Catalytic Performance Investigation Of Novel Rare Earth-pd-Based Catalysts For Methanol And Formic Acid Direct Electro-oxidation

Direct methanol fuel cell(DMFC)has received enormous attention due to high energy density,low emissions,and abundance of methanol.It is expected to become a highly-efficient and clean future energy source for portable electronic devices and automobiles.However,challenge issues such as low catalytic activity of anode catalysts for the electrooxidation of methanol,susceptibility of the catalysts to be poisoned by CO-like intermediates formed in the methanol oxidation reaction and high cost of noble metal platinum(Pt)-based catalysts,as well as the mixed potential caused by methanol crossover have greatly hindered the commercialization of DMFC.In addition,direct formic acid fuel cell(DFAFC)also has similar strengths and weaknesses as DMFC.In this work,inspired by synergistic effect of the catalyst components,a simple simultaneous co-reduction method was used to prepare palladium(Pd)-rare earth catalysts,which were supported on carbon black,activated multi-wall carbon nanotubes(aMWCNTs),functionalized aMWCNTs and Ti4O7,respectively.The structure and morphology of the prepared catalysts were characterized by X-ray diffraction(XRD)and transmission electron microscope(TEM).The valence state changes of the surface elements were studied by X-ray photoelectron spectroscopy(XPS).The electrocatalytic performances of these catalysts for methanol/formic acid electrooxidation were characterized by cyclic voltammetry(CV),linear sweeping voltammetry(LSV),CO stripping experiments and chronoamperometric(CA).(1)Firstly,Pd-Er/C catalysts with different Pd/Er ratios were prepared by a sodium borohydride reduction method.The results of XRD and TEM show that Pd-Er nanoparticles can be dispersed uniformly on carbon black with little agglomeration.Compared with other catalysts,Pd-4Er/C catalyst has higher catalytic activity and better stability for methanol electrooxidation.It can be seen that the addition of Er can remove CO-like intermediates,which can be partially explained by the bi-functional mechanism.XPS results show that the addition of Er can increase the content of Pd(0).In other words,Er has a "metallization”effect on Pd,which is the other reason for the improvement of catalytic performance of Pd catalysts.(2)In addition,the Pd-Yb/C catalysts were also prepared.When the addition amount of Yb was 2%,the maximum catalytic activity of methanol electrooxidation could be achieved.However,the catalytic activity is slightly inferior to Pd-4Er/C catalyst.Thus,the performance of Pd-Er catalysts was further studied in the following part.(3)Based on the excellent catalytic performance of the Pd-4Er/C catalyst,Pd-4Er/aMWCNTs catalyst was prepared.However,the Pd-Er particles supported on aMWCNTs have a significantly more serious degree of agglomeration,which may be due to the hydrophobicity of aMWCNTs according to FTIR analysis.The catalytic activity of Pd-4Er/aMWCNTs catalyst for methanol electrooxidation is lower than that of Pd-4Er/C catalyst,which is caused by the agglomeration of nanoparticles.After functionalized with chitosan(CS),the prepared Pd-4Er/aMWCNTs-CS catalyst exhibited better catalytic performance for methanol electrooxidation than Pd-4Er/C catalyst,which can be ascribed to the improved dispersion of the nanoparticles and the increased content of Pd(0)due to the presence of CS.(4)The Pd-Er nanoparticles were supported on Ti4O7 by a co-reduction method.Comparing the electrochemical testing results of Pd-4Er/Ti4O7 with Pd-4Er/C,it is found that carbon black support performs better in alkaline methanol solution than Ti4O7 support.According to our previous researches,the catalytic activity of Ti4O7 supported Pd-based catalysts for direct electrooxidation of formic acid is higher than that of carbon black supported Pd-based catalysts.This may be related to the ionic crystal structure of Ti4O7 with strong polarity,and its adsorption capacity for formic acid is significantly better than that for methanol.In order to verify whether the Pd-Er bimetallic catalyst also conforms to this conjecture,the electrooxidation activity of Pd-Er catalysts supported on two different supports for formic acid was tested.It was found that the Ti4O7 supported catalysts performed better than the carbon black supported catalyst.Therefore,the effect of the support is closely related to the electrolyte solution.