Preparation And Investigation Of The Composite Support Materials Based On TiO₂NTs As Pd Catalyst Support For The Methanol Oxidation Reaction

In recent years,fuel cells have been extensively developed as clean and sustainable energy sources.Therefore,fuel cell technologies have acquired special attention.Direct methanol fuel cells(DMFCs)have been widely investigated as portable power sources due to intrinsic advantages,such as large energy density,low operating temperature,no humidification requirements,and minimal emission of pollutants in the environment.However,slow methanol oxidation kinetics and poisoning of electrode surfaces are the biggest hurdles that need to urgently cross by developing new catalysts for commercializing the DMFC technology.Currently,Platinum(Pt)and Pt-based electrocatalysts are predominant as the best electrocatalyst for methanol oxidation reaction(MOR)at room or moderate temperature.However,their high price,natural scarcity,and CO poisoning lead to seeking for suitable alternates.Among various candidates,Palladium(Pd)is a considerable good substitute for commonly-used Pt as an electrocatalyst in fuel cells.However,several challenges of Pd and Pd-based electrocatalysts still exist for the commercialization of DMFCs.The development of suitable support materials is another strategy to improve the methanol oxidation performance of electrocatalysts and realize the commercialization of DMFCs.Typically,various carbon materials have been used as support materials.However,the carbon support materials are susceptible to degrade during the MOR process,and cannot improve the activity of the catalysts.Titanium dioxide nanotubes(TiO2NTs)has been investigated as a kind of advanced support materials for electrocatalysts.However,relatively lower electrochemical activity and electrical conductivity of TiO2NTs limit its practical application in fuel cells.Therefore,the development of new support materials for fuel cell catalysts with low-cost,high surface area,high electrical conductivity,suitable porosity,and long-term stability still remains a big challenge.Therefore,in the work,we propose a new effective approach to improve the performance of TiO2NTs by introducing other materials within the reduction TiO2NTs matrix.A series of new Ti/r-TiO2NTs composite support materials were obtained.In the next step,Pd nanoparticles were coated onto Ti/r-TiO2NTs nanocomposites by pulse electrodeposition(PED).The electrocatalytic activity and stability of the newly prepared catalytic electrode toward MOR was evaluated in DMFCs.The main contents of dissertation are listed as follows:1.Our results demonstrate a simple electrodeposition synthesis method and the use of Poly(5-aminoindole)(PAIn)modified Ti/TiO2NTs as a support material of Pd NFs for the MOR in DMFCs.The incorporation of PAIn can further improve the conductivity and electrochemical catalytic activity of the TiO2NTs matrix,facilitating the generation of the Pd NFs and avoiding the growth and agglomeration of metals into larger particles.During MOR,the electrocatalytic mass activity of the Ti/TiO2NTs/PAIn/Pd is 6.38 and 6.08 times higher than commercial Pd black and Pt black.The improved electrochemical performance and stability of Ti/TiO2NTs/PAIn/Pd electrode are attributed to the unique flower-like nanostructures and the good synergistic effects between Pd,PAIn,and Ti/TiO2NTs.2.The Ti/r-TiO2NTs/Ni/CeO2 composite support materials were fabricated via a simple two-step electrodeposition method.Then a unique corrugated dendritic Pd loaded on the Ti/r-TiO2NTs/Ni/CeO2 support is successfully achieved by PED.Electrocatalytic studies,evaluated from the electro-oxidation of methanol in alkaline media,show that the Ti/r-TiO2NTs/Ni/CeO2-Pd electrode exhibited higher peak current,lower onset potential,higher stability,and excellent poison resistance for MOR.The mass activity of Ti/r-TiO2NTs/Ni/CeO2-Pd electrode is about 1776 mA·mgPd-1,which is～10.89 times higher than Pd/C(Pd/C 163 mA·mgpd-1).After successive CV tests of 1000 cycles,Ti/r-TiO2NTs/Ni/CeO2-Pd electrode still retained 88.9%of its initial current(Pd/C 11.8%).The excellent MOR performance of the modified electrode can be attributed to the synergetic effects of the incorporated Ni and CeO2 and the unique dendritic morphology of Pd nanostructures.3.A novel catalyst support was prepared by modifying Ti/r-TiO2NTs with MOF derivatives.Then Ni-MOF on the surface of Ti/r-TiO2NTs electrode was calcined to form a porous carbon framework containing Ni(Ni-PCs).The original flower-like morphology of Ni-MOF was perfectly kept.The prepared Ti/r-TiO2NTs/Ni-PCs appeared as an efficient platform for electrodeposition of Pd.Electrochemical measurement suggests that theTi/r-TiO2NTs/Ni-PCs/Pd catalysts demonstrate dramatically enhanced electrocatalytic activity for MOR(2890 mA·mgPd-1),which is～17.7times higher than Pd/C(Pd/C 163 mA·mgPd-1).After the 1000 cycles of scanning,Ti/r-TiO2NTs/Ni-PCs/Pd lost 9%of initial activity,which is better than the 88.2%loss of Pd/C.This work might provide a new strategy for the construction of an excellent MOR support materials.