Study On Synthesis And Characterization Of Pd/C Catalyst For Direct Formic Acid Fuel Cell

To solve the problems of energy shortage and environmental pollution in the world, the direct formic acid fuel cell were paid much attention and investigated widely.They havewide applications in the portable equipment,electric car and field power etc.due to thelow-pollution,abundant sources,high energy efficiency,the easy storage and transportation of the fuel.However,the low electrochemical activity and high cost of the electrocatalysts are still the key issues hindering the commercial application of fuel cell.Therefore,the improvement of the electrocatalytic activity of the electrocatalysts and the decrease of the loading mass of noble metals are the effective routes for the commercial application of fuel cells. In this thesis, we prepared highly activity Pd/C catalyst and Pd nanoparticle catalysts within carbon nanoparticle-chitosan host films using simple mehtods and study the effect of pH on the Pd/C performance.Also the activation activity energies for formic acid oxidation were studied by electrochemical measures.The main progress of this paper is summarized as follows.(1) An simple synthesis for highly dispersed and active Pd/C catalyst for formic acid electro-oxidation. we report an experimentally simple process to prepare highly dispersed Pd/C catalyst without the use of any stabilizing agent. The [Pd(NH3)4]2+ ion is synthesized with gentle heating in aqueous ammonia solution without formation of Pd(OH)x complex intermediates. The adsorbed [Pd(NH3)4]2+ on the surface of carbon is reduced in situ to Pd nanoparticles by NaBFH4 and H2.The Pd/C catalyst using H2 as reduciton has better formic acid oxidation than the Pd/C catalyst reduced by NaBH4 because t average size of Pd/C catalyst reduced by H2 are smaller than Pd/C catalyst reduced by NaBFH4. The average size of Pd/C catalyst reduced by H2 are 1.3nm at 200℃and 2.2nm at 600℃. The average size of Pd/C catalyst reduced by NaBH4 is 4.7nm.(2) Electrochemical determination fo activation energies for formic acid oxidation o on Pd/C catalyst in acidic electrolytes. The activation energies for formic acid oxidation o on Pd/C catalys decrease with potential increase at positive-going potential scan.And The activation energies for formic acid oxidation o on Pd/C catalys increase with potential increase at negative-going potential scan. The activation energies for formic acid oxidation o on Pd/C catalys are higher in H2SO4 than that in HCO4 because of adsorption characteristics of H2SO4.(3) Study on pH effect on the Pd/C catalyst for formic acid oxidation. Cyclic voltammograms and Chronoamperometry measurements show that activation energies for formic acid oxidation o on Pd/C catalyst increase with pH increase in acidic electrolytes.The stability of Pd/C catalyst increase with pH increase in in acidic electrolytes because the Pd/C catalysts were oxidied easily at low pH values. We think the formate is a reactive intermediate in the formic acid oxidation and the formic acid is oxidized throught "formate path".(4) Growth and Characterization of Palladium Electrocatalyst in a Carbon Nanoparticle-Chitosan Host for Formic Acid Oxidation. It has been shown that Pd nanoparticle catalysts are reproducibly formed within carbon nanoparticle-chitosan host films and on different types of substrate electrodes. These nanocomposite catalyst films can be applied to different types of electrodes and they are highly active for the oxidation of formic acid. The effect of the solution pH and hydrodynamic flow were investigated and a localized resistance effect introduced by CO2 gas bubble formation has been proposed as dominating factor in limiting the catalyst turn over at high current densities.