Air PEM. Load. Cathode

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1 Water Management E Kumbur and MM Mench, The Pennsylvania State University, University Park, PA, USA & 2009 Elsevier B.V. All rights reserved. Introduction uel cells hold great promise to meet the basic requirements of many future energy conversion systems. The factors driving the promotion of fuel cell technology arise from the converging needs for decreased pollution, economic stability, and global security. or decades, various fuel cell systems have been developed, with some reaching limited market penetration. or example, more than two hundred and forty 200-kWe phosphoric acid based fuel cell systems developed by ONSI orporation (now United Technologies orporation uel ells) for stationary power applications have been sold and are in service since 1992, with >95% service reliability. Other types of fuel cell systems including the alkaline electrolyte fuel cell, solid oxide fuel cell, and molten carbonate fuel cell have been, or continue to be, developed. However, ubiquitous integration into the global power production profile has not yet occurred for a variety of reasons beyond the scope of this article. In particular, the high cost of materials or manufacturing, low durability, and lack of hydrogen infrastructure represent major bottlenecks that must be overcome. Among the various fuel cell types, hydrogen-based proton-exchange membrane fuel cells (PEMs) are the most suited for many stationary power, portable power, and automotive applications because they offer low operating temperature (o100 1), high efficiency (>50%), rapid startup time, and suitable transient response characteristics. igure 1 shows a schematic of a typical PEM assembly. In a PEM, the electrolyte ( mm thick) is a flexible polymer membrane. The catalyst layer (L) electrodes (B10 20 mm thick) consist of nanosized (B2 5 mm diameter) catalyst particles that are generally based on the noble metal platinum. These catalysts are typically supported on larger carbon particles (B45 90 mm diameter). The fuel (typically hydrogen (H 2 )) and oxidizer (typically air) are distributed through separate channels and flow parallel to the electrode surface, reaching the reaction site by diffusive and convective transport through a porous hannel Land Bipolar plate L H 2 Air PEM e Load e Anode athode igure 1 Schematic of a typical proton-exchange membrane fuel cell (PEM) assembly and components. L, catalyst layer;, diffusion medium, PEM, polymer electrolyte membrane. Reproduced with permission from Mench MM (2008) uel ell Engines. New Jersey: John Wiley & Sons Inc. 828

2 uel ells Proton-Exchange Membrane uel ells Water Management 829 carbon diffusion medium (), which is also referred to as the gas diffusion layer (GDL) or porous transport layer. This critical component functions to deliver reactant to, and product away from, the electrodes, as well as provide high electrical conductivity with low contact resistance and appropriate heat transfer. At the anode, hydrogen is split into hydrogen ions and electrons by an electrochemical oxidation reaction. The semipermeable polymer membrane is conductive to H þ ions but not electrons; therefore, only H þ ions migrate to the cathode, whereas the electrons flow through the and channel structure. The electrons passing through the external circuit reunite with H þ ions and oxygen molecules at the cathode, undergoing an electrochemical reduction reaction. As a result, water vapor is generated at the cathode. The fuel cell generates waste heat as a result of ionic, electronic, kinetic, and mass transport losses. Although convenient in terms of pollution, the fact that water is produced and must be properly managed represents a major technical challenge. A complex relationship exists between the water content and the performance of a PEM. The water generated by reaction needs to be both removed and retained at the same time. The fluoropolymers and hydrocarbon membranes conventionally used for the electrolyte require moisture to be ionically conductive; thus, local moisture is critical to operation. However, the water vapor generated by the electrochemical reaction often condenses into liquid phase. The presence of excess liquid water in the, L, or flow channel can block available pathways for reactant flow, thereby hindering the transport of reactant to the electrochemically active sites. Real-time performance loss and operational instability resulting from liquid water accumulation are generically referred to as flooding, although flooding losses can result from local and discrete accumulations in the electrodes,, flow channels, or along interfaces between the and L as illustrated in igure 2. Although short-term performance loss is to be avoided, in the long term, excess liquid overhead, even if it is not responsible for immediate performance loss, can also result in reduced long-term performance through several mechanisms: 1) Electrolyte internal stresses : Locations within the fuel cell can have highly inhomogeneous water distributions and temperatures. High levels of internal hannel low hannel flooding Water low channel hannel low Diffusion media flooding Water Reactant flow L atalyst layer atalyst layer flooding L Water Electrochemical reaction sites igure 2 Illustration of possible locations of flooding in a proton-exchange membrane fuel cell (PEM) including catalyst layer (L), diffusion media (), and channel level flooding. Interfacial accumulation and reactant restriction is another mode of flooding-related performance loss not shown.

