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FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Freeze Operational Conditions
Freeze Operational Conditions M Oszcipok, Nucellsys GmbH, Nabern, Germany R Alink, Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany & 2009 Elsevier B.V. All rights reserved.
Introduction The development activities for proton-exchange membrane fuel cells (PEMFCs) in the early 1980s focused on performance improvement by means of better materials and flow field design at a single-cell level. Later, optimization of the operating strategies was investigated to develop PEMFC stacks with high power densities. Many stationary, automotive, and portable demonstration systems were presented in the 1990s. With a more and more sophisticated development level, new challenges such as lifetime requirements, costs, manufacturability, recycling, and many more came up. Among these ‘2nd generation’ problems, the subzero operation of PEMFCs is still one of the most challenging. The initial development was mainly driven by the automotive industry, which considers PEMFC systems as a serious alternative for fossil fuel-powered combustion engines. In this context, the US Department of Energy proposed a guideline with freeze start targets for 2010, such as a start-up ability to 50% power from 20 1C within 30 s. To become a serious alternative, fuel cell-powered vehicles have to fulfill the same requirements as conventional IC engines. Even when the temperature is far below 0 1C, the driver expects immediate power after start and, therefore, warm-up periods of some minutes are not acceptable. By contrast, portable outdoor fuel cell systems do not require very short response times and warming up is more acceptable due to the targeted applications. Here, the start-up strategies can be better tailored to prevent potential degradation caused by temperatures below 0 1C. A review of the literature on subzero operation shows that start-up effects and degradation mechanisms occurring in PEMFCs are not yet fully understood and that the conclusions of different research groups are not always in good agreement with each other.
Proton-Exchange Membrane Fuel Cells and Systems under Freezing Conditions Impact on Material Functionality Proton-exchange membrane fuel cells consist of very different materials to ensure an optimum functionality. The main component is the ionic membrane with a thickness between 15 and 175 mm. The membrane is
coated with a catalyst layer, which consists of a mixture of platinum alloy on carbon particles in an electrolyte matrix. The catalyst layer is a porous structure with a thickness of a few micrometers. The adjacent gas diffusion layer (GDL) is a porous structure of carbon fibers of B100 mm, and is relatively stiff. Finally, there are the gas distribution plates, which are made out of graphite compounds, hard graphite, and also metal. There are a number of other components such as sealing, end plates, compression hardware, fittings, and tubes, all of which can be more or less affected by low temperatures. This article focuses only on the main fuel cell components and their problems related to freezing temperatures. Electron conduction
Produced electrons in the PEMFC are conducted by the carbon particles of the catalyst layer, the carbon fibers of the GDL, and finally they reach the conductive gas distribution plate. The main functionality of carbon materials is to conduct electrons and heat. This functionality does not change significantly with temperature that are not below 0 1C. This is also valid for metallic gas separator plates. Reactant distribution
Reactants supplied to the PEMFC need to be distributed uniformly over the catalyst area. The distribution is achieved by the channels of the gas distribution plates (flow fields). The fine distribution occurs in the porous GDLs and finally in the porous catalyst layer. The change in the mechanical properties of these components below 0 1C under dry conditions is negligible. Therefore, the porous characteristics themselves do not change and reactant distribution is not affected by the material properties. However, when product water is taken into account, the reactant distribution ability of the fuel cell components can change strongly especially below 0 1C. The impact of product water during subfreezing operation will be discussed in detail in the section Fuel Cell Operation. Proton conduction
Protons produced at the anode electrode have to migrate toward the cathode electrode where they can react with oxygen. Protons are conducted by the ionic membrane and the ionomer in the catalyst layer when the ionomer is hydrated.
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According to Arrhenius’ law, the activation energy for proton conduction strongly depends on temperature. Furthermore, the amount and the aggregate state of water in the ionomer change significantly with temperature, influencing the proton conduction additionally. Water inside a fully hydrated membrane can exist in three forms. In the microchannels of the membrane, which connect hydrophilic micelles, there is free, bulk (liquid) water, as long as the membrane interface is in contact with liquid water. The microchannels can exhibit two states according to the membrane model of A. Z. Weber and J. Newman, which accounts for Schro¨der’s paradox. If liquid water at the interface is present, the channels are open and fully evolved; when the interface is dry or steam-saturated, the channels are collapsed and not evolved. Differential scanning calorimetry (DSC) on Nafions membranes showed that free water in the microchannels freezes slightly below 0 1C and shows a sharp peak in the DSC curves. Loosely bound water (second form) interacts weakly with the polymer chain and freezes in a broad temperature range below 0 1C. In the third form water within the membrane is strongly bound to the polymer chain and is assumed not to freeze at subzero temperatures. With decreasing temperature, the proton conductivity is reduced and consequently the ionic high-frequency resistance (HFR) of the membrane increases as depicted in Figure 1. Fuel Cell Operation Water production and influence of capillary pressure
To elucidate the phenomena of water production and the influence of capillary pressure in PEMFCs when they are operated below 0 1C, it is beneficial to recapitulate that PEMFCs produce water in the cathode catalyst layer during operation. Under normal operating conditions above 0 1C, the water balance in the catalyst layer equilibrated. The competing mechanisms of water production, on the one hand, and water diffusion/evaporation/electroosmotic drag, on the other hand, are in equilibrium during steady-state operation. Water produced in the cathode catalyst layer can diffuse and evaporate through the GDL into the gas channels of the cathode flow field, where it is transported away by the airflow. Additionally, product water diffuses back through the membrane toward the anode electrode, driven by the humidity gradient across the membrane. Depending on the operating conditions, such as current, gas flow rates, and temperature, the pores of the catalyst layer are partially filled with product water (see Figure 2). When liquid water is present in the catalyst layer, which in general is the case after standard operation, the
8.0 0.23 Ω cm−2 @ 45 °C 0.39 Ω cm−2 @ 60 °C 0.49 Ω cm−2 @ 60 °C 0.76 Ω cm−2 @ 60 °C 1.39 Ω cm−2 @ 60 °C
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Figure 1 The ionic membrane and contact resistance HFR increases with decreasing temperature. The results are based on a single cell equipped with a W.L. GORE& MEA (Type Primea 5510, 35 mm membrane thickness) and FCCT Freudenberg gas diffusion layer (GDL) (Type FCX 0026, 10%) HFR, highfrequency resistance; polytetrafluoroethylene (PTFE) with different initial HFRs at 60 1C/45 1C prior to freezing. HFR, highfrequency resistance.
microchannels in the membrane are fully evolved (Figure 3(a)). With decreasing temperature, the humidification level of the membrane changes significantly at 0 1C: water starts to freeze in the catalyst layer and, because of capillary pressure change, water is drained out of the membrane, and finally the membrane channels collapse (Figures 3(b) and 3(c)). This effect can be measured by means of the ionic membrane and catalyst resistance. Figure 4 shows an Arrhenius plot of the HFR of a six-cell PEMFC stack that was frozen to 40 1C from different initial humidification levels. When the stack is frozen after normal operation, the resistance decreases abruptly near 0 1C (Figure 4, blue curve) because of the effect described above. When the stack is heated up again, the ice in the catalyst layer melts, and as soon as liquid water is in contact with the membrane the channels evolve again and the ionic resistance increases step by step. The hysteresis effect can be explained by the crystallization enthalpy during the ongoing melting and freezing process of water in the temperature range between 0 and 40 1C. For comparison, the same stack was dried prior to freezing. The initial conductivity was much lower, no hysteresis was observed, and the conductivity curve showed no step-like change around 0 1C (Figure 4, green curve).
