Electrochemical Synthesis Of Porous Palladium Nanomaterials And Studies On Their Electrocatalytic Performance

Electrocatalytic reactions have extensive applications in fuel cell,clean energy,CO₂ immobilization,pollutant degradation,etc.fields.In these applications,catalysts served as the most important part enable the functioning of the whole system.The catalysts with remarkable performance can reduce the energy losses and increase the reation rates,which are very important to improve the electrocatalysis technology.Noble metals such as Pt and Pd are qualified in many kinds of electrocatalytic reactions because of their good corrosion resistance and high catalytic activities.However,the limited source and high price preclude their large-scale application.Therefore,special attention is paid to reduce the loading of noble metals and to enhance their catalytic activities.Up to now,noble metals prepared through strategies of controlling the surface lattice plane and modifying the composition can not achieve excellent catalytic activies and stabilities simultaneously.While,this issue can be well solved by introducing porous structures in noble metals.In this work,by focusing on preparing controllable porous structures and taking Pd as the research target,we prepared three kinds of porous nanomaterials with varied structures,which were ordered mesoporous film,free-standing nanotube arrays and free-standing mesoporous nanotube arrays.The effects of preparation processes on their morphologies and structures were systematically studied,and the growth mechanism were thoroughly discussed.The electrooxidation of formic acid and formate were choosen as a model to prove that the controllable porous structures could significantly enhance the electrocatalytic performance of noble metal nanomaterials.The major work was as follows:1.The ordered mesoporous Pd film was prepared by electrodeposition method with an in-situ formed liquid crystal template.During electrodeposition process,the template formed reversed double diamond(Q₂₂₄)liquid phase which contained two continuous channels.Pd could only fill one of them due to the relatively large diameter of its precursors,and finally resulted in a film with single diamond(Q₂₂₇)nanostructure.The continuous channels in the film were completely connected to the outer solution,making it easier for liquid mass transfer.The film had an electrochemical active surface area?ECSA?of 90.5 m²/g and a catalytic activity toward formic acid of 3.34 A/mg,both of which were much higher than those of commercial Pd/C and non-porous films.It was shown that changing the temperature and the potential in electrodeposition process would have an apparent effect on the film's morphology.The perfect ordered mesoporous structure could be obtained only in the condition of 20?and 0.1 V.If the temperature rose or the deposition potential dropped,the mesopores would lose their order.On the contrary,if the film was deposited at a relatively high potential,the spherical mesoporous nanoparticles could be obtained.Afterwards,electrochemical tests were performed on these films having different morphologies.It showed that the film with ordered mesoporous structure had the highest ECSA and catalytic activity toward formic acid,while others with disordered mesopores couldn't reach this level.This result proves that the ordered mesoporous structure is very useful to improve the performance of catalysts.2.The free-standing Pd nanotube arrays were prepared by electrodeposition method with an AAO template.The nanotubes had a very regular shape and were opened on both sides.The arrays could be transported to any substrate without damaging the array structure.The effects of deposition conditions such as electrolyte concentration,deposition potential,and deposition time on the morphology of these arrays were studied to realize their growth mechanism.Only in mild conditions,when the growth rate of Pd along the axis of the template pores was identical to that of ions'diffusion can we produce the thin-walled nanotubes.Otherwise,the nanotubes would turn into nanowires in electrolytes with high concentration or turn to irregular dendrites at low deposition potentials.Electrochemical tests were conducted on the free-standing Pd nanotube arrays.The results showed that their ECSA was 43.7 m²/g,and their electrochemical activities toward formic acid and formate were 0.95 A/mg and 1.00A/mg,respectively.Furthermore,they could retain 96.7%of the activity after 50catalytic cycles in formate solution.3.Based on the free-standing Pd nanotube arrays,free-standing Pd mesoporous nanotube arrays were also prepared by combining AAO with the soft template.The using of soft template led to the formation of abundant mesopores in the nanotubes.Meanwhile,it also confined the diffusion of ions in the solution.As a result,the electrolyte concentration had less effect on the morphology of the arrays,while the deposition potential had a greater one.The nanotubes would turn into hollow nanowires if they were deposited at lower potentials.The electrochemical tests showed that the mesoporous nanotube arrays had a larger ECSA?82.7 m²/g?and higher catalytic activities toward formic acid?3.64 A/mg?and formate?3.87 A/mg?than those of the normal nanotube arrays prepared before,not only owing to their larger specific surface area,but also to the numerous defects produced by the mesopores.In addition,they could sustain 92.6%of their initial activity after 50 catalytic cycles in formate solution,showing their excellent cyclic stability.