v.d.11 development of novel non-pgm …...preparation conditions for sample prepared by msu using...

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FY 2012 Annual Progress Report DOE Hydrogen and Fuel Cells Program

Sanjeev Mukerjee Department of Chemistry and Chemical Biology, Northeastern University (NEU)Boston, MA 02115Phone: (617) 373-2382 Email: [email protected]

DOE Managers HQ: Kathi Epping MartinPhone: (202) 586 7425Email: [email protected]: David PetersonPhone: (720) 356-1747Email: [email protected]

Contract Number: DE-EE0000459

Subcontractors: • UniversityofNewMexico,Albuquerque,NM(UNM)

(Prof. Plamen Atanassov)• MichiganStateUniversity,EastLansing,MI(MSU)

(Prof. Scott Barton)• UniversityofTennessee,Knoxville,TN(UTK)(Prof.ThomasZawodzinski)

• NissanTechnicalCenter,NorthAmerica(NTCNA),Detroit,MI(Dr.KevAdjemian)

• BASFFuelCells,Somerset,NJ(BASF)(Dr. Emory De Castro)

• LosAlamosNationalLaboratory,LosAlamos,NM(LANL)(Dr.PiotrZelenay)

Project Start Date: August 1, 2010 Project End Date: August 1, 2014

Fiscal Year (FY) 2012 Objectives

Theobjectiveofthisprojectistodesignnonplatinumgroup metal (PGM)-based materials and supporting gas transportlayer,bothintheinterfacialreactionlayerbetweentheelectrodeandmembraneaswellasintheunderlyinggasdiffusionmedium,formeetingandexceedingDOEgoals for application in solid polymer electrolyte fuel cells. Thisprojectisfocusedonmaterialsdevelopmentandisassisted by advanced analytical tools, computation, and testing for improving the design via critical understanding of electrocatalysis in these novel structures.

Technical Barriers

ThisprojectaddressesthefollowingtechnicalbarriersfromtheFuelCellssectionoftheFuelCellTechnologiesProgram Multi-Year Research, Development and Demonstration Plan:

(B) Cost (eliminate precious metal loading of catalysts)(C) Performance(increasethespecificandmassactivitiesof

catalysts)(A) Durability (increase the durability/stability of catalysts

withcycling)

Technical Targets

Table 1. Progress towards Meeting Technical Targets for Non-PGM Electrocatalysts for Transportation Applications

Characteristic Units 2017 Target NEU 2012 status

Specific Activity @ 80oC, 150 kPa, H2/O2, 100% relative humidity (RH)

A/cm3

A/cm2300 A/cm3 (internal

resistance free)100 mA/cm2 (internal

resistance free)

130 A/cm3

105 mA/cm2

Durability at 80oCCycling: Catalyst Durability

% loss of activity

5% <1%

Durability at 80oCCycling: Carbon Corrosion Durability

% loss of activity

10% <50% Partially

recoverable

FY 2012 Accomplishments

Crosslaboratorymeasurementofawidevarietyof•samples emanating from diverse choice of polymer precursorsatMSU,NEU,UNMindicateaconfluenceintermsofperformance;theseareveryclose(withintheerrors inherent in these measurements) to the current state-of-the-art materials recently reported by our partnersatLANL[1].OneofthemostrecentsamplesreportedbyLANLunderthelabelLANL-2howevershowsa~49mVloweroverpotentiallossascomparedtothe other materials.Wide variety of polymeric precursors indicating such •confluenceofinherentoxidationreductionreaction(ORR)activitystronglysuggests,atleastqualitatively,evolutionof a common active site as a result of pyrolysis conditions.Extrapolationoffuelcellderivedinternalresistance•(iR) corrected data currently indicates volumetric performance in the range of 160-175 A/cm3. Geometric area activity (iR-free) is currently 105 mA/cm2.