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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Freeze Operational Conditions
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Figure 2 Schematic of water production in the pores of the cathode catalyst layer, and competing transport mechanisms. GDL, gas diffusion layer.
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Figure 3 Freezing process in a proton-exchange membrane fuel cell (PEMFC) after normal operation, when the catalyst layer is partially flooded with liquid water; lB22 (a). With decreasing temperature, water freezes in the catalyst layer first and is drained out of the membrane (b), the channels in the membrane collapse (c) and finally the membrane humidification level is significantly decreased; lB16. –3.5 –4
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start-up) and heated up again after operation. During cold start-up, product water diffuses into the membrane even when the temperature is below 0 1C and increases its conductivity until it is fully humidified. After this, the stack is heated up and finally exhibits a much higher conductivity than before the experiment.
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Figure 4 Arrhenius plot showing the specific ionic conductivity of a six-cell proton-exchange membrane fuel cell (PEMFC) stack frozen in different freezing experiments. Even below 0 1C, a dry membrane can take up product water. The results can be explained by the model of A. Z. Weber and J. Newman, which accounts for the Schro¨der’s paradoxon for Nafion membranes.
Taking both effects into account, it is now possible to explain the behavior of the specific ionic resistance during a cold start-up (Figure 4, red curve), where the stack was dried prior to freezing, then operated (cold
When starting a fuel cell under subzero conditions, water is produced and stored in the membrane until saturation is reached. If the heat released during start-up operation is not sufficient to raise the electrode temperature above 0 1C, water will drain out, accumulate inside the electrode, blocking oxygen pathways to the active zones, and finally inhibit the electrochemical reaction. Figure 5 shows potentiostatic start-up experiments at different temperatures and demonstrates the phenomena described above very clearly. The 8 1C curve shown in Figure 5 (blue) represents a worst-case scenario where the complete pores of the electrode are blocked by ice. In a PEMFC stack, this scenario is fairly unlikely. But what could happen is that locally the electrodes of some cells within the stack could be blocked by ice, and in fact not only the cathode but
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Figure 5 Start-up experiments with a single proton-exchange membrane fuel cell (PEMFC) at 300 mV. At 40 1C, oxygen reduction reaction (ORR) is not hindered by product water, and the current density level after 2 min is stable around 0.23 A cm 2. At 2 1C, the cathode electrode is partially blocked by liquid water, but equilibrium is reached after 14 min with 0.17 A cm 2. At 8 1C, ice blocks the cathode electrode, ORR decreases finally, and the cell current goes to 0 A cm 2.
also the anode electrode could be blocked. Often the end cells of the stack are endangered because of the temperature gradient over the stack during start-up. The impact of partially frozen electrodes can be severe and will be discussed later. In critical cases, the GDL and the flow field channels could also be blocked by ice. The result of blockage is reactant starvation, which could lead to severe degradation (see Article 3). Optimized System Architecture and Start-Up Strategies Shutdown
Degradation by low temperatures after shutdown is mainly caused by the volume expansion of residual water during phase transition from liquid to solid phase. This can happen when the fuel cell is shut down after normal operation without applying any mitigation strategies. The most common way in fuel cell technology to overcome this problem is to remove residual water not only from the stack, but also out of all critical components of the fuel cell system by purging with reactant gases after operation. Analysis of driving cycles and temperature profiles in different North American regions led to the conclusion that sufficient insulation around an automotive fuel cell system can also prevent the stack from going below 0 1C. Other groups propose to purge with antifreeze solutions during shutdown to depress the freezing point of water. But removing residual water by purging with reactant gases after operation seems to be the simplest and most promising way to prevent degradation by freezing and to prepare for a successful cold start-up.
Purging time in the anode loop has to be as short as possible to maintain a high fuel efficiency of the fuel cell system, but it has to be long enough to remove a sufficient amount of residual water to prevent degradation of the porous layers by freezing. For this purpose, recent work concentrates on modeling the water transport processes within the GDL and electrodes during purging, to predict the residual water content and water location after the purging step. To ensure a less extensive purging and uniform water distribution, the present models and analyses recommend purging directly after shutdown, when the PEMFC is still at high temperature, using reactant gases and high flow rates. Some studies report that degradation in the temperature range down to 20 1C could be successfully prevented if the fuel cell is purged until the relative humidity of the outlet air drops below 17%. Similar to the PEMFC itself, freezing of water has to be prevented in critical locations of the peripheral fuel cell system, like tubes or valves, which could also lead to a malfunction during restart. Often condensers are integrated in the reactant supply loops, where liquid water can be separated from the gas stream. Figure 6 shows a scheme of a portable PEMFC system for outdoor applications. For shutdown, the six-cell PEMFC stack is purged with ambient air and also with hydrogen, which is recirculated in the anode loop. In the water separator, residual water is condensed and the anode side of the stack will be kept at a defined humidification level. To assist the start-up from subzero temperatures, active electrical heaters are implemented powered by a battery. Automotive PEMFC systems are certainly more complex, but the shutdown strategy is often comparable.
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Figure 6 Portable proton-exchange membrane fuel cell (PEMFC) system by Fraunhofer ISE. The system is able to start up and operate at 20 1C using active electrical heating for start-up and reactant gas purge for shutdown.
Subfreezing start-up
Subfreezing start-ups can be generally divided into passive start-ups, where no additional energy is needed and active start-ups, where additional heat is introduced into the stack and/or the fuel cell system. Owing to efficiency reasons, it might not be reasonable to heat up large fuel cells to 0 1C before restart; here the heat generated from the fuel cell itself is used for warming up. For such socalled passive cold start-up, it is beneficial to adjust high flow rates of dry gases to carry out product water and to operate the fuel cell with poor efficiency at low voltage to generate as much heat as possible. During cold start-up, single-cell voltage distribution in stacks is by far not as evenly distributed as during normal operation and the lowest single-cell voltage has to be monitored carefully. Some automotive fuel cell developers report on startup times within 10 s to reach 0 1C from 15 1C. Then their fuel cell systems are able to deliver 50% of rated power and more. Such cold start-ups can be supported by the stack and system architecture. The target for automotive applications is to realize hundreds of start-ups up to 30 1C. Active methods facilitate start-ups from subfreezing temperatures by heating up the PEMFC to temperatures above 0 1C before starting. To apply these methods, additional active heating devices are used such as electronic heaters, latent heat storages, or catalytic burners.
For large PEMFC systems, passive start-up strategies seem to be the most efficient, whereas for small-scale PEMFC systems active assisted strategies might be superior with respect to efficiency. Nevertheless, additional volume, weight, and energy consumption have to be considered for each system separately. One main factor to improve the start-up time is the thermal heat capacity of the stack. The ratio of active area to overall plate area should be as large as possible to produce heat over the complete area of the gas distribution plate. The plates themselves should be made as thin and light as possible, which can be realized using metallic bipolar plates. Also the temperature distribution in the stack is very important. Often the cells in the middle of the stack heat up very fast, whereas the end cells lag behind owing to heat losses over the end plates. In such cases, water produced during start-up can leave warm areas of the cell but can refreeze again in colder locations. Efficient cooling loops can help to mitigate this problem by equalizing the temperature distribution. Optimized components design
By appropriate material choice, it is possible to design a desired humidity gradient over the fuel cell, which promotes cold start-up. A freezing optimized membrane– electrode assembly (MEA) should work with very dry catalyst layers and GDL, a membrane that could absorb a
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large amount of water, and a flow field that carries out water droplets very fast. For instance, hydrophobic catalyst layers and GDLs that are treated with polytetrafluoroethylene (PTFE) absorb less water, but thicker membranes more water. Gas distribution plates can also be surface modified to repel water easily out of the stack. By such designs, a PEMFC can be optimized for cold start-up. But most often, cold start-up is given low priority compared to performance, cost, and lifetime.
