【作者】 梁营；

【导师】 廖代伟；

【作者基本信息】 厦门大学 ， 物理化学， 2007， 博士

【摘要】 直接甲醇燃料电池（DMFC）具有燃料便宜、易于储存和携带、理论比能量高,近乎零污染排放等优点,是各种小型便携式电源的理想动力源之一。而实际DMFC性能和理论性能还有较大的差距,制备和寻找高活性的电催化剂是提高DMFC性能的重要途径,也成为研究者们孜孜以求的目标。目前,Pt基催化剂仍然是DMFC中最广泛被使用的电催化剂,通常Pt基催化剂负载在导电的高比表面的碳载体上,碳载体的种类、表面性质等在很大程度上影响着Pt基催化剂的分散程度、颗粒大小和分布等。如何制备粒径适宜、分布均匀的Pt基纳米颗粒,如何有效地提高贵金属的利用率对DMFC的性能具有重要的意义。碳纳米管有着不同于传统的碳材料的特征,在燃料电池领域正受到研究者们广泛的关注。本文主要对DMFC阳极电催化剂进行了研究,从载体入手,首先研究了负载型高分散性Pt基催化剂的制备,后考察了不同管径和管长碳纳米管负载的Pt基催化剂的性能,接着开发了碳纳米管和Vulcan XC-72碳黑复合载体负载的Pt基催化剂。在DMFC电池方面,本文最后对被动式DMFC的性能进行了初步研究。在燃料电池催化剂的制备方面,本文优化了微波协助乙二醇还原方法来制备Vulcan XC-72负载的Pt和PtRu催化剂。我们成功地合成出粒径细小,分布较窄（2-5 nm）,高分散性的纳米颗粒。研究结果表明反应溶液的pH值对催化剂颗粒以及金属负载情况影响很大,金属纳米粒子粒径随着溶液pH值的增加而减小,平均颗粒范围在3.5-1.8 nm之间。此外,当溶液pH值超过11时,由于Pt、Ru粒子被保护剂过保护以及颗粒比较小造成金属担载量减小。对制备PtRu和Pt催化剂来说,适宜的pH值分别在9和10左右。电化学表征显示,实验室制备的催化剂具有优异的性能,对甲醇氧化的活性和商品化E-Tek催化剂相当。PtRu/C催化剂比Pt/C催化剂具有更好的甲醇氧化活性,PtRu/C和Pt/C催化剂对甲醇氧化的起始电位分别为0.19 V和0.32 V,峰电位分别在0.53 V和0.62 V。但在金属含量都为20%的情况下,Pt/C催化剂对甲醇氧化的峰电流比PtRu/C催化剂的高。CO溶出实验显示,PtRu/C催化剂比Pt/C催化剂具有更低的CO氧化峰电位,以上结果间接地说明了PtRu催化剂促进甲醇氧化的双功能机理。碳纳米管具有好的电子传导性,特殊的管状结构以及更少的硫含量,作为燃料电池Pt基催化剂的载体有着独特的性能,成为现阶段研究的热点。本文首次系统地研究了不同管径和管长多壁碳纳米管（MWCNTs）负载的Pt及PtRu催化剂的性能。从TEM分析中发现,Pt基颗粒担载在较大管径、较短管长的MWCNTs上时,部分金属颗粒可以负载在管的内壁和管口上,从而增加了金属的分散性和利用率。CV和XPS结果显示,MWCNTs表面含氧官能团量随着管径和管长的减小而增加,但表面官能团并不是影响甲醇氧化活性的主要因素。电化学性能测试结果显示,甲醇的氧化活性主要是和金属的分散性相关,而金属的分散性和MWCNTs的管长和管径有着很大的关系。负载在管径为30-50 nm、管长约50μmMWCNTs上的PtRu纳米粒子的电化学比表面（ECSA）为57.6 m²g^-1。,而负载在同样管径、较短管子（约2μm）MWCNTs上的PtRu纳米粒子的ECSA达到74.4m²g^-1。此外,当MWCNTs管径相似时,Pt纳米粒子负载在较短MWCNTs上的特定活性是负载在较长MWCNTs上活性的1-4倍,从中分析我们可以得到碳纳米管的边缘效应,即碳纳米管管口原子的助催化作用,管口原子比基面原子使碳纳米管具有更高的性能。文中最后得到,合适管径（30-50 nm）、更短管长MWCNTs负载的Pt基催化剂具有最好的金属分散性和甲醇氧化活性。虽然MWCNTs具有一系列的优点,但本文研究发现MWCNTs负载的Pt基催化剂的分散性不如Vulcan XC-72负载的催化剂好,为了有效地利用两种载体的优点,接着本文开发了以碳黑Vulcan XC-72和多壁碳纳米管（MWCNTs）为复合载体的Pt及PtRu催化剂。利用BET、TEM、XRD、XPS等多种手段研究了催化剂的形貌、合金程度及表面状态等。结果表明相对于Vulcan XC-72负载的Pt基纳米粒子,MWCNTs负载的金属纳米粒子产生部分团聚现象,而复合载体负载的Pt基颗粒大部分担载在Vulcan XC-72上,少量负载在MWCNTs上,从而使金属粒子有较小的粒径,较少团聚体生成,增加了电化学比表面积。相比较Vulcan XC-72和MWCNTs,复合载体和PtRu金属之间的相互作用更强,PtRu/C+MWCNTs中具有更高的PtRu合金程度。我们对催化剂进行甲醇氧化的电化学表征,结果显示,复合载体负载的Pt基催化剂具有比单一载体负载的催化剂更好的甲醇氧化活性。在相同条件下,Pt/C+MWCNTs催化剂对甲醇氧化峰电流活性达到600 mAmg^-1,是Pt/C催化剂活性的1.4倍。PtRu/C+MWCNTs催化剂对甲醇氧化的峰电流密度为256.4 mA mg^-1,高于PtRu/C催化剂的活性(206.7 mA mg^-1),而PtRu/WCNTS和PtRu/C对甲醇氧化的活性相当。当PtRu/C+MWCNTs催化剂实际用作DMFC的阳极催化剂时,相比于其它单一载体负载的阳极催化剂,电池能得到更高的功率密度。通过一系列的分析和表征,本论文认为复合载体兼具了两种载体的优点,对甲醇氧化活性的增加主要归于催化剂更高的电化学表面积,高的合金程度及增加的金属利用率。被动式DMFC不需要泵等额外的辅助设施,结构简单,易于携带,使电池的重量和体积更容易进行微型化和商业化,被认为是最具有市场化潜力的一种燃料电池。而对被动式DMFC的研究目前还处在初步发展阶段,本论文的最后对实验室组装的被动式DMFC的性能进行了初步的研究。在室温下工作,两电池组峰值功率达到80 mW,开路电压达到1.2 V。甲醇浓度对电池性能影响较大。由于浓差极化和甲醇渗透的共同影响,随着甲醇浓度的增加,电池性能先上升后下降。本实验结果表明,在低电流密度下,低浓度甲醇具有较大的开路电压,在高电流密度下,1.0 M和2.0 M甲醇浓度下电池较早出传质极化,3.0 M甲醇浓度下电池具有最佳性能。此外,实验研究发现提高温度和空气的流动速度都可以很大程度地增加电池性能。对被动式DMFC的长期性能的研究结果显示,一次性注入3 ml 3.0 M甲醇溶液,在100 mA电流下放电,电池电压为0.2 V,单电池可连续稳定地工作4个小时。更多还原

【Abstract】 Direct methanol fuel cell（DMFC） is one of the most desirable power source for small-scale power units because of inexpensive fuel,ease of handling,high theoretical energy density and near-zero pollutant emission.However,there is large distance between the practical DMFC performance and the theoretical performance. Synthesizing and searching for electrocatlasyts with high activity are the important approach to improve DMFC performance,which also become to be the endlessly goal for researchers.At present,Pt-based catalysts are still used as electrocatalysts most widely in DMFC.Generally,Pt-base catalysts are prepared supported on carbon materials of conductive and high surface area.The support type and surface chemistry have a significant effect in the resulting metal dispersion,particle sizes and distribution.How to synthesize Pt-based nanoparticles with appropriate particle