【作者】 王家钧；

【导师】 尹鸽平；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2008， 博士

【摘要】 直接甲醇燃料电池（DMFC）甲醇来源丰富、价格便宜、易于贮存和运输,因此在便携式电源中具有广泛应用前景。尽管目前DMFC在技术上已经取得了很大进展,但要实现其真正商业化应用,依然有许多问题需要解决,诸如燃料电池阴极较低的反应性能、电池寿命不高、甲醇通过膜的扩散引起的混合电位等。解决这些问题的途径之一就是开发更有效的阴极催化剂。本文主要在阴极催化剂的活性、电化学稳定性以及耐甲醇等方面展开研究。为了提高Pt/C催化剂的活性,分别对浸渍还原法、胶体法和离子交换法几种催化剂制备方法进行研究。在浸渍还原法工艺优化过程中,分别从还原剂、缓冲溶液的加入、常温浸渍时间几个影响铂粒径的主要因素进行研究,结果表明,采用HCHO作为还原剂,用Na₂CO₃/NaHCO₃做为缓冲溶液,通过15 min短时间浸渍可以制备出平均粒径更小,分散性更好的Pt/C催化剂。在改进的胶体法制备研究中,发现采用柠檬酸三钠作为稳定剂,可以获得粒径更小的铂胶粒,其中30 mass%的Pt/C催化剂平均粒径约为2.4 nm,而50 mass%的Pt/C催化剂的平均粒径为3.2 nm,催化剂的粒径明显小于传统方法制备的催化剂。在离子交换法工艺研究中发现,离子交换法可以在碳纳米管（CNT）上实现铂的均匀分散,通过重复离子交换法,可以制备出平均粒径为3.4 nm,15.4 mass%的Pt/MWNT催化剂,也可以制备出平均粒径为2.6 nm,19.2 mass%的Pt/SWNT催化剂,其对氧还原反应的催化活性明显高于传统方法制备的催化剂,而且铂在催化剂中的利用率也有很大提高。研究了碳的腐蚀对催化剂稳定性的影响。比较研究了Vulcan XC-72（XC-72）和Black Pearl 2000 （BP-2000）载体在恒电位1.2 V条件下的腐蚀行为,通过循环伏安和X射线光电子能谱（XPS）分析表明,BP-2000具有更高的腐蚀速度,相应的Pt/C催化剂的稳定性也通过加速老化测试进行研究,发现Pt/BP-2000催化剂测试后,其电化学活性面积损失为40.9 % ,而Pt/XC-72仅为20.6 %,因此Pt/XC-72催化剂具有更高的电化学稳定性,这主要是由于其载体XC-72的高稳定性。同时,通过恒电位1.2 V下氧化120 h比较研究了两种常见碳纳米管（Multi-walled carbon nanotubes, MWNT和Single-walled carbon nanotubes, SWNT）的稳定性,发现经过120 h氧化后,SWNT表面含氧量增幅明显高于MWNT,由于SWNT具有更高的有效比表面积以及表面应力,因此稳定性较低,相应的Pt/SWNT催化剂也具有较低的电化学稳定性,经过加速老化测试后,Pt/SWNT催化剂电化学活性面积降低约40%,而Pt/MWNT则为约25%。在进一步提高催化剂稳定性的研究中,采用高度石墨化的多壁碳纳米管（HG-MWNT）作为载体,可以提高催化剂的电化学稳定性。通过在2800℃下对化学气相沉积制备的MWNT进行热处理,可以提高MWNT的石墨化程度。通过XRD和Raman分析表明,经过热处理获得的HG-MWNT的石墨化程度达到95.3%,而未处理的MWNT仅为39.5%。在常温下经过加速老化测试后,Pt/MWNT催化剂性能衰减了59.7%,而Pt/HG-MWNT催化剂仅衰减了39%。对功能化后的HG-MWNT进行热处理,可以进一步提高催化剂的稳定性,在60℃加速老化测试后,Pt/Ox-HG-MWNT的电化学活性面积衰减了55%,而Pt/T -Ox-HG-MWNT仅为37%,这主要是由于热处理可以除去碳载体表面不稳定的含氧官能团,从而提高铂颗粒的稳定。另外,研究也发现,通过Pt-Co合金可以提高Pt/C催化剂的稳定性,而且通过温和的热处理,提高PtCo合金化程度,也可以提高催化剂的稳定性。在阴极耐甲醇催化剂的研究中,对常见的Pt-Co/C和Pt-Ni/C催化剂进行研究发现,在常温和60℃下,两种合金催化剂均表现出比Pt/C催化剂更高的耐甲醇性能,其中Pt-Ni/C催化剂比Pt-Co/C催化剂具有更高的耐甲醇行为。同时,研究也发现,Au对甲醇氧化反应几乎没有催化活性,而对氧还原反应具有一定的催化反应,通过采用胶体法,制备出粒径约3～5 nm的Au/C,提高催化剂的比表面积,可以提高Au/C催化剂对氧还原反应的催化活性。据此,研究设计了一种新型的Pt/Au/C催化剂,EDX和TEM等分析表明,大部分的Pt颗粒被Au分隔开,电化学测试表明,这种新型的催化剂具有很好的耐甲醇行为,同时对氧还原反应具有更高的催化活性,因此是潜在的DMFC阴极催化剂。更多还原

【Abstract】 The direct methanol fuel cell （DMFC） is a good candidate as a power source for applications in transportation and in portable electronic devices because methanol is an abundant, inexpensive liquid fuel, and it is easy to store and transport. Although good progress has been made in the development of DMFCs, the commercialization of DMFCs is, however, still hindered by a number of basic problems, including the poor kinetics of both the cathode reaction, the poor durability or short life time, and the cross-over of methanol from the anode to the cathode through the proton exchange membranes. To avoid these problems, one strategy is the development of oxygen reduction catalysts, which have a high catalystic activity for oxygen reduction reaction （ORR）, a high methanol tolerance, and a high electrochemical stability. The work in this dissertation is devoted to these issues.To improve the performance of Pt/C catalyst, the impregnation method, colloid method, and ion-exchange method were investigated, respecticely. In the study of the impregnation method, the