Investigation On Preparation And Electrochemical Performance Of Co-deposited Pt-H_xMoO₃

Recently direct methanol fuel cell (DMFC) has been received widespread attention due to the abundant source, the low price, the safety of the storage and transportation of the fuel. Nevertheless, the step toward the practical application has not been gone as fast as expected. There are two reasons for this slow progress. One is low electrocatalytic activity of the anodic catalyst, the other is the poisoning of the anode catalysts by the adsorbed carbonyl species derived from methanol oxidation. In this thesis, in order to improve the electrocatalytic activity of the Pt, the electrochemical co-deposition methods by cyclic voltammetry and potentiostatic were used to prepare catalysts Pt, Pt-H_xMoO₃ and Pt-H_xMoO₃-Nafion. Several electrochemical methods including cyclic voltammetry and potential step, as well as physcial analytical methods including SEM, IR and EDX were used to analyze the morphology of catalyst and the electrocatalytic activity and stability for the methanol oxidation. Besides, the electrocatalytic mechanism of hydrogen molybdenum bronze was discussed on the base of results obtained. The main results are as follows:1. Platinum and platinum-hydrogen molybdenum bronze with various amount of platinum and/or hydrogen molybdenum bronze could be prepared on a glass carbon electrode. The real surface area of the prepared platinum increases with increasing cycle number for electrode preparation and the electrocatalytic activity of the catalysts toward methanol oxidation only depends on the real surface area of platinum. However, the electrocatalytic activity of platinum-hydrogen molybdenum bronze is related to the ratio of platinum and molybdenum. It reaches its maximum when the molar ratio of platinum and molybdenum in the solutions for the electrode preparation is 2:1. In this case the oxidation peak current of methanol on the platinum-hydrogen molybdenum is 1.63 times that on the platinum. Considering the dependence of the oxidation peak current of methanol on the real surface area and the ratio of platinum and molybdenum, it is found that there are double effects of hydrogen molybdenum bronze in the improvement of electrocatalytic activity of platinum. One is dispersing effect, the real surface area of platinum is increased by the co-deposition of platinum and hydrogen molybdenum bronze. The other is proton spillover effect, the hydrogen molybdenum bronze continually accepts proton from the methanol oxidation on platinum by the oxidation and reduction of couple H_xMO₃/H_yMO₃ (y＜x＜2), and makes it easier for the oxidation of adsorbed intermediates on platinum.2. The methanol oxidation of platinum prepared by CV on glass carbon took place in the alkaline solution at the potential 600mV more negatively than in the acid solution. The proton spillover effect from the redox couple H_xMO₃/H_yMO₃ (y＜x＜2) works in the acid solution while it does not in the alkaline and neutral solutions. The hydrogen molybdenum bronze is not stable in acid, neutral andkaline solutions, and the higher the acid concentration, the worse the stability is.3. Pt-H_xMoO₃-Nafion was prepared by cyclic voltammetry method. Compared with electrode Pt-H_xMoO₃, the electrocatalytic activity and stability of Pt-H_xMoO₃-Nation toward methanol oxidation were improved. The elecrocatalytic activity of Pt-H_xMoO₃-Nafion reaches its maximum when the content of Nation in solution is 0.012%(wt) when it is prepared by cyclic voltammetry. In this case the oxidation peak current of methanol on Pt-H_xMoO₃-Nafion is 1.758 times that on Pt-H_xMoO₃, and the electrocatalytic activity was not dropped after 5000 seconds when it was polarized in lmol/LCH₃OH+0.5 mol/L H₂SO₄ solution at 0.3V(vs Hg-Hg₂SO₄). The content of Nation in Pt-H_xMoO₃-Nafion is 0.57%(mol) from the EDX analysis. From the SEM image of the surface of Pt-H_xMoO₃-Nafion, it can be seen that the particles on Pt-H_xMoO₃-Nafion is more uniform than that on Pt-H_xMoO₃.