sizes, homogenous distribution and how to improve noble metal utilization effectively have important sense for DMFC.Lately,carbon nanotubes have attracted more and more attention in fuel cell due to its different characteristics from the traditional carbon supports.This dissertation mainly focused on the anode electrocatalysts for DMFC, being based on the supports,firstly studied the preparation of Pt-based catalysts with high dispersion,then investigated the Pt and PtRu electrocatalysts supported on multi-walled carbon nanotubes（MWCNTs） with various diameters and lengths,and then exploited the Vulcan XC-72 carbon and MWCNTs composite supported Pt-based catalysts.For the DMFC,last part of this paper investigated the performance of passive DMFC.For the preparation of DMFC catalyst,the dissertation optimized the microwave-assisted polyol reduction process to prepare Vulcan XC-72 supported Pt and PtRu catalysts.We synthesized the nanoparticles with small particle sizes and narrow particle distribution（2-5 nm） successfully.The experiment results indicated that the pH value of solution made important influence on the metal particle sizes and the metal loading.The metal particle sizes decreased with the increase of solution pH,the average particle sizes were in the range from 3.5 nm to 1.8 nm.Moreover,when the solution pH exceeded 11,the metal loading decreased caused by small particles and the competition reduction of Pt and Ru ions.The appropriate pH values were about 9 and 10 for PtRu/C and Pt/C catalysts,respectively.The electrochemical results indicated that the prepared Pt/C and PtRu/C catalysts had excellent activity and showed comparable methanol electrooxidation activity with the commercial E-Tek catalysts.PtRu/C catalyst had higher methanol oxidation activity than Pt/C catalyst. The onset potential for methanol oxidation was 0.19 V and 0.32 V,the peak potential was 0.53 V and 0.62 V for PtRu/C and Pt/C catalysts,respectively.But when the metal loading was 20%,the Pt/C catalyst showed the higher peak current than PtRu/C catalyst.CO stripping test also showed that PtRu/C catalyst exhibited the lower peak potential for CO oxidation than Pt/C catalyst.The results demonstrated indirectly the bifunctional mechanism of PtRu catalyst.Carbon nanotubes has the extraordinary property as supports for Pt and Pt alloy catalysts in fuel cell due to its good electronic conductivity,unique tubular structure and low sulfur content.Pt and PtRu electrocatalysts supported on MWCNTs with various diameters and lengths were studied systematically firstly in the paper.It can be found from TEM that part of metal particles can deposit on the inner walls of MWCNTs with large tube diameter and short tubes,which increased the metal dispersion and utilization.The results of CV and XPS showed that the amounts of surface oxides of MWCNTs increased with the decrease of the diameters and lengths, but the amounts of surface oxides were not the mainly factor on the methanol electrooxidation activity The electrochemical results showed that methanol electrooxidation activity was influenced mainly by the dispersion of metal in this work,and the metal dispersion connected with the tube diameters and lengths of MWCNTs primarily.PtRu nanoparticles supported on long MWCNTs（50μm） with a diameter of 30-50 resulted in the electrochemical surface area（ECSA） of 57.6 m²g^-1, while the ECSA reached 74.4 m²g^-1 when PtRu supported on short MWCNTs（2μm） with the same diameter.In addition,the specific activities of Pt supported on short MWCNTs were 1.4 times of Pt supported on long MWCNTs.It can be deduced the edge effect of MWCNTs,namely,the intrinsic activity of the ends of