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Microscale and Macroscale Aspects of Water Management Challenges in PEM
Fuel Cells
Satish G. Kandlikar a
a
Mechanical Engineering Department, Rochester Institute of Technology, Rochester, New York, USA
Online Publication Date: 01 July 2008
To cite this Article Kandlikar, Satish G.(2008)'Microscale and Macroscale Aspects of Water Management Challenges in PEM Fuel
Cells',Heat Transfer Engineering,29:7,575 — 587
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Heat Transfer Engineering, 29(7):575–587, 2008
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Copyright ISSN: 0145-7632 print / 1521-0537 online
DOI: 10.1080/01457630801922246
Microscale and Macroscale Aspects
of Water Management Challenges in
PEM Fuel Cells
SATISH G. KANDLIKAR
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Mechanical Engineering Department, Rochester Institute of Technology, Rochester, New York, USA
Water management is critical to the successful implementation of proton exchange membrane (PEM) fuel cells in the
automotive transportation sector. Liquid water appears in the fuel cells not only from the water generated at the cathode
catalyst layer but also as a result of condensation of water vapor within the humidified gases. Topics of intense current interest
include the microscopic flow of liquid water through the membrane, catalyst layers, and gas diffusion layers; the macroscopic
interaction between water and the gas flow at the gas diffusion layer interface; and the two-phase, multicomponent flow
through the gas channels. Recent work published in this area is reviewed and recommendations for future work are outlined.
INTRODUCTION
Automotive application of fuel cells is being pursued worldwide to develop hydrogen as a replacement fuel for the current petroleum products. Hydrogen can be produced from different energy sources, and is expected to play a major role in
the future, especially in the transportation sector [1]. A number
of engineering disciplines are involved in enabling hydrogenbased fuel cell technology. The role of effective water management in proton exchange membrane fuel cell (PEMFC) in
particular has been identified as a major research area related
to its performance, durability, and subfreezing operation of
PEMFC.
Water management in PEMFC is receiving considerable attention as seen in the number of recent publications. The objective of this review article is to provide a comprehensive summary
of research published mainly in the last three years on this topic
and present recommendations for future research in this area.
Earlier work in this area is summarized extensively in handbooks, such as Handbook of Fuel Cells [2].
Operation of a Proton Exchange Membrane (PEM) Fuel Cell
Figure 1a shows a schematic of a single fuel cell assembly. It
consists of two bipolar plates that provide channels for gas distriAddress correspondence to Satish G. Kandlikar, Mechanical Engineering
Department, Rochester Institute of Technology, Rochester, NY 14618 USA.
E-mail: [email protected]
bution over the gas diffusion layers (GDL). The space between
the two adjacent channels in the bipolar plate is called the land.
The lands provide a pathway for free electrons between the GDL
and the bipolar plates. The GDL distributes the gases uniformly
over the anode and cathode catalyst layers (ACL and CCL). A
microporous layer (MPL) often is employed between the GDL
and CCL to improve the water management in this region. A
membrane sandwiched between the ACL and CCL provides a
pathway for the transport of hydrogen ions (H+ ) from the anode
to the cathode side. Nafion (DuPont, Wilmington, DE, USA) is
a commonly employed membrane material. The catalyst layers
are made of proton conducting ionomers, carbon particles, and
platinum catalyst particles. Figure 1b shows a schematic of the
catalyst layer, showing the ionomers (providing ion pathways)
and the platinum particles (catalyst) dispersed on the carbon
particles (providing electron pathways). The GDL is made of a
porous electrically conducting material such as carbon paper or
cloth, and is specially coated with PTFE (Teflon) to enhance the
hydrophobicity. The subassembly, consisting of the membrane,
ACL, and CCL is referred to as the membrane electrode assembly (MEA). Sometimes the MPL and GDL are also included as
a part of the MEA.
The electrochemical reaction takes place at the catalyst layers as illustrated in Figure 1c. Hydrogen ions and free electrons are produced at the ACL during the oxidation of hydrogen. The electrons travel through the anode side GDL/MPL and
the bipolar plate, go through an external load, and then are returned to the CCL through the cathode side bipolar plate and
GDL/MPL. The air flowing in the cathode side channels provides
575
576
S. G. KANDLIKAR
liquid water content in the channels and other regions of the
fuel cell.
Water is essential for keeping the membrane and the catalyst
layer ionomers conductive for transport of hydrogen ions, but
excess water must be removed to avoid flooding in various regions. Effective water management thus is critical to the efficient
operation of a PEM fuel cell.
SOURCES AND TRANSPORT OF WATER
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Water exists in three forms in a PEM fuel cell:
Figure 1 Basic elements of a PEMFC. a) Schematic showing the bipolar
plates and MEA; b) Details of the catalyst layer, membrane, and GDL interfaces;
c) electrochemical reaction in a PEMFC. Not to scale.
oxygen, which diffuses through the GDL and MPL, and reacts
with hydrogen ions and free electrons at the CCL, generating
heat and water as the reaction products. Effective removal of
heat is essential to control the cell and gas stream temperatures,
which, in turn, govern the humidity of the gas streams and the
heat transfer engineering
(i) As water vapor within the gas streams. In a mixture of
a gas and water vapor, the amount of water vapor (humidity ratio) is determined by its partial pressure, which
cannot exceed the saturation vapor pressure of water at
the mixture temperature. A decrease in the temperature,
or an increase in the humidity ratio of the gas stream due
to either (a) depletion of the reactants (hydrogen on the
anode side or oxygen in the air on the cathode side) or
(b) introduction of water vapor from the electrochemical
reaction beyond the saturation limit, results in condensation of the water vapor. Conversely, an increase in temperature may cause the saturation vapor pressure to rise
and provide a potential for further evaporation of liquid
water.
