Preparation And Electrocatalytic Performances Research Of Pd Catalysts Supported On Carbon Composites

Polymer electrolyte membrane fuel cells?PEMFCs?as a kind of green and portable electronic device has been widely studied in recent years.The high cost of noble metal is the main factor hindering development of PEMFCs.Direct formic acid fuel cells?DFAFCs?have attracted more and more interest because of its non-toxic,high theoretical open circuit voltage,low fuel permeability and so on.Pd catalysts are superior in formic acid electrooxidation compared with that of Pt catalysts,because of their low price and high catalytic efficiency.However,the catalytic activity and durability of the Pd-based catalysts still need to be improved.The supported Pd-based catalyst,both Pd metal and support affect the catalytic activity and stability.In this paper,starting from the catalyst support,different carbon supports on the catalyst performance are studiedfor developing DFAFCs anode Pd catalysts with high activity and high stability.Specific research contents are as follows:Different concentrations of 1,4-benzoquinone?C₆H₄O₂?were used to modify Vulcan XC-72 carbon black and then the impregnation method was used to prepare the nano-Pd supported catalyst for the formic acid electro-oxidation.Electrochemical measurements show that the catalyst prepared by modified benzoquinone catalyzes formic acid oxidation compared well to the Pd/Vulcan XC-72 catalyst.Different concentrations of benzoquinone have different effects on the Pd catalyst,and the best concentration is 3 mmol L^-1?Pd/C-3?.Cyclic voltammetry?CV?and CO-stripping measurements results show that the maximum current densities for formic acid oxidation decrease in the order of Pd/C-3>Pd/C-2>Pd/C-4>Pd/C,which are in fairly good agreement with the ECSA.The catalytic mass activity of the Pd/C-3 catalyst(0.936 A mg_Pd^-1)towards formic acid electrooxidation is 2.8 times that of the Pd/C catalyst(0.312 A mg_Pd^-1).The ECSA of the Pd/C-3 catalyst(81.13 m² g^-1)is 2.8times that of the Pd/C catalyst(52.60 m² g^-1),revealing an efficient utilization of the Pd atoms in the Pd/C-3 catalyst.Conversion to specific activity,the peak current density?specific activity?of the formic acid oxidation on the Pd/C-3 catalyst(11.53 A m^-2)is about 2.0 times higher than that on the Pd/C catalyst(5.94 A m^-2).The ratio of I₃₆₀₀/I₁₀ for Pd/C-3 and Pd/C catalyst is calculated to be 7.5%and 0.8%according to the chronoamperometry?CA?measurement which indicated that the catalytic stability of the Pd/C-3 was stronger than that of the Pd/C catalyst.In order to further study the mechanism of benzoquinone modification improving the performance of Pd catalyst for formic acid oxidation,we used the best concentration of benzoquinone to modify Vulcan XC-72 and prepare the Pd/C-3catalyst by liquid reduction method.The supports and catalysts were studied by X-ray diffraction?XRD?,Raman spectroscopy?Raman?,X-ray photoelectron spectroscopy?XPS?and transmission electron microscopy?TEM?.The results showed that modification of benzoquinone increased oxygen content on the surface of the carbon support.The proportions of different oxygen functional groups on the carbon surface were changed:C-OH and C-O-C increased while C=O and COO decreased.This change is beneficial to deposit Pd metal particles on the support surface,increase the corrosion resistance of the support,expose more Pd?100?surface and enhance electrocatalytic performance for the formic acid oxidation.We reported an F-doped carbon support Pd catalyst?Pd/C-F?for formic acid electrooxidation.F-doped carbon support was obtained by simple pyrolysis to the mixture of ammonium fluoride?NH₄F?and Vulcan XC-72 under N₂ at 900?.Electrochemical measurements demonstrated that the oxidation current density of the Pd/C-F catalyst(0.86 A mg_Pd^-1)is 2.8 times higher than Pd/C catalyst(0.31 A mg_Pd^-1)for formic acid oxidation.Electrocatalytic stability also increased,the current densities at the Pd/C-F and Pd/C catalyst electrodes at 3600 s decayed to 24.70%and0.85%of the initial values?at 10 s?,which indicated that the stability of Pd/C-F catalyst was enhanced.Uniformly dispersed catalyst nanoparticles were prepared on C-F support and their average particle sizes smaller than Pd/C catalyst according to TEM.The XPS results indicated that the F atom in the C-F catalyst mainly exists in semi-ionic,and semi-ionic C–F bonds are capable of polarizing the C atom in a greater extent,which benefits the dispersion of Pd nanoparticles on support.The CO-stripping voltammetry results demonstrated that Pd/C-F has much better resistance to CO poisoning.Electrochemical measurements show that the Pd/C-F catalyst exhibits superior electrocatalytic activity and stability than traditional Pd/C catalyst towards formic acid oxidation,which may be attributed to the better resistance to CO poisoning of the Pd/C-F catalyst and the high dispersion of Pd nanoparticles.