Phenomenological Modelling of GDL in LT_PEMFC

I. SCHMIDHALTER; P. AGUIRRE

Buenos Aires

Congreso; 20th Topical Meeting of the International Society of Electrochemistry; 2017

In this paper, we introduce a 1D model of an elemental portion of the Gas Diffusion Layer (GDL) in Low Temperature Proton Exchange Membrane Fuel Cells (LT_PEMFC), taking into account the following phenomena: inert gases concentration gradients, water condensation and evaporation, convective flows and diffusive flows in gas phase, liquid water flow across GDL, liquid water hold-up, and mass transference across membrane-GDL interphase. Integral balances are proposed for finite volumes in which the GDL thickness is divided. Gas streams leaving a finite volume section must be saturated when a liquid water stream leaves the same section. In cases in which there is no leaving liquid stream, leaving gases may be saturated or undersaturated. Local water hold-up depends upon local liquid water flow rate. Boundary conditions for GDL are related to gas flow rate and gas composition in the chamber, electrochemical reaction rate at the Catalytic Layer, and water flow across GDL-membrane at this interphase. The solution to the equation system is analyzed and the number of elements is adjusted to obtain errors lower than 0.5% in numerical results. Changes in boundary condition values dramatically affect gas composition profiles and water condensation/evaporation processes through the GDL. Either convective or diffusive flows dominate the transport of reactants from the chamber to the membrane, depending on the parameters of those boundary conditions. Furthermore, for some conditions, water hold-up and both flows, diffusive and convective, are non-linear along GDL thickness. Other parameters that greatly influence concentration and flow profiles are identified by sensitivity analysis. A list ranking these parameters according to their relative Lagrange multiplier values is presented, void fraction, tortuosity, and Bruggeman coefficient being among the most important. Lagrange multiplier can be obtained for different optimizing objective functions, i.e.: maximizing reaction rate or minimizing partial pressure drop of reactant. Each objective reveals a different ranking list of the model parameters. Void fraction, tortuosity, Bruggeman exponent, and net water mass transfer across the membrane interphase are always at the top of these lists. Transport rates of reactants from the chamber to the Catalytic Layer are limited by effective diffusion which depends on the void fraction available for gas phase, which, in turn, depends on water hold-up. The greater the reaction rate, the greater the water hold-up in GDL. The model here developed allows computing the maximum transport rate for a given set of model parameters. This value imposes an upper bound on current density. Besides indentifying the importance of theses parameters, water management schemes could be developed on the basis of the model predictions in order to avoid GDL flooding or membrane drying processes. Based on this work, a phenomenological model for a whole LT_PEMFC will be presented in future works.