(ii) As liquid water in the catalyst layers, gas diffusion layers,
gas channels, and headers. The state of water produced as
a result of the chemical reaction is intimately linked to the
heat removal process. The local humidity ratio as well as
the temperature in the gas phase are high in the catalyst
region as water is produced and the gases are continually
consumed by the reaction. Flooding is generally a concern
on the cathode side because water is produced here. On
the other hand, dehydration is of greater concern on the
anode side as water is available only from the humidified
hydrogen stream in the channels or from the back diffusion
process through the membrane.
(iii) As absorbed water in the membrane. The proton conductivity of the membrane depends on its water content.
The water transport mechanisms in the membrane include: (a) back diffusion of water from the cathode to
the anode as a result of the water concentration gradient and (b) electro-osmotic drag on water molecules from
the flow of hydrogen ions from the anode to the cathode
side.
The transport of water in the membrane and the ionomers
occurs at a molecular level, while its transport through the porous
structures of the ACL, CCL, MPL, and GDL takes place at
a microscopic level, depending on the pore sizes. Removal of
water from the GDL surface and in the channels occurs at a
microscopic or a macroscopic scale, depending on the channel
vol. 29 no. 7 2008
S. G. KANDLIKAR
dimensions. This article presents a critical review of literature
covering water transport in a fuel cell at both microscopic and
macroscopic scales.
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TRANSPORT OF WATER IN THE CATALYST LAYERS
Presence of water in the ACL and CCL is of critical importance. Dehydration causes a loss of proton conductivity in
the ionomers and the membrane, thus hurting cell performance
[3]. It also leads to contraction of ionomers away from other
components in the catalyst layer with a subsequent reduction
in the reaction area. In addition, the ACL- and CCL-membrane
interfaces become susceptible to cracking and peeling [4]. Under certain conditions, the increase in ionic resistance due to
reduced ionic pathways can lead to local heating, potentially
burning holes through the membrane. Such holes would provide a flow pathway for gases to move across the membrane,
causing undesirable effects [5].
The CCL is perhaps the most critical region from a water management standpoint in a fuel cell. Here the transport of hydrogen
ions, electrons, and oxygen gas to the three-phase interface, and
removal of product water, pose a significant challenge. When
CCL becomes flooded, oxygen has to diffuse through a thicker
layer of water surrounding these reaction sites in order for the
reaction to continue, leading to performance degradation. The
CCL has been treated as a thin layer in the earlier literature on
this topic, but recent efforts are aimed at incorporating its structural details. Eikerling [6] used a statistical theory of random
composite materials to relate the CCL material characteristics,
such as composition, pore structure, and wettability to the effective transport properties (such as diffusion coefficients) that
influence the reaction process. The CCL plays a key role in water generation and its relative distribution toward the membrane
and gas diffusion layer.
Since the electrochemical reactions occur in the interfacial
three-phase contact line region, it is important to understand the
microscopic interaction of water with the hydrophilic and hydrophobic surfaces presented by Nafion and the carbon particles.
Yu et al. [7] studied the contact angles of microdroplets on the
surface of a catalyst coated membrane (CCM) and noted that the
contact angle decreased with time when a droplet was placed on
a dry CCM. The CCM swelled during this period. They related
the performance of the fuel cell to the microscopic wetting characteristics of four different catalyst coated membrane samples.
It was noted further that the localized degradation was caused
due to the presence of silicon, which came from the sealing material and preprocessing on these sites, where a contact angle
of 33◦ (hydrophilic) or lower was observed. It should be noted
that the design of such experiments is quite complex due to the
dynamic nature of evaporation and condensation processes. An
environmental scanning electron microscope (ESEM) was used
to capture the images under carefully controlled temperature and
humidity conditions.
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577
The presence of wetting regions on the CCL is needed to
improve the performance at low humidity conditions. Jung et al.
[8] added hydrophilic SiO2 to the catalyst layer and observed
performance improvement. For removing excess water at high
humidity conditions, or at higher loads, Wang et al. [9] used
permanent magnet particles to drive the water away from the
CCL region.
Research Needs—Water Transport in the Catalyst Layers
The microscopic nature of the interactions of water and the
catalyst layer components needs to be clearly understood for effective water management in the ACL and CCL. Recent efforts
have been directed to measure the contact angles at microscale
within the porous structure and relate them to the overall cell
performance. The difficulties in accessing these interior regions
have prevented us from knowing the characteristics of water in
different phases within the microscopic passageways in these
porous materials with varying surface energies. Separate experiments focused on understanding the mechanistic behavior of
water in these configurations will lead us to develop better water management strategies at microscale and improve the dry and
wet operations in the catalyst layers. New strategies to control
the water movement (e.g. using magnets and hydrophilic particles) need to be considered in more depth. It is also important
to investigate the cost-effectiveness, durability, and long-term
performance of these strategies.
