Sulfur-containing Molecular-assisted Synthesis Of Graphitic Carbon Supported Pt Catalysts And Their Properties For Fuel Cell

Polymer electrolyte membrane fuel cells(PEMFCs)represent a clean energy technology that hold promise for deployment in low-carbon transportation.Reducing the amount of platinum used in the cathode catalysts without sacrificing performance and durability is crucial for achieving widespread commercialization of PEMFCs.One fundamental consensus in this endeavor is the need to control the particle size of platinum(Pt)to attain a balance between activity and durability in catalyzing oxygen reduction reaction(ORR).In this regard,PEMFCs cathode catalysts are typically designed as porous carbon black(CB)supported Pt nanoparticles,where the carbon supports feature high surface areas that facilitate the preparation of Pt nanoparticles with relatively small sizes and uniform distribution.However,the high-surface-area carbon black supports are vulnerable to electrochemical corrosion under real fuel cell operating conditions.Especially during the start-stop operation,the cell voltage will rapidly shift to 1.2～1.5 V vs.RHE(Reversible hydrogen electrode)due to the reverse current decay mechanism,which greatly accelerate the corrosion of carbon support,and eventually lead to severe catalyst attenuation.While replacing traditional high-surface-area carbon black as a support with highly graphitic carbon materials is an effective way to solve carbon corrosion problem,graphitic carbon usually has a lower surface area and lacks defect structures,which makes it difficult to achieve effective control over Pt particle size and dispersion.To tackle this challenge,graphitic carbon supported small-sized Pt nanoparticles were prepared by the method of sulfur-containing small molecule-assisted impregnation in this paper,this catalysts showed excellent activity and corrosion resistance in H2-air fuel cell tests.The main contents are as follows:1.A sulfur-containing small molecule-assisted impregnation method for preparing small-sized Pt nanoparticles on graphitic carbons were developed.The graphitic carbons with varying degrees of graphitization were prepared by subjecting commercial carbon black to heat treatment at different temperature.The fact that an elevated level of graphitization as the temperature increased,was evidenced by methods such as X-ray diffraction(XRD)and Raman spectroscopy.The uniform dispersion of Pt particles was hindered following graphitization treatment due to the reduction in surface area,porosity,and defect sites in carbon support.While the size control of Pt particles supported on graphitic carbons could be achieved by sulfur-containing small molecule-assisted impregnation method.Compared to the conventional impregnation method without the addition of sulfur-containing small molecules,this method resulted in the synthesis of smaller Pt particles supported on graphitic carbon treated at 3000℃,with an average particle size of 4.60 nm calculated by XRD full width at half maximum and 3.80 nm according to statistics from the transmission electron microscopy(TEM)images.Detailed investigations were further carried out on the formation mechanism of small Pt particles loaded on graphitic carbon,using advanced characterization techniques,such as X-ray photoelectron spectroscopy(XPS)and high-angle annular dark field-scanning transmission electron microscopy(HAADF-STEM).It was found that sulfur-containing small molecule could coordinate with Pt during the impregnation process,and subsequently converted into sulfur-doped carbon shell coated on Pt nanoparticles during the following thermal reduction treatment.The formation of sulfur-doped carbon shell could significantly suppress the migration and aggregation behavior of Pt nanoparticles on the graphitic carbon,thereby achieving effective size control.2.The ORR catalytic activity and durability of the catalysts in PEMFC was then evaluated.Compared to the catalysts prepared by the traditional impregnation method,the catalysts prepared by this method exhibited a high electrochemical surface area(ECSA)of 40 m2 gPt-1,and an enhanced H2-air fuel cell performance with a high current density of 402 and 1197 mA cm-2 at 0.7 and 0.6 V vs.RHE,respectively.The results of accelerated stress test(AST)verified the pivotal role of the degree of graphitization in determining the corrosion resistance of the carbon support.In particular,the potential loss at 1.5 A cm-2 was only 10 mV for the graphitic carbon(treated at 3000℃)supported Pt catalyst after the durability test at 1.0-1.5 V vs.RHE.