| Since the destruction of environment and the challenge to thermal electric generation, fuel cell, which was born in 60s, has attracted considerable attention. Direct methanol fuel cell (DMFC) has high toxicity which contains a stimulating effect on the optic nerve and may cause blindness to limit its application. Direct ethanol fuel cell (DEFC) uses alcohol instead of the toxic methanol in the fuel cell. Ethanol is alternative to methanol because not only it is a hydrogen rich liquid to deliver 12e- per ethanol molecule providing higher energy density but also it can be obtained by fermentation process from renewable resources like corn and has very low chemical toxicity.The anode catalysts of fuel cell have being one of hot research area. For ethanol oxidation, there are several steps:the wholly dehydrogenation, the C-C bond cleavage and the oxidation of adsorbed CO to complete convert ethanol into CO2. PtRu is the most successful catalyst in the methanol and CO electrooxidation. But leaching of Ru during the reaction remains a challenging problem. Pt and Rh have very close atomic radius and enthalpy of vaporization so that the change of their mix enthalpy is nearly zero. PtRh catalysts also have high activity and become a hot issue. Further more, Rh also activated the cleavage of C-C bond.In this work, taking graphite powder as the carrier and H2PtCl6 as the precursor, Pt5/C substrate (Pt loading is 5wt.%) was prepared by chemical deposition under untrasond promotion and then characterized by cyclic voltammetry (CV) and X-ray diffraction (XRD). Ethanol electrooxidation was used as a probe reaction to test the catalytic activity of the obtained Pt5/C substrate. The effects of calcination temperature, reduction temperature and the volume ration of glycol and water in the dispersion media on the catalytic performance were investigated. Bimetallic catalysts PtRh/Pt5/C with different deposit structure (Rh0.5/Pt5/C, Pt0.1Rhx/Pt5/C and "sandwich" structure PtyRhxPty/Pt5/C) were prepared on the self-made substrate Pt5/C by calcination at 400℃followed force deposition in hydrogen(H2) bubbler 150℃. The obtained catalysts were characterized by cyclic voltammetry (CV), X-ray diffraction (XRD) and Transmission electron microscope (TEM) to research the effect of preparation atmosphere, different deposition structure and deposition amount to the PtRh/Pt5/C catalytic. The electrocatalytic activity was studied in ethanol oxidation reaction. From the researches, we may draw the conclusions:Calcined in air at 400℃for 4h where the volume ratio of glycol and water dispersion media is 2:1 shows better catalytic properties in ethanol oxidation. Pt5/C, which has similar crystal surface structure with pure Pt, is suggested the potential to replace platinum metal electrode in preparation of Pt-based surface bimetallic catalysts with high catalytic and low amount of active compounds.When deposited one layer Rh, bi-layer PtRh and "sandwich" PtRhPt on the substrate, there is regular change on the CV and XRD of catalysts. The surface of Pt0.1Rhx/Pt5/C and "sandwich" structure PtyRhxPty/Pt5/C is composed of pure Pt, pure Rh and PtRh alloy. The ratio of PtRh alloy is of a high level on the "sandwich" structures catalyst surface. It is indicated that the catalyst surface composition may change when the deposition way during preparation changed.The results of voltammograms profiles of ethanol oxidation on different catalyst show that the catalysts after reduction have better catalytic effect for ethanol oxidation. When deposited Rh on Pt5/C substrate, the peak potential of ethanol oxidation shifts negatively significantly with the decrease of peak current. When deposited different ratio of bi-layer PtRh and "sandwich" PtRhPt on Pt5/C, the catalysts catalytic for ethanol oxidation changes regularly. Rh could activate the cleavage of C-C bond and surpass Pt on oxygen absorption. However, the catalytic may decrease with excessive Rh as the weak ethanol absorption of Rh. So that the catalytic increases firstly and then decreases with the increase of Rh amount. When added an inner-layer Pt, the roughness of substrate surface rises, and the loss of mid-layer Rh reduces. As a result, Pt and Rh alloys better and the ethanol oxidation peak current increases. The surface of the prepared catalysts Pt0.1Rh0.5Pt0.1/Pt5/C-R with sandwich-like structure is showing the exceptional catalytic with ethanol oxidation peak and onset potential shifted to 30mV and 80mV negatively compared with the other prepared catalysts on the substrate Pt5/C, respectively. Moreover, the oxidation current increased 314%. |
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