【作者】 杨波；

【导师】 刘佩芳；

【作者基本信息】 武汉大学 ， 物理化学， 2010， 博士

【摘要】 燃料电池具有高效、清洁等优点,被认为是未来的备选能源之一。以氢为燃料的质子交换膜燃料电池(PEMFC)已经达到相当高的技术水平,但氢气储运的困难和安全问题成为其商品化的主要障碍。研究者们在企图解决这一问题的同时,开始寻求新的燃料,直接甲醇燃料电池(DMFC)和直接甲酸燃料电池(DFAFC)成为研究的热点。Pt/C、PtRu/C和Pd/C是上述三种燃料电池最常用的催化剂。无论何种燃料电池,都需要适于规模生产的高效电催化剂的制备方法。本论文工作对最适合批量生产的浸渍法进行较全面的研究,探究控制催化剂品质的关键,最终获得了高分散的PtRu/C、Pt/C和Pd/C催化剂,并对其物理化学性质及相关电催化进行了研究。主要的工作内容与结论如下：1、高分散、高载量PtRu/C催化剂的浸渍法制备及表征发展了一种简单的易实现规模化制备的催化剂合成方法,整个过程由“浸渍-干燥-氢气还原”三个步骤构成,无需过滤洗涤。即便采用含Cl-前驱体,也可获得分散度很高的金属载量为60wt%的PtRu/C催化剂。TEM分析表明,所制60wt%PtRu/C的金属粒径为1.5±0.5nm；通过EDAX, XRD, XPS和TGA/DTA等分析发现,制备的PtRu/C催化剂中含PtRu合金与非晶态RuOxHy。电催化研究表明,所制PtRu/C对甲醇氧化具有优越的性能,可能与催化剂中含非晶态RuOxHy有关。2、PtRu/C催化剂的热重分析热重分析(TGA)是文献中用于指认PtRu催化剂中RuOxHy组分的常用实验方法,通常将150-600℃下的催化剂失重归结为RuOxHy的失水。我们通过TG-FTIR联用分析发现,在上述温度区间催化剂失重的主要产物是C02,没有发现可以检测的H2O。因此此温度范围内催化剂的失重应该归因于碳载体在PtRu催化下的氧化,而且氧的来源主要是催化剂中Pt表面的氧与氧化钌中的氧。此研究对PtRu/C催化剂的热重行为产生了不同于文献的认识,TGA并非分析PtRu/C催化剂中非合金钌组分的有效方法。3、高分散Pt/C催化剂的浸渍法制备对Pt/C催化剂浸渍法制备过程中的关键实验参数进行探究,发现获得高分散Pt/C催化剂的关键因素包括：(1)采用大比表面积的载体有利于获得小粒径的Pt/C。(2)热浸渍和超声结合搅拌是我们的方法与传统浸渍法的重要区别,也是获得高分散Pt/C的关键。(3)氢气还原的温度须控制在80-150℃之间,过高的还原温度导致Pt粒径增大。(4)浸渍后凝胶态的含水量对Pt粒径有影响,水碳质量比在5-20范围内,催化剂的粒径为2.5nm左右。4、Pd/C催化剂的浸渍法制备以PdCl2为前体,采用浸渍法制备Pd/C催化剂很难获得高的分散度。研究发现,Cl-的存在和还原气体的种类是影响Pd/C粒径的两大因素。采用Pd(NO3)2为前体,并以Ar+H2混合气或CO代替纯氢气作为还原剂,可以显著提高Pd/C催化剂的分散度。优化条件下制得的20wt%Pd/C的Pd粒径为3.5nm,10wt%Pd/C的Pd粒径为2.7nm。5、Pd/C催化剂的电催化粒度效应研究对粒径分别为2.7、3.5、4.7、6.1、8.1nm的Pd/C催化剂进行氢氧化、氧还原、甲酸氧化等反应的粒度效应研究。结果表明,Pd粒径越小,催化剂与氧原子结合力越强,氧还原反应动力电流密度随着粒径增大而提高。对于氢氧化反应,随着粒径增加,反应交换电流密度增大；4.7nm Pd/C的氢氧化交换电流密度为0.21mAcm-2,约为Pt的百分之一。对于甲酸氧化反应,4.7nm的Pd/C催化剂具有最高的质量比活性和面积比活性。更多还原

