Numerical Study On Mass Transport Characteristics In A Vapor Feed Passive Direct Methanol Fuel Cell

Vapor feed passive direct methanol fuel cell(DMFC)has been recognized as an alternative for the traditional powering portable device due to its high energy density,environmental friendliness and high efficiency,compact structure,ease to be portable and so on.However,the vapor feed passive DMFC still has many problems such as methanol crossover and lack of water at the anode,which inhibits its widespread applications.To ensure high power density and stable operation,water and methanol management is critically important for vapor feed passive DMFC.In this case,it is necessary to have a deep understanding of the methanol and water transport and electricity generation characteristics of vapor feed passive DMFC,and to study the effect of key factors on the mass transport and cell performance,which will provide theoretical basis for optimizing water and methanol management method.Vapor feed passive DMFC involves the methanol and water transport in the complex porous structure,which is coupled with the electrochemical reaction.Especially the mass transport behaviors in this type of fuel cell are significantly different from those in a conventional diluted methanol feed DMFC.For example,water required for the anode reaction is all provided by the methanol solution in the convectional diluted methanol feed DMFC,while it is partly or all provided by the cathode in a vapor feed passive DMFC.In this case,the mass transport in this type of fuel cell is of importance.However,the experimental method is difficult to directly observe the internal mass transfer process and the influence of related parameters.Therefore,the simulation approach has become the important method to study the mass transport and electricity generation characteristics,to optimize the fuel cell structure and to select suitable operation parameters.However,the existing numerical simulation on the vapor feed passive direct methanol fuel cell usually neglects the transport of water supplied by the anode side during the DMFC operating with high concentration methanol,and ignores the correlation between various influencing factors.In this thesis,therefore,the mass transport characteristics and its influence on the cell performance is numerically studied.Moreover,the influence of the variation of multiple parameters on the performance of the vapor feed passive DMFC is also numerically studied.Main outcomes of this thesis are summarized as follows.(1)A two-dimensional two-phase(gas and dissolved phases)mathematical model is developed to simulate the mass transport processes in a vapor feed passive DMFC operating with concentrated or neat methanol.This model simultaneously considers the actual physical process of pervaporation,in which the methanol vapor and water vapor are supplied to the anode.As a result,the mass transport process of this type of fuel cell can be described more accurately.(2)For vapor feed passive DMFC operating with concentrated methanol,numerical results also indicate that the methanol concentration,vaporizer open area ratio and the relative humidity of the ambient air have significant effect on the performance of vapor feed passive DMFC.It can be seen that the cell performance at high current densities can increase with an increase in the methanol concentration or vaporizer open area ratio.The effect of methanol concentration and vaporizer open area ratio on water mass transport in the cell is different.It is found that the maximum power density is different with the combination of the methanol concentration and the opening rate,and the fuel utilization ratio is different.If the cell operated with smaller methanol concentration but bigger open ratio,the performance can be better than the cell operated with bigger concentration but smaller open ratio.Under all methanol concentrations,the increase of the humidity can well address the issue of the lack of water at the anode,thereby improving the cell performance.It is found that the effects of the methanol concentrations on the cell performance under bigger humidity are more remarkable.(3)For the vapor feed passive DMFC operating with neat methanol,almost all the water required for the anode reaction comes from the cathode.The methanol transport is mainly controlled by the anode structure,and the gas transport layer(VTL)plays a significant role in reducing the anode methanol concentration at the anode catalyst layer.The water content in the membrane is increased with increasing the relative humidity,lowering the anode overpotential and ohmic loss and thereby improving the cell performance.When the ambient temperature is increased,the cell performance increase at first and then decrease,indicating that the temperature is not as high as possible.(4)For the vapor feed passive DMFC operating with neat methanol,the increase of open ratio of the vaporizer is beneficial for the improvement in the cell performance,but at the same time it will also increase the methanol crossover and reduce the fuel utilization.At different open ratios,the decrease of the membrane thickness facilitate the reduction of the transport resistance of water through the membrane,which increases the water content in the membrane,but its effect on methanol crossover is slight.As a result,the cell performance is increased with reducing the membrane thickness.It is also found that increasing the thickness of the water management layer(WML)will increase the resistance of water transport from the cathode to the environment,and thereby increase the water content in the cathode catalyst layer(cCL),membrane and anode catalyst layer(aCL),but have no significant effect on the methanol and oxygen transport,thereby improving the cell performance.Therefore,the water and methanol management can be optimized by adjusting the open ratio of the pervaporation membrane,reducing the membrane thickness and increasing the thickness of the water management layer.