Controllable Synthesis Of Pt-based Multimetallic Core-shell Nanocrystals For Oxygen Reduction

The energy crisis and environmental pollution caused by the massive consumption of fossil fuel are increasingly serious,and thus the development of environmentally friendly green and clean energy is quite imminent.As one of the clean energy technologies,proton-exchange membrane fuel cells(PEMFCs)are getting more and more attention due to their significant advantages such as high energy conversion efficiency,high energy density and low or zero pollutant emission.However,the sluggish reaction kinetics of ORR at the cathodes in PEMFCs often requires a relatively high load of Pt to achieve a desirable fuel cell performance in the practical applications,which is severely restricted by the high cost and scarcity of Pt.As such,tremendous efforts have been employed to develop active,stable,and economical Pt-based catalysts towards ORR over the past decade.One of the most popular methods is the deposition of a thin or even atomic layer thick Pt shell on the surface of a core made of less noble and/or low-cost metals to form core-shell nanocrystals.Such Pt-based core-shell electrocatalysts not only maximize the utilization efficiency of Pt,but also offer great opportunity to tune the ORR performance in terms of activity and stability through the ligand and geometry effects,which has received abundant research interests recently.This dissertation focuses on controllable synthesis of Pt-based core-shell nanocrystals and their use as ORR electrocatalysts in PEMFCs.By tunning the composition,structure and morphology,Au-Pd@Pt icosahedral and Pd-Cu@Pt truncated octahedral core-shell nanocrystals were successfully synthesized through a simple wet-chemical method.In addition,their catalytic performances in terms of activity and stability for ORR were systematically investigated.The main innovative results are summarized as follows:(1)A three-layer core-shell icosahedral nanocrystals with an Au-Pd alloy core,a Pd interlayer and an ultra-thin Pt shell were successfully generated by using the epitaxial growth of Pt on the AuPd icosahedral seeds under thermodynamic control.By varying the molar ratio of the Au and Pd salt precursors,the thickness of the Pd interlayers could be adjusted to 12,6,3,and 0 atomic layers,respectively.All Au-Pd@Pt nanocatalysts outdistanced Pt/C for ORR activities while a volcano trend could be obtained from the relationship between ORR activities and the thickness of Pd interlayers.Specifically speaking,Au60Pd40@Pt icosahedra with about six atomic layers of Pd interlayers displayed the highest area specific activity and mass activity,which are 7.5 and 11.6 times higher than those of Pt/C,respectively.Significantly,Au60Pd40@Pt icosahedra remained most of the activity(91.4%)after 50,000 cycles,exhibiting extremely excellent activity and stability.(2)Pd2Cu@Pt,PdCu@Pt and PdCu2@Pt truncated octahedral core-shell nanocrystals were successfully synthesized through the epitaxial growth of Pt on the PdCu seeds under thermodynamic control.Such three nanocrystals exhibited superior ORR performances relative to commercial Pt/C.Among them,Pd2Cu@Pt nanocrystals with a Pd interlayer showed the highest area specific activity(0.46 mA/cm2)and mass activity(0.59 mA/?gpt),which were superior to PdCu@Pt and PdCu2@Pt without a Pd interlayer.The introduction of inexpensive Cu not only improved the ORR activities,but also significantly reduced the cost of the catalysts.