Polymer Electrolyte Fuel Cell Degradation

460 Pages·2011·22.655 MB·472\460

by  Matthew Mench, Emin Caglan Kumbur And T. Nejat Veziroglu (Eds.)

Polymer Electrolyte
Fuel Cell Degradation
Matthew M. Mench
Emin Caglan Kumbur
T. Nejat Veziroglu
AMSTERDAMlBOSTONlHEIDELBERGlLONDON
NEWYORKlOXFORDlPARISlSANDIEGO
SANFRANCISCOlSINGAPORElSYDNEYlTOKYO
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Preface
Thefieldoffuelcellscienceandtechnologycontinuestoevolverapidly,with
conversion to the commercialization stage already well underway. For
example,in2009over24,000fuelcellunitsfromvariousmanufacturerswere
shipped worldwide. This represents a 40% increase compared to 2008 [1]. In
the last decade alone, the cost of mass production (e.g. 500,000 units) of
transportationfuelcellsystemshasdroppedbynearlyanorderofmagnitude,to
anestimatedcostof$45perkWin2010[1].Allothermetricsofperformance
have continued to improve as well, including operational stability, cold-start
capability and survivability, and operating efficiency. The cost of hydrogen
production and storage has also continually dropped, and is now already at
levels that are considered economically competitive with existing sources in
certain markets. Even as budget constraints and political considerations have
limited expansion in the United States, the rest of the world has continued to
accelerate hydrogen and fuel cell development, with numerous plans for
continued growth and commercialization inAsia andEurope.
While real commercialization has already begun, there are still significant
development challenges, such as long-term durability, that require advanced
researchandengineeringsolutions.Thesubjectoflong-termdurabilityoffuel
cell systems, and in particular polymer electrolyte fuel cells (PEFCs), is the
subject of this book. It should be noted that this book does not cover system-
relatedissuessuchasblowersorpumps,butinsteadfocusesonthefundamental
andpracticalissuesofthestackitself.Allmaterialsandlayersinthestackfrom
the electrolyte to the bipolar plates are covered, as well as computational and
experimental methods, and the protocol used to evaluate and predict degra-
dation.Presently,degradationisobservedinallmaterialsinvolved,sothatnone
can be overlooked, and no one breakthrough will solve all of the industrial
needs.Althoughadvances indurabilityhavebeen rapid,theyhaveoftenbeen
founded on trial-and-error, and the fundamental science has lagged behind.
WheresignificantprogresshasbeenmadethroughmoreEdisonianapproaches,
more academic studies are needed. Conversely, the academic studying these
issuesneedstounderstandtheunderlyingappliedissuesandthebigpicture.To
respond to the underlying need for a combined exchange of applied and
academic information, this book is designed as a compilation of the state-of-
the-art fundamental knowledge and applied practice of some of the top
international experts in the field, both in industry and academia. The focus,
whereverpossible,isonthefundamentalunderstandingandunderlyingphysics
of the degradation processes involved inside the fuel cell stack, so that it is
timeless,andnotafunctionofaparticulardesign.Thecombined applied and
ix
x Preface
academicnatureofthebookmakesitsuitableasareferenceinagraduateclass
studyingthe topic, orasaneducationalsummaryandreference forpracticing
engineers.
Chapter 1 presents an overview of the durability status and United States
Department of Energy future targets for PEFCs, and is authored by Professor
E.C.KumburandhisstudentsatDrexelUniversity.Thischapterisprovidedto
givethereaderageneraloverviewofthepresentstate-of-the-artandthefuture
requirementsneededtoensurecompetitivesystemsinavarietyofapplications,
beforedivingintotherestofthemorespecificmaterial.Ofcourse,themoment
itwaswritten,thestate-of-the-artbecameoutofdate,sothereadershouldrefer
to the online material available from the Department of Energy or other
resources for amoretimelyreference.
Chapter 2 is written by top researchers at General Motors Fuel Cell
Activities,andcoversphysicalandchemicaldegradationofthemembrane.The
membraneisperhapsthemostdifficultandcomplexmediumtodesignforhigh
durability. Coupled physicochemical degradation occurs, and there is always
a push to move to higher temperature, lower humidity environments that
exacerbate these mechanisms. This important chapter summarizes the multi-
disciplinary understanding that overlaps fields of electrochemistry, transport,
mechanicalengineering,andpolymerscience,allofwhichareneededtofully
understand the underlyingphysicsof thismaterial.
