PEM fuel cell voltage transient response to a thermal perturbation

Abstract

This paper describes a transient model predicting PEMFC voltage response to a step change in the cooling water temperature. Its objectives are to put forward the main transport parameters and their corresponding time scales. The fuel cell is assumed isothermal with a time constant τ t . The temperature variations result from the production of heat by the exothermic chemical reaction and by internal heat dissipation, and from heat transfer with the cooling circuit. The effects of temperature on fuel cell performances are taken into account through the variations in its thermodynamic voltage, in the kinetics of the half-reactions, and in the membrane ionic resistance. A dynamic and one-dimensional simulation of water transport in the membrane by electroosmotic drag and by diffusion is carried out: the relative humidity of gases varies with the cell temperature under the assumption that their specific humidity (i.e., the vapor content in the gas diffusion layers) remains unchanged. Two time constants characterize mass transfer in the membrane by water diffusion ( τ d) and by electroosmosis ( τ e). The Péclet number Pe which is equal to the ratio between τ d and τ e allows the comparison of the magnitude of these two transport mechanisms, both depending on current density and on the other operating conditions. The results of the model are compared to a set of experimental results obtained with a cell composed of a Nafion 115 membrane, and fed by hydrogen and pure oxygen. The average current density is 4000 A m −2. In these conditions, the smallest time constant is the one characterizing the fuel cell thermal response τ t (16 s). Therefore, the fuel cell voltage response to a temperature step occurs in two stages, the first one corresponding to the thermal regime. The second stage concerns water transport in the membrane; the best fit between numerical and experimental results yields to a Péclet number of about 16, which makes electroosmosis the most significant phenomenon.