Ternary Pd-based Electrocatalysts And Their Catalytic Behaviors Toward Oxidation Of Formic Acid And Reduction Of Oxygen

It is urgent to explore safe,efficient and clean renewable energy source for solutions of?energy shortage?and?environmental pollution?.Direct formic acid fuel cell?DFAFCs?has attracted much attention for its advantages of high energy conversion efficiency,safe and non-toxic,environmentally friendly,convenient storage and transportation of liquid fuel for wide applications in the field of portable power sources and vehicle powers.DFAFCs usually used noble metal platinum?Pt?as the electrode catalyst due to its slugish kinetics.In addition,the use of Pt results in the high cost and is easily poised by intermediate species in the anode formic acid oxidation reaction?FAOR?.Furthermore,the stability of Pt cathode is poor under the high potential condition,which results in the degradation of the oxygen reduction reaction?ORR?catalytic performance.Palladium?Pd?is very promising substitute for Pt as a DFAFCs catalyst,since it has similar crystal structure and electronic structure to that of platinum,but is less expensive and better anti-poisoning than Pt.Tailoring the compositions,morphologies and structures of nanostallline catalysts in the nanoscale during the synthesis for Pd-based catalysts with high catalytic activity and stability has become one of the research focuses in the field of DFAFCs.Nevertheless,Pd is still expensive and its electrochemical stability needs to be further improvement,therefore,it is of great significance for exploration of the fundamentals and its practical applications of low Pd catalyst with high activity and stability in DFAFCs.In this thesis,we have designed and prepared a series of highly performance ternary Pd-based alloy catalysts by tailoring compositions and structures of nanomaterials.Furthermore,the electrocatalytic behavious are discussed and the enhancement mechanisms of electrode kinetics as well as investigated.The main research contents and results of this paper are as follows:?1?Preparation of low cost and high performance anode FAOR catalyst is of great significance to promote the commercialization of DFAFCs.The third chapter presents the preparation of ternary Pd-Co-P catalyst by combining of solvothermal reduction and solid phosphating reactions.The results display that the electrochemical active surface area of Pd-Co-P nanocatalyst is lager than that of Pd-P/C,PdCo/C and commercial Pd/C catalysts.In addition,the mass current density of formic acid oxidation at 0.65 V of the ternary Pd-Co-P/C catalyst is 1.90,3.14 and 6.07 times larger than that of Pd-P/C,PdCo/C and commercial Pd/C catalysts,respectively.The stability of the catalyst in comparison to the commercial Pd/C is also greatly improved.The enhanced catalytic performance of the Pd-Co-P/C catalyst can be ascribed to the interaction betwwen Pd and Co,P for rationally modified an electronic structure of Pd promoting the removal ability of intermediate toxic product CO_ad.Meanwhile,the surface of Pd-Co-P/C catalyst became rougher and has defect with vacancy,which provide much more available catalytic active sites,thus greatly improving the catalytic activity and stability of ternary Pd-Co-P/C catalyst for FAOR.?2?Tailoring the compositions,morphologies and structures of nanocatalyst is the most common approach to improve its catalytic performance.The fourth chapter presents the preparation of ternary PdIrCu nanosheets by a one step solvothermal reduction method and studies its electrocatalytic performance toward formic acid oxidation.The mass current density of formic acid electrooxidation at 0.5 V of ternary PdIrCu/C(1.38 A mg_Pd^-1)catalyst is 1.17 and 4.31 times larger than that of PdCu/C(1.18 A mg_Pd^-1)and commercial Pd/C(0.32 A mg_Pd^-1)catalysts,respectively.The enhanced catalytic performance of the catalyst contributed to the interaction between Pd and Ir,Cu for rationally modified the electronic structure of Pd fot improved electron transfer rate.Further,the presence of Ir atoms can dissociate the water molecules and produce-OH functional groups at a lower potential,which is helpful to promoting the oxidation of the intermediate CO_ad and removing from the active sites,thereby enhancing catalytic activity and stability toward formic acid oxidation reaction.?3?The sluggish kinetics of oxygen reduction reaction at the cathode is one of the obstacles for development of DFAFCs.Previous researches have demonstrated that the unique concave-convex nanostructures can expose highly active sites and promote the bridge and lateral adsorption of oxygen atoms,which could enhance the catalytic performance toward oxygen reduction reaction.Fifth chapter presents preparation of ternary PtPdCu multicubes with three dimensional hierarchically porous concave-convex structure electrocatalyst by a facile solvothermal co-reduction method with tailoring the elemental components,the optimal Pt₄₅Pd₃₀Cu₂₅/C(0.47 A mg^-1_Pt+Pd)multicubes catalyst result in 1.68-fold and 2.35-fold increase in ORR mass activity compared to Pt₇₅Cu₂₅/C(0.28 A mg_Pt^-1)and commercial Pt/C(0.20 A mg_Pt^-1)catalysts at0.9 V?vs RHE?,and shows an excellent long term stability.Both experimental result and theoretical calculations reveals that the performance enhancement is mainly caused by Pd and Cu jointly modified electronic structure of Pt,which strengthened O atoms and weakened OH_ad species for adsorption as well as facilitates the charge transfer rate.In addition,the unique porous concave-convex surface structure provides a large number of active sites for catalytic reaction,which accelerated mass transport process.In brief,we have prepared three different Pd-based alloy nanocatalysts and investigated their electrocatalytic behavious.The results reveal that tailoring the compositions,morphologies and structures of alloy nanocatalysts can accelerate the interfacial charge transfer rate,and is favor to enhancement of the electrocatalytic activity and stability.This work provides a new approach for design and synthesis of low cost and high performance DFAFCs catalysts,thus holding great promise to the large-scale commercialization of DFAFCs.