| Fuel cells with a high efficiency and low/zero emission, have been attracting more and more attention in recent decades due to high-energy demands, fossil fuel depletions, and environmental pollution throughout the world. For hydrogen gas fed fuel cells at their current technological stage, hydrogen production, storage, and transportation are the major challenges in addition to cost, reliability and durability issues. Direct methanol fuel cells (DMFCs), using liquid and renewable methanol fuel, have been considered to be a favourable option in terms of fuel usage and feed strategies. Compared to hydrogen-fed fuel cells, which have a reforming unit, or low capacity in the hydrogen storage tank, DMFC uses a liquid methanol fuel, which is easily stored and transported and simplifies the fuel cell system.However, the electrooxidation of methanol is a very complex reaction, during which many intermediates and poisoning species are involved. In an acidic medium, the most efficient catalyst for this reaction is platinum and its alloys. The electrocatalytic activities of platinum particles torwards methanol oxidation are dependent on many factors. Of these, the supporting materials and their surface conditions are essential for the Pt catalyst to produce high catalytic activity. The supporting materials with high surface area are essential to reduce the noble Pt metal loading under the condition of keeping the high catalytic activity.In this paper, polyaniline(PANI), poly(o-toluidine)(POT), poly(o-methoxyaniline)(POMA), multi-walled carbon nanotubes (MWCNT) and graphite carbon paste electrodes doped with multi-walled carbon nanotubes (MWCNT), graphene(GRA), activated batteries carbon(YBC) and activated capacitors carbon(YEC), respectively, containing various mass fractions, were chosen as supporting materials for the platinum nanoparticles. The effect of the supporting materials and their preparation conditions on the electrocatalytic activity of the supported platinum nanoparticles for the methanol oxidation has been investigated in the following four aspects.(1) A comparative study on the electrocatalytic activities of Pt/PANI/GC, Pt/POT/GC and Pt/POMA/GC torwards the electrooxidation of methanolPANI, POT and POMA with various film growth charges were characterized by Cyclic voltammetry(CV), in situ UV-vis spectroscopy and electrochemical impedance spectroscopy(EIS), respectively, in order to investigate their different film structures and conductivities. Pt/PANI/GC, Pt/POT/GC and Pt/POMA/GC with various film growth charges were prepared by a constant potential method. Morphologies of the three kinds of electrodes ware characterized by SEM and their electrochemical active surface areas (EASA) and electrocatalytic activities toward the electrooxidation of methanol were also evaluated by the CV. The results show that the platinum nanoparticles electrodeposited on the PANI/GC, POT/GC and POMA/GC with various film charges exhibit different morphologies, respectively, and the electrocatalytic activities for the electrooxidation of methanol on these three kinds electrodes increase with their film growth charges at first, then followed by a drop. Except Pt/POMA/GC, the variation trends of the electrocatalytic activity of the electrooxidation of methanol versus the film thickness for the both Pt/PANI/GC and Pt/POT/GC are in great consistency with that of their EASA, Pt/POT/GC shows the best electrocatalytic activity towards the methanol oxidation followed by the Pt/PANI/GC and Pt/POMA/GC.(2) The effect of the preparation conditions of Pt/POT/GC on the electrocatalytic activities torward the electrooxidation of methanolA composite catalyst, Pt/POT/GC, has been successfully prepared by the electrochemical method(cyclic voltammetry and potentiostatic method) and used for the electrooxidation of methanol in acidic media. The electrocatalytical activity of the as-prepared Pt/POT/GC was characterized by cyclic voltammetry and the influence of electrode preparation parameters such as the potential for Pt nanoparticles deposition, method for POT preparation and the POT film thickness have also been investigated. The results show that the best electrodeposition potential for Pt nanoparticles with best electrocatalytic activity for the electrooxidation of methanol is -0.1V. The electrocatalytic activity of Pt/POT/GC varied with its film preparation condition. When Pt/POT/GC electrode was prepared using POT electropolymerized at 0.85V with a film growth charge of 4.5mC, it has the best electrocatalytic activity towards the electrooxidation of methanol.(3) Electrodeposition behaviors of platinum nanoparticles on POT/MWCNT/GC and their electrocatalytic activities toward the electrooxidation of methanolPt/POT/MWCNT/GC electrodes were successfully prepared by a potentiostatic method and the morphologies of the platinum nanaoparticles on them ware characterized by SEM. The influence of POT film structure and its film growth charge on the electrocatalytic activity of Pt/POT/MWCNT/GC towards methanol oxidation and the kinetics of electrooxidation of methanol on Pt/POT/MWCNT/GC were investigated by cyclic voltammetry and constant potential method. Compared with GC, the polymerization rate of POT electrodeposited on MWCNT/GC is much faster and the electroactivity of the POT/MWCNT/GC is also much higher, due to the existence of MWCNT which increase the electrodeposition surface area of POT. The electrodeposition behavior of platinum nanoparticles changed with the POT film growth charges would affect their morphologies, particle sizes and dispersities electrodeposited on POT/MWCNT/GC and would further affect their electrocatalytic activities toward the electrooxidation of methanol. when the film growth charge of POT is 1.0mC, the best electrocatalytic activity of methanol oxidation on Pt/POT(1.0mC)/MWCNT/GC is obtained. The kinetical study show that the electrooxidation of methanol on the Pt/POT/MWCNT/GC mainly is diffusion-controlled process when the potential scan rate is between 10 and 100mV-1, However, the diffusion-controlled process and the adsorption process are concurrence when the potential scan rate is between 200 and 1200mVs-1. (4) platinum nanoparticles electrodeposited on various doped graphite carbon paste electrodes and their electrocatalytic activities toward the electrooxidation of methanolVarious liquid oil-bound carbon pastes have been prepared from the mixtures of graphite powders and paraffin(CPE), multi-walled carbon nanotubes and graphite powders and paraffin(MWCNT+CPE), graphene and graphite powders and paraffin (GRA+CPE), activated batteries carbon and graphite powders and paraffin (YBC+CPE), activated capacitors carbon and graphite powders and paraffin(YEC+CPE), respectively, keeping the mass of carbon mixture materials 0.5g and the mass of paraffin 0.325g. POT/CPE and POT/YBC(14%)+CPE(14% refers to the mass fraction of YBC in the carbon mixture materials) with various POT film growth charges were also frabricated. Platinum nanoparticles were successfully electrodeposited on Y+CPE(Y refers to the various carbon materials mentioned above except graphite powders), POT/CPE and POT/POT/YBC(14%)+CPE by a constant potential method. The morphologies of platinum nanoparticles ware characterized by SEM and their EASA and electrocatalytic activities towards the electrooxidation of methanol were evaluated by the CV. The results show that the composite electrode Pt/POT(6.5mC)/YBC(14%)+CPE has best electrocatalytic activity torwards the electrooxidation of methanol. |
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