The Preparation And Investigation Of Anode Catalyst With Siver-based Core-shell Structure For Direct Borohydride Fuel Cell

Direct borohydride fuel cell(DBFC),with alkaline aqueous solution of NaBH4 as fuel and O2 or H2O2 as oxidant,is one of the most exciting energy technologies because of its high theoretical open cell voltage(1.64V)and high energy density(9.3Wh/g).In theory,the oxidation of BH4-can transfer eight electrons.However,the complete 8e-exchange is not achieved on most of the metals studied in relation to BH4-oxidation,because of the borohydride hydrolysis.This hydrolysis can decrease the utilization of BH4-.Therefore,the key factor for the application of DBFC is to prepare anode electrocatalysts that has high selectivity and high catalytic activity for improving the kinetic parameters of BF4-oxidation and inhibiting the hydrolysis of BH4-.Among the anode metals,Ag is presented as an non-catalytic material with respect to the hydrolysis of BH4-.However,slow electrode kinetics and low power densities indicate that Ag can not be used alone.In this paper,carbon supported M@Ag(M=Cu,Ni)core-shell catalysts are prepared by a successive reduction method and are investigated by physical and electrochemical measurement.First,the Cu@Ag/C core-shell nanoparticles with different atomic ratios are prepared by the successive reduction.The morphology and structure are investigated by transmission electron microscopy(TEMM),X-ray diffraction(XRD),and energy dispersive spectroscopy(EDS).Electrochemical characterizations are performed by cyclic voltammetry(CV),chronoamperometry(CA),linear scan voltammetry with rotating disc electrode(LSV RDE),in situ Fourier transform infrared(FTIR)spectroscopy,and fuel cell test.The results show that the Cu?Ag/C nanoparticles are core-shell catalysts,and the average size is approximately 17.8 nm.As shown by the electrochemical measurements,the BH4-oxidation reaction mechanism of Gu@Ag/C is similar to that of Ag/C,that is BH4?BH3,ads? BH3OHads-? BO2.The kinetics of BH4-oxidation are faster for Cu?Ag/C than for Ag/C.Among all the as-prepared catalysts,the Cu2@Agi/C catalyst presents the highest catalytic activity.The BH4-oxidation on Cu2@Agi/C surface could transfer 4.2 electrons.The DBFC using Cu2@Ag1/C as anode electrocatalyst and Pt mesh(1cm2)as cathode electrode obtains the maximum anodic power density as high as 6.17 mW cm-2.On the basis of the above research,the Ni@Ag/C core-shell nanoparticles with different atomic ratios are subsequently prepared by the successive reduction.The morphology and structure are investigated by TEM,XRD,and EDS.Electrochemical characterizations are CV,CA,LSV RDE,and fuel cell test.The catalytic behaviors and main kinetic parameters(e.g.Tafel slope,number of electrons exchanged,exchange current density and apparent activation energy)toward BH4-oxidation are determined.The results show that the the average size of Ni@Ag/C is approximately 15 nm.The kinetics of BH4-oxidation are faster for Ni@Ag/C than for Ag/C.Among the as-prepared catalysts,the highest transition electron value and the lowest apparent activation energy are obtained on Ni1@Ag,/C,the values are 4.8 and 20.23 kJ mol-1,respectively.In the paper,the results show that the catalytic activity of M@Ag/C core-shell catalyst is better than that of Ag/C.It can be attributed to the result of an electronic or a geometric effect.When the metallic bond among Ag atoms and Cu or Ni atoms is formed,the d-band of Ag over-layer is broadened and the average energy is lowered.The ratio of Cu:Ag 2:1 and Ni:Ag 1:1 may offer a moderate interaction with BH4-and show high electrocatalytic activity toward BH4-.