Applied Thermal Engineering 29 (2009) 1276–1280

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Thermal management issues in a PEMFC stack – A brief review of current status Satish G. Kandlikar *, Zijie Lu Department of Mechanical Engineering, Rochester Institute of Technology, 76 Lomb Memorial Drive, Rochester, NY 14623, USA

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Article history: Received 24 January 2008 Accepted 13 May 2008 Available online 21 May 2008 Keywords: PEM fuel cell Thermal management Review

a b s t r a c t Understanding the thermal effects is critical in optimizing the performance and durability of proton exchange membrane fuel cells (PEMFCs). A PEMFC produces a similar amount of waste heat to its electric power output and tolerates only a small deviation in temperature from its design point. The balance between the heat production and its removal determines the operating temperature of a PEMFC. These stringent thermal requirements present a significant heat transfer challenge. In this work, the fundamental heat transfer mechanisms at PEMFC component level (including polymer electrolyte, catalyst layers, gas diffusion media and bipolar plates) are briefly reviewed. The current status of PEMFC cooling technology is also reviewed and research needs are identified. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction With the world concern about the environmental pollution and fossil fuel depletion, alternative clean energy solutions are in urgent demand. The hydrogen fuel cell, primarily the proton exchange membrane fuel cell (PEMFC), is a promising energy conversion system for future automobiles and stationary applications. The reaction in a PEMFC is chemically simple, with hydrogen molecules splitting into hydrogen ions and electrons on the anode while protons re-combine with oxygen and electrons into water and release heat on the cathode. However, a fuel cell can be very complicated and delicate mechanically due to the specific requirements of high power output (fast reaction and dynamics), longevity and economical effectiveness. Fig. 1 shows a schematic of a PEMFC together with the electrode reactions. Generally, a proton exchange membrane (PEM) is used as a proton conductor in a PEMFC, a catalyst layer containing platinum and/or platinum alloy is used to catalyze the electrode reactions, a gas diffusion medium, often comprising a microporous layer (MPL) and a carbon fiber based gas diffusion layer (GDL), is used to effectively transport reactant gases and electrons as well as remove product water and heat, and finally a flow field plate is needed to uniformly distribute the reactant gas. For a fuel cell stack, some additional components such as cooling plates are necessary. Significant technical challenges exist for PEMFC technology before it can be commercialized. Among them, the proper thermal management has been recognized as one of the most critical technical issues that must be resolved [1,2]. A PEM fuel cell produces an amount of waste heat similar to its electric power output, thus limiting its energy efficiency to about 50% [1]. This means that for an * Corresponding author. E-mail addresses: [email protected] (S.G. Kandlikar), [email protected] (Z. Lu). 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.05.009

automotive fuel cell engine of 100 kW the heat dissipation rate is also 100 kW. Moreover, PEM fuel cells tolerate only a small temperature variation. Current PEM fuel cells operate in the temperature range of 60–80 °C. This range is dictated by the material properties of the PEM, most commonly Nafion (a trademark of E.I. DuPont de Nemours, Wilmington, DE). The requirement of good hydration of Nafion (in order to have high proton conductivity) limits the maximum fuel cell operating temperature to about 80 °C [3]. On the other hand, a cell temperature below 60 °C may lead to water condensation and flooding of electrodes, with a resultant voltage loss caused by added resistance to reactant mass transport. A low operating temperature is also undesirable as determined from consideration of proton conductivity and electrochemical reaction kinetics. These stringent thermal requirements present a significant heat transport problem. In this paper the published literature on PEM fuel cell (PEMFC) thermal management is briefly reviewed, the heat transfer issues in individual PEMFC component are outlined, and different practical cooling systems are discussed.

2. Thermal balance and temperature distribution in PEMFC 2.1. Heat generation Heat generation in a PEMFC includes entropic heat of reactions, irreversibilities of the electrochemical reactions and ohmic resistances, as well as water condensation [1,4,5]. The entropic heat is representative of the entropy change of the electrochemical reaction and must be supplied to or removed from the electrode compartment to maintain constant temperature when current is flowing. The irreversibility of the electrochemical reactions inside a fuel cell, i.e. the overpotential on anode and cathode, induces a

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Fig. 1. Schematic representation of a PEMFC and the electrode reactions.

significant heat generation. Due to the splitting of the fuel cell reaction into two electrode reactions, it is readily apparent that heat is generated on both anode and cathode. However, the asymmetry of the entropy change and overpotential on the two electrodes leads to a greater heat generation at cathode [6–9], which imposes a challenge for maintaining a uniform temperature distribution across the MEA. The ohmic heat results from both the proton current and the electron current. Higher conductivity is always desired in reducing the ohmic losses. The sum of the entropic heat, irreversible reaction heat and ohmic heating is comparable to the power output of a PEM fuel cell. Roughly, they account for 55%, 35% and 10% of the total heat release, respectively [1].

