalysts in this paper,a series of experimental studies on three kinds of lanthanide metal oxides and vanadium oxides show that this noble metal/metal oxide/carbon black three-phase composite catalyst has great potential in ORR field depending on interfacial interaction,electron transfer and synergistic effect. |
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