Research On Water And Heat Transfer Characteristics Of Open Cathode Proton Exchange Membrane Fuel Cell Stack Integrated With Vapor Chambers

The open cathode proton exchange membrane fuel cell is widely used because of the advantages of simple structure and high energy utilization,but there are some problems,such as insufficient heat dissipation capacity and uneven temperature distribution in the fuel cell.In addition,water management problems such as flooding or low water content can also arise in different operating environments.To address the above problems,this paper integrates five vapor chambers into an open cathode proton exchange membrane fuel cell stack with 10 cells to improve thermal management;the mechanism of water and heat transfer and the electrical and thermal characteristics in steady and dynamic processes of open cathode proton exchange membrane fuel cells integrated with vapor chambers are analyzed by theoretical model,experimental study and numerical simulation.The specific research content is as follows:The output performance of the open cathode proton exchange membrane fuel cell and its internal water heat transfer process are analyzed.The effects of temperature and humidity changes on the water content and output voltage are discussed.The steady-state water content of the proton exchange membrane is mainly determined by the concentration difference diffusion,electro-osmotic drag effect and the adsorption or desorption of water inside the membrane;decreasing the ambient temperature,increasing the ambient humidity and the operating current increase the steady-state water content.It is found that decreasing the ambient temperature,increasing the ambient humidity and operating current can increase the steadystate water content.The matching scheme between the vapor chambers and the fuel cell stack is analyzed by combining the heat balance,i.e.,the ratio of the number of vapor chamber and single cell is 1:2.By solving the pressure balance equation of the fluid inside of the vapor chamber,it is found that the hydrogen distribution holes on the vapor chamber will increase the pressure loss of the fluid and decrease the heat transfer limit,but the decrease is less than 1.5%.The effects of different humidity supply gas,initial wetting degree of membrane and loading current amplitude on the start-up process of open cathode proton exchange membrane fuel cell are experimentally investigated.It is observed that the voltage dropped sharply after start-up and then slowly recovered to the steady-state value.The decrease of the initial water content of the membrane and the decrease of the humidity of the reaction gas or the increase of the start-up current will aggravate the magnitude of the voltage drop.The Electrochemical Impedance Spectroscopy technique is used to reveal the steady-state and dynamic electrical output characteristics under different humidity gas supply.Combined with the analysis of highfrequency resistance,it is found that the sharp voltage drop is caused by a dramatic increase in the ohmic impedance of the membrane at the moment of loading.After more than 100 lowhumidity gas supply start-up tests,the catalyst coated membrane is irreversibly damaged and the maximum output power decreases by 15.8%.Scanning electron microscopy technique is used to analyze the damage mechanism of the catalyst coated membrane after low humidity gas supply start-up,and it is found that the surface structure of the cathode catalytic layer is damaged and cracks appear.The effects of the vertical vapor chamber on the output voltage,temperature distribution,temperature uniformity and dynamic electrical and thermal characteristics of the open cathode PEMFC are investigated.When cooling the condensation section of the vertical vapor chamber,the integration of the vertical vapor chamber can effectively improve the thermal management of the fuel cell stack.The operating temperature of the stack at 20 A is reduced by 5 °C,the output voltage is increased by 4.61%,and the in-plane temperature difference of single cell is only 0.5 °C.Because the cathode flow channels and the cooling channels are independent,the effects of the cathode fan and cooling fan on thermal characteristics are discussed.Reducing the cathode air flow can improve the heat dissipation ratio of the VCs,thereby improving the temperature uniformity of the fuel cell stack.The electrical output characteristics and temperature distribution of the open cathode PEMFC integrated with horizontal VCs are measured and analyzed.Because the heat transfer of condensation section of VCs would be influenced by the cathode air flow,the influence of cooling air and cathode air on the heat transfer performance of the open cathode PEMFC integrated with horizontal VCs is investigated.The experimental results find that when the air is pressed into the stack,the condensation section of the VCs is downstream of the air,and the air flowing through it is at a higher temperature.Therefore,the heat cannot be discharged from the VCs.In contrast,when air is drawn into the stack,the thermal performance of the stack is better,and it runs at a lower temperature with better temperature uniformity.It is proposed and verified that the electrical thermal performance is improved by periodically switching the air flow direction,and the voltage output can be improved by about 5% by switching the cathode air flow direction in a 30 s cycle compared with the suction mode.The transient simulation model coupling water and heat management based on the open cathode PEMFC with VCs is constructed.Based on this model,the variation law of electrical and thermal characteristics with time during the start-up process is analyzed,and the effects of ambient temperature and humidity and the magnitude of load change on the water content of the membrane,liquid water saturation and the change of electrical output performance during the loading process of the fuel cell are studied and compared,and the fuel cell stack under different cathode air flow and cooling air flow working conditions are analyzed.The electrical efficiency and the exergy efficiency of the system are analyzed.The results show that the energy utilization efficiency of open PEMFC with embedded soaking plate cathode can be improved by rationally distributing the power of cathode fan and cooling fan.