Chapter 3 is written by Dr Shyam Kocha, formerly of Nissan Motor
Company and presently with the National Renewable Energy Laboratory
(NREL) in the United States. Based on his dozen years of experience in the
research and development of PEMFCs in industry (UTC Fuel Cells, General
Motors and Nissan), Dr Kocha’s chapter summarizes the fundamental under-
standing of the modes, mechanisms, and mitigation strategies involved in
degradation and durability of the electrocatalyst and carbon-based support
structure.Thisshouldbeofgreatinteresttoalargecross-sectionofengineers,
sincethedrivingforcesforthevariousdegradationmodesaffectmanyaspects
of the system and materialdesign.
Chapter4ofthisbookisbelievedtobeauniquecontributiontoliteraturein
thisfield,asitexploresourunderstandingofdegradationinthefuelcelldiffusion
medialayer.WrittenbyProfessorBrunoPolletoftheUniversityofBirmingham
in the United Kingdom, it summarizes results from industry and academics in
thisrelativelyunexploredtopic,andshouldbeofgreatvaluetothecommunity.
Chapter5movestotheedgeofthefuelcell,andexaminesthebipolarplate
degradationmechanisms.WrittenbyProfessorH.Tawfik,ofFarmingdaleState
University, Dr Y. Hung of Brookhaven National Laboratory, and Professor D.
Mahajan of Stony Brook University, this chapter is perhaps the first such
summaryavailableintheliterature.Aspecialfocusonmetallicbipolarplatesis
given, as they represent the least expensive alternative available and are
therefore more likely to be implemented on a massive scale, especially in
automotivebased applications.
Preface xi
InChapter6,thesubjectmattermovesawayfromaspecificmateriallayer
andinsteadreviewsthephenomenonoffreeze-relateddegradationinpolymer
electrolyte fuel cells. Written by Professor M. M. Mench of the University of
Tennessee, Knoxville and his Ph.D. student A. K. Srouji, the field of freeze-
related damage and mitigation strategies is reviewed. This topic is extremely
richinfundamentalscienceandcomplextransportphenomena,andshouldbe
of interest to engineers involved in the design of any system exposed to
extremely cold environments.
Chapter 7 is an extremely unique contribution, covering the practical
experimental diagnostics and durability testing protocols used at one of the
leadingfuelcelldevelopers;UTCPower.WrittenbyRobDarling,RyanBalliet
andMikePerryofUTCPower,thischapteriscompletelyfresh,andnotfound
inotherliteraturetodate.Sinceitcoversprotocolsdevelopedtoexperimentally
understand degradation phenomena, it should be of particular interest to
practicing engineers andindustrialdevelopmentprograms.
Chapter 8 was developed to cover some unique areas of experimental
diagnostics that can be used to identify and understand degradation mecha-
nismsindifferentfuelcellmaterials.Writtenbywell-establishedexpertsinthis
field at The University of Delaware, this chapter should be of great use to
graduatestudentsandfaculty,aswellaspracticingengineersseekingnewtools
for analysis.
The final chapter of this book also represents a unique contribution in the
challenging field of computational modeling of PEFC durability. Due to the
disparate timescale between real-time performance changes and various
degradationmodes,thisareaisoneoftheleast exploredanddifficultareas in
the field of fuel cell science. Written by Dr Yu Morimoto of Toyota Motor
Company,thischaptershowsnewcontributionstowardestablishingacompu-
tationalframeworkforpredictionofvariousmodesoffuelcelldegradation,and
should beof great interesttomodelersand experimentalists alike.
The editors of this book hope that the reader finds this compilation both
relevant and enlightening, and welcome any feedback to help us further
contributeto thisevolvingfield.
[1] UnitedStatesDepartmentofEnergy,September2010.
Chapter 1
Durability of Polymer Electrolyte
Fuel Cells: Status and Targets
E.A. Wargo, C.R. Dennison and E.C. Kumbur
ElectrochemicalEnergySystemsLaboratory,DepartmentofMechanicalEngineering
andMechanics,DrexelUniversity,Philadelphia,PA,USA
1. BACKGROUND
Growing concerns over energy supply and surrounding security and environ-
mental issues have placed considerable emphasis on the development of
alternatives to conventional energy sources. Much of the world’s energy is
derivedfromfossilfuels;coal,petroleum,andnaturalgas.IntheUnitedStates,
forexample,in2009over50%ofthecountry’selectricitywasgeneratedbycoal-
firedpowerplants,whichconsequentlycontributed40%ofthenation’scarbon
dioxide (CO ) emissions [1]. Figure1.1(a) shows recentworld wideconsump-
2
tionofthetopfiveenergysources,withnon-renewables(suchaspetroleum,coal,
and natural gas)displaying a more dramatic increase than renewables (such as
hydroelectricity).Thedemandforenergy–petroleuminparticular–hasbeenon
the rise since the early 1980s, following the 1970s oil crisis. In 2009 over
84 million barrels of oil were consumedinternationally each day, comparedto
58millionin1983[2].Allregionsoftheworldhaveseensignificantgrowthin
oildemand,butthistrendhasbeenmostnotableinAsia(Fig.1.1(b)).
The increase in fossil fuel consumption over the past 40 years and the
accompanying rise in carbon emissions have provided a substantial driving
force for the development of alternative energy systems. Fuel cells are an
alternative energy technology which show great potential, as they have the
ability to alleviate both consumption and emissions concerns. They can be
utilized for transportation, residential and commercial power, electronic
devices, and so forth.
Due to their numerous benefits and wide range of application areas, they
have attracted significant research and investment; specifically in polymer
electrolyte fuel cells (PEFCs). This forms the main emphasis of this book.
PEFCtechnologyishighlyefficient(83%theoretical,atroomtemperature[4]),
produces near-zero detrimental greenhouse gas emissions, operates at low
PolymerElectrolyteFuelCellDegradation.DOI:10.1016/B978-0-12-386936-4.10001-6
Copyright(cid:1)2012ElsevierInc.Allrightsreserved. 1
2 PolymerElectrolyteFuelCellDegradation
FIGURE 1.1 (a) World consumption of primary energy by energy type, 1980–2006 [2].
(b)Petroleumconsumptionbyregion,1980–2008[2].
(cid:1)
temperatures (generally less than 100 C), and features rapid start-up and
transientresponsecharacteristics.Inadditiontostationaryandportablepower
applications, PEFCs are a very promising alternative to internal combustion
engines for transportation applications. All of these reasons mean that the
technology has attracted much attention from governments, industrial devel-
opers,andresearchinstitutions,resultinginafocused,collectiveeffortaimedat
the development andcommercialization ofPEFC systems[3].
In order to be truly competitive, PEFCs must meet or exceed the techno-
logical advantages of heat engines and other conventional power systems, on
both the small and large scale. This includes balancing power output, system
lifetime,andcostforaspecificapplicationofinterest.Currentresearchefforts
Chapter | 1 DurabilityofPolymerElectrolyteFuelCells:StatusandTargets 3
arefocusedonimprovingeachoftheseaspects,whilerecognizingthecomplex
and coupled relationships between these characteristics. For example,
increasingthemembranethicknesswillincreaseitsdurabilityandlifetime,but
this requires more membrane material and lowers the cell’s performance via
higher protonic resistance, both of which elevate the overall cost [3]. A rela-
tively limited lifetime is currently one of the greatest shortcomings of PEFCs
comparedtoheatenginesandothercompetingtechnologies.Inordertoimprove
celllifetimewithoutsacrificingcostand/orperformance,muchattentionisnow
focusedonidentifyingandinvestigatingthefactorsthatimpactPEFCdurability.
2. DURABILITY TARGETS FOR PEFC TECHNOLOGY
DespitethepresentcostanddurabilitychallengesofPEFCtechnology,thereis
a strong consensus that it is a practical, marketable, alternative energy tech-
nology.Thisfactor,alongwiththeimpressiveadvantagesofPEFCs,hasledto
theestablishmentofseveralgovernmentprogramswhichhavebeendesignedto
facilitate the development andimplementationof PEFC power systems.Most
notablearetheeffortsoftheUnitedStatesDepartmentofEnergy,theEuropean
Commission, and Japan’s Ministry of Economy, Trade and Industry, each of
whichhasestablishedtargetsandtimelinesforthecommercializationofPEFCs
for a range of applications. The technical targets established by each of these
organizationswillbepresentedinSections2.1–2.3andcomparedinSection3
at the conclusion of the chapter. While the focus of this book is PEFC dura-
bility, due to the coupling between durability, cost, and performance, other
relevant targets are also presented to provide the reader with the complete
contextinwhichthese targetsmustbe achieved.
2.1. United States Office of Energy Efficiency and Renewable
Energy
TheUnitedStatesHydrogenEnergyProgramwasinitiallyauthorizedin1976
underthemanagementoftheNationalScienceFoundation,andtransferredto
the Department of Energy (DOE) in 1990. In 2005, the Office of Energy
Efficiency and Renewable Energy (EERE) released the Multi-Year Research,
Development and Demonstration Plan which established goals for the
Hydrogen, Fuel Cells and Infrastructure Technologies Program, and outlined
a researchand development plan for attaining thosegoals by 2015[5].
The EERE has identified specific long-term objectives for PEFC develop-
mentinvariousapplicationareas,namely:transportation,stationary,consumer
electronics, and auxiliary power units (APUs). Transportation systems are
expectedtooperateinenvironmentsbetween (cid:3)40and40(cid:1)C,while stationary
systems are expected to operate in environments between (cid:3)35 and 40(cid:1)C.
Recent (2005) performance milestones, as identified by the EERE, for effi-
ciency,durability, and cost are summarized in Table 1.1 [5].
4 PolymerElectrolyteFuelCellDegradation
TABLE1.1 2005PerformanceMilestones,USDepartmentofEnergy,
OfficeofEnergyEfficiencyandRenewableEnergy[5]
Application Efficiency Durability(Hours) Cost
Transportation 50% ~1,000 $110/kW
Stationary 32% 20,000 $2,500/kW
ConsumerElectronics(<50W) e >500 $40/W
AuxiliaryPowerUnit(3e30kW) 15% 100 >$2,000/kW
The EERE’s 2010 and 2015 targets for overall efficiency, durability, and
costaresummarizedinTable1.2[5].Aspreviouslynoted,thesecharacteristics
areinherentlycoupled,andmustbeconsideredtogether.TheEEREprojectsits
targets out to the year 2015, which agrees with the fuel cell development
timelines of other regional organizations throughout the world. It should be
noted that for transportation applications, the 2010 targets allow for external
humidification. However, the 2015 targets specify that the membrane must
function without external humidification, with an end of life (EOL) perfor-
manceloss oflessthan 5% beginningoflife (BOL) performance [5].
In order to achieve the objectives summarized in Table 1.2, the EERE
identifiestargetsforindividualfuelcellcomponents.TheEEREisspecifically
concerned with bipolar plate/membrane electrode assembly (MEA) develop-
ment for general applications, as well as membranes/electrocatalysts for
TABLE1.2 Long-termTargetsbyApplication,USDepartmentofEnergy,
OfficeofEnergyEfficiencyandRenewableEnergy[5]
Durability
Application Efficiency (Hours) Cost,2010 Cost,2015
Transportation 60% 5,000 $45/kW $30/kW
Stationary 40% 40,000 $750/kW1 e
ConsumerElectronics e 5,000 $3/W e
(<50W)
AuxiliaryPowerUnit 40% 35,000 $400/kW $400/kW
(3e30kW)
1Milestonedelayedfrom2010to2011duetoappropriationsshortfallandCongressionallydirected
activities[5].
Chapter | 1 DurabilityofPolymerElectrolyteFuelCells:StatusandTargets 5
TABLE1.3 TechnicalTargetsforMembranesforTransportation
Applications,USDepartmentofEnergy,OfficeofEnergyEfficiencyand
RenewableEnergy[5]
Characteristic Units 2005Status 2010 2015
Cost $/m2 25 20 20
Durabilitywithcycling1
Atoperatingtemperatureof(cid:4)80(cid:1)C Hours ~2,000 5,000 5,000
Atoperatingtemperatureof>80(cid:1)C Hours N/A 2,000 5,000
1BasedontheDOEstresstestprotocol[6].
transportation use. The EERE technical targets for membranes for trans-
portationapplicationsaresummarizedinTable1.3[5].Itisworthnotingthat,
to date, the EERE has not acknowledged any representative data available
for high temperature membranes (>80(cid:1)C). However, high temperature
membranes are still expected to meet the same durability criteria as low
temperature membranes by2015.
Inadditiontomembranes,electrocatalystsforautomotiveapplicationshave
been identified for development by the EERE (Table 1.4) [5]. The targets for
durability are based on an accelerated stress test protocol issued by the DOE
[6].Thisprotocolisintendedtoacceleratecomponentdegradationbyvarying
TABLE1.4 TechnicalTargetsforElectrocatalystsforTransportation
Applications,USDepartmentofEnergy,OfficeofEnergyEfficiency
andRenewableEnergy[5]
2005Status StackTargets
Characteristic Units Cell Stack 2010 2015
Cost $/kW 9 55 5 3
Durabilitywithcycling
Operatingtemp(cid:4)80(cid:1)C Hours >2,000 ~2,000 5,000 5,000
Operatingtemp>80(cid:1)C Hours N/A N/A 2,000 5,000
Electrochemicalarealoss % 90 90 <40 <40
Electrocatalystloss mVafter100 >30 N/A <30 <30
hoursat1.2V