Preparation and Electrochemical Properties Investigation of Phosphorus and Sulfur Doped Graphene Supported Pt Catalysts

Author:An Mei Chen

Supervisor:du chun yu


Degree Year:2019





Direct methanol fuel cells(DMFCs)has attracted more attention from researchers all over the world due to its advantages of high energy density,safe fuel storage and transportation,low cost and less pollution.At present,Pt/C catalyst is commonly used as anode catalysts for DMFCs.However,the high cost,poor CO anti-toxicity ability and stability of Pt/C catalyst directly affect the market application of DMFCs.Therefore,it is urgent to design a catalyst with low cost,easy preparation,high catalytic performance,strong anti-poisoning and high stability.It has been found that Pt nanoparticles are susceptible to agglomerate,fall off and be poisoned in the process of anodic methanol oxidation.In this study,the P and S atoms single doped or co-doped graphene supported Pt nanoparticles catalyts were prepared,and their electrocatalytic performance in acidic media and mechanism were analyzed and studied.P doped graphene(PG)support was successfully prepared by thermal annealing using phosphoric acid and graphene oxide(GO)as precursors.TEM,Raman and XPS measurements show that P atoms were doped into graphene in the form of P-C bonds generating a large number of defect sites,which contribute to the uniform loading of Pt nanoparticles with a smaller particle size(2.36 nm),and effectively preventing the agglomeration of Pt.The Pt/PG catalyst presents over 1.2 and 2.6 times higher ESA and MOR mass activity than Pt/C,respectively.In addition,low electronegativity of P enables electron donation to Pt,reducing the d-band center of Pt and thus enhancing the anti-CO toxicity of Pt/PG catalyst.S doped graphene(SG)support was successfully prepared by thermal annealing using sulfuric acid and GO as precursors.TEM,Raman and XPS tests showed that S atoms were doped into graphene in the form of S=C bond forming a large number of defect sites,which were beneficial to uniformly anchoring Pt nanoparticles with smaller particle size(2.47 nm).Compared with Pt/C catalyst,Pt/SG catalyst exhibited outstanding electrocatalytic activity and stability,in which ESA increased by 12.1%,methanol oxidation mass activity increased by 1.3 times and CA mass activity increased by 2.3 times.In addition,Pt/SG catalysts exhibit stronger CO tolerance,which is mainly attributed to the electronic effect that S atoms can transfer polarized electrons,resulting in the formation of more metal Pt to enhance methanol oxidation performance and a large number of oxygen-containing groups to promote the oxidation of CO adsorbed on the active sites of metal Pt,thereby enhancing the CO tolerance of Pt/SG catalyst.In order to further improve the kinetics of methanol oxidation of doped catalysts.S and P co-doped graphene(SPG)support was successfully prepared by two-step thermal annealing using sulfuric acid,phosphoric acid and GO as precursors.TEM,Raman and XPS showed that S and P atoms were successfully incorporated into graphene in the form of S=C and P-C bond,respectively.The doping content of P atom(1.9%-4.6%)was significantly increased by S atom doping,which produce a large number of defect sites and then loading Pt nanoparticles uniformly with smaller particle size(2.16 nm).Compared with Pt/PG and Pt/SG catalysts,the Pt/SPG catalyst exhibits over 1.2 and 1.3times higher ESA,1.4 and 1.7 times higher MOR activity,respectively.At the same time,Pt/SPG shows desirable stability toward the MOR.More importantly,the co-doping of S and P atoms leads to the further increase of C-OH bond,which promote the oxidation of CO,thus enhancing the CO tolerance of Pt/SPG catalyst.In order to further overcome the stacking problem caused by van der Waals force between GO.In this study,SPG was assembled into 3D-SPG by solvothermal method.SEM and BET measurements showed that 3D-SPG had typical 3-D porous network structure.This unique structure not only significantly increases the specific surface area(568.6 m2 g-1)of 3D-SPG and helps to enhance the loading of Pt nanoparticles,but also effectively promotes the contacts between reactants and active sites,thus efficiently enhancing mass transfer.Compared with Pt/SPG catalyst,the Pt/3D-SPG catalyst exhibits over 1.2 times higher ESA,2.4 times enhanced methanol activity,as well as superior stability.In addition,the 3-D porous network structure of Pt/3D-SPG catalyst not only contributes to the rapid release of CO from the surface of metal Pt,but also exposed more hydroxyl groups to promote CO oxidation,thus exhibiting stronger CO tolerance.