High Performance Platinum-based Medium Entropy Alloy And Effect Of Alloying On Electrocatalytic Performance

Hydrogen energy is one of the most important ways for dealing with energy shortages and environmental issues since it is carbon-free and clean.In hydrogen-oxygen fuel cells,due to the complex and slow kinetics of the cathode oxygen reduction reaction（ORR）,high-efficiency catalysts are required to participate in the reaction to greatly improve the conversion efficiency of fuel cells.At present,commercial platinum-carbon（Pt/C）catalysts still face problems such as low performance and poor stability,which seriously restrict the commercial application of hydrogen-oxygen fuel cells.Therefore,the development of high efficiency Pt-based alloy catalysts is one of the current research hotspots.The multi-component designed medium entropy alloys can effectively regulate the electrocatalytic activity and stability of Pt-based catalysts.Based on this,a series of medium entropy Pt-based alloys with precise composition and structure were designed by combining experimental synthesis and theoretical simulation calculation.Starting from the aspects of micro-strain,structural defects and electronic structure at the atomic level,the internal relationship between alloy composition,structure and performance is discussed.The main research results obtained are as follows:（1）Pt Cu Sn and Pt Cu W medium entropy alloys with precise compositions and structures were designed using density functional theory.The alloy has a typical face-centered cubic（FCC）structure,and there are Pt skin structure and subsurface Sn/W segregation phenomenon on the surface.The ORR electrocatalytic performance of Pt Cu Sn in acidic solution is better than that of pure Pt and Pt Cu by calculating the kinetic current of the ORR process.This is because the reasonable compressive strain existing in the alloy moderately negatively shifts the d-band center of Pt atoms,thereby optimizing the adsorption energy of ORR intermediates and accelerating the kinetic process.In addition,the vacancy energy of Pt atoms on the surface of the alloy reaches1.99 e V,which is higher than that of other catalysts,reflecting higher chemical stability.The experimentally prepared Pt Cu Sn@C exhibits a half-wave potential as high as 0.92 V,and still retains 85%mass activity after 5000cycles of cyclic voltammetric accelerated degradation tests.The experimental results are basically consistent with theoretical predictions.（2）Using the ethylene glycol thermal reduction system,a W microalloying medium entropy Pt Cu Co alloy（W-Pt Cu Co）was successfully prepared for the first time using a stepwise synthesis strategy.The alloy catalyst presents a typical worm-like morphology with a particle size of2.5-6.5 nm.The surface of the catalyst has abundant W-terminated stable defect structures,including terraces,twisted edges and corners.Under acidic conditions,the half-wave potential of the alloy is as high as 0.957 V,and the mass activity reaches 1.01 A mg _Pt^-1,which is 7.8 times higher than that of commercial Pt/C.After a 30,000 cycle cyclic voltammetry accelerated degradation tests,the mass activity decreased by only 3%,and its high catalytic performance was mainly due to the defect microstructure and local electronic reconstruction induced by W microalloying.Among them,the W-capped terrace and corner microstructures are the main active sites of ORR,and the existence of W also improves the stability of the active sites.（3）In addition,the application and performance characterization of medium entropy platinum alloy catalysts in proton exchange membrane fuel cell（PEMFC）devices were carried out.In the H₂-O₂single cell test,the peak power density of the W-Pt Cu Co catalyst was as high as 2.09 W cm^-2,significantly higher than that of commercial Pt/C.To further investigate the effect of catalyst surface purification on performance,W-Pt Cu Co was subjected to prolonged acid treatment.After 24 h treatment in perchloric acid solution,the performance improved significantly.When the cathode Pt loading is 0.3 mg _Ptcm^-2,the specific mass power density of the H₂-O₂single cell can reach 4.76 W mg _Pt^-1.After optimizing the type of carbon support and ionomer ratio,when the cathode loading is only 0.1 mg _Ptcm^-2,the specific mass power density of the single cell is as high as 6.97 W mg _Pt^-1.This indicates that the catalyst has good commercial application value.