Preparation Of Iron-based Catalysts For Direct Methanol Fuel Cells And Their Electrochemical Properties

The rapid consumption of fossil fuels and mankind’s thirst for renewable energy sources have prompted researchers to conduct intensive research into new energy sources.Direct methanol fuel cells（DMFC）have the advantages of low temperature and fast start-up,clean and environmentally friendly fuel and simple cell structure,but their cathode oxygen reduction reaction（ORR）kinetics are slow,which seriously affects the efficiency.Currently,platinum（Pt）-based catalysts are commonly used in DMFC cathodes,which have good catalytic activity for ORR,but H₂O_2,ads are produced as by-products during the reaction,which reduce the effective active area of the electrocatalytic reaction and even lead to catalyst poisoning and deactivation.In addition,the commercialization of DMFC catalyzed by Pt-based materials is hindered by the following reasons:（1）low reserves of the precious metal Pt and high cost;（2）methanol permeation causes“fuel crossover phenomenon”and“mixing potential”,resulting in lower DMFC output power and system life is reduced.Therefore,it has become a major research direction to explore relatively inexpensive and methanol-resistant non-precious metal catalysts to replace Pt-based catalysts and avoid the dependence on the precious metal Pt.In recent years,iron-based catalytic materials have been widely used as ORR catalysts due to their good catalytic activity,easy availability of raw materials and low prices.The aim of this thesis is to solve the problems of insufficient activity,poor stability and methanol resistance encountered by iron-based catalytic materials in the oxygen reduction reaction.The electrode materials such as iron carbide,Co-doped iron carbide and Fe PNC loaded with nitrogen and carbon materials were prepared by grinding,heat treatment and acid washing,and the effects of different nitrogen and carbon sources,second metal doping and heteroatom P doping on the electrode materials were investigated.（1）A series of iron carbide catalysts（Fe₃C/N_m-C_n）loaded with nitrogen-containing carbon materials were successfully prepared under different nitrogen and carbon sources,and the effects of nitrogen and carbon sources on the catalyst structures and electrochemical properties were investigated.The experimental results showed that the Fe₃C/N-C catalysts synthesized with melamine as the nitrogen source and glucose as the carbon source had more defects and tubular structures were generated,which were favorable for the oxygen reduction reaction.Electrochemical tests showed that the half-wave potential of the catalyst Fe₃C/N-C was 0.73 V.The current density retention was 89.23%after 4 h of operation,and the current percentage was 89.15%after the addition of methanol.The stability and methanol resistance of the catalyst were better than 20%Pt/C,but the catalytic activity needed to be improved.（2）A series of cobalt-doped iron carbide catalysts（Co_x-Fe₃C/N-C）loaded with nitrogen-containing carbon materials were successfully prepared by introducing cobalt nitrate hexahydrate as the cobalt source and pyrolyzed under N₂ conditions,and the influence of the doping amount of the second metal element Co on the structure and properties of Fe₃C/N-C was investigated.The Co_0.4-Fe₃C/N-C catalysts prepared at the optimum cobalt content are rich in pore structure,and the synergistic interaction between the Co species highly dispersed in the carbon material and the active component Fe₃C enhances oxygen adsorption and optimizes the oxygen intermediate,so that the bimetallic doping exhibits better electrochemical performance than the monometallic doping.In addition,the main phase of the material turned into iron phosphide（Fe P and Fe₂P）loaded with nitrogen-containing carbon materials.Electrochemical tests showed that the half-wave potential of the catalyst Co_0.4-Fe₃C/N-C was 0.86 V.The current density retention was 95.08%after 4 h of operation,and the current percentage was 97.60%after the addition of methanol.The catalytic activity reached a level comparable to that of 20%Pt/C,and the stability and methanol resistance were better than that of 20%Pt/C.This fully demonstrated that doping with cobalt could improve the catalyst activity,stability and methanol resistance.（3）The Fe PNC catalysts with more surface folds and larger specific surface area were prepared by the introduction of P element using phytic acid as the phosphorus source and pyrolyzed under N₂ conditions,and the effect of doping P element on the structure and performance of Fe₃C/N-C catalysts was investigated.The P atom increases the defect position by modulating the coordination environment of the central metal atom and the electronic structure of the carbon material surface,attracting electrons from neighboring iron,making it easier for oxygen to adsorb on the catalyst surface and promoting the oxygen reduction reaction.Electrochemical tests showed that the half-wave potential of Fe PNC catalyst was 0.86 V.The current density retention was 93.94%after 4 h,and the current percentage was 97.20%after the addition of methanol.The catalytic activity reached a level comparable to that of 20%Pt/C,and the stability and methanol resistance were better than that of 20%Pt/C.This indicates that the doping of P element can effectively improve the activity,stability and methanol resistance of the catalyst.