WATER TRANSPORT IN MEMBRANES
Water transport in the membrane occurs due to two mechanisms. The first one is called electro-osmotic drag, which refers
to the drag force exerted by the transport of hydrogen ions on
the water molecules in the membrane. This causes movement
of water from the anode to the cathode side. The other transport
mechanism, back-diffusion, is the result of the difference in the
water concentration on the two sides of the membrane. Due to
its production in the CCL, water content on the cathode side is
generally higher than on the anode side, and water diffuses from
the cathode to anode side through the membrane. These two
mechanisms play a critical role in maintaining a proper water
balance on the two sides of the membrane.
In order to accurately estimate the electro-osmotic drag and
water back-diffusion rates, it is essential to obtain the respective
coefficients. The electro-osmotic drag coefficient is defined as
the number of water molecules dragged by each hydrogen ion
as it moves from anode to cathode. Yan et al. [10] performed
experiments on a PEM fuel cell of 25 cm2 active area using
MEAs made by E-Tek (Nafion 117 membrane and a total Pt
loading of 1 mg/cm2 ). Their drag coefficient values ranged from
1.5 to 2.6 obtained over variable inlet humidity, although values
as high as 5 are reported by other investigators (e.g., Larminie
and Dicks [11]).
vol. 29 no. 7 2008
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S. G. KANDLIKAR
As discussed earlier, the water transport in the membrane
is due to electro-osmotic drag and back diffusion processes.
The three resistances in the back diffusion are: (a) absorption
of water at membrane/CCL interface on the cathode side; (b)
diffusion resistance in the membrane; and (c) desorption of water at membrane/CCL interface on the anode side. Ge et al.
[12] measured the absorption and desorption coefficients for a
Nafion 115 membrane. They found that the strongly hydrophobic features of the Nafion surface resulted in an absorption coefficient much lower than the desorption coefficient. Also, as the
thickness of the membrane is reduced, these interfacial resistances played a greater role on the water transport through the
membrane.
The electro-osmotic drag coefficient and the hydraulic permeability of Nafion were measured by Meier and Eigenberger
[13] and were found to be dependent on the water content. Based
on experimental observations, they concluded that the membrane structure also has an influence on these properties. Similar
findings were earlier reported by Okada et al. [14], who found
that the mobility of the ions is dependent on the microstructure
of the membrane pathways. The electro-osmotic drag is a result of the hydration of cations and hydrodynamically pushed
water molecules. They proposed a criterion for designing high
performance ion conducting membranes based on their findings. Karimi and Li [15] developed models for electro-osmotic
flow using Poisson-Boltzmann and Navier Stokes equations and
solved them numerically for a single pore over a range of conditions. Their results indicate that the electro-osmotic drag coefficient increases with pore diameter. The effect of pore structure
on the drag coefficients is an important area for further investigation.
Research Needs—Water Transport in Membranes
Water transport in membranes plays a crucial role in properly maintaining the membrane’s hydration level and providing
water in the ACL region. A clear understanding of the water
transport mechanisms in the membranes and their characterization as a function of the membrane and operating parameters
is needed in developing efficient water management solutions.
These characterizations may be conducted through simulated
experiments on the membrane and then verified during in situ
testing.
drophobic to aid in the formation of droplets and their expulsion
into the gas stream. The hydrophobicity also helps in preventing
the re-entry of water into the GDL and reducing condensation
of water vapor from the gas stream as droplets on the GDL surface. The pore sizes in the GDL are larger (due to these mass
transfer requirements) as compared to the catalyst layers, which
are populated by the ionomer pathways to conduct hydrogen
ions to the reaction sites. Details of the GDL material and its
characterization are given by Mathias et al. [17].
The contact angles of water on the GDL surface serves as
a useful indicator of the surface energy of the GDL, but the
measurements of these angles are affected by the roughness
due to fiber structure, especially when the droplets are of the
same dimensions as the fibers [18]. The contact angles at the
pore level are of interest when considering transport of water
through the GDL as these angles are indicative of the surface
energy inside the pores. Gurau et al. [16] employed a combination of the Wasburn method [19] and the Owens-Wendt
theory [20] to obtain the internal contact angles. Their setup
consisted of carefully measuring the water uptake of a GDL
sample using a tensiometer and other test liquids to obtain the
solid-vapor surface tension estimation for water and the GDL
elements. Figure 2 shows a SEM image of a GDL made of Toray
carbon paper with microscopic water droplets adhering on the
fibers.
Flooding is of great concern in the cathode side GDL. Yamada et al. [21] experimentally measured the extent of flooding
by switching the operation of the fuel cell from the conventional flow field to interdigitated flow field and measuring the
pressure drop. This pressure drop measured immediately after
switching the flow fields is indicative of the level of flooding
with the conventional channels. At high current densities, they
noted that the flow field was flooded with water. Such testing
is possible only with experimental units with switchable flow
fields.
