Study On The Enhancement Of Methanol Electrooxidation By Transition Metal Phosphide/carbon Carrier

With the rapid development of industrialization,people's demand for energy is increasing.The traditional fossil energies,such as coal,oil and natural gas,are constantly mined,which results in the energy reserves decline sharply and creates many irreversible harms for environment.Thus,it is imminent to seek a novel environment-friendly energy for gradually replacing the fossil energy.Direct methanol fuel cell(DMFC)is considered to be an ideal portable energy alternative because of its unique advantages such as fuel renewable,low cost,safety,high energy density,zero emissions and easy operation.At present,although the researchs on DMFC have been relatively mature,the catalysts of DMFC anode are still show some issues for poor stability,easy CO poisoning and high cost.To improve the performances of catalyst support and endow catalyst support with excellent co-catalytic activity towards methanol oxidation reaction(MOR)is an effective way to strengthen the MOR electrocatalytic activity and durability of Pt-based catalysts.This study was mainly aims to research and prepare the catalyst support of DMFC anode and obtain a novel Pt-based catalyst support with excellent durability and co-catalytic activity towards MOR,and inspect the MOR electrochemical performance of as-synthesized support composites.In this work,transition metal phosphides with Pt-like properties were chosen as object.By varying the molar ratio of chelating agent(citric acid,CiA)and supplemental carbon source(glucose,Glu),we prepared molybdenum phosphide/porous carbon(MoP/C)and tungsten phosphide/3D-tufted porous carbon(WP/C)composites.The Pt-MoP/C and Pt-WP/C catalysts with Pt loading of 5 wt.%were prepared using the sodium borohydride reduction method and researched their activity and stability toward MOR.Moreover,we discussed the influence of passivation for catalytic performance and the MOR co-catalytic mechanism of MoP and WP.Furthermore,X-ray diffraction(XRD),nitrogen adsorption/desorption,scanning electron microscopy(SEM),X-ray photo electron spectroscopy(XPS)and tansmission electron microscopy(TEM),and other means were used to analyze the physical and chemical characterization of as-synthesized supports and catalysts.The main research contents and results are shown as follows:(1)Through one step carbonization method,molybdenum phosphide/porous carbon(MoP/C)composites as Pt-supports and co-catalysts for MOR are prepared by varying the molar ratio of CiA to Glu.Pt-MoP/C-2(CiA:Glu=2)has far higher electro-catalytic activity(platinum electrochemical surface area 103.90 m2 gpt-1,mass activity 680.77 mA mgPt-1)and durability for MOR than those of commercial Pt/C(mass activity 480.73 mA mgPt-1).Because of the close integration between MoP and carbon and the ligand effect of Mo-P bonds,MoP/C can maintain a commendable stability for facilitating the fast transport of electron.A fast oxidative removal of CO can reduce the adsorption of poisoning species(CO and CHO)onto the surface active sites of Pt and lead to the regeneration of active sites in Pt-MoP/C.Furthermore,the surface of MoP/C contains abundant absorbed-OH(hydrophilic)and oxygen-containing functional groups originating from the O2 passivation,which promotes the dispersion of deposited Pt and the mass transfer of fuel.This study provides useful information for the design and preparation of a state-of-the-art anodic electro-catalyst support with promising co-catalytic effect for MOR.(2)By varying the molar ratio of CiA to Glu,tungsten phosphide/3D-corrugated porous carbon(WP/C)composite as Pt-support and co-catalyst for MOR is prepared via a synchronous synthesis method.Porous 3D-tufted structure and high surface area of WP/C with abundant oxygen-containing groups(such as C-O-C,C-O-H,or C-OH)can significantly improve the exposure of active sites,which enlarge the contact area with electrolyte and facilitate the mass transfer and absorption of methanol for promoting the MOR activity in acidic electrolyte.Pt-WP/C exhibits a considerably higher mass activity(1559.3 mA mgpt-1)for MOR than that of Pt/C(488.2 mA mgPt-1),owing to the special activity of W?+ and P?-sites for the decomposition reaction of water.With the introduction of W species,more available P species(passivated or not)are activated for further enhancing the co-catalytic activity of WP for MOR.Furthermore,the CO tolerance and durability of Pt-WP/C are also remarkable,which should benefit from the fast surface transport of adsorbed CO on different crystalline faces of WP and the extremely stable WP-C structure originating from the existence of P-P chains between the adjacent WP particles,respectively.The design of the porous structure and co-catalytic effect of this catalyst support(WP/C)provides a promising method to drastically enhance MOR activity.