Controlled Synthesis Of Noble Metal Nanocatalysts And Study Of Their Electrocatalytic Properties

Direct liquid fuel cells（DLFCs）are considered as the most potential fuel cell technology by having low operating temperatures,high conversion efficiencies,and minimal emission levels.However,the reaction kinetics of the cathode oxygen reduction reaction（ORR）and anode liquid fuel oxidation reaction（FOR）is slow,reducing the power output of battery.At present,noble metal（Pt,etc.）is still the catalyst with the best performance for FOR and ORR reactions,but its limited reserves,low catalytic activity and stability,which also hinder the development of DLFCs.To solve these problems,a series of noble metal-based nano-catalysts with different components and structures were constructed by low cost,green and efficient method by introducing inorganic salt as stabilizing assistant or structural guide agent.Introducing chemical dealloying post-treatment and segmental heating process into the preparation of catalyst not only forms structural defects to achieve electronic structure control of surface precious metal elements,but also produces a large number of active sites.Furthermore,the electrocatalytic activity and stability of different noble metal-based nano-catalysts of small organic molecules（methanol,formic acid）oxidation and oxygen reduction were systematically studied.The main research contents are as follows:（1）Based on conventional citrate reduction method,the inorganic salt sodium nitrate（Na NO₃）was introduced to prepare ultra-small,clean surface,stable and highly monodispersed Pt nanoparticles（NPs）.And the specific complexations of nitrite ions（NO₂^-）possibly altered the reaction kinetics and lowered the growth rate of Pt NPs by retarding the reduction reaction.The average size of Pt nanoparticles was 2 nm.The carbon nanotube（CNT）supported Pt electrocatalysts（Pt/CNT）prepared in this work were used in the cathodic oxygen reduction exhibited the cell performance of 744 m W?cm^-2（maximum power density）,compared with commercial Pt/C catalyst（MEA-Pt/C-JM）in the same operating conditions,which increased by 7%.The optimized Pt/CNT catalysts exhibited high mass activity and moderate activity decay after 10,000 times of potential cycling.（2）Three-dimensional porous Pt Cu nanonetworks（NCNs）With unique high interconnection and interweaving were prepared by inorganic salt-mediated self-assembly method using inorganic salt Al Cl₃ as structure directing agent and urea as reducing agent.The inducing effect of Al³⁺was used to promote the aggregation of small nanoparticles to form the network framework,and Cl^-was used to promote the anisotropic growth of the catalyst producing more active sites.Then,chemical de-alloying process was carried out by reflux in acetic acid to remove the surface Cu oxide and change the electronic structure of the surface platinum.In the Electrochemical evaluation,Pt₃Cu₁NCNs due to its high specific surface area,high porosity,rich edge/Angle atoms as active sites and the synergistic effect of Pt-Cu alloy,exhibited a 96%increase in electrochemical active area,and the highest mass activity was 3.13and 2.60 times respectively for methanol oxidation reaction（MOR）(1.257A mg _Pt^-1)and oxygen reduction reaction（ORR）(0.651A mg _Pt^-1)compared with commercially available Pt/C catalysts,respectively.In addition,the Pt skins formed during the dealloying process also helps the catalyst to deliver excellent stability.Pt₃Cu₁ NCNs remains 75%of its initial mass activity after 10000 potential cycles,which was better than 59%of commercial Pt/C catalysts.（3）The Pd Cu hollow porous nanoparticles（Hollow Pd Cu）having abundant twin boundaries with an average diameter of 37.2 nm and an average shell thickness of 4.6 nm were prepared by step-by-step heating process and chemical dealloying post-treatment process using the bubbles（NH₃ and CO₂）generated by urea decomposition as the structure directing agent.The step-by-step heating process（TP）could promote the anisotropic growth of Pd Cu precursor during the reduction process,and generate a rich defect catalyst with unique twin interface.The chemical dealloying process with mild acetic acid leaching was performed to remove surface organic poisons together with Cu oxides,leaving with some structural defects and realizing surface Pd recombination.Therefore,in the electrochemical evaluation,hollow Pd₃Cu₁-TP exhibited higher mass activity and higher electrochemical active area,formic acid oxidation mass activity was achieved 1206.2 A g^-1,which was 2.2 times higher than that of commercial Pd black(540.9 A g^-1),and it had the highest i₀,the lowest R_ct,the lowest Tafel slope.In terms of stability,the chronoamperometry proves that the optimized Hollow Pd₃Cu₁-TP stability was better than commercial Pd black.