Preparation Of Doped Carbon-Supported Nano-Palladium Catalysts And Kinetic Study Of Formic Acid Electrochemical Oxidation

Direct formic acid fuel cells(DFAFCs)have a wide range of applications in portable power supplies due to their high energy density,the convenience of using liquid formic acid(HCOOH),and the simplicity of power system integration.At present,direct formic acid fuel cells have been made considerable progress in the research and engineering application.However,the electrocatalytic activity and stability of the anode catalyst of direct formic acid fuel cells need to be improved.In order to increase the carbon-supported Pd(Pd/C)catalyst for the electrocatalytic performance of formic acid oxidation in DFAFC,the modification and doping of the Pd group are developed to improve the catalyst.In this paper,carbon materials modified with heteropolyacids were prepared,and Pd was loaded on heteropoly acid doped carbon support.Electrochemical methods(cyclic voltammetry,chronoamperometry,rotating disk voltammetry,etc.)were used to study the oxidation of formic acid oxidation,and acquired kinetic parameters.At the same time,other metal components were introduced to obtain bimetal alloy,and the properties of the binary metal catalyst were studied by physical characterization(XPS,Raman,SEM,etc.)and electrochemical methods.(1)The phosphotungstic acid and carbon are mixed and heated to obtain a phosphotungstic acid modified carbon support,and the catalyst is prepared by liquid phase reduction method,using palladium chloride solution as a precursor and sodium borohydride as a reducing agent to prepare Pd/PWA-C.The XPS test indicated that the 3d5/2 binding energy(BE)of Pd(0)changed from 335.40 eV of Pd/C to 335.78 eV of Pd/PWA-C,indicating that the modification of PWA increases the binding energy of the catalyst.CO stripping test showed a significant negative shift of 780 mV of Pd/PWA-C catalyst relative to the peak potential of 804 mV of Pd/C,indicating that the modification of PWA can weaken the adsorption binding energy of CO and Pd.CV tests with different sweep speeds indicate that formic acid oxidation is a different reaction path at different potentials.The electron transport coefficients of Pd/PWA-C and Pd/C is α=0.2,and the diffusion coefficients of Pd/C and Pd/PWA-C were L59×10-5 cm2 s-1 and 6.45×10-5 cm2 s-1,respectively,about 4.4 times that of the Pd/C catalyst.The Tafel slopes of the Pd/PWA-C and Pd/C catalysts were 160 mV dec-1 and 188 mV dec-1,respectively.The EIS test at different temperatures measured the electron transfer resistance of the Pd/PWA-C catalyst from 44.47 Ω at 30℃ to 28.36 Ωat 60℃.The significant attenuation of the electron transfer resistance can be attributed to the HCOOH and the reaction intermediates.At the same time,the diffusion coefficients of Pd/C and Pd/PWA-C were 3.69×10-6 cm2 s-1 and 4.87x10-6 cm2 s-1,respectively.CV scanning at different temperatures indicates that formic acid oxidation is particularly sensitive to temperature on the Pd/PWA-C electrode due to its apparent activation energy as a function of potential.The diffusion coefficient of formic acid on the Pd/C catalyst was calculated from the fitting curve of the Koutecky-Levich diagram by RDV,is(1.57±0.11)x 10-7 cm2 s-1,and the formic acid diffusion coefficient of the Pd/PWA-C catalyst was(3.47±0.65)×10-6 cm-2 S-1,indicating that the PWA-modified Pd/C catalyst can promote the diffusion step and thus enhance the electrode kinetics of formic acid electrooxidation.(2)A catalyst with a PdCu atomic ratio of 1:2,1:1,2:1 was prepared by a modified liquid phase reduction method.The 20 of the Pd(111)crystal plane in the XRD test indicates that the Cu atoms were embedded in the Pd lattice to form a Pd-Cu alloy with a face-centered cubic structure.As a result of TEM,it was found that the Pd1Cu2/PWA-C catalyst had the smallest average particle diameter and the narrowest particle size distribution.TheID/IG values of the Raman test PWA-C and XC-72 were 1.08 and 1.15,respectively.The CV results showed that when Pd:Cu=1:2,the current density of Pd/C catalyst is the highest after Cu doping and PWA modification,and the peak current density is 7.08 times of Pd/C.The CA results show that Pd1Cu2/PWA-C,Pd1Cu1/PWA-C,Pd2Cu1/PWA-C,Pd/PWA-C,Pd/C five currents at 3600 s(I3600)and 10 s(I10),The ratios are 7.0%,0.6%,0.5%,0.3%,and 0.1%,respectively.It is shown that the PWA modified Pd:Cu=1:2 alloy catalyst has better stability to formic acid.At the same time,CO dissolution showed a significant negative shift in the peak potential of the PdCu alloy catalyst with respect to the peak potential of Pd/C of 804 mV.The stability and activity of the PdCu/PWA-C catalyst for the formic acid oxidation is mainly attributed to the synergistic effect between PWA and PdCu alloy.