Numerical Simulation Study On Mass Transmission In Transient Passive Direct Methanol Fuel Cell

As an important part of Fuel cells,the direct methanol fuel cell(DMFC)has many advantages such as higher energy density,easier storage of liquid methanol fuel and cleaner recycling.These features make DMFC a promising power source for portable applications.Expecially,passive direct methanol fuel cell has reduced the consumption of auxiliary equipment heat and make the equipment simple,which became the hotspot of research of people gradually.However,the fuel is self-feeding when passive direct methanol fuel cell operates,mass and heat transport can not reach the optimal state,which influence cell performance.So the numerical simulation study on the passive DMFC can gain better understanding of the characteristics of mass transport and electrochemical reaction,optimize the structure of the electrode and operating conditions,save more time and cost,which has a great significance for high performance of a passive DMFC.Firstly,a transient multiphase model of passive direct methanol fuel cell was developed and analysised the effects of current densities,the voltage,the miro-porous layer and fuel feeding conditions during the whole operation processes from full of methanol to empty.The results show that methanol crossover rate is limited to ensure the high enengy efficiency of a passive DMFC in appropriate current density and cell voltage.The anode MPL produces high flow resistance in the surface between MPL and GDL,which could improve fuel efficiency by decreasing the methanol crossover rate.Increasing the size of fuel tank can improve the energy density generally,but has negligible effect on the fuel and energy efficiency.Secondly,considering the actual operation in the fuel cell process,portable DMFCs will operate in different orientation.Based on the transient multiphase model for passive DMFC,the effects of cell orientation on cell performance are studied.Numerical simulation results showed that the passive DMFC with vertical orientation performs better at lower current density due to the lower methanol crossover rate.With an increased in current density,a large amout of water produced in cathode is hard to be removed in vertical orientation,the passive DMFC with horizontal orientation gains a better performance.In addition,the hydrophobic microporous layer(MPL)has different effects on the passive DMFC with different orientation.Finally,the paper gives a method of feeding with pure methanol,the performance and discharing time of the passive DMFC is improved by increasing the methanol fuel concentration.A transient model was presented to investigate the internal mass transport phenomena for passive vapor-feed direct methanol fuel cell.The pervaporation membrane and vapor transport layer(VTL)are considered for the formation and transport of methanol vapor.The results showed that the cell performance could be improved by increasing current density,decreasing open radio of the vaporizer or increasing membrane thickness due to the reduced methanol crossover rate.The cathode MPL is useful in enhancing water recovery flux and decreasing water losses.The transient simulation is analysised by optimizing the operating conditions and cell structures in the paper,which give better understanding the characteristics of mass transport and provide a better theoretical basis for transient operation of passive DMFC.