Preparation And Application Of Pd-Based Alloy Electrocatalysts For Formic Acid Oxidation

Direct formic acid fuel cells?DFAFCs?empoly liquid formic acid as fuel,which is non-toxic and possesses lower fuel crossover than methnol.Consequently,there are many research about DFAFCs.However,rate of the anodic reaction?formic acid oxidation reaction,FAOR?is rather slow.Catalyst,as one of the core component for fuel cell,can accelerate formic acid oxidation,which is of great significance to promote the commercialization of DFAFCs.It necessitates to investigate higher efficient catalysts since the activity and stability of common Pt/C and Pd/C catalysts cannot meet the standard of commercial applications of DFAFCs.In recent years,a large number of literatures have pointed out that combining the advantages of two or more metals to form alloys can help enhance catalytic performance.In this thesis,catalysts are designed and prepared in terms of improving catalytic activity and enhancing stability.The main contents are as follows:Firstly,aimed to decerase the amount of Pt and increase catalytic activity,surfactant-free Pd_xPt_y/C with ultra-low Pt catalysts are successfully synthesized by H₂ reduction in ethylene glycol assisted with ultrasonication vibration at room temperature.Nanoparticle agglomeration in the course of preparation has been sufficiently curbed by strong mechanical ultrasonication,which produced"clean"active site in Pd_xPt_y/C surface.Catalysts were characterized by X-ray diffraction,X-ray photoelectron spectroscopy and transmission electron microscopy,indicating that Pd and Pt precursors have been completely reduced to produced alloy nanoparticles.The Pd-Pt alloy nanoparticles are around 4.2 nm and highly dispersed.Electrochemical evaluation results showed that mass activity of homemade Pd/C and Pd₇₅Pt₁/C was 581 and 1572 A g^-1,respectively,demostrating that the addition of ultra-low amount of Pt can significantly boost catalytic activity for FAOR.At the same time,Pd₇₅Pt₁/C(59.6 m² g^-1,1572 A g^-1)displayed a higher electrochemically active surface area?ECSA?and mass activity than Pd₇₅Pt₁/C-cit(43.3m² g^-1,1245 A g^-1).It proves that surface of as-prepared catalysts by mechanical ultrasonication was much cleaner and good for boosting catalytic performance for FAOR.In order to further screen the best catalysts,the Pd/Pt atomic ratio was changed.Screened Pd₁₀₀Pt₁/C registered a mass activity of 3171 A g^-1 for FAOR,which lists one of the best results reported so far and surpassesd that of a commercial Pd/C by 5.6 times.Pd₁₀₀Pt₁/C and Pd₇₅Pt₁/C also displayed higher catalytic for ORR than that of commerical Pd/C.Repeated cyclic voltammetry and chronoamperometry evaluations also proven that optimized Pd₁₀₀Pt₁/C displayed better stability than commercial Pd/C.Secondly,a series of Pd_xCu_y/C catalysts were prepared by one-pot hydrothermal method using dimethylformamide?DMF?and ethylene glycol?EG?as solvents.The morphologies and exact metal loadings of catalysts were characterized by TEM,XRD and ICP-AES.Afterwards,as-synthesized Pd_xCu_y/C catalysts were electrochemically dealloyed to obtain D-Pd_xCu_y/C catalysts with porous structure.Electrochemical evaluation demostrated that D-Pd_xCu_y/C catalysts possessed rather rough surface and high ECSA.Mass activity of D-Pd₁Cu₄/C was2611 A g^-1,3.3 times higher than that of commercial Pd/C.After dealloying,D-Pd₁Cu₄/C catalysts showed excellent stability.The mass activity remained 30%after 150 cycles of repeated cyclic voltammetry,whereas a commercial Pd/C-Aldrich benchmark showed retained16.5%activity.On the one hand,the Pd–M nanoalloys could improve stability because of ligand effect and strain effect.On the other hand,the porous structure that resulted from electrochemical dealloying prolonged residence time at the active surface.The cyclic voltammetry and TEM images were used to investigate electrochemical dealloying mechanism.With repeated cyclic voltammetry scans incerasing,original Cu-rich and Pd-poor surface of Pd₁Cu₄ gradually evolved into a porous structure with Pd-rich surface,accompanied by successive Cu dissolution.