3 830 uel ells Proton-Exchange Membrane uel ells Water Management stresses can develop inside the membrane, because the electrolyte membrane swells significantly in the presence of water vapor and, even more, when in contact with liquid-phase water (a condition known as Schroeder s paradox). As a result, accelerated physicochemical degradation of the L and interfaces can result. 2) Ionic contamination : Ionic impurities from metals are most easily transported in liquid droplets and will readily absorb into the fuel cell electrolyte, because it is an ionic conductor. Ionic impurity in the membrane greatly reduces conductivity and water transport, thereby reducing performance, even with very minute quantities of impurities. Postmortem testing of membrane electrode assemblies (MEAs) has demonstrated the presence of a surprising array of impurities, such as calcium, iron oxides, copper, magnesium, and various other metals. As a result, most fuel cell systems are designed to avoid contact of the reactant flow stream with any metal connectors or couplings, and special plastics deemed compatible for fuel cell systems have been developed. 3) rozen condition damage : Residual water in the Ls at shutdown to a frozen state has been shown to cause irreversible damage to the L and L interface under certain conditions; therefore, adequate purge at shutdown is essential to remove residual water even if it is not responsible for flooding performance loss. Typical water overhead stored in the membrane, porous media, and flow channels in an operating PEM is around 5 20 mg cm 2, depending on flow field design, operating conditions, and materials. Although the adequate removal of the product water is essential, adequate water in the membrane electrolyte is also a necessity for achieving high performance, because membrane proton conductivity depends on hydration of the membrane. The paradox is that although the fuel cell is a net producer of water, water is typically carried into the fuel cell by humidification of the reactant gas streams to avoid local dry-out. Because of these conflicting requirements, the efficient operation of PEM requires a delicate balance between membrane hydration and the local avoidance of cathode flooding to achieve high ionic transport and low mass transport resistance. In order to identify the requirements of proper microfluidic management and understand the global water balance in polymer electrolyte membrane (PEM) fuel cells, we need to first understand the modes of local water transport in the components of PEM. It must be emphasized that flooding is a localized phenomenon. Any local loss of active catalyst sites increases the charge transfer burden on the remaining sites, thereby reducing overall performance. Water Transport in the Polymer Electrolyte Membrane To date, most modern solid electrolytes used in the PEMs are perfluorinated ionomers with a fixed side chain of sulfonic acid bonded to the inert, but chemically stable, polymer polytetrafluoroethylene (PTE) structure, as illustrated in igure 3. The development of hydrocarbon- and alkaline-based membranes is also under way, and these structures also need some water for sufficient conductivity. One widely used example of this type of membrane is Nafion s, manufactured by E. I. DuPont de Nemours and ompany. In terms of chemical composition, these membranes consist of two very different substructures: (1) a hydrophilic and ionically conductive phase due to the attached sulfonic acid groups and (2) a hydrophobic and relatively inert polymer backbone, which is not ionically conductive but provides chemical stability and durability. When the Nafion structure is hydrated, the hydrophilic sulfonic acid chains imbibe water and enable the motion of H þ ions. Depending on the hydration level, there are two modes of proton transport in the electrolyte: (1) vehicular or diffusion mechanism (occurs at low hydration level) and (2) Grotthuss or protonhopping mechanism (occurs at high hydration level). These modes are illustrated in igure 4. In the vehicular diffusion mode, the ionically conductive SO 3 chains are distributed as generally isolated clusters in the membrane, and proton transport mechanism from one cluster to another is dominated by the diffusion mechanism. In the Grotthuss mechanism, the ionically conductive SO 3 clusters share increased connectivity in a highly hydrated environment, and the proton can be transported through a more efficient proton-hopping mechanism. Water Uptake Water uptake (l) of the membrane is defined as the number of water molecules per sulfonic acid site: l ¼ H 2O SO 3 H The water uptake of a Nafion membrane at 30 1 in contact with a gas-phase flow is l ¼ 0:043 þ 17:18a 39:85a 2 þ 36:0a 3 for 0rar1 ½2Š a ¼ RH ¼ y vp P sat ðtþ where a is the water activity, i.e., the relative humidity (RH), y v is the mole fraction of vapor, P is the total pressure of the air vapor mixture, and P sat represents the ½1Š ½3Š

4 uel ells Proton-Exchange Membrane uel ells Water Management 831 H + O O S O Sulfonated side chain O PTE O Repeating O O O O O S O O S O O H + O H + igure 3 Schematic of fluoropolymer membranes with connected sulfonated side chains. PTE, polytetrafluoroethylene. Reproduced with permission from Mench MM (2008) uel ell Engines. New Jersey: John Wiley & Sons Inc. saturation pressure at a given temperature (T). Although the relationship given in eqn [2] was derived for 30 1, it has been applied to higher temperature conditions. or a fully humidified condition, the relationship in eqn [2] gives a maximum achievable water uptake value (l) of 14; however, this value actually decreases with increasing temperature, to a value around l ¼ 10 at When the membrane is equilibrated with liquid water, the water uptake of expanded Nafion is much higher, approaching a value of l ¼ 22. The sharp difference in water uptake characteristics of Nafion between water- and vaporequilibrated conditions is a result of Schroeder s paradox. Schroeder s paradox is critical, because the abrupt change in water content results in a similarly abrupt variance in ionic conductivity, membrane swelling, and other important transport parameters. The ionic conductivity of the membrane (s i ) can be defined in terms of water uptake as s i ðs cm 1 1 Þ¼exp T ð0: l 0:003 26Þ ½4Š where T is in kelvin. The correlation given in eqn [4] was derived based on conductivity measurement from 20 to 90 1 and a fully humidity range, and predicts a value of 10 S cm 1 proton conductivity for a well-hydrated PEM membrane. The sensitivity of ionic conductivity in response to a change in RH is shown in igure 5. Although operating at high RH is beneficial, at high temperature, the water vapor mole fraction becomes excessively large in the gas phase, which reduces reactant availability and performance. Water lux through the Polymer Electrolyte Membrane Water is transported across the membrane via four different modes, as illustrated in igure 6. Electro-osmotic drag (potential-driven flow) Electro-osmotic drag is the flux of water resulting from a polar attraction of the water molecules to the positively charged protons moving from the anode to the cathode through the electrolyte. When H þ ions migrate from the anode to the cathode, they tend to attract and drag water molecules along with them. The number of water molecules transported per hydrogen proton (H þ ) is called the electro-osmotic drag coefficient (x). onsidering the direction of proton flux in the membrane, the transport of water molecules via electro-osmotic drag always occurs

5 832 uel ells Proton-Exchange Membrane uel ells Water Management igure 4 Schematic of (a) Grotthuss and (b) vehicular water motion mechanisms. Reproduced with permission from Mench MM (2008) uel ell Engines. New Jersey: John Wiley & Sons Inc. from anode to cathode, and can be described as n w;e-o mol s 1 i ¼ x where i is the local current density, is araday s constant, and x is the electro-osmotic drag coefficient. ½5Š Although there is some discrepancy in the measured values of the drag coefficient among different groups and between membrane materials, in general, a value of x ¼ 1.0 for lo14 is appropriate. It has been shown that significantly higher values of x ¼ 2 5 result from a liquid-equilibrated membrane.