Degradation due to Ice Formation The section ‘Proton-Exchange Membrane Fuel Cells and Systems under Freezing Conditions’ describes the importance of water balance in the PEMFC for subzero operation. When a PEMFC is started or shut down in a subzero environment, the product water can freeze, causing mechanical stress as a result of phase change and the associated volume expansion within the porous structures of the cell layers. This article provides a survey of the possible irreversible damage caused by water freezing in the structures of a PEMFC. Impact on Components Membrane
Ionomer membranes like Nafion are not solid and they are full of microchannels where water can be stored. In the literature review, there are several investigations about the material properties under freezing conditions, but serious degradation of the material itself is not reported. For instance, Nafion membranes did not suffer any material change when they were subjected to freezing down to 40 1C in a fully humidified state. However, when frozen in a dry state, changes in mechanical properties, swelling behavior, and gas permeability were observed, indicating rearrangement processes in the ionomer molecular structure. Water within the membrane maintains plasticity during freezing and prevents damage.
thermal and water movement effects can lead to movement of water from the warmer side (membrane) toward the colder side of the layer structure (electrode). When reaching the interface between the two layers, ice lens formation sets in and leads to mechanical stress and finally to delamination at the interface. If the electrode detaches from the membrane, ohmic losses will increase, which can be measured by the HFR. However, several investigations on PEMFCs point out that this degradation mode does not take place in every case and depends on the humidity of the fuel cell prior to freezing, water distribution, cooling rate, and the material combination/properties. Electrode
Owing to the porous structure of the electrode, product water accumulates in the pores during operation of the fuel cell. The brittle structure of the carbon/catalyst/ electrolyte mixture of the electrode is endangered to undergo mechanical degradation by the volume expansion of water when freezing in the electrode pores (see Figure 7). Cracks within the structure or even complete detachment of parts of the electrode can be the consequence of mechanical degradation (see Figure 8). Besides increasing ohmic losses and decreasing electrochemical surface area owing to a loss of electrode surface, cracks within the electrode can have a strong negative impact on water transport within the cell.
Interface membrane/electrode
Membrane and electrode are a relatively damageable assembly because of the difference in their stiffness and the manufacturing process of the MEA. Mechanical stress by ice formation or shrinking/swelling during purge and start-up within the electrode can lead to delamination in catalyst-coated membranes (CCMs). Another effect leading to electrode delamination is known from road construction and is called frost heave. Similar to fuel cells, roads consist of multiple layers with different porosities partially filled with water or humid air. When subjected to subfreezing temperatures, coupled
Figure 7 Microcavities on membrane–electrode assembly (MEA) electrode surface after 70 cold start-up and freeze/thaw cycles. The microcavities are the result of mechanical stress in the electrode owing to the phase change of water. This degradation type is characteristic of subzero operation and is not observed during normal operation above 0 1C even over hundreds of hours. Gas diffusion layer (GDL) fiber imprints are visible as well.
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Freeze Operational Conditions
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electrodes, the GDL is by far not so at risk to suffer any degradation by ice formation as the electrodes. Impact on Performance
Figure 8 Electrode surface before (a) and after (b) 10 freeze/ thaw cycles in an environmental scanning electron microscope (ESEM; ex situ experiment). Cracks grow and parts of the electrode detach because of the mechanical stress by ice formation.
Gas diffusion layer
Possible degradation mechanisms in GDL materials by water freezing inside the porous structure are carbon fiber breakage, loss of hydrophobicity by detachment of Teflon, or detachment of fibers by loss of binder. Carbon fiber breakage will result in increasing ohmic losses and changes in the water transport properties of the GDL. If Teflon detaches from the surface of the carbon fibers, the loss of hydrophobicity will result in increasing water accumulation and as a result reduced oxygen diffusion to the active zones. Although the flexibility of the GDL is higher than that of the
Subfreezing operation of PEMFC systems can have minor and also very strong influence on the fuel cell performance. Some effects are reversible whereas some are not. As mentioned above, ice formation in the porous structures of the cell layers slowly leads to irreversible morphological changes. Reduction of electrochemical surface area can result in lower performance, and changes in hydrophobic properties have a great impact on the water equilibrium in the layer structure. The capillary transport of water from the three-phase zone through the electrode to the GDL strongly depends on the pore size distribution. Cracks within the electrode or GDL fiber breakages lead to a shift of pore size distribution toward larger diameters, resulting in a reduced capillary pressure and higher water saturation. The maximum power of fuel cells is most often limited by the amount of oxygen, diffusing through the GDL and the electrode to the active zones. If the pathways of oxygen are blocked by residual water, maximum power and efficiency are reduced. The impact of porosity changes is important for transient and ramped operation of fuel cell systems. For steady-state operation, readjustment of operating conditions can be an appropriate corrective action to counterbalance the effects induced by subfreezing operation. Figure 9 shows a load jump experiment of a six-cell portable fuel cell stack before and after 70 subzero exposures (i.e., combination of freeze start-up and freeze/ thaw cycles) without readjusted operating conditions. The voltage drop after the load jump is larger and the HFR drops significantly to a lower level after the subzero exposures. Therefore, the duration for reaching steadystate voltage after the load jump is significantly prolonged owing to higher initial water saturation of the cathode electrode/GDL. By extended steady-state operation after the load jump, the performance can be fully recovered. But if the fuel cell is always operated in a transient mode, the time for recovery would probably be too short and the fuel cell system power could appear to be effectively degraded. Another very severe point to be considered is reactant starvation, which is very typical for PEMFCs under freezing conditions. Starvation can occur not only in PEMFC stacks, where local temperature distribution during freeze start-up can vary strongly, but also in FC systems. Liquid product water can freeze in colder regions of the stack and lead to ice blockage of the flow field channels of the anode and/or the cathode side. Air starvation in the cathode causes a single-cell potential of 0 V, whereas starvation on the anode side could result in a
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Figure 9 Load jump from 0.2 to 0.5 A cm 2 with a portable fuel cell stack. After 70 freeze/thaw cycles, the stack takes significantly longer time to reach the voltage level than before the freezing experiments. HFR, high-frequency resistance.
massive electrolysis. The electrolysis problem is very severe and can lead within few seconds to a local overheating in the cell. As a result, the membrane can melt locally and the whole stack can be damaged. The only way to stop this undesired operation is to interrupt current demand immediately. But detection of local electrolysis is very difficult. Starvation can also be induced by the system. When all the gas supply loops in the PEMFC system are cold during freeze start, water vapor that comes out of the stack can condense, freeze, and block tubing or valves. Therefore, the critical parts of PEMFC systems are often thermally connected to the heat source in the system, which is the PEMFC stack. If this is not possible, critical components can be heated separately.
