Durability of PEFCs at High Humidity Conditions

Journal of The Electrochemical Society, 152 ~1! A104-A113 ~2005!0013-4651/2004/152~1!/A104/10/$7.00 © The Electrochemical Society, Inc.

A104

Durability of PEFCs at High Humidity ConditionsJian Xie,a,* ,z David L. Wood III, a,c,* David M. Wayne,b

Thomas A. Zawodzinski,a,d,* Plamen Atanassov,c,* and Rodney L. Borupa,*aLos Alamos National Laboratory, Electronic and Electrochemical Materials and Devices Groupand bPit Disposition Science and Technology Group, Los Alamos, New Mexico 87545, USAcDepartment of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque,New Mexico 87131, USA

This work addresses issues of long-term durability of hydrogen-air polymer electrolyte fuel cells~PEFCs!. The chromium in aPt3Cr binary alloy catalyst has been found to migrate from cathode to anode during the course of life testing when operating withinthe oversaturated, or high-humidity, gas feed regime~one or both inlet feeds with a dew point equal to or higher than cell operatingtemperature! above 1 A/cm2 current density. Other major factors such as membrane degradation, dissolution of catalyst-layerrecast ionomer, catalyst oxidation, and catalyst agglomeration/growth have been identified as simultaneous, gradual processes thatcan lead to long-term PEFC failure.In situ cyclic voltammetry measurement of electrochemically active catalyst surface areashows a continuous decrease, revealing that catalyst agglomeration and/or growth may be a major cause of membrane electrodeassembly degradation during middle-term life tests~i.e., operation times up to about 2000 h! under high-humidity conditions.Membrane and/or recast ionomer degradation was confirmed by the presence of fluoride and sulfate anions in the cathode outletwater. Scanning and transmission electron microscopy observation of a tested MEA suggest the loss of carbon-supported catalystclusters and possible dissolution of recast Nafion ionomer.© 2004 The Electrochemical Society.@DOI: 10.1149/1.1830355# All rights reserved.

Manuscript submitted January 30, 2004; revised manuscript received June 21, 2004. Available electronically December 1, 2004.

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Component lifetimes of hydrogen-air polymer electrolytecells ~PEFCs! have significant implications on the commercial scess of PEFC systems for both stationary and transportationcations. Durability improvement of PEFC components can etively reduce the cost of implementing PEFC systems, mawidespread commercialization more realistic due to a reducedreplacement frequency. Although there have been extensive son the improvement of performance of PEFCs, little has beenlished by comparison with regards to durability studies.1-4 PEFCdurability has remained a difficult topic because of the lengthyration of the test time required~i.e., several thousand hoursmore!, the complexity of failure analysis, the overlapping of cauof failure, and the difficulty ofin situ, nondestructive examinationthe structure of electrocatalysts and membrane electrode asse~MEAs!.

One major methodology involved in long-term durability studhas been the so-called post-mortem examination of individboiled components. The nature of lamination and layer integrof the MEA and gas diffusion layer~GDL! makes this approach bitself somewhat ambiguous and many of the results are subjcareful re-evaluation. Another common approach within the indis to operate a cell or stack for a given period of time, anddisassemble the components for post-mortem examination.methodology only gives insight at the point of disassembly andnot provide information on how the post-mortem situation arosdegradation mechanisms over time. Durability data is often acclated in bulk to determine the expected power loss over timesources of catastrophic failure for prototypes or nearly commeready sets of components; however, rarely is this long-term pemance data examined in detail and correlated with the numoff-line experiments required to understand the origin of the dbility performance obtained. Although this sort of analysis is becial for small-scale production and marketing of PEFC technoit falls well short of what is required in order to push the genfundamental understanding of the technology ahead and legitPEFCs as a leading alternative energy source. What is requirecombined, committed, and sustained effort utilizing all three of tmethodologies.

* Electrochemical Society Active Member.d Present address: Department of Chemical Engineering, Case Western Rese

versity, Cleveland, OH 44106-7217.z E-mail: [email protected]

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The membrane electrode assembly~MEA! is the core of thPEFC, where the chemical energy of the fuel~i.e., hydrogen! isconverted into electricity through the electrochemical oxidatiofuel at the anode and the electrochemical reduction of oxygencathode. The thin-film decal method of MEA preparation usethis work, and developed at Los Alamos National Labora~LANL !,5,6 has been widely utilized in recent hydrogen and dmethanol PEFC research. An MEA made by this method consitwo thin catalyst layers directly bonded onto either side of a polyelectrolyte membrane by hot pressing. Nanoparticle preciouscatalyst supported on microparticle carbon aggregates, togethethe recast Nafion ionomer network, form a composite catalystThe recast ionomer network has three important functions, wconsist of holding the catalyst particles together, forming a conous pore-volume network, and providing a path for proton tran

There is an established common opinion on some of theimportant possible causes of PEFC failure and/or performancewhich is briefly reviewed. Degradation of the ionomer membwill occur over prolonged testing periods, which results in a grareduction of ionic conductivity, an increase in total cell resistaand a loss in net power output. The membrane is not only attaby the intermediate radicals of the electrochemical oxygen redureaction~ORR!, but also by the chemically oxidizing environm~air/oxygen! at the cathode side and the chemically reducing eronment~hydrogen! at the anode side.7-9 As the membrane structuis compromised~thinning, etc.!, by whatever mechanism, increagas diffusion through the membrane from one electrode to an~primarily hydrogen crossing to the cathode because of theeffective diffusivity of hydrogen molecules! will lower both thethermodynamic, or voltage, efficiency and fuel efficiency. Morevere performance loss from gas crossover arises from pinholmation in the membrane, which causes more extensive reactaning via counter-diffusion through the hole. Pinhole formationtopic of utmost importance to industrial researchers and towartablishing product reliability, but the data and findings are cloguarded. As a result, the mechanism~s! by which these holes forover long testing times remains, for the most part, unknown togeneral PEFC community.

Due to the nanometer size of the metal catalyst particles~typi-cally 2-6 nm in diameter supported on carbon! and their high surfacenergy, they will tend to agglomerate or grow~sintering, ripeningcoalescence, etc.! into larger particles, especially in a wet operaenvironment. This process may result in a loss of electrochemactive surface area due to catalyst ripening.10,11Although it has no

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Journal of The Electrochemical Society, 152 ~1! A104-A113 ~2005! A105

yet been reported, the recast Nafion ionomer commonly uslow-temperature PEFC electrocatalyst layers may also be subto the same degradation and/or dissolution processes as thatmembrane~i.e., Nafion or other structures containing a perfluorofonic acid polymer!. The recast Nafion may even suffer more intechemical degradation than a Nafion membrane because ogreater mechanical integrity of the membrane and the fact tharecast ionomer is located directly adjacent to the ORR activelyst sites, resulting in more exposure to intermediates.