6 uel ells Proton-Exchange Membrane uel ells Water Management 833 σ i (Scm 1 ) Relative humidity igure 5 Specific conductivity (s i ) of Nafion as a function of environmental humidity at Diffusivity 10 6 (cm 2 s 1 ) igure 7 content. 353 K 303 K Membrane water content, λ (H 2 O/SO 3 H) Membrane water diffusivity as a function of water Anode PEM electrolyte Electro-osmotic Drag H 2 O diffusion athode H 2 O generation the urther Reading section) reported a corrected ickian diffusion coefficient for 1100 EW Nafion, which accounts for the effect of temperature and membrane swelling as a result of water uptake: D w ðcm 2 s 1 Þ¼3: l 1 þ exp 0:28l exp 2436 for 0olr3 TðKÞ ½7Š H + transport H 2 O vapor inlet/ humidified flow Temperature/ heat flux H 2 O vapor inlet/ humidified flow D w ðcm 2 s 1 Þ¼4: l 1 þ 161exp l exp 2436 for 0olr17 TðKÞ ½8Š H 2 O hydraulic permeability (liquid and gas phase) igure 6 Illustration of different modes of water transport inside the fuel cell. PEM, proton-exchange membrane fuel cell. Diffusion in Nafion (concentration-driven flow) The diffusion of water through the polymer membrane occurs as a result of a concentration gradient across the membrane, and can be modeled as a single-component diffusion mechanism according to ick s law: n w; diff ðmol s 1 Þ¼ D w D c a Dx where D w is the diffusion coefficient (a function of local water content of the membrane (l)) and D c a /Dx is the water concentration gradient across the membrane of thickness Dx. The water diffusion in the ionomer phase, D w, has been measured as a function of ionomer water content by several groups. On the basis of the measured water self-diffusion data, Montupally and coworkers (see ½6Š This nonlinear relationship is plotted in igure 7 for two different temperatures. It should be noted that the diffusion coefficient reported by different groups actually varies over several orders of magnitude. The reason for this is not yet resolved, although it is likely due to the differences in the measurement approach. Some recent studies (see Benziger and coworkers in the urther Reading section) suggest that the discrepancy is a result of relatively slow membrane interfacial uptake of water. Water concentration is usually higher at the cathode because of water generation in the L. The transport of water from cathode to anode is termed back-diffusion, and it can play an important role in maintaining a uniform water distribution across the membrane during operation. In an operating PEM, electro-osmotic drag can result in anode dry-out, whereas the cathode tends to get flooded as a result of the cumulative effect of electro-osmotic drag and water generation. In that sense, back-diffusion of water from cathode to anode serves to compensate the water loss of the anode, and flatten the water activity profile across the membrane, especially for thinner membranes.

7 834 uel ells Proton-Exchange Membrane uel ells Water Management Hydraulic permeability (pressure-driven flow) Hydraulic permeation of water through the membrane can occur as a result of a pressure difference between the anode and the cathode, and can be described using Darcy s law: n w-hydraulicðmol s 1 Þ¼ k m DP c a Dx where k is the effective permeability of the membrane, m is the liquid viscosity, Dx is the membrane thickness, and DP c a is the pressure difference between the cathode and anode, which can be a gas-phase difference or a liquidto-liquid (capillary) pressure difference. The gas-phase difference is typically small because the anode and cathode pressures are usually similar, so this effect can be ignored. In the liquid phase, however, a capillary pressure difference between the anode and the cathode can result in a net flux of water across the electrodes. Thermo-osmosis (temperature-driven flow) A fourth mode of transport in the membrane is thermoosmosis, which is a temperature-driven flow through the membrane. This mode of transport has not commonly been included in the analysis of normal operation, because this effect is obscured by the net diffusive and electro-osmotic drag transfer, although it is a known phenomenon in the polymer community. Under startup or shutdown, the net water flux from this mode can be significant. The thermo-osmotic diffusion coefficient has been directly measured for a reinforced membrane in the author s laboratory, and is shown in igure 8. The coefficient has been determined to be a function of ½9Š membrane type, preconditioning, and heat treatment, but is always in the direction of cold to hot side for fuel cell membranes. Net Water lux oefficient The net water flux through the membrane can be described as a combination of various modes of transport explained earlier, and a net drag coefficient, a d, can be defined to account for the cumulative effect of all transport modes: n w-net a c ¼ n w;e-o þ n w-hydraulic þ n w;diff þ n w;temp n w-net a c ¼ a d ia ½10Š ½11Š In an ideal situation for water management, a d would be uniformly 0.5 (i.e., toward anode) along the electrode. In this case, the water generated by the reaction is exactly balanced by directing half of the generated water to the anode side. In practice, for the thin (B15 25 mm) membranes that are used in automotive applications, the high back-diffusion to the anode nearly compensates for the electro-osmotic drag overall, yielding zero or slightly negative value of net drag. On the contrary, for thicker membranes, which are generally preferred in stationary power application, the overall net drag coefficient can be slightly positive. The assumption of uniform net drag within the fuel cell is rarely justified. igure 9 shows the measured net drag coefficient distribution along the flow channel of a PEM operating at either relatively dry anode or dry cathode inlet conditions, with a thin 18-mm electrolyte 7 Thermo-osmotic diffusivity, Log 10 ( D ) (kg m 1 Ks) y = R ² = Reinforced membrane Inverse temperature (K 1 ) igure 8 Measured thermo-osmosis diffusion coefficient for 18-mm-thick dry reinforced membrane.

