Polymer electrolyte fuel cell transport mechanisms: a universal modelling framework from fundamental theory

Abstract

A mathematical multi-species modelling framework for polymer electrolyte fuel cells (PEFCs) is presented on the basis of fundamental molecular theory. Characteristically, the resulting general transport equation describes transport in concentrated solutions and also explicitly accommodates for multi-species electro-osmotic drag. The multi-species nature of the general transport equation allows for cross-interactions to be considered, rather than relying upon the superimposition of Fick's law to account for the transport of any secondary species in the membrane region such as hydrogen. The presented general transport equation is also used to derive the key transport equations used by the historically prominent PEFC models. Thus, this work bridges the gap that exists between the different modelling philosophies for membrane transport in the literature. The general transport equation is then used in the electrode and membrane regions of the PEFC with available membrane properties from the literature to compare simulated one-dimensional water content curves, which are compared with published data under isobaric and isothermal operating conditions. Previous work is used to determine the composition of the humidified air and fuel supply streams in the gas channels. Finally, the general transport equation is used to simulate the crossover of hydrogen across the membrane for different membrane thicknesses and current densities. The results show that at 353 K, 1 atm, and 1 A/cm2, the nominal membrane thickness for less than 5 mA/cm2equivalent crossover current density is 30 μm. At 3 atm and 353 K, the nominal membrane thickness for the same equivalent crossover current density is about 150 μm and increases further to 175 μm at 383 K with the same pressure. Thin membranes exhibit consistently higher crossover at all practical current densities compared with thicker membranes. At least a 50 per cent decrease in crossover is chieved at all practical current densities, when the membrane thickness is doubled from 50 to 100 μm.

Multiscale Modeling of Single-Phase Multicomponent Transport in the Cathode Gas Diffusion Layer of a Polymer Electrolyte Fuel Cell

“…Electrochemical treatments are generally based on dilute solution theory or concentrated solution theory and describe the transport characteristics of potentially multi-phase, multi-component flows through porous and quasi-porous regions of the multi-layered cell [3,4]. The detailed electrochemical treatment usually limits the mathematical treatment to one-dimension (through the thickness of the cell) [5,6] or two-dimensions (through-the thickness of the cell and along the channel length) [7,8,9,10,11].…”

“…If species 1 is assumed to the lightest fluid with a molecular weight of M 1 , specie 3 the heaviest with a molecular weight of M 3 , and the molecular weight of species 2 M 2 which lies between them, the velocities along which the particles of specie 1 move are…”

“…The one-dimensional electrochemical model employed for the current study is based on the General Transport Equation (GTE), which is derived from fundamental molecular theory [3,4]. In general, the GTE describes the movement of a species as part of a multi-component concentrated solution system due to the following modes of transport:…”

Polymer Electrolyte Fuel Cell Transport Mechanisms: Simulation Study of Hydrogen Crossover and Water Content

“…Springer et al [7] In our previous work, we established a validated general transportation equation that describes the physical behaviour of a constituent species within a fluid mixture and which also provides a theoretically consistent meaning for the benchmark fuel cell modelling approaches listed above [14,15]. The general transport equation was applied to simulate transport across the membrane as an inter-dependant, simultaneous fourcomponent system comprising of water, hydrogen ions, hydrogen and electrolyte [15].…”

“…The general transport equation was applied to simulate transport across the membrane as an inter-dependant, simultaneous fourcomponent system comprising of water, hydrogen ions, hydrogen and electrolyte [15].…”