The resistance to water flow through GDL was measured by
Benziger et al. [22]. They applied hydrostatic pressure to force
water through the GDL. Pressures of 5-10 kPa were required to
overcome the surface energy of the Teflon coated GDL/water
interfaces in the largest pores. Such measurements can provide
TRANSPORT OF WATER IN THE GAS DIFFUSION AND
MICROPOROUS LAYERS
Gas diffusion layers help in distributing the reactant gases
uniformly over the catalyst layers as well as in transporting water back from the reaction sites into the gas stream. The in-plane
porous pathways (in the same plane as the GDL) help in distributing the gases over the catalyst layer underneath the land
area. The surface of the GDL exposed to the gas stream is hyheat transfer engineering
Figure 2 SEM image showing water droplets on the individual fibers in a
GDL [16].
vol. 29 no. 7 2008
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S. G. KANDLIKAR
information on the diameters of pores available for water
transport at specific pressure drop values. At higher pressures,
pores with smaller diameters also became active. The pore diameters varied from 20 μm to 250 μm, and Benziger et al. [22]
estimated that the water flows through less than 1% of the void
volume in the GDL. Wang et al. [9] implemented two different
levels of porosity along the water flow length in the GDL, called
bi-functional pore structure, and found it to be effective in water removal from the GDL as well as gas transport through the
GDL to the reaction sites. Earlier in 2003, Nam and Kaviany [23]
modeled the formation and distribution of water by determining
the effective diffusivity as a function of local porosity and local
water saturation. They applied the hydrodynamics of capillary
two-phase flow in hydrophobic media, along with a water balance from the chemical reaction and the local condensation rate,
to obtain water distribution as a function of the GDL material
characteristics. Their results indicate that a two-layer structure
provides a jump in the saturation condition across the fine and
coarse layers and helps in transporting water from the fine to
the coarse layer. By placing the fine layer (the MPL) adjacent to
the catalyst layer, water was efficiently removed from the reaction sites, resulting in enhanced cell performance. Pasaogullari
and Wang [24] developed a model to analyze the water transport in the GDL and concluded that the transport is governed
by the capillary forces. Par et al. [25] confirmed the effect of
two different pore sizes, and observed that condensed water was
difficult to remove from the GDL-catalyst layer interface. They
suggested that shear forces and evaporation may be responsible
for the water movement rather than the capillary force alone.
Shi et al. [26] employed fractals to model the GDL permeability
with two fractal dimensions: one using the size of the capillary flow pathways relative to their population, and the other
based on the tortuosity of the pathways. Their model was validated with the experimental data for Toray carbon paper. Zhan
et al. [27] applied a one-dimensional model and concluded that
under steady-state conditions water transport increases as the
contact angle and porosity increase, and as the GDL thickness
decreases. Their analysis showed the efficacy of the MPL and
porosity gradient in improving the water drainage performance
of a GDL.
Pressure drop measurements in interdigitated flow fields provide a simple method for the measurement of the flooding level
in the GDL as noted earlier. Lin and Nguyen [28] used this
method to compare the effect of GDL thickness and its hydrophobic polymer content on the flooding levels. Their results
indicate that hydrophobic pores promote gas flow, while hydrophilic pores aid the water transport in the GDL. There is
some optimal ratio of their distribution that gives a good balance
between the gas flow and water transport from cell performance
viewpoint. The presence of an MPL between the GDL and CCL
also was shown to reduce the flooding [29]. Lin and Nguyen
[28] also noted that without the MPL on the face of the GDL,
a thinner GDL was more susceptible to flooding compared to a
thicker GDL (in the range of 180 μm to 370 μm investigated
heat transfer engineering
579
Figure 3 Different pathways followed by water during its passage through
the GDL [31].
by them). Similar results were obtained by Weber and Newman
[30].
Visualizing the flow of water within a GDL pore structure
poses challenges due to the difficulty in gaining proper optical
access within the GDL. Studying the behavior of water droplets
on the surface of a free-standing GDL in the air provides some
useful information, but transport within the porous GDL matrix
cannot be revealed by this technique. Litster et al. [31] developed
a fluorescence microscopy technique to study the dispersion of
water within the GDL. A fluorescent dye was introduced in the
water and was forced through a 1-mm diameter inlet on one face
of the GDL. Using an appropriate exciting frequency of light to
activate the dye fluorescence, dispersion of water through the
Figure 4 Schematic showing water transport mechanism in a GDL [31].
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S. G. KANDLIKAR
porous GDL microstructure was observed through a microscope
mounted with a CCD camera. Figure 3 shows the various pathways taken by the water as it flowed through the porous structure,
and Figure 4 shows a schematic of the idealized liquid transport
mechanism proposed by Litster et al. [31]. Water enters through
a pore at the surface and then travels through a GDL porous pathway until it finds an opening, or breakthrough, to the other side
of the GDL. The different pathways of water shown in Figure
3 represent some of the dead end as well as successful transport pathways. The new mechanism proposed by Litster et al.
[31] is quite different from the earlier mechanisms proposed by
Udell [32], Nam and Kaviany [23], and Passaogullari and Wang
[24]. According to the Litster et al. [31] model, the liquid is
transported by fingering and channeling and features numerous
“dead ends,” where water in a dead end recedes when an adjacent breakthrough channel forms. The convergence of smaller
capillaries into a larger capillary is not supported by their model.