8 uel ells Proton-Exchange Membrane uel ells Water Management Net drag coefficient (α d ) Net drag for the full cell α d = ractional distance from the channel inlet igure 9 Measured local and net effective drag coefficients along the gas flow channel of a proton-exchange membrane fuel cell (PEM) operating at 0.7 V, with a dry anode inlet and cathode inlet at 50% relative humidity (RH) at The net drag is negative, indicating a net drag of water toward the dry anode. Toward the exit, however, the local effective drag becomes positive. membrane. Even though the electrolyte is very thin, the net drag coefficient is not near zero as a result of the initial imbalance between anode and cathode humidity. or this dry anode inlet case, the overall net drag coefficient is 0.27, representing a net back-diffusion flow toward the anode. However, after the initial 40% of the fuel cell flow channel, the net flux is slightly positive, toward the cathode, because the anode has become humidified, reducing the diffusion concentration gradient. Water Transport in Porous omponents of Proton-exchange Membrane uel ells Water transport mechanism in the porous components of PEMs, i.e., in the L, microporous layer (MPL), and, is a critical topic of interest. A description of the porous components is summarized in this section. atalyst Layer The L is the heart of the PEM, producing power and water, as well as much of the waste heat, especially at the cathode. atalyst layers in PEMs consist of a porous, three-dimensional structure with a thickness of 5 30 mm (see igure 10). Owing to its vital role in facile transport of ions, electrons, reactants, and products, the L has a highly porous structure (typical porosity ). The L typically contains a considerable fraction of ionomer (up to B30% by weight (wt)) to form the ionic pathways for effective proton transport to or from the main PEM. Some fraction of the pores may also contain hydrophobic PTE to promote local water removal and enhance the reactant gas transport. Because of the existence of ionomer, catalyst, and PTE coating, the L consists of a igure 10 Transmission electron micrograph of a 40 wt% platinum/chromium catalyst on carbon support. Reproduced with permission from Thompsett D (2003) Pt alloys as oxygen reduction catalysts. In: Vielstich W, Lamm A, and Gasteiger HA (eds.) Handbook of uel ells undamentals, Technology and Applications, vol. 3, ch. 37, pp New Jersey: John Wiley & Sons Inc. mixed pore network, having both hydrophobic and hydrophilic surfaces and mixed ionic/electronic conductivity. Diffusion Media The provides pathways for gas transport to the L, as well as enables transport of product water, electrons, and excess heat of the reaction from the L. The is typically constructed from electrically conductive macroporous substrates with varying degrees of mixed wettability, such as a nonwoven carbon paper, felt, or a woven carbon cloth. All types of have complex internal structures with pore size ranging from a few microns to tens of microns. The porosities vary between 40 and 90%, and thicknesses between 90 and 500 mm. igure 11 shows the scanning electron microscopy images of nonwoven carbon paper and a woven carbon cloth. loth s are as flexible as any textile, whereas paper s are fairly brittle because of the presence of thermoset polymer resin, and can easily be broken under strain. elt s are in between paper and cloth s in terms of flexibility. These two types of have different

9 836 uel ells Proton-Exchange Membrane uel ells Water Management SE 22-Dec-04 L01 WD 8.6 mm 20.0 kv x µm SE 22-Dec-04 P01 WD 8.7 mm 20.6 kv x µm (a) (b) igure 11 Scanning electron micrograph of nonwoven fiber carbon paper diffusion medium structure: (a) nonwoven paper and (b) woven carbon cloth. Reproduced with permission from Mench MM (2008) uel ell Engines. New Jersey: John Wiley & Sons Inc. characteristics of spatial uniformity and degree of anisotropy. Because of the hydrophilic nature of these packed carbon fibers, the carbon fiber substrates are typically treated with a nonuniform coating of PTE to achieve the desired wettability for effective water removal. As a result, the internal pore structure exhibits mixed wettability characteristics, with different fractions of hydrophobic and hydrophilic (B20 40%) and partially hydrophobic/hydrophilic (mixed) pores, depending on the PTE additive amount. Microporous Layer A thin (5 20 mm thick), highly dense, and almost completely hydrophobic MPL with pore sizes ranging from 100 to 500 nm is commonly introduced between the and L for water management and electrical conductivity enhancement. There are two basic types of MPLs: slurry based and polymer sheet type. The slurry-based MPLs are generally coated directly to the catalyst side of the surface, and consist of carbon particles (5 20% of weight), polymeric binder, and PTE (5 20% of weight). The other type of MPL is a porous polymer sheet bonded to the outer surface of the L. The MPL was originally designed to provide improved electrical conductivity between the L and, but has shown an improved water management during PEM operation. In addition, the MPL also functions to protect the L and membrane from carbon fiber intrusion damage from the. Modes of water transport in proton-exchange membrane fuel cell porous media To date, the science of multiphase flow through thin-film mixed wettability porous media such as the L and is not yet well developed; therefore, much of the present level of understanding is based on the application of porous media theory from civil and petroleum engineering studies of flow of water or oil through packed soil beds. The reader is referred to the urther Reading section for this background material. Even though the traditional approaches adopted from other disciplines provide useful starting point, there are key differences between the transport characteristics of common soils and the fuel cell porous media that must be considered: 1. The transport length scale in fuel cell media is much smaller. Therefore, the porous media in fuel cells (i.e.,, MPL, and L) has a very large surface area to volume ratio, indicating the importance of interfacial effects, which are not treated in the bulk flow theory adopted from soil science. 2. Soil science literature has very limited treatment of vaporization/condensation, which has a major impact on low-temperature fuel cells. 3. Most soil science studies have been conducted with hydrophilic media, whereas the PEM medium typically has a highly heterogeneous surface energy distribution. 4. Most soil science modeling/work is performed in saturated domain (i.e., the pores of the soil are completely filled with fluid), whereas fuel cell porous materials are nearly always only partially saturated. 5. The models and physics attempt to account for bulk flow, while generally ignoring the orphan droplets trapped in the isolated pores, which are likely to be common in fuel cell media, as a result of condensation and mixed wettability. When fuel cell operations are considered, the liquid water transport within the pores of the fuel cell porous media can be driven by several forces:

