Analytical Pore Scale Modeling of the Reactive Regions of Polymer Electrolyte Fuel Cells

Abstract

This paper analyzes the effects of the catalyst layer porous structure on the performances of polymer electrolyte membrane fuel cells. Comparing the characteristic lengths of the porous structure with the characteristic lengths of the diffusion phenomena shows that the oxygen and hydrogen concentrations in the electrolyte phase change significantly at the pore scale level; therefore, the related diffusion phenomena need a nonhomogeneous description. These rapidly varying concentrations are coupled to the cell potentials through the reaction rate expression, i.e., the Butler-Volmer equation. Thus, to employ a macrohomogeneous description of the fuel cell without loss of accuracy, it is necessary to find an effective expression for the reaction rate which does not depend explicitly on the rapidly varying concentrations. This is done here through an analytical averaging procedure and results in an effective Butler-Volmer expression that includes implicitly the effects of nonhomogeneity of the porous structure. This expression is compared with the ordinary Butler-Volmer expression and with the agglomerate models in the literature. The former turns out to be valid only in the limit of low current densities, and the latter only in the high porosity limit. Finally, the effective Butler-Volmer expression is inserted in the framework of macrohomogeneous models. From the analysis of the model results, one can conclude that the effects of the porous structure on the cell performances are crucial for the correct description of the cell concentration polarization and the estimation of the effective Tafel slope at high current densities.The main goal of fuel cell modeling is the description of the device performances starting from the underlying physical phenomena, material parameters, and operating conditions. The models should be as simple as possible to reduce the numerical complexity but accurate enough to describe correctly the fuel cell operation. Some assumptions are often used to simplify the mechanistic models such as: 1 one-dimensional ͑1D͒ geometry, constant gas porosity, fully hydrated membrane, isothermal conditions, steady-state operation, and homogeneity of the media. Qualitative considerations can be used to estimate the applicability range of such assumptions. For example, the 1D geometry approximation can be applied when the gas concentrations do not vary too much along the flow channels, as in the case of high stoichiometric flow ratio, and when channels and ribs are sufficiently thin to render homogeneous the delivery of electrons and reactants. Where the approximations are no longer applicable, the underlying assumptions must be relaxed, and the models become more complex. In the literature, several models have been presented with the aim of going beyond the following approximations: 1D geometry, 2-4 constant gas porosity in the diffusive region, 4-6 fully hydrated electrolyte membrane, 7,8 isothermal conditions, 8,2 and steady-state operation. 9,4 Although these extensions are straightforward, relaxation ...

Macroscopic Modeling of Polymer-Electrolyte Membranes

“…where bulk signifies the concentration outside the film, the reference concentration is that in the membrane in equilibrium with the reference pressure, film δ and A are the thickness and specific external surface area of the film, respectively, and (66) In the third model [56,57,76,103,[194][195][196][197][198][199], denoted PEA, the porous-electrode equations are used and the reaction site is assumed to be a spherical agglomerate composed of supported catalyst, membrane, and possible gas micropores. For this model, the current balance for the ORR is given by…”

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

“…Initially, they adopted the BV model and successfully improved the model predictions at higher current densities [22]; the initial BV models did not predict well the polarizations at higher current densities caused by cathode flooding. Later work used the improved BV model to analyse the effect of the porous structure of the catalyst layer on cell performance [23], and later work focused on optimizing the BV model for faster computation by eliminating non-linear terms [24].…”

Analysis of a cathode catalyst layer model for a polymer electrolyte fuel cell

“…The central question is, at given operating conditions, where within the CL the reaction takes place, and how is the reaction distribution sensitive to the operating conditions and CL composition. CL models have recently received increasing attention (Eikerling et al, 2005;Pisani et al, 2003;Siegel et al, 2004). The models range in complexity from simplistic yet analytical (Eikerling et al, 2005) to comprehensive and complex (Pisani et al, 2003;Siegel et al, 2004).…”

“…CL models have recently received increasing attention (Eikerling et al, 2005;Pisani et al, 2003;Siegel et al, 2004). The models range in complexity from simplistic yet analytical (Eikerling et al, 2005) to comprehensive and complex (Pisani et al, 2003;Siegel et al, 2004). Eikerling et al (2005) has modelled the impedance of the layer with only two effects, oxygen diffusion in the pores and ohmic losses in the PEM.…”