State-of-the-Art Technology In recent years, operation of various fuel cell systems has been demonstrated and claimed to be able to be started up at subfreezing temperatures. Detailed strategies for freeze start-up are most often not described by fuel cell industry for proprietary reasons. All major automobile companies are working on fuel cell technology and the cold start ability still remains a challenge. Nevertheless, automotive fuel cell stack and system developers like Ballard, Proton Motor, NuCellSys, Honda, Toyota,
General Motors, and others have successfully demonstrated the freeze start ability of their stacks/systems up to 30 1C in recent years. The long-term goal for freeze start-up is 40 1C, which represents the automotive standard. Improvement of start-up time is reached by freeze optimized operating strategies and by reduction of the thermal mass of the PEMFC systems. Degradation still seems to be a severe problem, to a lesser extent from the aspect of dramatic performance loss, but primarily from worse dynamics. One example for a successful approach to meet subfreezing requirements is the commercial PEMFC stack PM200 of Proton Motor Fuel Cell GmbH. Studies to investigate the degradation by 36 freeze/thaw cycles included a drying step, cooling down to 20 1C, and detailed characterization after thawing. The examination showed that purging step up to 1 min could successfully prevent damage by freeze/thaw cycling. Figure 10 depicts the stack voltage at nominal power during the examination and shows negligible changes during the 36 freeze/thaw cycles. The number of publications and patents on freeze start-up and the increasing trend reflect the importance of the topic for the automotive industry. In recent years, the portable fuel cells also expanded their field of application by being able to operate under ambient conditions, including temperatures below 0 1C. Besides preseries PEMFC systems like a 50 W system presented
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Freeze Operational Conditions
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Figure 10 Performance of a PM200 fuel stack by Proton Motor after repeated freeze/thaw cycling.
by Fraunhofer ISE in 2005, commercial suppliers such as Flexiva also envision freeze start-up to supply outdoor applications like mobile homes, monitoring and navigation systems, or meteorological stations with power. A detailed investigation of the cold start effects and degradation mechanisms is required as not all effects are fully understood. Fuel cell materials and components can be improved to withstand freezing and further development is needed. Especially for the topic PEMFC freeze start-up, the gap between industrial and scientific research is still large. The numerous industrial patents are in contrast to few and sometimes contradicting scientific publications. In recent years, however, international scientists have recognized subzero operation as a very important topic and intensified their activities. However, with the current methods and the knowhow gained in the previous years, freeze start is no more a big, unsolved problem for PEMFCs and systems for commercialization.
Nomenclature Symbols and Units i t T r
current density (A cm 2) time (min) temperature (1C) specific ionic conductivity of MEA (S cm 1)
Abbreviations and Acronyms CCM DSC ESEM GDL HFR MEA ORR
catalyst-coated membrane differential scanning calorimetry environmental scanning electron microscope gas diffusion layer high-frequency resistance (O cm2) membrane–electrode assembly oxygen reduction reaction
See also: Electrodes: Porous Electrodes; Electrolytes: Polymer; Fuel Cells – Proton-Exchange Membrane Fuel Cells: Bipolar Plates; Catalysts: Life-Limiting Considerations; Cathodes; Cells; Gas Diffusion Layers; Membrane: Life-Limiting Considerations; Membrane– Electrode Assemblies; Membranes; Membranes: Ambient Temperature; Overview Performance and Operational Conditions; Stacks; Systems; Measurement Methods: Structural and Chemical Properties: Scanning Electron Microscopy.
Further Reading Alink R and Oszcipok M (2008) Degradation effects in polymer electrolyte membrane fuel cell stacks by sub-zero operation – an in situ and ex situ analysis. Journal of Power Sources 182: 175--187. Ballard Power Systems Inc., Burnaby, BC V5J5j9 (2000) Methods for Improving the Cold Starting Capability of an Electrochemical Fuel Cell. CA Patent US 2003077487 (A1). Ballard Power Systems Inc., Burnaby, BC (2001) Method of Reducing Fuel Cell Performance Degradation of an Electrode Comprising Porous Components. US Patent US 6306536 (B1). Borup R, Mayers J, Pivovar B, et al. (2007) Scientific aspects of polymer electrolyte fuel cell durability and degradation. Chemical Reviews 107: 3904--3951. Bradean R, Haas H, Desousa A, et al. (2005) Models for predicting MEA water content during fuel cell operation and after shutdown. AIChE 2005 Annual Meeting. ACS Annual Meeting-Fuel Processing Session I: Modeling and System Integration, vol. T1, p. 322c. Cincinnati, OH, USA, 30 October to 4 November. Cappadonia M, Erning JW, Saberi Niaki SM, and Stimming U (1995) Conductance of Nafion 117 membranes as a function of temperature and water content. Solid State Ionics 77: 65--69. Cho EA, Ko JJ, Ha HY, et al. (2003) Characteristics of the PEMFC repetitively brought to temperatures below 0 1C. Journal of the Electrochemical Society 150: A1667--A1670. Energy Partners, L.C., FL 33407 (2000) Freeze Tolerant Fuel Cell System and Method. Patent WO 0065676 (A1). Ge S and Wang CY (2007) Characteristics of subzero startup and water/ice formation on the catalyst layer in a polymer electrolyte fuel cell. Electrochimica Acta 52: 4825--4835. General Motors Corporation, Detroit, MI (1999) Freeze-protecting a fuel cell by vacuum drying. Patent CA2323036 (A1). Guo Q and Qi Z (2006) Effect of freeze–thaw cycles on the properties and performance of membrane-electrode assemblies. Journal of Power Sources 160: 1269--1274. He S and Mench MM (2006) One-dimensional transient model for frost heave in polymer electrolyte fuel cells. Journal of the Electrochemical Society 153: A1724--A1731. Hermansson A and Guthrie WS (2005) Frost heave and water uptake rates in silty soil subject to variable water table height during freezing. Cold Regions Science and Technology 43: 128--139. Hou J, Yu H, Zhang S, et al. (2006) Analysis of PEMFC freeze degradation at 20 1C after gas purging. Journal of Power Sources 162: 513--520. Jaouen F and Lindbergh G (2003) Transient techniques for investigating mass-transport limitations in gas diffusion electrodes. Journal of the Electrochemical Society 150: A1699--A1710.
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Kong CS, Kim DY, Lee HK, Shul YG, and Lee TH (2002) Influence of pore-size distribution of diffusion layer. Journal of Power Sources 108: 185--191. Kumbur EC, Sharp KV, and Mench MM (2007) On the effect of Levrett approach for describing the water transport in fuel cell diffusion media. Journal of Power Sources 168: 356--368. Lee C and Merida W (2006) Gas diffusion layer durability under steadystate and freezing conditions. Journal of Power Sources 164: 141--153. McDonald RC, Mittelsteadt CK, and Thompson EL (2004) Effects of deep temperature cycling on Nafion 112 membranes and membrane electrode assemblies. Fuel Cells 4: 208. Mukundan R, Kim YS, Garzon F, and Pivovar B (2005) ECS Transactions 1(E): 403. Oszcipok M, Hakenjos A, Riemann D, and Hebling C (2007) Start up and freezing processes in PEM fuel cells. Fuel Cells 7: 135--141. Oszcipok M, Riemann D, Kronenwett U, Kreideweis M, and Zedda M (2005) Statistic analysis of operational influences on the cold start behaviour of PEM fuel cells. Journal of Power Sources 145: 407--415.
Oszcipok M, Zedda M, Hesselmann J, et al. (2006) Portable proton exchange membrane fuel-cell systems for outdoor applications. Journal of Power Sources 157: 666--673. Oszcipok M, Zedda M, Riemann D, and Geckeler D (2006) Low temperature operation and influence parameters on the cold start ability of portable PEMFCs. Journal of Power Sources 154: 404--411. Pesaran A, Kim GH, and Gonder J (2007) PEM fuel cells operation and durability at subfreezing temperatures: Issues and solutions. In Proceedings of the 3rd Annual International Fuel Cells Durability & Performance 2007, Conference Documentation–Stationary, Automotive & Portable, p. 205. Brookline, MA, USA. Wang H, Hou J, Yu H, and Sun S (2007) Effects of reverse voltage and subzero startup on the membrane electrode assembly of a PEMFC. Journal of Power Sources 165: 287--292. Weber AZ and Newman J (2003) Transport in polymer-electrolyte membranes. Journal of the Electrochemical Society 150: A1008--A1015.
