【作者】 牛秀红；

【导师】 刘世斌；

【作者基本信息】 太原理工大学 ， 高分子化学与物理， 2007， 硕士

【摘要】 甲醇质子交换膜燃料电池（亦称直接甲醇燃料电池，简称DMFC）是以液体甲醇为燃料，无需复杂的燃料重整，具有能量密度高，无污染，环境友好等优点，是燃料电池技术发展的重点。然而，DMFC存在两个重大问题：一是阳极电催化剂的催化活性偏低；二是“甲醇的渗透”，即甲醇由阳极透过膜电解质到达阴极，在阴极氧还原反应和甲醇氧化反应同时发生，形成“混合电位”，导致阴极电位负移和电池效率显著降低。一般，“甲醇渗透”问题可采用低甲醇渗透率电解质或抗甲醇性氧还原电催化剂的方法解决。就新型氧还原电催化剂而言，必须具有以下两个特点：①高抗甲醇性，即甲醇的吸附和氧化不影响氧还原反应；②与Pt相比，催化剂对氧还原反应具有高交换电流密度。M_xRu_ySe_z（M=Mo、W、Fe、Ni）类催化剂在氧还原活性、抗甲醇性、稳定性及价格方面优势突出，具有良好的发展前景。但在活性上与Pt／C相比仍有一定差距，因此，进一步提高活性是其应用的关键，展开相关方面的研究对该催化材料的应用具有重要理论和实际意义。M_xRu_ySe_z类催化剂，氧电还原反应主要在Ru核表面进行，加入配体元素形成M-Ru合金，可改变Ru的d电子结构或增加活性中心，改变氧分子、甲醇分子的吸附能，从而提高氧电还原的催化活性，降低甲醇电氧化的活性。因为Pt的d％值较小，氧在Pt活性位上具有较高的吸附能，在M_xRu_ySe_z内掺杂适量Pt可为催化剂表面增添高效的弱化O-O键的Griffiths吸附活性位，可望明显改善活性，同时保持原催化材料的抗甲醇性。为此，本论文采用低温（139℃）合成方法制备掺杂Pt的Ru-Se纳米簇合物Pt-M-Ru-Se（M=Mo、W）催化剂，采用XRD、EDS、XPS表征结构及表面组成，旋转圆盘电极的动电位和交流阻抗表征催化剂活性及抗甲醇性。结果表明：（1）Pt-Mo-Ru-Se纳米簇合物的催化性能与掺杂Pt的比例有明显关系。无甲醇时Mo-Ru-Se的氧还原起始电位为-0.1 V，峰电流为96 mA·mg^-1，而掺杂5 wt．％Pt的Mo-Ru-Se起始电位提高为-0.05 V，峰电流增大为213 mA·mg^-1，达到Pt峰电流的70％，催化活性因掺杂了Pt明显提高。Mo-Ru-Se纳米簇合物中掺杂Pt后起始还原电位正移，反应过电位减小，动电位曲线的峰电流增大。（2）W-Ru-Se纳米簇合物掺杂Pt后其催化性能明显改善。无甲醇时W-Ru-Se的氧还原起始电位为-0.1 V，峰电流为140 mA·mg^-1；而掺杂5wt．％Pt的W-Ru-Se起始电位提高为0.15 V，过电位减小250 mV，峰电流增大为310 mA·mg^-1，是W-Ru-Se的2.2倍，Pt的1 03％，催化活性大大提高。有甲醇存在时，Pt-W-Ru-Se（5 wt．％Pt）的活性及抗甲醇性远远优于Pt；与W-Ru-Se相比其活性明显提高，而抗甲醇性没有得到显著增强。总之，M-Ru-Se（M=Mo，W）掺杂Pt的催化剂的活性存在Pt-W-Ru-Se＞Pt＞Pt-Mo-Ru-Se＞Mo-Ru-Se＞W-Ru-Se的关系。（3）催化剂结构对还原氧性能有一定的影响。在催化剂的制备过程中，加入Pt（CO）₂与其它反应物的时间间隔越短，催化剂活性越高，抗甲醇性也越强。更多还原

