Study On Carbon Nanotubes Supported PtRu Based Anode Catalysts For Direct Methanol Fuel Cells

Direct methanol fuel cell (DMFC) is the most promising power source for electric cars as well as various portable equipments due to its favorable advantages, such as high specific energy and power densities, less environmental impact, room temperature operation, cheap liquid fuel, compatibility to the current gas system, and so forth. At present, the performance and cost of DMFC still have not met the need of commercial applications because of some big technical problems, such as low activity of the anode catalysts and methanol crossover. The problem of the low activity of the anode catalysts is especially more serious, leading to the high metal loading and high cost of DMFC, which restricts its commercial applications. In order to solve this problem, the main research contents of this thesis is to develop the anode carbon nanotubes (CNTs) supported PtRu based catalysts with simple preparation procedure, low cost and better catalytic activities, and to study the relationship between the methanol electro-oxidation activities and the crystal structure, particle seizes, oxidation states as well as the interaction of metal nanoparticles on the catalyst surface, by means of various electrochemical methods and catalyst characterization techniques. The home-made CNTs supported PtRu based catalysts were used as the anode catalysts to prepare membrane electrode assemblies (MEAs) and a single DMFC, then the effect of various operating conditions on the performance of DMFC was investigated, and the electrochemical characteristics of the DMFC anode were explored by AC impedance spectroscopy.The PtRu/CNTs and novel PtRuNi/CNTs and PtRuMo/CNTs catalysts were synthesized by microwave assisted polyol reduction method, and the pretreatment condition of CNTs, the preparation conditions of the catalysts and the optimal composition of PtRuMo/CNTs catalyst were studied. The CNTs treated by the ultrasonic frequency of 60 kHz in mixed acid solution resulted in the optimal content of the oxygen containing groups on the surface, and the prepared PtRu/CNTs catalyst showed better methanol electro-oxidation activity when compared to the commercial PtRu catalyst and PtRu/C catalysts supported on CNTs treated with other ultrasonic frequencies. The pH value of the reaction solution was 3.86 when the initial pH value was 9, at this condition, there was appropriate amount of stabilizer glycolate anion in the solution and better absorption environment for metal nanoparticles, therefore the synthesized PtRu/CNTs catalyst showed the best methanol electro-oxidation activity. The heat treatment of the catalysts could facilitate the removal of the remaining organics to enhance the catalytic activities; however, heat treatment with higher temperatures could increase the particle sizes of the metal nanoparticles and decrease the content of active species RuOxHy, resulting in lower catalytic activities. It was found that the optimal heat treatment temperature is 160℃. The results of stationary polarization cures and CO stripping voltammograms of methanol electro-oxidation over CNTs supported PtRu based catalysts revealed that the order of decreasing activities of the catalysts was PtRuMo/CNTs > PtRuNi/CNTs > PtRu/CNTs. PtRuMo/CNTs catalyst not only showed better CO-tolerant ability, but also had larger electrochemical active surface area. A combinatorial method was used to selected the optimal composition for PtRuMo/CNTs catalyst, and the results demonstrated that the optimal atomic ratio of Pt:Ru:Mo was 6:3:1.The methanol electro-oxidation performance of CNTs supported PtRu based catalysts was investigated by AC impedance spectroscopy, and equivalent circuits were established to module the impedance data. The results showed that the value of the electron transfer resistance Rct of PtRuMo/CNTs catalyst was the smallest among the three CNTs supported PtRu based catalysts, revealing that PtRuMo/CNTs had the best activity for methanol electro-oxidation. The Rct and inductive resistance Lco decreased with increase in the potentials and methanol concentrations, which means that the reaction rates of methanol dehydrogenation as well as the COad oxidation increase as the potentials and methanol concentrations increase. At low potentials (350~400 mV), most of the catalyst surface is occupied by COad which can't be oxidized to CO2 in time, resulting in the poison of the catalysts, at this condition, the rate determining step is methanol dehydrogenation reaction; as the potentials increase (450~600 mV), the amount of OHad produced on the RuMoOx sites increase, so COad is gradually oxidized, therefore the released Pt active sites can continually absorb and oxidize methanol molecules, leading to the appearance of the inductive effect, at this condition, the rate determining step is COad oxidation reaction.The results of XRD, FT-IR and TG measurement showed that the ultrasonic treatment had no impact on the crystal structure of CNTs, and the oxygen containing groups, such as hydroxyl, carboxyl, and so forth, were produced on CNTs surface; the content of the oxygen containing groups first increased and then decreased with the increase in the ultrasonic frequencies, reaching a maximum value at the ultrasonic frequency of 60 kHz. The results of XRD, TEM and XPS characterizations revealed that there were crystalline face-centered cubic phase of Pt in all the catalysts; no crystalline Ru, Ni, Mo and their oxides were detected in the XRD spectra; the metal particle sizes decreased as the pH value of the reaction solution increased, while the metal particle sizes increased as the heat treatment temperature of the catalysts increased. The metal particles were well dispersed on the CNTs surface with little agglomeration, and the particle sizes were centered in the range of 2~4 nm. There were multi-value states of Pt, Ru, Ni and Mo species in the catatlysts, however no Ni(0) and Mo(0) metals were found. The addition of Ni and Mo to the PtRu catalyst made the binding energies of Pt 4f shift towards lower binding energies, demonstrating that the electron densities of Ni and Mo move to that of Pt.The home-made CNTs supported PtRu based catalysts were used as the anode catalysts to prepare MEAs and a single DMFC. The results showed that the highest power densities at 60℃of the single DMFC with PtRu/CNTs,PtRuNi/CNTs and PtRuMo/CNTs catalysts were 55.88, 57.60, 61.32 mW/cm2, respectively. The DMFC with PtRuMo/CNTs catalyst showed the best cell performance, which reached the highest power density level, reported in China, of the single DMFC with the imported commercial PtRu anode catalyst. The MEA activation of discharging with small current density improved the three reaction zone and Nafion distribution in the electrode to enhance the electrochemical active surface area of the catalysts. The appropriate operating conditions for DMFC were: methanol concentration of 2~2.5 M, methanol flow rate of 1~2 mL/min, oxygen flow rate of 100~150 mL/min, under these conditions, the highest power densities of DMFC at temperature of 25, 40 and 60℃were 23.54, 38.24 and 61.32 mW/cm2. The results of AC impedance spectroscopy of the DMFC anode showed that the charge transfer resistance Rct, anode/membrane interface resistance Ri, catalyst layer resistance Rc and inductive resistance Lco decreased as the potential and cell temperature increased, and the anode/membrane interface tended to capacitance characteristic, while the reaction zone of catalyst layer extended and changed to a porous structure. The increase of cell temperature enhanced the proton migrating ability of proton exchange membrane (PEM) and reduced the PEM resistance Rm.