Pd-based/N-doped Graphene Nanocomposite Synergetic Catalytic Effect Enhanced Electrooxidation Of Small Organic Molecules In Electrochemical Performance

The rapid increase in energy demands,greenhouse gas emissions and depletion of fossil fuels make unquestionably direct alcohols fuel cells(DAFCs)attractive technology due to their low emission,no toxicity,highly efficient fuel utilization and environmentally-friendly operations.Some poisonous intermediates will be generated and strongly adsorbed on catalyst surface in the process of electro-oxidation,blocking the surface active sites from further catalysis.This results in a dramatic decrease in electrochemical activity of the catalyst.Therefore,careful electrocatalyst design strategies must be applied to strengthen the activity and anti-poisoning issues associated with both the catalyst metal particles and carbon supports to achieve practical adoption targets.As the electrocatalysis is a surface reaction,the eletrocatalytical performance is largely determined by the shape and composition of catalysts.Therefore,optimizing the structure of Pd,such as alloying surface decorating and chosing supports etc.,becomes the key point in the explorion of highly active and stable catalysts.Due to its planer structure,huge specific surface area,high electronicconduction rate,and corrosion resisting,graphene is considered suitable as the anode catalyst support in direct alcohol fuel cells(DAFCs).The incorporation of electron-rich nitrogen atoms into graphene can not only improve the dispersion state of the nanoparticles on the graphene surface,but also modify the surface structure of carbon materials and strengthen interaction between metal nanoparticles and supports.Pb and Sn were chosen to prepare the two basic catalytic systems of PdPb/NG and PdSn/NG by a one-step reduction method in this work.However,the synergetic effect of N-doping and second active component towards noble metal on small organic molecule electrooxidation have been rarely reported until now,which will be revealed in this study.Moreover,two improvements for each basic catalytic system were devoted to the performance enhancement and the action mechanism on these catalysts was deeply discussed.The main results are as follows:(1)In this work,a series of palladium and palladium-lead nanoparticles supported on active carbon,graphene and nitrogen-doped graphene are synthesized via a one-step reduction method.AAS,XRD,TEM and XPS are used to characterize the catalysts.The results indicate that metal nanoparticles are more uniformly dispersed on the surface of N-doped graphene than those on graphene,without any aggregation.Various electrochemical techniques are carried out to evaluate the electrocatalytic ethanol oxidation activity and durability.The peak current for ethanol electrooxidation of Pd/N-doped graphene increases to 70.2 mA cm-2,obviously higher than that of Pd/Graphene(38.0 mA cm-2)and even surpasses that of Pd/C(51.9 mA cm-2).N-doped graphene support not only possesses faster dehydrogenation but provides an electron effect to Pd.Introduction of Pb into the catalyst causes the formation of abundant oxygenated species on the catalyst surface at low potential.Based on the synergistic effect of N and Pb towards Pd particles,the PdPb/N-doped graphene catalyst(Pd:Pb ?8:1.0)exhibits remarkably enhanced activity up to 152.3 mA cm-2 for ethanol oxidation,which is 4.0 and 2.9 times higher than that of Pd/Graphene and Pd/C,respectively.The catalytic durability and stability are also greatly improved.(2)A series of palladium-based catalysts of metal alloying Sn and/or(N-doped)graphene support with regular enhanced electrocatalytic activity were investigated.ICP-AES,XRD,TEM and XPS are used to characterize the catalysts.Various electrochemical techniques are carried out to evaluate the electrocatalytic ethanol oxidation activity,durability and electrochemical performance.The peak current density(118.05 mA cm-2)of PdSn/NG is higher than the sum current density(45.63+47.59 mA cm-2)of Pd/NG and PdSn/G.It reveals a synergistic electrocatalytic oxidation effect in PdSn/N-doped graphene Nanocomposite.On this basis,the PdSn/NG also showed the enhanced catalytic durability and anti-poisoning.(3)Further,extend experiments show this multisource synergetic catalytic effect of metal alloying and N-doped graphene support in one catalyst on small organic molecule(methanol,ethanol and Ethylene glycol)oxidation is universal in PdM(M=Sn,Pb)/NG catalysts.The high dispersion of small nanoparticles,the altered electron cloud density and Pd(0)/Pd(II)ratio of Pd on the catalyst by strong coupled the metal(Sn,Pb)alloying and N-doped graphene are responsible for the multisource synergistic catalytic effect of PdM(M=Sn,Pb)/NG.Our results support a new concept to explore new anode catalyst in fuel cell and are applicable to the design of other catalysts for direct alcohols fuel cells(DAFCs).