【Abstract】 Fuel cells are an efficient, green power source. The hydrogen powered proton-exchange-membrane fuel cells (PEMFC) has seen great advance in recent decades. However, the commercialization of PEMFC is still hampered by a few very challenging factors, among which is the storage and transportation of hydrogen. As an alternative resolution, direct methanol fuel cells (DMFC) and direct formic acid fuel cells (DFAFC) are developed to make use of the hydrogen-rich liquid fuels. In these types of fuel cells, the catalysts are somewhat different, involving Pt/C, PtRu/C, and Pd/C, but the synthetic method for catalysts may share common features. In particular, a simple and efficient method suitable for massive production is highly demanded.This thesis is designed to be an in-depth study of the impregnation preparation for PtRu/C, Pt/C, and Pd/C catalysts. Controlling parameters of this method have been systematically investigated, with rich characterizations on the resulting highly-dispersed catalysts. Relevant electrocatalytic processes, such as the methanol oxidation reaction (MOR), oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), and formic-acid oxidation reaction (FAOR), are also studied. The main results are summarized as follows: 1. Preparation and characterization of highly dispersed PtRu/C catalystsAn improved impregnation method was developed, involving three steps: impregnation, drying, and hydrogen reduction. According to HRTEM analyses, the resulting 60 wt% PtRu/C catalysts are highly dispersed with particle size of 1.5±0.5 nm. Further EDAX, XRD, XPS, and TGA/DTA characterizations show that in addition to PtRu alloy, the catalyst also contains amorphous RuOxHy, which turns out to be a key factor for the promotion in the catalytic activity (CA) toward the MOR. 2. Thermogravimetric analysis of PtRu/C catalystsThe thermogravimetric analysis (TGA) was a commonly-adopted method to identify the presence of RuOxHy in PtRu/C catalysts; the weight loss between 150℃and 600℃has been assigned to the loss of structural water of RuOxHy. We have combined TGA and FTIR to unravel the origin of the weight loss within this temperature range, and found that the resulting gas was CO2 rather than H2O, which can only be attributed to the oxidation of the carbon support by the oxygen species on Pt surface or in the amorphous ruthenium component. This finding is opposite to the observation in previous reports, and points out that TGA may not be an adequate method for analyzing the amorphous ruthenium component in PtRu/C catalysts.3. Optimized preparation of highly dispersed Pt/C catalystsWith systematic optimization on the impregnation method, we find the keys to achieve highly dispersed Pt/C catalysts, which include:(1) Large specific surface area of the carbon support is necessary. (2) The hot impregnation and the sonication are featured procedures of our method to achieve the high dispersion. (3) The temperature of hydrogen reduction is better to be 80-150℃, above which larger Pt particles will result. (4) The water content in the gel obtained from the impregnation step is a key factor; a water/carbon ratio of 5-20 results in a Pt particle size of ca.2.5 nm.4. Preparation of highly dispersed Pd/C catalystsWe find that the precursor and the reducing atmosphere are key factors for attaining highly dispersed Pd/C catalysts. Using Pd(NO3)2, rather than PdCl2, as the precursor and Ar+H2 gas or CO instead of pure H2 as the reductant, the particle size of Pd can be 3.5nm and 2.7nm in 20wt% Pd/C and 10wt% Pd/C, respectively.5. Study on the particle size effects of Pd/C catalystsPd/C catalysts with Pd particle size of 2.7,3.5,4.7,6.1, and 8.1 nm were used to study the particle size effects on fuel cell reactions. It was found that, the smaller the particle size of Pd, the stronger the adsorption of the Oads, and the lower the CA toward the ORR. For the HOR, larger Pd particles will give higher exchange current density (i0); the i0 on 4.7nm Pd/C is 0.21 mAcm-2, one hundredth of that of Pt. For the FAOR,4.7nm Pd/C exhibits the highest mass-specific and surface-specific CA.更多还原