Operation under demanding conditions beneficial to systemsign is yet another source of durability challenges, especially foMEA. For example, Endoth and co-workers found that membdecomposition is more sever when operating under dry cond~those of lower inlet relative humidities and lower gas pressu!than wet conditions.12 The drier the reactant streams, the simplerhumidification subsystem is to integrate, and the lower the catgas pressure, the smaller the parasitic power losses are fromcompressor. Furthermore, freeze-thaw cycling, set-point cyctransient operation, and improper shutdown play important rolecause of their potential impact on the hydration state, morphoand stability of the MEA subcomponents. Although Wilsonet al.examined the effects of freeze-thaw cycling on MEA integrity,effects of this cycling process on long-term performance wasinvestigated.1 In addition to these system considerations, lenoperating times may introduce structural changes to the catalysers or the MEA in general, even under moderate or nonaggreconditions. Therefore, the target lifetime of a particular PEFC acation may impose inherent durability considerations not relevaanother application. Target automotive lifetimes are 50001.5 A/cm2 or more, ambient cathode relative humidity, and ab150 kPa absolute gas pressures. Target stationary lifetime40,000 h at about 0.6 A/cm2, full cathode humidity, and neaambient gas pressures.

Because there are so many interrelated issues involveddurability analysis of a PEFC, many broad studies addressingthese factors and operating conditions are needed. This studsents the results from a comprehensive and systematic survamination of PEFC durability under high-humidity operating cotions, focusing on the MEA.

Experimental

MEA preparation.—MEAs were prepared by the Los AlamNational Laboratory~LANL ! standard decal method.5,6,13 Catalysinks were prepared by mixing a carbon-supported catalyst witherol ~Fisher Scientific, NJ!, 5% Nafion solution~1100 EW, SolutionTechnologies, Inc., PA!, and 1.0 M tetrabutylammonium hydroxi~TBAOH! in methanol~Aldrich, WI!. The inks were stirred ovenight to form a well-mixed dispersion, which was then brupainted onto a decal surface of fiberglass coated with Teflon~forMEAs denoted as type I!. Each brushed decal was heated in an oat 140°C for 30 min. This process of painting and heatingrepeated until the desired catalyst loading was reached. Anodcathode catalyst decals were then hot-pressed onto either sidsodium-form Nafion 112 membrane~E.I. du Pont de Nemours aCompany, DE! at 454 kg/cm2 and 210°C for 5 min. After hot presing, the decals were peeled off, forming a three-layer MEA, wwas boiled for 2 h in 0.5 Msulfuric acid followed by two additionahours of boiling in DI water. Finally, the MEAs were dried onheated vacuum table at 60°C for 30 min.

Catalyst layers for MEAs denoted as type II were preparedactly as described above~for type I MEAs!, with two exceptions. Ablade-coating machine with an X-Y recorder was used, which dan aluminum blade over the catalyst ink and onto the decal surather than the brush-painting method. The blade height wastrolled to ensure the desired catalyst loading. Second, the dwith their respective catalyst layers for type II MEAs were ovdried at 80°C for 30 min prior to hot pressing.

The catalyst constituents were different for each type of MType I MEAs, with a PtRu anode~54% PtRu /C, Tanaka Kikin-
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zoku Kogyo, Japan! and a Pt2CoCr cathode~20% Pt2CoCr/C,E-TEK, Natick, MA!, contained areal loadings of 0.18 a0.43 mg Pt/cm2 for the anode and cathode, respectively. TypMEAs, with a Pt anode~20% Pt/C, E-TEK, Natick, MA! and aPt3Cr cathode~20% Pt3Cr/C, E-TEK, Natick, MA!, contained arealoadings of 0.206 0.01 mg Pt/cm2 for both the anode and cathoThe Nafion content in all catalyst layers was controlled to 28 wThe geometrically active area was 50 cm2 for type I MEAs and5 cm2 for type II MEAs. A Nafion 112 membrane was used for bMEA types. Carbon-cloth backings with a wet-proofing treatm~ELAT Version 2.0, E-TEK, Natick, MA! were used as both tanode and cathode GDLs. The wet-proofing treatment consistedouble-sided, hydrophobic microporous layer~MPL! coating, wherthe MEA side contained a thick MPL and the flow-field side ctained a thin MPL.

For the purpose of comparison of structural integrity ofMEAs after preparation, a non-hot-pressed MEA was subjectthe same boiling procedure as the type I and II MEAs used fowork. This non-hot-pressed MEA was made using conventionarect methanol fuel cell~DMFC! technology and consisted of watbased, unsupported catalyst inks that were hand painted donto the membrane and allowed to air dry.

Catalyst surface area measurement.—The electrochemically ative surface area of the cathode catalyst layer was measuredinsitu hydrogen adsorption-desorption cyclic voltammetry~CV!. Aftera 12 h MEA conditioning period at a constant voltage of 0.6 V,cell was subjected to an initial active surface area measurePeriodically ~roughly every 100 operating hours! the constancurrent operating mode was stopped to measure the catalyst sarea. The cathode was purged with nitrogen for about 45 minthe open-circuit voltage~OCV! dropped to just below 0.1 V befomeasurements were taken. The anode was kept at the same hyflow rate as during the life test, serving as both the referencecounter electrodes, and the cathode was fed with humidifiedgen at the corresponding volumetric flow rate of air. Cathode vmmograms were measured in sequence within a scan range of1.0 V at a scan rate of 100 mV/s. The catalyst active surfacewas calculated by integration of the total quantity of electricity~inCoulombs! under each of the adsorption and desorption peaksthe measured CV curves between 0.1 and 0.4 V.14-17 These quantties were converted to true surface area using the specific capa210m C/cm2 of Pt for the hydrogen adsorption-desorption reacand the values presented represent the average of the adsorptdesorption areas.

Cathode water analysis.—Water was collected from the cathogas outlet of the cell. To avoid any cross-contamination of thelected water from other tests~after the MEA break-in period!, eachcell was operated with air and hydrogen under life-test condifor at least 24 h, so the fuel cell test system~tubing, flow fields, anhumidifiers! were thoroughly rinsed prior to the collection of wasamples. Water was then collected every 100 constant-currentof operation and analyzed using a ThermoFinnigan-MAT Elemehigh-resolution inductively coupled plasma mass spectrom~ICPMS! and a Dionex DX-100 ion chromatograph. For collecand storage of the outlet water, glass Qorpak bottles with a porafluoroethylene~PTFE! lid liner were used and carefully rinswith ultrapure DI water~3 times!, then with isopropyl alcohol~2times!, and subsequently dried with a high-purity N2 stream.

For the ICPMS chemical analysis, 20 mL of each sample~undi-luted! were poured into an acid-washed Teflon bottle, acidifiedSeastar grade HNO3 ~to 2% by volume!, spiked with 1 ppb In-11~internal standard!, and allowed to sit overnight. Blanks, standaand samples were nebulized at 100 mL/min, using a polyfluoran~PFA! self-aspirating nebulizer, into a PFA spray chamber.ICPMS analytical settings are summarized in Table I. Measure

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Journal of The Electrochemical Society, 152 ~1! A104-A113 ~2005!A106

quantified isotopes and sample times at low, moderate, andmass resolution settings (M /DM 5 ;300, 4000, and 7500! aresummarized in Table II. Analytical blanks were quantified by alyzing a solution of 2% Seastar grade HNO3 in Milli-Q deionizedwater ~18.3 MV cm! before and after calibration, and every fisample thereafter. The microcapillary tube to the nebulizerrinsed in a separate container of 2% Seastar grade HNO3 in the samedeionized water for at least 1 min between each sample. Allreported here are blank-corrected.

Each ICPMS analysis represents the average of nine indivanalyses~three passes and three runs! in each resolution mode. Dtection limits for the solution-based ICPMS technique are 5Five standards containing each analyte element at 5 ppt, 100ppb, 10 ppb, and 1 ppm were used to calibrate the analysweighted calibration scheme was used, and all elements yielstraight-line fit to an accuracy ofr 2 5 0.95 or better. Both analoand ion-counting detection were used. Error bars shown repr6 one standard deviation.

For the ion chromatography analysis, the anion exchange coof the Dionex DX-100 was an AS14 with an AG14 guard coluThe eluent was 5 mM Na2B4O7 • 10H2O ~sodium tetraborate, Cetified A.C.S. Grade! adjusted to pH 9.5-10.5 with 4 M NaOH ~so-dium hydroxide, reagent grade or better!, and the eluent suppresswas an ASRS-II~anion self-regenerating suppressor!. A sample in-jection volume of 25 mL was used with an eluent flow rate ofproximately 1 mL/min and detection method of conductivity. Tstandards curve was prepared from stock 1000 ppm standardchased from VHG Labs, and check standards were prepared f1000 ppm standard purchased from SPEX CertiPrep. Analysicertainty was610% or less.

SEM analysis of MEAs.—A 10 3 10 mm sample was cut frothe middle of a MEA for surface examination with a Philips XLscanning electron microscope~SEM! coupled with energy dispersispectroscopy~EDS! capability~Philips Electronics, Eindhoven, TNetherlands!. For cross section examination, samples were frozliquid nitrogen and cut with a prefrozen pair of scissors. To impimage resolution and prevent charging of the sample by electhe sample cross sections were sputtered with a thin~2-3 nm! goldfilm.

Table I. Element 2 ICPMS parameters and settings.

Gas flows and RF Cool gas 16 L/minAuxiliary gas 0.9 L/minSample gas 0.976 0.02 L/minRF power 10706 20 W

Lens voltages Extraction lens 22000 VFocus 21415 V

X-deflection 22.45 VY-deflection 7.05 V

Shape 145 VOther settings Take-up time 60 s

Internal standard 115InLock mass @40Ar36Ar#

Table II. ICPMS analytical methods for PEFC cathode outletwater solutions.

Resolution setting Isotope Sample tim~s

Low mass resolution(M /DM ; 300)

23Na, 63Cu, 101Ru, 195Pt,208Pb

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TEM analysis of MEAs.—A small piece of sample was cut frothe MEA and embedded in epoxy resin before sectioning wdiamond knife on a Reichert OMU3 ultramicrotome at room tperature. The thin sections were then examined with a Hitachi2000 200-kV, cold field emission TEM. The HD-2000 was equipwith a secondary electron detector, a bright-field detector, ahigh-angle annular dark-field~HA-ADF! detector. See Blomet al.18

for a detailed description of the TEM procedure used in this w

GDL analysis.—MPL surfaces of tested GDLs were analyusing laser-ablation-assisted, high-resolution ICP~ThermoFinnigan-MAT, Element 2!. Single GDL samples weplaced in the laser ablation chamber, which was then sealed, pof air, and filled with argon~Ar!. The laser ablation micropro~New Wave/Merchantek-EO GLUV 266X Nd-YAG!, which con-tains a neodymium-doped yttrium aluminum garnett laser soremoves the top layer of the sample as a fine powder. This powthen entrained in Ar gas, conveyed to a Tracey spray chambeinjected into the ICPMS ion source for ionization and mass anaA coiled tubing insert (id5 0.75 mm) was used to remove laparticles (.;1.0mm in diameter! from the Ar stream. Laser paraeters and ICPMS analytical settings are summarized in Table

Analytical blanks were quantified by analyzing the Ar strefrom the laser ablation cell with the laser turned off. Blanks wdetermined before and after each sample analysis~two laser ablatioruns per sample!. A 4 to 6 mm2 rectangular area is ablated insingle pass with the laser in rastering mode, where ablation decontrolled to 20-30mm to ensure only the outermost surface ofGDL is sampled. No preablation or surface cleaning was perforSite selection, laser focusing, and ablation control are monitorreal time by a video camera aimed down the laser axis. Each asis represents the average of 25 individual analyses~five passes anfive runs!. Detection limits for the laser-ablation ICPMS techniare 5 to 40 ppb for most elements.

No standards for trace elements in PEFC GDLs currently eand no attempt was made to acquire calibrated, quantitativeelement data. All laser ablation data are qualitative, and standas a basis for comparison within the set of GDL samples. The bsubtracted ion-beam intensity for each element~at each mass reslution setting! is presented as a ratio to that of carbon. The datahigh degrees of uncertainty~typically 20 to 30% for one standadeviation! relative to most solid samples analyzed using laablation ICPMS, which is likely due to localized lateral inhomoneities in element concentration within the GDL, and to the friapowdery nature of the samples themselves.

GDL and water blank samples.—Since there are no standamethods of quantifying trace impurities in PEFC effluent watewithin the components themselves, a customized method for aing blank samples was developed. First, a mock cell was built w

Table III. Laser ablation ICPMS parameters and settings.

GLUV 266X laser-ablationsettings

Raster spacing 50 mm

Beam diameter 40 mmLaser output 50%Scan speed 100 mm/s

Repetition rate 20 HzTotal laser energy 1.4 6 0.1 mJ

Gas flows and rf Cool gas 16 L/minAuxiliary gas 0.9 L/minSample gas 0.7 6 0.05 L/minRF power 11606 25 W

Lens voltages Extraction lens 22000 VFocus 213506 50 V

X-deflection 21.806 0.25 VY-deflection 6.706 0.45 V

Shape 1356 5 VOther settings Sampling time 0.01 s

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included all components except the MEA, with a thin sheet of Psubstituted instead. Then the cell was exposed to identical ctions as those under normal life-test operation~i.e., temperaturepressure, and inlet humidity!. This exposure was continued for ab120 h while the water was collected. At the end of this periodwater was saved and denoted as the water blank sample for thstand and cell hardware, and the GDLs were removed and deas anode and cathode GDL blank samples. Through this procthe amount of background content of a particular element waslyzed through the same solution-based ICPMS and laser-abICPMS methods.

Life testing and cell impedance.—All MEAs were tested inconstant-current mode at 80°C using a single-cell test station~FuelCell Technology Corp., NM! at 0.40 A/cm2 for type I (Pt3Ru4 /Canode, Pt2CoCr/C cathode, and 50 cm2 active area! and 1.07 A/cm2

for type II ~Pt/C anode, Pt3Cr/C cathode, and 5 cm2 active area!. Fortype I MEAs gas flows rates were controlled to 4203 1026 ~3.0stoichiometric ratio! and 21753 1026 ~6.6 stoichiometric ratio!standard cubic meters per minute~scmm! for hydrogen and air, respectively. For type II MEAs the anode flow rate was 1333 1026

scmm of hydrogen~3.6 stoichiometric ratio!, and the cathode florate was 5503 1026 scmm of air~6.2 stoichiometric ratio!. Bothgas streams were prehumidified with sparging bottles at a temture of 80°C (;100% relative humidity wherepsat 5 47.363 103 Pa, or 6.869 psi! for air and 105°C (@100% relative humidity wherepsat 5 120.863 103 Pa, or 17.529 psi! for hydrogen. Humidifier efficiencies of these test stands have been shown to b100% when checked. Back pressure of each stream was h2063 103 Pa gauge, or 30.0 psig, or 2.80 atm absolute~at 2300meters, or 7500 feet, elevation atmospheric pressure is only3 103 Pa absolute, or 11.1 psia!. Polarization curves were altaken at roughly 100 h intervals prior to performing the surfacemeasurements.

In addition to the OCV periods associated with the schedstoppages of constant current operation for polarization curves,trochemically active surface area, and cathode water collectiocells also experienced some unintentional OCV periods. Wheerating for long periods of time in constant-current mode, cellsbe automatically set to OCV when the voltage value drops belowlower limit ~typically 0.1 to 0.2 V! for brief periods, usually lesthan 1.5 s~commonly referred to as low-voltage spikes!. This soft-ware control mode acts as an extra layer of protection for a duity test cell operating under constant current and, when the latory is unattended, can result in the cell idling at OCV for a peof hours. These stoppages are yet another common obstaclelecting large amounts of durability data and are not included inlife test data presented here for the sake of clarity.

Cell impedance, often referred to as high-frequency resis~HFR! was measured at 8 kHz~to eliminate concentration polariztion and Faradaic effects! every 5-20 h. This measurement was pformed by superimposing an 8 kHz sine wave signal~with voltageamplitude of 10 mV! over the dc cell voltage measured in constcurrent mode. The cell current density was chosen automaticathe LabVIEW control software and was set to about 0.5 A/cm2 forthe impedance measurement. The ac signal was generatedDAQCard ~National Instruments! and sent to the cell via the eletronic load. Impedance was then obtained from the return signa shunt resistor.

Methodology and choice of conditions.—These operating condtions were chosen to intensify the degradation of the MEA by intionally setting the anode and cathode streams at high humiditels, by setting the current density close to the diffusion coregion, and by flowing high stoichiometric ratios of reactants.presence of liquid water and excess reactants at high current dpresents one form of conditions that is expected to induce signidegradation of the MEA and GDL in commercial systems. Ethough membrane degradation is greatly affected by the voltag

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ference between the anode and cathode~i.e., higher voltage difference with lower current density!, it is expected that high curredensity~lower voltage difference! will intensify the chemical degradation of the membrane due to the much higher electrochereaction rates. Higher reaction rates include a higher generatioof oxidative radicals and a higher proton flux throughmembrane.9 For the same reasons, the recast ionomer in the calayers may experience similar degradation since the electrochereactions take place at the interface between the catalyst clusteionomer network. High liquid water content may facilitatemovement of ions and particles, and hence the agglomeratigrowth, of the catalyst nanoparticles, also leading to a deteriocatalyst layer.

Since the use of SEM and TEM to examine the MEA invodisassembling the MEA from the single-cell hardware and pcally cutting off a piece of material, continuous monitoring offace and structural changes by microscopy during the courselife test is impractical. The approach taken here for type II Mwas to make only a single batch of 13 MEAs~all at the same time!with statistically similar catalyst loading parameters. Then tnearly identical MEAs were tested in separate cells, increasingtest time by 500 h increments for each cell. In addition, GDLswith type II MEAs all came from a single batch, which is a kfactor in minimizing statistical cell-to-cell variance in PEFCs. Atend of each life test, the MEA was disassembled and examinSEM and TEM, with the assumption that any physical changesbe due to the increased testing time and not key cell-to-cell stcal variables. Performance of the type II MEAs was monitoevery ;100 h by measuring polarization curves, active catasurface area throughin situ CV, and trace components of cathooutlet water by ICPMS and ion chromatography. Thus, degradof the MEA was continuously monitored throughout the courslife testing.

Results and Discussion

The intent of this study was to compare the individual findfrom two separate durability examinations of type I and II MErespectively, with each examination conducted at different pointime. The experimental methodology and test protocols describthe section above for type II MEAs represents an improvedexpanded methodology over that used for type I MEAs. In addthe particular differences between the type I and II MEAs~particu-larly the catalyst types! are not relevant to the discussion presehere, as the purpose of this investigation was to obtain a comhensive survey covering a wide range of factors and to validatutility of the methodology used for type II MEAs.

Surface and structural change.—A life test of type I MEA(Pt3Ru4 /C anode and Pt2CoCr/C cathode! is shown in Fig. 1. Thvoltage decreased at a rate of 0.06 mV/h through 1916 h~from 0.684to 0.570 V!. Cell impedance slowly increased at a rate6.3mV cm2/h from 0.092 to 0.104V cm2 within the same timperiod, but it increased rapidly to 0.170V cm2 over the subseque228 h. MEA failure in this test was due to a power outage aroun1916 h point, which likely caused drying and decomposition oMEA, resulting in significantly increased cell impedance.

Events of an unmonitored power outage under constant cuoperation with our control methodology include an outage pulsubsequent power spike with the electronic load still drawingcurrent set point, and a safety shut-off of the H2 supply. Air supplyis continued when the compressor power is restarted, but with n2supply until the alarm is manually reset. The load box continudraw current until the control software reads that the low-vollimit has been exceeded. In this case, the voltage reading wasonly when the data file recording loop finished, with a data ptaken every 5 min. Therefore it is likely that this cell experiencreverse polarization for up to 5 min with no hydrogen supply. Adental interruptions of this type~as shown in Fig. 1! are a commoobstacle to durability analysis and demonstrate that proper smanagement is critical to maximizing the life of PEFCs.

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The gradual linear increase in cell impedance of 6.3mV cm2/hshown in Fig. 1 for the type I MEA has not been reported for MEcontaining pure Pt~or Pt/C! cathode catalysts. We believe thishavior is partially due to oxidation of the tertiary alloy cathocatalyst, Pt2CoCr/C, and that the oxidation is indirectly responsfor part of the impedance increase, in addition to the normallyserved gradual increase due to membrane and recast ionomeradations. Divalent Co21 and Cr21 ions formed during the oxidatioof the Pt-alloying components could be diffusing into the memband exchanging irreversibly with protons at the SO3

2 sites.19 Sinceprotons are transported along the SO3

2 sites of the membrane, tcharge-transport pathways containing a Co21 or Cr21 cation wouldbecome inaccessible to protons. Therefore, this situation wousult in higher cell impedance due to reduced proton conductivithe membrane.

Figure 1. Life test results of type I MEA under constant current modevoltage decay and cell impedance change as a function of time: MEAmetric active area5 50 cm2; current5 20 A (0.40 A/cm2); anode/cathodflow rate5 420/21753 1026 scmm; back pressure5 2.8 atm abs. ~30psig!; anode/cathode humidity5 100%/100%; humidifier temperatur5105/80°C ~anode/cathode!; cell operating temperature5 80°C; cell im-pedance~HFR! was measured at 8 kHz; anode catalyst: 54% Pt3Ru4 /C withloading of 0.18 mg Pt/cm2; cathode catalyst: 20% Pt2CoCr/C with loading o0.43 mg Pt/cm2; N112 membrane.

g-

-

There are significant changes in surface features of thisafter life testing. The surface of the cathode catalyst layer berough, shown by mud-cracking features in the Fig. 2a SEM anaas compared to the smooth surface of a fresh MEA shown in Fi(;200 times magnification!. Surface erosion is also observed in F2c in comparison to the fresh MEA surface in Fig. 2d (;6500 timesmagnification!. It is clear that either catalyst particles or recNafion ionomer, or both, are lost from the surface of the MEA athe 2200 h life test, suggesting possible dissolution of the ionoA structural change caused by loss of the recast ionomer couldin deterioration of the integrity of the catalyst layer, a drop in bionic and electronic conductivities, loss of catalyst metal cluster~orcarbon support!, and increased resistance within the interfacial zof the membrane and catalyst layer. Although the changes in sufeatures shown here are of the cathode, similar processes mayat the anode, too.

In addition, a substantial morphological change within the tyMEA after life testing has been observed. Figure 3a shows animage of a fresh MEA cross section, and both the anode and cacatalyst layers are seen as smooth and uniform with no craccavities. Slight catalyst agglomeration can be seen for eachelectrode~arrows denoted as A and B for the anode and cathrespectively!. After life testing, however, significant further aggloeration and/or crystallite growth transpired, especially for the code catalyst layer~seen as large, dark areas in Fig. 3b!. Both catalyslayers became irregularly shaped with respect to thickness ahomogeneous in texture. The anode catalyst layer developedcracks and showed thinning in some regions and clumping in oAs for the cathode, the original texture uniformity of the catalayer completely disappeared, leaving behind portions of diffesizes with varying degrees of catalyst agglomeration. A consideportion of the cathode layer is seen fully detached from the elecmatrix ~denoted by arrow C in Fig. 3b!, clearly showing the pronounced impact of long-term testing on catalyst layer morphoHowever, it is not known to what extent that the power oumentioned above affected the final morphological state of the tMEA catalyst layers.

Shown in Fig. 4 is a comparison of TEM images taken fortype I MEA after testing for 2200 h~Fig. 4a! and for a fresh typeMEA ~Fig. 4b!. It is clearly seen that significant agglomeratigrowth of metal catalyst clusters occurred over the course of thtest ~metallic particles are seen as bright white!. Figure 4b showthat the fresh MEA had a noticeably nonuniform metal particle

Figure 2. SEM micrographs of catalylayer surfaces for type I MEA:~A! surfaceafter 2200 h life test~mud-cracking feature!, ~B! fresh surface before testing,~C!after 2200 h life test~erosion!, and ~D!fresh surface before testing.

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Figure 3. SEM micrographs of catalyst layer cross section for type I M~a, top! cross section of fresh MEA before testing,~b, bottom! cross sectioof MEA after 2200 h life test with arrows denoted as A and B showcatalyst agglomeration/growth in the anode and cathode catalyst layespectively. Arrow denoted as C shows a segment of the catalyst layer thcompletely detached.

Figure 4. TEM ~HA-ADF! micrographs of catalyst metal clusters for tyI MEA: ~a! metal catalyst particle distribution after 2200 h life test showagglomeration/growth and~b! metal catalyst particle distribution befotesting.

that smaller~higher surface area! metal clusters are either aggloerating with other small clusters or being consumed by the lones present from the onset~ripening, etc.!.

The hot-pressing step for MEA preparation used instudy5,6,13enhances the integrity of the catalyst layer structure, cpared to MEAs made by the alternative method without hot pre~described in the Experimental section!. MEAs made by the lattemethod did not survive even a 2 h boiling test in DI water, withcatalyst particles readily detaching from the MEA. No damageobserved to the hot-pressed MEAs during similar boiling testsconversion to the proton form. Often MEAs made without thispressing method exhibit lifetimes of only several hundred hourder typical DMFC operating conditions. Examining the microstture, the non-hot-pressed MEA shows acceptable features foaccess to the electrode surface~seen in Fig. 5a!. These non-carbosupported catalyst particles are loosely connected to each otheplenty of space exists between them, providing pathways for gawater transport. This cauliflower-shaped geometry should facilitgas-electrode reaction because its hemispherical shape prabundant reaction surface area for oxygen reduction. By contracatalyst layer of the MEA made by hot pressing and usingcarbon-supported catalyst is more compressed, and these carbgregates, on which catalyst clusters reside, are closely conneceach other~as shown in Fig. 5b!. Despite the reduced mass-transpathways within the hot-pressed catalyst layer, improved duraand structural integrity of this layer are the key factors in its loterm performance behavior.

