【作者】 李莉；

【导师】 魏子栋；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2010， 博士

【摘要】 质子交换膜燃料电池(PEMFCs)的大规模商业化要求降低催化剂成本,提高催化剂的活性、稳定性和寿命。目前阴极催化剂主要以Pt金属为主,以现有Pt地球的存储量计算,燃料电池要商业化,Pt的担载量必须降低十倍,因此需要寻找有效的替Pt催化剂。此外,燃料电池在动态负载下造成的催化剂腐蚀、流失;阴极氧还原本身较慢的反应动力学速率;以及氧还原中间物种对金属催化剂和载体的腐蚀等因素,均需要设计和制备出高活性、高稳定性和长寿命的催化剂。直接甲醇燃料电池(DMFCs)中甲醇较慢的氧化动力学速率也是制约DMFCs商业化的原因之一,如何有效提高阳极催化剂的抗CO中毒性能,提高催化剂活性是DMFCs面临的主要问题之一。基于上述问题,本文结合理论计算和实验方法对非铂催化剂、高稳定性催化剂、甲醇氧化结构敏感反应以及PtM阳极催化剂进行研究,了解催化剂-载体相互作用及电子结构对催化剂催化行为的影响。一方面从实验现象出发,采用量化计算探讨宏观现象背后的量子化学原因,为实验进一步改进提供理论依据;一方面则从理论计算出发,探索甲醇催化剂机理和导致催化剂活性降低的本质原因,提出设计高活性和提高催化剂稳定性的理论依据,用于指导实验,并采用实验进行验证。首先,针对Pt、Pd对氧气还原(ORR)催化活性随着载体从C到TiO2改变而发生变化的实验现象,采用密度泛函方法(DFT)从理论角度研究了C和TiO2载体对Pt和Pd催化氧还原活性的影响。首先,在外加电场情况下,计算了电子给体(催化剂)与受体(氧气)之间轨道对称性、能级差以及轨道重叠程度。发现与C(110)载体相比,TiO2(110)载体可以有效地增大Pd/TiO2 HOMO轨道的空间尺寸,克服了Pd/C的HOMO与O2的LUMO空间尺寸悬殊,重叠性小,因而电子转移困难。其次,计算了ORR中间物种(Oads)在不同催化剂表面的吸附能,发现Oads在Pt/TiO2上的吸附能大于Pd/TiO2。计算的差分电子密度与分态密度显示,由于Pt与TiO2(110)表面Ti的强相互作用,Pt的d带空穴值增加,增强了Oads的吸附,阻碍了ORR后续反应的进行;而Pd与TiO2表面O的强相互作用,则削弱了中间物种Oads在Pd上的吸附,使ORR后续反应顺利进行。其次,通过将碳纳米管巯基化,制备出Pt/SH-MWCNTs催化剂。催化剂活性和稳定性测试发现:Pt/SH-MWCNTs能较好的保持催化氧还原活性,且其Pt的电化学活性表面积(ECSA)在循环伏安(CV)扫描600圈后仅降低了15%,商业化的JM-Pt/C和Pt/FMWCNTs的ECSA均降低了35%。量化计算发现,巯基化碳管S-SWNTs具有较O-SWNTs更高的抗腐蚀性能,且能提高负载Pt催化剂的抗氧化性。主要原因是:与羟基相比,巯基不仅能提供催化剂的沉积活性位,还能有效保持碳纳米管本身的完美构型、维持碳管本身的抗腐蚀性能,并降低负载Pt催化剂上含氧物种的吸附,从而提高Pt纳米粒子的氧化电位。羟基虽然也能提供催化剂沉积的活性位,但其造成碳纳米管的缺陷容易受到含氧物种的攻击,而发生氧化腐蚀。再次,甲醇的电化学氧化在Pt电极上为结构敏感反应,其不仅依赖于甲醇的脱氢步骤,还依赖于欠电位沉积Hupd的脱附、水的活化和CO的氧化。据此,本节采用电势相关的DFT方法计算Pt(111)和Pt(110)-(1×1)面上Hupd的脱附、水的活化离解和CO的氧化过程。研究发现:Pt(111)上Hupd的脱附电位较Pt(110)上更负,Hupd的脱附与H原子与Pt表面的相互作用有关;Pt(111)上水的活化离解发生电位与Pt(110)-(1×1)相同,水的离解主要受到Pt表面剩余电荷的影响;在低电位下Pt(111)更利于CO的氧化;OHad的的产生能有效推动CO的氧化去除,但OHad的形成还应与CO的产生、氧化速率匹配,过剩的OHad以及产物会占据催化剂的活性位置,从而阻碍和抑制甲醇的吸附和CO的氧化。最后,通过理论建模计算分析,从Zn、Cd、Pb、Bi、Tl、Hg、Cu、In金属中筛选出Pb、Bi、In三种掺杂金属具有提高Pt催化甲醇活性的能力。并以Pb为例,计算比较了Pt(110)和PtPb(110)上Hupd的脱附、CO和含氧物种的吸附。理论计算发现:Pb并不利于PtPb(110)上Hupd的脱附;PtPb(110)对CO的吸附较Pt(110)略有减小;对O的吸附显著增强,且Pb为含氧物种的富集源,能有效促进邻近Pt位上CO的氧化脱附,从而提高PtPb(110)的催化活性。欠电位沉积法制备的upd-Pb/Pt电极与纯Pt电极在酸性和碱性介质中甲醇的催化活性研究进一步证实了上述计算结果,即Pb修饰增强Pt催化活性的主要原因是Pb利于含氧物种的形成和吸附。更多还原

【Abstract】 Fuel cells have been recognized as clean energy-converting devices due to their high efficiency and low emission, which have taken great advances in recent decades. Despite these efforts, there are several scientific and technological difficulties hampering the widespread commercialization of fuel cells, such as (1) high Pt loading on the electrodes; (2) poor stability of the Pt-based electrocatalysts; (3) low dynamic reaction rate on anode and cathode fuel cells.Based above problems, in this dissertation, quantum chemistry and experiment method are combined to study Pt-free, highly stabile and active catalysts for oxygen reduction reaction and methanol electroxidaiton. On the one hand, based on the experiment results, theory calculation is used to establish the quantum chemistry explanation behind the experimental phenomena. On the other hand, theory calculation is performed to explore the reaction mechanisms