【作者】 程年才；

【导师】 潘牧；

【作者基本信息】 武汉理工大学 ， 新能源材料， 2010， 博士

【摘要】 为了解决当前世界面临的能源短缺和环境污染两大难题,直接甲酸燃料电池以其具有燃料来源广、能量转化率高、低污染、储存和运输方便等优点,在便携式电源、电动机车和野外电站等领域具有广阔的应用前景,已经得到了世界范围的关注和重视。但是,电极电催化剂的活性较低及高昂的价格仍是阻碍燃料电池商业化发展的关键问题之一。提高催化剂活性、降低贵金属用量是推动燃料电池商业化发展的重要途径。本论文采用简单的制备工艺制备了高性能的Pd/C催化剂及纳米Pd薄膜复合催化剂,并对甲酸在催化剂上的氧化性能影响因素进行了研究。主要研究进展如下：(1)采用无任何保护剂或表面活性剂的方法制备高度分散的纳米Pd/C催化剂。通过氨络合[PdCl4]2-离子并在微波加热的条件下直接形成[Pd(NH3)4]2+络合物,没有产生沉淀。通过[Pd(NH3)4]2+与载体表面带负电荷的基团相互作用来控制Pd催化剂制备过程中颗粒粒径,并采用氢气还原法还原Pd／C催化剂。氢气固相还原法中,吸附在载体表面的[Pd(NH3)4]2+还原过程中不易迁移,原位还原成Pd颗粒,粒径较小,200℃还原温度下的Pd/C催化剂颗粒平均粒径为1.3nm,600℃时为2.2nm。同时还尝试采用液体还原法还原Pd/C催化剂。但由于Pd离子因为浓度的改变或者溶液中Pd离子还原过程中会发生迁移,从而合成的Pd颗粒粒径比氢气固相还原法大,液相还原法制备的Pd／C催化剂颗粒平均粒径为4.7nm。(2)甲酸在Pd/C催化剂上电氧化的活化能研究。正电势方向扫描,甲酸在Pd/C催化剂氧化的活化能随电势增大而逐渐减小,而负电势扫描过程中,甲酸氧化的活化能扫电势增大而增大。因为在负电势扫描过程中,Pd／C催化剂上氧化物对甲酸的氧化有一定的促进作用。H2SO4电解质中的甲酸氧化活化能比HClO4高,因为H2SO4为吸附型电解质,吸附在催化剂表面上的电解质会降低催化剂的性能。电解质的浓度对甲酸在Pd/C催化剂上氧化活化能影响很小。(3)pH对Pd/C催化剂甲酸氧化性能的影响研究。循环伏安和计时安培法测试显示在pH2到pH6的范围内,Pd/C催化剂对甲酸氧化电催化性能随着pH的升高而增大,大概在0.30V vs SCE左右,Pd/C催化剂的甲酸电催化氧化性能达到最大。Pd/C催化剂的耐久性随着电解质pH的增大而增强,这主因为要pH越小,酸性越大,对催化剂和载体的腐蚀越大,加剧了Pd催化剂和载体表面的氧化,使载体上的催化剂颗粒更容易迁移、氧化和长大。研究结果显示：HCOO为甲酸氧化过程中活性物质,甲酸在Pd催化剂上是通过“甲酸根途径”被氧化成CO2,甲酸氧化性能随着pH增大而增大。(4)Pd纳米薄膜电催化剂的制备及对甲酸氧化性能的研究。我们在carbon nanoparticle-chitosan host films中原位合成Pd纳米颗粒,并在不同电极上测试了Pd催化剂的性能。本研究中我们发现壳聚糖网络有利Pd颗粒的合成。通过电化学测试发现,合成的复合Pd纳米薄膜催化剂有很好的甲酸氧化性能。同时还研究了溶液pH和动力学液体的条件下对甲酸氧化的性能的影响,提出了由于电极中生成的CO2气泡而增加的阻抗效应为主要因素限制催化剂的性能的理论。更多还原

【Abstract】 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.更多还原