Modeling of the planar self-breathing PEMFC

This paper presents a model-based analysis of a proton exchange membrane fuel cell (PEMFC) with a planar design for the power supply of portable applications. The cell is operated with hydrogen and it consists of an open cathode side allowing for passive, self-breathing operation. This planar fuel cell is fabricated using printed circuit board (PCB) technology. Long-term stability of this type of fuel cell has been demonstrated. A stationary, two-dimensional, isothermal, mathematical model of the planar fuel cell is developed. Fickian diffusion of the gaseous components (O2, H2, H2O) in the gas diffusion layers and the catalyst layers is accounted for. Transport of water is considered in the gaseous phase only. The electrochemical reactions are described by the Tafel equation. The potential and current balance equations are solved separately for protons and electrons. The resulting system of partial differential equations is solved by a finite element method using the software FEMLAB (COMSOL Inc.). Three different cathode opening ratios are realized and the corresponding polarization curves are measured. The measurements are compared to numerical simulation results. The model reproduces the shape of the measured polarization curves and comparable limiting current density values due to mass transport limitation are obtained. The simulated distribution of gaseous water shows that an increase of the water concentration under the rib occurs. It is concluded that condensation of liquid water may occur under the rib leading to a reduction of the open pore space accessible for the gas transport. Thus, a broad rib does not only hinder the oxygen supply itself, but may also cause additional mass transport problems due to condensation of water