and design the highly active catalysts. And then the results of theory calculation are validated by the experiment at last.Firstly, experimental phenomenon shows that catalytic activity of Pt and Pd for oxygen reduction reaction (ORR) changes with catalyst supports from C to TiO2. Based on the phenomenon, density function theory (DFT) was used to elucidate the cause behind the difference in catalysis caused by catalyst supports. At first, factors closely associated with the first electron transfer of the ORR were assessed in the light of quantum chemistry. Then intermediate (atomic oxygen, O) adsorption strength on the catalyst surface was calculated. The results show that, in terms of minimum energy difference, the best orbital symmetry match, and the maximum orbital overlap, TiO2 does bring about a very positive effect on catalysts Pd/TiO2 for the first electron transfer of the ORR. Especially, TiO2 remarkably expands the space size of Pd/TiO2 HOMO orbital and improves orbital overlap of Pd/TiO2 HOMO and O2 LUMO. The analysis of deformation density and partial density of state shows that the strong interaction between Pt and Ti leads to a strong adsorption of intermediate O on Pt/TiO2, but the strong interaction between Pd and surface O causes positive net charge of Pd and a weak adsorption of intermediate O on Pd/TiO2. Thus, the ORR can proceed more smoothly on Pd/TiO2 than Pt/TiO2 in every respect of maximum orbital overlap and rate delay by intermediate O. The research also discloses that several factors lead to less activity of TiO2 supported Pt and Pd catalysts than the C-supported ones for the ORR. These factors include the poor dispersion of Pt and Pd particles on TiO2, poor electric conduction of TiO2 carrier itself, and bigger energy difference between HOMO of TiO2 carried metallic catalysts and LUMO of O2 molecule due to electrons deeply embedded in the semiconductor TiO2 carrier.Secondly, carbon nanotubes (CNTs) have large surface area, excellent conductivity, and high level of chemical stability. However, the interaction between the CNTs and Pt catalysts is still one of the main problems which induce the short lifetime of catalysts. In present work, thiol (-SH) is introduced into multiwalled carbon nanotubes (MWCNTs) to synthesize the Pt/SH-MWCNTs catalysts. It shows high durability during 600 repeated potential cycles. The platinum ECSA of the Pt/SH-MWCNTs decreases about 15% even after 600 cycles, while the JM-Pt/C and Pt/FMWCNTs catalysts have lost about 35% of their platinum ECSA. The high stability of Pt/SH-MWCNTs still has high catalytic activity for ORR. To compare the effect of thiol and hydroxyl group on stability of catalysts, and explore the stability mechanisms of Pt/SH-MWCNTs caused by thiol, density functional theory (DFT) calculations are further performed. As revealed by migration and agglomerate active energy calculation for Pt particles on O-SWNTs and S-SWNTs, the interaction between the Pt and O-SWNTs or S-SWNTs