Numerical Simulation On The Multi-component And Multi-phase Transport In Micro Direct Methanol Fuel Cells Operating With High Fuel Concentration

With rapid development and increasing popularity of intelligent and portable devices in the modern society,it is very urgent to seek the advanced micro power sources with high efficiency,stability and convenience.Micro fuel cell technology can directly convert the chemical energy stored in the fuels into electric energy via electrochemical reactions,offering the remarkable advantages of high conversion efficiency,high flexibility,good adaptability,and environmental friendliness,etc.Hence,micro fuel cell is one of ideal micro power sources.Currently,extensive efforts are mainly devoted to the improvement in the cell performance via the preparations of the high-performance catalysts and electrode materials,as well as the optimization of the cell configurations and operating conditions.It is still short of deep understanding of the coupling mechanism among the two-phase flow,heat and mass transfer,charge transfer and electrochemical reactions,leading to poor theoretical foundation for the design and practical application of the high-performance micro fuel cells.To address this issue,this thesis is devoted to comprehensive numerical simulation studies on the characteristics of the heat and multicomponent transfer coupled with the electrochemical reactions occurring in the micro fuel cells based on the multiphysics coupling principle.Firstly,a two-phase mass transfer model for the microfluidic fuel cell（MFC）with a flow-over anode was developed based on the enhancement of proton transfer,through which the interplay between the two-phase mass transfer in the anode catalyst layer and electrolyte flow channel and the electrochemical reactions was explored.In the face of the low specific energy of this type fuel cell caused by the use of low fuel concentration,a passive liquid-fed micro direct methanol fuel cell（DMFC）operating with high methanol concentration was constructed.An isothermal two-phase mass transfer model was developed based on the mixed wetting of the porous media layers,in which effects of the capillary convection at the external interfaces of the porous anode and cathode,as well as the phase change of the condensable species inside the electrodes were also considered.Then,considering the effect of the thermal effect in the DMFC on the cell performance,a non-isothermal model for a passive liquid-fed DMFC operating with high methanol concentration was also developed to investigate the heat and mass transfer characteristics due to the cell thermal effect and its effect on the cell performance.Afterwards,to further improve the specific energy of the fuel cell system,the neat methanol operation was accounted.Aiming at the water and heat management in a passive vapor-fed DMFC operating with neat methanol,a non-isothermal model describing the heat and mass transfer coupled with the electrochemical reactions,in which the water transfer between different phases was considered.The coupling mechanism between the heat and mass transfer and electrochemical reactions was studied under the condition of no water actively supplied to the anode.Finally,to address the issue of the low water content in the ionomers encountered in a passive vapor-fed DMFC operating with neat methanol,a passive vapor-fed MFC with a liquid electrolyte stream was constructed,which combined the advantages of good heat dissipation and water retention due to the liquid electrolyte and high specific energy of the passive vapor-fed DMFC operating neat methanol.A non-isothermal model for this type fuel cell was also developed to study the effect of the liquid electrolyte stream on the cell performance and temperature adaptability.Main outcomes of this thesis are summarized as follows.（1）Two-phase flow and mass transport in the microfluidic fuel cell with a flow-over anode:The simulation results reveal that due to addition of a transport barrier layer（TBL）with low fuel diffusivity at the cathode,the effect of the fuel crossover on the cathode performance could be effectively suppressed in case the blank liquid electrolyte stream was removed and the distance between the cathode and anode was shortened.The effective fuel diffusivity and proton conductivity in the anode catalyst layer（ACL）tended to decrease with the accumulation of gaseous CO₂ generated by the methanol oxidation reaction,leading to the extremely low fuel concentration within the ACL and thereby limiting the cell performance improvement.The two-phase flow in the electrolyte flow channel（EFC）will decrease the local proton conductivity but enhance the convection effect.Benefited from the declined Ohmic loss and the enhanced mass transfer by higher flow velocity,better performance of a MFC with a narrower EFC can be achieved.Although the increase in the fuel supply concentration as well as the decrease of the TBL thickness will facilitate the electrochemical reaction and proton transfer,the degradation in the peak output power density can be witnessed once the fuel crossover to the cathode aggravates due to excessive fuel concentration or the extremely thin thickness of the TBL.