Research on Stability and Activity of Anode Catalyst for Direct Liquid Fuel Cell

Author:Yu Xin Xin

Supervisor:cheng han song


Degree Year:2019





In recent years,with the development of economy and society,application of the traditional energy has brought a series of problems to our society,so development and application of new energy has becoming a urgently problem to be solved.Fuel cell is a clean energy.Its products are clean.Fuel sources of fuel cells are extensive.It has been studied more and more widely.Proton exchange membrane fuel cell(PEMFC)with advantages of low operating temperature,high energy density and clean protect,has become an efficient and convenient energy supply equipment.Proton exchange membrane fuel cell(PEMFC)can convert the chemical energy stored in fuels transformed into electrical energy.When the fuel cell works,the electrochemical oxidation of fuel occurs at lower anode potential,releasing protons.And protons are transmitted to the cathode through the proton membrane.At the same time,oxygen is electrochemically reduced at the cathode,releasing electrons,and electrons are transmitted through the external circuit to generate current.According to the different fuels used in proton exchange membrane fuel cell,it can be divided into hydrogen-oxygen fuel cell,direct methanol fuel cell,direct formic acid fuel cell and so on.Due to the high cost of hydrogen during producting and transportation,there are many studies on direct methanol fuel cells and direct formic acid fuel cells.Platinum-based catalysts are the most active and stable catalysts in fuel cells.And Platinum-based catalysts are also the most widely used catalysts at present.However,the carbon monoxide,recoganized intermediate product of methanol oxidation,is easily adsorbed on the surface of platinum to cover the active site of platinum,which affects the electrochemical performance of the catalysts and reduces the activity and durability of catalysts.This paper will focus on improving the activity,durability and oxidize carbon monoxide of methanol oxidation intermediates.Aimed at above problems,we synthesized four catalysts based platinum and palladium.And the catalysts were characterized by X-ray photoelectron spectroscopy(XPS),transmission electron microscopy(TEM),High-Angle Annular Dark Field scanning-transmission electron microscopy(HAADF-STEM),particle size and morphology.At the same time,the electrochemical activity,durability and carbon monoxide oxidation of the catalyst were tested.The dissertation will start from the following four sections.(1)Study on the stability and activity of CB/Pt coated with PBI and PVP.In order to solve the problem that Pt was easy poisoned by CO which was the intermediate product of methanol oxidation,adding Ru element to form the CB/PtRu catalyst is considered to be an effective way to solve the problem.CB/PtRu catalyst has also been commercialized.However,Ru is easily lost in acidic environment,which seriously affects the activity and stability of catalysts in fuel cell applications.At the same time,Ru element is also a precious metal.The use of Ru element increases the cost of catalyst.PBI and PVP were common macromolecule materials.In this chapter,we synthesized two kinds of catalysts CB/PBI/Pt/PVP and CB/PBI/PVP/Pt.The structure and morphology of the catalysts were analyzed and compared with commercial CB/PtRu catalysts.The results showed that the two samples performing better durability,but in the sample CB/PBI/Pt/PVP PVP layer covered part of the active site of Pt so decreased the activity of Pt.CB/PBI/PVP/Pt performed high activity and high stability.The utilization rate of Pt in CB/PBI/PVP/Pt reaches 94%,while the utilization rate of Pt of CB/PtRu and CB/PBI/Pt/PVP catalysts were only 81%and 55%.At the same time,CB/PBI/PVP/Pt also has good performance in CO oxidation experiment,methanol oxidation experiment and battery test.Therefore,compared with CB/PBI/PVP/Pt and CB/PtRu catalyst,the activity and stability of CB/PBI/PVP/Pt catalyst have been greatly improved on the basis of reducing the cost of catalyst,and it has great potential in fuel cell application.(2)Study on the stability and CO oxidation of CeOx decorated Pt catalyst.A new catalyst CB/PBI/Pt was synthesized by hydrothermal method.Its activity was higher than that of commercial catalyst CB/Pt.The activity and durability of CB/PBI/Pt were greatly improved by coating cerium oxide on the surface of CB/PBI/Pt.The analysis of the morphology and structure of the catalyst showed that the addition of CeOx had negligible effect on the morphology and particle size of the catalyst particles.The presence of cerium oxide layer was detected by high resolution electron microscopy.Ce(OH)3 is easily formed in cerium oxide with high oxygen vacancy.XPS tests show that Pt-O-Ce chemical bond was formed in cerium oxide coated catalyst.The high oxygen vacancy of cerium oxide and the formation of Pt-O-Ce chemical bond are conducive to the CO conversion to CO2,the intermediate product of methanol oxidation into carbon dioxide.Therefore,the existence of cerium oxide can improves the stability of the catalyst and the performance of methanol oxidation.(3)Study on the electrochemical performance of CeOx coated CB/PtRu.In order to solve the problem that Ru in CB/PtRu was easily losing,we modified the surface of commercial CB/PtRu catalyst and coated it with cerium dioxide.Due to cerium dioxide has the ability to store oxygen,it can quickly remove the CO adsorbed on the catalyst surface and protect the catalyst from the poisoning of CO.Cerium dioxide was coated on the surface of the catalyst by hydrothermal reaction.The existence of cerium dioxide layer was demonstrated by high resolution transmission electron microscopy(HRTEM).Results of electrochemical experiments show that the performance of the catalyst coated with cerium dioxide has been improved,and the performance of the CB/PtRu catalyst has been significantly improved in the durability test,especially after the initial 600 durability cycles.Because cerium dioxide can promote the rapid oxidation of CO to carbon dioxide,its stability has been greatly improved.The methanol oxidation experiment shows that the peak current value of methanol oxidation is 2.8 times that of CB/PtRu when the content of cerium dioxide is20%.The experimental results show that the CO oxidation and single cell performance of CB/PtRu@CeO2 are improved after coating with cerium dioxide.(4)Inverstigation of the activity and stability of Pd Catalyst coated with carbon layer.The Pd-based catalyst CB/PBI/Pd@C was synthesized by glucose-coated and carbonization.The experimental results show that the carbonization process of glucose can promote the transition of Pd from Pd(Ⅱ)to metal Pd(Pd(0)),then increase the content of Pd(0)in the catalyst.Pd(0)has high activity and stability,so the existence of carbon layer improves the activity of catalyst.The stability test in sulfuric acid showed that CB/PBI/Pd@C still remain 60%activity after 100 durability cycle tests,while CB/PBI/Pd had no catalytic activity after 100 durability cycle tests.CB/PBI/Pd@C also has high catalytic activity and stability in alkaline solution.The results show that CB/PBI/Pd@C not only has high catalytic activity,but also has lower starting potential,which means that the fuel oxidation on the surface of CB/PBI/Pd@C is easier.