Numerical Simulation Of Mass Transfer In Passive Direct Methanol Fuel Cells

Direct methanol fuel cell (DMFC) is an electrochemieal energy-conversion device which directly uses methanol as fuel. DMFC has received much attention as a renewable energy source for portable applications that have the advantages of simple in system, high energy density, and convenient fuel storage and transportation. Compared with the active DMFCs making the fuel cell system complex, the passive DMFCs without using moving parts such as pumps, fans and blowers have been appealing and studied. In an effort to resolve a series of processes, including mass transfer, heat transfer and electrochemical reactions, mathematical modeling of the passive DMFC system plays an import role, as it can provide a powerful and economical tool to elucidate the complex transport processes, which is rather difficult to be studied experimentally.Firstly, a numerical model is developed to simulate the three-dimensional (3D), two-phase, transient characteristics in anode side of a fully passive direct methanol fuel cell. The results indicate that the distributions of methanol and the gas-void fraction are caused by both nature convection and slight force convection accompanied by a complicated interaction between the chemical reaction and the gravitational force; the resistance of mass transfer mainly occurs in the porous medium rather than that in tank; the gaseous velocity in anodic porous medium affects mass transfer in the passive DMFC, however it is more important that the appropriate magnitude of liquid velocity should be studied in-depth for enhancing mass transfer of liquid in anodic porous medium of passive DMFCs.Then, based on above-mentioned 3D mathematical model, the effects of the porosity of anode catalyst layer (ACL), current density and cell orientation on two-phase mass transport are investigated. The numerical results indicate that the porosity has a little impact on the transfer of gas and liquid in anode side; the decrease of current density reduces the generation rate of CO2 and consumption rate of methanol solution, which leads to reduction of the magnitude of both gaseous velocity and liquid velocity; natural convection in the vertical cell is stronger than in the horizontal cell, so vertical operation can increase the mass transfer rate of methanol from the fuel tank to ACL and CO2 emitted to the ambient.Finally, a three-dimensional transient model of two-phase mass transfer is described to investigate the fluid flow and species transport in cathode side of passive direct methanol fuel cells. The effect of the gravitational force on the distribution of gas and water volume fraction in the fuel cell is analyzed, and the effect of cathode structural parameters of the passive DMFC such as the porosity of porous catalyst layer on the two-phase mass transfer is investigated using the presented model. The paper also discusses the effects of some operation conditions on the mass transfer in cathode side of fuel cells based on the simulation results, including current density and operation pressure conditions. The model developed here is useful for the basic understanding of three-dimensional transport phenomena in fully passive DMFCs, and for the optimization of cell design and operating conditions.