Synthesis Of Binary Pd-M(Sn,Pb) Nanoalloys And Their Applications In Oxygen Reduction

Fossil fuel consumption brings the world's energy crisis and environmental pollution problems.Fuel cells,as energy storage and conversion devices,are the most potential candidates for mitigating energy problems.The oxygen reduction reaction?ORR?mainly occurs at the cathode of the fuel cell,but its slow dynamics has always been a bottleneck problem of the fuel cell.Although it has been reported in the literature that a variety of Pt-based catalysts have been synthesized to accelerate ORR kinetics,their disadvantages,such as limited reserves,high cost and poor stability,seriously hinder the development of their commercialization.Therefore,there is an urgent need to design and synthesize low-cost,high-activity,good-stability electrocatalysts to accelerate the reaction kinetics and reduce the electromotive force of the reaction.Metal palladium?Pd?has been considered as the most promising alternative to Pt,with abundant storage and activity comparable to Pt,but the catalytic activity of pure Pd is far less than that of Pt catalyst,and its stability can be improved.In view of the current research status of Pd-based catalysts,we synthesized nano-network intermetallic phase Pd₃Pb and a series of Pd_xSn_y nanocrystals and related nanohybrids.Through the corresponding test instruments,the morphology characteristics were observed,the corresponding components were determined,and the mechanism of the reaction was explained.The corresponding ORR test proves that nanocrystals and corresponding nanohybrids have excellent electrocatalytic activity and stability.The specific synthesis method and test results are as follows:?1?The surfactant polyethylene glycol monohexadecyl ether is first dispersed in the solvent of DMPU,then ODE is added to form a uniform micelle solution,and then a suspension of Pd?acac?₂and Pb?acac?₂ is added.The intermetallic phase Pd₃Pb network nanostructure was synthesized by hydrothermal method under the reduction of borane-tert-butylammonium complex.During the reaction,the growth process of nano-network Pd₃Pb was explored by the change of reaction time and temperature,the results showed that the optimal reaction temperature and reaction time were 180 ^oC and 10 h.TEM and HRTEM demonstrate the synthesis of intertwined multilayer network structures with a large number of holes.The results show that intermetallic phase Pd₃Pb has excellent catalytic performance,after the acid etching,the ORR half-wave potential of the network intermetallic phase Pd₃Pb is further improved.The porous network structure and high specific surface area,which not only facilitates the transport of electrons and electrolytes,but also increases the possibility of active site exposure.?2?The element Sn and the element Pb belong to the same group element and have similar physicochemical properties.Therefore,we used a simple one-pot non-aqueous solvothermal method to changed the molar ratio of the Pd and Sn metal precursors used to synthesize a series of Pd_xSn_y NCs?Pd₃Sn,Pd₂Sn and Pd₃Sn₂?electrocatalysts.XRD proves that Pd-Sn nanocrystals with different components and phase structures are obtained,such as cubic phase Pd₃Sn NCs,orthorhombic phase Pd₂Sn NCs and hexagonal phase structure Pd₃Sn₂ NCs.The results show that the order of ORR activity of different components Pd_xSn_y NCs is:Pt/C>Pd₃Sn>Pd₂Sn>Pd₃Sn₂ NCs,the cubic phase of Pd₃Sn NCs has the best catalytic activity and good stability.XPS analyzed the chemical state and surface electronic structure of Pd₃Sn NCs,and proved that Pd and Sn exist mainly in the form of simple substance.Compared with the electron binding energy?BE?of pure Pd,the peak position of Pd 3d is shifted to the high BEs direction,confirming the electron transfer in the obtained sample was transferred from Pd to element Sn,and the related kinetic tests also confirmed that strong polarization occurred between Pd and Sn.?3?Although Pd_xSn_y NCs has good ORR activity,its activity is lower than that of Pt/C catalyst,and its stability needs to be improved.On the basis of the previous work,we have achieved the improvement of catalytic activity and stability by the addition of carriers to form nano-hybrids.First,the carboxyl-functionalized carbon nanotubes?CNTs?are dispersed in a mixed solvent of octadecene and oleylamine,and a palladium acetate and a tetrabutyltin metal precursor are added,using a simple one-pot non-aqueous solvothermal method to synthesize a series of different components Pd_xSn_y/CNTs?Pd₃Sn/CNTs,Pd₂Sn/CNTs,Pd₃Sn₂/CNTs,PdSn/CNTs?nanohybrids.TEM and HRTEM indicated that Pd₃Sn NCs was firmly anchored on the side of CNTs,and some nanocrystals were embedded in the surface of CNTs,which enhanced the interaction between Pd₃Sn NCs and CNTs and contributed to the improvement of electrocatalytic stability.The ORR performance test results showed that the addition of CNTs improve the catalytic activity and stability.Among the different nano-hybrids,Pd₃Sn/CNTs have the best catalytic activity,and the performance in alkaline electrolyte exceeds that of commercial Pt/C.In addition,the anti-methanol test results show that the anti-methanol performance of Pd₃Sn/CNTs is better than that of commercial Pt/C.XPS and corresponding kinetic tests showed a similar"double volcano"shape between the Pd content of the nanohybrids and the Pd 3d binding energy?BE?and mass specific activity?j_m?,suggesting that the electronic structure of the catalyst can be effectively adjusted by changing the components of Pd and Sn in the alloy.Due to its binding energy value maintains a dynamic balance between the chemical adsorption of O₂ molecules and desorption of intermediate oxygen species.Therefore,Pd₃Sn/CNTs NHs showed the best catalytic performance.