| 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.
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