Introduction The development activities for proton-exchange membrane fuel cells (PEMFCs) in the early 1980s focused on performance improvement by means of better materials and flow field design at a single-cell level. Later, optimization of the operating strategies was investigated to develop PEMFC stacks with high power densities. Many stationary, automotive, and portable demonstration systems were presented in the 1990s. With a more and more sophisticated development level, new challenges such as lifetime requirements, costs, manufacturability, recycling, and many more came up. Among these ‘2nd generation’ problems, the subzero operation of PEMFCs is still one of the most challenging. The initial development was mainly driven by the automotive industry, which considers PEMFC systems as a serious alternative for fossil fuel-powered combustion engines. In this context, the US Department of Energy proposed a guideline with freeze start targets for 2010, such as a start-up ability to 50% power from 20 1C within 30 s. To become a serious alternative, fuel cell-powered vehicles have to fulfill the same requirements as conventional IC engines. Even when the temperature is far below 0 1C, the driver expects immediate power after start and, therefore, warm-up periods of some minutes are not acceptable. By contrast, portable outdoor fuel cell systems do not require very short response times and warming up is more acceptable due to the targeted applications. Here, the start-up strategies can be better tailored to prevent potential degradation caused by temperatures below 0 1C. A review of the literature on subzero operation shows that start-up effects and degradation mechanisms occurring in PEMFCs are not yet fully understood and that the conclusions of different research groups are not always in good agreement with each other.
Proton-Exchange Membrane Fuel Cells and Systems under Freezing Conditions Impact on Material Functionality Proton-exchange membrane fuel cells consist of very different materials to ensure an optimum functionality. The main component is the ionic membrane with a thickness between 15 and 175 mm. The membrane is
coated with a catalyst layer, which consists of a mixture of platinum alloy on carbon particles in an electrolyte matrix. The catalyst layer is a porous structure with a thickness of a few micrometers. The adjacent gas diffusion layer (GDL) is a porous structure of carbon fibers of B100 mm, and is relatively stiff. Finally, there are the gas distribution plates, which are made out of graphite compounds, hard graphite, and also metal. There are a number of other components such as sealing, end plates, compression hardware, fittings, and tubes, all of which can be more or less affected by low temperatures. This article focuses only on the main fuel cell components and their problems related to freezing temperatures. Electron conduction
Produced electrons in the PEMFC are conducted by the carbon particles of the catalyst layer, the carbon fibers of the GDL, and finally they reach the conductive gas distribution plate. The main functionality of carbon materials is to conduct electrons and heat. This functionality does not change significantly with temperature that are not below 0 1C. This is also valid for metallic gas separator plates. Reactant distribution
Reactants supplied to the PEMFC need to be distributed uniformly over the catalyst area. The distribution is achieved by the channels of the gas distribution plates (flow fields). The fine distribution occurs in the porous GDLs and finally in the porous catalyst layer. The change in the mechanical properties of these components below 0 1C under dry conditions is negligible. Therefore, the porous characteristics themselves do not change and reactant distribution is not affected by the material properties. However, when product water is taken into account, the reactant distribution ability of the fuel cell components can change strongly especially below 0 1C. The impact of product water during subfreezing operation will be discussed in detail in the section Fuel Cell Operation. Proton conduction
Protons produced at the anode electrode have to migrate toward the cathode electrode where they can react with oxygen. Protons are conducted by the ionic membrane and the ionomer in the catalyst layer when the ionomer is hydrated.
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According to Arrhenius’ law, the activation energy for proton conduction strongly depends on temperature. Furthermore, the amount and the aggregate state of water in the ionomer change significantly with temperature, influencing the proton conduction additionally. Water inside a fully hydrated membrane can exist in three forms. In the microchannels of the membrane, which connect hydrophilic micelles, there is free, bulk (liquid) water, as long as the membrane interface is in contact with liquid water. The microchannels can exhibit two states according to the membrane model of A. Z. Weber and J. Newman, which accounts for Schro¨der’s paradox. If liquid water at the interface is present, the channels are open and fully evolved; when the interface is dry or steam-saturated, the channels are collapsed and not evolved. Differential scanning calorimetry (DSC) on Nafions membranes showed that free water in the microchannels freezes slightly below 0 1C and shows a sharp peak in the DSC curves. Loosely bound water (second form) interacts weakly with the polymer chain and freezes in a broad temperature range below 0 1C. In the third form water within the membrane is strongly bound to the polymer chain and is assumed not to freeze at subzero temperatures. With decreasing temperature, the proton conductivity is reduced and consequently the ionic high-frequency resistance (HFR) of the membrane increases as depicted in Figure 1. Fuel Cell Operation Water production and influence of capillary pressure
To elucidate the phenomena of water production and the influence of capillary pressure in PEMFCs when they are operated below 0 1C, it is beneficial to recapitulate that PEMFCs produce water in the cathode catalyst layer during operation. Under normal operating conditions above 0 1C, the water balance in the catalyst layer equilibrated. The competing mechanisms of water production, on the one hand, and water diffusion/evaporation/electroosmotic drag, on the other hand, are in equilibrium during steady-state operation. Water produced in the cathode catalyst layer can diffuse and evaporate through the GDL into the gas channels of the cathode flow field, where it is transported away by the airflow. Additionally, product water diffuses back through the membrane toward the anode electrode, driven by the humidity gradient across the membrane. Depending on the operating conditions, such as current, gas flow rates, and temperature, the pores of the catalyst layer are partially filled with product water (see Figure 2). When liquid water is present in the catalyst layer, which in general is the case after standard operation, the
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Figure 1 The ionic membrane and contact resistance HFR increases with decreasing temperature. The results are based on a single cell equipped with a W.L. GORE& MEA (Type Primea 5510, 35 mm membrane thickness) and FCCT Freudenberg gas diffusion layer (GDL) (Type FCX 0026, 10%) HFR, highfrequency resistance; polytetrafluoroethylene (PTFE) with different initial HFRs at 60 1C/45 1C prior to freezing. HFR, highfrequency resistance.
microchannels in the membrane are fully evolved (Figure 3(a)). With decreasing temperature, the humidification level of the membrane changes significantly at 0 1C: water starts to freeze in the catalyst layer and, because of capillary pressure change, water is drained out of the membrane, and finally the membrane channels collapse (Figures 3(b) and 3(c)). This effect can be measured by means of the ionic membrane and catalyst resistance. Figure 4 shows an Arrhenius plot of the HFR of a six-cell PEMFC stack that was frozen to 40 1C from different initial humidification levels. When the stack is frozen after normal operation, the resistance decreases abruptly near 0 1C (Figure 4, blue curve) because of the effect described above. When the stack is heated up again, the ice in the catalyst layer melts, and as soon as liquid water is in contact with the membrane the channels evolve again and the ionic resistance increases step by step. The hysteresis effect can be explained by the crystallization enthalpy during the ongoing melting and freezing process of water in the temperature range between 0 and 40 1C. For comparison, the same stack was dried prior to freezing. The initial conductivity was much lower, no hysteresis was observed, and the conductivity curve showed no step-like change around 0 1C (Figure 4, green curve).
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Figure 2 Schematic of water production in the pores of the cathode catalyst layer, and competing transport mechanisms. GDL, gas diffusion layer.
