Modelling and simulation of the laboratory low temperature proton exchange membrane and direct methanol fuel cells

Abstract

Proton exchange membrane fuel cells (PEMFCs) are promising candidates as power sources due to their high energy conversion efficiency, power density and low pollutants emission. Water management is of vital importance to achieve maximum performance and durability from PEMFCs. The main object of this work was to develop a mathematic model to better understand the water transport in PEMFCs under practical conditions. The aim is to enhance the output power of fuel cells by establishing effective water removal and distribution strategies. A single-phase flow, along the channel, isothermal model of a PEMFC is developed and validated against experimental data. Reactant flow and diffusion are simulated using the Navier-Stokes equation and Maxwell-Stefan equation, respectively. Water transport through the membrane is described by the combinational mechanism in which electro-osmotic drag, back diffusion and hydraulic permeation are all included. Agglomerate assumption is applied for the catalyst layer structure. This model is used to study the effects of the catalyst layer properties on cell performance. The model indicates that the rapid decrease in current density at lower cell voltage is due to an increased oxygen diffusion resistance through the ionomer film. A two-phase flow, across the channel, isothermal model is developed. The water phase transfers between water vapour, dissolved water and liquid water are taken into account and liquid water formation and transport are introduced. Liquid water occupies the secondary pores of the cathode catalyst layer to form a liquid water film on the outer boundary of the ionomer film. This model is used to study the influence of catalyst layer parameters and operating conditions on the cell performance. The model provides useful guidance for optimisation of the ionomer volume fraction in the cathode catalyst layer and the relative humidity of the cathode gas inlet. A two-phase flow, across the channel, non-isothermal model is developed. The model considered the non-uniform temperature distribution within the fuel cell. The modelling results show that heat accumulates within the cathode catalyst layer under the channel. Higher operating temperatures improved the fuel cell performance by increasing the kinetic rate, reducing the liquid water saturation on the cathode and increasing the water carrying capacity of the anode gas. Applying higher temperature on the anode and enlarging the width ratio of the channel/rib could improve the cell performance. A multi-variable optimisation of the cathode catalyst layer composition is represented by a surrogate modelling. Five design parameters, platinum loading, platinum mass ratio, ionomer volume fraction, catalyst layer thickness and agglomerate radius, are optimised by a multiple surrogate model and their sensitivities are analysed by a Monte Carlo method based approach. Two optimisation strategies, maximising the current density at a fixed cell voltage and within a specific range, are implemented for the optima prediction. At higher current densities, cell performance is improved by reducing the ionomer volume fraction and increasing the catalyst layer porosity. The one-dimensional, isothermal, time dependent and steady state models for the anode of a direct methanol fuel cell (DMFC) are developed. The two models are based on the dual-site mechanism, in which the coverage of intermediate species of methanol, OH and CO on the surface of platinum and ruthenium are included. Both the effect of operating conditions and electrode parameters are investigated. The distributions of methanol concentration and overpotential inside the electrode are represented and the current densities predicted by the intrinsic and macro kinetics are compared. From the analysis of the different models developed in this thesis, the main results can be summarised as: (1) Mass transport resistance resulted from the oxygen diffusion through the ionomer film surrounding the agglomerate is the main reason for the rapid fall of current density at lower cell voltage. (2) Ionomer swelling has a significant effect on fuel cell performance because it resulted in a decrease in the porosity and an increase in the ionomer film thickness, leading to an increase in the oxygen transport resistance. (3) Catalyst layer composition has a vital impact on the utilisation of the platinum catalyst and cell performance. (4) Heat accumulates within the cathode catalyst layer under the channel. Applying higher temperatures on the anode optimises the temperature distribution within the MEA and improves the cell performance. (5) Cell performance is improved by enlarging the width ratio of channel/rib. However, the improvement is limited by the sluggish oxygen reduction reaction. (6) For the methanol oxidation reaction in a Pt-Ru anode, the intrinsic current density is determined by the coverage ratios of the intermediate species. The structure and property of the electrode also play an important role in determining the anode performance of a DMFC.