10 uel ells Proton-Exchange Membrane uel ells Water Management apillary action : This is a result of a pressure difference between the phases, and dominates for small pores. 2. Gravitational forces : Gravitational effects are typically very small compared to the capillary forces because of the small pore size, although gravity affects the flow of liquid in the channels and manifolds. 3. onvective forces : The importance of this effect depends on the flow field design. An interdigitated flow field is designed to force flow through the, which can assist in removing stored water. 4. Evaporation/condensation : These processes have a major effect because of the low operating temperature of PEMs. 5. Interfacial effects : The interfacial surface area and morphology within the different layers of fuel cell porous media have a substantial effect on the accumulation and storage of liquid water. Of these considerations, the interfacial transport and phase-change-related flow are the least studied, and considerable uncertainty still exists. apillary Pressure-Driven low The most important relationship that must be established for an accurate prediction of the liquid gas capillary transport is the relationship between the capillary pressure (P ) and the liquid saturation (s l ). In the cathode, gas-phase pressure generally remains constant because of the low viscosity; therefore, P ¼ P l P g where - DP E - DP l ½12Š Several empirical and semiempirical expressions are available in soil science literature. As many porous media share similar characteristic behavior, for PEM porous materials, a generic Leverett approach from soil science has been commonly used to describe the capillary transport behavior of the fuel cell porous media. This semiempirical correlation relates the capillary pressure and saturation data for clean unconsolidated sands of various permeability and porosity by means of defining a dimensionless capillary pressure function: P ¼ g cos y e k 1=2JðsÞ ½13Š where k, e, and y are permeability, porosity of the porous media, and a representative contact angle, respectively. J(s) represents the Leverett J-function for scaling drainage capillary pressure curves. Jðs 1 Þ ¼ 8 >< >: 1:417ð1 s 1 Þ 2:120ð1 s 1 Þ 2 þ1:263ð1 s 1 Þ 3 if yo901 ) hydrophilic 1:417s 1 2:120s 2 1 þ 1:263s3 l if y > 901 ) hydrophilic ½14Š Although this Leverett approach serves as a useful starting point, the applicability of the generic Leverett approach function to the highly anisotropic thin-film porous materials such as in PEMs has been challenged. In particular, the definitions of wetting and nonwetting phases used to determine capillary pressure and surface tension angle are taken as a statistical average over the entire medium, obscuring local effects, which differ from the whole. Because the droplet/bubble sizes are on the same order of magnitude as the thickness, the complex bimodal (hydrophobic and hydrophilic) pore size distribution, and droplet eruption from the surface has been observed to be a highly localized discrete event, a volume-averaged approach such as this may not be appropriate at all. Several recent studies have presented modified versions of this relationship (see the urther Reading section). One such approach derived from the direct capillary pressure saturation measurements of different paper-based s tailored with PTE content ranging from 0% to 20% of weight at different operating conditions is shown as rffiffiffiffi P ¼ ð293=tþ 6 gðtþ 2 0:4 e c fflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflffl} k Temperature effect fflfflfflffl{zfflfflfflffl} ompression effect Kðs nw Þ fflffl{zfflffl} Mixed wettability ½15Š where, e c, k, g, and T represent the compression pressure, compressed porosity, absolute permeability, surface tension, and temperature, respectively; K(s nw ) represents a modified function: K ðs nw Þ¼ð%wtÞI0:0469 0:00152 ð%wtþ 0:0406 s 2 nw þ 0:143 s 3 nw m þ 0:0561 lns nw for 0os nw r0:50 ½16Š where %wt and s nw represent PTE weight percentage of the and nonwetting liquid saturation, respectively. A similar approach for the L has not yet been developed. It should be noted that this relationship covers capillary transport in the hydrophobic regions of the only, whereas liquid water will tend to first accumulate in hydrophilic regions. This residual water in hydrophilic pores can be significant (up to 20% or more), and is removed only by evaporative or convective forces. The MPL is almost completely hydrophobic, and capillary pressures are so high because of the small pore size in these regions that the MPL in any practical situation will have almost no saturation of water. This useful feature provides an open pathway for gas transport across the L interface, which is one of the main performance-enhancing features of the MPL. Modeling approaches to flow in the and the L have included Leverett-based average approaches modeling capillary flow in the hydrophobic pores only, with hydrophilic

