Preparation And Electroactivity Of Palladium-based Multi-metal Nano-electrocatalysts

Study on the electro-oxidation of alcohol and formic acid is of much significance in the field of fuel cells.Although Pt and Pt-based catalysts possess efficient electroactivity towards oxidation of alcohol and formic acid,the commercial application of metal Pt has been limited because of its high price,scarce resources,and easy poisoning.The price of metal Pd is relatively cheap,and Pd catalysts have exhibited great electrocatalytic activity for oxidation of alcohol and formic acid in alkaline media.Therefore,Pd catalyst is a better candidate than Pt catalyst for alcohol and formic acid oxidation.In this paper,a series of Pd-based catalysts have been prepared,and their electrocatalytic activities for the oxidation of alcohol and formic acid have been studied.Furthermore,a membraneless direct alcohol fuel cell has been constructed by using the prepared Pd-based catalysts as the anode.The main contents are as follows:?1?Multi-walled carbon nanotube?CNT?supported Pd-Ag-Sn nanocatalysts including Pd/CNT,Pd₇Ag₃/CNT,Pd₇Sn₂/CNT,Pd₇Ag₁Sn₂/CNT,Pd₇Ag₂Sn₂/CNT and Pd₇Ag₃Sn₂/CNT,have been prepared in a mixture solution of ethylene glycol andwater by NaBH₄ reduction method.The samples have been characterized by using scanning electron microscopy?SEM?,transmission election microscopy?TEM?,X-ray diffraction?XRD?,and X-ray photoelectron spectroscopy?XPS?.Compared to the Pd/CNT and Pd₇Ag₃/CNT or Pd₇Sn₂/CNT catalysts,metal particles of the ternary Pd₇Ag₂Sn₂/CNT catalyst are more uniformly dispersed on the surface of CNTs and present the size of ca.2.4 nm.Both binary Pd-Ag and ternary Pd-Ag-Sn catalysts exhibit the alloying of Pd and Ag.Electrochemical activities of the prepared catalysts for ethanol oxidation in alkaline media were investigated by cyclic voltammetry?CV?,chronoamperometry?CA?and electrochemical impedance spectroscopy.Results show that Pd₇Ag₂Sn₂/CNT catalyst reveals the highest current density of ethanol oxidation among the prepared catalysts and presents a high steady-state current density for ethanol oxidation from CA measurement,which is 32.3 times higher than the commercial Pd/C.Also,Pd₇Ag₂Sn₂/CNT and Pd₇Ag₃Sn₂/CNT catalysts show a large ethanol oxidation peak current density?against Pd loading?of 2.29 and 2.30 A mg^-1,indicating the high usage efficiency of Pd particles for ethanol oxidation.?2?The electrocatalytic activity of as-synthesized Pd-based catalysts towards formic acid oxidation in both H₂SO₄ and NaOH solutions was investigated using cyclic voltammetry?CV?and chronoamperometry?CA?.In both acidic and alkaline media,the ternary Pd-Ag-Sn/CNT catalysts present higher anodic current density for formic acid oxidation than Pd/CNT and binary PdAg/CNT or PdSn/CNT catalysts.Among the prepared catalysts,the Pd₇Ag₂Sn₂/CNT catalyst displays the highest HCOOH oxidation current density of 108.8 mA�cm^-2 in 0.5mol�L^-1 H₂SO₄ solution or 211.3 mA�cm^-2 in 1mol�L^-1 NaOH solution,corresponding to the Pd mass current density of 1364 mA�mg^-1 or 2640 mA�mg^-1,respectively.These currents are extremely larger than those obtained from the commercial Pd/C.Results exhibit the excellent electrocatalytic activity of the ternary Pd₇Ag₂Sn₂/CNT catalyst towards formic acid oxidation.?3?Pd and PdNi nanostructured particles have been deposited on the carbon paper?CP?by a novel and simple method of in situ deposition.The prepared catalysts were named as Pd-situ/CP,PdNi-situ/CP.The as-formed Pd and Pd-Ni nanoparticles are well dispersed with the sizes of 15²4 nm for Pd-situ/CP,8¹1 nm for Pd₉Ni₁-situ/CP,and 8¹4 nm for Pd₈Ni₂-situ/CP.Their electroactivity for ethanol oxidation in 1 mol L^-1 NaOH solution was investigated with CV,CA,and EIS.The samples fabricated by this in situ deposition method?Pd-situ/CP,Pd₉Ni₁-situ/CP,and Pd₈Ni₂-situ/CP?exhibit both higher electroactivity and higher electrocatalytic stability for ethanol oxidation than the Pd/CP obtained by the conventional NaBH₄ reduction method.This new method of preparation of catalyst nanoparticles is simple and hardly leads to the loss of catalyst particles due to the omitted filtration step of catalyst particles.?4?The Co/PANI-PPY catalyst was obtained by high temperature pyrolysis of the the precursor of Co-doped aniline and pyrrole copolymer?PANI-PPY?.Then,the as-synthesized catalyst was mixed with the acidified CNTs,and the mixed black powder was ball-milled to obtain a Co/PANI-PPY/CNT catalyst.The electrocatalytic activity of the prepared catalysts for oxygen reduction reaction?ORR?was tested by using linear sweep voltammetry?LSV?.The results show that the onset potential of ORR on the two catalysts is around-0.07 V.The limiting diffusion current density of ORR on the Co/PANI-PPY and Co/PANI-PPY/CNT catalysts is 3.61 and 3.82 mA�cm^-2,respectively,indicating that Co/PANI-PPY/CNT presents better electrocatalytic activity for ORR.Then,a membraneless direct alcohol fuel cell has been assembled using Co/PANI-PPY and Co/PANI-PPY/CNT as cathode catalysts and Pd/C,Pd/CNT and Pd₇Ag₂Sn₂/CNT as anode catalysts,respectively.The discharge performance of membraneless direct alcohol fuel cells has been tested by using ethanol,n-propanol and isopropanol as fuels,respectively.The results show that the fuel cell using Pd₇Ag₂Sn₂/CNT as anode catalyst and Co/PANI-PPY/CNT as cathode catalyst exhibits the highest power density value and most stable discharge performance.The maximum power density of the membraneless direct alcohol fuel cells with the fuels of ethanol,n-propanol and isopropanol is1.62,1.71 and 0.82 W�g^-1?Pd?,respectively,and the current density at the end of the discharge is 12.8,16.5 and 6.4 mA�cm^-2,respectively.Compared with the test results of the membrane direct alcohol fuel cells,the membraneless alcohol fuel cells also present good performances,as well as the low cost.