Synthesis And Characterization Of Electrocatalysts Based On Crystalline Phase Control And Their Electrocatalytic Behavior

Nanocatalysts play a crucial role in electrocatalytic reactions.In the past decades,the activity,selectivity,and stability of catalysts have been improved by controlling the composition,facet orientation,and crystalline phase structure of catalysts.Crystalline phase control is one of the most effective methods to modulate catalytic performance by changing the arrangement of atoms,crystal cell structure,and energy band structure of catalysts.In this paper,several new crystalline materials with potential applications in electrocatalytic reactions,namely ordered alloys,4H crystalline materials,and amorphous materials,are investigated in terms of the synthesis of ordered alloys,the stability of 4H crystalline phases,the structure of the crystal facets under electrocatalytic conditions,and the ’structure-effect relationship’ of the catalysts.The main research contents are as follows:(1)Synthesis,characterization,and oxygen reduction(ORR)behavior of PtCu ordered alloys:Pt-based ordered alloys exhibit superior activity and stability in ORR.However,the synthesis of Pt-based ordered alloys usually requires a high-temperature annealing process,which inevitably leads to nanoparticle sintering and thus reduces the specific mass activity of the catalyst.We have invented a method for the lowtemperature synthesis of PtCu ordered alloys,namely the hydrogen reduction and deposition method.The PtCu ordered alloy is prepared by reduction and deposition of Cu2+using hydrogen atoms generated by the dissociation of hydrogen on the surface of Pt particles to form Pt core Cu shell structure(Pt@Cu),followed by annealing at a relatively low temperature(500℃).By comparing the changes in the physical parameters such as composition,particle size,crystalline phase,and orderliness of the alloy particles after annealing at different temperatures for the Pt@Cu nanoparticles synthesized by the reduction and deposition method and the samples synthesized by the impregnation method,it was found that the process of diffusion of Cu atoms on the carbon support takes place at high temperatures,and its energy barrier is higher than that of the crystalline phase transition of the PtCu alloy from a disordered solid solution structure to an ordered alloy structure.The core-shell structure of Pt@Cu nanoparticles effectively avoids the process of Cu atoms diffusion on carbon support when forming PtCu alloys,eliminating the highest energy barrier in the formation of PtCu ordered alloys,thus achieving the low-temperature synthesis of PtCu ordered alloys,and thus successfully avoiding the sintering that occurs during high-temperature annealing.By varying the annealing temperature,we achieved the synthesis of PtCu ordered alloy samples with different degrees of order and found that the ordered alloy PtCu-H2-600 sample exhibited higher ORR activity(specific mass activity:1.67 A mgPt-1 and specific area activity:1.24 mA cm-2@0.9 VRHE)than that of PtCu disordered alloy and Pt/C.We also found a linear relationship between the orderliness of PtCu ordered alloys and the specific area activity of ORR and constructed a preliminary ’structure-effect relationship’ between the ORR activity and the orderliness of the alloys,which can provide a reference for the design and preparation of higher activity ORR catalysts.(2)Synthesis,surfactant removal,structure,and electrocatalytic behavior of 4H crystalline gold nanowires(4H Au NWs):4H Au NWs with the unique ’ABCB’ close packing sequence of Au atoms were first successfully synthesized in 2015.And it has attracted much attention for its superior catalytic performance to its FCC counterparts face-centered cubic(FCC)Au nanoparticles(Au NPs)).However,few previous studies have focused on the influence of surfactant(oleylamine molecules)adsorbed on the 4H Au NWs on their catalytic performance.We have tried to remove the adsorbed oleylamine molecules by acetic acid,electrochemical cleaning,and diethylamine substitution,and systematically investigated the performance of electrocatalytic CO oxidation.It was found that the 4H crystalline phase transformed into the more thermodynamically stable FCC crystalline phase.The nanowires transformed into dumbbell-shaped large particles after removing surfactants by any method.In addition,we also characterized the crystal surface structures of the above samples using the surface structure-sensitive lead-underpotential deposition technique(Pb-upd)and found that the dumbbell-shaped large particles have mainly defect-rich Au(111)facets,while the small-sized gold particles have mainly defect-rich Au(110)facets.Moreover,the small particles exhibit higher CO oxidation activity due to their higher content of Au(110)facets.This was further confirmed by designing calibration experiments for the selective deposition of Pb2+on Au(110)faces inhibiting their CO oxidation activity.(3)Study of the intrinsic mechanism of the oxygen evolution reaction(OER)of amorphous iridium nanosheets(A-Ir Ns):A-Ir Ns exhibits higher activity in the OER reaction than crystalline IrO2.Although previous studies have characterized its amorphous structure and the change of iridium elemental valence during OER by transmission electron microscopy,X-ray absorption spectroscopy,etc.,the oxidation process and the adsorption process of oxygen-containing species on the surface of A-Ir Ns are still unclear.We tried to use electrochemical impedance spectroscopy(EIS)to analyze the adsorption processes of surface oxygen-containing species on A-Ir Ns and crystalline IrO2 during OER and found that the oxidation of Ir0 to Ir4+on A-Ir Ns occurred firstly before OER,which in turn changed the capacitive properties of the system.The results of the model simulation indicate that the adsorption resistance of A-Ir Ns is smaller than that of N-IrO2 during the surface oxidation process,which indicates that its surface has a faster oxygenated species adsorption/desorption process.However,the impedance of OER process suggests that both A-Ir Ns and N-IrO2 have faster adsorption/desorption processes of oxygenated species in the OER process,and the rate of adsorption/desorption of oxygenated species may not be the key factor to determine the OER rate,but the coupling process may be the critical factor to determine the OER reaction rate.