11 838 uel ells Proton-Exchange Membrane uel ells Water Management regions represented by an arbitrarily chosen irreducible saturation of around 10%. Other approaches that attempt to realistically follow the fluid interface involve percolation-based pore level models, again borrowed from soil and oil recovery science. A powerful new approach that uses the lattice Boltzmann method to simulate the multiphase flow is also under development. The reader is referred to the urther Reading section for more details. To date, however, modeling approaches have not yet fully resolved experimental observations and further work is needed. Phase-hange-Driven low A largely ignored but important mode of transport in the porous media is that related to phase change. Although assumptions of thermodynamic equilibrium are useful for modeling purposes, in reality, small local changes in surface energy and temperature result in the potential for significant amounts of water transport through phasechange-related mechanisms. As the provide the dominating thermal resistance in the cell, in general, the largest temperature gradient exists in this material, which can amount to >5 1 during operation. As water is produced in the L, it will condense at locations with suitable surface conditions and temperatures, potentially in the L or in the. Surface impurities, roughness, and hydrophobicity affect the nucleation site condensation rate and temperature. Another mode of transport that is beginning to be understood is the so-called heat-pipe effect. Because the channels are a cooler location than the L, condensation of water on the channel walls can cause local desaturation of the gas phase, drawing more water out of the liquid phase, and thus forming a continuous transport pathway from the liquid-saturated regions to the condensation plane, as illustrated in igure 12. This effect is particularly relevant during shutdown and startup, where significant plate-to-plate variation in temperature can exist. During cooling, even small gradients have been shown to draw a very significant amount of water from the porous media into the colder channel locations. This effect should be delineated from membrane thermo-osmosis, which has been shown to occur from cold to warm locations in sulfonated fluoropolymer membranes. low is exclusively from the hot to cold locations with the heat-pipe effect. igure 13 shows a side-on view neutron imaging picture of flow in a fuel cell initially with liquid on the left side of the image, and a hydrophobic with MPL. When there is no temperature gradient, there is no flow from the liquid- to gas-phase sides. However, when a modest temperature gradient is introduced, as shown, liquid flow moves rather quickly to the colder location. This effect has been quantified for a typical PEM membrane and a 5% wet-proofed SGL 10BB with MPL material set, and typically dominates the modest thermo-osmotic flow in the opposite direction. The rate of transport by this mode is a function of absolute temperature, temperature gradient, and material set, and has recently been shown by the author s laboratory to be well fit using an Arrhenius expression to relate transport rate to the temperature gradient and average temperature. Interfacial Effects Perhaps the least understood fundamental issue regarding water transport is the role of the various interfaces. As discussed, the normal and perpendicular land interfaces with the have been shown to play a key role in water removal from under the lands of the. The membrane L interface is a complex three-dimensional structure that changes with water content as a result of ionomer swelling. Most models deal with the L only as an infinitely thin layer, and attempts at resolving the L generally assume homogeneous properties with empirical relationships. The L interface is also of particular Water transport due to capillary action Evaporation Vapor diffusion ondensation igure 12 Illustration of heat-pipe effect between diffusion media (s) and flow channel.

12 uel ells Proton-Exchange Membrane uel ells Water Management 839 relevance because there is little known and experimental visualization tools have yet to achieve the resolution required to investigate these ultrathin regions. or an unbonded, delamination as a result of freeze thaw or mechanical stresses has been shown to occur in extreme circumstances (see igure 14), which can lead to severe performance degradation. The larger relative gap size Hot (70 ) side initially liquid filled older (60 ) side initially dry created serves as a local pooling location for water because of the low relative capillary pressure, which can partially flood the electrode. Additionally, the current redistribution required by the lack of physical contact will result in increased ohmic losses. Because of this, as well as manufacturing assembly concerns, it is common to hotpress the onto the L to create an intimate bond. In this case, the interfacial behavior is even less understood. or a bonded MEA, ionomer and catalyst are squeezed into the gaps along the MPL L interface during the hot press, creating a complex structure. During operation, the morphology at this interface will change as a result of swelling and dry-out, and may alter the transport at this interface in ways not yet understood. 1 min: old side is starting to fill. 4 min: old side is completely filled. Modern View of Transport and looding in the Porous Media As discussed in Introduction, performance reduction from flooding can be a result of (1) channel, (2), (3) L, or (4) interfacial water accumulation at discrete locations within the fuel cell. The exact nature of the liquid water structure in the and L is not precisely known, but it is generally believed from capillary theory that the liquid water flows through large hydrophobic and hydrophilic pores, and the gas phase flows through small hydrophobic pores. However, evaporation and condensation are also important, because a unified understanding based on capillary pressure arguments alone has been shown to be insufficient. igure 13 Side-view image of water transport by heat-pipe effect taken with neutron imaging. In 4 min, the initially gas-filled colder flow channels are completely filled by heat-pipe flow from the liquid-filled warmer side channels. Description of hannel looding hannel level flooding is commonly observed under low stoichiometry or low current conditions, when the stack temperature is relatively cool and the channel flow rates are insufficient to clear slug accumulations. onsidering that most fuel cell stacks contain more than 100 plates mounted in parallel hydraulically (generally in series electrically), any slight perturbation in the pressure drop igure 14 Scanning electron micrograph of interfacial delamination caused by freeze thaw cycling in a proton-exchange membrane fuel cell (PEM).

13 840 uel ells Proton-Exchange Membrane uel ells Water Management across a single cell can result in a drastic reduction of flow from the manifold, further exacerbating the flooding problem and eventually resulting in parasitic and potentially damaging voltage reversal in the flooded cell from reactant starvation. In the channels and manifolds, gravity can assist in draining the excess liquid, depending on cell orientation. In general, any gravity-unfavorable design features of the flow field are to be avoided (see igure 15), for example, any wells that would tend to accumulate water. Stack orientation and flow field design are generally arranged so that the natural progression of droplets in the channels is at worst neutral and at best assisted by gravitational forces. In neutral or unfavorable orientations, excess liquid water removal is dominated by drag forces. As the becomes saturated with liquid water, droplets form at discrete locations along the surface and increase in size, eventually turning into water slugs in the flow channel. On the anode side, channel blockage can cause voltage reduction and fuel starvation for the L, leading to the oxidation of carbon support and accelerated degradation. Surface conditions of the channel can also Pooling location Neutral zone avorable zone (gravity effect) Neutral zone igure 15 Illustration of favorable, neutral, and unfavorable channel configurations based on gravity and local pooling. influence the water management. Although it would seem logical that a hydrophobic surface would be favorable for water removal, testing has revealed that flooding is worse and performance less stable with hydrophobic channel walls. The increased flooding is believed to be a result of restraining capillary force projected from the land channel interface into the that prevents regular drainage into the channel. The increased instability is a result of droplet accumulation that causes sporadic pressure variation, rather than the smoothly varying pressure change resulting from a liquid film buildup along a hydrophilic channel wall, as shown in igure 16. Description of Diffusion Media looding Diffusion media flooding can occur as a result of excess water accumulation by condensation or through capillary introduction from the L as a result of insufficient channel gas flow rate. Diffusion media flooding occurs most commonly under high current conditions. Pure capillary flow will follow the path of least resistance, first moving to and filling connected hydrophilic pores. Thus, the PTE additive in the (typically 5% or more by wt) is essential to avoid flooding. At the L where water is generated, heat is also generated, and the resulting flux of water can condense either within the L or within the MPL. By condensation, water can be introduced into the hydrophobic areas of the and MPL without first filling all hydrophilic pores. Neutron imaging has revealed that liquid accumulation under lands is common, which tend to be much colder than channel sections because of enhanced heat transfer. onnected liquid in hydrophobic pores will build up a positive capillary pressure favoring expulsion to lower low channel low channel Hydrophilic channel walls Hydrophobic channel walls igure 16 medium. Illustration of water droplet distribution inside flow channels having different wettability characteristics., diffusion