【Abstract】 The use of methanol as energy carrier and its direct electrochemical oxidation in a direct methanol fuel cell （DMFC） represents an important challenge for the polymer electrolyte fuel cell technology, since the complete system without a reformer and further gas treatment steps.However, there are two main technique problems in direct methanol fuel cell. One is anode catalysts with low activity for methanol oxidation, the other is the methanol crossover through the polymer electrolyte leads to a mixed potential at the cathode, which results from the oxygen reduction reaction （ORR） and the methanol oxidation occurring simultaneously. This effect causes a negative potential shift at the cathode and a significant decrease of performance in a DMFC. For the latter, the problem should be solved by using electrolytes with lower methanol permeability or by developing new cathode catalysts. So a new type catalyst must have two characteristic, first, cathode catalysts in a DMFC should show a high methanol tolerance, that means the oxygen reduction will not be affected by the adsorption and oxidation of methanol. Secondly, the catalysts should show a higher exchange current density for oxygen reduction compared to Pt.There are some kind of methanol-tolerant catalysts. For example, Pt-M alloy, macrocyclic complexes, transition metal oxides, transition metal sulfides based on Chevrel phases, amorphous transition metal sulfide phases, etc. The prevailing view still considers carbon-supported Pt alloy to be the most efficient cathode atalysts for oxygen reduction. But there are two disadvantages: low methanol-tolerant and economic feasibility. Contrary, in recent years, amorphous transition metal sulfide phase catalysts have been proved that they have more superiorities in activity, methanol-tolerant, stability and the price. So transition metal sulfide phase catalysts were considered a substitute of Pt in the future.Ruthenium-chalcogenide clusters M_xRu_ySe₂ （M = Mo, W, Fe, Ni, 0.02<x<0.04, 1<y<3, z=2） have shown high catalytic activity, methanol-tolerant stability and low price. In contrast to platiunum, there is some gap in oxygen reduction in activity. So It is necessary to increase the activity and investigate theory and applications.M_xRu_ySe_z compounds have a core of ruthenium atoms, which is the center of oxygen reduction. Doped ligand element and ruthenium can formate M-Ru alloy, which change d-state of Ru or increase activity site. Because Pt have high activity site for oxygen reduction, doping Pt to Mo-Ru-Se、W-Ru-Se improve catalytic activity. So the Pt-M-Ru-Se （M = Mo、W） catalysts were synthesized by low temperature （139℃） method and characterized by XRD, EDS and XPS. The activities were measured by potentiodynamic and AC impedance techniques.The results show that:①The activity and methanol-tolerant on Mo-Ru-Se catalysts doped with platinum were greatly heighten, and the oxygen reduction reaction on them has higher current density and lower overpotential than that of platinum-free Mo-Ru-Se catalysts under the same conditions. The peak current density of platinum-free Mo-Ru-Se is 96 mA·mg^-1 and the potential is -0.1V. The optimum of Pt loading is about 5 wt.%, and the peak current density is 213 mA·mg^-1, approach 70 % that of Pt, and the potential is -0.05 V.②The oxygen reduction performance is greatly affected by W-Ru-Se catalysts doped with platinum. The current density is 140 mA·mg^-1 and the potential is -0.1V in 0.5 mol·L^-1 H₂SO₄. Under the same conditions, Pt-W-Ru-Se （5 wt.%Pt）, the peak current density is 310 mA·mg^-1, approach 2.2 times that of W-Ru-Se, 103 % that of Pt, and the potential is 0.15 V. In the presence of CH₃OH, Pt-W-Ru-Se （5 wt.%Pt） is better than Pt in activity and methanol-tolerant, but comparing to W-Ru-Se, Pt-W-Ru-Se （5 wt.%Pt） is not markedly improved in methanol-tolerant.In short, the current densities have the relation of i_Pt-W-Ru-Se > i_Pt > i_Pt-Mo-Ru-Se> i_Mo-Ru-Se> i_W-Ru-Se.③The structure of catalyst affect catalytic performance of oxygen reduction. In the synthesis process of catalyst, the shorter interval of adding Pt（CO）₂ and other reactants, the better activity of catalyst and methanol resistance.更多还原