【作者】 叶飞；

【导师】 林维明；

【作者基本信息】 华南理工大学 ， 工业催化， 2009， 博士

【摘要】 直接甲醇燃料电池(DMFC)具有高的比能量和比功率,环境污染少,可在室温下工作,甲醇价格低廉及可与现有加油系统兼容等优点,是最有希望用于电动汽车及各种可移动设备的动力电源。目前DMFC的性能和成本仍不能满足商业化应用的要求,主要是因为技术上仍存在较大问题,如阳极催化剂活性低、甲醇渗透等。尤其是阳极催化剂活性低的问题更加突出,导致贵金属使用量高,DMFC成本高,限制了它的商业化应用。鉴于此,本论文的主要研究内容是开发容易制备、成本较低和具有较好催化活性的阳极碳纳米管(CNTs)负载PtRu基催化剂,借助各种电化学研究方法和表征手段研究催化剂上纳米粒子的晶体结构、粒径、各组分价态及相互作用与其甲醇电氧化活性的关系;以自制的CNTs负载PtRu基阳极催化剂制备膜电极,并组装单电池,探讨操作条件对电池性能的影响,采用交流阻抗谱分析阳极的电化学行为。以碳纳米管为载体,采用微波协助乙二醇还原法,制备了PtRu/CNTs催化剂和新型的PtRuNi/CNTs、PtRuMo/CNTs催化剂,考察了CNTs的预处理条件和催化剂制备条件,优化了PtRuMo/CNTs催化剂的组成。在混酸溶液中以超声频率为60kHz的超声波处理的CNTs具有最佳的含氧基团,用其作为载体制得的PtRu/CNTs催化剂的甲醇电氧化活性好于其它频率处理的CNTs负载PtRu催化剂及商品PtRu/C催化剂。合成溶液的pH=9时,反应后溶液pH=3.86,溶液中存在适量的乙醇酸根稳定剂,也提供了更好的金属纳米粒子吸附环境,制得的PtRu/CNTs催化剂具有最佳的甲醇电氧化活性。热处理有利于残留有机物的脱除,提高催化剂的活性;但过高的处理温度会增大金属粒径,减少活性物种RuOxHy的含量,降低催化剂的活性,得出的最佳热处理温度为160℃。稳态极化曲线和CO溶出伏安曲线测试表明,PtRu基催化剂的催化活性顺序为PtRuMo/CNTs > PtRuNi/CNTs > PtRu/CNTs。PtRuMo/CNTs催化剂不仅具有较高的抗CO中毒能力,还有较大的电化学活性表面积。采用组合方法筛选出PtRuMo/CNTs催化剂最佳组成,原子比Pt:Ru:Mo为6:3:1。采用交流阻抗谱研究了CNTs负载PtRu基催化剂的甲醇电氧化性能,并建立等效电路对阻抗谱数据进行模拟。结果表明,PtRuMo/CNTs对应的电荷转移电阻Rct值最小,具有最好的甲醇电氧化活性。Rct和感抗Lco都随着电位和甲醇浓度的增加而减少,表明甲醇脱氢和COad氧化反应都随着氧化电位和甲醇浓度的增加而加快。在低电位下(350~400mV),大部分催化剂表面的活性位被COad占据,不能及时氧化成CO2,造成催化剂中毒,此时甲醇脱氢反应为速率控制步骤。随着电位的增加(450~600mV),RuMoOx位上生成的OHad增加,COad逐渐被氧化,释放出来的Pt活性位能够继续吸附和氧化甲醇分子,导致感抗效应的出现,COad氧化反应变成速率控制步骤。XRD、FT-IR和TG测试表明,超声处理不影响CNTs的晶体结构,在其表面产生了如羟基、羧基等含氧基团。随着超声频率的增加,CNTs表面含氧基团的含量先增加后下降,超声频率为60kHz处理时达到最大值。XRD、TEM和XPS分析结果显示,PtRu基催化剂均为Pt面心立方晶系,谱图中没发现Ru、Ni、Mo及其氧化物的晶相存在,PtRu金属粒径随着pH的增加而降低,随热处理温度的升高而增加;金属粒子在CNTs载体上均匀分布,团聚较少,金属粒径主要集中在2~4nm。PtRu基催化剂中Pt、Ru、Ni和Mo均存在多种价态物种,未发现Ni(0)和Mo(0)金属,Ni和Mo的添加使Pt 4f结合能向低结合能方向移动,表明Ni和Mo的电子云密度向Pt偏移。以自制的碳纳米管负载PtRu基阳极催化剂制备膜电极,并组装单电池。结果表明,60℃时使用PtRu/CNTs、PtRuNi/CNTs和PtRuMo/CNTs催化剂的单电池最高功率密度分别为55.88和57.60和61.32mW/cm2,使用PtRuMo/CNTs催化剂的电池显示出最高的放电性能,已达到国内报道的使用进口商品PtRu催化剂的电池最高功率密度水平。通过小电流放电活化膜电极,改善了电极三相反应区域和Nafion的分布状况,从而提高了催化剂的电化学活性表面积。单电池适宜的运行条件为:甲醇浓度2~2.5M,甲醇流量1~2mL/min,氧气流量100~150mL/min,在此条件下单电池在25℃、40℃和60℃下的最高功率密度分别为23.54、38.24和61.32mW/cm2。阳极交流阻抗谱分析表明,电荷转移电阻Rct、阳极/膜界面电阻Ri、催化层电阻Rc和低频电感Lco都随着电位和温度的增加而下降,其中阳极/膜界面逐渐偏向电容特征,而催化层反应区扩大,变成多孔结构。温度升高,提高了膜的质子迁移能力,降低了膜电阻Rm。更多还原

【Abstract】 Direct methanol fuel cell (DMFC) is the most promising power source for electric cars as well as various portable equipments due to its favorable advantages, such as high specific energy and power densities, less environmental impact, room temperature operation, cheap liquid fuel, compatibility to the current gas system, and so forth. At present, the performance and cost of DMFC still have not met the need of commercial applications because of some big technical problems, such as low activity of the anode catalysts and methanol crossover. The problem of the low activity of the anode catalysts is especially more serious, leading to the high metal loading and high cost of DMFC, which restricts its commercial applications. In order to solve this problem, the main research contents of this thesis is to develop the anode carbon nanotubes (CNTs) supported PtRu based catalysts with simple preparation procedure, low cost and better catalytic activities, and to study the relationship between the methanol electro-oxidation activities and the crystal structure, particle seizes, oxidation states as well as the interaction of metal nanoparticles on