Investigation On Pt/C Cathode Catalyst For Direct Methanol Fuel Cell

The direct methanol fuel cell (DMFC) is a good candidate as a power source for applications in transportation and in portable electronic devices because methanol is an abundant, inexpensive liquid fuel, and it is easy to store and transport. Although good progress has been made in the development of DMFCs, the commercialization of DMFCs is, however, still hindered by a number of basic problems, including the poor kinetics of both the cathode reaction, the poor durability or short life time, and the cross-over of methanol from the anode to the cathode through the proton exchange membranes. To avoid these problems, one strategy is the development of oxygen reduction catalysts, which have a high catalystic activity for oxygen reduction reaction (ORR), a high methanol tolerance, and a high electrochemical stability. The work in this dissertation is devoted to these issues.To improve the performance of Pt/C catalyst, the impregnation method, colloid method, and ion-exchange method were investigated, respecticely. In the study of the impregnation method, the effects of the reducing agents, buffer solution, and impregnation time on performance of Pt/C catalyst for ORR were investigated. It was found that the highly dispersed Pt/C catalyst with smaller particle size could be obtained with the reducing agent of HCHO in the Na₂CO₃/NaHCO₃ buffer solution after the 15 min impregnated time. An improved colloid method was used to prepare the Pt/C catalyst with high Pt loading. The result showed that the Pt nanoparticles were highly dispersed on the carbon support with the novel synthesis method with sodium citrate as the stabilizing agent, and then depositing the Pt nanoparticles on the carbon support. The as-prepared Pt/C catalyst had a narrow particle distribution with smaller particle size (2.4 nm for 30 mass%, 3.2 nm for 50 mass% Pt/C catalyst). Highly dispersed platinum supported on Multi-walled and single-walled carbon nanotubes (MWNTs and SWNTs) as catalysts were prepared by ion-exchange method. The homemade Pt/MWNTs and Pt/SWNTs underwent a repetition of ion exchange and reduction process in order to achieve an increase of the metal loading. The catalysts give a Pt loading of 15.4 mass % for Pt/MWNTs, 19.2 mass% for Pt/SWNTs. The mean particle sizes of Pt/MWNTs and Pt/SWNTs catalysts are 3.4 nm and 2.6 nm. The as-obtained Pt/CNTs catalyst prepared by the ion exchange method gave a higher electrocatalytic activity for oxygen reduction and a higher Pt utilization efficiency in comparison to the one obtained by conventional method.The effect of carbon black support corrosion on the stability of Pt/C catalyst was investigated. The corrosion behaviour of Vulcan XC-72 (XC-72) and Black Pearl 2000 (BP-2000) was investigated using accelerated degradation test (ADT) by applying a fixed potential of 1.2 V. Cyclic voltammograms (CV) and X-ray photoelectron spectroscopy (XPS) results indicated that a higher oxidation degree appears on the Black Pearl 2000 (BP-2000) support. A potential cycling test from 0.6 to 1.2V was applied to the system to investigate the disabilities of Pt/C catalysts. The electrochemical measurement indicated a higher EAS degradation rate (40.9%) for Pt/BP-2000 after ADT, while it was 20.6% for Pt/XC-72 catalyst. The higher degradation rate of Pt/BP-2000 catalyst mainly resulted from the lower corrosion resistance of BP-2000. The electrochemical corrosion behaviors of single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) were also investigated with potentiostatic oxidation at 1.2 V for 120 h. The results indicated that the increase in oxygen content on the SWNTs surface was higher than that on the MWNTs after 120 h oxidation. SWNTs exhibited higher electrochemical stability than MWNTs, which was due to the higher effective accessible surface area and local strain energy for SWNTs. Thus the degradation rate of the performance for the Pt/SWNTs catalyst (40%) was larger than that of the Pt/MWNTs catalyst (25%).Some routes for the improvement of the stability of Pt/C catalyst were investigated. The stability of Pt/CNT could be improved by the further graphitization of CNT. The highly graphitized multiwalled carbon nanotubes (HG-MWNT) were obtained by heat treatment at 2800°C upon the as-obtained chemically vapor deposited multiwalled carbon nanotubes (CVD-MWNT). The graphitization behavior was studied by X-ray diffraction and Raman spectroscopy. The results indicated that the obtained HG-MWNT had a high degree of graphitization (95.3%), while it was only 39.5% for the as-obtained MWNT. Electrochemical investigation suggested that the HG-MWNT had a lower corrosion rate than the original MWCNT, which could be attributed to the less surface defects on the HG-MWNT with the increase of the graphitization degree. The durability of the corresponding Pt/CNT catalyst was discussed. The results revealed that Pt/HG-MWNT using the highly graphitized carbon nanotubes as the supporting material had a higher electrochemical stability, which was due to the lower corrosion rate of HG-MWNT and the stronger interaction between metal and carbon support. The stability of Pt/CNT could also be improved with the support of heat-treated functionalized HG-MWNT. The ADT results at 60°C indicated that the EAS loss for Pt/Ox-HG-MWNT (functionalized HG-MWNT) is 55%, while that is only 37% for Pt/T-Ox-HG-MWNT (heat-treated functionalized HG-MWNT). The unstable carbon oxides (-COOH) could be decomposed by the heat-treatment, and the remained stabler carbon oxides improved the stability of the Pt/CNT catalyst. In addition, the stability of Pt catalyst could also be enchanced by the Pt-Co alloy. Furthermore, the PtCo/C alloy catalyst could be further improved by the increase of the alloy extent with the heat-treatment in moderate temperature.The methanol tolerance behavior of the cathode catalyst for DMFC was investigated. The electrocatalysis of the oxygen reduction reaction on carbon supported Pt and Pt–Co (Pt/C and Pt–Co/C) alloy electrocatalysts was investigated in sulphuric acid with the presence of methanol (both at room temperature and 60°C). A higher methanol tolerance of the binary electrocatalysts than Pt/C was observed. Furthermore, as compared to Pt-Co/C catalyst, Pt-Ni/C catalyst exhibited higher methanol tolerance. Au itself was not active to methanol oxidation reaction. Au nanoparticles with small particle size (3⁵ nm) obtained by colloid method were found to enhance their catalytic activity for oxygen reduction reaction. A novel Pt/Au/C catalyst was prepared by depositing the Pt and Au nanoparticles on the carbon support. EDX and TEM results revealed that Pt nanoparticles supported on carbon supports were separated by Au nanoparticles. The electrochemical analysis indicated that the novel catalyst showed the enhanced methanol tolerance while maintaining a high catalytic activity for the oxygen reduction reaction. It is well known that Au itself is not active to methanol oxidation reaction and its addition will part block contact between Pt nanoparticles and methanol molecules, which suppresses methanol oxidation on the Pt/Au/C catalyst. Therefore, the high methanol tolerance could be ascribed to the unique surface structure of the Pt/Au/C catalyst.