Electrocatalytic Performance Of Polyamine Functionalized Noble Metal Nanocrystals

Oxygen reduction reaction(ORR)and hydrogen evolution reaction(HER)are important half-reactions of fuel cells and water electrolysis respectively,which are important components of future new energy conversion devices.So far,noble metals nanocrystals,such as Pt,Pd and their alloys,are still the most active ORR and HER catalysts.However,the development of them is severely limited by its price and earth-abundant.Rationally controlling the morphology,chemical composition,and interfacial property of Pt-based and Pd-based nanocrystals can remarkably enhance their electrocatalytic performance,such as activity and durability.In this thesis,we focus on the design of functional Pt-based and Pd-based nanocatalyst,which is a versatile and effective strategy for ORR and HER.The superior performance generally originates from the unique metal-organic interface,including the electron interaction between metal atoms and organic polymers,the synergistic effect between metal atoms,as well as the unique structure size effect,crystal surface effect.The details are as follows:(1)The surface chemical functionalization of noble metal nanocrystals is a promising strategy for improving the catalytic/electrocatclytic activity and selectivity of noble metal nanocrystals.In this work,we successfully synthesize the polyallylamine(PAA)with different molecular weight functionalized Pt nanodendrites(Pt-NDs)using a facile hydrothermal reduction method.The morphology and surface composition are investigated by transmission electron microscopy,element map,and thermogravimetric analysis.Furthermore,we detailedly investigate the effect of the molecular weight of PAA on the electrochemical property of the functionalized Pt-NDs.Electrochemical measurements show only low molecular weight PAA functionalized Pt-NDs allow electrolytes to access freely the Pt sites.Meanwhile,the low molecular weight PAA functionalized Pt-NDs show the excellent selectivity and activity for the oxygen reduction reaction in the presence of methanol.(2)The electrocatalytic performance of noble metal nanocrystals highly depends on their surface structure and interface structure.Effective surface/interface control over noble metal nanocrystals can significantly improve their electrocatalytic activity,durability,and selectivity for the various important electrochemical reactions in low-temperature polymer electrolyte fuel cells.In this work,the PAA functionalized Pt nanocrystals with long-spined sea urchin-like morphology(Pt-LSSUs@PAA)have been synthesized successfully through a simple chemical reduction route.The high branching degree of Pt-LSSUs@PAA nanocrystals and the sheet morphology of the branches effectively improve the utilization of the Pt metal.The particular 3D interconnected architecture of Pt-LSSUs@PAA nanocrystals significantly enhances the electrochemical stability.Loose-packed PAA layers on the surface of Pt-LSSUs@PAA nanocrystals efficiently modify the electronic property of Pt atoms and serve as barrier networks to restrain the accessibility of alcohol molecules.As a result,the as-prepared Pt-LSSUs@PAA nanocrystals show the high activity,excellent durability,and particular alcohol tolerance for the oxygen reduction reaction in acidic media.(3)Owing to alcohol crossover problem,improving the selectivity of cathodic electrocatalysts for the ORR is one of the key challenges for the commercial viability of direct alcohol fuel cells.Pd is highly efficient Pt-alternative elelctrocatalyst for the ORR in alkaline media but it is also highly active for the alcohol oxidation reaction(AOR)in alkaline media.In this work,we demonstrate an efficient water-based synthesis of Pd nanowires(Pd-NWs)in presence of polyethyleneimine(PEI)with branched structure.Experimental results show the formation of Pd-NWs originates from oriented attachment.During the synthesis,Pd-NWs are simultaneously functionalized by PEI due to strong Pd-N interaction,which is confirmed by molecular dynamics simulation and various physical characterizations.PEI layers on Pd-NWs efficiently serve as "molecular window gauze" to physically block the access of alcohol molecules to Pd sites but allow the access of oxygen molecules owing to their difference in molecular size,resulting in excellent alcohol tolerance of Pd-NWs for the ORR in alkaline media.(4)Rationally controlling the morphology,chemical composition,and interfacial property of metal nanocrystals can remarkably enhance their electrocatalytic performance,such as activity,selectivity and durability.In this work,we develop a facile functional molecule assisted cynaogel-reduction method to synthesize successfully the PAA functionalized PdCo alloy nanonetworks(PdCo-NNW@PAA)inorganic-organic nanohybrids.The solid,double-metal and three dimensional backbone properties of jelly-like K2PdCl4/K3Co(CN)6 cyanogel intermediate contribute to the high alloying degree and network structure of PdCo-NNW@PAA nanohybrids.During the reduction,PAA molecules not only serve as surfactant to decrease the particle size but also act as functional molecule to modify the metal surface.The ORR polarization and chronoamperometry tests demonstrate that PdCo-NNW@PAA nanohybrids have outstanding electrocatalytic activity,excellent resistance to alcohol crossover effect,and good durability towards the ORR in alkaline media.(5)Tailoring the size,controlling the morphology,and designing the metal-organic interface are three promising strategies to improve the catalytic performance of monometallic noble metal nanocrystals.In the "hydrogen economy" society,the water electrolysis is viewed as one of the most promising technologies for the hydrogen production.The design and synthesis of highly active and durable electrocatalysts for the HER is vitally important for development of "hydrogen economy".In this work,we synthesize successfully the PAA functionalized Pt tripods(Pttripods@PAA)with ultrathin and ultralong branches through a facile chemical reduction method in a PAA aqueous solution.The morphology,structure,and composition of Pttripods@PAA are fully investigated by various physical techniques.Characterization results reveal that ultrathin and ultralong branches at Pttripods@PAA have the concave structure with high-index facets and PAA strongly binds on the Pt surface as a surface proton carrier.Impressively,Pttripods@PAA display unexpected activity for the hydrogen evolution reaction in an acidic solution with an onset reduction potential at+19.6 mV versus a reversible hydrogen electrode due to the increase in the local proton concentration on the Pt surface,which significantly outperforms currently reported monometallic Pt electrocatalysts.(6)The electrocatalytic HER is highly promising green method for the sustainable and efficient hydrogen production.So far,Pt nanocrystals are still the most active electrocatalysts for the HER in acidic medium,although the tremendous search for alternatives have been done in past decade.In this work,we synthesize the PEI functionalized Pt superstructures(Pt-SSs@PEI)with tetragonal,hierarchical,and branched morphology by a facile wet chemical reduction.A series of physical characterizations are conducted to investigate the morphology,electronic structure,surface composition,and formation mechanism of Pt-SSs@PEI.Impressively,the as-prepared Pt-SSs@PEI show unprecedented onset reduction potential(+64.6 mV vs.reversible hydrogen electrode)for the HER in strong acidic medium due to the protonation of-NH2 groups at PEI adlayers on Pt surface,which outperforms all currently reported HER electrocatalysts.