The reader is referred to their time-sequenced images and model
description for further explanation on the various pathways followed by the water.
Buie et al. [33] employed a novel electro-osmotic pumping
technique to remove water from the GDL on the cathode side of a
fuel cell. Electro-osmotic pumping employs an electrical voltage
to induce the flow of ions present in an electrical double layer,
and water molecules are dragged along with the ions. Water is
thus transported from the GDL to the channels by placing an
electro-osmotic pump between the GDL and the bipolar plate.
The power consumption of such devices is quite small, but their
effective use could yield significant performance improvement
through reduced flooding.
Based on the available literature, a mechanistic description
as shown in Figure 5 may be offered at this time. The water
produced at the CCL is expected to be in the vapor phase due
to the local heat generation at the reaction sites as also modeled by Eikerling [6]. The water vapor then diffuses through
the GDL toward the channels because of the difference in the
partial pressures of the water vapor-gas mixture at the CCL and
in air channels. At some location along the flow, water vapor is
condensed into liquid phase forming a condensation front and
releasing latent heat. The local temperature and saturation water vapor pressure profiles determine the exact location of the
condensation front. It is desirable that a condensation front is
formed at the interface between the MPL and GDL or at some
location within the GDL. If the condensation occurs at the CCL,
it will cause flooding because a much higher pressure is required
at the CCL to overcome the capillary forces at the hydrophobic
pores of the MPL for water to enter and flow toward the GDL.
MPL acts as a surface tension based gate that permits the transportation of water vapor from CCL toward GDL. However the
flow of water from GDL back toward CCL is prevented because
of the surface tension forces acting on the finer hydrophobic
pores of MPL. This mechanistic description also allows for the
countercurrent flow of reactant gases and liquid water through
different regions of GDL and MPL.
Research Needs—Water Transport in GDL
Water transport in the GDL is closely linked to its structure
and the surface energy of its constituents. It provides a capillary structure for effective water movement. Current models rely
largely on water transport studies in porous materials (such as
soils), whereas it is recognized that the flow in the GDL structure
is different. The model proposed by Litster et al. [31] provides a
new direction in this field. The effect of porosity gradient, local
interactions of water and the GDL matrix in microsize passages,
and condensation/evaporation phenomena at these scales on hydrophobic and hydrophilic surfaces need to be understood from a
fundamental perspective. Further research also is warranted on
the microporous layer and development of effective, low-cost
water removal strategies.
DROPLET REMOVAL AND TWO-PHASE FLOW IN THE
GAS CHANNELS
Flooding in Gas Channels
Water emerging through the network of pores in a GDL appears on its surface and is exposed to the gas stream. It is desirable to remove this water as quickly as possible to avoid flooding
in the channels. There are at least four reasons why the flooding
in the channels is undesirable:
Figure 5 A mechanistic description of water transport within a fuel cell.
heat transfer engineering
(i) Flooding in the channel causes a liquid film to cover the
GDL surface and prevent access of gases to the reaction
sites.
(ii) Flooding in the channels may lead to a partial or complete
blockage of the channel for gas flow, thereby starving the
reaction sites fed by the channel.
(iii) Flooding leads to a reduction in the power generation and
nonuniform current distribution in a fuel cell.
vol. 29 no. 7 2008
S. G. KANDLIKAR
581
(iv) Some of the purging techniques employed at the shutdown
may not be adequate to remove the water in the flooded
regions.
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The presence of water in the form of a film is undesirable
on the GDL surface. A film prevents the access of gases to the
reaction sites. A hydrophobic GDL surface promotes formation
of droplets which can be more easily removed by the flowing
gas stream. The two-phase flow in the channels is different from
the two-phase flow that generally is studied in the literature. The
main differences are listed below:
(i) The water content gradually increases along the flow length
with the introduction of water resulting from reaction, while
the gas flow rate decreases as the hydrogen and oxygen are
consumed by the reaction.
(ii) Water comes out from the GDL surface at preferred pore
locations, which are random. These droplets may be in the
center of the channel, or close to the walls in the corner.
Their location will influence their growth and departure
characteristics.
(iii) Water is present in the form of growing droplets, which
periodically are stripped off by the gas, are displaced in the
different areas of the channels (such as corners), as droplets
attached on the channel walls, or as a film.
(iv) The droplets stripped in the upstream flow region may coalesce with the droplets present in the channels and create
slugs of water.
(v) The flow may encounter localized regions where water is
taken away or is introduced into the flow as a result of
evaporation, condensation, water generation, and sudden
discharge or absorption of water within the GDL.
Droplet Removal Process
The advancing and receding contact angles of a droplet on a
GDL surface play a significant role on the departure mechanics.
Figure 6 Typical droplet shape and advancing contant angles, θA , and receding contact angles, θR , at departure observed by Borrelli et al. [35].
heat transfer engineering
Figure 7 Effect of superficial flow velocity, jG , on the departure droplet diameters of a water droplet at departure from the GDL surface [35].
A number of investigators have studied the droplet departure
sizes and contact angles in single channel studies. A few studies
have focused on observing the motion of water by using transparent fuel cells. Such efforts by Feindel et al. [34] employing
machined Delrin (an acetal resin engineering plastic by DuPont)
with gold contacts provided information on the water distribution in the channel and over the GDL surface.
