Electrocatalytic Performances Of Mixed Heteropolyacids Modified And Doped Carbon Supported Pd Catalysts For Formic Acid Oxidation

The direct formic acid fuel cell?DFAFC?is considered as a promising power sources due to its high energy conversion efficiency,low-pollution,low operating temperature,the easy storage and transportation of the fuel.At present,it is reported that the electrocatalytic activity of Pd catalyst for formic acid oxidation is higher than that of Pt catalyst.However,the stability of Pd anodic catalyst towards formic acid oxidation is poor.In order to improve the stability of Pd catalyst,two kind of modified and doped carbon support were developed and investigated in the dissertation.In this paper,activated carbon modified with mixed heteropolyacids and doped by phosphorus were prepared.And the Pd nanoparticles were anchored on the as-prepared carbon support by an improved liquid reduction method.The performances of catalysts were characterized by a series of chemical?cyclic voltammetry,chronoamperimetry,electrochemical impedance spectroscopy and so on?and physical?XRD,TEM,XPS and so on?methods.The main contents are as follows:?1?Activated carbon was modified with mixed phosphotungstic acid?PWA?and phosphomolybdic acid?PMA?by impregnation.The effects of different molar ratios of PWA and PMA(n_PWA:n_PMA=2:1,1:1,1:2,3:0,0:3,0:0)on the catalytic activity of the formic acid oxidation were investigated.TEM results showed that uniformly dispersed Pd nanoparticles were prepared.The average particle sizes of the corresponding catalyst Pd/W2M1–C is the smallest?3.36 nm?when the molar ratio of PWA and PMA is 2:1.CV measurements demonstrated that the electrocatalytic activity of Pd/W2M1–C catalyst was enhanced as a factor of 2.17 than Pd/C catalysts when the molar ratio between PWA and PMA is 2:1.CA results showed that the values of I₃₆₀₀/I₁₀ between the Pd/W2M1–C and Pd/C catalysts are 5.21%and 0.05%,respectively.A negative shift?41 mV than Pd/C?of CO stripping current peak was observed for the Pd/W2M1–C catalyst,which shifted the most among the as-prepared catalysts.In addition,faster charge-transfer kinetics of the formic acid oxidation for the Pd/W2M1–C catalyst was found through electrochemical impedance spectroscopy?EIS?measurements.The better electrocatalytic activity and stability of Pd/W2M1–C catalyst towards formic acid oxidation may be attributed to excellent Pd nanoparticles dispersing on the W2M1–C support and the synergistic effect of the PWA and PMA modification.?2?A new kind of phosphorus contained carbon support?P–C–X,X=700,800,900??was fabricated by calcining the mixture of the carbon and triphenylphosphine under nitrogen.The Pd/P–C–X and Pd/C catalysts were synthesized by an improved liquid reduction method when the P–C–X and C were used as supports.The effects of different calcining temperatures on the catalytic activity of the formic acid oxidation were investigated.Well-dispersed Pd nanoparticles were obtained and their particle sizes are in the range of 3.5⁴.5 nm.CV measurements demonstrated that when the mixture of carbon and triphenylphosphine were annealed at 800?,the activity of the corresponding catalyst?Pd/P–C–800?was the highest.The formic acid oxidative current of Pd/P–C–800 and Pd/C catalyst was determined as 19.45 A m^-2 and 6.19 A m^-2,respectively.Furthermore,the onset oxidation potential of formic acid oxidation on the Pd/P–C–800 shifted negatively ca.31 mV compared to the Pd/C catalyst.CA results showed that the values of I₃₆₀₀/I₁₀ between the Pd/P–C–800 and Pd/C catalysts are 18.4%and 5.9%,respectively.A negative shift?76 mV than Pd/C?of CO stripping current peak was observed for the Pd/P–C–800 catalyst,which shifted the most among the as-prepared catalysts.Moreover,faster charge-transfer kinetics of the formic acid oxidation for the Pd/P–C–800 catalyst was determined through EIS measurements.The better catalytic performance of the Pd/P–C catalysts are ascribed to the doping of phosphorus in the carbon support.