Preparation Of Pd-based Catalysts As Highly Methanol-Tolerant Electrocatalysts For The Oxygen Reduction Reaction

Direct methanol fuel cell(DMFCs)is a device that can convert chemical energy into electrical energy by means of electrochemical oxidation of methanol to CO2 and H2O.DMFCs have potential application value in commercial energy storage equipment due to advantages of the abuntant fuel storage resources,high safety in the transportation process,simple operation,no external reforming treatment.But to commercialise DMFCs still faces many challenges and the defects including diffusion of methanol from the anode to cathode,poor chemical reaction rate of cathode,and catalyst poisoning have limit its large-scale application.Oxygen reduction reaction(ORR)in the DMFCs cathode is the rate controlling step of the electro catalytic reaction,and the poor ORR will inhibit the methanol oxidation efficiency of the anode.However,ORR has a low reaction rate no matter in acidic or alkaline environments.The main reason for the slow reaction rate is the diffusion of methanol in the battery.In addition,the catalyst is the key to improve the electrocatalytic performance of ORR.By changing composition and morphology of the catalysts,it can improve the catalytic activity and even the methanol resistance.Therefore,the developing of high efficient,low cost and methanol-tolerant ORR electrocatalysts has become an critical issue in the field of DMFCs.At present,the research on ORR electrocatalysts is mainly divided into three categories:"platinum-based catalyst","non-platinum catalyst" and "non-noble metal catalyst".Considering the two aspects of reducing the cost of catalyst and improving the methanol tolerance,the "non-platinum catalyst"and "non-noble metal catalyst" have a greater advantage and more bright application prospects.This paper mainly studied on the DMFCs cathode ORR electrocatalyst,emphatically explored the Pd-based catalyst and containing Co non-noble metal nitrogen doped carbon composite catalyst preparation and their electrocatalytic performance for ORR in alkaline environment.Research paper mainly consists of the following two parts:Part One:We mainly outline the DMFCs composition,structure,working principle,advantages,disadvantages and applications,etc.In addition,the reaction pathway,mechanism and research progress of DMFCs cathode and anode electrocatalyst are introduced in detail.Part Two:Research papers,including preparation of hexagonal Pd nanoparticles as highly methanol-tolerant electrocatalysts for the oxygen reduction reaction;synthesis of reduced graphene oxide supported high dispersion of PdCux nanoparticles with enhanced methanol tolerance and electrocatalytic activity for the oxygen reduction reaction;preparation of cobalt and nitrogen co-doped nano-porous carbon composite as highly methanol-tolerant electrocatalysts for the oxygen reduction reaction.Specific content is as follows:1.Hexagonal Pd nanoparticles(Pd-HGNs)have been synthesized by simply hydrothermal treatment mixture containing the cationic surfactant cetyltrimethyl-ammonium bromide(CTAB)with the metal precursor H2PdCl4,a little ferric chloride,and the reductant formaldehyde.Scanning electron microscopy(SEM),transmission electron microscopy(TEM)are used respectively for detecting the morphology of the catalyst.The crystal structure and composition of the catalyst are characterized by X-ray powder diffraction(XRD).X-ray photoelectron spectroscopy(XPS)furture to analyse valence of metal surface and elements composition.Characterization and electrochemical test results show that:1)The presence of a small amount of ferric lions was proved to be critical for the formation of the Pd-HGNs;2)The prepared Pd-HGNs has great crystalline state;3)Pd-HGNs/XC-72 and Pd/XC-72 have close electrocatalytic activity for ORR and Pd-HGNs/XC-72 has better electrocatalytic stability.4)Pd-HGNs/XC-72 catalyst has outstanding resistance to methanol performance.2.PdCu2 alloy nanoparticles(NPs)dispersed on reduced graphene oxide(PdCux-rGO)electrocatalysts have been successfully synthesized by "one pot,two-step" strategy.SEM,TEM and high-resolution transmission electron microscopy(HRTEM)are used respectively for detecting the morphology of the catalyst.XRD and XPS furture analyse valence of metal surface and elements composition.The results show that the rGO with thin sheet structure combine with PdCu nanoparticles strongly and PdCu with small particle size uniform dispersed on the surface of the rGO.The results of cyclic voltammetry(CV)and linear sweep voltammetry(LSV)suggest that compared with other catalysts,PdCu2-rGO catalyst with enhanced catalytic activity for ORR is close to the electrocatalytic performance of Pt/XC-72,which can be explained by the synergistic effect of alloying and particle size effect.On the other hand,changing the methanol concentration in electrolyte solution PdCu2-rGO catalyst shows poor oxidation efficiency for methanol,which indicate that PdCu2-rGO catalyst has relatively high catalytic selectivity to improve methanol tolerance.3.Melamine as nitrogen source and carbon source,cobalt acetate and copper acetate as precursor fully mix in ethanol to obtain the powder solid mixture.The mixture was subjected to successive two-step high temperature pyrolysis.In this process,melamine will pyrolysis,at the same time,a part of Co2+ will be converted to Co3O4,and the other part of Co2+ will combine with copper to form CoCu nanoalloy.Finally,dealloying to the substance after pyrolysis,a new type of CoCu/Co3O4/N/HC composite catalyst was obtained.The morphology,composition and structure of the as-prepared electrocatalysts are characterized by SEM,TEM,XRD and XPS.Furthermore,Raman spectroscopy is used to determination the degree of graphite for catalyst.Electrochemical testing results show that different pyrolysis temperature of the catalyst exhibits different electrocatalytic ORR performance and the secondary pyrolysis temperature for 1000? for preparation of CoCu/Co3O4/N/HC-1000 catalyst with the highest catalytic activity.Compared with Pt/XC-72 catalyst,the initial potential and half wave potential of CoCu/Co3O4/N/HC-1 000 catalyst for ORR are greatly improved.Stability test shows that after the 2000 CV cycle,the limit current density loss only 9.7%,while Pt/XC-72 catalyst loss 40.8%.In addition,change the methanol concentration in the electrolyte solution,the CoCu/Co3O4/N/HC-1000 catalyst exhibited significant methanol tolerance.This highly electrocatalytic ORR activity,stability and good methanol tolerance make it has great application potential in DMFCs as cathode catalyst.