Phase Change in a Polymer Electrolyte Fuel Cell

Abstract

Stable high performance in a polymer electrolyte fuel cell ͑PEFC͒ requires efficient removal of product water and heat from the reaction sites. The most important coupling between water and heat transport in PEFC, through the liquid-vapor phase change, remains unexplored. This paper sheds light on physical characteristics of liquid-vapor phase change and its role in PEFC operation. A two-phase, nonisothermal numerical model is used to elucidate the phase-change effects inside the cathode gas diffusion layer ͑GDL͒ of a PEFC. Locations of condensation and evaporation are quantified. Operating conditions such as the relative humidity ͑RH͒ of inlet gases and materials properties such as the thermal conductivity of GDL are found to have major influence on phase change. Condensation under the cooler land surface is substantially reduced by decreasing the inlet RH or increasing the GDL thermal conductivity. The RH effect is more pronounced near the cell inlet, whereas the GDL thermal conductivity affects the phase-change rate more uniformly throughout the flow length.Liquid-vapor phase change, or specifically evaporation/ condensation, plays a pivotal role in the coupled water and thermal management of polymer electrolyte fuel cells ͑PEFCs͒. Although a large body of literature exists to explore two-phase transport in PEFCs analytically, experimentally, and numerically, 1-14 direct examination of the phase change phenomena has been absent. This work sheds light on the physical characteristics of phase-change phenomena and their role in fuel cell operation. Another objective is to prompt the development of in situ and direct measurements of the phase-change rate as an important parameter to the fundamental understanding and design improvement of PEFCs.In two-phase modeling of PEFCs, local thermodynamic equilibrium is often assumed between the liquid and gas phases at the evaporation/condensation interface. This assumption implies that negligibly small supersaturation of water vapor exists in the gas phase when condensation or evaporation occurs. The chemical equilibrium occurs at the microscale and in the vicinity of an interface. The existence of interfacial equilibrium at a phase-change interface does not mean a zero phase-change rate. Indeed, a finite phasechange rate can coexist with the interfacial equilibrium condition. Analogously, an extremely facile electrochemical reaction yields negligibly small overpotential or an equilibrium condition, but there is still a reaction current occurring at the electrochemical interface.In general, the phase-change rate can be expressed as 15-18where H is directly proportional to the area of the interfacial surface where phase change occurs. In porous layers of fuel cells, the liquidvapor interfacial area is usually sufficiently large thatthus yielding equilibrium phase change. If H is not sufficiently large, there exists nonequilibrium phase change. Wang et al. 15 theoretically showed that the coefficient H is proportional to aD H 2 O /r p , where r p is the pore radius, a is th...

Impact of Liquid Water on Reactant Mass Transfer in PEM Fuel Cell Electrodes

“…The model is based on a percolation approach to approximate the configuration of liquid water resulting from cluster growth at isolated nucleation sites and subsequent coalescence and percolation of growing clusters. This simplified approach begins by assuming condensation commences over the cool flow field rib areas 24 and randomly assigns a small fraction (arbitrarily chosen as 10%) of the pore throats in this region as nucleation sites. Invasion percolation (IP) described in the previous section proceeds from each nucleation site simultaneously in a manner similar to the invasion from the CL-GDM injection face.…”

Liquid water transport mechanism in the gas diffusion layer

“…Therefore, the capillary pressure vs. liquid water saturation curve, providing a constitutive relationship to simulate the liquid water transport in the cathode, has received considerable attention [1,2]. However, recent studies show that the vapor condensation obviously affects the liquid water transport in the cathode [3][4][5], i.e., the capillary pressure driven flow is not the only physical phenomenon in cathode.…”

Determination of Water Evaporation Rates in Gas Diffusion Layers of Fuel Cells

“…The shape of the evaporation front was only postulated without actually visualizing it. Also, the available amount of numerical studies on phase-change in GDL [20][21][22] is limited. In the last few years, there have been rapid developments in the application of X-ray tomographic microscopy (XTM) for visualizing water distribution in GDL.…”