Chapter 6 Freeze Damage to Polymer Electrolyte Fuel Cells Abdul-Kader Srouji 1 and Matthew M. Mench 2* 1 Fuel Cell Dynamics and Diagnostics Laboratory, and Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, PA, USA, 2 Electrochemical Energy Storage and Conversion Laboratory, and Department of Mechanical Aerospace and Biomedical Engineering, The University of Tennessee, TN, USA 1. INTRODUCTION The Department of Energy (DOE) 2015 technical target requires that fuel cell vehicles should deliver 50% of rated power in 30 seconds from a cold start at 20 C, with less than 62.5 J/We parasitic energy input for start-up and shut-down [1]. With those targets, they should also be able to start from 40 C after being soaked at this temperature for 8 hours. DOE targets for an 80 kW e (net) fuel cell system for automotive applications are summarized in Table 6.1. Dozens of studies have been performed to examine various aspects of fuel cell material compatibility and performance degradation in freezing environ- ments. The damage resulting from a frozen soak, from freeze/thaw (F/T) cycling, and from frozen start have been examined. The purpose of this summary is to explore the damage resulting from freeze/thaw conditions. A detailed summary of damage observed due to some aspects of a frozen envi- ronment from various published studies is shown in Table 6.2. Generically, damage resulting from a frozen environmental condition is due to water generated at the cathode during sub-zero operation, or the existence of liquid water that resides in the membrane, porous media after shut-down. Liquid water that freezes in the channels and internal manifolds can hinder cold start and reactant flow, resulting in exacerbated degradation due to cell voltage reversal and carbon corrosion or local fuel starvation. An important result from accumulated studies is that, for conventional fuel cell materials and designs, no significant damage is observed from simply cycling the fuel cell material to subzero conditions without start-up operation or liquid water before freeze. This indicates that freeze-related damage can be eliminated through proper Polymer Electrolyte Fuel Cell Degradation. DOI: 10.1016/B978-0-12-386936-4.10006-5 Copyright Ó 2012 Elsevier Inc. All rights reserved. 293
Transcript
Chapter 6
Freeze Damage to PolymerElectrolyte Fuel Cells
Abdul-Kader Srouji1 and Matthew M. Mench2*1Fuel Cell Dynamics and Diagnostics Laboratory, and Department of Mechanical and Nuclear
Engineering, The Pennsylvania State University, PA, USA, 2Electrochemical Energy Storage and
Conversion Laboratory, and Department of Mechanical Aerospace and Biomedical Engineering,
The University of Tennessee, TN, USA
1. INTRODUCTION
The Department of Energy (DOE) 2015 technical target requires that fuel cellvehicles should deliver 50% of rated power in 30 seconds from a cold startat �20�C, with less than 62.5 J/We parasitic energy input for start-up andshut-down [1]. With those targets, they should also be able to start from�40�C after being soaked at this temperature for 8 hours. DOE targets for an80 kWe (net) fuel cell system for automotive applications are summarized inTable 6.1.
Dozens of studies have been performed to examine various aspects of fuelcell material compatibility and performance degradation in freezing environ-ments. The damage resulting from a frozen soak, from freeze/thaw (F/T)cycling, and from frozen start have been examined. The purpose of thissummary is to explore the damage resulting from freeze/thaw conditions. Adetailed summary of damage observed due to some aspects of a frozen envi-ronment from various published studies is shown in Table 6.2. Generically,damage resulting from a frozen environmental condition is due to watergenerated at the cathode during sub-zero operation, or the existence of liquidwater that resides in the membrane, porous media after shut-down. Liquidwater that freezes in the channels and internal manifolds can hinder cold startand reactant flow, resulting in exacerbated degradation due to cell voltagereversal and carbon corrosion or local fuel starvation. An important result fromaccumulated studies is that, for conventional fuel cell materials and designs, nosignificant damage is observed from simply cycling the fuel cell material tosubzero conditions without start-up operation or liquid water before freeze.This indicates that freeze-related damage can be eliminated through proper
TABLE 6.1 DOE Technical Targets for Automotive Applications: 80-kWe (net)
Integrated Transportation Fuel Cell Power Systems Operating on Direct
Hydrogena. As Reported in [1]
Characteristic Units
2003
Status
2005
Status 2010 2015
Energy efficiencyb at 25% ofrated power
% 59 59 60 60
Energy efficiency at rated power % 50 50 50 50
Power density W / L 440 500 650 650
Specific power W / kg 420 470c 650 650
Costd $ / kWe 200 110e 45 30
Transient response (time from10% to 90% of rated power)
seconds 3 1.5 1 1
Cold start-up time to 50%of rated powerat �20�C ambient temperatureat þ20�C ambient temperature
secondsseconds
12060
20<10
305
305
Start-up and shut-down energyf
from �20�C ambient temperaturefrom þ20�C ambient temperature
MJMJ
N/AN/A
7.5N/A
51
51
Durability with cycling hours N/A ~1,000g 5,000h 5,000h
Unassisted start from lowtemperaturesi
�C N/A �20 �40 �40
aTargets exclude hydrogen storage, power electronics and electric drive.bRatio of DC output energy to the lower heating value of the input fuel (hydrogen). Peak efficiency
occurs at about 25% rated power.cBased on corresponding data from the DOE report to account for ancillaries.dBased on 2002 dollars and cost projected to high-volume production (500,000 systems per year).eStatus is from 2005 TIAX study and will be periodically updated.fIncludes electrical energy and the hydrogen used during the start-up and shut-down procedures.gDurability with cycling is being evaluated through the Technology Validation activity. Steady-state
stack durability is 20,000 hours.hBased on test protocol issued by DOE in 2007.i8-hour soak at stated temperature must not impact subsequent achievement of targets.
294 Polymer Electrolyte Fuel Cell Degradation
design, materials, and operating protocol. Based on the summary of availablestudies shown in Table 6.2, not all environments, materials, or designs result indamage. In fact, there has been a seemingly high discrepancy between theresults of different studies, which suggests there is still much to learn about the
TABLE 6.2 Summary of Observed PEFC Damage Due to Frozen Environments from Various Sources
Reference Test Mode PEM CL MEA DM
Test conditions
Results
T range
(�C)Number
of cycles
Purge/no
purge
Wilson et al.1994 [62]
In-situ F/T Nafion 117 20wt% Pt/C(0.16mg/cm2)
Decalprocessa
ELAThydrophobiccarbon cloth
�10/80 3 No purge(wet)
No performance loss
McDonaldet al. 2004[17]
Ex-situ F/T Nafion 112 0.4mg Pt/C/cm2
N/A None �40/80 385 Dry state(l<3)
No significant physicaldamage change in themolecular level
In-situ F/T Nafion 112 0.4mg Pt/C/cm2
N/A Carbon paper 385 Dry state(l<3)
No significant physicaldamage change in themolecular level
Liu 2006 [2] Ex-situ F/T(immersion)
Nafion 112 N/A N/A None �40/50 10 Immersedin water
Severe CL lossSevere deformation of MEA
DSM N/A N/A None 10 Immersedin water
No observable loss
In-situ F/T Nafion 112 N/A N/A N/A 40 N/A No performance lossNo ECSA loss
DSM N/A N/A N/A 40 N/A No performance lossNo ECSA loss
(Continued ) 295
Chap
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6Freeze
Dam
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Polym
erElectro
lyteFu
elCells
TABLE 6.2 Summary of Observed PEFC Damage Due to Frozen Environments from Various Sourcesdcont’d
Reference Test Mode PEM CL MEA DM
Test conditions
Results
T range
(�C)Number
of cycles
Purge/no
purge
Pattersonet al. 2006[63,64]
In-situ F/T N/A N/A N/A N/A �40/25 63 N/A No performance loss
Interfacial delaminationDM/CLDeformation of stiff diffu-sion media
SGL 10BB
SGL 25BC
SGL 10BA
aDecal printing (TBAþ form catalyst) and then hot pressing at 200 �C.b20% PTFE treatment with MPL.
Abbreviation: F/T: Freeze/thaw thermal cycling DSM: Dimensionally stable membrane GDE : Gas diffusion electrode, PEM: Polymer electrolyte membrane.cCatalyst ink sprayed on DM and then hot pressing at 140�C.dSprayed on DM and then hot pressed.
300
Polym
erElectro
lyteFu
elCell
Degrad
ation
301Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
genesis and causes of freeze related damage. There are some commonalitiesbetween the studies. When freeze related damage was observed, it wasgenerally observed at the following locations:
l The membrane in the form of pinholes on the surface.l The catalyst layer (CL), in the form of local cracks as well as interfacial
delamination between the CLjmembrane and/or CLjDM interface, andloss of electrochemical surface area (ECSA).
l The diffusion media, via cracking of the microporous layer (MPL) andinterfacial delamination with the CL, or loss of hydrophobicity.
The effect of different component characteristics on sustaining damage wasstudied [2,3,4]. One important result concludes that properly drying a cell beforesub-zero cool down and freeze prevents observable physical damage. However,over-drying or non-uniform drying of the membrane during shut-down has beenshown to cause an uneven stress distribution in the membrane and result inaccelerated membrane degradation [5]. Although freeze-related damage can beeliminated by completely purging the cell, thismitigationmethod is generally tootime consuming, parasitic, and potentially damaging to the membrane to be ofuse. Much more study of purge, evaporative removal, and knowledge of thelocations and sources of freeze-relatedwater damage is needed. FromTable 6.2, itcan be seen that damage is not uniformly observed and varies as a function of:
1. Material sets – Microporous layer (MPL)jCL combinations appear to impactresults. This is a function of drainage at shut-down, interfacial contact, anddistribution of compression pressure. Stiff or bonded DM appear to best miti-gate damage because they can reduce interfacial liquid accumulation whichhas been shown to cause freeze delamination in some cases [3,4].
2. Cell design – The channel/land design also appears to impact damage. Inparticular, for a traditional channel/land configuration, low under-channelcompression exacerbates damage. The wider the channel, the worse thedamage appears to be. Figure 6.1 shows the result for freeze/thaw cyclingto �30�C from a wet state with 2mm wide lands. This testing qualitativelyconfirmed the results of computational simulation [6–8] which predictedthat the major damage locations from a macroscopic perspective appearunder the channel along various material interfaces in the cell.
3. Shut-down protocol – Obviously, the source of damage is water. Upon cool-down, condensation, diffusive flow, temperature gradient driven flow, andcapillary transport take place. To avoid damage or hindered cold-start,residual water inside the CL should be removed. This can be accomplishedwith a variety of methods discussed in Section 4 of this review.
4. Location in stack – Anode end cells in particular have been shown byseveral groups to suffer aggravated damage compared to center and cathodeend stack plates. This is apparently a result of greater heat transfer from endplates and concomitant phase-changed induced (PCI) motion [9].
Channel Land
FIGURE 6.1 SEM image of
cross-section of membrane
electrode assembly after 100
freeze/thaw cycles to �30�C.Delamination damage is seen
under the channel but not land
locations due to the overburden
pressure from the land [4].
302 Polymer Electrolyte Fuel Cell Degradation
Although many studies have been conducted to investigate freeze damage ondifferent components of a polymer electrolyte fuel cell, there are significantvariation in the results. Much, but likely not all, of this variation can be ascribedto the non-standardized testing procedures, cell designs, and materials used.There is still a clear need to resolve these discrepancies with fundamentalunderstanding of the physicochemical mechanisms involved, so that optimizeddesigns, materials, and protocols can be developed. The motivation of thisreview is to better understand and codify the existing data so that conflictingconclusions from the various studies can be better explained. Additionally,methods and potential concepts to mitigate freeze-induced damage arediscussed. The operation of start-up from a frozen state and possible damageresulting exclusively from a frozen start is another topic that is not within thescope of this review.
2. COMPUTATIONAL MODEL EFFORTS
Several publications based on computational models for predicting the keyparameters and conditions for freeze damage in PEFCs have been developed.These are based on porous media flow theories, and frost heave formation inthin cracks [6–8]. Models to predict the formation of ice, without the onset ofdamage, have also been developed, but are not within the subject of this reviewand are not summarized here. Damage resulting from ice formation can beseparated into two different phenomenological categories:
1. damage due to ice formation and expansion from a liquid to solid state; and2. damage due to an ice lens, which develops sufficient phase pressure to phys-
ically separate interfacial surfaces.
Damage due to the approximately 8% volume expansion of the ice phase isimagined by many to be the only possible mechanism for damage, but
303Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
experimental and computational evidence now suggests that ice lens forma-tion is responsible for much of the observed freeze damage. In fact, the porousmedia in fuel cells (CL and DM) are typically far from a full-saturation state,and another 8% expansion as the ice forms during a slow cooling at shutdownis not likely to cause severe morphological damage. In contrast, an ice lenscan cause plastic or elastic delamination and deformation, and can form asa result of even slight interfacial water accumulation, or from water expulsionfrom the membrane under decreasing temperatures [6–8]. The presence ofinterfacial accumulation of liquid has been suggested or observed by severalindependent studies [10–12]. After ice nucleation, whether or not an ice lenscontinues to grow depends on the ice phase pressure, the overburden pressure,the hydraulic availability of liquid water flowing to the ice lens location, andthe heat transfer rate from the porous media. The critical ice pressure for icelens formation is referred to as the local overburden pressure Povbd. Over-burden pressure is a function of the assembling pressure Passm or the trans-mitted channel pressure Pch, depending on whether the location is under theland or the channel respectively, the material tensile strength sts and the shearstress ssh [6]. The possible cases are listed in Table 6.3. The impact of DMstiffness is discussed in the following section of this review.
The large overburden pressure restrains macroscopic ice lens formation.However, local delamination at the catalyst level could still occur due to highlocal ice-phase pressure. Figure 6.2 is a schematic representing potentiallocations of freeze/thaw damage according to a computational model, whichincluded the impact of water motion in the ionomer and porous media duringshut-down to a frozen state [6–8]. The results are in qualitative agreement withthe observed F/T damage on SEM for materials which underwent ex-situ F/Tcycling. Ice growth leading to damage most likely occurs under the channel andat the interface between CLjDM and CLjMembrane. The CLjMembrane icegrowth is highly dependent on the freezing temperature depression propertiesof Nafion membrane. Non-freezing water flowing out of the membrane wouldimmediately freeze upon contact with the catalyst layer. The maximum ice lensgrowth at this location would therefore depend on the initial water content of
TABLE 6.3 Possible Locations of Ice Lens Growth with their Respective
Overburden Pressure Based on Data from [6]
Position Under BP Under CH
Within DM, CL or Nafion Povbd ¼ Passm þ sts þ pch Povbd ¼ pch þ sts þ ssh
At interface between bipolarplate(BP)/DM, DM/CL orCL/Nafion
Povbd ¼ Passm þ pch Povbd ¼ pch þ ssh
FIGURE 6.2 Schematic showing the potential locations of freeze/thaw damage [6].
304 Polymer Electrolyte Fuel Cell Degradation
the membrane, and the membrane type, as represented in Fig. 6.3. Reduction ofthe liquid water in contact with the ionomer at shut-down to a frozen state is thekey to avoid damage, as the liquid contact is responsible for higher free-watercontent in the membrane, which is responsible for local degradation – as dis-cussed. It should be noted that extensive ex-situ testing revealed that no damagewas observed when F/T cycling in a purely gas phase (but vapor-saturated)environment [13].
3. MODES OF DEGRADATION
In this section, the various modes of observed freeze-related physicochemicaldamage are discussed. Where possible, the phenomena responsible for thedamage are described. The section is divided into subsections based on
FIGURE 6.3 Maximum thickness of ice lens that could be formed by water expelled from Nafion
during freezing, as a function of the initial water content and membrane type [6].
305Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
components for convenience, although it is likely that the observed damage isinterrelated.
3.1. Membrane
Water freezing in the PEFC can damage the electrolyte membrane indifferent ways. Physical damage is observable with a scanning electronmicroscope (SEM). Increased roughness, cracks and pinholes were observedin a membrane after in-situ operation at sub-zero temperatures ranging from�5�C to �15�C [14]. The same was observed of cells in a fuel cell stack thatwere freeze/thawed 20 times at �20�C [15], and idle stacks during winter orlong duration installation under freezing temperatures [16]. Figure 6.4 showsSEM images of damage to a Nafion membrane after being stored andoperated at sub-zero ambient temperatures as low as �15�C [14]. The MEAwas assembled by spraying the catalyst on wet-proofed carbon paper andthen hot pressing the electrodes on the Nafion. After operation the electrodeswere separated from the membrane and the polymer electrolyte was exam-ined with an SEM. Increased membrane roughness is observed (Fig. 6.4(c))after sub-zero operation compared to a membrane operated at room tem-perature (Fig. 6.4(b)). At higher magnification, (Figs. 6.4(d) and (e)) micro-cavities and pinholes at the cathode outlet region of the membrane areclearly visible after sub-zero operation. This type of damage can lead toperformance loss through increased hydrogen crossover and loss of catalystactivity.
In the electrolyte, water content (l) is defined as the number of moles ofwater per mole of sulfonic acid group in the electrolyte. Membrane hydration iselementary to ionic conductivity, but over-hydration and subsequent freeze cancause damage. A dried MEA with l < 4 (water weight percent less than 6%),did not experience freeze-damage [17–20]. However, this state has difficulty ingenerating current from a frozen state because of low ionic conductivity[21,22]. Under sub-freezing conditions, Mukundan et al. [20] determined that7<l<12 in Nafion� resulted in optimal conductivity. Later, Tajiri et al. [22]observed similarly low ohmic losses for 6.2<l<14 in a W.L. Gore Primea�
MEA.Water in the membrane can exist in a non-freezing state or a freezing state.
Water in Nafion is subjected to a fluorocarbon environment and bonds stronglyto cations and ion exchange sites which prevents it from freezing to �120�C[23]. Other water molecules, interacting weakly with ions and cations exchangesites, freeze on reaching �20�C. Free water inside the membrane behaves likebulk water on the surface of the membrane and freezes immediately at 0�C [23].Since the strongly bonded water in the membrane has some freezing pointdepression, it should not cause damage as it does not freeze. Differential scan-ning calorimetry (DSC) has been used to measure the amount of unfrozen waterin fine grained media as a function of temperature. In addition, the finer the
(a) (b)
(c)
(e)
(d)
FIGURE 6.4 Effect of sub-zero temperature on membrane (a) Virgin Nafion membrane,
(b) membrane after operation at room temperature, (c) membrane after operation at �15�C,(d) membrane from cathode outlet regions after operation at �15�C and (e) membrane from
cathode outlet regions after operation at �15�C. Images from [14].
306 Polymer Electrolyte Fuel Cell Degradation
grains are, the greater the freezing point depression compared to free standingwater. DSC data from the literature [24,25,26] characterizing water compositionin Nafion has been extrapolated by He and Mench [6] and is shown in Fig. 6.5.It was shown that the weakly bonded water in Nafion pores sized 2 nm corre-sponds to a freeze point depression of 24.5 K. It was hypothesized by He et al.that this weakly bound water comes out of the membrane and results in freezingdamage at the interface between the membrane and the catalyst layer. An
FIGURE 6.5 Unfrozen water versus temperature curves derived from Nafion DSC data of ref
24,25,26. [6].
307Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
experimental study by Pineri and co-workers appears to confirm this [27]. In thePineri work, X-ray diffraction results indicate that some of thewater desorbs outof the membrane below 0�C. Damage could also be a result of the membraneswelling and contraction with temperature. The Pineri result suggests that thedamage is more likely a result of water outflow than membrane swelling. Theend result is the same for either case, and indicates that a major source ofpotential damage is excess membrane water resulting from liquid water incontact with the electrolyte at shut-down to a frozen state.
A study by Liu measured the strain before failure of Nafion 112 membraneafter dry and wet F/T cycles. The results are summarized in Table 6.4. Themembrane after dry F/T cycles from �40�C to 80�C ruptures under signifi-cantly less strain than a membrane not subjected to F/T, indicating someinternal change in structure. Wet F/T cycling did not show any additional
TABLE 6.4 Percent Elongation at Break of Nafion 112 Membranes Before/
After Dry/Wet F/T Thermal Cycling Compiled from [28]
Material Percent Elongation at break
Before Cycling After 385 F/T Cycles (Dry)
Membrane (Machine direction) 1290 40
Membrane (Cross direction) 320 25
Before Cycling After 200 F/T Cycles (Wet)
Membrane (Machine direction) >300 >300
Membrane (Cross direction) >300 >300
308 Polymer Electrolyte Fuel Cell Degradation
potential, as the strain after 200 wet F/T cycles was the same as the strain ofa new membrane [28]. The authors suggest that water in the membrane relievesstructural change that can occur during freezing, because it makes chainmovements more facile. It therefore prevents the membrane from becomingbrittle. This effect may also contribute to the observed exacerbated damagefrom uneven dry-out during purge of large stack plates. Areas of high local dry-out can suffer from reduced plasticity in the membrane on subsequent purges.
3.2. Catalyst Layer Damage
Maintaining catalyst layer integrity throughout operation is of critical impor-tance to a fuel cell performance, since other non-freeze related degradationmodes commonly cause significant irreversible damage to the catalyst,membrane, and support structure [29]. The catalyst layer (or electrode) isa porous media covering both faces of the membrane, with a typical thicknessrange of 5–30 mm and porosity of 0.4–0.6. A surface morphology character-ization of catalyst layer was performed by Hizir et al. [30]. Local catalystcracks with large relative dimensions O(mm) compared to pores are oftenobserved after F/T and sub-zero operations of fuel cells. Because the catalystlayer is between the membrane and the gas diffusion media, interfacialCLjmembrane and CLjDM delamination is often observed due to ice lensformation. In addition, because the catalyst layer is a reaction site, any damageto will most likely lead to a loss in electrochemically active area. However,there exist conflicting results, as some researchers observed damage at theelectrodes in the form of lost ECSA, physical cracking or pulverization of theelectrode, while other studies did not show any damage. The work of Kim et al.[3,4] investigated those conflicting conclusions by studying the effect of fuelcell component structures, DM stiffness/thickness and membrane rigidity onthe impact of freeze thaw (F/T) damage on the electrodes. Kim et al. deter-mined that a stiff DM with a thin, reinforced membrane was the best config-uration to mitigate damage from a freeze/thaw environment. DM thickness wasnot found to play a significant role in freeze/thaw damage.
3.2.1. Electrode Cracking
Extensive cracking of the electrode structure has been observed on MEAssubject to freeze-thaw cycles in both ex-situ and in-situ conditions. Figure 6.6 isan SEM image from the work of Guo and Qi and depicts the change in surfacemorphology of a commercial MEA frozen after being subjected to ambienttemperature and RH (Fig. 6(a)) versus a similar MEA that was frozen afterbeing hydrated in water at 80�C for 10 minutes (Fig. 6(b)). Each were cycledsix times between 20 and�30�C and soaked at�30�C for 6 hours during everycycle [18]. In Fig. 6.6(a) the electrode is smooth and almost no damage can beseen as a result of the six freeze thaw cycles. However, severe damage isapparent in Fig. 6.6(b) with cracks and obvious separation of the catalyst
(a) (b) FIGURE 6.6 SEM of the cathode side of freestanding MEAs after six freeze-thaw cycles between
20 and�30�C: (a)MEAwas only exposed to ambient temperature and relative humidity before going
through the freeze-thaw cycles; 100x magnification; (b) MEA that was fully hydrated in water at
80�C for 10min before going through the freeze/thaw cycles; 50x magnification. Images from [18].
309Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
surface. In the upper left corner, a total detachment of the catalyst material fromthe membrane can also be seen. Alink et al. also made a similar observation ex-situ with freeze/thaw cycling down to �40�C of a wet gas diffusion layer andMEA assembly without compressive forces [31]. In fact, without assemblycompression the mechanical bond is very weak and repeated testing fromvarious groups shows that detachment readily occurs.
Alink et al. also used a commercial typeMEA for in-situ freeze/thaw cyclingwith compressive forces. One assembly was exposed to fully humidified reac-tants at the cathode and anode before being cycled. SEM showed catalystdamage and fracture, although catalyst layer segregation was not as noticeableas in the ex-situ experiment. This could be due either to the fact that an MEA inan assembled fuel cell is subjected to pressure from the backing plates, or thefact that the membrane water uptake is less when subjected to vapor instead ofliquid [31] – or indeed both. This is in agreement with other research thatexplains how water drains and freezes in the catalyst layer below 0�C whenmembrane water content has reached a maximum [31]. This is also due to thefact that some water inside the membrane does not freeze, as explained inSection 2. Additionally, upon freezing in the electrode pores, water expands involume and may generate micro-cracks on the surface of the catalyst [18,20,31].
Delamination of the catalyst layer from both the membrane and the DM sideswas shown to occur in several publications after the cell is subjected to sub-zerooperation or brought to a frozen state without effective purging [3,4,14–16,20].The catalyst layer and the DM are both porous media and permeable to gas.With pore sizes ranging from several nanometers to hundreds of micrometers,
(a)
(c) (d)
(b)
FIGURE 6.7 Effect of sub-zero temperature on MEAwith cloth DM. (a) Virgin MEA, (b) MEA
after operation at room temperature, (c) MEA after operation at �15�C. Images from [14].
310 Polymer Electrolyte Fuel Cell Degradation
water confined in those pores experiences a freezing point depression of only 2to 4�C, which is not enough to prevent freeze damage [32]. The cathode side ismore prone to separation as water is generated in the cathode catalyst layer bythe oxygen reduction reaction. Figure 6.7 shows the evolution of a virgin MEA(Fig. 6.7(a)) upon operation at room temperature (Fig. 6.7(b)) where nodelamination is observed from liquid water. Delamination on both themembrane and DM side became apparent after operation at�10�C (Fig. 6.7(c))and �15�C (Fig. 6.7(d)). The delamination normally occurs under a channellocation, where overburden pressure is low. For this reason, open or mesh flowfield designs have an intrinsic advantage over conventional channel land designfor limiting freeze/thaw damage.
To better understand CL delamination and how fuel cell components canhelp to promote or mitigate freeze damage, Kim et al. investigated the effect ofDM stiffness, DM thickness and membrane rigidity on freeze/thaw damage inan ex-situ environment. The results are summarized below.
Effect of DM Stiffness
Frost heave formation and volume expansion of frozen water can induce shearforce on the catalyst layer leading to interfacial delamination. Frost heaving is
311Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
a phenomenon more complex than volume expansion of frozen water. Primaryand secondary heave can occur whether or not an ice fringe exists, depending onthe thermal andmass transport conditions. As shown in Fig. 6.8, the frozen fringeis the transition two-phase zone between 0% and 100% ice at the freezing front.Primary heave refers to frost heave with no frozen fringe, where no ice willpenetrate into the unfrozen area. During secondary heave, the ice lens grows andpenetrates into the frozen fringe [16]. Two test cells, one with flexible cloth DMand another with stiff carbon paper type DM, were F/T cycled 30 times from�40 to 70�C [4]. They are both shown in Fig. 6.9. Excessive surface damage andCLjDM delamination were observed on the catalyst layer of the cell assembledwith cloth DM (Fig. 6.9(a)), while no cracks were observed on the cell using stiffcarbon paper DM (Fig. 6.9(b)). A stiffer diffusion media more uniformlytranslates the compressive forces from under the land to under the channels andtherefore provides more deformation resistance when subject to ice growthpressure. The stronger compression to the CL surface can also reduce interfacialwater accumulation at shutdown. This also means that the channel width andchannel/land ratio are important parameters in uniformly spreading thecompressive forces; relatively wide channels will promote DM deformation.Figure 6.10 shows a calculated compression distribution from a common feltDM (SGL 10BB) onto the CL. To obtain the non-homogeneous compressionpressure data required for the simulation, the DM thickness versus compressionpressure data given in [33] is used to evaluate the non-homogeneous strain in theDM layer. Finally, using the DM strain data, the compression information underone land-channel configuration is extracted from the stress-strain data of the DMgiven in [34]. The DM thickness measurement was performed ex-situ for one setof land and channel (each of length 1mm), and Fig. 6.10 shows the variation ofthe non-homogeneous compression pressure from mid-land to mid-channellocation with a span of 1mm. As can be seen, even for a stiff DM, thecompression pressure on the catalyst layer drops off very sharply in the channel
(a) (b) FIGURE 6.8 Comparison of (a) primary and (b) secondary frost heave. Images from [6].
(a) (b) FIGURE 6.9 Surface images of MEAs cycled 30 times between �40 and 70�C with negligible
cracks in the virgin catalyst layer and 18mm reinforced membrane. Images shown correspond to
locations under channel. (a) CARBEL-CL (cloth type) DM and (b) SGL 10BB (non-woven felt
type) DM. Images from [4].
312 Polymer Electrolyte Fuel Cell Degradation
region. This is the reason delamination damage is most likely in this location.Based onmodeling from S.He et al, the calculated ice-phase pressure rarely getsover 2MPa, which is at the high range of the normal compression experiencedunder a land in a typical fuel cell.
Effect of DM Thickness
Kim et al. also compared the effect of carbon paper DM thickness on a 35 mmreinforced membrane known to be sensitive to damage [4] and an 18 mm
Mid-land to mid-channel span (mm)
0.0 0.2 0.4 0.6 0.8 1.0
Co
mp
ressio
n p
ressu
re (M
Pa)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
FIGURE 6.10 Calculated compression distribution from a common DM onto the CL.
313Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
reinforced membrane known to be less sensitive, as seen in Fig. 6.11. Thenon-woven felt type is stiffer due to its more three dimensional lattices. Bothcells with 18 mm membrane showed no physical damage with either thincarbon paper DM (235 mm) of non-woven paper type (Fig. 6.11(a)) or thickercarbon (415 mm) non-woven felt type DM (Fig. 6.11(b)) respectively. This isa result of stiff DM applying some compressive force under the channels.Interestingly, the stiff DM did not prevent damage on the thicker membrane(Fig. 6.11(c)) and the thicker stiff DM showed as much damage (Fig. 6.11(d)).It was concluded that a thickness from 235mm to 415mm of non-woven typewas not significant to mitigate the observed physical damage to the electrodesurface [4].
Effect of Membrane Rigidity and Thickness
A test cell with 18 mm non-reinforced membrane and carbon paper DM wasF/T cycled 30 times between �40�C and 70�C [3]. Fig. 6.12 shows inter-facial delamination under the channel. Although the DM was stiff carbon
(a) (b)
(c) (d)
FIGURE 6.11 Cross-sectional images of MEAs with negligible virgin cracked catalyst layers, F/T
with SGL 25BC DM; (d) 35mm reinforced membrane with SGL 10BB DM. Images from [4].
FIGURE 6.12 SEM image of F/T cycled non-cracked CL with 18mm non-reinforced membrane
under the channel location [3].
314 Polymer Electrolyte Fuel Cell Degradation
paper, the non-reinforced membrane promoted delamination. Membranereinforcement is used to make a membrane mechanically stronger and moredurable without significantly changing its conductive capabilities. The mostcommon methods include adding a strong polymer such as expandableporous polytetrafluoroethylene or other fibers, resulting in a membranecomposite [35]. When using a thicker 35 mm reinforced membrane withcarbon paper DM, as shown in Fig. 6.13, frost heave damage is visible underthe channels. Although the thicker membrane is reinforced, it is a bigger
FIGURE 6.13 SEM image of F/T cycled non-cracked CL with 35mm reinforced membrane
under the channel location [3].
315Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
reservoir for water and by itself a source of water for damage in the CL [3].Therefore, the best material combination to mitigate freeze-damage wasfound to be a non-cracked virgin catalyst layer on a reinforced, thinmembrane, assembled with stiff diffusion media. Although this freeze-tolerable design reduced freeze damage under worst case scenarios of directliquid contact with the ionomer at freeze, irreversible damage was stillpresent, highlighting the importance of liquid removal from the catalyst layerbefore shut-down to a frozen state.
3.3. Loss of Electrochemical Surface Area
Besides physically observable damage, performance is directly relevant to theelectrochemical surface area (ECSA) at the electrodes, which can be measuredin-situ with cyclic voltammetry [18,20,32,36]. Even without major observablemorphological damage, ECSA loss has been observed in F/T testing. After 20freeze/thaw cycles (20 to�30�C at fully humidified state) Guo and Qi observedECSA loss at both electrodes. ECSA decreased by 23% at the cathode and by15% at the anode, as seen in Fig. 6.14. This difference could be due to storage ofgeneratedwater in the cathode CL due to previous operations. However, the shortterm performance of the cell did not show much change [18]. Hou et al. inves-tigated freeze degradation using 20 freeze/thaw cycles between �20�C and60�C. The cell was operated at 60�C, purged by gases at 25�Cwith 58%RH aftereach operation, and then frozen to�20�C. Cyclic voltammetry (performed onlyat the cathode) showed that values of ECSAfluctuated between 45.4 and 51.2m2/gcat and did not decrease progressively after each cycle [19]. Interestingly, thisfluctuation did not alter the performance curves after each freeze/thaw cycle.Although the ECSA measurement fluctuation could be from the experimentaldevice, cyclic voltammetry is a transient test and it is possible to have detectedstructure alteration or liquid water transients blocking the triple-phase boundarywhichwould not affect a steady-state performance test.Mukundan et al. [20] alsoperformed ECSA measurements to compare the durability of their Los AlamosNational Lab (LANL)-made MEAs to MEAs from W.L. Gore. The W.L. GoreMEAs showed >50% loss in the catalyst surface area after five cold starts at�10�C, while LANL-prepared MEA showed negligible loss. In this study,catalyst layer morphology is obviously important for durability at freezingconditions; as previously discussed, water in smaller pores may not freeze atconditions inwhichwater in larger pores does. This loss in ECSA after cold startswas not observed at the anode, clearly because water is generated at the cathodeside. Ge and Wang [32] and Srouji [36] made the same observation regardinglack of damage at the anode from cold starts. Ge andWang have also recorded 1to 3% of Pt area loss at the cathode per cold start performed at�10�C, althougheach cold start was interrupted by a thaw and operation at 70�C, making the cellgo to a freeze down process before each cold start [32]. Srouji recorded 4.4% ofPt area loss at the cathode after 25 consecutive cold starts at�10�C. The protocol
FIGURE 6.14 Cyclic voltammograms of (a) cathode and (b) anode of an MEA after 0–20
freeze/thaw cycles at fully humidified state. Images from [18].
316 Polymer Electrolyte Fuel Cell Degradation
developed for rapid consecutive cold starts with known initial membrane watercontent is described in detail by Chacko et al. [37], and is capable of isolatingsub-zero operation damage from thewater generated at the cathode from residualwater damage resulting from the freeze down process itself. A challenge in cyclicvoltammetry studies is to correlate ECSA loss with performance loss duringsteady-state operation.
317Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
3.4. DM Fracture and Loss of Hydrophobicity
Some failure of the gas diffusion media has been observed as an apparentconsequence of F/T cycles. Mukundan et al. [20] observed DM failure after 10F/T cycles down to �80�C. However, Yan et al. [14] witnessed no damage toDM after exposure to normal conditions but noticed an increase in porosity anddarkness in color after sub-zero temperature exposure. They attributed thisphenomenon to ice forming in the DM. Results from neutron imaging studieshave shown that the saturation of the DM materials rarely can exceed 30%, sothat the additional 8% volume expansion from freezing should be tolerable inthe overall structure. However, some damage can occur if the water is locallyconfined by an enclosed pore structure or surrounding ice. Although the DMprovides a stiff support for the MEA and a hydrophobic barrier, it does storea considerable amount of water in the CL after shut down [31]. Evidence ofreduced hydrophobicity from exposure to freezing conditions with high liquidsaturation has also been observed. The damage to the DM in freeze conditionsneeds to be investigated more for a more complete understanding.
4. METHODS OF FREEZE DAMAGE MITIGATION
There are various concepts for preventing the fuel cell system damage causedby freezing, as well as a damage-free rapid start-up in sub-freezing conditionsthrough good energy/power management. Pesaran et al. [38] categorizeda review of solutions into two strategies: ‘Keep Warm’ where the system usesenergy during vehicle parking and ‘Thaw and Heat at Startup’ which consumesenergy mostly at vehicle startup. A summary of the various approaches isshown in Fig. 6.15. Intellectual properties have been developed for most if notall of them, and a compilation of 160 patents for freeze damage mitigation arelisted and summarized in reference [38]. In that same milestone report, it isconcluded that the correct use of insulation around the stack components candelay stack freeze by several days after it is shut down.
Residual water reduction and evaporation during shut-down before the fuelcell is frozen can be achieved by several methods, including:
1. convective purge;2. vacuum purge;3. capillary drainage;4. thermally driven drainage; and5. combinations of the above.
No more than 62.5 J/We should be consumed during cold start-up, based on theDOE goals for parasitic losses. The ultimate goal is a non-parasitic shut-downwith no damage.
Clearly, the key to shut-down is proper removal of liquid which is in contactwith the ionomer without producing overly dry areas of the membrane. Several
Insulation Box
Vacuum Insulation
Catalytic Burner
Low-Power Stack Operation
Electric Heater
Temperature Sensing
Freezing Judgment
Controller
Energy Use Minimizing
Discon. Reactants Humid.
Vacuum Drain
Humidifier
Combusting H2 in Channels
Reaction Heat at Catalyst
Wire Heating in MEA
Coolant Heating
Hot Air Blowing
Thawing Tank
Burner, Compressor
• Water Draining & Purging
• Insulation
• Energy/Power Source
• Sensing & Control
• Strategy for Using Energy
• Re-Humidification
• Internal Heating
• External Heating
• Component
Keep-WarmDuring Dwelling(Moist Parking)
Thawing andHeat Up at Start(Drain Parking)
Hybrid: FC &High Power ES(Battery/Ucap)
Determining StrategiesBetween the Two
FIGURE 6.15 Method and technology chart for fuel cell start from subfreezing environment [38].
318
Polym
erElectro
lyteFu
elCell
Degrad
ation
319Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
studies agree that an MEA equilibrated to 80~95% relative humidity is bettersuited for rapid cold start, since this provides some storage for generated waterduring start-up and assures limited liquid-phase ionomer contact at freeze.However, this is difficult to achieve in practice without an exceedinglylong purge, high parasitic losses, or distributed stresses which can lead todegradation and the fact that it is progressively more difficult to removewater with decreasing temperature, to maintain a dryer than saturated state [39].
A suggested optimal purge strategy is to keep purging until the water in thechannels and diffusion media is removed, while water is still largely present inthe membrane [5]. An MK 9 series 10 cell stack used in this study had anoptimal purge duration of 88 seconds with dry air and H2 at 89 L/min and25 L/min respectively; both at 70�C and 1.6 bar. However, this can be difficultto achieve in full size stack plates. Cho and Mench showed that for certainconditions, the water content in the cell is not correlated with high frequencyresistance (HFR) during purge, and is not a good metric of water removal fromthe cell. Figure 6.16 shows this [40]. For this plot, data were taken usingneutron imaging to record total liquid water content, and HFR, to recordaverage membrane resistance. Different combinations of anode and cathodeinlet relative humidity were used to purge a 250 cm2 full size fuel cell stackplate. Each test began from the same initial conditions. All comparative purgeswere operated at the same flow rates relative to each other. As can be seen fromthe figure, a full humidity (100/100% RH anode/cathode) purge results in waterremoval from the cell, indicating significant accumulation in the channels can
FIGURE 6.16 Cell water amount from neutron radiography with respect to membrane resistance
at different operating conditions during purge [40].
320 Polymer Electrolyte Fuel Cell Degradation
be removed due to non-evaporative effects such as shear. Due to back diffusionand the initial water distribution, there is a sharp difference in the membranedry-out compared to a dry anode or cathode purge. As discussed, membranenon-uniformities in water content have been shown to exacerbate damage andshould be avoided. Recent work has shown a novel composite purge approachcan most efficiently remove water content while preventing membrane dry-out[41,42], as shown in Figs 6.17(a) and (b). The characteristic water removalbehavior during gas purge was analyzed using neutron radiography (NR) andHFR, as shown in Figs 6.17 (a) and (b). NR is used for quantifying the total
Purge time (minute)
0
Liq
uid
w
ater a
mo
un
t (kg
m
-3
x 10
-3)
35
38
41
44
47
50
High purge flow rate
Medium purge flow rate
Low purge flow rate
Composite purge flow rate
Purge time (minute)
Cell resistan
ce (m
• )
0
2
4
6
8
10
12
High purge flow rate
Medium purge flow rate
Low purge flow rate
Composite purge flow rate
(a)
(b)
3 6 9 12 15
0 3 6 9 12 15
FIGURE 6.17 Water removal behavior of fuel cell during purge: (a) variation of water amount in
the cell and (b) variation of total cell resistance [41].
321Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
amount of water residing in all the components of the fuel cell, whereas HFR isutilized to indicate variation of water content in the membrane. Therefore, bycomparing both data sets during purge, water removal behavior can beunderstood in detail. As shown in Figs 6.17 (a) and (b), a high flow rate purgewas very fast and efficient for decreasing the residual water in the cell, butincreased the cell resistance substantially, raising issues of possible degradationof the membrane and high energy consumption. For a relatively low flow ratepurge, the cell resistance did not increase severely, but water removal from thecell was not efficient. However, in the case of a composite purge with mixedpurge flow rates (high flow rate for 1min., medium flow rate for 3min., and lowflow rate for 10min.), the water removal rate from the cell was almost identicalto the medium flow rate case, but with reduced membrane resistance increase(�91%) and less energy consumption (�24%). More details of this can befound in ref. [41].
A typical convective method of removing residual water during shut-downis purging with hot dry gas, which is effective in rapidly evaporating residualliquid water from inside the DM or CL and the channels. The convenience ofthis method depends on the reactant gas flow field patterns. It often leads tonon-uniform water distribution, which can result in rapid degradation of theMEA. For example serpentine flow field patterns have more water content nearthe outlet and suffer from dry out at the gas inlets. This results in mechanicalstress causing physical degradation. Serpentine flow fields also suffer fromwater accumulation around the 180� turns, as shown in Fig. 6.18. This accu-mulation would tend to damage the cell upon freeze or prevent proper start-upvia channel blockage. However, parallel flow fields have less resistivity to fluidmotion and hence mitigate non-uniformity [43]. This general effect has beenobserved consistently in both small and full size stack designs, leading toa modern design paradigm that seeks to straighten the flow field as much as
FIGURE 6.18 Neutron radiograph showing
a tendency for water accumulation at corners and
switchbacks in the fuel cell flow channel [43].
322 Polymer Electrolyte Fuel Cell Degradation
possible and manage water content through thermal or other transport mech-anisms to eliminate these effects and reduce water content.
Dry purging should be done with careful attention to the purge gastemperature. AnMK 513 series single cell of Ballard Power Systems Inc. [5,44]experienced freeze damage after a dry hot purge (dry N2 purge was conductedright after operation for one minute at 85�C on both sides). On the other hand,no damage occurred when the cell was cooled down to ambient temperatureand then purged with cold dry N2. Although no reason behind this observationwas disclosed, it’s important to note that the MK 513 series cell has very long,parallel flow channels. The hot purge may have over-dried the MEA near theinlet and then cooled and wetted the MEA near the outlets, leading to freeze-damage. However, a cold purge induces less gradients of moisture leading toslower evaporation but less damage and a more uniform water distribution. Thekey point is removal of liquid water in contact with the catalyst layer withoutinducing damage from uneven stress caused by drying.
Although water removal is necessary for freeze damage mitigation, it isvery important not to over-dry the MEA. A dehydrated membrane will havea very low electric conductivity and cold start-up will not be possible. HFRmeasures the ionic resistance and therefore can be used as a diagnostic tool todetermine optimal purge duration. HFR is not affected when the purgingprocess removes residual liquid water from the channels and DM. Cell resis-tance starts to increase when water removal is initiated at the membrane level.An optimum strategy is to stop the purge at the inflection point of the resistanceversus time curve [5] as show in Fig. 6.19 for an Mk9 10-cells stack.
Vacuum purging was proposed as a method for drying out the DM and MEAof a cell [45], before shut-down in a freezing environment, since water is moreeasily drained at higher temperatures because of better evaporation. It’s
FIGURE 6.19 HFR change with purge time [5].
323Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
preferable to start vacuum purging as soon as the cell is shut down from itsoperating temperature. Vacuum drying at a higher temperature dictates the needfor a smaller vacuum pump which is typically already onboard a vehicle forother purposes. Although this approach is shown to result in damage mitigation,and may be appropriate in certain niche applications, in general it is notgenerally believed to be practical in operating systems.
Temperature-gradient driven water transport is an attractive non-parasiticwater drainage method during fuel cell shut-down. The use of engineeredtemperature gradients within the stack has been demonstrated to prevent freezedamage [46,47]. There are two basic modes of temperature-gradient driven fluxof water that are relevant at shut-down; thermo-osmotic transport in themembrane, and phase-change-induced (PCI) flux through the open voids.Thermo-osmosis in the membrane is the water flux observed when water withdifferent temperatures is separated by the membrane [44,46,48–54]. Thermo-osmotic water flux in fuel cell membranes is from the cold to the hot side, anddepends on the difference in entropy between water stored in the membrane andwater external to the membrane [53]. Unbound water transport is thermody-namically favored in the direction with increasing entropy [52,53]. Kim et al.further investigated water flux through the membrane and concluded that waterflux is proportional to temperature difference as shown in Fig. 6.20, andinversely proportional to membrane thickness as seen in Fig. 6.21. An Arrhe-nius rate law was determined to capture this transport mode.
PCI flow occurs with the presence of a temperature gradient and gas phasein the CL, MPL or main DM and dominates once irreducible saturation is
FIGURE 6.20 Thermo-osmotic water flux in Nafion 112 membrane [55].
FIGURE 6.21 Comparison of thermo-osmotic water flux of membranes [55].
324 Polymer Electrolyte Fuel Cell Degradation
attained in the porous media [54–56]. PCI flow is strongly dependent onaverage membrane temperature and temperature gradients [54–56]. The effectof DM/CL thermal mass was negligible. M. Khandelwal and Mench showedthat thermo-osmotic flow can either assist or oppose PCI flow depending onthe hydrophobic properties of the membrane [57]. In fuel cell media, itgenerally opposes the PCI flow. Thus, a residual water content in the warmerelectrode can result under significant temperature gradients, which would tendto occur near the end plates of a stack. Therefore, to minimize water in thecathode CL, thermo-osmosis flux across the membrane is very important tohelp freeze durability. Both PCI flow and thermo-osmosis in variousmembranes and DM material sets have been experimentally investigated andquantified, and it was determined that both modes of transport can be well-correlated using Arrhenius rate laws as shown in Fig. 6.22. Although the typeof reinforcement in the membrane has some impact, thermo-osmosis is fairlyconstant for perfluorosulfonic type membranes, but significantly less thanregular concentration-based diffusion. Therefore, this mode of transport is notnormally critical during operation, given the existing high range of uncertaintyin published diffusivity values. However, during shut-down to a frozen state,thermo-osmosis can become important, as it can counteract the PCI flow,which moves liquid toward the cold location. The result of the interaction canbe a residual frozen water saturation in the warmer-side catalyst layer of theMEA, as has been shown via recent modeling of this effect [57]. In general,the PCI flow is much more significant for even the small temperature gradi-ents expected during shut-down between MEA components [56]. Several fuelcell manufacturers have also investigated this effect, and Ballard suggested
0.0026
-8.5
-8.0
-7.5
-7.0
-6.5
T=10KT= 5KT= 3K
Reciprocal Temperature (1/K)
Diffu
sivity, lo
g10|D
| (kg
/m
-K
-s)
Phase change
induced flow
(hot to cold)
Thermo-osmosis
(Cold to Hot)
NR-MEA/SGL10BB Y = -1.486 - 1870X R-MEA/SGL10BB Y = -1.5974 - 1849.2X R-PEM A/SGL10BB Y = -1.6477 - 1858.9X Nafion 112/SGL10BB Y = -2.1236 - 1723.6X R-PEM A/SGL10AA Y= -4.767 - 998X
0.0028 0.0030 0.0032 0.0034 0.0036
FIGURE 6.22 Correlated thermo-osmosis and PCI flow relationships based on an Arrhenius rate
law. Data from different temperature gradients all conveniently collapse into a single curve for
a given material set [54,55].
325Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
a unique stack design to promote internal temperature gradients near end-platelocations to avoid damage and promote reliable start-up from a frozencondition [46].
4.1. Damage Mitigation via Material Choice and Design
Although the various presented methods of mitigation are useful, in principleno action would be needed at shut-down if the operational overhead of liquidwater was reduced to a value below that at which damage occurs. That is, ifproper materials and design to reduce the liquid water overhead can be chosen,the required parasitic purge can be reduced. Work by Turhan et al. has shownthat water content in a fuel cell can be reduced by as much as 50% with littleperformance change, just by changing the DM thickness and channel/landdesign [58]. The following parameters have been determined from accumu-lated research to be key controlling parameters in the water content in the fuelcell porous media and flow channels:
1. The operating conditions: It should be noted that high current does notnecessarily result in higher water content. In fact, the high channel flowrates and heat produced by inefficiency often reduce water content ascurrent increases. Low current conditions often have the greatest total storedwater content in the fuel cell.
2. The thermal boundary conditions and heat transport: PCI flow plays a crit-ical role in water distribution, as proven by various studies. Water
326 Polymer Electrolyte Fuel Cell Degradation
distribution and storage can be controlled through manipulation of thisboundary condition via coolant channel design or material selection.
3. The material choices: Tremendous shifts in water content at similar oper-ating conditions have been observed depending on the thickness and typeof diffusion media and other components.
4. The channel/land interface: The shape and surface energy (e.g. contactangle) have been shown to be critical in the drainage of liquid from accumu-lation under the lands, as described in [58]. This impact should not be over-looked in terms of expected water content and freeze effects. In general,liquid flow across this interface is dominated by capillary action, so thatthe interface shape, roughness, contour, and surface energy are importantaspects of drainage. A hydrophilic interface is preferred to allow drainagefrom the DM into the channel.
5. Manifold design: The ability for water to drain from the internal channelstructure into the main manifold is a key factor. In this location, even a smallamount of water can impede the ability to properly start-up from a frozenstate. Thus, it is critical that this location remain free of accumulation atshut-down.
6. Channel design: As described, there is a general desire to reduce the numberof flow switchbacks and flow deceleration points to avoid water accumula-tion. Thus, the general design paradigm from this result is to straighten theflow path as much as possible, and maintain water balance through othermeans such as boundary temperature control.
7. Channel shape and surface energy: It has been shown by many researchersin the fuel cell and micro-fluidics field, that there is a clear relationshipbetween water retention in the channels and channel shape and surfaceenergy. However, using hydrophobic channels is not a particularly goodsolution since it restricts water removal from the DM and can result in oper-ational instabilities related to the creation of multiple slugs of water [59].
Many of these parameters are not included in modern computational models.Thus, there is still a tremendous discrepancy between the water distributionpredicted and that which is observed in practice [60]. Clearly, much additionalresearch is needed before this can be fully resolved.
4.2. Comments on Proper Conditions for ExperimentalTesting of Freeze/Thaw
As discussed, there are discrepancies between the results in literature forseemingly similar testing. The issues which result in these discrepancies include:
1. Differences in the experimental configuration or materials. As discussed,the stiffness of the DM, membrane thickness and type, as well as channelto land width ratio and compression play a strong role in the developmentof freeze/thaw damage.
327Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
2. For single cells, it is imperative that precise thermal boundary conditions aremaintained. Single-cell testing has traditionally taken place using a heatingcartridge to maintain temperature, but neutron imaging has shown this toresult in very different internal water distribution than if coolant channelsare used. For accurate testing at the single cell level, it is imperative thatcoolant-based or other type boundary temperature control is used that issuperior to cartridge heaters, which provide inconsistent and non-represen-tative heating and cooling behavior.
3. The shut-down procedure used in the laboratory is obviously critical andshould be carefully considered and controlled in terms of thermal boundaryconditions. One of the main differences between single cell and stack cellfreeze/thaw testing is that in a single cell, both sides are colder than thecenter of the cell at shut-down. This results in PCI flow removal of waterfrom both electrodes, and mitigation of freeze damage compared to an in-stack cell, where the temperature gradient is in one direction on both sidesof the membrane. In-stack cell testing can be simulated by using dualcoolant controlled boundary conditions. If separate coolant flows areused, a temperature gradient representative of any particular location inthe fuel cell stack can be simulated.
4. The initial conditions before shut-down and purge to a frozen state shouldalso be carefully maintained. That is, the same shut-down procedure,executed on two similar cells with a different operational history, will resultin a different final condition before freeze. This discrepancy can be elimi-nated by ending a cyclewith a pre-shut-down step. By operating at a selectedknown condition for a significant period of time and then initiating shut-down, the previous operational history effects can be effectively erased,and the cells will be shut-down from a precise initial condition. Forexample, for a 50 cm2 active area single cell, operation at 0.6 V for 30minutes before initiating the purge protocol should eliminate any differ-ences that might arise from operational conditions before purge.
5. SUMMARY AND FUTURE OUTLOOK
This chapter has examined the results of published studies that examinephysicochemical degradation in polymer electrolyte fuel cells resulting froma shut-down to a frozen state. Damage caused from a frozen start-up is out ofthe scope of this publication, but deserves additional attention in the literature.Ultimately, no damage as a result of freezing to �40�C was observed for anycommon fuel cell materials if there was no contact with liquid water. Thisresult indicates that damage-free shut-down to a frozen state is possiblethrough proper engineering of operational and shut-down protocol, materials,and design. Achieving a damage-free frozen condition is difficult, however,because the time for purge should be short, and parasitic losses should beminimized.
328 Polymer Electrolyte Fuel Cell Degradation
Four to six years ago, the major automotive manufacturers reportedsuccessful start-up of their respective fuel cell vehicles in relation to the issueof freeze. It seems that their approach was through good systems engineeringleading to a multitude of patents. In fact, sustaining a freezing environment isnot the challenge holding up fuel cell vehicle commercialization. On the otherhand, the 2007 DOE fuel cell technical plan reports that clear results ofdegradation rate over a 5,000 hour lifespan (150,000 miles equivalent) of anautomotive stack have not been declared, although it is estimated to be< 20%. The ultimate goal is 5% performance degradation at the end of lifeof a stack subjected to the full range of external environmental conditions(�40 to 40�C).
From a summary of the existing literature, damage to the fuel cellcomponents is a result of water expansion upon freezing, and a frost-heavedelamination mechanism unrelated to the expansion process. Electrochemicalsurface area (ECSA) reduction has been commonly measured as a function offrozen conditions. Physical damage to PEFC components were identified toinclude membranejCL delamination, CLjDM delamination, and local poredamage in porous layer (CL and DM), and some membrane cracking. Lossof DM hydrophobicity and some morphological changes have also beenobserved, including some instances of DM punch-through from iceformation.
A key source of freeze damage is now known to be the result of liquid watercontact with the ionomer in the CL and membrane at shut-down. After a frozencondition is reached, the excess water uptake in the membrane can causesignificant local delamination damage along the CL interface.
Key factors which influence the degree of damage include the compressiondistribution on the MEA, membrane type and thickness, diffusion mediastiffness, and shut-down conditions. Designs which limit areas of lowcompression are better suited for a frozen environment. Stiff diffusion mediamaterials and thinner membranes with reinforced structure offer the greatestresistance to damage by limiting expansion and contraction forces, potentialinterfacial accumulations of water, and membrane-based sources of waterunder a frozen state.
The ideal shut-down condition appears to be one in which the membranephase is slightly and uniformly under-humidified, ensuring a low level of liquidaccumulation and contact with the ionomer. Overly drying the membraneresults in a poor cold start and potential membrane damage from internal stressgeneration.
In order to achieve the desired shut-down condition of a slightly anduniformly under-humidified membrane, a simple, low temperature dry purge iseffective, but too time consuming and parasitic to achieve desired levels ofperformance in practical operating systems. Various different purge approachesand damage mitigation techniques have been developed. Among the mostpromising is the use of controlled temperature gradients to assist liquid water
329Chapter | 6 Freeze Damage to Polymer Electrolyte Fuel Cells
drainage via a phase change induced flow. Engineering design of typical stackcomponents or coolant flow can induce sufficient gradients during shut-down toassist water removal from porous media into the channels, which can then beflushed by a short blast purge under low temperature conditions that will notharm the membrane.
Many of the discrepancies between the experimentally observedphenomena can be attributed to the different materials, channel/land configu-rations and operational protocols. It is critical in freeze/thaw testing to achieveproper thermal boundary conditions and initial conditions for shut-down toa frozen state to assure reliable data and interpretation. A key differencebetween single cell and in-stack data is the thermal boundary conditions, whichcan control the final distribution of liquid going into a frozen state. A dualcoolant system can be used to achieve near isothermal controlled boundaryconditions, as well as to simulate accurate conditions for in-stack cells witha single laboratory cell.
Although much work has been done to identify and explain freeze damagein PEFCs, there is still work to be done. The role of the CLjDM interface hasbeen shown to be critical, yet little is known about the in-situ nature of thisinterface, particularly under dynamic operating conditions. Understanding thenature of materials and design so that the residual liquid water overhead in thefuel cell can be reduced before shut-down is perhaps more critical to achievedesired performance levels. If cells can be designed to have greatly reducedstored water content during operation, less is obviously required of the shut-down. Finally, a more complete knowledge of the nature of condensation andevaporation in fuel cell media is required for accurate modeling. Currently,models are constructed based on thermodynamic driving force of saturationpressure gradients and no information on the potentially important effects ofsurface energy or morphology are included.
ACRONYMS
CL
Catalyst Layer CV Cyclic Voltammetry DM Diffusion Media DSC Differential Scanning Calorimetry ECSA Electrochemical Surface Area F/T Freeze/Thaw GDL Gas Diffusion Layer HFR High Frequency Resistance LANL Los Alamos National Lab MEA Membrane Electrolyte Assembly MPL Microporous Layer PEFC Polymer Electrolyte Fuel Cell PEM Polymer Electrolyte Membrane RH Relative Humidity SEM Scanning Electron Microscopy
330 Polymer Electrolyte Fuel Cell Degradation
REFERENCES
[1] US Department of Energy, Hydrogen Fuel Cells and Infrastructure Technologies Program.
