Preparation Of Transition Metal Oxide Supports And Their Application In Oxygen Reduction Electrocatalysts

The kinetics of the oxygen reduction reaction at the cathode of a fuel cell is slow,requiring the participation of a catalyst to accelerate its reaction process.Currently,the most widely used cathode catalyst is carbon supported Pt based catalyst.Under fuel cell operating conditions,electrochemical corrosion of carbon supports can cause dissolution,agglomeration,and detachment of Pt catalyst particles.This leads to problems such as low utilization efficiency of Pt nanoparticles,reduced electrochemical active area,and degradation of catalytic materials.This in turn affects its activity and stability,ultimately leading to poor performance and shortened lifespan of fuel cells.Transition metal oxides have better stability compared to carbon,and their application as supports for electrocatalysts is expected to enhance stability.The strong metal support interaction（SMSI）between the metal and the oxide can inhibit the aging and agglomeration of catalyst particles and improve the dispersity of the catalyst.However,oxides typically have a smaller surface area and poor conductivity.This limits its application as electrocatalyst supports.In this article,a suitable material was used to disperse the precursor of meta titanic acid to prepare Ti_nO_2n-1 materials.It has a high specific surface area and excellent electrochemical stability and conductivity.The Pd/Ti_nO_2n-1 and Ag/Ti_nO_2n-1 catalysts with high activity and stability were obtained by using Ti_nO_2n-1 supports to support metal Pd that has similar chemical characteristics and electronic structure to Pt,as well as cheap metal Ag.This is helpful to replace the scarcity Pt based catalyst.The main research contents are as follows:（1）Using organic macromolecules as surfactants,Ti_nO_2n-1materials with different n values were obtained by sol-gel method and carbothermal reduction method.Titanium oxide materials with rich mesopores and high specific surface area(150～250 m² g^-1)can be obtained by dispersing meta titanic acid precursors with suitable matrix materials.This overcomes the problem of small specific surface area of Ti_nO_2n-1prepared by ordinary carbothermal reduction method.The conductivity of Ti_nO_2n-1is higher than that of semiconductor Ti O₂,which is more suitable for use as a support for electrochemical catalysts in fuel cells.（2）Series of Pd/Ti_nO_2n-1 catalysts were prepared by in-situ growth method to reduce Pd nanoparticles onto Ti_nO_2n-1 materials.The Pd nanoparticles on the Ti_nO_2n-1 are uniformly distributed and have a small particle size.Pd/Ti_nO_2n-1exhibits better ORR catalytic performance than commercial Pd/C.This indicates that titanium oxide supports can generate strong metal support interaction with Pd particles.ORR catalytic activity can be improved by adjusting the electronic structure of Pd.Among them,Pd/Ti₆O₁₁ exhibits optimal catalytic activity and good durability.Mass activity at 0.85 V is 6.4times greater than commercial Pd/C.The increase in activity is due to the highest content of oxygen vacancies and Pd（0）components in Pd/Ti₆O₁₁,as well as the high conductivity of Ti₆O₁₁.After 5000 cycles of cyclic voltammetry scanning,the half wave potential of Pd/Ti₆O₁₁ only shifted negatively by 13 m V.The electrochemical active area decreased by only 12%compared to the initial value.The mass activity at 0.85 V is 7.4 times the Pd/C after stability testing.This indicates that it has good stability.Preliminary research on its stability mechanism found that,the interaction between Pd and the support and the good stability of Ti₆O₁₁ itself endow the catalyst with good durability.（3）Series of Ag/Ti_nO_2n-1 catalysts were prepared by in-situ growth method and explore the influence of synthesis factors such as Ti_nO_2n-1 supports,types of reducing agents,and the amount of sodium citrate on the morphology characteristics and catalytic ORR performance of silver based catalysts.Finally,the optimized Ag/Ti₅O₉ catalyst with the best performance was synthesized with an ORR half wave potential of 0.80 V.After 4000 cycles of cyclic voltammetry scanning,its half wave potential decreased to 0.73 V,higher than 0.67 V of Ag/C tested under the same conditions.The high activity and stability of Ag/Ti₅O₉stems from its unique wormlike morphology and stable Ti₅O₉support.