Liquid Water Transport in Gas Diffusion Layer of Polymer Electrolyte Fuel Cells

Abstract

High-current-density performance of polymer electrolyte fuel cells (PEFCs) is known to be limited by transport of reactants and products. In addition, at high current densities, excessive amount of water is generated and condenses, filling the pores of electrodes with liquid water, and hence limiting the reactant transport to active catalyst. This phenomenon known as "flooding" is an important limiting factor of PEFC performance. In this work, the governing physics of water transport in both hydrophilic and hydrophobic diffusion media is described along with one-dimensional analytical solutions of related transport processes. It is found that liquid water transport across the gas diffusion layer (GDL) is controlled by capillary forces resulting from the gradient in phase saturation. A one-dimensional analytical solution of liquid water transport across the GDL is derived, and liquid saturation in excess of 10% is predicted for a local current density of 1.4 A/cm2. Effect of GDL wettability on liquid water transport is explored in detail for the first time. Furthermore, the effect of flooding on oxygen transport and cell performance is investigated and it is seen that flooding diminishes the cell performance as a result of decreased oxygen transport and surface coverage of active catalyst by liquid water. © 2004 The Electrochemical Society. All rights reserved.