Synthesis Of Heteroatoms Doped Intermetallic Compounds/Carbon Based Nanocomposites By Pyrolysis Of Metal Organic Frameworks And Their Electrocatalytic Properties

Intermetallic nanoparticles(NPs)have proven to exhibit superior catalytic activity and stability to solid solution alloy due to their unique highly ordered atomic structure.Recently,encapsulating functional NPs with a carbon layer of a suitable thickness can not only prevent nanoparticle agglomerat:ion and transition metal dissolution effectively,but also significantly modify the electronic structure of nanomaterials,which plays a crucial role for enhancing their performances in elecltrocatalytic reactions.Of note,the electronic structure and surface structure of the catalyst can be changed by doping appropriate amount of p-zone elements(B,N,P,etc.)in metal NPs or carbon support,thus affecting its electrochemical performances.However,it is a great challenge to construct such heteroatom-doped carbon-encapsulated metal catalyst composites by a simple method to date.Metal organic framework(MOF)is a kind of porous coordination polymer with a periodic structure,which are formed by self-assembly of metal ions or metal clusters with an organic ligand.On the one hand,MOF contains a large number of cavities,which can accommodate other materials(such as nanoparticles,nanoclusters,quantum dots,enzymes,etc)to form composites with special functions.On the other hand,MOF has adjustable structure,abundant ligands and lots of heteroatoms,which are easy to dope the products during pyrolysis.Therefore,MOF is considered to be ideal precursor for the preparation of heteroatom-doped metal/C composites.In this thesis,we proposed a facial synthesis route to obtain heteroatom-doped carbon-encapsulated intermetallic NP(PtZn@NC)composites via pyrolysis of Pt@ZIF-8.Besides,we systematically studied the performances of the PtZn@NC composites in electrocatalytic reaction such as oxygen reduction(ORR)and hydrogen evolution reduction(HER).The main research contents are summarized as follows:(1)We investigated the influences of pyrolysis temperature and Pt loading amounts on the composition and structure of products.Through pyrolysis of 10%-Pt@ZIF-8 precursors at 800 ℃,intermetallic PtZn NPs that formed via alloying of Pt NPs with Zn were encapsulated by a layer of thin N-doped carbon shell.With the increasing of temperature and Pt loading amount in precursors,the structure of alloy NPs transformed from intermetallic PtZn alloy to fc structured PtZn alloy,while the particle size significantly increased.The electrochemical test results demonstrated that the as-prepared best catalyst 10%-PtZn@NC-800 exhibited outstanding activity and stability in the ORR reaction under acidic conditions.The half-wave potential was 0.912 V(vs.RHE),which is more positive 26 mV than the commercial Pt/C.And the mass activity and specific activity of 10%-PtZn@NC-800 were nearly 3 times and 5 times as high as that of commercial Pt/C,respectively.After 5000 cycles,the half-wave potential was only shifted by 1 mV.In addition,the synthesized PtZn@NC catalysts exhibited excellent properties in anode reactions such as formic acid and methanol oxidation.(2)We investigated the influences of external dopants on the composition and structure of products.When ammonia borane was used external dopant precursor,B atoms were doped into the carbon matrices of PtZn@NC catalysts.In contrast to this,P atoms were doped into the lattice of PtZn alloy NPs of PtZn@NC catalysts when NaH2PO2 was used external dopant precursor.In the two cases,with pyrolysis temperature increasing,a phase transformation of alloy NPs occurred accompanying with a significant increase in particle size.Such heteroatom doped PtZn@NC catalysts exhibited significantly enhanced activity in the HER reaction under alkaline conditions compared to undoped counterparts.Specifically,the overpotential at current density of 10,50,and 100 mV cm-2 was smaller than that of commercial Pt/C,as well as much lower than that of undoped PtZn@NC catalyst.These findings well demonstrated that ideal electrocatalysts with excellent activity and stability could be produced by rationally engineering the interface structure between active metal NPs and carbon supports.And this work provides us a new engineering strategy to acquire highly active and stable multifunctional catalysts for PEMFCs.