Optimization Of Core-shell Like PtNi Alloy Catalyst And Boron-doped Carbon Support For PEMFCs

Hydrogen energy is a kind of clean secondary energy,which plays an important role in solving energy crisis and environmental pollution problems.Proton exchange membrane fuel cells（PEMFCs）is a type of energy conversion device that can efficiently convert hydrogen energy into electrical energy.The development of high-efficiency and low-cost PEMFCs is of great significance.Membrane Electrode Assembly（MEA）is the chip of PEMFCs,which is the interface of hydrogen and oxygen for electrochemical reaction,accounting for 40-60%of the cost of fuel cell.Electrocatalyst is one of the core materials in MEA,which is mainly platinum catalyst supported on carbon material.However,the high cost of platinum increases the cost of PEMFCs,which limits the commercial application of PEMFCs.Therefore,the main research direction of this project is to improve the electrocatalytic performance and stability of platinum catalyst,reduce the amount of platinum,then extend the lifetime and reduce the cost of MEA.In order to solve the core problems of catalysts for PEMFCs,boron-doped carbon support and PtNi alloy catalyst were selected as the main research contents in this study.The following work has been carried out:（1）A novel synthesis method of boron-doped carbon black B-ECP using sodium borohydride as structure directing agent was proposed.There is no need to open the covalent bond between the carbon atoms,for the B element is incorporated into the carbon nanospheres.Therefore,it is easier to realize industrial mass production without complex preparation conditions such as high temperature.The Pt particles supported on B-ECP have smaller particle size,higher electrochemical mass activity(0.286A·mg _Pt^-1)and surface area(95.62 m²·g _Pt^-1).The results of TEM and XRD show that the B-doped carbon support has a thinner flake structure,which plays an important role in improving the size and dispersion of platinum nanoparticles.It is also found that boron can contribute to the oxygen reduction reaction in acidic environment of PEMFCs due to its low electronegativity.In addition,the accelerated durability test in the high potential range shows that the introduction of boron can greatly improve the stability of carbon support in the acidic environment of fuel cell and thus improving the electrochemical performance of the catalyst during long-term operation.（2）PtNi/C alloy nano-catalyst with metal loading more than 50 wt.%and core-shell like structure was prepared by high-temperature annealing and acid pickling with Ni as the second metal.The physical characterization of the catalyst by TEM and XRD shows that high temperature annealing can effectively improve the alloying degree of the catalyst.At the same time,dealloying by acid treatment can effectively remove the unstable Ni in the catalyst,so as to improve the corrosion resistance and stability of the catalyst.The results of electrochemical tests also show that the alloy structure is superior.The mass specific activity of PtNi/C alloy catalyst reaches 0.574 A·mg _Pt^-1,which is 2.73 times of that of the commercial Pt/C catalyst with high metal loading.The ECSA and MA of PtNi/C alloy catalyst only decreased by 10.2%and 31.2%respectively after accelerated durability test with 30K cycles.The power density of PtNi/C alloy catalyst exhibits good performance in the practical application of membrane electrode test,and the power density reached 1.022 W·cm^-2 at 0.65 V.The introduction of Ni not only improves the oxygen reduction activity of the catalyst,but also reduces the cost.The two optimization schemes proposed in this study are simple in preparation process and easy to mass production,which not only improve the oxygen reduction performance and stability of the catalyst,but also reduces the catalyst load on the MEA.Thus,the cost of PEMFCs is reduced,the power density is increased,the volume and weight are reduced,and the lifetime is prolonged,which can promote the application of PEMFCs.At the same time,the theoretical exploration and verification of this study combined with the experimental phenomena also provide a reference for the future research in the field of catalysts for fuel cells.