Borrelli et al. [35] studied the droplet departure diameter, and
advancing and receding contact angles over a GDL surface in a
1-mm square Lexan (a polycarbonate plastic resin by General
Electric, Fort Edward, NY, USA) channel. Water was introduced
through a water manifold placed on the opposite side of the
GDL. The air flow rates were typical of those employed in fuel
cell applications. A schematic of a side view of a droplet at
departure is shown in Figure 6. The receding edge is seen to
exhibit a wetting behavior (contact angle less than 90◦ ), while
the advancing edge is seen to exhibit a non-wetting behavior
(contact angle greater than 90◦ ). Figure 7 shows the variation of
departure diameter as a function of the superficial air velocity.
As expected, the departure diameter decreases with an increase
in the superficial air velocity. The contact angle variation with
departure diameter also was reported. The advancing contact
angles, θA , remain relatively constant at fairly high values, in
the range of 120◦ to 140◦ , whereas the receding contact angles,
θR , are consistently lower, spread over a wide range from 20◦
to 80◦ . The contact angles are a function of the GDL surface
characteristics as well.
Hidrovo et al. [36] conducted experiments to study the water film thickness and its relation to the evaporation, condensation, and flow patterns in a 0.5-mm gas channel made in silicon.
Micro-PIV, fluorescence microscopy, and high-speed imaging
were employed to understand the basic water-air interactions at
these scales.
Kumbur et al. [37] conducted experiments to study the droplet
behavior and instability in a rather large rectangular channel of
5 mm × 4 mm (compared to less than 1-mm channels employed
in the automotive fuel cell applications) with the GDL placed
at the bottom of a channel with a transparent top cover. The
droplet size and contact angles were measured by employing a
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S. G. KANDLIKAR
prism along the channel length. Their findings also indicate the
dependence of the contact angles and departure diameters on
each other, as well as on the GDL surface and the flow Reynolds
number. Similar conclusions were drawn by Zhang et al. [38]
who observed the accumulation of water in the corners of the
channel at low air flow rates, and by Hickner and Chen [39] who
correlated a dimensionless droplet height with the contact angle hysteresis (difference in the advancing and receding contact
angles).
Novel techniques for droplet removal early during their
growth was considered by a few investigators. Palan and Shepard [40] and Palan et al. [41] applied structural and acoustic
vibration to aid in the droplet removal by atomizing or moving
the droplets on the GDL surface and channel walls.
The droplet departure models reported in the literature consider the surface tension and drag forces on a droplet emerging
from a GDL surface (e.g. He et al. [42], Chen et al. [43], Kumbur et al. [37], and Zhang et al. [38]). A detailed comparison
of these models is presented by Schillberg and Kandlikar [44].
Readers are referred to this reference for a more in-depth review
of droplet dynamics and modeling.
Pressure Drop Measurements
The pressure drops were also measured by Borrelli et al.
[35] in their ex situ experimental setup using a GDL as one
surface in a 1-mm Lexan channel. These were found to be
considerably higher when compared to the predicted pressure
drops from the two-phase correlations from literature using
the average channel mass quality. Introduction of water at discrete locations, accumulation of water in the channels at low
gas velocities, and the presence of small growing droplets in
the flow field were believed to be the main reasons for this
deviation.
English and Kandlikar [45] conducted pressure drop experiments with 1-mm square Lexan channels coated with a hydrophobic coating, and presented a modification of the Mishima
and Hibiki [46] correlation for estimating pressure drop in small
channels. Water flowed as droplets rather than in a film on the
hydrophobic surfaces, with somewhat higher pressure drops.
Although the channel pressure drop measurement in the gas
channels can serve as an indicator of flooding in the channels,
it does not provide information regarding the location of the
flooding zone. Nonetheless, such overall flooding information
in a channel can be integrated with the performance plots to
identify the regions where flooding may become a concern. Such
a diagnostic tool was studied by Ma et al. [47].
Neutron Imaging of Water in Channels
Neutron imaging is a very useful tool in locating the presence
of water. It was developed for visualizing water distribution in
an operating PEM fuel cell by Geiger et al. [48] and Satija et al.
heat transfer engineering
Figure 8 Neutron radiograph images obtained by Trabold et al. [51].
[49]. It has been employed successfully by a number of investigators (e.g. Owejan et al. [50] and Trabold et al. [51]). This
technique provides images showing the location of water in the
flow fields. Figure 8 is an image of a cathode side flow field
showing dry operation near the inlet region and water flooding near the outlet region. The effects of gas stream humidity,
flow field design, and header design have been investigated to
aid in the overall design and operational strategy for automotive
fuel cells. The availability of the neutron imaging facility [52]
and the freeze chamber at the National Institute of Standards
and Technology (NIST) is a very useful feature for fuel cell researchers for studying the freezing characteristics. Kowal et al.
[53] used the neutron imaging technique to identify water retention locations under the land area and under the gas channels.
They showed that the distribution in the GDL was a function
of the GDL matrix and the current density. Application of this
technique in measuring the water retention in the membrane also
is of interest as investigated by Ludlow et al. [54].