transfer in the catalyst layer and the gas diffusion layer. Additional complication comes from the fact that the heat and water transports in PEMFC are inherently coupled: (i) evaporation and condensation processes are, respectively accompanied with absorption and release of latent heat; (ii) water and heat transport occur in conjunction with each other due to a heat pipe effect (a temperature gradient induces phase change and net mass transfer of water) [5,8]; and (iii) the saturation vapor pressure is strongly dependent on local temperature. The coupled water and heat transport processes in a PEMFC are further explained in a later section. 2.3. Fuel cell stack cooling

2.2. Heat transfer in PEMFC components The heat generated in a PEMFC is generally removed by the cooling system or transferred by conduction–convection across the faces of the stack. Heat removal rate is dictated by the thermal properties of individual components used in a PEMFC, i.e. polymer electrolyte, catalyst layer, gas diffusion layer (GDL) and bipolar plate (BPP). The heat conductivities of these materials have been experimentally measured [10,11] and are summarized in Table 1. However, these data are all obtained from ex situ measurement and their corresponding values in in situ operation need to be confirmed by more accurate real-time measurements. Different heat transfer mechanisms have been observed in different components of a PEMFC. Heat transfer through the polymer electrolyte is almost completely via heat conduction, while both conduction and convection have significant contribution for heat

Two factors are critical in designing a cooling system for PEM fuel cells: first, the nominal operating temperature of a PEMFC is limited to about 80 °C. This means that the driving force for heat rejection is far less than that in a typical internal combustion engine cooling system. Second, nearly the entire waste heat load must be removed by an ancillary cooling system since the exhaust streams contribute little in the heat removal. These two factors account for the need to have relatively large radiators in automotive fuel cell systems, and providing space for the radiators and the associated air handling ducts represents a significant design challenge [12]. A common fuel cell stack cooling method involves design of the bipolar plate such that there are internal cooling channels between the anode and the cathode [13]. With this approach, the channel geometry is designed to accommodate the heat transfer medium

Table 1 Heat conductivities of individual PEMFC components and their chemical compositions Component

Chemical composition

Heat conductivity (W/m K)

References

Polymer electrolyte Catalyst layer Gas diffusion layer Bipolar plate

Perfluorosulfonic acid membranes Mixture of platinum nanoparticles, carbon nanopowders and ionomer material Composed of microporous layer and carbon fiber substrate Graphite composite

Nafion 115: 0.2 dry Nafion: 0.16 0.27 0.22 30

[10,11] [11] [11]

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of choice. Various liquids have been used, with differences related mostly to cooling capacity, cost, corrosion control, or adjustment of electrical conductivity [14–19]. New concepts have been proposed to facilitate fuel cell stack cooling. In one cooling method, which relies on evaporative processes, liquid water is directly introduced into the reactant flows to provide both the humidification of PEM and evaporative cooling [20,21]. A novel design has been proposed and advanced by UTC Power, in which the bipolar plates are fabricated from a porous, hydrophilic material. This allows direct water exchange with the cathode side of the MEA [22,23]. 2.4. Non-uniform temperature distribution in PEMFC The balance between the heat generation and the heat removal rates determines the steady-state operating temperature of a PEMFC. A peak temperature is expected within the cathode catalyst layer due to the large amount of heat generation from the electrochemical reaction. This has been verified by experimental measurements [10] and various thermal models [1,5,24]. A temperature gradient of a magnitude of 5 °C has been found to exist across the cathode [25,10,26]. This temperature gradient has a significant influence on the water and heat transport in a PEMFC. The thermal and electrical conductivities of GDL are likely anisotropic, which complicates the thermal measurements and heat transfer modeling [27]. 3. Coupled heat and water transport in A PEMFC The heat transfer in a PEMFC is inherently coupled with the water transport and phase change dynamics. According to Eikerling [28], in spite of a PEMFC operating at 80 °C and near atmospheric pressure, water vapor is favored in the cathode catalyst layer for several reasons: (a) the nanometer scale of the pores in cathode catalyst layer (CCL) [29,30,6,7] greatly increases the saturation vapor pressure and the water evaporation dynamics and (b) the local high temperature in the CCL further increases the saturation vapor pressure and water evaporation rate. The first point is best highlighted by an example: the equilibrium vapor pressure, calculated from the Kelvin equation [31], in a micropore with a diameter of 20 nm increases by 11% at 25 °C and by 9% at 80 °C when compared to a flat surface. For the hydrophobic pores in a CCL, as demonstrated by Yu et al. [32], the saturation vapor pressure is further increased. Eikerling [28] demonstrated that at current density up to 1 A/cm2 the evaporation rate in the porous