Decay of electrochemically active surface area.—A life test oftype II MEA is shown in Fig. 6. The constant-current performadecay exhibited by this MEA tracks closely with that shown fortype I MEA in Fig. 1 ~with the exception of the cell impedanbehavior!, dropping about 54 mV during the 1000 operating hoThe most significant finding is that the electrochemically activeface area of the type II cathode catalyst layer continuously decrfrom the onset at a calculated rate of 7.143 1023 m2/g Pt/h ~from23 to 16 m2/g Pt over 1000 operating h!, as shown in Fig. 7. Thdecrease may be caused by the combined effects of (i ) metal catalyst cluster agglomeration10,11as seen in Fig. 4; (i i ) oxidation of thebinary Pt3Cr/C catalyst~possibly leading to ripening!; and (i i i ) lossof support particles and metal clusters from the surface of thelyst layer~Fig. 2!. The decrease in active surface area suggesteither agglomeration/growth of metal catalyst clusters beginsthe initial testing of the MEA (,100 h), or degradation of the recionomer causes loss of contact with catalyst particles. The asurface area steadily declined until the end of the life test, indicthat one or both of these degradation mechanisms continuedpresent throughout the testing period. Several desirable featuthe design of electrocatalysts and MEAs are not only to makelyst particles with high surface area, but also to prevent mcluster agglomeration/growth and loss of catalyst/ionomer cowhen operating the MEA.

-s

Figure 5. SEM micrographs of catalyst layer cross sections:~a! MEA madeby non-hot-processing method and PtRu catalyst with loading of 6Pt/cm2; ~b! MEA made by hot-pressing decal method and 20% Pt3Cr/Ccatalyst with loading of 0.20 mg Pt/cm2.

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Dissolution, or oxidative corrosion, of catalyst metal is anomechanism by which active surface area would decrease. In adto the SEM observations of the MEA surface of Fig. 2 and csection of Fig. 3, and the TEM images in Fig. 4, loss of catametal clusters from the type II MEA is confirmed by the ICPanalysis of the cathode outlet water as shown in Fig. 8. Theshows the presence of Pt and Cr in the cathode outlet watethough only in the range of 100s of ppt~background Pt content wdetermined to be about 7 ppt!. From Fig. 8 it is seen that the cocentration of Cr decreased noticeably within the range of 600 toh. Also observed is a substantial,ca. two-fold increase in Pt concetration at 700 h, which corresponds to the sharp drop in activelyst area~at 700 h! in Fig. 7. We further believe that the steenegative slope in the active catalyst area~Fig. 7! through 500 h idue to a higher degree of Cr oxidation, corresponding to the mhigher levels of Cr seen in the outlet water during the sameperiod ~Fig. 8!. Then it appears that the Pt oxidation may hbecome more prevalent during the 500-900 h range. Viewed anway, the Cr/Pt mass ratio in the cathode outlet water remainedtively stable at an average value of about 6:1 through 500 h,dropped down to an average value of about 1.5:1 for the remaof the life test. These observations could be partially attributed t

Figure 6. Life test results of type II MEA under constant current mode-voltage decay and cell impedance change as a function of time: MEAmetric active area5 5.0 cm2; current5 5.34 A (1.07 A/cm2); anodecathode flow rate5 133/5503 1026 scmm; back pressure5 2.8 atm abs~30 psig!; anode/cathode humidity5 100%/100%; humidifier temperatur5105/80°C ~anode/cathode!; cell operating temperature5 80°C; cell im-pedance~HFR! was measured at 8 kHz; anode catalyst: 20% Pt/C; cacatalyst: 20% Pt3Cr/C; areal loadings of 0.206 0.01 mg Pt/cm2 for bothelectrodes; N112 membrane.

Figure 7. Change in electrochemically active surface area of a type II Mcathode catalyst layer during the course of 1000 h of life testing~measuredby in situ hydrogen adsorption-desorption CV!. Operating conditions anMEA compositions are the same as those listed in Fig. 6.

-

-

r-

r

morphological and structural properties of the carbon-suppPt3Cr clusters, having a mixture of both amorphous and crphases and containing both CrOx and PtCrOx .20 A possible drivingforce for the cathode Pt dissolution could have been the OCVods~mentioned in the Experimental section!, which, in effect, act along-term cycles between lower and higher cell voltage. DarlingMeyers have shown that potential cycling promotes Pt oxidatithe cathode,21,22 which then allows for the possibility of dissolutiand/or migration. Potential resistance to oxidation of cathode Ploy catalysts is shown here to be key to their long-term operviability.

To monitor the durability of the type II MEA, a polarizaticurve was measured every 100 h of constant-current operatio~seeFig. 9a!. The most significant change is seen in the mass-tranregion ~current density. 0.8 A/cm2!, with a noticeable degreechange seen in the kinetic region as well~current density, 0.2A/cm2!, and a negligible change is seen in the Ohmic regionshown in Fig. 9b, the kinetic performance reached maximum vat about 200 h, and then began a steady decline through thethe 1000 h life test, likely caused by the loss in cathode acatalyst area and the different rates of dissolution of Pt and Crcathode catalyst alloy. These different rates of dissolution of PCr would change the Pt/Cr ratio in the cathode, and performanthe Pt-Cr alloy catalyst is known to be dependent on this ratio

The peak in kinetic performance of Fig. 9b is attributed toeffect of conditioning an MEA. A break-in, or massaging, perioan MEA is commonly accepted in both industry and researchronments as a necessary aspect of using a fresh MEA, typlasting anywhere from 2-12 h. We believe that these changes dbreak-in could last much longer, as the data in Fig. 9b suggestin general this phenomenon is not well understood. This effect calso explain the gradual decrease in cell impedance from theof testing through about 400-500 h, shown in Fig. 6, before it ba subsequent increase around 500-600 h. Nearly identical beregarding the kinetic performance and cell impedance trendsobserved for another type II MEA tested for only 500 h, verifythe reproducibility of the trends~data has not been included!. It isnot known why a similar decrease and subsequent increaseimpedance was not observed for the type I MEA.

The following theory is proposed to address the observarelated to the MEA break-in behavior by explaining a posschange in the catalyst layer structure that may occur. The Nmembrane and catalyst-layer ionomer swell by continuous wup-take during initial operation, likely expanding and contacting

Figure 8. Change in elemental Pt and Cr concentrations~in picograms ometal per gram of water! of the cathode outlet water for a type II MEduring the course of 1000 h of life testing~analyzed by ICPMS!. Operatingconditions and MEA compositions are the same as those listed in Fig

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another more intimately during the process. Possible structuraentation effects on the membrane/recast ionomer may help tomize protonic and electronic conductivities. Swelling of the catalayers may induce a change in overall pore structure of the laallowing for more access of reactant gases to electrocatalytic sinanostructural change within the membrane as a result of a sushigh electrical field could result in increased proton conductivitthe ionic domains. These theories need to be investigated anstantiated by experiment to help truly explain the MEA conditionphenomenon.

The decline of mass-transport performance as seen in Fimay be caused, at least in part, by morphological changes ocatalyst support~carbon! and metal catalyst clusters. The macomponent of state-of-the-art PEFC electrocatalysts is the C suphase~typically up to 80 wt %!, and the widely used support marial, Vulcan XC-72, has an average~mass-based! particle size of 34nm.23 This size is roughly two orders of magnitude larger than tcal metal catalyst cluster sizes of 2-10 nm. Certain oxycontaining functional groups, such as carbonyls and quinonesalso exist on the surfaces of many C-based materials.24 Under thestrong and prolonged oxidative conditions existing at an operPEFC cathode, these surface groups originally present withunused MEA could oxidize to carboxyl groups or phenols~respectively!, which are hydrophilic. As a result, the cathode catalyst lwould become more hydrophilic, causing a gradual increase inode water-uptake over the course of life testing. Similar pheno

Figure 9. Polarization data for a type II MEA during 1000 h of life testi~a, top! IR-corrected polarization curves;~b, bottom! IR-corrected kinetiperformance. Operating conditions and MEA compositions are the sathose listed in Fig. 6.

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may proceed within the GDL carbon phases and could also shoas performance deterioration in the mass-transfer region.

The total change in cell impedance of the type II MEA is sm~found to be 4.8% based on a linear data fit!. Even for the relativellarger change in cell impedance during the life test of the tyMEA, it is still only an increase of 13% over about 2000 h. Thimpedance changes alone do not explain the gradual voltageobserved during the two life tests. Since the type II MEAsome 30% of the cathode active surface area over 1000 hsuggests that during moderate-term life tests~<2000 h! the majocause of MEA degradation is agglomeration/growth of metallyst clusters.

Membrane degradation.—Cell impedance ~HFR! consistsmainly of the proton ionic resistances of both the membranerecast ionomer of the electrocatalyst layers, with a minor conttion to the measured value consisting of the pure electrical~electrontransport! resistances. So two main causes of increased cell imance over time are (i ) membrane degradation resulting in increabulk resistance or increased resistance at the interfacial zonemembrane and catalyst layers, and (i i ) degradation of the catalylayer recast ionomer resulting in increased resistance at the intbetween catalyst clusters and the ionomer network.25-30 If thechange in ionic resistance of the catalyst layers is smaller thaof the membrane~as shown by Saabet al. for lower ionomer contents and as may be suggested by the SEM data in Fig. 2 showchange in morphology localized to the cathode catalyst surf!,then measured cell impedance during life testing should reflchange primarily due to the membrane. Therefore, the contingradual cell impedance increase shown in Fig. 1~type I MEA! up tothe point of the power outage event around 1800 h, would susome degradation of the membrane took place. Cell impedanthe type II MEA~Fig. 6! decreased slightly due to the initial hydtion of the membrane~conditioning, massaging, incubating, et!,remained unchanged through about 700 h, then slowly increuntil the end of the test at 1000 h.

The differing overall cell impedance trend between the two Mtypes is attributed to the combined effects of different currentsity and hydration state of the two MEAs. One membrane degtion mechanism is the chemical attack by hydroxyl and hydrooxyl radicals generated by the reaction of H2 and O2 at the cathodfrom hydrogen crossover through the membrane.9 Higher currendensity means a higher ORR rate, which would result in mothese radicals generated. Consequently, chemical degradationmembrane should become more intense at higher current dfrom the increased supply of cathode radicals.

Even though the two MEA types were operated under theinlet relative humidities and cell temperature,i.e., the same initiaconditions for isothermal membrane-water-sorption equilibriumwater content inside the membrane of the type I MEA wouldhave been different from that of the type II MEA. Since the wconcentration profile of the membrane depends on water sofrom the vapor phase, water flux by electro-osmotic drag fromode to cathode, and water back-diffusion from cathode to anodsignificant difference in current density between the two MEA tywould have yielded a much different hydration state of the restive membranes. As Springeret al. pointed out in their modelinwork of the water concentration profile in a Nafion 117 membra25

a current density increase of 50% over the original value wresult in a 144% decrease in maximum water content of the mbrane at the anode side. As a result, the difference in water cotration along the direction of membrane thickness from anodcathode would be much greater for the higher current densityAlthough Nafion 112 was used in this work and is significathinner ~50 mm vs. 175 mm for N117!, the general findings oSpringeret al. still apply, with the main difference being a flatwater concentration profile with decreasing membrane thickTherefore, the higher current density used with the type II Mresulted in a lower hydration state of the membrane and mo

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tense degradation beyond the 500 h point from the long-term eof current-density hydration. Between 500 and 900 h, cell imances increased 3.2% for the type II MEA~at 1.07 A/cm2) and only2.1% for the type I MEA~at 0.40 A/cm2). This explanation agreewith the findings of Endothet al., who reported that membrane deradation is more severe at low hydration than at high hydrabased on cathode inlet dew points.12

Ion chromatography analysis of the cathode outlet watertype II MEA ~see Fig. 10! shows the presence of fluoride (F2) ionsand sulfate (SO4

22) ions, confirming at least partial degradationthe membrane and/or recast ionomer. The fluoride ion concentwas consistently higher than that of the sulfate ion~average background content of F2 and SO4

22 was found to be 0 and 101 pprespectively!, which is qualitatively consistent with the findingsBlom et al.18 who investigated the interface of the membranethe catalyst layers. In their work, it was observed through csectional X-ray mapping that there was a depletion of elemfluorine and an accumulation of elemental sulfur within a narband of the interfacial region between the membrane and cacatalyst layer after some 300 h of MEA durability testing. The2

ion concentration increased by a factor of.2 at about 800 operatinhours~denoted with an additional arrow in Fig. 10!, correspondinto the observed voltage drop and cell impedance increase after~shown in Fig. 6!. These collective observations suggest more mbrane degradation after 700 h and the need for further investig

Dissolution of Nafion ionomer.—The Nafion 112 membrane usfor these tests is relatively stable and difficult to decompose~i.e.,chemically break down the polymer structure! or dissolve~i.e., sol-vate fragments of the oligomers! in water at conditions of ambiepressure and 100°C. Dissolution of a Nafion membrane occursit is boiled at 210°C at 68 atm~1000 psi! for at least 2 h. The recaNafion ionomer network of the catalyst layer is not as stable amembrane counterpart.~In our laboratory at LANL it has been cofirmed that dissolution of recast Nafion ionomer happens durDMFC life test by recasting an ionomer film directly from the caode product water.! During a H2 /air PEFC life test, the recast ionmer of the cathode catalyst layer is soaked and washed continuby both product water generation and inlet water flow, especiaan 80°C dew point and 2063 103 Pa gauge~30 psig! gas pressureIt is then reasonable to assume that dissolution of recast ionomthe catalyst layer may occur at these prolonged operating condAlthough the Fig. 2 SEM micrographs presented here sugg

Figure 10. Change in concentration of fluoride (F2) and sulfate (SO422)

anions during the course of 1000 h of life testing for a type II MEA~ana-lyzed by ion chromatography!. Operating conditions and MEA compositioare the same as those listed in Fig. 6.

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.

possible loss of ionomer from the catalyst layer surface, a stunow underway at LANL to use neutron scattering and reflectomwith the goal of analyzing Nafion ionomer decomposition proddirectly in collected outlet water. This analytical technique maable to distinguish between dissociation components from mbrane degradation and those from recast ionomer degradation

Migration of Cr from cathode to anode.—The cathode catalyfor type II MEAs is a platinum-chromium alloy (Pt3Cr), and oxidation and dissolution of the alloying component can occur understrong oxidative conditions as in a PEFC cathode catalyst layer~par-ticularly in a prolonged life test!. To study the stability of this allocatalyst, the surfaces of both the anode and cathode GDLs in cwith the electrocatalysts were examined using laser ablation ICwith the intent of determining if any dissolved Cr from the cathcatalyst layer ends up in the GDLs. With this technique thebeam is controlled to ablate a small area of GDL to a depth of 2mm, so the elemental make-up within a shallow surface layer canalyzed. Anode and cathode GDL results are shown in Figwhich indicates that there is a high Cr content in the as-recGDL ~existing as a natural trace impurity of carbon and grapproducts!. Initial Cr content for both the anode and cathode Gdecreases substantially after only about 120 h of exposure toating conditions in actual cell hardware~obtained from the ceblank without drawing current, as shown in Fig. 11!. Based on thidata, it appears that the majority of elemental Cr found in theode outlet water during life testing~Fig. 8! is Cr from the cathodelectrocatalyst and not from the intrinsic GDL materials. AlmosCr is present in either GDL after 500 h of life testing at 1.07 A/c2.We believe that the Cr originally present in the as-receivedmaterial is washed away under the generous humidity condsometime within 500 operating hours. Furthermore, there wasnificant increase in Cr content after 1000 h~shown in Fig. 11! forboth the anode and cathode GDL, suggesting Cr entrapmentthe GDLs from the Pt3 Cr alloy between 500 and 1000 operathours. Since Pt was the only metal in the anode catalyst, thcreased Cr content of the anode GDL after 500 h must havefrom the cathode side. In other words, Cr migrated from the catelectrocatalyst through the membrane to the anode side. Ththat the Cr content in the cathode GDL was found to be higherthat in the anode GDL after 1000 h also supports the theorymigration from cathode to anode. If the origin of the 1000 h Cr wthe cathode catalyst layer, it would follow that less Cr wouldpresent at the anode side due to the added mass-transfer resof the membrane and the greater migration distance.

Cr migration from cathode to anode has not been previoreported, although migration of Ru from a PtRu anode alloy31 andmigration of Pt from a pure Pt cathode21,22 have been reported. Rmigrated from the anode side of a DMFC through the membrathe cathode side, and Pt migrated from the cathode side of an H2 /airPEFC into the membrane, respectively. Migration of Cr in thisrection ~cathode to anode! is thermodynamically favorable duethe potential gradient between anode and cathode set up belectrical connection of the fuel cell. In addition, the concentragradient of dissolved Cr across the membrane would also be aing force for diffusion. Divalent Cr21 cations could also exchanirreversibly with protons in the membrane causing a net increaproton transport resistance.19 These phenomena could also hapwhen the cell is at open circuit~as discussed in the Experimensection!, with an even higher potential gradient between anodecathode, and could have also contributed to the type II MEA imance increase beyond;500 h~discussed above for Fig. 6!. Stabilityof alloy catalysts in PEFCs remains a prominent issue toachieving required commercial lifetimes, and needs further invgation. An attempt is planned to reproduce these laser abICPMS results suggesting Cr crossover using either X-ray flucence spectroscopy, secondary-ion mass spectroscopy, or eprobe microanalysis and TEM X-ray mapping.

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Conclusions

Life tests of two different types of MEA have been carriedunder high-humidity conditions~i.e., over-saturated gas feed streaand high current density operation!. TEM and SEM examinationsthe MEA revealed the agglomeration and/or growth of metal catclusters and the possible loss of recast Nafion ionomer fromcathode catalyst surface. The electrochemically active surfaceof the cathode catalyst layer decreased continuously durincourse of life testing. This loss was caused by metal-catalyst cagglomeration and/or dissolution~ripening, etc.! of metal clusterfrom the catalyst layer. One major cause of PEFC performanceradation at high-humidity conditions during middle-term life te~i.e., up to around 2000 h! has been shown to be the catalyst cluagglomeration/growth phenomena. The presence of fluoride anfate anions in the cathode outlet water suggests degradation ofthe Nafion 112 membrane or catalyst-layer recast ionomer, orChromium in the cathode catalyst alloy migrated through the mbrane to the anode side, which raises a concern for Pt-alloyed

Figure 11. Elemental changes in Cr content within the outermost MPL/Gsurface layers in contact with the respective electrocatalyst layers duricourse of 1000 h of life testing~analyzed by laser-ablation-assisted ICPM!:~a, top! Cr in cathode GDL, and~b, bottom! Cr in anode GDL. Operatinconditions and MEA compositions are the same as those listed in Fig

a

r

-

l-r

.

-

ode catalyst materials used during long-term PEFC operationture work will consist of experiments to further distinguish betwthe four complementary processes of catalyst metal-clagglomeration/growth, dissolution/ripening of catalyst metalticles, membrane structural degradation, and catalyst-layerionomer degradation.

Acknowledgments

This work was supported by the U.S. Department of EneOffice of Hydrogen, Fuel Cells and Infrastructure Technologythe Office of Basic Energy Sciences-Chemical Sciences. Ionmatography analysis was completed by Patrick Martinez, andanalysis was carried out by Douglas Blom of Oak Ridge NatiLaboratory. Acknowledgement is also given to Mahlon WilsonFrancisco Uribe for helpful discussions.

Los Alamos National Laboratory assisted in meeting the publiccosts of this article.

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Durability of PEFCs at High Humidity Conditions

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