14 uel ells Proton-Exchange Membrane uel ells Water Management 841 pressure areas such as hydrophilic sections or the flow channel. Because the MPL is much more hydrophobic than any other component, water is forced toward the channels from these locations. hannel-to-land ratio and land channel interfacial properties have also been shown to be important. The channel-to-land ratio is critical because the heat transfer profile between the and the channel or the lands is very different. onvection into the channels is significantly less efficient than direct conduction into the lands, so locally cool regions that tend to accumulate liquid water typically exist under the lands. As the water accumulates under the land, it is pushed by capillary forces laterally under the channels, where it erupts into the channel along the interface between the land and the channel. There is some discrepancy in the literature between the particular distribution and mode of motion and rejection within the. One conceptual model assumes that the liquid water in the saturates in a tree-like structure, with large connected channels soaked with water and smaller branches emanating from the main structures. More recently, an alternate view that explains the experimentally observed presence of droplets that erupt from the and are removed, or erupt and recede on the surface, assumes that the liquid water in the follows a fingering pathway, where drainage of one finger results in the recession of other fingers. Recent experimental observations more strongly support the eruptive model. Assembly compression and operating temperature also play a major role in the flooding. or instance, the placed in a fuel cell stack assembly is exposed to nonuniform compression, which in turn can generate strong local stresses that can alter the morphological structure and consequently the multiphase transport characteristics of the. It has been established that for a given thin-film fuel cell, any compression is followed by a decrease in porosity and an increase in tortuosity, and some studies have suggested that permanent damage to stiff papers results in a local decrease in effective hydrophobicity, which results in the experimentally observed behavior of sporadic liquid water eruption along the channel land interface. In addition, the discontinuity of the surface contact area at the flow channel interface creates inhomogeneous compression distributions, yielding substantial changes in local physical properties of the. The portion of the in contact with the landings is subjected to higher compression, whereas the portion under the flow channel experiences less compression and tends to intrude into flow channels, thus resulting in increased reactant flow field pressure drop. The axial variation of the compression in the cell assembly yields discrete regions that have different capillary transport characteristics. Description of atalyst Layer looding atalyst layer flooding can occur as a result of liquid condensation and pore filling or localized film formation, resulting in a significant decrease in the diffusion of reactant gases to the catalyst sites. Two common approaches to reduce the catalyst flooding are (1) to introduce PTE additive to the L and (2) to use an MPL. ompared to the extensive efforts to understand the role of s and flow channels, few fundamental studies have been performed to analyze the flooding mechanisms in the cathode L of a PEM. In most computational models, the L is treated as an infinitesimally thin interface, even though it acts as a vital component for the conversion of liquid water to vapor, regulating the water flux across the entire porous electrode of a PEM. Some studies have shown that the location of liquid flooding between the L and can be controlled by PTE content. A more hydrophilic electrode compared to the results in predominantly L flooding. Description of Interfacial looding At equilibrium, a pressure balance in the gas and liquid phases across interfaces must exist. Therefore, for a perfect interface, we can predict the water distribution and flooding locations depending on the hydrophobicity of the mating surfaces and the pore size distribution. igure 17 shows the liquid water saturation distribution in the porous media assuming an isothermal condition with no phase change and hydrophobic surfaces at steady state. In steady state, there is a flow of condensed water from the catalyst to the channel and out of the fuel cell as ia 2 0 S W S MPL S GDL ia 2 at SS L MPL GDL hannel igure 17 Illustration of the typical water distribution in the fuel cell porous media under steady state (SS), after sufficient time to allow equilibrium in the porous media between stored and flowing liquid. The discontinuities in the saturation level are a result of the changing pore size and hydrophobicity between the different layers. Hydrophobic layers are shown. L, catalyst layer; GDL, gas diffusion layer; MPL, microporous layer. Reproduced with permission from Mench MM (2008) uel ell Engines. New Jersey: John Wiley & Sons Inc.