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Figure 3 Freezing process in a proton-exchange membrane fuel cell (PEMFC) after normal operation, when the catalyst layer is partially flooded with liquid water; lB22 (a). With decreasing temperature, water freezes in the catalyst layer first and is drained out of the membrane (b), the channels in the membrane collapse (c) and finally the membrane humidification level is significantly decreased; lB16. –3.5 –4
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start-up) and heated up again after operation. During cold start-up, product water diffuses into the membrane even when the temperature is below 0 1C and increases its conductivity until it is fully humidified. After this, the stack is heated up and finally exhibits a much higher conductivity than before the experiment.
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Figure 4 Arrhenius plot showing the specific ionic conductivity of a six-cell proton-exchange membrane fuel cell (PEMFC) stack frozen in different freezing experiments. Even below 0 1C, a dry membrane can take up product water. The results can be explained by the model of A. Z. Weber and J. Newman, which accounts for the Schro¨der’s paradoxon for Nafion membranes.
Taking both effects into account, it is now possible to explain the behavior of the specific ionic resistance during a cold start-up (Figure 4, red curve), where the stack was dried prior to freezing, then operated (cold
When starting a fuel cell under subzero conditions, water is produced and stored in the membrane until saturation is reached. If the heat released during start-up operation is not sufficient to raise the electrode temperature above 0 1C, water will drain out, accumulate inside the electrode, blocking oxygen pathways to the active zones, and finally inhibit the electrochemical reaction. Figure 5 shows potentiostatic start-up experiments at different temperatures and demonstrates the phenomena described above very clearly. The 8 1C curve shown in Figure 5 (blue) represents a worst-case scenario where the complete pores of the electrode are blocked by ice. In a PEMFC stack, this scenario is fairly unlikely. But what could happen is that locally the electrodes of some cells within the stack could be blocked by ice, and in fact not only the cathode but
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Figure 5 Start-up experiments with a single proton-exchange membrane fuel cell (PEMFC) at 300 mV. At 40 1C, oxygen reduction reaction (ORR) is not hindered by product water, and the current density level after 2 min is stable around 0.23 A cm 2. At 2 1C, the cathode electrode is partially blocked by liquid water, but equilibrium is reached after 14 min with 0.17 A cm 2. At 8 1C, ice blocks the cathode electrode, ORR decreases finally, and the cell current goes to 0 A cm 2.
also the anode electrode could be blocked. Often the end cells of the stack are endangered because of the temperature gradient over the stack during start-up. The impact of partially frozen electrodes can be severe and will be discussed later. In critical cases, the GDL and the flow field channels could also be blocked by ice. The result of blockage is reactant starvation, which could lead to severe degradation (see Article 3). Optimized System Architecture and Start-Up Strategies Shutdown
Degradation by low temperatures after shutdown is mainly caused by the volume expansion of residual water during phase transition from liquid to solid phase. This can happen when the fuel cell is shut down after normal operation without applying any mitigation strategies. The most common way in fuel cell technology to overcome this problem is to remove residual water not only from the stack, but also out of all critical components of the fuel cell system by purging with reactant gases after operation. Analysis of driving cycles and temperature profiles in different North American regions led to the conclusion that sufficient insulation around an automotive fuel cell system can also prevent the stack from going below 0 1C. Other groups propose to purge with antifreeze solutions during shutdown to depress the freezing point of water. But removing residual water by purging with reactant gases after operation seems to be the simplest and most promising way to prevent degradation by freezing and to prepare for a successful cold start-up.
Purging time in the anode loop has to be as short as possible to maintain a high fuel efficiency of the fuel cell system, but it has to be long enough to remove a sufficient amount of residual water to prevent degradation of the porous layers by freezing. For this purpose, recent work concentrates on modeling the water transport processes within the GDL and electrodes during purging, to predict the residual water content and water location after the purging step. To ensure a less extensive purging and uniform water distribution, the present models and analyses recommend purging directly after shutdown, when the PEMFC is still at high temperature, using reactant gases and high flow rates. Some studies report that degradation in the temperature range down to 20 1C could be successfully prevented if the fuel cell is purged until the relative humidity of the outlet air drops below 17%. Similar to the PEMFC itself, freezing of water has to be prevented in critical locations of the peripheral fuel cell system, like tubes or valves, which could also lead to a malfunction during restart. Often condensers are integrated in the reactant supply loops, where liquid water can be separated from the gas stream. Figure 6 shows a scheme of a portable PEMFC system for outdoor applications. For shutdown, the six-cell PEMFC stack is purged with ambient air and also with hydrogen, which is recirculated in the anode loop. In the water separator, residual water is condensed and the anode side of the stack will be kept at a defined humidification level. To assist the start-up from subzero temperatures, active electrical heaters are implemented powered by a battery. Automotive PEMFC systems are certainly more complex, but the shutdown strategy is often comparable.
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Figure 6 Portable proton-exchange membrane fuel cell (PEMFC) system by Fraunhofer ISE. The system is able to start up and operate at 20 1C using active electrical heating for start-up and reactant gas purge for shutdown.
Subfreezing start-up
Subfreezing start-ups can be generally divided into passive start-ups, where no additional energy is needed and active start-ups, where additional heat is introduced into the stack and/or the fuel cell system. Owing to efficiency reasons, it might not be reasonable to heat up large fuel cells to 0 1C before restart; here the heat generated from the fuel cell itself is used for warming up. For such socalled passive cold start-up, it is beneficial to adjust high flow rates of dry gases to carry out product water and to operate the fuel cell with poor efficiency at low voltage to generate as much heat as possible. During cold start-up, single-cell voltage distribution in stacks is by far not as evenly distributed as during normal operation and the lowest single-cell voltage has to be monitored carefully. Some automotive fuel cell developers report on startup times within 10 s to reach 0 1C from 15 1C. Then their fuel cell systems are able to deliver 50% of rated power and more. Such cold start-ups can be supported by the stack and system architecture. The target for automotive applications is to realize hundreds of start-ups up to 30 1C. Active methods facilitate start-ups from subfreezing temperatures by heating up the PEMFC to temperatures above 0 1C before starting. To apply these methods, additional active heating devices are used such as electronic heaters, latent heat storages, or catalytic burners.
For large PEMFC systems, passive start-up strategies seem to be the most efficient, whereas for small-scale PEMFC systems active assisted strategies might be superior with respect to efficiency. Nevertheless, additional volume, weight, and energy consumption have to be considered for each system separately. One main factor to improve the start-up time is the thermal heat capacity of the stack. The ratio of active area to overall plate area should be as large as possible to produce heat over the complete area of the gas distribution plate. The plates themselves should be made as thin and light as possible, which can be realized using metallic bipolar plates. Also the temperature distribution in the stack is very important. Often the cells in the middle of the stack heat up very fast, whereas the end cells lag behind owing to heat losses over the end plates. In such cases, water produced during start-up can leave warm areas of the cell but can refreeze again in colder locations. Efficient cooling loops can help to mitigate this problem by equalizing the temperature distribution. Optimized components design
By appropriate material choice, it is possible to design a desired humidity gradient over the fuel cell, which promotes cold start-up. A freezing optimized membrane– electrode assembly (MEA) should work with very dry catalyst layers and GDL, a membrane that could absorb a
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large amount of water, and a flow field that carries out water droplets very fast. For instance, hydrophobic catalyst layers and GDLs that are treated with polytetrafluoroethylene (PTFE) absorb less water, but thicker membranes more water. Gas distribution plates can also be surface modified to repel water easily out of the stack. By such designs, a PEMFC can be optimized for cold start-up. But most often, cold start-up is given low priority compared to performance, cost, and lifetime.
Degradation due to Ice Formation The section ‘Proton-Exchange Membrane Fuel Cells and Systems under Freezing Conditions’ describes the importance of water balance in the PEMFC for subzero operation. When a PEMFC is started or shut down in a subzero environment, the product water can freeze, causing mechanical stress as a result of phase change and the associated volume expansion within the porous structures of the cell layers. This article provides a survey of the possible irreversible damage caused by water freezing in the structures of a PEMFC. Impact on Components Membrane
Ionomer membranes like Nafion are not solid and they are full of microchannels where water can be stored. In the literature review, there are several investigations about the material properties under freezing conditions, but serious degradation of the material itself is not reported. For instance, Nafion membranes did not suffer any material change when they were subjected to freezing down to 40 1C in a fully humidified state. However, when frozen in a dry state, changes in mechanical properties, swelling behavior, and gas permeability were observed, indicating rearrangement processes in the ionomer molecular structure. Water within the membrane maintains plasticity during freezing and prevents damage.
thermal and water movement effects can lead to movement of water from the warmer side (membrane) toward the colder side of the layer structure (electrode). When reaching the interface between the two layers, ice lens formation sets in and leads to mechanical stress and finally to delamination at the interface. If the electrode detaches from the membrane, ohmic losses will increase, which can be measured by the HFR. However, several investigations on PEMFCs point out that this degradation mode does not take place in every case and depends on the humidity of the fuel cell prior to freezing, water distribution, cooling rate, and the material combination/properties. Electrode
Owing to the porous structure of the electrode, product water accumulates in the pores during operation of the fuel cell. The brittle structure of the carbon/catalyst/ electrolyte mixture of the electrode is endangered to undergo mechanical degradation by the volume expansion of water when freezing in the electrode pores (see Figure 7). Cracks within the structure or even complete detachment of parts of the electrode can be the consequence of mechanical degradation (see Figure 8). Besides increasing ohmic losses and decreasing electrochemical surface area owing to a loss of electrode surface, cracks within the electrode can have a strong negative impact on water transport within the cell.
Interface membrane/electrode
Membrane and electrode are a relatively damageable assembly because of the difference in their stiffness and the manufacturing process of the MEA. Mechanical stress by ice formation or shrinking/swelling during purge and start-up within the electrode can lead to delamination in catalyst-coated membranes (CCMs). Another effect leading to electrode delamination is known from road construction and is called frost heave. Similar to fuel cells, roads consist of multiple layers with different porosities partially filled with water or humid air. When subjected to subfreezing temperatures, coupled
Figure 7 Microcavities on membrane–electrode assembly (MEA) electrode surface after 70 cold start-up and freeze/thaw cycles. The microcavities are the result of mechanical stress in the electrode owing to the phase change of water. This degradation type is characteristic of subzero operation and is not observed during normal operation above 0 1C even over hundreds of hours. Gas diffusion layer (GDL) fiber imprints are visible as well.
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electrodes, the GDL is by far not so at risk to suffer any degradation by ice formation as the electrodes. Impact on Performance
Figure 8 Electrode surface before (a) and after (b) 10 freeze/ thaw cycles in an environmental scanning electron microscope (ESEM; ex situ experiment). Cracks grow and parts of the electrode detach because of the mechanical stress by ice formation.
Gas diffusion layer
Possible degradation mechanisms in GDL materials by water freezing inside the porous structure are carbon fiber breakage, loss of hydrophobicity by detachment of Teflon, or detachment of fibers by loss of binder. Carbon fiber breakage will result in increasing ohmic losses and changes in the water transport properties of the GDL. If Teflon detaches from the surface of the carbon fibers, the loss of hydrophobicity will result in increasing water accumulation and as a result reduced oxygen diffusion to the active zones. Although the flexibility of the GDL is higher than that of the
Subfreezing operation of PEMFC systems can have minor and also very strong influence on the fuel cell performance. Some effects are reversible whereas some are not. As mentioned above, ice formation in the porous structures of the cell layers slowly leads to irreversible morphological changes. Reduction of electrochemical surface area can result in lower performance, and changes in hydrophobic properties have a great impact on the water equilibrium in the layer structure. The capillary transport of water from the three-phase zone through the electrode to the GDL strongly depends on the pore size distribution. Cracks within the electrode or GDL fiber breakages lead to a shift of pore size distribution toward larger diameters, resulting in a reduced capillary pressure and higher water saturation. The maximum power of fuel cells is most often limited by the amount of oxygen, diffusing through the GDL and the electrode to the active zones. If the pathways of oxygen are blocked by residual water, maximum power and efficiency are reduced. The impact of porosity changes is important for transient and ramped operation of fuel cell systems. For steady-state operation, readjustment of operating conditions can be an appropriate corrective action to counterbalance the effects induced by subfreezing operation. Figure 9 shows a load jump experiment of a six-cell portable fuel cell stack before and after 70 subzero exposures (i.e., combination of freeze start-up and freeze/ thaw cycles) without readjusted operating conditions. The voltage drop after the load jump is larger and the HFR drops significantly to a lower level after the subzero exposures. Therefore, the duration for reaching steadystate voltage after the load jump is significantly prolonged owing to higher initial water saturation of the cathode electrode/GDL. By extended steady-state operation after the load jump, the performance can be fully recovered. But if the fuel cell is always operated in a transient mode, the time for recovery would probably be too short and the fuel cell system power could appear to be effectively degraded. Another very severe point to be considered is reactant starvation, which is very typical for PEMFCs under freezing conditions. Starvation can occur not only in PEMFC stacks, where local temperature distribution during freeze start-up can vary strongly, but also in FC systems. Liquid product water can freeze in colder regions of the stack and lead to ice blockage of the flow field channels of the anode and/or the cathode side. Air starvation in the cathode causes a single-cell potential of 0 V, whereas starvation on the anode side could result in a
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Figure 9 Load jump from 0.2 to 0.5 A cm 2 with a portable fuel cell stack. After 70 freeze/thaw cycles, the stack takes significantly longer time to reach the voltage level than before the freezing experiments. HFR, high-frequency resistance.
massive electrolysis. The electrolysis problem is very severe and can lead within few seconds to a local overheating in the cell. As a result, the membrane can melt locally and the whole stack can be damaged. The only way to stop this undesired operation is to interrupt current demand immediately. But detection of local electrolysis is very difficult. Starvation can also be induced by the system. When all the gas supply loops in the PEMFC system are cold during freeze start, water vapor that comes out of the stack can condense, freeze, and block tubing or valves. Therefore, the critical parts of PEMFC systems are often thermally connected to the heat source in the system, which is the PEMFC stack. If this is not possible, critical components can be heated separately.
State-of-the-Art Technology In recent years, operation of various fuel cell systems has been demonstrated and claimed to be able to be started up at subfreezing temperatures. Detailed strategies for freeze start-up are most often not described by fuel cell industry for proprietary reasons. All major automobile companies are working on fuel cell technology and the cold start ability still remains a challenge. Nevertheless, automotive fuel cell stack and system developers like Ballard, Proton Motor, NuCellSys, Honda, Toyota,
General Motors, and others have successfully demonstrated the freeze start ability of their stacks/systems up to 30 1C in recent years. The long-term goal for freeze start-up is 40 1C, which represents the automotive standard. Improvement of start-up time is reached by freeze optimized operating strategies and by reduction of the thermal mass of the PEMFC systems. Degradation still seems to be a severe problem, to a lesser extent from the aspect of dramatic performance loss, but primarily from worse dynamics. One example for a successful approach to meet subfreezing requirements is the commercial PEMFC stack PM200 of Proton Motor Fuel Cell GmbH. Studies to investigate the degradation by 36 freeze/thaw cycles included a drying step, cooling down to 20 1C, and detailed characterization after thawing. The examination showed that purging step up to 1 min could successfully prevent damage by freeze/thaw cycling. Figure 10 depicts the stack voltage at nominal power during the examination and shows negligible changes during the 36 freeze/thaw cycles. The number of publications and patents on freeze start-up and the increasing trend reflect the importance of the topic for the automotive industry. In recent years, the portable fuel cells also expanded their field of application by being able to operate under ambient conditions, including temperatures below 0 1C. Besides preseries PEMFC systems like a 50 W system presented
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Figure 10 Performance of a PM200 fuel stack by Proton Motor after repeated freeze/thaw cycling.
by Fraunhofer ISE in 2005, commercial suppliers such as Flexiva also envision freeze start-up to supply outdoor applications like mobile homes, monitoring and navigation systems, or meteorological stations with power. A detailed investigation of the cold start effects and degradation mechanisms is required as not all effects are fully understood. Fuel cell materials and components can be improved to withstand freezing and further development is needed. Especially for the topic PEMFC freeze start-up, the gap between industrial and scientific research is still large. The numerous industrial patents are in contrast to few and sometimes contradicting scientific publications. In recent years, however, international scientists have recognized subzero operation as a very important topic and intensified their activities. However, with the current methods and the knowhow gained in the previous years, freeze start is no more a big, unsolved problem for PEMFCs and systems for commercialization.
Nomenclature Symbols and Units i t T r
current density (A cm 2) time (min) temperature (1C) specific ionic conductivity of MEA (S cm 1)
Abbreviations and Acronyms CCM DSC ESEM GDL HFR MEA ORR
catalyst-coated membrane differential scanning calorimetry environmental scanning electron microscope gas diffusion layer high-frequency resistance (O cm2) membrane–electrode assembly oxygen reduction reaction
See also: Electrodes: Porous Electrodes; Electrolytes: Polymer; Fuel Cells – Proton-Exchange Membrane Fuel Cells: Bipolar Plates; Catalysts: Life-Limiting Considerations; Cathodes; Cells; Gas Diffusion Layers; Membrane: Life-Limiting Considerations; Membrane– Electrode Assemblies; Membranes; Membranes: Ambient Temperature; Overview Performance and Operational Conditions; Stacks; Systems; Measurement Methods: Structural and Chemical Properties: Scanning Electron Microscopy.
Further Reading Alink R and Oszcipok M (2008) Degradation effects in polymer electrolyte membrane fuel cell stacks by sub-zero operation – an in situ and ex situ analysis. Journal of Power Sources 182: 175--187. Ballard Power Systems Inc., Burnaby, BC V5J5j9 (2000) Methods for Improving the Cold Starting Capability of an Electrochemical Fuel Cell. CA Patent US 2003077487 (A1). Ballard Power Systems Inc., Burnaby, BC (2001) Method of Reducing Fuel Cell Performance Degradation of an Electrode Comprising Porous Components. US Patent US 6306536 (B1). Borup R, Mayers J, Pivovar B, et al. (2007) Scientific aspects of polymer electrolyte fuel cell durability and degradation. Chemical Reviews 107: 3904--3951. Bradean R, Haas H, Desousa A, et al. (2005) Models for predicting MEA water content during fuel cell operation and after shutdown. AIChE 2005 Annual Meeting. ACS Annual Meeting-Fuel Processing Session I: Modeling and System Integration, vol. T1, p. 322c. Cincinnati, OH, USA, 30 October to 4 November. Cappadonia M, Erning JW, Saberi Niaki SM, and Stimming U (1995) Conductance of Nafion 117 membranes as a function of temperature and water content. Solid State Ionics 77: 65--69. Cho EA, Ko JJ, Ha HY, et al. (2003) Characteristics of the PEMFC repetitively brought to temperatures below 0 1C. Journal of the Electrochemical Society 150: A1667--A1670. Energy Partners, L.C., FL 33407 (2000) Freeze Tolerant Fuel Cell System and Method. Patent WO 0065676 (A1). Ge S and Wang CY (2007) Characteristics of subzero startup and water/ice formation on the catalyst layer in a polymer electrolyte fuel cell. Electrochimica Acta 52: 4825--4835. General Motors Corporation, Detroit, MI (1999) Freeze-protecting a fuel cell by vacuum drying. Patent CA2323036 (A1). Guo Q and Qi Z (2006) Effect of freeze–thaw cycles on the properties and performance of membrane-electrode assemblies. Journal of Power Sources 160: 1269--1274. He S and Mench MM (2006) One-dimensional transient model for frost heave in polymer electrolyte fuel cells. Journal of the Electrochemical Society 153: A1724--A1731. Hermansson A and Guthrie WS (2005) Frost heave and water uptake rates in silty soil subject to variable water table height during freezing. Cold Regions Science and Technology 43: 128--139. Hou J, Yu H, Zhang S, et al. (2006) Analysis of PEMFC freeze degradation at 20 1C after gas purging. Journal of Power Sources 162: 513--520. Jaouen F and Lindbergh G (2003) Transient techniques for investigating mass-transport limitations in gas diffusion electrodes. Journal of the Electrochemical Society 150: A1699--A1710.
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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Freeze Operational Conditions
Kong CS, Kim DY, Lee HK, Shul YG, and Lee TH (2002) Influence of pore-size distribution of diffusion layer. Journal of Power Sources 108: 185--191. Kumbur EC, Sharp KV, and Mench MM (2007) On the effect of Levrett approach for describing the water transport in fuel cell diffusion media. Journal of Power Sources 168: 356--368. Lee C and Merida W (2006) Gas diffusion layer durability under steadystate and freezing conditions. Journal of Power Sources 164: 141--153. McDonald RC, Mittelsteadt CK, and Thompson EL (2004) Effects of deep temperature cycling on Nafion 112 membranes and membrane electrode assemblies. Fuel Cells 4: 208. Mukundan R, Kim YS, Garzon F, and Pivovar B (2005) ECS Transactions 1(E): 403. Oszcipok M, Hakenjos A, Riemann D, and Hebling C (2007) Start up and freezing processes in PEM fuel cells. Fuel Cells 7: 135--141. Oszcipok M, Riemann D, Kronenwett U, Kreideweis M, and Zedda M (2005) Statistic analysis of operational influences on the cold start behaviour of PEM fuel cells. Journal of Power Sources 145: 407--415.
Oszcipok M, Zedda M, Hesselmann J, et al. (2006) Portable proton exchange membrane fuel-cell systems for outdoor applications. Journal of Power Sources 157: 666--673. Oszcipok M, Zedda M, Riemann D, and Geckeler D (2006) Low temperature operation and influence parameters on the cold start ability of portable PEMFCs. Journal of Power Sources 154: 404--411. Pesaran A, Kim GH, and Gonder J (2007) PEM fuel cells operation and durability at subfreezing temperatures: Issues and solutions. In Proceedings of the 3rd Annual International Fuel Cells Durability & Performance 2007, Conference Documentation–Stationary, Automotive & Portable, p. 205. Brookline, MA, USA. Wang H, Hou J, Yu H, and Sun S (2007) Effects of reverse voltage and subzero startup on the membrane electrode assembly of a PEMFC. Journal of Power Sources 165: 287--292. Weber AZ and Newman J (2003) Transport in polymer-electrolyte membranes. Journal of the Electrochemical Society 150: A1008--A1015.