MWCNTs.The atoms of ends in MWCNTs had higher properties compared with the carbon atoms of side walls.It can be obtained lastly that the Pt-based nanoparticles supported on short MWCNTs with a diameter of 30-50 nm showed the best metal dispersion and highest methanol oxidation activity.Although MWCNTs had a serious of advantages,we found that the MWCNTs supported Pt-based catalysts yielded poorer dispersion than Vulcan XC-72 supported catalysts.To take use of the advantages of the two supports effectively,the dissertation then exploited Vulcan XC-72 carbon and MWCNTs composite supported Pt and PtRu catalysts.The dispersion,alloy degree and the surface states of prepared catalysts were characterized by BET,TEM,XRD,XPS.The results indicated that, compared to the Vulcan XC-72 supported Pt-based nanoparticles,MWCNTs supported metal nanoparticles resulted in aggregates partly.For the composite supported catalysts,most of metal particles deposited on the Vulcan XC-72,a small quantity of particles was supported on MWCNTs,which resulted in small particles size,a few aggregates and enhanced ECSA.Compared with Vulcan XC-72 and MWCNTs,there were the stronger interaction of composite supports and metal,which made composite supported catalysts PtRu/C+MWCNTs had the higher alloy degree.It can be found by electrochemical results that,compared to the single supported catalysts,the composite supported catalysts exhibited superior electrocatalytic activity for methanol electrooxidation.At the same conditions,the methanol oxidation activity at peak potential for Pt/C+MWCNTs catalyst got 600 mAmg^-1,which was 1.4 times to the activity of Pt/C catalyst of 430 mAmg^-1.The methanol oxidation peak current density of PtRu/C+MWCNTs catalyst(256.4 mAmg^-1) was higher than that of PtRu/C catalyst(206.7 mA mg^-1),while the PtRu/MWCNTs catalyst showed the comparable activity to PtRu/C catalyst.The DMFC performance test showed that composite supported PtRu anode catalysts yielded higher power density than single carbon supported anode PtRu catalysts.Through analysis and characterization,we attributed the higher methanol oxidation activity of composite supported catalysts to the higher electrochemical surface area,higher alloy degree and the enhancement of metal utilization. Passive DMFC have been considered as one of the most potential fuel cells for commercialization because of its much simpler structures with no pumps or other auxiliary devices,ease of carrying,easiness of micromation and commercialization of eight and volume.However,studies of passive DMFC are at initial progress stage,in the last part of this dissertation,the performance of home-made passive DMFC was studied primarily.The maximal power of two-cell stacks was 80 mW and the open circuit voltage was 1.2 V at room temperature.The methanol concentration made significant influence on the cell performance.The performance of cell ascended at the beginning then dropped with the increase of methanol concentration,which was caused by anode concentration polarization and methanol crossover.Experiment results showed that at low current density,low methanol concentration exhibited the larger open circuit voltage,but at high current density,DMFC with 1 M and 2 M methanol resulted in transport polarization earlier and 3 M methanol yielded the best DMFC performance.In addition,the increase of cell temperature and flowing air can improve the cell performance evidently.The study of long-operation performance indicated that single cell can work with the cell voltage 0.2 V stably and continuously for 4 h at 100 mA discharge at room temperature under the condition of infusing 3 ml 3 M methanol one-off.更多还原