15 842 uel ells Proton-Exchange Membrane uel ells Water Management (a) Top view (b) Side view igure 18 Scanning electron micrographs of proton-exchange membrane fuel cell (PEM) catalyst layer (L) with relatively largescale macro cracking. slugs. Discontinuities in the liquid saturation as a result of pore size variations result in a saturation jump for the same capillary pressure. At each interface, the liquidphase capillary pressure is balanced (in steady state). Although this approach is appropriate for perfectly mated surfaces, in reality, there is a distribution of pores in the and MPLs, and the L or MPL can have significantly large cracks and gaps between interfaces in the catalyst surface, as shown in igure 18. Thus, it is unlikely that such abrupt discontinuities in saturation, as shown in igure 17, really exist. atalyst layer surface cracking is typically present from manufacture and is a result of the presence of volatile compounds in the catalyst slurry or the manufacturing process. rom a multiphase flow perspective, this situation is very different from a continuous homogeneous phase, as commonly modeled, and some larger gaps and cracks may dominate flow physics in these regions. These cracks and gaps are orders of magnitude larger (they can be up to B10 mm wide) than the normal pore size in the L. Thus, these cracks are regions of reduced capillary pressure that promote local liquid pooling. In terms of gas-phase transport, these macro cracks may enhance reactant transport to catalyst regions by reducing flow resistance. These cracks increase the effective L porosity, enabling high reactant species flux to the catalyst surface. In PEMs, the interface between the channel, land, and, and the interface between the and L can also affect transport of liquid. If there is a gap, this can serve as a pooling location for water. Neutron imaging has also been used to confirm the important role of the land interface in storing the liquid water at equilibrium. or fuel cells with otherwise identical operating conditions, the greater the number of hydrophilic land interfaces, the lesser the liquid water content in the. On the contrary, hydrophobic land surfaces have been shown to retain water under the lands and promote flooding by restricting drainage from the, as discussed. Overall View of looding igure 19 is an illustration of the typical water distribution in the fuel cell porous media under the lands and channels. As the coldest location during operation is generally under the lands, water vapor tends to condense in these cold spots. As the saturation increases, water pushes out laterally from under the lands and either erupts along the channel land interface or forms connections between the lands under the channels, resulting in flooding and performance degradation. The removal of water from under the lands into the channels is important to avoid flooding, because lateral connectivity between the water under adjacent lands in the can induce severe performance loss through reactant blockage. Thus, larger channel-to-land width ratios provide enhanced resistance to flooding. Besides under the lands, there are other locations in a fuel cell where liquid water tends to accumulate. These locations have been identified primarily using neutron imaging, which enables a direct nonintrusive quantification of the liquid water content in the operating fuel cell and is used by several research institutions for this purpose. Liquid accumulation also commonly occurs around channel switchbacks, as shown in igure 20. This is a result of flow recirculation, stagnation, and pressure drop at locations of sudden momentum reversal. Additionally, the local flow separation near the corner accelerates the core flow, promoting annular flow of liquid water. Interestingly, even in very dry operating conditions, accumulation of liquid water under the lands has been observed, as shown in igure 20, implying that effective

16 uel ells Proton-Exchange Membrane uel ells Water Management 843 Land Land igure 19 Illustration of the typical liquid water accumulation behavior under the lands and channels in a proton-exchange membrane fuel cell (PEM)., diffusion medium. change and temperature gradients. Under current, the polarization losses will generate a majority of the heat dissipated by the fuel cell. As most of the entropy change and activation polarization are generated at the cathode, this is typically the hottest location in the fuel cell, up to 5 1 or more under high current depending on the thermal properties of the. At the cathode L, vaporphase water will be transported into the electrolyte and back to the anode or outward to the cathode flow channel. As the temperature cools down, a saturation plane will develop in a location preferably where liquid water condensation occurs. The location of condensation will have a tremendous effect on the flooding. or the mixed wettability L, the condensation would tend to take place in the predominantly hydrophilic pores first, flooding them, while the hydrophobic pores remain mostly liquid free. If the condensation plane is beyond the L and in the microporous or diffusion layer, the highly hydrophobic nature of the MPL will prevent backflow into the L. igure 20 Neutron radiograph showing liquid water accumulation along the corners of a 1801 switchback in an operating proton-exchange membrane fuel cell (PEM). Water is often observed to preferentially accumulate at switchback locations and along the channel walls. conduction heat transfer occurs through the lands, and access to the channel is blocked. While isothermal conditions may be true at low current density, higher current densities include phase Overall Role of Materials in Water Management and looding On the basis of the current understanding of the gas- and liquid-phase transport in the fuel cell, a unified view of the role of the various porous media can be constructed, and is shown in igure 21. Because of the lack of direct experimental observation of minute, small MPLs and Ls, there exist several different theories to resolve the

17 844 uel ells Proton-Exchange Membrane uel ells Water Management MPL Membrane MPL H 2 in apillary flow from MPL Liquid H 2 O out Restriction of H 2 O vapor out O 2 in Anode athode L L Potential condensation plane igure 21 Overall schematic showing the roles of the different porous media in a proton-exchange membrane fuel cell (PEM). L, catalyst layer;, diffusion medium; MPL, microporous layer. observed influences of the material properties on fuel cell performance. Microporous Layer or the same interfacial liquid pressure and hydrophobicity, the media with the smallest hydrophobic pores (L) will have the lowest liquid saturation. If there were similar levels of hydrophobicity in each layer, the capillary pressure gradient will drive the liquid flow from the L through the MPL and into the flow channel. Acknowledging the relatively smaller pores and more hydrophobic pore structure of the MPL, the MPL can act as a barrier for liquid flow unless the breakthrough pressure is reached. Typically, MPLs have a breakthrough pressure around 5 10 kpa. Some studies have shown that the MPL can act to force liquid water toward the anode, preventing dry-out. This can happen once the L is completely flooded and cannot store more water to overcome the breakthrough pressure of the cathode MPL. athode-side Microporous Layer An MPL on the cathode side has been experimentally determined to enhance performance under high current density and high humidity conditions, where L and flooding is prevalent. As the performance in this situation is generally limited by the flooding and not the gas-phase oxygen transport, the additional diffusion resistance of the MPL does not significantly reduce performance. Several experiments have demonstrated that the coated with MPL is observed to have a more uniform water distribution in the MEA. Existence of such a fine layer is found to prevent drying out of the membrane and reduce the flooding of the L. The wetting characteristics of the MPL are found to cause a discontinuity in the liquid saturation at the MPL interface, yielding a reduction in the amount of liquid water in the cathode by directing the flow to the anode side. It is believed that the MPL serves as a highly hydrophobic boundary to prevent water accumulation, forcing water back to the anode. This back-diffusion of water is observed to be improved by increasing MPL thickness, rendering the MPL more hydrophobic, and decreasing the pore size and bulk porosity of the MPL. The backflow of water would occur primarily through the hydrophilic pore network in the L, because complete pore saturation in the L would result in nearly total performance loss, and high saturation in the hydrophobic pores of the L would likely overcome the breakthrough pressure of the MPL, resulting in sporadic liquid slug emission, a phenomenon that has been observed to occur experimentally. However, with no MPL