Effect Of Titanium Oxide/Carbon Composite Supports On Performance Of Pt Electrocatalyst For Methanol Oxidation

Direct methanol fuel cell(DMFC) is considered to be a potential power source for portable electronic devices and electric vehicles due to its high power density and low weight and volume. The promise of practical application is mainly hindered by the low activity and poor durability of Pt-based catalysts. In this paper, Pt-based catalysts are prepared by the microwave-assisted polyol process. The research is focused on the special structure titanium oxide supports. Through the improvement of experiment method, the titanium oxide supports of different morphologies are gradually developed for high activity and durability of Pt-based catalysts.Firstly, anodization method was employed to obtain Ti O2 nanotubes(TNTs) under two different conditions of low voltage(LV) and high voltage(HV). The mixture of carbon black and TNTs is used as catalyst support. Ti O2 nanoparticles(TNPs) were used instead of TNTs as a reference. The performances of Pt/C-TNTs catalysts are studied in detail. The results show that the conductivity of supports is better using TNTs than TNPs. The presence of carbon greatly improves the conductivity of catalysts and the dispersion of Pt nanoparticles. TNTs have high corrosion resistance in acidic and oxidative environments and metal support interaction with Pt. Pt/C-TNTs catalysts exhibit higher performance of methanol electrooxidation than Pt/C-TNPs and commercial Pt/C. Compared with the Pt/C-TNTs-LV and Pt/C-TNTs-HV catalysts, the latter shows higher activity of methanol electrooxidation while the former shows higher durability of methanol electrooxidation. Taking into accout activity and durability, both of them are comparable. However, compared with TNTs-LV, TNTs-HV can be obtained in shorter time, which is more suitable to practical application.Since the yield of anodization method was too little to meet the application requirement, hydrothermal method was employed to obtain titanium oxide nanostructure. The effects of p H values of acid treatment and calcination temperature on structure and performance are investigated. Hydrogenotitanate/titania composite nanotubes(HTNTs) were synthesized at various p H values of acid treatment. The change of HTNTs components is investigated by X-ray diffraction(XRD) and energy dispersive analysis of X-ray(EDAX), which reveal that the products are gradually converted from ―H2Ti2O5·H2O‖ phase to ―anatase Ti O2‖ phase with the decrease of p H values. The distribution and electrocatalytic properties of platinum nanoparticles are sensitive to the p H value of acid treatment. The Pt/C-HTNTs catalyst prepared at the acid treatment p H value of 7 exhibits better electrochemical activity and stability than other catalysts. Sodium titanate/titania composite nanotubes/nanorods(STNS) were synthesized at the acid treatment p H value of 7 by annealing in the range of 300-700 oC. The changes of STNS in composition and morphology are investigated by XRD and transmission electron microscopy(TEM). The results reveal that the composition of STNS changes from ―Na2-xHxTi2O5‖ to ―Na2Ti6O13‖ and its morphology changes from nanotube to nanorod with the increase of calcination temperature. The products of 400 oC and 600 oC correspond to the intermediate states of reactions. The variations of catalytic activity and stability of Pt/C-STNS catalysts show the interesting ―M‖ shape with the increasing of annealing temperature of STNS. The Pt nanoparticles supported on STNS-400 nanotubes and STNS-600 nanorods exhibit more uniform dispersion and superior electrocatalytic performance for methanol electrooxidation. The main reason seems to be that both of them are multiphase composites with a large number of phase interfaces and crystal defects, which is conducive to the deposition of Pt nanoparticles. In addition, the presence of ―anatase Ti O2‖ phase can enhance the electrochemical performance due to the metal-support interaction. Compared with commercial Pt/C, the activity of methanol electrooxidation on Pt/C-STNS-600 catalyst is about 1.3 times and the durability increases about 10 % by amperometric i–t curves.In order to achieve the uniform combination of carbon and titania, the core/shell structured carbon-coated Ti O2 nanowires(TNWs@GC) were synthesized by in-situ carbonizing glucose by two-step hydrothermal reaction and subsequent calcination. Then TNWs@GC is used as efficient Pt-based catalyst support. The physical characterization confirms that it has the special core/shell structure. The glucose content and carbonization temperature greatly affect the graphitization degree, porosity and surface chemical property of carbon shell. Electrochemical measurements indicate that when the optimized carbon content and calcination temperature are 60 % and 800 oC, the Pt/TNWs@GC catalyst exhibits the best activity and durability of methanol electrooxidation. Compared with commercial Pt/C for methanol electrooxidation, the activity of Pt/TNWs@GC catalyst is about 1.4 times. The cycling durability of methanol oxidation increases by 6.9 %. It is noteworthy that the activity of Pt/TNWs@GC after 800 cycling is equivalent to the initial activity of commercial Pt/C. The enhanced performance is attributed to the design of special core/shell structure. The carbon shells coating on the surface of Ti O2 nanowires can greatly improve the electronic conductivity and suppress crystal growth of Ti O2 during calcination. Meanwhile, a large number of defects within the carbon shells are conducive to the dispersion of Pt nanoparticles. In addition, the core of Ti O2 nanowires can produce the synergetic effect with Pt nanoparticles.To further improve the performance of catalyst, the 3D-network structure with high porosity, good flexibility, large surface area and efficient transport channel was constructed. The unique 3D-network structured carbon-coated Ti O2 nanowires(TCN) were synthesized by in-situ carbonizing resorcinol–formaldehyde polymer(RF) and used as highly efficient Pt-based catalyst supports. Physical characterizations confirm the 3D-network structure. The carbon network derived from RF carbonization plays a critical role in facilitating the final morphology. The effects of RF content and calcination temperature on structure and performance are investigated in detail. When the optimized mass ratio of titania and RF is 5:10 and the optimum calcination temperature is 800 oC, the Pt/TCN catalyst exhibits the best activity and durability of methanol electrooxidation. Strikingly, the Pt/TCN catalyst exhibits far better electrochemical active specific surface area than the commercial Pt/C. Its activity and durability for methanol electrooxidation are about 1.5 and 5.7 times higher than the commercial Pt/C, respectively. More importantly, the Pt/TCN catalyst in the single DMFC exhibits higher polarization current and power density than Pt/C. The enhanced performance could be attributed to the design of special 3D-network structure, the numerous anchoring sites for Pt deposition, as well as the synergetic effect between Pt and Ti O2.