structure is sufficient to convert all the product water from liquid state to vapor phase. Owejan et al. [33] studied the water transport in gas diffusion medium with and without MPL and concluded that the water flow through the MPL may also be in the vapor form. This is understandable given that the fine pores in MPL have a pore size of about 0.1 lm. The fibrous GDLs, on the other hand, seem much less suited to maintain the water in vapor form because their pores or channels have much larger sizes, in the magnitude of 10 lm [34]. A detailed review of the transport mechanisms of water is given by Kandlikar [35]. The role of microporous layer is critical in water management as it acts like a surface tension based gate that prevents the backflow of liquid water from GDL into the CCL, while facilitating flow of water vapor away from CCL into GDL [36–39]. Fig. 2 shows a proposed transport model for water from its generation at CCL to its removal in the gas channels. When the high temperature water vapor in the CCL passes through the microporous layer and the GDL, it is cooled and partly condensed into liquid water. Condensation in the MPL with finer pores is less desirable than in the GDL which has a coarser pore structure. A condensation front therefore is expected to exist under optimum conditions somewhere at the interface between the MPL and the GDL or within the GDL as shown in Fig. 2, depending on the local temperature. As a consequence, the flow of vapor from the reaction sites and its subsequent condensation inside the GDL enhances the heat transfer due to release of latent heat during condensation. The thermal gradients across CCL/MPL/GDL also play a significant role in the water transport toward the gas channels and its condensation within the GDL matrix. More experiments and modeling are warranted to better understand the water vapor transport in CCL and MPL and water condensation in GDL. 4. Thermal management issues in A PEMFC Improper thermal management will induce various thermal issues. Among them the electrolyte dehydration and cathode flooding impose the most critical challenges to PEMFC operation. Several causes have been identified for the PEM dehydration and consequently increased proton conduction resistance. A PEMFC operated at low relative humidity, high air stoichiometric ratio (high air flow rates) and/or high temperatures is readily subject to membrane dehydration due to the insufficient water supply to PEM. The second possible reason is due to the electro-osmotic drag, i.e. water molecules are pulled from anode to cathode by the flow of protons. The electro-osmotic drag leads to the dehydration of

Fig. 2. A proposed CCL water transport mechanism showing the electrode reaction and the transport of product water in a catalyst layer. Not to scale.

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the membrane at the anode side [40–45], which is worsened at high current densities. The cathode flooding, caused by liquid water accumulation in the pores of the cathode electrode (including catalyst layer, MPL and GDL) and consequently hindering oxygen transport to the catalyst sites, is one of the most common issues in water and thermal management. Flooding may occur when a PEMFC is operated at moderate or high current densities and/or with fully humidified reactants. The flooding phenomenon is dominated by the condensation/evaporation processes in the cathode [46], which is strongly affected by the temperature distribution. The cathode flooding phenomenon has been the focus of a large number of modeling and experimental studies [5,47,48,2,49–52]. However, a basic understanding is still lacking with respect to various issues, such as the exact location of the vapor–liquid interface, the water saturation inside GDL, the liquid water transport mechanism in GDL, etc. Another important, but often overlooked thermal issue is the non-uniform temperature distribution in the membrane, which exists at both the direction through the membrane [10,5,47,50,51] and along the flow length [53]. This temperature non-uniformity, on the order of several degrees C, has considerable impact on the water content of the membrane and the current density uniformity [54,55]. 5. Concluding remarks Thermal management is a key design consideration in PEMFCs, from the microscopic scale of the catalyst particles and microporous layer, the component level gas diffusion layers and bipolar plates, to the integration of the fuel cell stack with various external subsystems. The temperature and water vapor pressure profiles within the membrane–electrode assembly dictate the phase of water present in various regions and its transport from the PEM to gas channels. In this sense, fuel cell thermal and water transport mechanisms are intimately interlinked, and one cannot study the fuel cell performance without considering the heat transfer mechanism. As fuel cells continue to develop toward commercialization, there are a number of critical research needs related to thermal management. Some of the important needs identified in the above review are: 1. Accurate experimental data for the thermal conductivity of different components are needed for thermal and water transport modeling. Particular focus is needed on understanding multidimensional effects in components which are highly anisotropic. 2. New materials are desired for certain individual components. For example, a new PEM that can operate at higher temperature (above 100 °C) and low relative humidity, a new catalyst material or design that provides better mass and heat transport properties, and a new GDL that further mitigates water accumulation. 3. A clear understanding of the fundamental water and heat transfer mechanisms in each component is needed. 4. Defining a set of easily measurable and rigorously meaningful transport parameters for the catalyst layer, microporous layer and GDL is critical in developing better water and heat transport models. 5. The transient heat transfer problems, such as the start-up, shutdown, and freeze-thaw cycling, need to be accurately modeled for developing appropriate control algorithms, especially in the case of automotive systems which encounter highly dynamic load profiles.

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