V.D.11 Development of Novel Non-PGM Electrocatalysts for Proton Exchange Membrane Fuel Cell Applications

Mukerjee – Northeastern UniversityV.D Fuel Cells / Catalysts

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DOE Hydrogen and Fuel Cells Program FY 2012 Annual Progress Report

Durability measurements conducted in one of such •samples derived from Melamine polymeric precursor andironacetatefollowedwithplasmapyrolysisshowsexcellenttolerancetocatalystsstabilitytests(based on Nissan protocol, similar to DOE protocol). Carboncorrosiontestswhichinvolvesloadcyclingto 1.5 V vs. the reference hydrogen electrode (RHE) howeverindicatedsignificantlosses;thishoweverwasrecoverableover1,600cyclesbeyondwhichirreversiblelosses occurred.Understandingofthenatureoftheactivesitewas•significantlyadvancedinthisreportingperiodwithidentificationofadualsitemechanismwhereintheN2+2sitewasresponsiblefortheinitialadsorptionandreductionofoxygentoperoxidemoietiesfollowedwithasecondcascadestepoffurtherreductionoftheperoxideincloselysurroundingFe-N2 sites. Such formulationofthemechanismwassupportedwithinsitu X-ray absorption spectroscopy (XAS) and targeted electrochemical probe measurements.

G G G G G

Introduction Recentreports[1-2]haveclearlydemonstratedthe

significantadvancementsmadeinenablinggoodoxygenreductionactivitybyFe-basednon-PGMcatalysts.ThesesocalledFe-Nx based systems have evolved over several decadesofintenseworkleadinguptothecurrentstateoftheart,reportedrecentlyinreferences[1-2].Thisreportprovidesforthefirsttimeacomprehensiveviewof(a)confluenceofORR activity derived from materials prepared using a variety ofpolymericprecursormaterialsviz.thecurrentstateoftheart[1-2]bythreedifferentuniversitygroups(b)excellentdurability in terms of catalyst stability (via DOE and Nissan protocols) and recovery of losses incurred during carbon corrosionmeasurementwhenplacedunderload(c)detailedunderstanding of nature of active site and electrocatalytic pathwayasdistinctfromtheparallelpathwayinalkalineelectrolytes.

Approach Theapproachadoptedinthisreportingperiodinvolved

(a)materialpreparationusingofawidevarietyofpolymerprecursormaterialsinconjunctionwithFe-containingsaltspyrolyzeduptotemperaturesof700-800oC under inertatmospherefollowedwithrepeatedsteps(uptothree)of etching (in acid) and repyrolysis. Rotating ring disc electrode(RRDE)andfuelcelltestsusingwellestablishedprotocols for cross laboratory performance comparison andextrapolationofvolumetricactivity(fromiR-correctedfuelcelldata).Durabilitymeasurementsusingtwowell

established protocols (catalyst durability) and carbon corrosion tests (both DOE protocols) and investigation on the nature of active site and ORR electrocatalysis steps as measuredusinginsitusynchrotronspectroscopyattheFeKedge under actual cell operational conditions.

Results

Figure1showsthreedifferentpolymerprecursorstartingmaterials,namelypolyaniline(LANL),polyvinylguanidine(NEU)and1aminoanti-pyrine(UNM).ThesewerepolymerizedoneitherKetjenblack600(LANLandNEU) or templated on high surface area silica moieties. AllofthepolymerizationswereinitiatedusingwellknownproceduresreportedinourJulyinterimreport.Extentofpolymerizationhasbeenpreviouslycalibratedusingreferencemolecularweightmeasurementsconductedwiththeaidofmassspectrometry.AllthepolymerizationstepswereconductedinthepresenceofFesalts(typicallyacetate)andinonecasealsowithCosalt(alsoacetate).Followingpolymerizationwithsimultaneousincorporationofthemetalsalt,eachsamplewaspyrolyzedunderinertgas(Ar)inatemperature range of (700 to 800oC);thiswasfollowedbya series of acid leach (0.5 M H2SO4) and repyrolysis steps (uptothreetimes)beforefinalformationofthecatalystmaterial.InthecaseoftheSitemplatematerialsfromUNM,theinitialpyrolysisstepwasfollowedwithanacidetchstepusingdiluteHF(0.05M).ThisprovidedforaninsituopenframeworkcarbonstructuretoformalongwiththeevolutionoftheFe-NxstructureaswasthegoaloftheLANLandNEUsamplesalbeitwithouttheSitemplate.IntheMSUsamplecaseshowninFigure3,asinglehigh-pressurepyrolysisstepwasadoptedusingnitrogenprecursor(Melamine),carbon(KetjenblackorVulanXC-72)andthemetalsaltFe2+ acetate. Figure2,showsacrosslaboratoryORRmeasurementusingaglassycarbon-basedRRDEexperimentwithhighloadingof catalysts mentioned above (600 µg/cm2).AlsoshownaretheresultsofsamplesobtainedfromLANL,namelythenewsamplereferredtoasLANL-2andthepreviouslyreportedmaterial[1]referredtoasPANI-Fe-Coderivedusingpreparation conditions mentioned above. As can be seen fromFigure2,measuredusing0.1MHClO4 (900 rpm) under oxygensaturationconditions,roomtemperature,allsampleswiththeexceptionofanewhithertounreportedmaterialfromLANLlabeledasLANL-2performedsimilarly.TheLANL-2materialshoweda~40mVinitiallowerORRoverpotential.Inadditiontothematerialsmentionedabove,Figure2alsoshowstheperformanceofanearlierreportedmaterialreferredtoasPEIbFefromNEU,whichconstitutesthepolyethyleneiminepolymerinconjunctionwithFeacetatepreparedinexactlysimilarconditionsasreportedabove.Suchcloseperformancelevelsobservedforawidevariety of samples prepared using different procedures, LANL,NEUvs.UNMandMSUsamplesstronglyindicatespossible evolution of common active site as a result of such

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V.D Fuel Cells / CatalystsMukerjee – Northeastern University

Figure 1. Representation of chemistry and pyrolysis conditions used by cross laboratory partners, UNM, LANL, and NEU

Figure 2. Cross laboratory comparison of ORR activity measured using rotating ring disk electrode technique, performance comparison shown at 900 rpm. Electrolyte is 0.1 M HClO4, room temperature with standard catalyst loading of ~600 µg/cm2. Corresponding Tafel slopes and kinetic activity are shown in associated plot and table.


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pyrolysis.ItshouldbenotedthatattemptstopreparesuchFe-based(mostlikelyFe-Nx) type materials have a three decade history of evolution in activity. One important distinctionbetweenthecurrentmaterialsandthewealthofdata reported earlier is the use of starting materials. Most of theearlierworkusedmetalhemetypemacrocyclessuchasFeorCocontainingphthalocyaninesorporphyrins.InthecurrentevolutionofmaterialsshowingtrueenhancementofbothonsetofORRaswellasinherentactivityemanatefromslowevolutionofFe-Nx structures via pyrolysis of polymericprecursorswithnoexistingFe-Nstructures.Thisisanimportantdistinctiontomakeasthiswillbelaterusedtoexplainthereasonforenhancedperformanceofsuchmetalpolymer composite pyrolysis derived materials as opposed to previous attempts involving heme-containing macrocycles. Figure3providesasnapshotofdurabilityandfuelcellperformancereportedviaMSUpreparedsample,whereintheextrapolatedvolumetricactivity(A/cm3)showsvaluesinthe range of 160-170 A/cm3.Whilenotstellarthisiswithinthe DOE mandated target of 300 A/cm3. More importantly

howeverthecatalystdegradationmeasurementsconductedusingNissanprotocols(squarewaveswith3secondonand3secoffperiodsbetweenapotentialrangeof0.6and1.0V)indicated no degradation; in fact over the short range of time (1,600cycles)therewasaslightimprovementofperformance(Figure3).Subjectingthesamemembraneelectrodeassembly (MEA) to carbon corrosion test using DOE protocols(1.0to1.5Vloadcyclingtests)showedimmediateloweringofperformance,anoccurrencenotsurprisingconsidering the most likely active site formation involving a local graphene structure. Most surprising observation howeverwastherecoveryobservedwhentheMEAwasleft under load at 400 mA/cm2.Thisrecoverylastedthroughtill1,600cyclesafterwhichirrecoverablelosseswereobserved.Figure4representsaproposedmechanismdrawingdistinctionbetweentheobservedactivitydifferencesobservedbetweenthetwoextremeendsofthepHscale,athighpHORRactivity(900rpm)isalwaysbetterwitha~150-200mVloweroverpotential(seelowerlefthandplot).Thisisclearlyobservedintherotatingdiskelectrodeprofiles

Figure 3. Preparation conditions for sample prepared by MSU using high pressure pyrolysis of nitrogen precursor Melamine in conjunction with carbon support (Ketjen black 300) and Fe salt (acetate), iR-free fuel cell performance projections to 0.8 V is also made for obtaining volumetric activity (Table 1). Durability measurements are shown for this sample measured at the Nissan technical center North America using DOE protocols for both catalyst stability and carbon corrosion. As shown, this sample showed excellent durability for catalyst stability test; carbon corrosion test however exhibited losses which however were recoverable till 1,600 cycles.

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V.D Fuel Cells / CatalystsMukerjee – Northeastern University

Conclusions and Future Directions

Conclusions

Cross-laboratory studies of materials emanating from the •pyrolysis of diverse polymer precursors in conjunction ofFeandCosaltsindicateacommonevolutionofactivesites for ORR reduction.Detaileddurabilitystudiesindicateexcellenttolerance•to catalyst stability tests, and relatively poor resistance tocarboncorrosiontestprotocols,thelatterhoweverisrecoverable over several cycles.Mechanismforanactivesiteisproposedwhichis•supported by activity observations at both ends of the spectrum.

ofarepresentativeFe-TPP(tetraphenylporphyrin)samplepyrolyzedat800oC on Ketjen black (600) support. When theelectrolyteisinsteaddosedwith0.1MH2O2, the higher pH response is better than O2. Corresponding behavior at lowerpHscalerepresentsonlyinitialdecompositionoftheperoxide.DetailedXASmeasurements(notshown)supportthemechanismshowninFigure4,whereinthisdifferenceinactivityoverthepHscaleisexplainedonthebasisofthefact that the initial turnover number for the adsorption of molecularoxygenonF-N2+2isextremelyfastbothinacidandalkalinepH;howeveritisonlyinthealkalinepHthattheperoxy-anionisstabilizedandallowsforthefurtherreductiontowater.IntheacidpHtheinabilityofsuchstabilizationthereforerequiresanadditionalsitewhereperoxy-anioncanbefurtherreduced.

Figure 4. Mechanistic interpretation of the active site based on RDE measurements at acid and alkaline pH with O2 and peroxide and in situ and operando synchrotron measurements conducted at Fe K edge (not shown) which explains the important distinction of observed activities at the two pH scales and need for a dual site mechanism in the acidic environment.


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FY 2012 Publications/Presentations 1. ‘FundamentalMechanisticUnderstandingofElectrocatalysisofOxygenReductiononPtandNonPtSurfaces:Acidvs.AlkalineMedium’,N.RamaswamyandS.Mukerjee,AdvancesinPhysicalChemistry,Vol.2012,ArticleID491604,(2012).

References 1. Wu,G.;More,K.L.;Johnston,C.M.;Zelenay,P.Science 2011, 332, 443.

2.Lefevre,M.;Proietti,E.;Jaouen,F.;Dodelet,J.P.Science 2009, 324, 71.

Future Directions

Principalfocusofthegroupwillincludemeetingand•exceedingDOEtargetsforORRactivityintermsofbothvolumetricandgeometricareas(seeTable1).MeetandexceedDOEdurabilitymilestonesformore•diverse materials.Furthertestthevalidityoftheproposedmechanism•usingfinalsetofinsituandoperando synchrotron XASdatainconjunctionwithdensityfunctionaltheorycalculations.

v.d.11 development of novel non-pgm …...preparation conditions for sample prepared by msu using...

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