Analysis of heat transport in a Proton Exchange Membrane fuel cell

American Journal of Applied Sciences, Jan, 2009 by Afshari E., Jazayeri S.A.

INTRODUCTION

The proton exchange membrane (PEM) fuel cell is a promising alternative to traditional power sources for a wide range of portable, automotive and stationary applications. The advantages of PEM fuel cell include the ability to provide high current densities at relatively low operating temperature, quick start-up, immediate response to changes in the demand for power and emission-free operation (1). The high cost and relatively low reliability of fuel cell are the limiting factors for their widespread use. A better understanding of operating conditions in PEM fuel cell is essential to the development and optimization of fuel cells, the introduction of cheaper materials and fabrication techniques and the design and development of novel architectures.

Thermal management is very important in overall cell performance. The increase in the cell temperature could be beneficial or harmful to fuel cell performance. Since it increases electrochemical reaction rate and higher mass transfer rate, but usually lowers cell ohmic resistance arising from the higher ionic conductivity of the membrane. Instead, it may lead to increased mass transport losses due to the increase in water vapor pressure. In addition it may cause drying out of the membrane, which in turn can result not only in reduced performance but also in eventual rupture of the membrane. The decrease in the temperature can be beneficial or harmful to fuel cell performance since the low temperature allows the PEM fuel cell to have a quick start-up time compared to other fuel cell systems. Instead it could hamper the reaction rate which in turn could increase the losses and flooding of the membrane (1). Therefore temperature changes could reduce the performance of the PEM fuel cell where thermal management is critically important. Because of the highly reactive environment and compact nature of a fuel cell, it is hard to conduct calorimetric measurements to obtain the thermal data within a fuel cell. The transport phenomena are quite complex due to the coupling of convective heat and mass transport with phase change, porous media and electrochemistry in a fuel cell. This information is usually sought through modeling or simulation. Recent works are directed towards better understanding of the cell operation and performance (2-11). Mostly the electrochemical reactions are assumed as an adiabatic process (2-3) and some work considered the thermal-fluid transport in the porous electrode (4-11). Researchers have developed mathematical models to gain qualitative insights into the processes involved in the PEM fuel cells. Many mathematical models have been presented, but these models either treated the catalyst layer as an ultrathin interface or did not fully account for the effect of liquid water in the PEM fuel cell. Although modeling of transport and electrochemical phenomena has been considered extensively in the literature and provides good insight on fuel cell operation, but still a complete model of non-isothermal, two-phase and single-domain to determine temperature and the effect of thermal management on the performance of PEM fuel cell dose not exist. In this work a non-isothermal, two phases and single-domain, one-dimensional with the coupled electrochemical and thermal phenomena and unsaturated reactant gas streams is modeled in a five-layer membrane-electrode assembly of a PEM fuel cell to analyze the impact of cell voltage, operating temperature, relative humidity and GDL thermal conductivity on thermal behaviors of PEM fuel cell.

Model description: A typical PEM fuel cell layout is given in Fig. 1. The computational domain consists of anode and cathode gas diffusion layers (aGDL, cGDL), anode and cathode catalyst layers (aCL, cCL) and a proton exchange membrane (PEM). Humidified hydrogen and saturated air are supplied by the anodic gas channel and the cathodic gas channel, respectively. In the anodic catalyst layer, hydrogen is consumed to form protons and electrons that protons carry the ionic current to the cathode. However the electrons travel through the conductive diffusion layer and an external circuit that finally produces electric work. In the cathodic catalyst layer, the electrochemical reaction not only consumes the oxygen but also produces the heat and water.

[FIGURE 1 OMITTED]

The assumptions used in the model are: the model is one-dimentional and steady state, the reactant gas mixtures are ideal gases, the gas flow is laminar and incompressible with variable density, the Soret, Dufour, gravity and radiation effects are neglected, the catalyst is uniformly distributed in both the cathode and anode catalyst layers, The gas diffusion layer is assumed to be isotropic and homogenous and the viscosity of the gas mixture is constant and calculated from the inlet conditions. In contrast to usual approach which employs separate differential equations for different regions, we have taken a single-domain approach in which a sin gle set of governing equations valid for all regions. Therefore, no interfacial conditions are required to be specified at internal boundaries between these regions. The governing equations include conservation of mass, momentum, ionic charge and energy as well as individual species. The conservation of mass for all gas species are presented in Eq. 1. The source terms reflect changes in the overall gas phase mass due to consumption or production of gas species resulting from reaction and mass transfer between the water in the gas phase and that dissolved in the polymer. The gas mixture density is expressed as the sum of the individual species concentrations multiplied by their respective molar masses: