Fuel Cell Perfluorinated Sulfonic Acid Membrane ... - ACS Publications

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Fuel Cell Perfluorinated Sulfonic Acid Membrane Degradation Correlating Accelerated Stress Testing and Lifetime Marianne P. Rodgers,* Leonard J. Bonville, H. Russell Kunz, Darlene K. Slattery, and James M. Fenton Florida Solar Energy Center, University of Central Florida, 1679 Clearlake Road, Cocoa, Florida 32922-5703, United States 4.1.5. Freeze/Thaw Cycling and Subfreezing Start-Up 4.1.6. High Cell Voltage 4.1.7. Voltage or Current Cycling 4.2. AST Protocols 4.2.1. Membrane Chemical Attack 4.2.2. Membrane Mechanical Failure 4.2.3. Cathode Catalyst Decay 4.3. Summary of Results from ASTs 4.4. Relationship between Current Degradation Mechanisms and AST Results 5. Comparison of Lifetime Tests and Accelerated Stress Tests 6. Conclusions Author Information Corresponding Author Notes Biographies References

CONTENTS 1. Introduction 2. Types of Degradation 2.1. Thermal Degradation of Membranes 2.1.1. Introduction 2.1.2. Degradation at High Temperatures 2.1.3. Degradation at Low Temperatures 2.1.4. Degradation with Freeze Cycling 2.1.5. Summary and Thermal Degradation Mitigation 2.2. Mechanical Degradation of Membranes 2.2.1. Cracks and Tears Caused by Local Stresses 2.2.2. Correlation of Tear Toughness and Microcrack Resistance 2.2.3. Creep 2.2.4. Effect of Humidity on Mechanical Degradation 2.2.5. Prevention of Mechanical Degradation 2.3. Chemical Degradation of Membranes 2.3.1. Radical Formation 2.3.2. Degradation Mechanisms 2.3.3. Methods of Measuring Degradation 2.3.4. Impact of Impurities on Chemical Degradation 2.3.5. Effect of Catalyst 2.3.6. Localization of Degradation 2.3.7. Prevention of Chemical Degradation 3. Lifetime Tests 3.1. Summary of Results from Lifetime Tests 3.2. Relationship between Lifetime Tests and Degradation Mechanisms 4. Accelerated Stress Tests 4.1. Stressors 4.1.1. Reduced Humidity 4.1.2. High Humidification 4.1.3. Humidity Cycling 4.1.4. High Temperature

© 2012 American Chemical Society

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1. INTRODUCTION Cost, durability, performance, reliability, efficiency, and size are some of the requirements that must be met before proton exchange membrane (PEM) fuel cells can be used more extensively. For automotive applications, PEM fuel cells will need to have a durability of 5000 h (150 000 miles), at a cost of $30/kW. Stationary PEM fuel cells will require 40 000−80 000 h durability at a cost of $750/kW.1 Because no moving parts are present, fuel cells are naturally reliable systems, although they can be susceptible to material degradation. Within a PEM fuel cell, components are subjected to a combination of strongly oxidizing and reducing conditions, strongly acidic conditions, liquid water, elevated temperatures, and high electrochemical potentials, as well as potential cycling, reactive radical species, high electric current, and large potential gradients.2 Typically, PEM fuel cells exhibit a steady, moderate loss in power output during operation, followed by a sudden failure. Even if the initial power output of the cell with the gradual loss is still high enough to effectively operate the device to which it provides power, with time, the cumulative effect of this steady loss can become so large that the cell can no longer deliver the required power.2 The steady, moderate loss of performance is usually a result of steady electrode degradation, due to carbon corrosion, platinum dissolution and deposition inside the membrane, platinum sintering, particle growth, and recrystallization, or membrane degradation including fluoride

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loss, splitting of the backbone, and losses of side chains, ultimately resulting in membrane defects. The sudden failure of cells, on the other hand, is the result of substantial gas crossover through membrane defects such as holes, cracks, or tears formed in the PEM. The lifetime performance of fuel cells depends upon the design and management of the components and interacting conditions. Extensive reviews focusing on several aspects of fuel cell durability have recently been published. Borup et al.2 summarized the fundamental aspects of PEM fuel cell durability and degradation. Wang3 presented fundamental fuel cell engineering models. Wu et al.4 surveyed mechanisms of degradation, degradation mitigation approaches, and theoretical modeling related to PEM fuel cell durability. De Bruijn et al.5 assessed the relative importance of degradation mechanisms of all cell components. Shao et al.6 summarized the difficulties in developing materials for high -temperature PEM fuel cells, particularly with respect to the material durability. Schmittinger and Vahidi7 presented an overview of concerns impacting PEM fuel cell lifetimes and long-term performance. Zhang et al.8 reviewed accelerated stress tests for PEM fuel cells, concentrating on durability issues. Other fuel cell durability reviews have focused on the durability of particular PEM fuel cell components, including membranes9,10 and electrocatalysts.11 These reviews can aid researchers in understanding where the field of fuel cell durability stands at present from many different perspectives. However, none of these reviews has sufficiently compared the results obtained from accelerated and lifetime durability tests, while giving a comprehensive overview of the current understanding of membrane degradation mechanisms. The membrane is the core element in PEM fuel cells, acting as both a separator and electrolyte.12 PEMs are generally ioncontaining polymers comprised of a hydrophobic polymer backbone and proton exchange sites (usually sulfonic acid groups). Membranes should have high proton conductivity (≥0.1 S/cm13) to maintain adequate currents with negligible losses in resistance, no electronic conductivity, chemical and electrochemical stability when using highly oxidizing and reducing operating conditions, sufficient mechanical stability, little dependence on humidity, low fuel and oxygen crossover to maximize Coulombic efficiency, and production costs that are suited to the anticipated applications.14,15 The PEM must be easily fabricated into high-performance membrane electrode assemblies (MEAs). During the fuel cell lifetime, these attributes must not be hindered, meaning, for example, that the proton conductivity should remain above 0.1 S/cm and its permeability to gases must continue to be negligible, that is, 100 °C to improve reaction kinetics, simplify cooling systems, facilitate water management, and increase tolerance to CO. Higher temperatures also allow coproduction of electricity and heat, which can offer several benefits. However, the disadvantages to high-temperature operation include higher component degradation rates, increased corrosion, and reduced water content in the membrane and ionomer.17 The reduced water content can lead to increased membrane degradation and decreased 6077

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flow channels to eliminate water from the channels before freezing.48 After operation and purging during each of 55 freeze/thaw cycles, performance loss at 0.5 A/cm2 was negligible, and only 0.2 mV/cycle was lost at 1.0 A/cm2.

channels and result in partial or total reactant starvation. Due to volumetric expansion during the freezing process, water remaining in the catalyst-coated membrane (CCM), gas diffusion layers (GDLs), and gas flow channels can damage each component and interface. Many researchers38−41 have suggested that water in the membrane exists in three different states: (1) nonfreezing water is strongly associated with the polymer chain, (2) freezable loosely bound water is weakly associated with the polymer chain or interacts weakly with nonfreezing water, and (3) free water is not associated with the polymer chain and behaves like bulk water. Nonfreezing water content is independent of the degree of sulfonation in the membrane, whereas the amounts of freezable loosely bound water and free water both increase with increasing degree of sulfonation in the membrane.40,41 Because freezing temperatures are depressed, ∼24.5 K in Nafion membranes, at subfreezing temperatures water inside the membrane flows out and instantly freezes on the membrane surface and in the electrodes.42 Water freezing on, rather than inside, PFSA membranes results in damage to the membrane− electrode interface. Through modeling, He and Mench43 correlated the thickness of ice formed on the PEM surface with initial hydration. The ice layer becomes thicker as PEM thickness and initial water content increases. For example, Nafion 112, which is ∼50 μm thick, has an ice thickness of 5 μm, while Nafion 117, which is ∼180 μm thick, has an ice thickness of ∼18 μm (water content of Nafion = 20 molH2O/ molSO3−). 2.1.4. Degradation with Freeze Cycling. It is necessary that membranes are stable in changing temperatures for their use in the automotive and portable power industries. They must be able to tolerate cycling between low and high temperatures. The issue with temperature cycling is that the phase transformations and volume changes of water resulting from freeze/thaw cycles decrease the membrane’s lifetime. When freeze/thaw cycles are performed on MEAs, the cycles of ice forming and melting on the membrane surface can separate the electrode from the membrane or the gas diffusion layer (GDL),44,45 resulting in thermal and electrical contact losses in the MEA. After an unhumidified Nafion 112 membrane contained in an MEA was cycled between +80 and −40 °C 385 times, no membrane tearing was observed, although the ionic conductivity, impermeability to gases, and mechanical strength of the PEMs were damaged. When fully humidified membranes were tested, they suffered no loss of mechanical strength, which was ascribed to rearrangement of the membrane at the molecular level after the freeze/thaw cycles.46 Plug Power cycled from −30 to 20 °C and observed cracking of fully hydrated membranes.47 However, when the hydration of the membrane was lower, the cracks were less serious after shut-down. 2.1.5. Summary and Thermal Degradation Mitigation. To become commercialized, it is necessary that fuel cells can function at a wide range of temperatures and under temperature cycling conditions. Unfortunately, these conditions can accelerate membrane degradation. Although degradation of membranes at high temperatures in PEM fuel cells can be severe, it does not need to be a major issue because operating and start−stop procedures can be established that keep the majority of fuel cell operation at temperatures below the point at which degradation occurs. One way to mitigate degradation during operation at subfreezing temperatures is by purging the

2.2. Mechanical Degradation of Membranes

Early life failures in PEM fuel cells are usually caused by mechanical degradation. Examples of forms of mechanical degradation include cracks, tears, punctures, pinholes, and delamination between the membrane/electrode and electrode/ GDL interfaces.31,49−52 Mechanical degradation can take place at some stage in membrane fabrication and processing, MEA fabrication and processing, or fuel cell operation and may also result from defects innate to the membrane. PEM stack durability is often limited by the membranes’ ability to defend against mechanical degradation. When defects in the membrane, such as perforations, pinholes, or cracks occur, the reactant gases cross over to their respective opposing electrodes and react at the catalyst. This produces a mixed potential on the electrodes that offsets the cell’s electrochemical reaction, and cell voltage drops. When the reactant gases react at the catalyst, the highly exothermic direct combustion generates hot spots. The production of heat from this reaction may lead to softening or melting of the PEM, furthering the degradation and further increasing gas crossover. A cycle will occur49 with increasing gas crossover and defect generation, accelerating MEA degradation. Prevention of defect formation and the resulting increase in crossover are vital to the lifetime of the membrane. 2.2.1. Cracks and Tears Caused by Local Stresses. Membrane preparation must be executed carefully, because small perforations or tears may be incurred by foreign particles from fabrication. Cracks and tears may also develop at any areas where the membrane experiences increased stress, including reactant inlets, edges, or the border of the flow field’s lands and channels.54 At the edge of the flow channel, the PEM may stretch due to compression from the flow field lands, especially when reactant pressures on each side of the membrane are different. Another area where cracks are commonly propagated in the membrane is at the edge of where reactions take place. Failure near the edges could be caused by increased reactant crossover due to defective gasket sealing or hot spot formation.55 Stresses also develop locally from swelling differences in the membrane at the edge of where the reactions take place, from manufacturing imperfections, or from GDL and electrode misalignment.49 Sompalli et al.56 examined the effect of potential distributions induced by electrode overlap on membrane failure at catalyst layer edges in PEM fuel cells and discovered that in all cases, membrane failure only occurred in the edges where only one electrode was present. When the cathode was larger than the anode with fully humidified conditions, cells failed within 200 h, compared with lifetimes >2000 h when the anode was larger than the cathode with fully humidified conditions. However, at low RH, membrane degradation accelerated dramatically when the anode was larger than the cathode. Several researchers have determined that OCV conditions develop at locations where the cathode overlaps the anode.57,58 For these reasons, it is important to carefully align the electrodes during fuel cell assembly. 2.2.2. Correlation of Tear Toughness and Microcrack Resistance. Several researchers show that microcrack resistance correlates well with tear toughness.9 Crack resistance in PEMs is most commonly evaluated using stress−strain 6078

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peripheral edges of the MEAs. Reduction of the swelling coefficient of the membrane would result in reduction of dimensional changes and, therefore, mechanical stress on the membrane, leading to extended membrane life.68−70 Kolde et al.67 found that, during RH cycling, membranes that are dimensionally stable in the in-plane directions exhibited improved the cell reliability. Membrane reinforcement with a porous material with high dimensional stability (porous polyethylene or polytetrafluoroethylene (PTFE)49) improves the dimensional stability of the membrane. Reinforced membranes have increased stability and longer lifetimes66 at elevated temperature and low humidity.49 Liu et al.49 suggest that defects formed in the catalyst-coated membrane (CCM) become stress points that can be propagated through fuel cell operation or environmental changes. Reinforcing the membrane slows or prevents this process. Gore Fuel Cell Technologies membranes reinforced with e-PTFE had a longer lifetime49 than unreinforced membranes, even when the Gore membrane was one-third the thickness of the unreinforced Nafion membrane. The unreinforced membrane experienced a sudden catastrophic failure, while the reinforced membrane exhibited a gradual decline in performance. Carbon nanotubes and particulated fillers such as metal oxide/phosphate or silica also have been recently considered for use as reinforcing agents.51,60,72−75 These reinforcements have been reported to decrease membrane creep and increase morphological stability at elevated temperature. Adjemian et al.76 determined that the performance of a Nafion membrane reinforced with silicon oxide remained constant after durability testing at 130 °C while the cell performance of unmodified Nafion decreased significantly in less than an hour. Another method of preventing mechanical damage to the membrane at the edge of the reaction area is protecting the outer perimeter of an MEA with gasket seals,77−79 adhesively bonded layers,80 or plastic spacers.81 These protective layers increase crack resistance at the perimeter of the active area of the MEA.

curves, even though membrane tear tests are likely more applicable to actual failure mechanisms.59 As temperature and water hydration of Nafion increase, its tensile strength and modulus decrease. Elongation at break of Nafion is relatively constant with temperature, hydration, and strain rate and is typically in the range of 200−350%.2 Although cast Nafion tears isotropically, the extruded membranes have lower tear toughness (elongation at break) along the machine direction than perpendicular to the machine direction. Tears occurred in the machine direction for extruded membranes that were tested as MEAs under accelerated conditions, and no dependence on flow field orientation was shown during testing.59 2.2.3. Creep. Creep is the tendency of a solid material to gradually move or deform permanently under the continued application of stresses. During normal fuel cell operation, depending on the stack design, the MEA is often subjected to constant compressive pressure between the bipolar plates, causing the PEMs to creep. Polymer creep leads to irreversible thinning of the membranes and, ultimately, failure (for example, formation of cracks or pinholes) and, when compounded with other degradation routes, is an important degradation factor. Membrane thinning will be more extensive at regions with greater stress. Satterfield et al.60 reported that the creep response of an extruded Nafion 115 membrane increased with increased temperature. Hydrated samples had high initial creep rates, but as time increased, the creep rate slowed. On the other hand, dry samples had low initial creep rates, but creep rates increased as time increased. Although these are ex situ experiments, the results correlate well with typical membrane degradation behavior. For example, with mild fuel cell operating parameters (≤80 °C and high humidity), the creep rate of Nafion is slow, so it can take thousands of hours before the mechanical integrity of the membrane is compromised by the cell’s compressive forces, resulting in catastrophic failure. However, as the glass transition temperature of the PEM (which is related to the ionic mobility of the main and side chains,61,62 and depends on both temperature and RH) is approached, the mechanical creep increases, which can result in failure in less than 100 h with hot/low RH conditions. 2.2.4. Effect of Humidity on Mechanical Degradation. As humidity in the cell changes, the membrane swells and shrinks. Throughout fuel cell operation, the mechanical integrity of the PEM is damaged as a result of dimensional changes due to nonhumidification, 63 low humidification,17,18,53,64 and RH cycling.65 The membrane in an assembled fuel cell is stressed as a result of contraction with low RH conditions and compression during swelling with wet conditions. Insufficient humidification is also damaging to the membrane, because membranes become brittle and fragile with low water content. Several researchers17,18,65,66 observed higher degradation under low humidity and humidity cycling conditions. For example, Huang et al.65 observed reduced strain to failure and microcrack formation after RH cycling for 50 h. 2.2.5. Prevention of Mechanical Degradation. Although membrane mechanical degradation is usually the limiting parameter in determining fuel cell lifetimes, there are many ways to avoid or at least minimize this degradation. Impurities should be minimized and great care should be taken when assembling and disassembling fuel cells. Several researchers have improved membrane mechanical durability by improving membrane dimensional stability to swelling,67−70 reinforcing the membranes,49,51,60,71−76 and protecting the

2.3. Chemical Degradation of Membranes

Many factors contribute to the chemical decomposition of PEMs including reactant gas crossover, radical formation, Pt dissolution and redeposition, and transition metal ion contaminants. 2.3.1. Radical Formation. Thinning of membranes and detection of F− in the water from the fuel cell exhaust provide evidence that the polymer is being chemically degraded.82 Peroxyl (OOH•) and hydroxyl (OH•) radical attack on the remaining H-containing terminal bonds on polymer end and side chains83 is commonly believed to be the primary degradation mechanism. There have been two mechanisms proposed for the formation of these radicals. In the first mechanism, radicals are generated by the decomposition of hydrogen peroxide. Formation of hydrogen peroxide can take place84−86 at the cathode, where it is produced chemically (eq 1), or at the anode, where it is formed chemically or electrochemically (eq 2 and eq 3). A hydroxyl radical is generated by the decomposition of the H2O2 (eq 4). At the cathode: H 2 + O2 → H 2O2

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(chemically formed)

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Figure 2. Propagation of side chain fluororadical to induce main chain scission proposed by Coms.84 Reprinted with permission from ref 84. Copyright 2008 Electrochemical Society.

At the anode: H 2 + O2 → H 2O2

(chemically formed)

2H+ + O2 + 2e− → H 2O2

(2)

(electrochemically formed) (3)

E° = 0.672 V 1 H 2O2 → HO• 2

where X represents the species responsible for membrane degradation. Depending on the catalyst location, the source of H2, O2, or both is from the reactant crossover through the membrane. Regardless of how the hydroxyl and peroxyl radicals are formed, it appears that they are responsible for ionomer degradation.35 Cipollini93 proposed that the peroxyl radical is less reactive than hydroxyl radical and will attack only the polymer end group, not the C−S or C−O−C bonds, and suggested that the relative rate constants of reaction on energetic grounds are as follows:

(chemically formed)

(4)

For the second radical generation mechanism, LaConti et al. proposed the following in eqs 5−9:28,87 H 2 → 2H•

(via Pt catalyst)

H• + O2 (diffused through PEM to anode) → HO2•

•OH + H 2O2 > •OH + −COOH

(5)

> •OOH + −COOH

(6)

≫ •OH + −CS

HO2• + H• → H 2O2

The PFSA membrane degradation is proposed to proceed via the following three steps, which will be summarized in the following sections: A. An unzipping reaction at unstable carboxylic acid polymer end groups B. A radical attack of the C−S bond in the side chain C. Scission of the main chains of the PFSA polymer 2.3.2. Degradation Mechanisms. 2.3.2.1. End Group Unzipping. Chemical degradation of PEMs is thought to be initiated through the carboxylate end groups formed during PFSA polymer synthesis, which contain H-bonds that are vulnerable to radical attack.35,94 Radicals abstract the hydrogen, which initiates the unzipping reaction, producing a perfluorocarbon radical (eq 11).2 The perfluorocarbon radical may then combine with a hydroxyl radical, producing an alcohol that rearranges to yield hydrogen fluoride and an acid fluoride (eq 12). The acid fluoride is hydrolyzed, generating another carboxylate end group along with another equivalent of HF (eq 13).

(can diffuse into PEM, especially at points where degradation has already begun)

H 2O2 + M2 + → M3 + + OH• + OH−

(7) (8)

OH• + H 2O2 → H 2O + HO2• (hydrogen peroxide radical attacks PEM)

(9)

In the above mechanism, oxygen molecules diffusing through the membrane form active radicals on the platinum catalyst at the anode due to incomplete ORR.88,89 The OH• radical has been detected, via spin trapping, on the cathode side of a PEM fuel cell using electron paramagnetic (EPR) spectroscopy. Hydroxyl and superoxide radicals are regular intermediates of the ORR reaction, which is not a problem as long as they remain attached to the catalyst surface. Although H2O2 is generally considered to be indirectly (H2O2 decomposition to active oxygen radicals) responsible for membrane degradation,28,35,90 recent results have shown that production of H2O2 may not be the dominant source of PEM chemical degradation in an operating fuel cell and that alternative species are produced at the catalyst from the H2 and O2 reaction.91,92 A general form of the overall reaction, which is actually a complex multistep reaction, is shown in eq 10.

H 2 + O2 → X

R f CF2COOH + OH• → R f CF2• + CO2 + H 2O (11)

R f CF2 + OH → R f CF2OH → R f COF + HF •

•

(12)

R f COF + H 2O → R f COOH + HF

(13)

Because the carboxylate end group is regenerated in eq 13, the reaction can propagate, and after the radical depolymerization

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Figure 3. Mechanism of degradation of Nafion sulfonic acid group proposed by Ghassemzadeh et al.97 Energetics are from DFT calculations by Yu et al.102 Reprinted with permission from ref 97. Copyright 2010 American Chemical Society.

Figure 4. Proposed degradation mechanism by Yu et al.102 involving OH radical attacking the Nafion sulfonic acid group. Reprinted with permission from ref 102. Copyright 2011 American Chemical Society.

located on the −SO3 group as −SO3H. The hydroxide radical can abstract the proton to form −SO3•.84 The −SO3• weakens the C−S bond, which will fragment to form the perfluororadical (Figure 2). The resulting side chain terminal fluororadical has been reported previously in ESR studies.100 Once formed, the fluororadical will follow the degradation scheme outlined in Figure 2, eventually forming an oxyradical that is bonded to the main chain of the PFSA backbone. This oxyradical can fragment to yield an acyl fluoride and another fluorocarbon radical. In this mechanism, initiation of side chain degradation only occurs in very dry conditions, although propagation of the degradation through the unzipping process can occur during relatively wet conditions. The sulfonyl radical initiation mechanism described above as proposed by Coms84 is consistent with experimental observations. For example, perfluorosulfonic acid model compounds are unreactive in the solution Fenton’s test,66,95 which could be explained by the complete ionization of the sulfonic acid groups, which are thus unable to form a sulfonyl radical through reaction with a hydroxyl radical or with hydrogen peroxide. In addition, when all protons in a PFSA membrane were exchanged for alkali metal ions, the membrane was stable in OCV conditions.101 Based on the above mechanism, even though this metal-exchanged membrane cannot conduct protons, the reactive species are still produced but cannot damage the membrane because the acid group is deprotonated.

reaction begins at an end group, the PFSA will decompose to HF, CO2, and other compounds having low molecular weights. Model compounds having no carboxylate end groups decompose several orders of magnitude more slowly than compounds having carboxylate end groups.95 Curtin et al.35 minimized the H-containing reactive end groups of Nafion by exposing the polymer to fluorine gas. When membranes with 61% of the H-containing end groups removed were exposed to >50 h of Fenton testing, fluoride ion emission decreased by 56% compared with an untreated polymer. Similarly, Hicks eliminated −COOH containing end groups of the 3M perfluorinated ionomer and, after an accelerated life test, found an 89% improvement in lifetime.94 2.3.2.2. Side Chain Attack. Postfluorination of the ionomers has not led to significant improvements in membrane chemical stability during fuel cell operation.93 Additionally, when Conradi96 correlated the amount of fluoride released to the number of −COOH end groups, there was a nonzero intercept. These findings strongly suggest that in addition to the end group degradation mechanism, other degradation pathways must exist. In addition to the radical attack of the end group, several researchers84,93,96−99 proposed that, under certain conditions, hydroxyl radical attack of the C−S bond in the polymer occurs. Coms75 proposed two main chain scission mechanisms based on side chain attack, with the first based on sulfonyl radical initiation. Under dry conditions, the acidic proton in Nafion is 6081

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Figure 5. Proposed degradation by Yu et al. involving H radical attacking Nafion side chain. Reprinted with permission from ref 102. Copyright 2011 American Chemical Society.

Figure 6. Proposed mechanism for the side chain attack at the C−S bond. Reprinted with permission from ref 93. Copyright 2007 Electrochemical Society.

The hydrogen radical can also react with the sulfonate group,102 forming a radical product that will continue to decompose the Nafion side chain, similar to step 2a in Figure 4. This reaction results in the formation of H2SO3 and SO2, which were both observed in mass spectroscopy of the cathode exit gas. Cipollini offered an alternate side chain degradation mechanism.93 Peroxide generation and free-radical formation occur immediately as reactants are applied to the cell. Because there are more side chains than end groups and Pt deposits mainly in the hydrophilic regions of the membrane, in most of the locations where hydroxyl radicals are generated, no end group is present, enabling long reaction times of hydroxyl radicals with the side chains, which are present everywhere the hydroxyl radical is generated.93 The C−S bond of the side chain is the next lowest in energy after the end group and is capable of being broken by hydroxyl radicals.93 Based on this, Cipollini proposed that the C−S bond is slowly hydrolyzed to remove the sulfonate group and convert the adjacent −CF2 to a −COOH (Figure 6). The −COOH will then serve as another reaction center for the decomposition of the polymer analogous to the end group. The hydrolysis of C−S bonds, introducing new −COOH reaction sites into the polymer, will result in accelerated degradation. The −COOH reaction sites work their way up the side chain in a manner equivalent to that suggested above for radical attack of the end groups. Eventually, the backbone will be broken, introducing two more −COOH end groups onto the backbone. Several researchers104,105 proposed another membrane degradation mechanism, in which the ether linkages on the side chains are proposed to be the sites most vulnerable to radical attack, generating −COOH groups. This decomposition results in polymer molecular weight reduction and increased numbers of −COOH groups. Loss of a significant number of sulfonate groups or whole side chains would affect the proton conductivity of the membrane. Although membrane conductivity/resistivity is a property of the material, membrane conductance/resistance is dependent upon thickness. A decrease in membrane resistance

Also using a sulfonyl radical initiation mechanism, Ghassamzadeh et al.97 proposed the mechanism in Figure 3 to describe degradation of Nafion side chains in the presence of hydroxyl radicals. However, from enthalpy calculations using density functional theory (DFT), Yu et al.102 found high barriers for the final two steps of this mechanism, making it unlikely to occur at a typical 80 °C operating temperature. Kumar et al.103 proposed a similar degradation mechanism and also used DFT to calculate its energetics, finding three steps in the mechanism have high barrier values. To resolve the high barrier issue in the mechanisms proposed by Ghassemzadeh and Kumar, Yu et al.102 proposed a new side chain degradation mechanism that results in low barriers. They offered two mechanisms by which the hydroxyl radical can break the C−S bond, through attack on either the C or the S atom (Figure 4), and also looked at protonated and unprotonated sulfonic acids. They expect attack on the sulfur of the deprotonated sulfonic acid group to be the dominant first step. Following the C−S bond breaking, an unzipping reaction occurs where an epoxide is formed that breaks off from the side chain, and this propagates until all ether groups have been removed from the side chain. The epoxides can react with water to form tetra(ethylene glycol), which is seen in exit water by NMR. Alternatively, the tetrafluoroethylene can dissociate from the side chain (Figure 4, step 2b). Although the products from the mechanisms proposed by Yu et al.102 in Figure 4 agree well with exit water analysis for the most part, HF has been observed in the exit water stream but is not a product in the mechanisms proposed in Figure 4. Therefore, Yu et al. proposed a second Nafion side chain degradation mechanism with a low barrier that describes HF formation (Figure 5). In this mechanism, H2 crossover gas reacts with hydroxyl radicals to form a hydrogen radical. Although H• is not as aggressive as OH•, the hydrogen radical is very reactive and can react with fluorines bonded to secondary or tertiary carbons to form HF. After HF and the carbon radical are formed, the ether bond can break to form a ketone and a carbon radical. This mechanism explains the experimental observation that greater degradation occurs when there is increased H2 in the system. 6082

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CF2 + 2H 2 → CH 2 + 2HF

is observed when it is plotted against decay time for two Nafion 117 cells, but because the membrane thinned, when corrected for membrane thickness, the resistivity is constant. LaConti et al.28 established that the polymer equivalent weight was unchanged and that products containing fluorine, carbon, and sulfur were released in a fixed ratio. The implication of these results is that degradation destroyed entire polymer units, including the sulfonated groups, that the attack is localized, or that the number of side chains that are attacked is too small to make a measurable difference.93 2.3.2.3. Main Chain. In addition to a side chain degradation mechanism, Coms84 also proposed a main chain degradation mechanism. As described in step 1 of Figure 5, H•, a very reactive species capable of damaging PFSAs, is generated in the reaction of OH• with H2. Like in the side chain mechanism, the hydrogen radical can attack any secondary or tertiary C−F bonds on the main chain in PFSAs.106 A reaction mechanism of main chain scission through hydrogen radical abstraction of the F atom is shown in Figure 7. The mechanism proposed here is

(14)

The −CH2− groups resulting from this reaction will then be attacked by radicals. This exchange of fluorine for hydrogen in the membrane makes it more susceptible to radical attack. This theory is supported through examination of the anode side of an MEA sample using X-ray photoelectron spectroscopy (XPS) in which the −CF 3− peak shifts due to CF n group hydrogenation during fuel cell testing.110 2.3.3. Methods of Measuring Degradation. 2.3.3.1. Fluoride Release Rates. In the above degradation schemes, one product of the reactions is HF. Measuring the fluoride content of the water from the fuel cell exhaust and using the value to generate a fluoride release rate allows determination of the rate of PEM chemical degradation throughout fuel cell operation.66,111 The fluoride emission rate is recognized as an accepted gauge of the degradation rate and therefore the condition and longevity of the membrane. The analysis of fluoride content is generally carried out by collecting exhaust water periodically and using ion chromatography or a fluoride ion selective electrode.49,66,87,90,110 Additionally, continuous monitoring has been carried out using a process based on a fluoride ion selective electrode.112 In a membrane that is 25 μm thick, the initial fluorine content is ∼3.8 mg F/cm2.66 Therefore, the average release rate of 0.01 μg F/(cm2 h) when using very mild conditions18,66 results in loss of 2% of the total fluorine content after operating for 6000 h. A fluoride release rate of 3 μg F/(cm2 h) using harsh conditions would result in complete fluoride loss within 1200 h of operation. Generally, the pH and pF of the effluent water correlate very well, meaning that pH decreases often signify increased degradation of the membrane. However, the fluoride emission rate data can show substantial scatter.66 2.3.3.2. Hydrogen Crossover Rates. Increased gas crossover results from both increased chemical and increased mechanical degradation. The hydrogen crossover can be measured in situ electrochemically. 113 Oxygen crossover is not typically evaluated in situ, but the oxygen permeability in PEMs is generally half that of hydrogen.20,28,114,115 To measure the hydrogen crossover, hydrogen is applied to the fuel cell anode, nitrogen is used to purge the cathode, and a voltage is applied. The applied voltage oxidizes the hydrogen that permeates through the PEM to the cathode. The current generated based on the hydrogen oxidation depends on the hydrogen crossover rate through the PEM. The hydrogen crossover rate is affected by the hydrogen partial pressure at the anode, temperature, and membrane thickness and permeability. For a Nafion 112 membrane, which is ∼50 μm thick, a typical beginning-of-life crossover current at atmospheric conditions is 1 mA/cm2, which converts to 2.6 × 10−13 mol H2/(cm kPa s).5 Researchers at W.L. Gore and associates found that fuel cell life of their GoreSelect membrane was limited to 26 300 h due to hydrogen crossover leading to membrane degradation. During the life test, which was three years of uninterrupted operation, the cell temperature was 70 °C, the pressures at the reactant outlets remained ambient, and stoichiometric quantities of reactants were supplied at 100% RH. In this case, end-oflife conditions corresponded with values of ∼13 mA/(cm2 bar).116 According to the DOE accelerated stress test protocols,117 a cell is considered to have failed when the crossover is above 20 mA/cm2.

Figure 7. Proposed main chain scission process initiated by fluorine atom abstraction by hydrogen radical by Coms.84 Reprinted with permission from ref 84. Copyright 2008 Electrochemical Society.

consistent with many experimental observations. For example, it has been observed that the number of carboxyl groups is much higher on the anode side of the MEA, which may be attributable to high H2 gas concentration.86,107 Another investigation observed that the degradation rate is more sensitive to hydrogen concentration than oxygen concentration, suggesting an additional kinetic role for hydrogen gas besides generation of reactive species at the catalytic surface.108 Additionally, in situ fuel cell ESR spin trapping experiments have detected the hydrogen atom.107,109 Besides radical formation, the presence of hydrogen gas could have other damaging effects. The PFSA polymer backbone may react upon exposure to H2 gas even without vulnerable functional groups, as follows:30 6083

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2.3.3.3. Analytical Studies. FTIR, nuclear magnetic resonance spectroscopy (NMR), and mass spectroscopy (MS) have been used to identify products from test effluent water and the residue of degraded MEAs. These methods have identified the major product as being derived from the side chain with features that are consistent with a mechanism that involves perfluorinated ionomer backbone degradation.66 2.3.3.4. Open Circuit Voltage. The open circuit voltage is another measure of reactant crossover. However, it is not as selective or quantitative as the electrochemical hydrogen crossover method. The OCV provides a measure of the membrane’s condition and can be correlated to the fluoride release rate. Stabilized MEAs show a basically constant OCV and negligible fluoride emission.118 Under accelerated OCV testing at 80 °C/40% RH, Inaba et al. observed an average voltage degradation of 83 μV/h and an average fluoride release rate of around 0.1 mg/day.119 2.3.3.5. High-Frequency Resistance. Membrane resistance contributes significantly to the high-frequency resistance (HFR) of a PEM fuel cell. The GDLs and current collector plates also contribute to the HFR. Impedance spectroscopy is used to monitor HFR in situ.50,116 Even though the HFR measures membrane resistance changes that result from degradation, it is not a very informative indicator of the extent of degradation.87,116 The value of HFR depends on the mechanism of degradation. Chemical degradation can result in both reduced proton conductivity and membrane thinning, which compensate each other in terms of resistance. Additionally, if degradation is the result of chemical impurities, these can impact the HFR of the membrane. So, although it is worthwhile to monitor HFR, its value in membrane degradation characterization is limited. 2.3.4. Impact of Impurities on Chemical Degradation. PEM fuel cell contamination can also adversely impact cell performance and life. Contamination occurs when impurities infiltrate components of the cell or initiate chemical degradation and slow down the desired processes occurring in the cell. The contaminants either are introduced from cell components or are carried into the cell by the reactant gases. Even trace amounts of impurities degrade performance and durability considerably. Table 1 lists the most common impurities and their sources.120 The performance loss and degradation mechanisms due to these impurities vary, depending upon the chemical composition of the impurity. Contamination mainly impacts the PEM conductivity and the catalytic activity of both electrodes. The

mechanisms by which impurities affect membrane durability and performance generally include (1) a reduction in proton conductivity, (2) a reduction of water content, (3) a catalyst for Fenton’s reaction, and (4) a place where cracks occur. Cationic impurities, including alkaline metals and ammonium, can infiltrate the PEM, considerably reducing performance.22,121 Because the sulfonic acid group in PFSA membranes shows stronger affinity with many cations compared with H+, when cationic contaminants are present during fuel cell operation, many active sites will be occupied by ions, and therefore, membrane properties including proton conductivity, swelling, and H+ transference numbers will change linearly with the ionic charge of the cation. In their model, Kienitz et al.121 showed that the concentration of cationic impurities on the cathode side of the cell is always higher and therefore concluded that the decreased performance with cationic impurities results from reduced proton flux at the cathode. Contamination by metal ions, such as Fe2+ and Cu2+, arising from metal bipolar plate or end plate corrosion, can catalyze the reactions that result in radical formation, strongly accelerating the chemical degradation of membranes in a PEM fuel cell, as shown in following equations:119

potential impurities

H2S NH3 SOx NOx volatile organic compounds (VOCs) anions cations

(15)

Fe2 + + HO• → Fe3 + + OH−

(16)

H 2O2 + HO• → HO2• + H 2O

(17)

Fe2 + + HO2• → Fe3 + + HO2−

(18)

Fe3 + + HO2• → Fe 2 + + H+ + O2

(19)

The exchange of H+ with contaminant cations also accelerates and increases the extent of membrane dehydration, particularly near the anode.122 The membrane dehydration will result in membrane degradation. As summarized above, membrane chemical degradation mainly results from attack of the radicals created during PEM fuel cell operation. The presence of even small amounts of metal impurities can dramatically accelerate radical formation and consequently substantially accelerate membrane degradation. Therefore, decreasing the amount of metal impurities or incorporating a radical scavenger into the electrode or membrane can considerably decrease the membrane chemical degradation.10 2.3.5. Effect of Catalyst. The catalyst has a strong impact on membrane degradation. Experiments by Ghassemzadeh97 showed that Nafion does not degrade when it is exposed to hydrogen and oxygen gases without Pt. Similarly, membrane degradation studies at open-circuit conditions have revealed that membrane degradation is insignificant in the absence of H2, O2, or catalyst.123,124 With only a change in catalyst and all the other components of the fuel cell remaining the same, a 75 times lower membrane degradation rate (measured by F− release rate) has been demonstrated.125 When catalyst is coated on the anode only, or in the middle of a bilayer membrane, a long period of time is required for the membrane to start degrading.123 However, if Pt/C catalyst pre-exposed with O2 is used, the FER from the anode-only and bilayer membrane cells increases immediately and is comparable to that in the cathode-only mode. Thus, H2, O2, and catalyst are all required for membrane degradation.

Table 1. Origin of Common Impurities120

COx

H 2O2 + Fe2 + → HO• + OH− + Fe3 +

origin H2 production processes, carbon corrosion, fermentation, respiration, industrial processes H2 production, natural sources, byproduct of many industrial processes H2 production, farming industrial processes, fuel combustion fuel combustion, farming farming, burning of biomass, fuel combustion airborne salts, halide containing educts used to synthesize fuel cell catalysts fuel cell components, fuel cell accessories, electrocatalysts in fuel cells, airborne 6084

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chemical23,24) involving protons, as shown in eqs 20 and 21. In the Darling−Meyers model, although the oxide layer prevents platinum dissolution, the oxide formation kinetics are slow compared with the dissolution rate, so bare platinum is exposed to corrosive potentials with rapid changes in potential.

Hydrogen and oxygen appear to react at the catalyst to form a species that attacks the polymer and degrades the membrane. Carbon as a catalyst support does not significantly influence the reaction (comparable FER with Pt-black catalyst),123 although some surface characteristics of the Pt catalyst appear to influence the reaction. Because some researchers feel that degradation rates do not depend on the Pt/C catalyst location (anode, cathode, or in the membrane),101 the reaction is most likely chemical in nature and not potential-dependent. 2.3.5.1. Pt Band Formation. 2.3.5.1.1. Evidence of Pt Band Formation. Migration of catalyst particles into the membrane has been observed on numerous occasions.20,21,50,126−132 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are frequently used to investigate the size, shape, and distribution of platinum particles in the membrane and electrodes, and membrane and electrode thinning.5,93 2.3.5.1.2. Mechanism of Pt Band Formation. In a fuel cell, the cathode side of the MEA experiences a chemically oxidizing environment while the environment on the anode side is chemically reducing. Evidence shows that Pt dissolves from the cathode and deposits inside the membrane to form a Pt band.24,50,131,133,134 Guilminot et al.127 showed that Pt particles inside the PEM have a very different size and shape from that of fresh Pt/C electrocatalyst, which makes mobility of Pt nanoparticles from the electrode to the PEM very unlikely. In addition, hexachloroplatinate ions (PtCl62−) and tetrachloroplatinate ions (PtCl42−) were detected in the decantation solution of aged membranes, which demonstrated that Pt2+ and Pt4+ ions were also present inside the PEM. Because the membrane is electrically disconnected from the electrodes, the only possible reduction mechanism of Ptz+ ions is chemical reduction by hydrogen molecules permeating across the membrane. Pt Dissolution. Platinum solubility measurements provide evidence of platinum particle growth mechanisms. Pourbaix diagrams reveal that Pt metal is stable in solutions of a wide range of pH.135 The only exception is at 298 K when the pH < 1 and E > 0.9 V, where Pt cations are the stable form of Pt. Several articles have reported that platinum area loss over time in phosphoric acid fuel cells (PAFCs) is potential-dependent and that platinum is highly soluble in concentrated phosphoric acid at 176−196 °C and from 0.80 to 0.95 V.136−138 Pt solubility increases ∼100 times as potential increases from 0.7 to 1.0 V.139 Wang et al.140 reported that the concentration of dissolved Pt grew monotonically from 0.65 to 1.1 V and decreased when the potential rose above 1.1 V, which was ascribed to protective oxide film formation. The dissolution rates determined for static potentials are three to 4 orders of magnitude lower than those reported in the literature for potential cycling experiments. This means that, as a fuel cell is potential cycled (particularly from ∼0.7 V to OCV (∼0.96 V)), Pt will dissolve at OCV and then precipitate as the potential is reduced. These observations support the experimental results that Pt agglomerates grow with potential cycling rather than time at high potential.141 Darling and Meyers23,24 generated mathematical models for platinum dissolution and redeposition in PEM fuel cells. They claimed that potential, particle size, and oxide coverage ratio control the dissolution of platinum. Isolated Pt particles will not grow if Pt0 solubility is negligible. Platinum corrodes through an electrochemical/chemical mechanism (platinum dissolution is potential-dependent and platinum oxide dissolution is

Pt(s) + H 2O ↔ PtO(s) + 2H+ + 2e−

(20)

PtO(s) + 2H+ ↔ Pt2 + + H 2O

(21)

Pt Migration. The characteristics of the dissolved platinum species are not conclusively understood. From potential−pH diagrams, the dissolved species in acidic mediums are generally accepted to be a hydroxyl or aquo complex of Pt2+.139 Dissolution of Pt results in the formation of a supersaturated Ptz+ solution at the cathode. Very little is known about the nature of the counterions that accompany the Ptz+ species during their migration from the location where they dissolved. Because platinum salts that contain halides are commonly used in Pt nanoparticle synthesis, counterions such as chloride, bromide, or fluoride may remain on the carbon support and form complexes with Ptz+ species. Halides may also activate platinum oxide dissolution because the normal potential for platinum electro-oxidation is decreased in the presence of platinum complexants.142 Fluoride ions are very good candidates as the Ptz+ counterion because they have been detected in PEM fuel cell exhaust water87,90 and outlet gas,143 even without any natural contaminants on the carbon support. Sulfur-containing anions such as SO42−, which have also been detected in the cathode exhaust water may also act as the Ptz+ counterion.66,87,143 Sulfate species may be formed by oxidation of sulfur (a natural contaminant of carbon) in the presence of heat and water and have been revealed to be products of Nafion chemical/electrochemical decomposition.66,144 With the counterions, the Ptz+ ions are complexed onto high-surface-area carbon blacks to form extremely mobile species that can be transported across the PEM by way of electro-osmotic drag or diffusion.127 As platinum migrates into the membrane from the cathode, it is repeatedly oxidized/dissolved and reduced/deposited by diffusing O2 and H2, respectively. Pt particles are driven toward the anode by oxidation and dissolution from the crossover oxygen. The Pt ions are reduced by crossover hydrogen, stopping their movement toward the anode. Pt Deposition. Pt precipitates inside the membrane, inside the electrode, and at the cathode/membrane interface via reaction with crossover H2 (eq 22). The Pt precipitated inside the membrane/ionomer phase is electrically disconnected from the electrode and, therefore, does not participate in the electrochemical reaction.145 H 2 + Pt+2 → 2H+ + Pt

(22)

21

Bi et al. proposed that the location where platinum precipitates is primarily controlled by the amount of hydrogen permeating from the anode toward the cathode. When the oxidized platinum ions (Pt2+, for example) are chemically reduced by crossover hydrogen to precipitate platinum particles in the membrane, the hydrogen concentration is reduced significantly. According to Bi et al., calculation of the location where all reactant crossover would be depleted gives an estimate of the location of platinum precipitation, as illustrated in Figure 8. This location in the PEM depends upon the partial pressures of H2 and O2 and their permeability through the membrane.50 6085

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particles grow atomically when dissolved Pt ionic species are deposited on nuclei of previously deposited Pt. A uniform supply of Pt ions to the nuclei will result in an equilibrium crystalline shape, dependent upon the surface energy of the platinum, as depicted in the top scheme in Figure 10, which

Figure 8. A diagram of the partial pressures of hydrogen and oxygen across a fuel cell membrane, reproduced with permission from ref 21. Copyright 2007 Electrochemical Society. According to the authors, the Pt band forms when the partial pressures become zero.

Burlatsky et al.146 performed OCV tests using different hydrogen and oxygen partial pressures to intentionally change the site of the Pt band. They found that the Pt band forms where the potential in the membrane abruptly decreases from the cathode potential (∼0.9 to 1 V) to the anode potential (∼0 V). Few studies investigating the potential profile within a PEM fuel cell membrane have been published. Liu and Zuckerbrod147 used Pt microprobes to determine the potential profile inside the PEM of a fuel cell experimentally. They discovered that the potential was almost zero for ∼80% of the membrane thickness, and then rose sharply to the cathode potential (∼0.9 to 1 V). Atrazhev et al.148 generated a model explaining the PEM potential profile Liu and Zuckerbrod observed, assuming that the Pt microprobe was measuring a mixed potential resulting from the electrochemical reaction of crossover H2 and O2. They found that when the surface area of the Pt particle is small enough to not disturb the distribution of gases in the membrane, the mixed potential initially increases, then ranges from 0.3 to 0.4 V through most of the membrane, and last increases to the potential of the cathode. However, as platinum particles in the PEM become larger (>10 nm), the electrochemical reactions on the platinum increase in speed so that the process becomes diffusion limited, resulting in a sharp potential increase at ∼82% of the membrane thickness. Schematics of the potential profiles proposed in this model are shown in Figure 9.149 Even though this model was constructed particularly for OCV conditions, it provides valuable understanding of the Pt deposition processes occurring in the membrane. Pt particles detected in aged membranes are often flowershaped127 or faceted.25,50 Akita et al.133 proposed that Pt

Figure 10. Diagram representing potential growth routes of Pt agglomerates in the PEM, reproduced with permission from ref 133. Copyright 2006 Elsevier. In this drawing, the size of the arrows represent the relative supplies of Pt ions to the nuclei.

shows a diagram representing a potential growth route of Pt particles in the membrane, reproduced from Akita et al.133 They attributed cubic shaped Pt particles to Nafion acting as a surfactant and modifying the surface energy of the growing Pt particles. The flower-shaped patterns, consistent with dendritic growth, could result from a varying supply of Pt ions to the nuclei, which could be due to the ionic clusters in the PEM, as depicted in the bottom scheme in Figure 10.133 Ferreira et al.126 observed that dendritic Pt nanocrystals dominate where soluble Pt species are reduced and then incorporated onto a rough surface faster than they diffuse to the surface of the dendritic Pt nanoparticles. Faceted type Pt shapes dominate where the Pt ions diffuse in the ionomer faster than they add onto the faceted surfaces. Three distinct faceted shapes formed in the ionomer of PEM fuel cells, namely, truncated square cuboid, truncated octahedron, and truncated tetrahedron, which have increasing ⟨111⟩Pt to ⟨100⟩Pt surface ratios. The different faceted shapes have been attributed to differences in the local partial pressure of H2 at the Pt particle. 2.3.5.1.3. Effect of Operating Parameters on Pt Band Formation. Operating conditions, including the electrode potential, temperature, and humidity greatly influence catalyst degradation. In general, operation at high potentials and high RH and voltage cycling all accelerate Pt dissolution and sintering.25,29,150 Voltage cycling, rather than extended holds at constant potentials, of Pt/C catalysts in aqueous acids accelerates platinum dissolution.21,23,25,150−153 The cathode potential generally ranges from ∼0.6 to 0.95 V in an automotive fuel cell during normal operation. However, during start-up and shutdown, the voltage may increase to as high as 1.5 V.154 Although high voltages promote Pt oxide layer formation, which reduces the platinum dissolution rate,155 voltage cycling to continually form and remove this layer results in a very high dissolution rate. After cycling from 0.35 to OCV, Cho et al.153 observed severe degradation of the catalyst layers whereas MEAs under constant voltage experienced no degradation. Image analysis of a histogram resulting from a TEM image of an MEA that was tested in a PEM fuel cell, cycled from low to high current several tens-of-thousands times, found that 5−10 nm particles were uniformly distributed in the membrane, with a skewed (toward cathode) Gaussian distribution of larger

Figure 9. Potential inside the membrane, calculated according to Atrazhev et al.,148 and reproduced with permission from ref 149. Copyright 2009 Elsevier. Line 1 (···) represents the potential for a platinum particle that does not disturb the H2 and O2 concentrations in the membrane. Line 2 (−−−) represents the potential when the mixed potential is diffusion limited. Anode is on the left, and cathode is on the right. 6086

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particles.148 After potential cycling between 0.6 V and OCV, Schulenburg et al.152 observed formation of a Pt band in the middle of the membrane, which contained 16% of the initial cathode Pt loading. Because Pt is oxidized by water in a fuel cell, humidity is also an important factor affecting catalyst degradation.156 For example, operation at 25% RH rather than 100% RH largely reduces Pt surface area loss.29 The degree of platinum oxidation on PEM fuel cell cathodes increased considerably when RH increased from 20% to 72%.156 Platinum particle growth is accelerated in a liquid environment,157−160 which is likely because the particle growth activation energy is reduced. Water molecules can infiltrate between the metal and the support, lowering the bonding energy of the metal/support and facilitating the metal migration.161 Temperature also affects catalyst degradation; catalyst coarsening rates increased linearly with temperature.141 It was estimated that as temperature increased from 25 to 80 °C in an acidic solution at 0.9 V, the Pt2+ equilibrium concentration increased by ∼100 times and increased an additional ∼10 times as temperature increased from 80 to 120 °C.162 Generally, reaction rates increase as temperature increases and therefore Pt-particles grow faster at higher temperatures.141 To gain further understanding of how operating conditions impact Pt deposition in the membrane, Kim et al.128 varied operating parameters during long-term operation. Operation with 3/4 stoichiometric ratios of hydrogen and air at 80 mA/cm2 for 1784 h resulted in the formation of a wide Pt band with particles ranging from ∼30 to ∼70 nm near the cathode because hydrogen and oxygen partial pressures were relatively low. With the same reactant feed rates, but at OCV, a narrower Pt band with particles ∼100 nm was formed further from the cathode because of higher crossover reactant gas concentrations. When pure oxygen was supplied to the system, and the cell was operated under a constant current of 80 mA/cm2, the Pt band was still sharp with larger particles but was further from the cathode compared with the previous operation. When the concentration of supplied hydrogen to the system was reduced from a stoichiometry of 3 to 1.5 and then to 1.35, a diffuse band of Pt particles ∼25 nm formed throughout the membrane. A schematic summary of the impact of operating parameters on the size and distribution of platinum in the PEM based upon the results of the work discussed in this section is shown in Figure 11. When high reactant flows or OCV operation are used, a sharp, narrow Pt band with large particles forms close to the cathode. When the partial pressure of oxygen is considerably higher than that of hydrogen, the Pt band is still sharp and narrow but is further from the cathode. When the partial pressures of both reactants are low and with constant current operation, a diffuse band of Pt forms throughout the membrane. Voltage cycling results in the formation of small particles uniformly distributed throughout the membrane, with a Gaussian distribution of larger particles skewed toward the cathode. 2.3.5.2. Does the Pt Band Result in Membrane Degradation? The effect platinum catalyst has on the generation of free radicals and degradation of membranes is under debate. Watanabe et al.163,164 observed that Pt particles in the membrane reduced membrane degradation by scavenging H2O2 or radicals using both H2-rich and O2-rich conditions. They asserted that Pt dispersed in the membrane resulted in improved membrane durability.165 On the other hand, after investigating membranes with and without a

Figure 11. Size and distribution patterns of Pt in the PEM under various operation conditions. Adapted with permission from ref 128. Copyright 2008 Elsevier.

platinum band, Hasegawa et al.166 concluded that radical generation occurred at both band and electrode positions. Burlatsky et al.146 and Stucki et al.167 have proposed mechanisms of hydroxyl radical formation on platinum surfaces. Burlatsky et al. proposed that peroxide and hydroxyl radicals formed concurrently on platinum particles depending on the potential, with ∼0 to 0.4 V favoring peroxide and >0.5 V favoring hydroxyl radical. Many researchers have observed that Pt band formation results in increased membrane degradation. Ohma et al.168 observed that when a Pt band was visibly formed, the membrane around it was extensively degraded, while the membrane was relatively unchanged when no Pt band was formed. There was a strong correlation between the magnitude of the FER of the exhaust water collected from each electrode with the Pt band location.168 Atrazhev et al.148 also found, using TEM after OCV testing, that the Pt band location correlated with the magnitude of the FERs from both electrodes. When the Pt band was closer to the anode than the cathode, the anode side FER was greater than that from the cathode side.148 Zhao et al.169 found that when Pt was deposited into NRE 212 membranes, the FER was increased compared with pure NRE 212. Additionally, degradation was highest when Pt electrodes were coated on the membrane, suggesting that catalyst layer Pt accelerated PEM degradation. In summary, these researchers found that Pt band formation in the membrane enhances membrane decomposition, and that the magnitude of the FERs from each electrode, which is regarded as an indicator of membrane degradation, varies with the Pt band location. Based on these results, the Pt band formed in the membrane is a factor accelerating membrane degradation. In contrast to the above, Endoh et al.86 determined that the location of chemical degradation in the PEM did not correlate with the Pt band position, purified Pt black did not generate hydroxyl radicals, the membrane was not decomposed by Pt with a clean surface, and when 0.03 mg/cm2 Pt particles were dispersed in the membrane, they actually decreased membrane decomposition. Purified Pt black resulted in less chemical degradation when used in the electrode during an OCV test than Pt/C. From these results, the authors concluded that the Pt particles deposited in the PEM do not contribute to the degradation of the membrane. However, it is important to note that the contrasting results with regard to the effect Pt has on membrane degradation may be due to differences in size and 6087

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distribution of the particles. Bonakdarpour et al.170 observed that O2 molecules are more efficiently reduced to water, without a hydrogen peroxide intermediate, when the catalyst density increases. Gummalla et al.171 developed a physics-based theoretical model predicting chemical degradation of PFSA membranes during fuel cell operation depending on platinum particle size and spacing between particles. The authors showed that reactant crossover is stoichiometrically favorable for hydroxyl radical production in the same location where the Pt band in the membrane preferably forms, resulting in a highly localized radical attack. The effect of Pt particle size on fluoride emission with 2.5 and 0.25 μm spacing is plotted in Figure 12.

than those of Pt/C, the alloy had higher resistance to dissolution. Improved membrane stability was observed for a cell containing PtCo/C compared with a cell containing Pt/C and was attributed to formation of a Pt band in the cell containing Pt/C.27,130 While alloying appears to improve catalyst stability, it is unclear whether the stability improvement results from improved thermodynamic stability of the catalysts or from differences in the Pt dissolution/oxide formation kinetics. To fabricate catalysts with increased stability, the catalyst degradation mechanism must be better understood, as does the role of alloying on the fundamental properties of the catalyst. 2.3.6. Localization of Degradation. Because the anode and cathode represent reducing and oxidizing environments, respectively, it is conceivable that the degradation mechanisms at each electrode may be different.173 The question of whether membrane degradation is localized toward one of the electrodes in PEM fuel cells has provided conflicting reports. Some investigations demonstrate increased membrane degradation closer to the anode, others found more degradation close to the cathode, and others found no distinction between the electrodes. Most probably this is due to variations in test conditions. The FER depicts the total rate of ionomer decomposition throughout the cell/stack and does not accurately represent the localized rate of ionomer degradation, which relies on conditions such as local temperature, RH, and impurity concentration. Modeling results reveal that differences in total fluoride emissions from the anode and cathode result from differences in tortuosity of fluoride ion transport encountered when fluoride ions diffuse from the point where they are produced to either the anode or cathode channels.174 Even when tests are replicated that demonstrate reproducible membrane life, FERs fluctuate up to 10 times, which could signify that the MEA material degradation rate varies in different regions. In an early study by Gore,49 the fluoride emission rate, the hydrogen permeability, and lifetime could not be correlated. Their reinforced membranes had fluoride emission rates during testing at 90 °C/75% RH and 0.8 A/cm2, ranging from 0.2 to 0.6 μg F/(cm2 h).49 The hydrogen crossover rate was 2 mA/ cm2 at the beginning-of-life and rose during testing to 5−13 mA/cm2. End-of-life, marked by crossover of 13 mA/cm2, was reached after 600 to 1600 h. Gore attributed the lack of correlation in the degradation parameters to localization of the degradation. Although during fuel cell testing homogeneous thinning is generally observed,93 Stucki and Scherer167 observed inhomogeneous thinning of Nafion in a PEM electrolyzer. The uniform thinning of the membrane has been attributed to localized attack resulting from a homogeneous distribution of Pt particles and the nonuniform thinning/pore formation has been attributed to localized attack resulting from a nonhomogeneous distribution of Pt particles, such as those found at the Pt band.93 2.3.7. Prevention of Chemical Degradation. Several measures can be employed to reduce PEM fuel cell membrane degradation. Possible methods to prevent degradation include avoiding metal contamination; decreasing the permeability of gases through the membrane; and improving membrane stability chemically, physically, or through the incorporation of radical scavengers/inhibitors. 2.3.7.1. Radical Inhibitors. A major membrane chemical degradation mechanism is through oxidative degradation by

Figure 12. Fluoride emission rate as a function of platinum particle size in the membrane for two platinum particle spacing in the membrane. The simulation conditions are 30% RH, OCV, H2/O2, and 90 °C. Reprinted with permission from ref 171. Copyright 2010 Electrochemical Society.

In this model, the authors found that when the particles are more spread out in the membrane (2.5 μm spacing), the membrane degradation increases with particle size up to 25 nm, beyond which it decreases. The nonlinear behavior of membrane degradation with increasing particle size originates from two competing reactions: generation and quenching of radicals on platinum. Hydroxyl radicals escape from the platinum surface more slowly with increasing particle size. However, when the platinum particles are small, the platinum surface area is too small to generate radicals when the platinum particles are spaced far apart. When the platinum particles are closer together (0.25 μm spacing), the membrane degradation decreases with increasing particle size. These modeling results can resolve the contrasting conclusions on whether the Pt band accelerates membrane degradation. Because the Pt loading was 0.03 mg/cm2 (corresponding to 70 nm particles spaced 0.25 mm apart) in the test of Endoh et al., this would result in low degradation. However when Pt particles are smaller (500 h, it was likely that no perforations developed due to improper cell assembly. Because humidity was generally high and kept constant, the membranes should have suffered minimal mechanical stress. Additionally, increased crossover/shorts and membrane thinning (observed in experiments 1, 2, 5, 9, 12, and 13) can result from mechanical or chemical degradation of the membrane. However, the actual causes of these observed degradations have not been identified. Several experiments (2, 5, 8−18) resulted in reduced ECA, increased Pt particle size in the electrodes, electrode thinning, and identification of Pt in the membrane. The increased Pt particle size is caused by catalyst sintering and Ostwald ripening and could result in reduced ECA. Reduction of ECA and electrode thinning also result from Pt migration from the electrode into the membrane, which is exacerbated at high potentials and potential cycling conditions. Chemical degradation is likely a cause of the majority of the cell failures. The increased backpressure in experiments 7, and 13−19 would result in not only mechanical stress on the membrane but increased crossover, which would allow more instances of hydrogen, oxygen, and Pt being in contact and therefore increased radical formation and increased degradation. The increased crossover, membrane thinning, development of shorts, and fluorine and sulfur in the effluent water, which are observed in experiments 1, 2, 5, 9, 12, 13, 15, 17, and 18 could all be due to the chain scission and end group attack chemical degradation mechanisms. The increase in voltage decay rate that is observed after 1900 h in experiment 5 may also be explained with the radical degradation mechanism because as the membrane degrades more gases can crossover, resulting in increased formation of radicals and, therefore, increased voltage decay rates.

Table 3 summarizes the durability data of select membranes from fuel cell life tests. Although Stucki et al. saw lifetimes of 15 000 h using an electrolyzer with Nafion 117 under 80 °C continuous operation,167 most PFSA lifetimes were significantly lower at ≤5000 h when alternative conditions or thinner membranes were used.35,122 For the other lifetime tests, inlet gases were hydrogen, except for one case, when reformate was used, and air or oxygen; cell temperatures ranged from 60 to 80 °C; humidity ranged from dry H2 and 75% RH air to >100% RH; back pressures ranged from ambient to 2 bar on the anode and cathode; and the voltage or current density ranged from 0.01 to 1.07 A/cm2 or 0.6 to 0.8 V. For these samples, the voltage decay rate ranged from 0 to −60 μV/h.

4. ACCELERATED STRESS TESTS The evaluation of membrane durability under normal fuel cell operating conditions, as described in section 3, is not always practical because of the time and resources involved. However, it is believed that an indication of the degradation behavior of an MEA can be ascertained by examining its characteristics over a prescribed amount of time under accelerated degradation conditions.2 Accelerated testing of PEM degradation during the operation of fuel cells is complicated because there are distinct degradation paths operating to various degrees in parallel, and there are numerous avenues for introducing the active ingredients that promote and control each path. These multiple paths present significant obstacles for accelerated test programs.

Table 2. Voltage vs Time for Cycle Profile of US DOE Dynamic Stress Test step

duration (s)

V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

15 25 20 15 24 20 15 25 20 15 35 20 35 8 35 40

OCV 0.8 0.75 0.88 0.80 0.75 0.88 0.80 0.75 0.88 0.80 0.60 0.65 0.88 0.75 0.88

DOE protocol.117 Although this protocol does not simulate all issues that would be experienced under automotive operation (for example, fuel cell behavior under start/stop conditions), it provides valuable insight about fuel cell durability for automotive-type transients.

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180

180 130 33

25

25

H2/air

reformate (70% H2, 25% CO2, 2% air bleed)/air H2/air H2/O2 H2/air

H2/air

H2/O2

5, 53, 191 6, 192

10, 128

11, 128 12, 195

6091

13, 196 14, 50 15, 197 16, 25 17, 87 18, 87

7, 192 8, 193 9, 194

50

H2/air

4, 94

50 50

25 50 50

H2/air H2/air

H2/air H2/air H2/air

H2/air

25, 50, 127, 178 32

H2/air

50

50 25, 50

H2/air H2/air

2, 94 3, 35

180

thickness (μm)

H2/air

application type

1, 167

expt no., ref

80, 1.5, 1.5 80, 2, 2 80, 2, 2

80, 2, 2 80, 1.4, 1.4

1.2

70

70

70

80, 2, 2 75, 1, 1 60, 1.2, 1

75

80

80, 1, 1 65, 1, 1

80

temp (°C), Panode (bar), Pcathode (bar)

humidified >100% >100%

75% RH air and dry H2 100% 100%

humidified

humidified

humidified

100% 75%

80%

>100%

>100% 100%

humidification (% RH)

Table 3. Durability of Nafion Membranes under Fuel Cell Operation

1000 529 1000 1000 2000 1916 1000

0.18 A/cm2 (0.67 V) 1.07 A/cm2 1.06 A/cm2 0.2 A/cm2 0.4 A/cm2 1.07 A/cm2

1384

1350 1800 347, 892, 1397 1784

5000

2700

3400

2300 → 20000 3000 >2500

lifetime (h)

10 mA/cm2

80 mA/cm2 (0.7 to 0.8 V) 80 mA/cm2

0.4 A/cm2 0.35 A/cm2 0.18 A/cm2 (0.67 V)

1 A/cm (startup), cont 0.6 V cont 0.8 A/cm2, cont 0.55 A/cm2 (0.62 V) 0.3 A/cm2 (0.66 V) 0.3 A/cm2

2

test conditions

seals deteriorated cell resistance increased, loss of fluorine and sulfur, CO increased ∼30×, ECA decreased 25%

−33 μV/h −2.5 μV/h (0−1900) −50 μV/h (1900−2700)

ECA decreased −4 μV/h

−4 μV/h

ECA reduction, crossover

crossover, electrical short, membrane thinning

observed changes

0 to −2 μA/(cm2 h) 0 μV/h

voltage or current density decay

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Figure 13. Degradation effects, conditions, and mechanisms for perfluorinated membranes. Adapted with permission from ref 5. Copyright 2008 John Wiley and Sons.

and 70% humidified, completely humidified and dry, and dry and dry were used, operation continued for 3300 h, 1100 h, and 300 h, respectively. Using only slightly under-saturated conditions, Yu et al. noted a high degradation rate of ∼60 V/ h, and a large hydrogen crossover increase from 1.35 to 27 mA/ cm2.53 When operating at under humidified conditions (50 °C, 60% humidified cathode, 100% humidified anode), Hori et al.198 observed that the largest OCV drop and the highest H2 crossover rate occurred at the fuel outlet/air inlet, showing that fuel cell degradation takes place preferentially in under saturated regions of the cell. An explanation for the increased degradation with low humidity conditions has not been fully developed and is under debate by many researchers. Low-humidity conditions accelerate degradation as a result of both mechanical and chemical mechanisms. Yu et al.64 suggest that insufficient water content from low feed stream humidification accelerates membrane mechanical degradation, ultimately resulting in membrane cracks, holes, or tears and increased reactant gas crossover. Also suggesting that low humidities accelerate mechanical degradation of membranes, in a recent study it was proposed that drying can induce mechanical stress onto the MEA and that steadily reducing ductility, along with strains caused by drying the MEA in a constrained environment, results in mechanical failure of membranes.65 On the other hand, chemical degradation is also accelerated under low-humidity conditions. As was mentioned in section 2.3.2.2, Coms84 proposed that low humidities are required for hydrogen abstraction from the end group, which is an important first step in a mechanism involving side chain fluororadical propogation to induce main chain scission. In addition, the hydrogen peroxide decomposition to form hydroxyl radicals is sensitive to pH.199 Changes in membrane humidification would result in changes in proton activity in the membrane, and would, therefore, impact the rate of hydroxyl radical formation,

Acceleration of the membrane degradation rate would imply that the degradation mechanism also depends upon these operating conditions. Insight into the mechanism can be obtained by a detailed investigation of these operating conditions. To date, several accelerated stress test protocols for transportation applications have been defined and promulgated. Each of these has been found to accurately represent specific long-term failure modes, some for specific and distinct degradation paths and others for specific combinations of degradation mechanisms. The commonly known accelerated tests are the OCV tests, RH cycling tests, potential cycling tests, and drive cycle tests. For PEM MEAs, the OCV test is designed to measure chemical degradation independent of other degradation mechanisms, while the RH cycle test is designed to measure mechanical degradation. Neither test can be completely separated from thermal degradation mechanisms. Accelerated stress tests for other fuel cell applications, including stationary, need further development. Because the requirements for other fuel cell applications are different, the failure modes and, therefore, the ASTs must be different as well. 4.1. Stressors

A number of stressors are thought to accelerate membrane degradation as a result of different degradation mechanisms. The stressors, degradation mechanisms, and effects are summarized in Figure 13. 4.1.1. Reduced Humidity. Managing water content and transport is a key factor in optimizing fuel cell reliability and durability. Insufficient water content, either locally at specific sites within the cell or throughout the entire cell or stack, results in reduced membrane conductivity and therefore increased ohmic losses and decreased cell voltage.17 Operating continuously at lower than 100% RH can result in increased membrane degradation. Researchers at Ballard17 found that when anodes and cathodes that were completely humidified 6092

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increased hydrogen crossover compared with constant current operation.205 Nakayama et al. observed that as the ratio between OCV and load increased, the hydrogen crossover increase started earlier.205 The severe chemical degradation that occurs under high cell voltage conditions is generally attributed to the reactant gas partial pressures being at their maximum values, resulting in high gas crossover. As described above, hydrogen and oxygen crossover bring about formation of radical species, which are believed to chemically degrade the electrolyte membranes.206 During OCV testing, chemical degradation results in high fluoride ion release rates, membrane thinning, reduction in OCV,66,207 and platinum migration.192,208 Crossover gases create mixed potentials at the electrodes and, therefore, reduce the OCVs. This is described in eq 20, where the OCV is composed of the Nernst potential of the reaction, Eeq, and a term associated with losses due to hydrogen crossover, ηix. The voltage loss due to crossover hydrogen can be modeled using a Tafel-like expression as shown in eq 21209 where ECA is the electrochemical surface area per geometric surface area (mPt2/ cmgeo2), io is the exchange current density per cm2 of platinum (A/cmPt2), and icrossover is the crossover current per geometric area basis (A/cmgeo2).

even if the level of hydrogen peroxide in the membrane remains unchanged. Furthermore, H2O2 boils at a higher temperature than water (150 °C), and Inaba et al.119 observed that the amount of sulfate and ferrous ions both increased with operation at low RH, both of which would accelerate membrane degradation. 4.1.2. High Humidification. Like insufficient humidification, excess humidification also leads to many possible problems. Excess water content can impede reactant diffusion, especially on the cathode, increasing mass transport losses. High humidification also increases the leaching of contaminants out of system and stack components, therefore increasing occasions to transport these into the cell, which can result in many degradation effects, including a decrease in cell hydrophobicity.1,200 Because the ionomer swells with water uptake, exposure of the membrane to high RH can compressively stress the membrane,68 which is proposed as an important factor in membrane mechanical failures. High relative humidity also impacts membrane permeability and therefore chemical degradation. Generally, permeability of PFSA membranes increases with increasing relative humidity.201,202 Aoki et al.163 showed that increasing RH without varying reactant gas partial pressures resulted in increased fluoride emission, which was expected due to an increase in membrane gas permeability at higher RHs. They also postulated that there was some contribution from increased H2O2 penetration. 4.1.3. Humidity Cycling. Changes in RH may lead to membrane mechanical degradation during real life operation. The shrinkage in Nafion NRE111 membranes from water saturated to dry states at 25 and 110 °C is approximately 10% and 11%, respectively. Because in fuel cells membranes are constrained, the dimensional changes are restricted, generating internal stress and straining.65 If the membrane is nonuniform, such as if it contains a pinhole, the stress on the membrane is more severe. Researchers at General Motors Corporation cycled RH from 150% to 0% at 80 °C using air at both electrodes to isolate membrane mechanical failure from chemical degradation,29 and showed that the repeated swelling and shrinking caused by cycling from wet to dry conditions resulted in membrane failures. Tang et al.203 cycled membranes between 25% RH and water saturated at 90 °C and evaluated the impact of shrinkage on the mechanical stress and strain of the PEMs, finding that the maximum stress during RH cycling on a Nafion 111 membrane was as high as 3.1 MPa. 4.1.4. High Temperature. Elevated temperature operation (>100 °C) significantly shortens PEM lifetimes to less than a few hundred hours.204 High-temperature operation usually takes place in conjunction with low-humidity operation. Mechanical failures such as formation of pinholes/tears were often observed in post-mortem characterizations. 4.1.5. Freeze/Thaw Cycling and Subfreezing Start-Up. Subfreezing temperature operation of fuel cells can place severe stress on PEM fuel cells. To evaluate membrane durability with thermal cycling to below freezing temperatures, McDonald et al.46 performed 385 ex situ temperature cycles between −80 and 40 °C on humidified Nafion 112 membranes and observed no significant changes in ionic conductivity and mechanical strength. However, after dry freeze/thaw cycles, the mechanical strength, gas permeability, and water uptake of the membranes were damaged. 4.1.6. High Cell Voltage. Fuel cell operation at little or zero current results in increased degradation rates and

EOCV = Eeq − ηix ηix =

RT ln[icrossover /ECAIo) F

(23)

(24)

4.1.7. Voltage or Current Cycling. Voltage or current cycling a fuel cell increases platinum dissolution and redeposition.21,23,25,145−148 Although this at first appears to be a catalyst issue, which is not the focus of this paper, because the platinum redeposits in the membrane and therefore accelerates membrane degradation, it is pertinent. Darling and Meyers23,24 showed that although platinum is stable at both low and high potentials, it will dissolve rapidly when progressing from low to high potentials. Due to the rapid dissolution, Pt particles are often detected in the membrane after potential or current cycling. In the membrane, the Pt particles can accelerate membrane degradation, as described in section 2.3.5.2. 4.2. AST Protocols

Accelerated fuel cell stress tests are practical because lifetime tests using mild conditions can continue for thousands of hours. Although ex situ durability tests, such as membrane degradation in Fenton’s reagent, provide valuable information on chemical stability, the degradation mechanisms are not necessarily consistent with those in an operating fuel cell because the tests are performed in different environments. An alternative is to test the fuel cell under conditions that accelerate the membrane degradation rate. Membrane degradation in accelerated tests occurs much more quickly than when mild operating conditions are used, but the appropriateness of a specific stressor as a measure of membrane durability is not always apparent. The operating conditions for the ASTs use widely accepted degradation mechanisms to isolate effects and failure modes. In an AST, the fuel cell components, either in situ or ex situ, are subjected to controlled stressors to accelerate degradation. During or after ASTs, the performance degradation rate and level of damage to each component under prescribed conditions are evaluated to improve fuel cell lifetime prediction capabilities or to improve understanding of the degradation mechanisms. Generally, four different stressors or a 6093

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combination of these stressors is used in accelerated stress tests: (1) high temperature, (2) reduced humidity, (3) OCV, and (4) cycling (humidity, temperature, voltage, freeze/thaw, start/ stop). AST protocols are designed to evaluate fuel cell durability using conditions that encourage specific modes of degradation including dissolution of catalyst, corrosion of carbon, chemical decomposition of the membrane, and mechanical decomposition of the membrane. These modes of degradation occur simultaneously under normal fuel cell operation, particularly during transitory and cyclic conditions such as start-up/shutdown (fuel starvation during start-up), load cycling (corrosion of catalyst, catalyst deposition in the membrane, and mechanical stress on PEM), and partial power (high voltage and high PEM swelling). The fundamental failure mechanisms must be understood along with the impact the different degradation modes have on each other, such as the impact of membrane mechanical stress on membrane chemical degradation. AST protocols have been outlined by several groups, including the DOE, General Motors Corporation, and the Fuel Cell Commercialization of Japan. General AST protocols for degradation of membranes or MEAs both chemically and mechanically are outlined in Table 4. In general, chemical

4.2.2. Membrane Mechanical Failure. Because chemical degradation impacts the mechanical strength of the membrane, mechanical failure is strongly related to chemical attack. To initiate mechanical degradation without chemical degradation, membranes undergo relative humidity cycling. As RH conditions in the cell change, the membrane swells and shrinks, leading to mechanical failure because the membrane is restrained in the cell. Not only is the mechanical integrity of the membrane compromised through this stress test, but the relative volume changes between the membrane and catalyst layer can result in delamination of a catalyst layer from the membrane. 4.2.3. Cathode Catalyst Decay. As discussed above, platinum solubility depends on potential. In a fuel cell, the Pt solubility increases ∼100 times as potentials increase from 0.7 to 1.0 V.139 To accelerate catalyst degradation, fuel cells undergo potential cycling. As the fuel cell potential is cycled (particularly from ∼0.7 V to OCV (∼0.96 V)), Pt dissolves at OCV, and then precipitates out of solution when the voltage is reduced to less than 0.4 V. Potential cycling of a fuel cell results in electrode surface area loss, Pt agglomeration, and Pt dissolution and redeposition.

Table 4. General AST Methods for Membranes and MEAs in PEM Fuel Cells

Table 5 summarizes the durability data of select membranes from ASTs. Accelerated conditions used include high temperature, RH cycling, low RH, OCV testing, and potential cycling. Almost all degradation rates were greater than 10 μV/h, with some in the mV/h range. Although many different conditions were used for OCV testing, some trends can be noted regarding the effect of temperature, humidity, and pressure. The average degradation rate was 1.25 mV/h at 100 °C/25% RH, 1.06 mV/h at 100 °C/ 50% RH, and 0.5 mV/h at 100 °C/75% RH (experiments 1 and 9). The degradation rate was 2 mV/h at 80 °C/dry, 0.36 mV/h at 80 °C/25% RH, 0.083 mV/h at 80 °C/68% RH, and 0.025 mV/h in 80 °C/humidified conditions (experiments 7, 9, 22, and 28). At 80 °C/humidified conditions, the degradation rate was 0.5 mV/h under H2/O2 and 0.025 mV/h under H2/air (experiments 9 and 28). At 70 °C/humidified conditions, the degradation rate was 1.3 mV/h under H2/O2 and 0.050 mV/h under H2/air (experiment 3 and 27). Degradation rates during OCV testing are generally accelerated by increases in temperature, decreases in relative humidity, and increases in reactant partial pressure. During potential cycling, changes in inlet gases, backpressure, voltage ranges, humidity, and temperature make it difficult to observe trends in reaction conditions. Voltage-cycling studies in PEM fuel cells have been carried out with H2/N2, H2/air, or H2/O2 feeds, but the observed Pt area loss has been shown to be essentially identical at high RH.213 Pt dissolution from voltage cycling increases with larger potential windows25,141,151 and the shape of the cyclic voltage profile (symmetric vs asymmetric; square vs triangular) can further accelerate Pt dissolution.150,213 Pt losses from voltage cycling are less at low RH,186 which may be due to a shift of Pt oxide formation toward higher potentials.23 Relative humidity cycling tests took place at temperatures ranging from 65 to 100 °C, for times ranging from 17 to more than 200 h, and with a range of relative humidity windows and cycle times.

failure modes

available protocols

chemical stability

Fenton’s test: 30% H2O2, 20 ppm Fe , 85 °C, 3 cycles with fresh reagent 90 °C, OCV, 30% RH H2 and O2 fed to anode and cathode

210

80 °C, OCV, dry air and fully humidified H2 fed to cathode and anode 90 °C, OCV, 30% RH H2 and air or O2 fed to anode and cathode 95 °C, OCV, 50% RH H2 and air fed to anode and cathode 65 °C, RH cycling from 30% to 80% or from 80% to 120% at 30 min/step, air fed to anode and cathode 80 °C, RH cycling from 0 to 150% at 2 min/ step, air fed to anode and cathode 80 °C, RH cycling from 0 to 100% RH at 30 min/step, N2 fed to anode and cathode 80 °C, Load cycling from 10 to 800 mA/cm2 (7 min/3 min), H2 and O2 fed to anode and cathode, 50% RH

18

chemical/ electrochemical stability

mechanical stability

chemical and mechanical stability

4.3. Summary of Results from ASTs

ref 2+

65

117, 211 212 65 29 211 211

stability is assessed through OCV testing at reduced RH, while mechanical degradation is assessed by RH cycling. To evaluate membrane or MEA durability, F− release, OCV, hydrogen crossover, and high frequency resistance are monitored at least every 24 h for the chemical stability, while hydrogen crossover is monitored for the mechanical stability. 4.2.1. Membrane Chemical Attack. Operation of a fuel cell under OCV accelerates membrane chemical degradation. Acceleration of membrane degradation through this approach is attributed to the lack of consumption of reactant gases through the electrochemical fuel cell reaction, resulting in increased gas crossover. Application of this AST is complicated as a result of current distribution in the electrode and platinum transport into the membrane. 6094

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RH cycling

OCV

21, 65

22, 119

20, 65

19, 219

18, 132

17, 151

16, 218

potential cycling potential cycling OCV and humidity OCV

potential cycling OCV and RH cycling potential cycling and temp

14, 25

15, 217

potential cycling

potential cycling, cell temp, and humidity RH cycling and temp OCV

RH cycling RH cycling OCV, humidity, and cell temp OCV, humidity, and cell temp OCV, humidity, and cell temp

cell temp, humidity, and OCV cell temp, humidity, and potential cycling, OCV RH cycling

acceleration parameter

13, 21

12, 216

11, 186

10, 186

9, 186

8, 215

5, 214 6, 204 7, 18

3, 29, 143 4, 29

2, 214

1, 214

experiment number, reference

50

H2/O2

H2/air

30

25

25

50

H2/N2

H2/O2

50

H2/N2

30

50

H2/N2

H2/O2 and H2/N2

25

H2/air and H2/N2

25

23

H2/air

H2/air

50

50

H2/O2

N2/N2

50

50

25 25 50

H2/O2

H2/O2

H2/O2 air H2/air

50 25

50

H2/O2

H2/O2 H2/O2

25

membrane thickness (μm)

H2/O2

application type

80

65

90

90

80, ambient

65, 1, 1

130, 3, 3

90

80, ambient

65

75

100, 1.5, 1.5

100

80−120

90

100 90 80

70 80

100, 1.5

100, 1.5, 1.5

temp (°C), Panode (bar), Pcathode (bar)

150 h

OCV to 25 mA/cm2, 100 °C, 25% RH

OCV and 68% RH

From 30 to 80% RH and 80 to 120% RH

OCV, 30% RH

OCV, 30% RH

0.65 to 1.05 V, humidified

0.87 to 1.2 V, humidified

Cycling from 0.7 to 0.9 V under H2/O2; 0.6 to 1.2 V under H2/N2, and 0.7 V steady state, under H2/O2, 100% RH

Cycling from dry to 100% RH

Cycling from 0.6 to 1.0 V, humidified

Cycling from 0.87 to 1.2 V, humidified

81% RH

50 h (100 cycles) 60 days

48 h

24 h (21600 cycles) 48 h

1500 cycles and 100 h for steady state 2400 cycles

16.7 h

10000 cycles

3000 cycles

256 h

100 h

200 h 17 h 160 h

24 h 4000 cycles

60 h

70 h

degradation time

OCV; 82, 55, and 36% RH

hydrated and OCV RH cycles (150% RH to 0% RH, 2 min each RH cycles (0−100%) RH cycles (0−100%) OCV, dry air

potential cycles 1 min at 0.4 V and 1 min at 1V

100 °C, 25% RH, OCV

accelerated conditions

Table 5. Summary of Accelerated Durability Tests of Nafion Using In Situ Accelerated Tests suggested membrane failure mode

−83 μV/h

1−2 mV/h

@0.1 A/cm2 = 13 mV/cycle for Pt/C and 3.8 mV/ cycle for PtCo/C

132 mV/h

−0.8 mV/h

−0.36 mV/h for 80 °C/25% RH; −1.8 mV/h for 100 °C/25% RH, −1.06 mV/h for 100 °C/50% RH; −0.50 mV/h for 100 °C/75% RH −0.87 mV/h

CO increased 10×

membrane thinned, strain to break decreased >25× strain to break decreased 2−3×

Pt in membrane, ECA decreased 2× for Pt/ C 1.5× for PtCo/C ECA decreased 4×, Pt in membrane, cathode became thinner, Pt particle size increased Pt in membrane

F− detected, hydrogen crossover increased >20× catalyst size stayed constant under steady state, grew 1.3−2x under H2/O2 cycling and grew 2−3× under H2/N2 cycling

membranes thinned, H2 crossover increased, ECA decreased ECA decrease, more when N2 was used; Pt in membrane only when air was used, gas crossover increased ECA decreased >2×, Pt in membrane

membrane thinned, F− emission, gas crossover >2×

F− emission

F− emission for low RH

gas crossover gas crossover

hydrogen peroxide degradation fatigue, viscoelastic creep

−1.3 mV/h

−13.2 mV/h −2 mV/h

membrane thinning

membrane failure, increased gas crossover −1 mV/h

degradation rate

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4.4. Relationship between Current Degradation Mechanisms and AST Results

membrane thinned, F− emission CO increased >10×, ECA decreased 1.5×, F− detected F− detected, mechanical properties decreased, detection of small molecules ECA decreased 5×, Pt band Pt particle size increased, sharp Pt band ECA decreased 5× 2000 1500 2000 OCV and 100% RH OCV and humidified OCV and humidified

The failure modes that were observed in the ASTs in Table 5, including membrane thinning, gas crossover, detection of fluoride and sulfate ions, decrease in ECA, detection of Pt in membrane, and growth of catalyst clusters, were very similar to those observed for the lifetime tests, indicating that accelerated testing generally provides appropriate accelerating parameters. For the AST results, additional degradation modes including pinholes, tearing, and creep are also observed, which were not noted in the lifetime tests. When elevated temperature was used, the temperature was not high enough to thermally degrade the membranes but was high enough to accelerate any reactions taking place. RH cycling is designed to increase mechanical degradation of membranes, and this was done in experiments 4−6, 11, 15, and 21. The observed degradation modes, including fatigue, creep, crossover, and reduced mechanical strength could be a result of mechanical stress on the membrane, supporting the appropriateness of the protocol. Testing under OCV is designed to chemically degrade the membrane, and this was performed in experiments 1, 3, 7−9, 12, 15, 19, 20, 22, 23, and 25. The observed degradation modes of increased gas crossover, hydrogen peroxide degradation, F− emission, membrane thinning, and reduced mechanical strength are consistent with chemical degradation mechanisms. Potential cycling, which is designed to degrade the catalyst layer, took place in experiments 2, 10, 13, 14, 16−18, and 24. The observed degradation modes of decrease in ECA, Pt in membrane, increase in catalyst size in electrode, and cathode thinning are consistent with catalyst layer degradation. However, in this case, the observed degradation modes of membrane thinning, F− emission, and gas crossover are not necessarily consistent with catalyst layer degradation. These degradation modes are consistent with membrane degradation. The current degradation mechanisms propose that when the catalyst layer degrades and is deposited inside the membrane, as described in section 2.3.5.1.2.3, it may increase radical formation and therefore membrane chemical degradation. This means that the observed degradation modes are consistent with the current understanding of MEA degradation.

−50 μV/h −25 μV/h

−1.3 and 1.9 mV/h 45 and 72 h

5. COMPARISON OF LIFETIME TESTS AND ACCELERATED STRESS TESTS Because many different testing conditions (temperature, inlet gas, relative humidity, cycling conditions, membrane type, flow rates, back pressure, etc.) were used for the lifetime and AST experiments summarized in Tables 3 and 5, it is difficult to meaningfully compare the two types of tests. Ideally, only one variable would be changed for each test so that the accelerating factor for each variable could be clearly understood. Without this knowledge, and with the ranges in inlet gases, cell temperatures, humidity, back pressures, and voltages or current densities used for lifetime tests in Table 3, it is not possible to know what the baseline lifetime conditions should be. All the values in Table 3 will be used for baseline conditions to determine average degradation factors. In general, when degradation rates were calculated for the ASTs, they tended to be considerably higher than those from the lifetime tests, which is logical and indicates accelerated degradation. Voltage decay during lifetime testing generally ranged from 0 to 50 μV/h (Table 3). Throughout potential cycling, voltage

80, 1.5, 1.5 70 80, 1.5, 1.5 25 25 25 H2/air H2/air H2/air OCV OCV OCV 26, 20 27, 128 28, 25

50 25, 220

H2/O2

90

−62 mV/h 700 h 1000 h

OCV, 100% RH Cyclic from OCV to low, intermediate and high current, 100% RH OCV and 30% RH 90, 3, 3 80, 1.4, 1.4 50 50 H2/O2 H2/air

OCV potential cycling OCV 23, 20 24, 197

acceleration parameter experiment number, reference

Table 5. continued

application type

membrane thickness (μm)

temp (°C), Panode (bar), Pcathode (bar)

accelerated conditions

degradation time

degradation rate

suggested membrane failure mode

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Strain to break decreased 1.2%/h during RH cycling in experiment 24 (Table 5). During OCV testing in experiments 23 and 28, strain to break decreased 2.0% and 6−10%/h, respectively (Table 5). Unfortunately, little work has studied the effect of mechanical strength on lifetime testing, so it is not possible to generalize how much this degradation factor is accelerated through the ASTs. In summary, potential cycling accelerated voltage decay 10− 20 times, ECA losses 35 times, membrane thinning 35 times, gas crossover 2 times, and FER 25 times. Testing under OCV accelerated voltage decay 5−40 times, ECA losses 5 times, and gas crossover 3−12 times. RH cycling increased voltage decay 2−260 times, but generally there has not been enough characterization of RH cycling tests to quantify the extent to which it accelerates degradation.

decay ranged from 0.62 to 1 mV/h (Table 5). During OCV testing, voltage decay ranged from 0.36 to 2 mV/h (Table 5). During RH cycling, voltage decay ranged from 0.12 to 13.2 mV/h (Table 5). In general, voltage decay is accelerated 10−20 times by potential cycling, 2−260 times by RH cycling, and 5− 40 times by OCV testing. At OCV, voltage decay is mainly attributed to increases in hydrogen crossover. During OCV and potential cycling tests, the chemical degradation of the membranes would lead to increased hydrogen crossover, whereas during RH cycling tests, the mechanical degradation of the membranes would lead to increased hydrogen crossover. During lifetime testing, ECA losses for experiments 4, 14, 15, 16, 18, and 19 (Table 3) were 7.14 × 10−3, 0.020, 0.033, 5.6 × 10−3, 0.024, 0.018, and 0.030 m2/(g Pt h), respectively. The ECA losses for potential cycling for experiments 16 and 17 (Table 5) were 0.35 and 0.36 m2/(g Pt h), respectively. Throughout OCV testing, the ECA loss for experiment 15 (Table 5) was 0.087 m2/(g Pt h). In general, potential cycling accelerates ECA losses ∼35 times, and OCV testing accelerates ECA losses ∼5 times. The thickness changes during lifetime testing in experiments 9 and 15 (Table 3) were 0.011% and 0.024%/h, respectively. Throughout potential cycling, the thickness change in experiment 14 (Table 5) was 0.6%/h. During OCV testing, the thickness changes in experiments 15 and 23 (Table 5) were 1.2 × 10−3 and 5.0 × 10−3 %/h, respectively. In general, potential cycling accelerates membrane thinning around 35 times, and OCV testing does not consistently accelerate membrane thinning. The OCV tests that reported amounts of thinning may not have been severe enough to generate the predicted results. For example, in experiment 15, the RH was 81% RH, which may have been too high to accelerate membrane thinning, and in experiment 23, the test only took place for 48 h, which may not have been long enough to accelerate membrane thinning. Gas crossover increases during lifetime testing in experiments 12 and 15 (Table 3) were 1.4% and 0%/h, respectively. Throughout potential cycling, the gas crossover increase in experiments 13 and 27 (Table 5) were 2.7% and 1.7%/h, respectively. The gas crossover increases during OCV testing in experiments 12 and 15 (Table 5) were 17.5% and 4.6%/h, respectively. During RH cycling, the gas crossover increase in experiment 14 (Table 5) was 0%/h. In general, potential cycling accelerates gas crossover increases ∼2 times, and OCV testing accelerates gas crossover increases ∼3 to 12 times. Although no gas crossover increases were observed for RH cycling, because only one study evaluated the effect of RH cycling on gas crossover, it is not wise to make a general conclusion. During lifetime testing, the FER in experiment 17 (Table 3) was 1.2 × 10−5 ppm/(cm2 h). The FER throughout potential cycling experiment 27 (Table 5) was 3.0 × 10−4 ppm/(cm2 h). During OCV testing, the FER in experiments 11, 14, 26, and 28 (Table 5) were 1.0 × 10−5, 3.6 × 10−6, 9.4 × 10−7 and 1.1 × 10−5 (3.9 × 10−6 ppm/(cm2 h)) g/(cm2 h). In general, potential cycling increased the FER around 25 times, and OCV testing decreased the FER by ∼70%. Because in most cases reactant flow rates were not provided, it was not possible to convert from parts per million to grams, so the FER from OCV testing could only be compared with that from lifetime testing in one test; thus a meaningful conclusion cannot be made regarding the effect of OCV on accelerating FER.

6. CONCLUSIONS A fuel cell must have the ability to function in a broad range of conditions. Flexible operation of fuel cells includes both expected and a reasonable number of unforeseen conditions during the target fuel cell lifetime. The required flexibility depends on the application of the fuel cell. Conditions that must be considered include electrical load requirements, gas flow rates, reactant compositions, operating and environment temperatures and pressures, humidification levels, duty-cycle characteristics, and transient conditions. The desired fuel cell operating range includes temperatures from 100 °C; humidities from 0 to 100% RH, and potentials from 0 to >1.5 V. Additionally, the anode may experience both hydrogen and air simultaneously during start/stop cycles. The cycling can result in physical and chemical damage, sometimes with “catastrophic” results. To compare the results of accelerated and ex situ tests to actual cell performance decay, the cell decay mechanisms must be well-understood. Models of some of the decay modes have been developed, and they show a consistent but incomplete understanding of membrane decay. A true understanding of the mechanisms of decay is required to ensure that the protocols do not neglect potential decay modes or introduce modes that do not really exist. A difficulty of the modeling is that decay modes interact. Dissolution of the cathode catalyst affects membrane chemical attack, and this affects membrane mechanical failure. Models must be able to account for these interactions and be verified for actual cell operation. An overall predictive decay model can be developed by a combination of specific modeling and tests. The tests must be comprehensive enough so that statistical analyses can be used to predict the likelihood of failure. The OCV AST protocol is a valid method of accelerating and evaluating membrane chemical attack. According to the current understanding of PEM chemical degradation, the PEM is attacked by a hydroxyl radical formed by the reaction of H2 and O2 at the Pt catalyst. The membrane attack results in fluorine and sulfur loss from the membrane, membrane thinning, and reduced membrane mechanical integrity. Operation under OCV is understood to accelerate the membrane degradation because H2 and O2 are not being consumed by the fuel cell reaction. The manner for accelerating the mechanical aspect of decay has been with relative humidity cycling. Changes in RH simulate the changes in operating conditions that cause the membrane to swell and shrink. These changes can lead to mechanical failure since the membrane is restrained in the cell. 6097

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current, low power) results in decreasing temperature and rising RH (membrane swelling), the full load (low voltage, high current, high power) results in increasing temperature and lowering RH (membrane shrinking). If a stack integrator collects such data sets for its fleet of fuel cell engines, statistical theory and data mining methods can help analyze these data and discover inherent correlations between variables that can guide the design of alternative ASTs. For example, multifactor dimensionality reduction can detect and characterize the combination of these intensive variables that interact to influence a dependent variable, for example, end of life cell performance or membrane toughness.

This failure mode is strongly related to but separate from membrane chemical properties. The OCV, constant load, and drive cycle AST protocols are valid methods to accelerate and evaluate catalyst decay mechanisms. Several models have been generated on catalyst decay, accounting for the loss of the platinum catalyst and catalyst area. They address the catalyst and the catalyst supports, as well as new catalyst concepts. In this review, we have found that the AST protocols reproduced the failure mechanisms that are associated with testing in real time without introducing failure mechanisms that are not associated with testing in real time. It is recognized that the other components in the PEM fuel cell can be significant contributors to decay and decay mechanisms. However, in this paper, the focus is on the membrane and the membrane/ catalyst interaction, so analysis is limited to mechanisms in that area. We have attempted to determine the factor by which the accelerated tests amplify the time in the real lifetime tests by using the post-test evaluation of cells. This amplification of time depended on the type of accelerated test and the decay mode within that test. Only approximate amplifications were found possible. • Potential cycling accelerated voltage decay 10−20 times, ECA losses 35 times, membrane thinning 35 times, gas crossover 2 times, and FER 25 times. • OCV testing accelerated voltage decay 5−40 times; ECA losses 5 times; and gas crossover 3−12 times. • RH cycling accelerated voltage decay 2−260 times, but generally there has not been enough characterization of RH cycling tests to quantify the extent to what it accelerates degradation. Because the materials and hardware for each test were different and different lifetime and accelerated test protocols were used, as well as different methods and measurements to characterize the post-test results, it is difficult to conclusively assess the extent to which ASTs accelerate degradation. It is desirable that standard materials, tests, and methods are used to accurately assess the amplifications of degradation. In general, AST protocols have been intended to assess the durability of fuel cells under conditions that promote distinct degradation modes dominated by catalyst dissolution, carbon corrosion, membrane chemical decomposition, and membrane mechanical failure. In real-time degradation of the fuel cell, these degradation modes are occurring simultaneously, especially under transient and cyclic conditions, such as startup transient (fuel starvation), shut-down transient, load cycling (catalyst corrosion and membrane stress cycling), and partial power (high voltage and membrane swelling). Although it is necessary to understand the fundamental failure mechanisms, it is also necessary to understand interactions of the degradation modes, such as mechanical stress and chemical degradation. Design of experiments and alternative AST protocols that induce interactions of the degradation modes are also valuable and necessary for increased understanding of failure mechanisms and lifetime predictions. Degradation of a fuel cell is driven by many local intensive variables including temperature, humidity, voltage, current density, and concentration of gas species. In a real-time degradation process, such as a driving profile, the time history of these intensive variables will be stochastic in nature. However, there exist inherent correlations among these variables. For example, the partial load (high voltage, low

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Marianne Rodgers received her B.S. in Chemistry at St. Francis Xavier University in 2001. She completed her Ph.D. in Polymer Chemistry at Simon Fraser University in 2007 under the direction of Dr. Steven Holdcroft, studying structure−property relationships in several types of proton exchange membranes. She continued working in the area of proton exchange membrane fuel cells at the Florida Solar Energy Center as a post-doctoral researcher (2008−2009) and in her current position as an assistant research professor (2009−present). In this time, Marianne’s research has expanded to include additional aspects of fuel cell operation, including electrode fabrication, electrode characterization, durability testing, and performance testing. Marianne is the author or co-author of over 25 publications and more than 20 presentations.

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Leonard J. Bonville is currently a Research Specialist for the Center for Environmental Science and Engineering at the University of Connecticut, where he is the Director of the Fuel Cell Scale up laboratory. He is also employed as a Fuel Cell Research Specialist by the Florida Solar Energy Center at the University of Central Florida with a focus on fabrication, performance and endurance of proton exchange membrane fuel cells. Prior to joining the University of Connecticut, Mr. Bonville worked for 35 years at the International Fuel Cell (IFC) Division of United Technologies, where he held a series of positions of increasing responsibilities from Development Engineer to Vice President of Engineering. In addition, he was project manager working in Japan at Toshiba for 2 years. As Vice President of Engineering, he managed more than 120 engineers and scientists in the development and application of the IFC fuel cell data base for the manufacture and improvement of IFC fuel cell products. Mr. Bonville has 14 patents relating to fuel cells and has co-authored publications in both the industrial and the academic environments.

Dr. Darlene Slattery has a Ph.D. in chemistry from the Florida Institute of Technology and over 20 years of experience in all aspects of hydrogen including production, storage, and utilization in fuel cells. Dr. Slattery’s most recent assignment has been in the management of the Florida Solar Energy Center’s High Temperature, Low Relative Humidity Membrane program. FSEC has been the lead organization for the Department of Energy program that initially involved eleven other universities and companies. Dr. Slattery and her team have been responsible for the preparation and testing of membrane electrode assemblies fabricated from the membranes supplied by the other teams in the program. Dr. Slattery is the author or co-author of over 40 publications, which include peer-reviewed articles, conference proceedings, and two patents. She has also been an invited keynote speaker on hydrogen storage at Battelle Institute and Oak Ridge National Laboratory.

H. Russell Kunz is currently a Professor-in-Residence in the Chemical, Materials & Biomolecular Engineering Department at the University of Connecticut (UConn) and was also employed as an Assistant in Fuel Cell Research by the Florida Solar Energy Center at the University of Central Florida in the area of proton exchange membrane (PEM) fuel cells. Dr. Kunz’s areas of interest include many theoretical and experimental aspects of several types of fuel cells involving electrochemistry, thermodynamics, mass transfer, and chemical kinetics. His current research is being performed at UConn’s Center for Clean Energy Engineering. Dr. Kunz is internationally recognized for his accomplishments that have increased fuel cell life, raised performance, and reduced cost. He received the Research Award of the Energy Technology Division of the Electrochemical Society for his outstanding research contributions in the field of novel electrochemical energy technologies. Prior to joining UConn, Dr. Kunz was employed by United Technologies Corporation (UTC) directing research on several types of fuel cells. He has published many papers on alkaline, phosphoric acid, molten carbonate, and PEM fuel cells. He has 29 patents, mostly related to fuel cells. While at UTC, Dr. Kunz was an Adjunct Professor at the Hartford Graduate Center of Rensselaer Polytechnic Institute where he taught courses in Chemical Thermodynamics of Fuel Cells, Irreversible Thermodynamics, Convective Heat Transfer, Two-Phase Heat Flow, and Chemical and Phase Equilibria.

James M. Fenton has been Director of the University of Central Florida’s Florida Solar Energy Center since January 2005, where he leads a staff of 140 in the research and development of energy technologies that enhance Florida's and the nation's economy and environment and educate the public, students, and practitioners on the results of the research. In addition to his duties as FSEC Director, he leads a 12-member university and industry research team in a $19 million U.S. Department of Energy research program to develop the next generation proton exchange membrane (PEM) fuel cell automobile engine. Dr. Fenton also serves as a Professor in UCF’s Mechanical, Materials and Aerospace Engineering Department. Prior to joining FSEC, he spent 20 years as a Chemical Engineering Professor at the University of Connecticut. Dr. Fenton received his Ph.D. in Chemical Engineering from the University of Illinois in 1984 and his B.S. from UCLA in 1979. He is the author of more than 120 scientific publications and a number of book chapters and holds three patents. He was recently elected as Fellow of The Electrochemical 6099

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Society and was appointed as a member of the Florida Governor’s Action on Energy and Climate Change by Governor Crist.

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