Numerical Investigation Of Multiphase Mass Transport Characteristics In Direct Methanol Fuel Cells

A direct methanol fuel cell(DMFC)is an electrochemical energy-conversion device that converts chemical energy of liquid methanol into electrical energy directly DMFC is a clean energy device,comparing with the Li-ion batteries,which has many advantages,such as simple system structure,high energy efficiency and high energy density,and long operating time.In this study,we developed a two-dimensional multiphase passive DMFC and a three-demensional multi-phase active DMFC using ANSYS FLUENT,respectively.Based on the two numerical models,we optimize the electrode structure and various operating parameters and analysis the phenomenon on gas-liquid flow,the electrochemical reaction and crossover.It is helpful for understanding the multiphase transmission law and theoretical basis in DMFC.The contents of this work are as follows1.A two-dimensional multiphase passive numerical DMFC is developed.First we study the effect of micro-porous layer(MPL)insertion.It shows that the anode micro-porous layer(AMPL)greatly increases the mass transfer resistance,reducing the methanol concentration in anode catalyst layer(ACL)and methanod crossover effectively.And cathode micro-porous layer(CMPL)can increase the liquid pressure in cathode,driving part of the aqueous methanol flowing back to the anode.Second,considering the the different fabrication processes of electrode,the effects of the variable contact angle of diffusion layer(DL)and the MPL are also studied in detail It is found that with the contact angle increasing along the flow direction of methanol in anode diffusion layer(ADL),the flow resistance is increased and the liquid needs to flow from hydrophilic region to hydrophobic region,resulting in less methanol crossover.It also shows that a higher fixed contact angle of DL general leads to better performance and more methanol crossover,although it increases the flow resistance at the inlet,the capillary driver flow is enhanced inside the DL and the mass transport resistance at DL/MPL is reduced.The effects of the variable contact angle in MPL along the through-plane direction are different from the effects of contact angle in DL The bigger the contact angles of MPL could lead to increase the methanol transport resistance and decrease the methanol crossover.The thin and high hydrophibic MPL could act as a mass transport barrier mainly because of the contact angle differences across the DL/MPL and MPL/CL interfaces,which is the dominant factor determining the MPL function.2.A three-dimensional multiphase active numerical DMFC is developed.In this model,the Eulerian-Eulerian model is adoped to solve the multiphase flow in channels and two-fluid model are used to simulate the multiphase flow and mass transfer in porous electrodes.The effects of some operating parameters such as current density,methanol concentration and operation temperature on multiphase flow are analyzed in detail.The results show that the carbon dioxide in anode flow channel(AFC)increases gradually along flow direction and is mainly accumulated near the AFC/ADL interface.The distribution trend of the carbon dioxide produed in ACL is the same as in AFC,besides it is likely to accumulate under the inlet.Moreover,it shows that the higher current density is,the more carbon dioxide is,the less methanol crossover is.It shows that the higher methanol concentration is,the more methanol crossover is.And it also shows that the higher temperature is,the more methanol crossover is,the less carbon dioxide is in porous region and AFC for the dissolution3.The DL of DMFC features strong anisotropic characteristics because of its fibrous structure.The effects of anisotropy in cathode diffusion layers(CDL)on performance and multiphase flow are investigated based on the active DFMC model in detail.It is found that the anisotropic electric conductivity is helpful to predict accurate the electric distribution.Comparing the polarization curves with isotropic and anisotropic transfer in DL,it shows that the cell performance with isotropic DL is overestimated and the deviation is as much as 48%,especially the electric conductivity.Although the effect of anisotropic permeability and gas diffusion on the cell performance are much more neglible,they have dramatic effects on transfer phenomena.The results show that the lower permeability is in through-plane direction(Kthrough).the more water is in CCL,but less water intrudes across CCL/DCL interface It also shows that the lower permeability is in in-plane direction(Kin),the more liquid is accumulated under the rib is and the more non-uniform liquid is distributed in CDL for transport resistance in in-plane direction.Moreover,the gas transfer is overestimated by anisotropic gas diffusion characteristic.In conclusiton,the simulations show that the considering the anisotropic characteristic is very necessary.