Proton Conductivity And Thermal Conductivity Of Proton Exchange Membrane:Molecular Dynamics Simulation

With the increasing energy crisis in the world,fuel cell,as a new power supply device,has many advantages,such as high energy density,high energy conversion rate and environmental protection.Compared to other types of fuel cells,proton exchange membrane fuel cell with proton exchange membrane as solid electrolyte has the advantages of low operating temperature,high stability,high starting speed,which is considered to be the best alternative power source for electric vehicles,submarines,a variety of mobile power supply,power supply and fixed power supply etc.Membrane electrode is an important part of proton exchange membrane fuel cell(PEMFC)system,and proton exchange membrane is the key component of membrane electrode.It plays the role of separating anode and cathode reaction.It transfers the protons produced by anode to the combination of cathode and oxygen to form water.Therefore,it is required that the proton exchange membrane have high proton conductivity,good chemical stability and mechanical stability.This paper modified the side chain structure of the polymer membrane materials and form crosslinks,in order to reveal the different influence factors such as polymer side chain structure,cross-linking and temperature on membrane proton conductivity and thermal conductivity.And the reasons are analyzed.The main contents and conclusions are as follows:(1)Aiming at three proton exchange membranes with the same C-F backbone structure and different side chain structures,three dimensional initial models of proton exchange membrane for molecular dynamics simulation are built respectively.By selecting the COMPASS force field and using the calculation method of Newton’s laws of motion under molecular dynamics simulations,determine the cell model for research on three-dimensional periodic boundary condition.Structure optimization followed by the dynamic balance calculation,the trajectory obtaioned from the dynamic balance to calculation the radial distribution function and the mean square displacement,according to the calculated diffusion coefficient,proton conductivity and other parameters.Then the thermal conductivity of the three kinds of proton exchange membranes was calculated by the nonequilibrium molecular dynamics method,and then the parameters were characterized by the dynamic density of the vibration.The results show that: The Aciplex membrane form proton transport channels because it has longer side chain structure compared with the other two membranes.Therefore,Aciplex membrane has both the highest proton diffusion coefficient and the highest thermal conductivity,the proton diffusion coefficient of the Aciplex membrane is up to 0.22×10-5 cm2·s-1.In addition,the proton diffusion coefficient and thermal conductivity under different working temperatures are different.The higher the temperature,the greater the proton diffusion coefficient and thermal conductivity.(2)A proton exchange membrane model with the influence of crosslinked bonds was established.The calculation method used the Newtonian equations of motion and and the three-dimensional periodic boundary conditions,selecting the COMPASS force field.According to the principle of inter atomic distance would not less than 5?,random number of 0-30 crosslinkwere added to different proton exchange membrane cell models.The energy optimization is carried out to determine the suitable cell cell model,and then the dynamic equilibrium calculation is carried out to get the parameters of molecular displacement and energy density.The mean square displacement curves of hydrated hydrogen and water molecules were analyzed,and the proton diffusion coefficient and proton conductivity were obtained,and the radial distribution function was used to characterize them.In addition,the thermal conductivity of three proton exchange membranes with the influence of crosslinking bonds was calculated by the nonequilibrium molecular dynamics method.The results show that the formation of crosslinking bonds is beneficial for opening new channels for proton transport.Therefore,the proton conductivity and thermal conductivity increased with the increasing of crosslink number.Proton conductivity can reach up to 0.097 S·cm-1,and the thermal conductivity can reach up to 2.31 W·m-1·K-1.However,when the number of crosslinked bonds is too large,the channel blockage along the backbone of the main chain is caused and the transport of the proton is affected.Therefore,when the number of crosslinked bonds reaches a certain amount,the proton conductivity and thermal conductivity will not increase and then decline.It provides a new theoretical basis for the modification of other proton exchange membranes.