effects of the reducing agents, buffer solution, and impregnation time on performance of Pt/C catalyst for ORR were investigated. It was found that the highly dispersed Pt/C catalyst with smaller particle size could be obtained with the reducing agent of HCHO in the Na₂CO₃/NaHCO₃ buffer solution after the 15 min impregnated time. An improved colloid method was used to prepare the Pt/C catalyst with high Pt loading. The result showed that the Pt nanoparticles were highly dispersed on the carbon support with the novel synthesis method with sodium citrate as the stabilizing agent, and then depositing the Pt nanoparticles on the carbon support. The as-prepared Pt/C catalyst had a narrow particle distribution with smaller particle size （2.4 nm for 30 mass%, 3.2 nm for 50 mass% Pt/C catalyst）. Highly dispersed platinum supported on Multi-walled and single-walled carbon nanotubes （MWNTs and SWNTs） as catalysts were prepared by ion-exchange method. The homemade Pt/MWNTs and Pt/SWNTs underwent a repetition of ion exchange and reduction process in order to achieve an increase of the metal loading. The catalysts give a Pt loading of 15.4 mass % for Pt/MWNTs, 19.2 mass% for Pt/SWNTs. The mean particle sizes of Pt/MWNTs and Pt/SWNTs catalysts are 3.4 nm and 2.6 nm. The as-obtained Pt/CNTs catalyst prepared by the ion exchange method gave a higher electrocatalytic activity for oxygen reduction and a higher Pt utilization efficiency in comparison to the one obtained by conventional method.The effect of carbon black support corrosion on the stability of Pt/C catalyst was investigated. The corrosion behaviour of Vulcan XC-72 （XC-72） and Black Pearl 2000 （BP-2000） was investigated using accelerated degradation test （ADT） by applying a fixed potential of 1.2 V. Cyclic voltammograms （CV） and X-ray photoelectron spectroscopy （XPS） results indicated that a higher oxidation degree appears on the Black Pearl 2000 （BP-2000） support. A potential cycling test from 0.6 to 1.2V was applied to the system to investigate the disabilities of Pt/C catalysts. The electrochemical measurement indicated a higher EAS degradation rate （40.9%） for Pt/BP-2000 after ADT, while it was 20.6% for Pt/XC-72 catalyst. The higher degradation rate of Pt/BP-2000 catalyst mainly resulted from the lower corrosion resistance of BP-2000. The electrochemical corrosion behaviors of single-walled carbon nanotubes （SWNTs） and multi-walled carbon nanotubes （MWNTs） were also investigated with potentiostatic oxidation at 1.2 V for 120 h. The results indicated that the increase in oxygen content on the SWNTs surface was higher than that on the MWNTs after 120 h oxidation. SWNTs exhibited higher electrochemical stability than MWNTs, which was due to the higher effective accessible surface area and local strain energy for SWNTs. Thus the degradation rate of the performance for the Pt/SWNTs catalyst （40%） was larger than that of the Pt/MWNTs catalyst （25%）.Some routes for the improvement of the stability of Pt/C catalyst were investigated. The stability of Pt/CNT could be improved by the further graphitization of CNT. The