Further development (higher speed and better spatial resolution) of the neutron imaging technique is needed to enable
faster response for investigating transient as well as unsteady
behavior resulting from parallel channel instabilities. The ability of neutron imaging for studying the freezing conditions is
also promising. In conjunction with a freeze chamber, it will be
possible to visualize the residual water in the frozen state and
its response to different starting and purging protocols.
Additional Considerations for Two-Phase Flow in Gas Flow
Channels
The nonuniform flow distribution in parallel flow channels is
a major concern in flow field design. It is important that uniform
flow be first accomplished with single-phase flow to ascertain
that the flow resistances are well balanced. Further design considerations are needed for ascertaining flow uniformity under
two-phase flow conditions in the channels and headers. Barreras
et al. [55] employed planar laser induced fluorescence technique
to obtain the velocities in eighteen straight parallel channels.
Their experimental as well as numerical work indicated that
the gases flowed preferentially through the lateral channels with
pronounced velocity reductions in the central channels. Such
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S. G. KANDLIKAR
nonuniform distribution will lead to nonuniform current distribution and overall reduction in performance.
Effect of channel size is another major consideration. Making
channels smaller is beneficial in achieving uniform gas distribution over the catalyst layer, but the channel pressure drop
may become excessively large. Another concern is the flooding
characteristics of these channels. Cha et al. [56, 57] studied the
performance of a fuel cell with very small (100 μm and 20 μm)
channels and observed flooding in the channels. However, they
suggested using a thinner GDL to overcome the flooding problem. Proper matching of the GDL thickness with the channel
size is seen to be an important consideration.
The two-phase flow in channels is expected to be influenced
by the surface energies of the channel walls as well as that of
the GDL surface. Son and Allen [58] found that changing the
channel geometry and the contact angles during two-phase flow
in channels had a dramatic effect on the phase distribution and
pressure drop. Taniguchi and Yasuda [59], Cai et al. [60] and
Zhan et al. [27] studied the effect of making the channel walls
hydrophobic and found that it promoted droplet removal. Careful
assessment is needed in selecting the appropriate contact angles
for the GDL surface and the channel walls.
Two-phase flow in manifolds is also important from uneven
gas distribution and flooding considerations. A number of studies, for example, Jiao et al. [61, 62] report the results of numerical simulations with different flooding scenarios. Specific
experiments under actual operating conditions will allow us to
fully understand the design issues related to two-phase flow in
flow fields.
Research Needs—Water Transport in Flow Field Channels
The two-phase flow patterns in the gas flow channels determine the interactions between a growing droplet and the flow.
There is a clear need to develop an understanding of how the
surface energies of the channel walls and the GDL surface modify the two-phase flow and the droplet removal mechanisms.
The effects of channel geometry on the droplet removal process
needs to be explored. Flow instabilities in the parallel channels
also need to be investigated when dealing with multiple communicating channels in a fuel cell.
Pressure drop information is needed for design as well as
diagnostic purposes. In this regard, extensive measurements of
pressure drop under a known set of flow patterns and operating
conditions are needed. These measurements will be useful in
modeling of two-phase flows in fuel cell gas channels.
MODELING OF WATER TRANSPORT IN PEMFC
Since there are sometimes competitive aspects involved in
the overall performance of a fuel cell, a proper optimization
scheme is required to evaluate the net effect of varying specific
parameters on the overall fuel cell performance. Actually, this
heat transfer engineering
583
holds for many of the individual component-level performance
characterizations reported in the literature. The modeling tools
are useful in providing some of these answers.
A brief summary of some of the water transport models available in the literature is presented here. A comprehensive summary of the modeling efforts and fundamental aspects of modeling in different regions up to 2004 was presented by Wang [63].
A good introduction to the basic equations for mass transport
is given by Weber et al. [64]. In a subsequent article, Weber
and Newman [65] developed a mathematical model for transport of water in a PEM fuel cell. Their simulations agree well
with the experimental data over a range of operating conditions,
stoichiometry, temperature, humidity, and current densities. The
solution is effectively reduced to a two-dimensional formulation.
The drawback of their formulation is that it does not include the
kinetics in the catalyst layers. In another article, Weber and Newman [30] present equations coupling thermal and water transport
equations across the GDL from the membrane to the gas channels. Such models are useful in simulating the overall fuel cell
performance.
Wang et al. [66] developed a two-phase transport model in the
CCL by using a multicomponent mixture model for the porous
region. The model includes the oxygen concentration distribution, and accounts for the capillary forces in the porous region.
The catalyst layer, GDL, and the channel were coupled through
a transport model by You and Liu [67]. Hu et al. [68, 69] developed a similar model and predicted the water saturation curves
within the GDL to understand the effect of humidification in
interdigitated and conventional channels. Their model utilizes
the overall diffusion parameters in connecting the transport of
species through different regions of the fuel cell to the local
current density. Um and Wang [70] present a model for the
three-dimensional transport of water through an MEA under
isothermal conditions. Also, the interaction of liquid water with
the channels was ignored by considering it in the form of a fine
mist in the air. The model clearly showed the benefits of interdigitated flow fields in moving water away through the GDL.
Berning and Djilali [71] modeled the flow in the GDL and coupled it to the channel flow. A system level model incorporating
the temperature and pressure variations in the flow streams was
presented by Zong et al. [72]. A similar model was developed
by Matamoros and Brüggermann [73] and Meng [74].