the catalyst surface, by means of various electrochemical methods and catalyst characterization techniques. The home-made CNTs supported PtRu based catalysts were used as the anode catalysts to prepare membrane electrode assemblies (MEAs) and a single DMFC, then the effect of various operating conditions on the performance of DMFC was investigated, and the electrochemical characteristics of the DMFC anode were explored by AC impedance spectroscopy.The PtRu/CNTs and novel PtRuNi/CNTs and PtRuMo/CNTs catalysts were synthesized by microwave assisted polyol reduction method, and the pretreatment condition of CNTs, the preparation conditions of the catalysts and the optimal composition of PtRuMo/CNTs catalyst were studied. The CNTs treated by the ultrasonic frequency of 60 kHz in mixed acid solution resulted in the optimal content of the oxygen containing groups on the surface, and the prepared PtRu/CNTs catalyst showed better methanol electro-oxidation activity when compared to the commercial PtRu catalyst and PtRu/C catalysts supported on CNTs treated with other ultrasonic frequencies. The pH value of the reaction solution was 3.86 when the initial pH value was 9, at this condition, there was appropriate amount of stabilizer glycolate anion in the solution and better absorption environment for metal nanoparticles, therefore the synthesized PtRu/CNTs catalyst showed the best methanol electro-oxidation activity. The heat treatment of the catalysts could facilitate the removal of the remaining organics to enhance the catalytic activities; however, heat treatment with higher temperatures could increase the particle sizes of the metal nanoparticles and decrease the content of active species RuOxHy, resulting in lower catalytic activities. It was found that the optimal heat treatment temperature is 160℃. The results of stationary polarization cures and CO stripping voltammograms of methanol electro-oxidation over CNTs supported PtRu based catalysts revealed that the order of decreasing activities of the catalysts was PtRuMo/CNTs > PtRuNi/CNTs > PtRu/CNTs. PtRuMo/CNTs catalyst not only showed better CO-tolerant ability, but also had larger electrochemical active surface area. A combinatorial method was used to selected the optimal composition for PtRuMo/CNTs catalyst, and the results demonstrated that the optimal atomic ratio of Pt:Ru:Mo was 6:3:1.The methanol electro-oxidation performance of CNTs supported PtRu based catalysts was investigated by AC impedance spectroscopy, and equivalent circuits were established to module the impedance data. The results showed that the value of the electron transfer resistance Rct of PtRuMo/CNTs catalyst was the smallest among the three CNTs supported PtRu based catalysts, revealing that PtRuMo/CNTs had the best activity for methanol electro-oxidation. The Rct and inductive resistance Lco decreased with increase in the potentials and methanol concentrations, which means that the reaction rates of methanol dehydrogenation as well as the COad oxidation increase as the potentials and methanol concentrations increase. At low potentials (350~400 mV), most of the catalyst surface is occupied by COad which can’t be oxidized to CO2 in time, resulting in the poison of the catalysts, at this