Study On Carbon Nanotubes And Carbon Supported Anode Pt-based Catalysts For Direct Methanol Fuel Cell

Direct methanol fuel cell(DMFC) is one of the most desirable power source for small-scale power units because of inexpensive fuel,ease of handling,high theoretical energy density and near-zero pollutant emission.However,there is large distance between the practical DMFC performance and the theoretical performance. Synthesizing and searching for electrocatlasyts with high activity are the important approach to improve DMFC performance,which also become to be the endlessly goal for researchers.At present,Pt-based catalysts are still used as electrocatalysts most widely in DMFC.Generally,Pt-base catalysts are prepared supported on carbon materials of conductive and high surface area.The support type and surface chemistry have a significant effect in the resulting metal dispersion,particle sizes and distribution.How to synthesize Pt-based nanoparticles with appropriate particle sizes, homogenous distribution and how to improve noble metal utilization effectively have important sense for DMFC.Lately,carbon nanotubes have attracted more and more attention in fuel cell due to its different characteristics from the traditional carbon supports.This dissertation mainly focused on the anode electrocatalysts for DMFC, being based on the supports,firstly studied the preparation of Pt-based catalysts with high dispersion,then investigated the Pt and PtRu electrocatalysts supported on multi-walled carbon nanotubes(MWCNTs) with various diameters and lengths,and then exploited the Vulcan XC-72 carbon and MWCNTs composite supported Pt-based catalysts.For the DMFC,last part of this paper investigated the performance of passive DMFC.For the preparation of DMFC catalyst,the dissertation optimized the microwave-assisted polyol reduction process to prepare Vulcan XC-72 supported Pt and PtRu catalysts.We synthesized the nanoparticles with small particle sizes and narrow particle distribution(2-5 nm) successfully.The experiment results indicated that the pH value of solution made important influence on the metal particle sizes and the metal loading.The metal particle sizes decreased with the increase of solution pH,the average particle sizes were in the range from 3.5 nm to 1.8 nm.Moreover,when the solution pH exceeded 11,the metal loading decreased caused by small particles and the competition reduction of Pt and Ru ions.The appropriate pH values were about 9 and 10 for PtRu/C and Pt/C catalysts,respectively.The electrochemical results indicated that the prepared Pt/C and PtRu/C catalysts had excellent activity and showed comparable methanol electrooxidation activity with the commercial E-Tek catalysts.PtRu/C catalyst had higher methanol oxidation activity than Pt/C catalyst. The onset potential for methanol oxidation was 0.19 V and 0.32 V,the peak potential was 0.53 V and 0.62 V for PtRu/C and Pt/C catalysts,respectively.But when the metal loading was 20%,the Pt/C catalyst showed the higher peak current than PtRu/C catalyst.CO stripping test also showed that PtRu/C catalyst exhibited the lower peak potential for CO oxidation than Pt/C catalyst.The results demonstrated indirectly the bifunctional mechanism of PtRu catalyst.Carbon nanotubes has the extraordinary property as supports for Pt and Pt alloy catalysts in fuel cell due to its good electronic conductivity,unique tubular structure and low sulfur content.Pt and PtRu electrocatalysts supported on MWCNTs with various diameters and lengths were studied systematically firstly in the paper.It can be found from TEM that part of metal particles can deposit on the inner walls of MWCNTs with large tube diameter and short tubes,which increased the metal dispersion and utilization.The results of CV and XPS showed that the amounts of surface oxides of MWCNTs increased with the decrease of the diameters and lengths, but the amounts of surface oxides were not the mainly factor on the methanol electrooxidation activity The electrochemical results showed that methanol electrooxidation activity was influenced mainly by the dispersion of metal in this work,and the metal dispersion connected with the tube diameters and lengths of MWCNTs primarily.PtRu nanoparticles supported on long MWCNTs(50μm) with a diameter of 30-50 resulted in the electrochemical surface area(ECSA) of 57.6 m²g^-1, while the ECSA reached 74.4 m²g^-1 when PtRu supported on short MWCNTs(2μm) with the same diameter.In addition,the specific activities of Pt supported on short MWCNTs were 1.4 times of Pt supported on long MWCNTs.It can be deduced the edge effect of MWCNTs,namely,the intrinsic activity of the ends of MWCNTs.The atoms of ends in MWCNTs had higher properties compared with the carbon atoms of side walls.It can be obtained lastly that the Pt-based nanoparticles supported on short MWCNTs with a diameter of 30-50 nm showed the best metal dispersion and highest methanol oxidation activity.Although MWCNTs had a serious of advantages,we found that the MWCNTs supported Pt-based catalysts yielded poorer dispersion than Vulcan XC-72 supported catalysts.To take use of the advantages of the two supports effectively,the dissertation then exploited Vulcan XC-72 carbon and MWCNTs composite supported Pt and PtRu catalysts.The dispersion,alloy degree and the surface states of prepared catalysts were characterized by BET,TEM,XRD,XPS.The results indicated that, compared to the Vulcan XC-72 supported Pt-based nanoparticles,MWCNTs supported metal nanoparticles resulted in aggregates partly.For the composite supported catalysts,most of metal particles deposited on the Vulcan XC-72,a small quantity of particles was supported on MWCNTs,which resulted in small particles size,a few aggregates and enhanced ECSA.Compared with Vulcan XC-72 and MWCNTs,there were the stronger interaction of composite supports and metal,which made composite supported catalysts PtRu/C+MWCNTs had the higher alloy degree.It can be found by electrochemical results that,compared to the single supported catalysts,the composite supported catalysts exhibited superior electrocatalytic activity for methanol electrooxidation.At the same conditions,the methanol oxidation activity at peak potential for Pt/C+MWCNTs catalyst got 600 mAmg^-1,which was 1.4 times to the activity of Pt/C catalyst of 430 mAmg^-1.The methanol oxidation peak current density of PtRu/C+MWCNTs catalyst(256.4 mAmg^-1) was higher than that of PtRu/C catalyst(206.7 mA mg^-1),while the PtRu/MWCNTs catalyst showed the comparable activity to PtRu/C catalyst.The DMFC performance test showed that composite supported PtRu anode catalysts yielded higher power density than single carbon supported anode PtRu catalysts.Through analysis and characterization,we attributed the higher methanol oxidation activity of composite supported catalysts to the higher electrochemical surface area,higher alloy degree and the enhancement of metal utilization. Passive DMFC have been considered as one of the most potential fuel cells for commercialization because of its much simpler structures with no pumps or other auxiliary devices,ease of carrying,easiness of micromation and commercialization of eight and volume.However,studies of passive DMFC are at initial progress stage,in the last part of this dissertation,the performance of home-made passive DMFC was studied primarily.The maximal power of two-cell stacks was 80 mW and the open circuit voltage was 1.2 V at room temperature.The methanol concentration made significant influence on the cell performance.The performance of cell ascended at the beginning then dropped with the increase of methanol concentration,which was caused by anode concentration polarization and methanol crossover.Experiment results showed that at low current density,low methanol concentration exhibited the larger open circuit voltage,but at high current density,DMFC with 1 M and 2 M methanol resulted in transport polarization earlier and 3 M methanol yielded the best DMFC performance.In addition,the increase of cell temperature and flowing air can improve the cell performance evidently.The study of long-operation performance indicated that single cell can work with the cell voltage 0.2 V stably and continuously for 4 h at 100 mA discharge at room temperature under the condition of infusing 3 ml 3 M methanol one-off.