Some of the recent studies have included more detailed modeling of the transport processes within the MEA. For example,
Sivertsen and Djilali [75] and Um and Wang [70] have developed CFD-based models including the temperature and humidity variations in the gas streams. A more complete system level
simulation was developed by Yu et al. [76] for studying the effect of inlet gas conditions and other operating parameters on
the system level performance and efficiency. Another comprehensive model is presented by Shah et al. [77], who incorporated
the thermal aspects as well as the parameters defining the microstructures of the catalyst layer, membrane, and the GDL.
This model is able to predict the cell performance as well as
the effects of the GDL thickness, water activity, and the water
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S. G. KANDLIKAR
removal rate at the GDL-channel interface. This model is able to
connect the local microregion flow and reaction characteristics
with the conditions in the gas channel flow field. Pasaogullari
and Wang [78] developed a model to study the effect of porosity, thickness, and wettability of the MPL. They showed that the
presence of an MPL significantly reduces the liquid saturation
condition within the GDL and discourages flooding. In a subsequent article Pasaogullari and Wang [24] showed that the dryout
and flooding depend not only on the GDL material characteristics, but on the optimized combination of the operating and
structural parameters. Liu et al. [79] specifically modeled the
flooding phenomena within the GDL by considering the pore
size distribution.
There are a number of additional papers published in this area,
but they are not included here due to space constraint. A separate
in-depth review of these models is currently under progress.
Research Needs—Modeling of Water Transport in PEMFC
As seen from the brief overview of literature presented in the
above section, the modeling efforts are moving toward coupling
the two-phase flow in the channels to the microscale diffusive
flow within the porous structure of the catalyst, membrane, and
the GDL (as well as the microporous layer, if present). Characterization of the porous structure seems to be an area where
some innovative concepts would be helpful in modeling these
materials. It has been established that these porous materials
are quite different from the porous materials historically studied
in the earlier literature (e.g., soil). These new definitions integrating the hydrophobic and hydrophilic characteristics, pore
size distribution, flow lengths of the passageways, and crossconnectivity of the pores will be extremely helpful in modeling
and understanding the behavior of water transport within a GDL.
Another area where research efforts are needed is in connecting the two-phase flow patterns in the channel, channel flooding, and parallel channel instabilities, to the reaction kinetics.
Such comprehensive modeling will be helpful in identifying
potential flooding conditions under steady-state as well as transient conditions, and in evaluating different purging and freezestart conditions. The insight gained through these models is expected to provide directions for improvement in material and
operating conditions and various operational protocols based
on microscale as well as macroscale/system level performance
evaluation.
CONCLUDING REMARKS
A comprehensive review of the research work published in the
past three years on the water management issues at microscale
as well as at macroscale/system level is presented. The article is
subdivided into the following major subcomponents: the membrane, catalyst layers, GDLs, and the gas channels. Following a
heat transfer engineering
critical review in each section, future research needs are identified. A brief review of modeling is also presented.
Many of the articles reviewed here deal with a specific aspect of water transport in one of the fuel cell components, with
sometimes very narrow focus. However, collectively, these articles represent a significant enhancement to our understanding
of various water management issues at microscale. Combining
this understanding with the knowledge base available in the field
of two-phase flow at microscale and macroscale is expected to
yield novel techniques to provide answers in developing effective water management strategies in PEM fuel cells.
ACKNOWLEDGMENTS
The work was partly conducted under a Department of Energy
(DOE) Grant DE-FG36-07GO17018. The author is thankful to
Dr. Zijie Lu, postdoctoral research fellow, and Charles Schillberg, BS/MS student at RIT, for their valuable suggestions and
discussions in the preparation of this article. The support by Enrica Manos in the Mechanical Engineering Department at RIT
in the preparation of the manuscript is also gratefully acknowledged.
NOMENCLATURE
ACL
CCL
CCM
GDL
MEA
MPL
PEM
PEMFC
SEM
anode catalyst layer
cathode catalyst layer
catalyst coated membrane
gas diffusion layer (used interchangeably for gas diffusion medium or GDM in this article)
membrane electrode assembly
microporous layer
proton exchange membrane
proton exchange membrane fuel cell
scanning electron microscope
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Satish Kandlikar is the Gleason Professor of
Mechanical Engineering at RIT. He received his
Ph.D. degree from the Indian Institute of Technology in Bombay in 1975, where he served on
the faculty before coming to RIT in 1980. He
is involved in flow boiling and advanced singlephase and two-phase heat exchangers incorporating smooth, rough, and enhanced microchannels. His current work focuses on water management issues working in PEM fuel cells. He
has published over 180 journal and conference papers. Some of his accomplishments include, fellow member of ASME, Eisenhart Outstanding Teaching Award at RIT, Trustees Scholarship Award at RIT, and the IBM Faculty
Award. Dr. Kandlikar is Associate Editor of Journal of Heat Transfer and Heat
in History, Editor of Heat Transfer Engineering; he is a member of the editorial board of a number of international journals. He is the founder of the
annual ASME International Conference on Nanochannels, Microchannels, and
Minichannels.
vol. 29 no. 7 2008