Preparation And Electrocatalytic Performance Of Noble Metal Nanomaterials For Oxygen Reduction Reaction

The fuel cell is an electrochemical device,which can directly convert the chemical energy of the fuel into electrical energy through the chemical reaction at the interface of the electrode and electrolyte without going through the combustion process.Moreover,it is not limited by the Carnot cycle,so the energy conversion efficiency is very high without noise and pollution.Among various types of the fuel cells,the proton exchange membrane fuel cell(PEMFC)has been shown to be a promising power source for electric vehicles and stationary power generation programs.However,the high cost of current PEMFCs,insufficient power density,as well as their poor performance durability greatly limit their large-scale application.And thus,the modification and research of precious metal-based catalysts are particularly important.At present,researchers have introduced transition metals to reduce the loading of precious metals in catalysts,improve catalytic activity and stability to alleviate the impact on precious metals.On the other hand,efforts are being made to develop new types of precious metal-based-catalysts to replace traditional expensive platinum-based catalysts.Based on these aspects,we have done the following three research works:1.We reports a very simple and novel strategy to fabricate single Au atom doped carbon catalysts.A small amount of Au is doped into ZIF-8 by etching using the cation displacement reaction and the Kirkendall effect,and Then in the polycondensation process of dopamine,it is further etched by the strong chelation between the metal cation and the monomer to form a hollow material.Finally,the as-synthesized Au nanoclusters generated at initial stage are decomposed into Au mono-atoms by high-temperature calcination under a N₂ atmosphere,and Au mono-atoms were trapped and fixed by N defects formed by broken Zn-N bonds to produce Au monoatomic modified hollow MOF framework materials(Au-N-C).The introduction of Au single atom improves the stability of the material,and the hollow morphology is conducive to the exposure of more active sites and the improvement of mass transfer.The high-temperature pyrolysis of the N element that is originally rich in N and polydopamine as a nitrogen source in the MOF material is beneficial to improve the conductivity of the material.The half-wave potential of Au-N-C catalyst in 0.1 M KOH solution is 0.80 V,close to 0.83 V of commercial Pt/C,and the catalyst has good stability and good methanol toxicity resistance.2.We synthesized one-dimensional,PtNi nanowires via organic solvent reduction method,and then Ni-B film modified PtNi nanowires(PtNi-B NWs)were obtained by secondary reduction with sodium borohydride in the later stage.In the process of characterizing the morphology of the material by transmission electron microscopy,it was found that the obvious lattice fringes disappeared after the secondary reduction with sodium borohydride,indicating that part of the Ni in the alloy was reduced by sodium borohydride to form amorphous Ni-B membrane.In addition,PtNi-B NWs/C showed excellent electrochemical activity in the characterization of electrochemical performance.The measured half-wave potential during the catalytic oxygen reduction reaction in 0.1 M HClO₄ was close to 0.90 V,which was positively moved by 45 mV compared with that of PtNi NWs/C(0.855 V)without secondary reduction.Furthermore,the half-wave potential was only negatively shifted by 15mV after 2000 cycles of accelerated cycle test,indicating its excellent stability.3.PdCu ultrathin nanosheet-composed three-dimensional(3D)nanoflowers(Pd₁Cu_xNCFs)with different compositions were prepared by a facile one-pot hydrothermal method.Compared with traditional two-dimensional nanomaterials,their abundant routes for fast mass transport,high Pd atom utilization efficiency as well as the synergistic effect of PdCu bimetal endow Pd₁Cu_x NCFs with enhanced ORR performance under alkaline condition.In addition,we studied the effect of the dosage of Pd and Cu precursors on the ORR performance of the products.Electrochemical measurement results indicated that the optimal catalyst(Pd₁Cu_0.5NCFs,the molar ratio of Pd/Cu precursor is 1:0.5)exhibited the best ORR performance.A half-wave potential(E_1/2)of 0.937 V in alkaline medium was obtained,which is higher than that of commercial Pt/C(0.851V).Electrochemical accelerated durability test results indicated that the Pd₁Cu_0.5 NCFs could endure at least 1000 cycles with negligible activity decay,suggesting their excellent stability.At 0.90 V,the mass activity of Pd₁Cu_0.5 NCFs could achieve 1.09 A/mg _Pd,which is 14.5 times higher than that of commercial Pt/C.