Synthesis And Modification Of Palladium-based Electrocatalysts And Their Applications For Ethanol Oxidation Reaction In Alkaline Media

The rapid growth of the economy and society in China greatly promote the improvement of the living standard for all citizens.However,this booming development also brings about serious energy problems and environmental issues,making it urgent to push forward the wide application of clean energy.As a typical representative for the renewable energy,ethanol has a broad field of applications for its distinguished advantages including non-toxicity,high energy density?8.01 kWh/kg?,easy storage and delivery,renewability and environmental friendliness.As ideal devices for the energy conversion from ethanol to electricity,direct ethanol fuel cells?DEFCs?are very promising technologies for small and portable power supply equipments in the near future.Nowadays,the anion exchange membrane direct ethanol fuel cells?AEM DEFCs?,which adopt the alkaline media as electrolyte,are becoming a hot research topic because of the high performance through the application of alkaline media but lower cost from the use of cheaper anion exchange membranes and non-platinum electrocatalysts.The catalysts are always the key theme in the studies of ethanol oxidation reaction?EOR?especially for the anode catalysts,because they directly determine the stack performance of DEFCs.The obstacles that impede the mass application of EOR catalysts at present are ascribed to two aspects:On one hand,the poor stability resulting from serious degradation problem.On the other,the sluggish activity coming from low reaction efficiency.Both problems are disadvantageous toward the commercialization of AEM DEFCs.This study is in concerned with stability improvement in the first place.A series of Pd_xAu_y/C electrocatalysts with high stability and activity were successfully synthesized via a co-reduction method.The peak current density of the optimal sample PdAu/C is 41%higher than that of Pd/C,but only 12%degradation percentage after 3000 cycles in the cyclic voltammetry test.The physiochemical results reveal that the stability improvement owes to the alloy effect between Au and Pd,and the activity enhancement is in virtue of the advance of anti-poisoning ability electronically induced by the addition of Au.Moreover,in consider of the importance for kinetic activity,the modifications of the PdIr/C electrocatalysts were conducted on the Pd₇Ir/C as the substrate material with the best kinetic but moderately catalytic activity.The as-synthesized tri-metallic Pd₇Ir@Au/C electrocatalyst with the modification of Au submonolayer on Pd₇Ir nanoparticles was then synthesized by adopting Cu underpotential deposition combined with Au³⁺galvanic displacement.The Pd₇Ir@Au/C shows a reduced degradation percentage up to 27.2%compared to that of Pd₇Ir/C as anticipated.However,it not only surprisingly maintains a low onset potential?-0.68 V vs.MMO?as the substrate material Pd₇Ir/C,but also has an improvement of the peak current density over 131%.In short,Pd₇Ir@Au/C possesses good stability,excellent kinetic activity and strikingly improved catalytic activity.The physiochemical characterizations indicate that the improvement of stability can be attributed to the reduced surface free energy of the nanoparticles from the formation of near surface alloy among Pd,Ir and Au.Ir plays the major role in the amelioration for kinetic activity,because the presence of Ir and its oxides facilitates the adsorption of hydroxyl groups at lower potential,which promotes the fully oxidation of ethanol molecules.And the enhancement of catalytic activity should be ascribed to increasement of anti-poisoning ability electronically induced by the Au submonolayer.The results of the thesis show both the expected improvement of stability from Au addition as well as the unexpected positive influence toward the activity of Pd-based electrocatalysts.And there still remain great possibilities for the catalytic enhancement of Pd-based electrocatalysts.