highly graphitized multiwalled carbon nanotubes （HG-MWNT） were obtained by heat treatment at 2800°C upon the as-obtained chemically vapor deposited multiwalled carbon nanotubes （CVD-MWNT）. The graphitization behavior was studied by X-ray diffraction and Raman spectroscopy. The results indicated that the obtained HG-MWNT had a high degree of graphitization （95.3%）, while it was only 39.5% for the as-obtained MWNT. Electrochemical investigation suggested that the HG-MWNT had a lower corrosion rate than the original MWCNT, which could be attributed to the less surface defects on the HG-MWNT with the increase of the graphitization degree. The durability of the corresponding Pt/CNT catalyst was discussed. The results revealed that Pt/HG-MWNT using the highly graphitized carbon nanotubes as the supporting material had a higher electrochemical stability, which was due to the lower corrosion rate of HG-MWNT and the stronger interaction between metal and carbon support. The stability of Pt/CNT could also be improved with the support of heat-treated functionalized HG-MWNT. The ADT results at 60°C indicated that the EAS loss for Pt/Ox-HG-MWNT （functionalized HG-MWNT） is 55%, while that is only 37% for Pt/T-Ox-HG-MWNT （heat-treated functionalized HG-MWNT）. The unstable carbon oxides （-COOH） could be decomposed by the heat-treatment, and the remained stabler carbon oxides improved the stability of the Pt/CNT catalyst. In addition, the stability of Pt catalyst could also be enchanced by the Pt-Co alloy. Furthermore, the PtCo/C alloy catalyst could be further improved by the increase of the alloy extent with the heat-treatment in moderate temperature.The methanol tolerance behavior of the cathode catalyst for DMFC was investigated. The electrocatalysis of the oxygen reduction reaction on carbon supported Pt and Pt–Co （Pt/C and Pt–Co/C） alloy electrocatalysts was investigated in sulphuric acid with the presence of methanol （both at room temperature and 60°C）. A higher methanol tolerance of the binary electrocatalysts than Pt/C was observed. Furthermore, as compared to Pt-Co/C catalyst, Pt-Ni/C catalyst exhibited higher methanol tolerance. Au itself was not active to methanol oxidation reaction. Au nanoparticles with small particle size （3⁵ nm） obtained by colloid method were found to enhance their catalytic activity for oxygen reduction reaction. A novel Pt/Au/C catalyst was prepared by depositing the Pt and Au nanoparticles on the carbon support. EDX and TEM results revealed that Pt nanoparticles supported on carbon supports were separated by Au nanoparticles. The electrochemical analysis indicated that the novel catalyst showed the enhanced methanol tolerance while maintaining a high catalytic activity for the oxygen reduction reaction. It is well known that Au itself is not active to methanol oxidation reaction and its addition will part block contact between Pt nanoparticles and methanol molecules, which suppresses methanol oxidation on the Pt/Au/C catalyst. Therefore, the high methanol tolerance could be ascribed to the unique surface structure of the Pt/Au/C catalyst.更多还原