（2）Two-phase flow and mass transport in a passive liquid-fed micro DMFC operating with high methanol concentration:It is found that because of the variation in the wettability between the porous layers in the DMFC anode and cathode,there exists the jump of the liquid saturation between two distinct porous layers（i.e.diffusion layer,microporous layer and catalyst layer）.With the control of the physical properties of each porous layer,such as the thickness,porosity and hydrophobicity,etc.,lower liquid saturation can be expected in the anode diffusion layer（ADL）and microporous layer（AMPL）to increase the transport resistance of the liquid methanol,thus maintaining the adequate methanol concentration in the ACL while avoiding the serious fuel crossover.As much more gaseous CO₂ is generated through the methanol oxidation reaction under high current density condition,the increased gas pressure leads to the increase of liquid saturation in the porous media with the mixed wettability.It is also found that the electrochemical reaction rate does not significantly affect the two-phase flow in the cathode porous media.The peak output power density of such a DMFC is reduced because too high fuel concentration results in severe fuel crossover,which decreases the cathode performance.（3）Two-phase flow and heat and mass transport in a passive liquid-fed micro DMFC operating with high methanol concentration:The intrinsic thermal effect caused by the electrochemical reactions and fuel crossover will raise the operating temperature of the membrane electrode assembly（MEA）.The larger current density,the higher operating temperature.Since the liquid fuel exhibits good heat transfer performance,the temperature at the anode side is higher than that at the cathode side.Simulation results also indicate that the local concentration inside the catalyst layer under the rib is lower than the one under the channel because of the longer transport distance,which results in the lower temperature region inside the porous electrode under the rib because of low reaction rate and high thermal conductivity of the rid.The increase in the operating temperature will be beneficial for the proton transfer in polymer electrolyte,but too high temperature will lead to low water content in polymer electrolyte phase.Although the higher operating temperature will facilitate the transfer rate of each component and the electrochemical reaction rate,the fuel crossover to the cathode is dramatically deteriorating under high temperature condition and significant degradation will appear in the cathode performance.The lower thermal conductivity of the anode porous media tends to increase the operating temperature inside the MEA,which not only results in a larger oxygen diffusivity at the cathode but also enhance the liquid methanol transfer and evaporation.The enhanced evaporation allows for insignificant fuel crossover at high operating temperature.（4）Water and thermal management in a passive vapor-fed micro DMFC operating with neat methanol:For a passive vapor-fed DMFC operating with neat methanol without liquid water actively supplied to the anode,the thermodynamic state of water in the porous electrode mainly depends on the pressure difference between the water vapor partial pressure and the saturation vapor pressure under the corresponding temperature.The hydration of polymer electrolyte is better under the liquid water condition.But serious dehydration in polymer electrolyte will appear since the excessively high temperature makes it difficult for water vapor to condense into liquid water.Although the addition of the water management layer at the cathode will increase the diffusion resistance of oxygen to the cathode catalyst layer（CCL）,the increased transport resistance of water vapor to the surroundings also benefits the formation of liquid water.Accordingly,the improvement in the water retention will decrease the Ohmic loss and enhance the anode performance.An appropriate open ratio of the evaporator can maintain the adequate fuel concentration in the ACL to minimize the concentration polarization under high current density condition,but too large open ratio of the evaporator will lead to too high operating temperature,which causes severe dehydration of polymer electrolyte and the aggravated fuel crossover and thereby decrease the cell performance.（5）Two-phase flow and heat and mass transport in a passive vapor-fed micro MFC with an electrolyte flow channel:When the liquid electrolyte is in direct contact with the CCL,adjusting the hydrophobicity of each porous layer enables the formation of the appropriate gas-liquid two-phase distribution in the cathode porous layers.In this case,not only the full hydration of the polymer electrolyte but also the sufficient oxygen transport to the cathode catalyst layer can be ensured.For the passive vapor-fed MFC,the liquid electrolyte can remarkably improve the hydration of the polymer electrolyte and dilute the fuel concentration in the ACL,which can reduce the fuel crossover.The appropriate Pe number can effectively minimize the negative effect caused by too high operating temperature resulting from the thermal effect,which can greatly reduce the fuel crossover.In particular,when the environmental temperature is relatively high,the liquid electrolyte can ensure the hydration of the polymer electrolyte at the anode while maintain the cathode performance.As a result,the intrinsic high electrochemical reaction rate at both the anode and cathode can be remained at high operating temperature,which greatly improve the adaptability to the environmental temperature.