The Oxygen Reducation Reaction In HT-PEM Fuel Cells And Effects Of NH₃ On Its Performance

As an energy conversion device,proton exchange membrane fuel cell with its unique advantages such as high efficiency,zero pollution has been considered to be an excellent alternative energy sources.Currently,PEM fuel cells have been paid enough attention in the automotive sector,and gradually move towards their rightful place.HT-PEM fuel cells have the advantages of water and heat easy management,which make them get much attention,but immature technology hinder their application in the market.In this thesis,the effects of NH3 impurities in fuel on HT-PEM fuel cell performance were studied by experiment and using simulation software simulated oxygen reduction reaction characteristics which took place in HT-PEM fuel cell cathode.The results have some reference values in HT-PEM fuel cells'design and development.In this thesis,the history of fuel cell development,strengths and weaknesses of fuel cells and its application in practice were summarized.The components of PEM fuel cells,the working principle,development and the research progress of HT-PEM fuel cells were introduced.Then the thermodynamic theory,kinetic theory,calculation method of the amount of reactive gas and the efficiency of PEM fuel cells were summarized,and fuel cell test system was described in detail from software and hardware bases.In the end,the fuel cell system's operating methods and data processing methods were summarized as well.The v-i characteristic curve and AC impedances of HT-PEM fuel cell were tested by the fuel cell test system in this thesis.The equivalent circuit method was used to obtain the equivalent circuit element of HT-PEM fuel cell,and the catalyst layers after NH3 poisoning were scanned with the electron microscope.The effects of concentration of NH3,fuel cell temperature and using time on HT-PEM fuel cell performance were analyzed.It is observed that NH3 in hydrogen fuel changed the structure of the fuel cell electrode electrochemical reaction interface,hindered the proton transfer,resulting in a decline in HT-PEM fuel cell performance,and after poisoning NH3 bubbled into pure hydrogen,the performance of HT-PEM fuel cell continued to decline.It is concluded that NH3 in hydrogen fuel strongly poisons the HT-PEM fuel cell catalyst by reducing the electrochemical reaction interface in the fuel cell electrode,and it is found that toxic effect of NH3 to HT-PEM fuel cell is difficult to be restored.The damage is irreversible.Since the cathode oxygen reduction reaction has a great influence on the HT-PEM fuel cell performance,this thesis based on molecular dynamics and quantum mechanics,the HT-PEM fuel cell cathode catalyst Pt(1 1 1)plane and oxygen molecule model were established using Materials Studio.Based on density functional theory,oxygen adsorption on the catalyst Pt(1 1 1)surface and the reaction of oxygen and hydrogen atoms in HT-PEM fuel cell cathode were simulated using first-principles molecular dynamics.The results showed:there are three ways of oxygen's adsorption on Pt(1 1 1)surface.The adsorption needs longer time than any other reaction.The reaction process involves a two-electron reaction pathway and a four-electron reaction pathway at the same time.The second step of the reaction is the most complex and needs the longest reaction time in all reactions of O2 and H that occur on the surface of Pt(1 1 1).The second reaction is the whole decision-making step.The higher the fuel cell temperature is,the faster oxygen reduction reaction is for the analog reactions.