Preparation And Applications Of Electrocatalysts For Proton Exchange Membrane Fuel Cells

Fuel cells have been proven to be promising clean energy devices during the past few decades,among them,proton exchange membrane fuel cells?PEMFC?have attracted the most extensive attention.There are many different fuel resources for these fuel cells,for instance,hydrogen from high-pressure tank,hydrogen from methanol or ethanol reforming,methanol or ethanol as liquid fuels.Direct usage of liquid fuels such as methanol and ethanol not only removes the troublesome process of fuel reforming to produce hydrogen,but also reduces the loss of overall energy density.Therefore,there has been an increasing interest in the development of direct methanol fuel cells,particularly for applications to the electric vehicles.Although the technology has been advanced greatly in the past few decades,there are still major technical problems that hinder their commercialization.Among them,large-scale preparation of high-efficient catalysts is one of the core issues.Aimed at improving the activity of fuel cell electrocatalysts and utilization of Pt,various transition metals?such as Ni,Cu,Fe,etc.?have been doped to Pt to optimize the crystal structure and electronic structure of platinum-based catalysts.First,a nickel-phosphorus alloy?Ni-P?was uniformly adhered to the surface of the carbon nanotube?CNT?by electroless plating to prepare a uniformly dispersed intermediate Ni-P/CNT.Subsequently,a small amount of Ni was replaced by a displacement reaction to prepare a bimetallic catalyst Pt^Ni-P/CNT.The final catalysts were characterized by scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?.Catalytic activities towards methanol oxidation and hydrogen evolution reactions were evaluated and benchmarked with a commercial Pt/C catalyst.The electrochemical surface area of Pt^Ni-P/CNT?Pt loading is 12.1 wt.%?was 126 m²·g^-1 by cyclic voltammetry,higher than that of a commercial Pt/C-JM?Pt loading was 20 wt.%?is only 77.9 m²·g^-1.The Tafel slopes of Pt^Ni-P/CNT catalysts was found to be smaller than that of Pt/C-JM catalyst,indicating that the faster kinetics for hydrogen evolution reaction.Second,a highly active ternary platinum-based catalyst Fe-Cu-Pt/C-600oC was prepared by ethylene glycol reduction.The catalysts were characterized by SEM,TEM,XRD,TGA and XPS.The results showed that ternary alloy particles with an average diameter of 3-4 nm were formed on the carbon support and dispersed uniformly.Compared with the commercial catalyst Pt/C,the MOR activity of Fe-Cu-Pt/C-600oC is significantly improved,and the current density of Fe-Cu-Pt/C-d-600°C at 0.85 V is as high as 1106 mA·mg^-1,probably due to the combined electronic effect and bifunctional effect.The hydrogen evolution performance of Fe-Cu-Pt/C-b-900°C exceeds that of a commercial Pt/C,and the onset potential is 17 mV,which is lower than that of the commercial Pt/C?34 mV?.The overpotentials of Pt/C and Fe-Cu-Pt/C-b-900°C are 34 mV and 27 mV,respectively,for the current density reaching 10 mA·cm^-2.In addition,among all the catalysts,Fe-Cu-Pt/C-b-900°C displays the best CO poisoning resistance,and Fe-Cu-Pt/C-d-600°C shows a significant 22 mV shift to more-positive half-wave potential relative to the Pt/C catalysts.The CO poisoning resistance and ORR reaction rate of such catalysts are related to the Pt content in the alloy and the annealing temperature.Third,polyhedral copper nanoparticles with a good dispersion and uniform size?particle size<50 nm?were prepared at room temperature.This method was compared with other methods in the literature to highlight the simplicity of this method.Followed by galvanic replacement with platinum precursor,porous Cu-Pt electrocatalysts are prepared.Element mapping analysis and XRD indicated that Cu and Pt has formed a uniform alloy structure.The results show that this catalyst is prospective in fuel cell applications due to its high platinum utilization and catalytic activity.