On the water transport behavior and phase transition mechanisms in cold start operation of PEM fuel cell

An analytical dynamic model is proposed to predict the cold start behavior of proton exchange membrane (PEM) fuel cell. Water phase transition mechanisms have been reconstructed based on the five states of water migrating in the fuel cell, composed of water vapor, super-cooled water (liquid water), ice, membrane water (water dissolved in solid electrolyte) and frozen membrane water, in order to explore the reasonable water production and phase transition mechanism. The general water transfer behavior during cold start operation of PEM fuel cell finally evolves into four stages: (1) unsaturated water in ionomer and membrane water production; (2) water in ionomer reaching saturation; (3) over-saturated water in ionomer and quick desorption into liquid and vapor in pores; and (4) significant ice formation. Both non-equilibrium and equilibrium methods to simulate phase transition have been carried out to reveal water transport characteristics in the cold start operation. The assumptions associated with the liquid water and vapor production in the electrochemical reaction extensively involved in the previous modeling studies in the literature should be cautiously used, especially for the dynamic modeling studies. It is worthwhile addressing that the liquid water inside the cathode CL follows the slow increasing and rapid decreasing trend during the cold start operation, which indicates that the liquid water should mostly freeze at the shut-down moment. Furthermore, water prefers to freeze on the interface between catalyst layer (CL) and micro-porous layer (MPL), leading to the liquid water accumulation here migrating towards the membrane side.