| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系所 === 103 === Direct methanol fuel cell (DMFC) is a device converting fuel into electric energy, generally applied to the portable electronic products. Pt-Ru is the major binary alloy catalyst presently used in the methanol oxidation to delete CO poisoning effects which reducing Pt activity. According to our previous results, Pt nano-particles were successfully reduced on ferrous phosphate (Fe3(PO4)2·8H2O, Vivianite), assigned to Pt/Vi and then uniformly mixed with the carbon black to obtain Pt/Vi/C bi-function catalyst, revealing the excellent electrochemical activity in methanol oxidation without carbon monoxide (CO) poisoning effects. However, ferrous phosphate salts that formed by Iron(II) ammonium sulfate and Diammonium hydrogen phosphate has shown poor electronic conductivity and ion conductivity. Therefore, in this study, we attempted to prepare the precipitation of ferrous phosphate directly deposited on the acetylene carbon black, the surface of which was modified to generate more OH- radicals for strengthening the bonding between vivianite(Vi) and XC72 to improve its conductivity.
Three kinds of weight ratio of XC72 to Vi 1: 3, 1: 5 and 1: 10 were prepared. There were unobvious peak in XRD, referring to amorphous iron phosphate. Then H2PtCl6 were added in for platinum reduced on XCVi composite to form Pt/XCVi 1:3, Pt/XCVi 1:5 and Pt/XCVi 1:10 catalysts and finally annealed at 100℃ for 3 hrs. The features of these catalysts were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy – live fast Fourier transform (HRTEM-Live FFT), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-mass spectrometry (ICP-MS), Fourier transform infrared (FTIR) spectroscopy, membrane assembly electrode (MEA) and cyclic voltammetry (CV).
The electric resistance has been reduced from 268.0 of Pt/Vi to 162.5, 119.0 and 137.4 Ω of Pt/XCVi 1:3, 1:5 and 1:10, respectively. The bigger amorphous iron phosphate particles of Pt/XCVi 1:3 leads to the greater resistance among Pt/XCVi catalysts. The particle size of platinum was about 3.7 nm by XRD analyses and from 3.9 to 4.5 nm observed by TEM. The electrochemical surface area (ECSA) ranged from 441.25 to 649.18 (cm2/mg) while ranged from 623.19 to 1016.89 (cm2/mg) after two times XC72 added in and assigned to Pt/XCVi/C, analysed by cyclic voltammetry of hydrogen absorption / desorption reaction. In methanol oxidation reaction (MOR), Pt/XCVi1:5 illustrates the highest mass activity 132.55 (A/gPt) and the lowest on-set potential 0.25(V). No forward or reverse current peaks were observed, indicating no CO poison effects for all Pt/XCVi catalysts. For 1000 cyclic life tests, the retained capacity of Pt/XCVi 1:3 reached up to 96.12% compared with 81.45% of Pt/XCVi 1:3/C.
In membrane electrode assembly (MEA) tests, three prepared Pt/XCVi catalysts revealed the greater power density and open circuit voltage than Pt/C (Alfa) and Pt/Vi, since Fe3+ in these catalysts activated the water at low potential to yield Fe-OHads and detoxify the nearby nano-sized Pt poisoned by CO to create the opportunity for forming CO2 and H+, beside the enhanced conductivity. This argument also reasons results of MOR tests out. Obviously, both electric resistance and CO poisoning effect play the major role on the performance of catalysts in DMFC, and the Pt/XCVi improved from Pt/Vi has revealed the acceptable performance due to its enhanced conductivity and detoxifying function.
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