A pseudo two-dimensional two-phase proton exchange membrane fuel cell model and its applications for water management study

Abstract

Numerical studies of the water transport in proton exchange membrane (PEM) fuel cell are presented in this thesis in order to develop an effective active water management scheme for achieving an adequate water distribution in PEM fuel cell. A computational efficient two-phase gas channel model is derived in order to correlate the liquid water flooding in gas channel to the inlet operating conditions. The proposed model is then coupled to the membrane electrolyte assembly (MEA) model to develop a steady-state pseudo two-dimensional two-phase PEM fuel cell model as a framework for analyzing the role of inlet operating conditions. It is found that inlet operating conditions can have significant influences on the performance of PEM fuel cell and can act as effective control parameters for optimizing its performance. In particular, since the performance of PEM fuel cell is highly sensitive to the water distribution within the MEA and water flooding can have detrimental effects on both the performance and the lifetime of PEM fuel cell. The influences of the inlet relative humidity at anode and cathode are systematically investigated by the developed numerical model. A general trend is observed that the influences of the inlet relative humidity at anode and cathode are highly asymmetrical, so a fully humidified anode and dry cathode are generally preferred for maximizing the output power density of PEM fuel cell by means of achieving a good protonic conduction and an unimpeded diffusion of reactant gases. The usefulness of inlet relative humidity control as a simple and effective method for optimizing PEM fuel cell’s performance are demonstrated theoretically by using two examples, in which the use of inlet relative humidity control for maximizing the power density (in static condition) and extending the operating range of PEM fuel cell (in moving-static condition) are presented. In order to realize the proposed inlet relative humidity control, a fast two-phase dynamic PEM fuel cell model is further developed. A short computational time can be achieved by assuming that the concentrations of reactant gases are in steady state, while the dynamic responses of the water content in membrane and the liquid water saturationing as diffusion layer, which have significantly larger characteristic time constants, are considered. The asymmetrical influences of inlet relative humidity at anode and cathode and the dynamic responses of PEM fuel cell under step changes of current density are realistically captured by the model.