condition, the rate determining step is methanol dehydrogenation reaction; as the potentials increase (450~600 mV), the amount of OHad produced on the RuMoOx sites increase, so COad is gradually oxidized, therefore the released Pt active sites can continually absorb and oxidize methanol molecules, leading to the appearance of the inductive effect, at this condition, the rate determining step is COad oxidation reaction.The results of XRD, FT-IR and TG measurement showed that the ultrasonic treatment had no impact on the crystal structure of CNTs, and the oxygen containing groups, such as hydroxyl, carboxyl, and so forth, were produced on CNTs surface; the content of the oxygen containing groups first increased and then decreased with the increase in the ultrasonic frequencies, reaching a maximum value at the ultrasonic frequency of 60 kHz. The results of XRD, TEM and XPS characterizations revealed that there were crystalline face-centered cubic phase of Pt in all the catalysts; no crystalline Ru, Ni, Mo and their oxides were detected in the XRD spectra; the metal particle sizes decreased as the pH value of the reaction solution increased, while the metal particle sizes increased as the heat treatment temperature of the catalysts increased. The metal particles were well dispersed on the CNTs surface with little agglomeration, and the particle sizes were centered in the range of 2~4 nm. There were multi-value states of Pt, Ru, Ni and Mo species in the catatlysts, however no Ni(0) and Mo(0) metals were found. The addition of Ni and Mo to the PtRu catalyst made the binding energies of Pt 4f shift towards lower binding energies, demonstrating that the electron densities of Ni and Mo move to that of Pt.The home-made CNTs supported PtRu based catalysts were used as the anode catalysts to prepare MEAs and a single DMFC. The results showed that the highest power densities at 60℃of the single DMFC with PtRu/CNTs、PtRuNi/CNTs and PtRuMo/CNTs catalysts were 55.88, 57.60, 61.32 mW/cm2, respectively. The DMFC with PtRuMo/CNTs catalyst showed the best cell performance, which reached the highest power density level, reported in China, of the single DMFC with the imported commercial PtRu anode catalyst. The MEA activation of discharging with small current density improved the three reaction zone and Nafion distribution in the electrode to enhance the electrochemical active surface area of the catalysts. The appropriate operating conditions for DMFC were: methanol concentration of 2~2.5 M, methanol flow rate of 1~2 mL/min, oxygen flow rate of 100~150 mL/min, under these conditions, the highest power densities of DMFC at temperature of 25, 40 and 60℃were 23.54, 38.24 and 61.32 mW/cm2. The results of AC impedance spectroscopy of the DMFC anode showed that the charge transfer resistance Rct, anode/membrane interface resistance Ri, catalyst layer resistance Rc and inductive resistance Lco decreased as the potential and cell temperature increased, and the anode/membrane interface tended to capacitance characteristic, while the reaction zone of catalyst layer extended and changed to a porous structure. The increase of cell temperature enhanced the proton migrating ability of proton exchange membrane (PEM) and reduced the PEM resistance Rm.更多还原