Preparation And Oxygen Reduction Property Studies Of The Palladium-based Nanocatalysts

In recent years,with the increasing consumption of fossil fuels and the gradual deterioration of environmental problems,the development of clean and efficient renewable energy technologies is an inevitable choice for achieving sustainable social development.As a type of new energy conversion technology,fuel cells can directly convert chemical energy into electrical energy with high efficiency and less pollution.Among them,the proton exchange membrane fuel cell is expected to be widely employed in transportation and portable power generation devices due to its simple structure,low operating temperature,high power density and fast starting speed.Compared with the hydrogen oxidation reaction of the anode,the oxygen reduction reaction of the cathode involves multi-electron,multi-element chemical process with slow kinetics,which is the key to the performance of the fuel cell.Platinum-based materials are widely employed as conventional cathode catalysts of commercial fuel cell.However,platinum has a very low crustal content,high price?catalyst accounts for about 50%of the total cost of the fuel cell?,and relatively weak stability and anti-toxicity.The above defects are the main bottleneck restricting the development of fuel cells.Therefore,the development of a wide range of high-efficiency and stable non-platinum-based catalysts is essential for solving the large-scale commercialization of fuel cells.This study focuses on the research of metal palladium in two major directions.One is to synthesize palladium nanomaterials by employing specific biomolecules?polypeptides?as ligands,and to explore the relationship between polypeptide sequence,morphology and oxygen reduction catalytic performance.The second is to develop stable,high-efficiency and low-cost electrocatalysts by utilizing the synergistic effect between palladium and transition metals to promote the development of cathode catalysts for fuel cells.The specific research content includes the following four aspects:?1?A series of peptides such as Pd4,C6,C11,C6,11,A6C11 and C6A11 were used as ligands to synthesize palladium nanoparticles as high-efficiency oxygen reduction catalysts.By replacing amino acid groups at specific positions?six,eleven?,it could effectively regulate the binding mode of the peptides and the structural properties of the palladium nanoparticles.The circular dichroism spectrum showed that the C11,A6C11 and the corresponding palladium nanoparticles had the largest ellipticities,indicating that the surface structures were more disordered.For alkaline oxygen reduction,the catalytic performance followed the following rule:C11-,A6C11-PdNPs>C6-,C6,11-and C6A11-PdNPs>Pd4-PdNPs.C11-PdNPs and A6C11-PdNPs had the highest oxygen reduction activity,and their stability outperformed Pt/C and Pd/C,C11 and A6C11 not only improved the activity of palladium nanoparticles,but also ensured their stability.It was found that when the terminal?11 position?of the peptide was substituted by cysteine,the other amino acids were sufficiently released to provide sufficient space for electron transfer between oxygen and palladium,which promoted the oxygen reduction reaction.This study successfully established the structure-activity relationship between peptide sequence,morphology structure and catalytic performance,which had certain guiding significance for the rational design of bionic metal materials.?2?The peptide R5?SSKKSGSYSGSKGSKRRIL?was selected as a template,and its self-assembly property was utilized to guide the nucleation growth and form a peptide-palladium nanomaterial as an ORR catalyst.It was found that the ratio of palladium to peptide R5 can be adjusted to obtain a series of R5-Pd nanomaterials with different morphological structures.With the increase of palladium/R5 ratio,palladium nanoparticles were gradually transformed into palladium nanobelts and palladium nanonetworks.As for electrocatalysis,R5-Pd-90 exhibited the greatest activity and stability for oxygen reduction,meanwhile,it displayed the largest electrochemical activity area and mass activity,indicating that R5-Pd-90 had the largest intrinsic activity.The study showed that the peptide can directly regulate the structures and properties of the metal materials.When the peptide was excessive,the metal surface active sites were over-covered,which hinders oxygen reduction.When the peptide was too low,the metal particles became agglomerated due to lack protection of pepetide ligand,and some active sites began vanish,which also hindered oxygen reduction.The reasonable proportional regulation was critical to the structures and performances of the oxygen reduction catalysts.?3?The thermally stable metal-organic frameworks?ZIF-67?was employed a template to obtain cobalt nanoparticles embedded in nitrogen-doped porous carbon?Co NC?by calcining under an inert atmosphere,and then the chemically displacement reaction with Na₂PdCl₄ was carried out to form Co@Pd core-shell nanocomposities.For electrocatalysis,Co@Pd NC exhibited superior oxygen reduction and hydrogen evolution properties than Co NC and commercial Pd/C,showing perfect catalytic activity and stability.EDX,XPS and ICP-MS had proven that the atomic percentage of Pd is only 1.2%.The Co@Pd core-shell structure provided a large number of active sites for the reduction of oxygen,which improved the metal utilization.The synergistic effect between the core and shell accelerated the rupture of the O-O bond and promoted the four-electron transfer kinetics of oxygen reduction.At the same time,the structure of M-N-C could not only increase the stability of the palladium particles,but also accelerated the electron transfer between the metal palladium and oxygen molecules,reduced the oxygen dissociation energy,and promoted the oxygen reduction reaction.This research provided a certain reference value for the development of high-performance low noble metal catalysts.?4?Three type of palladium-zinc bimetallic composite nanoparticles?Pd/Zn=1/1?with similar particle sizes and different surface structures were synthesized,named PdZn_Disordered,PdZn_Ordered and Pd@Zn_Core-shell,and the effect of surface structure and atom mixting pattern were investigated.TEM,EDX,XRD and other means showed that PdZn_Disordered and PdZn_Ordered was amorphous and ordered alloy particles,respectively.For Pd@Zn_Core-shell,ordered-palladium-core and disordered-zinc-shell structures could be found.In the ORR electrochemical test,Pd@Zn_Core-shell exhibited the highest kinetics,intrinsic activity and stability.The performance of the three samples followed the order of Pd@Zn_Core-shell>PdZn_Ordered>>PdZn_Disordered,which was attributed to the ordered arrangement of metal atoms and the synergistic effect of core-shell structures.The uniform and ordered distribution of Pd atoms increased the utilization of active sites and accelerated the transfer,activation and dissociation of oxygen molecules.At the same time,the strong electron interaction between Pd@Zn core-shell structure reduced the d-band center of metal palladium,accelerated the fracture of O-O bond,reduced the formation of intermediate products,and improved the efficiency of oxygen reduction.Finally,the flocculated and disordered zinc shell incompletely surrounded in the periphery of the Pd core not only provided sufficient palladium active sites for the oxygen reduction,but also prevented the oxidation of palladium atoms by air,which was beneficial to the long-term stability of the Pd@Zn_Core-shell.