Liquid Water Transport in Gas Diffusion Layer
of Polymer Electrolyte Fuel Cells
Ugur Pasaogullari* and C. Y. Wang**
Electrochemical Engine Center and Department of Mechanical and Nuclear Engineering,
The Pennsylvania State University, University Park, Pennsylvania 16802, USA
High-current-density performance of polymer electrolyte fuel cells
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
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/cm
. 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.
Manuscript submitted June 26, 2003; revised manuscript received September 22, 2003. Available electronically February 5, 2004.
Polymer electrolyte fuel cells
have drawn much atten-
tion in the last decade as a promising candidate for high-efﬁciency,
low-emission power sources. High-current-density operation of
PEFCs, of special interest to vehicle applications, is prone to liquid
water formation due to excessive water generation at the cathode.
The ensuing two-phase transport of reactant and product species
becomes a limiting mechanism for cell performance, particularly at
high current densities,
. Therefore, a fundamental un-
derstanding of two-phase transport in porous gas diffusion layers
of PEFCs is essential in order to improve performance.
The importance of water management to PEFC performance is
repeatedly expressed in the literature.
A vast majority of currently
available polymer electrolytes requires hydration in order to provide
higher proton conductivity.
When the gas phase is saturated with
water vapor, water condensation takes place and resulting liquid
water starts to ﬁll the open pores of the GDL and cover the catalyst
particles, rendering them electrochemically inactive. This conﬂicting
requirement of membrane hydration and electrode ﬂooding avoid-
ance must be met simultaneously in order to achieve higher perfor-
mance. Flooding of electrodes is generally linked to high-current-