has indistinctive difference. The adsorption configurations of Oad atom on Pt5/S-SWNTs and Pt5/O-SWNTs show that S-SWNTs have stronger corrosion-resistance than that of O-SWNTs, becuase the S atom keeps the perfect structure of SWNTs. The lower-lying d-band center of Pt cluster on S-SWNTs can be considered as an indication for higher oxidation-resistance of Pt. The underlying mechanism, indicated by DFT calculations, could be ascribed to that the S atom of thiol group keeps not only the corrosion-resistance of carbon nanotubes, but also enhances the oxidation-resistance of Pt particles.Thirdly, methanol electrooxidation has been proven to be a surface structure sensitive reaction on platinum (Pt). It depends on desorption of underpotential deposition (upd) of hydrogen (Hupd), methanol dehydrogenation, OHad formation, and CO oxidation. The structure sensitivity of catalysts for methanol electrooxidation has hitherto not been well studied or understood. In this work, full potential-dependent periodic DFT calculations were performed to elucidate structure sensitivity and potential-dependent difference in Hupd desorption, water dissociation and CO oxidation over low Miller index planes (111) and (110)-(1×1) of platinum single-crystal electrode surface. The results show that the Hupd desorption is easier on Pt (111) than that on Pt (110)-(1×1), the water dissociation and OHad formation occur at same electrode potentials on Pt (111) as that on Pt (110)-(1×1), and the Pt (111) favors the CO oxidation at low potentials. At anodic potentials, the difference in Hupd adsorption and desorption potential on Pt surface is due to the difference in the adsorption energy on Pt surface, and the water dissociation and OHad formation deeply depend on the surface excess electrons. The relatively weak adsorption energy of OHad and CO on platinum surface would support an easy oxidation of CO on platinum electrode.Finally, theory calculation was performed to select the doped metal M to enhance the activity of Pt for CO and methanol oxidation. Three metals Pb, Bi and In were picked out. And then the Pb doped Pt (110) surface was calculated carefully to acquire the function of Pb on the activity of Pt (110) for methanol oxidation. The results show that the Pb dope to Pt (110) surface makes no good for Hupd desorption. The adsorption of CO on Pb doped Pt (110) surface is weakened in comparison with that on Pt (110). However, the adsorption of O on Pb doped Pt (110) surface is strengthened in comparison with that on Pt (110). What’s more, CO is hardly adsorbed on Pb but oxygen species have astrong adsorption on Pb. Thus, Pb doped Pt (110) surface would be more conducive for methanol oxidation because of oxygen species richening by Pb atoms. Underpotential deposition of Pb on Pt electrode was adopted to prepare upd-Pb/Pt electrode, on which methanol oxidation in acid and basic solution was investigated. The results were identical with the conclusions from theory calculation.更多还原