a practical approach in glycerol oxidation for the ... · development of a glycerol fuel cell by...

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Research Article

DOI: 10.21767/2471-9889.100018

Trends in Green ChemistryISSN 2471-9889

1© Under License of Creative Commons Attribution 3.0 License | This article is available in: http://green-chemistry.imedpub.com/archive.php

Quintero Pulido DF1, Ten Kortenaar MV2, Hurink JL1 and Smit GJM1

1 ElectricalEngineering,MathematicsandComputerScience,UniversityofTwente,Enschede,TheNetherlands

2 DrTenBV,Rondweg11M/N,8091XAWezep,TheNetherlands

Corresponding author: QuinteroPulido

[email protected]

ComputerScience,DepartmentofElectricalEngineering,MathematicsandComputerScience,UniversityofTwente,P.O.Box217,7500AEEnschede,TheNetherlands.

Tel: 0031621323089

Citation: PulidoDFQ,KortenaarMVT,HurinkJL,etal.APracticalApproachinGlycerolOxidationfortheDevelopmentofAGlycerolFuelCell.TrendsGreenChem.2017,3:1.

IntroductionThe growing amount of biodiesel produced worldwide leadstomore than twomillion tonsof glycerol entering themarketyearly [4]. Glycerol is used in the pharmaceuticals, cosmeticsand food industries. However, the current production ratealreadysurpassesthecapacitiesneededbytheseindustries.Thecountrieswithhighproductionofbiodiesel(e.g.USA,Germanyand Colombia) are facing serious challenges with the glyceroloverproductionchanging thevalueofglyceroldramatically [5].This specific situation has led to consistently generated low

glycerolprices,makingglycerolabio-wasteproductintheneedfornewmarketalternatives.Onealternativetovaloriseglycerolmightcomefromelectrochemicaloxidationwhichmaygivewayto oxygenated materials with higher market value (tartronicacid,dihydroxyacetone,andglycolicacid,amongothers)[6].Theelectrochemical oxidation of glycerolmay be performed usinga fuel cell. Fuel cells can convertdirectly a fuel intoelectricity[7].Onecommon fueluse in fuel cells isH2 and itsproductioniswellknowninchemicalproductionplants.However,tostorehydrogenisstillachallengeduetoitshighvolatilityandsafetyconcerns[8],onealternativetotheproblemistouseafuelthat

A Practical Approach in Glycerol Oxidation for the Development of A Glycerol Fuel Cell

AbstractTheresearchonelectrochemicalcarbonmoleculeoxidationstartedinthepastyearsasnewelectrochemistrieswere researched fornewfuelcells systemsandbatteriesthatmaylineupasbackupenergysupplyandstoragesystemsinoff-gridandon-gridmicrogrids,asmodeledbyourgroupattheUniversityofTwente[1-3].Byliningupthesetwodisciplineswehopetosupportthebridgebetween new electrochemical systems on one side, (pilot) productionwithpartnercompanies,prediction,andvalidationofsystemsuponimplementationinmicrogridsbyourgroup.

Theelectrochemicaloxidationofglycerol inalkalineaqueoussolutionhasbeenstudiedongoldandgoldcoatedmetals(Zn-AuandCu-Au)byvoltammetryandEIS(Electrochemicalimpedancespectroscopy)forpossibleuseinanewfuelcellas an outlet for the excess glycerol that is produced in the biodiesel industry.Theobservationsshowthatthegoldsurfacemaychangeuponcyclingbycyclicvoltammetry. Besides, the current density shows non-linear behaviorwith thesquarerootofthescanrate,implyingthatthereactionisnottotallycontrolledbydiffusion.EISanalysisusingtheEQUIVCRTsoftwarerevealedthatoneoutoftwenty tested equivalent circuits fitted the datawell at potentials of -0.05 V,-0.15Vand -0.25Vvs.Ag/AgCl, identifying resistorsandaWarburgelement inparallelwiththedoublelayercapacitance,theelementsarepossiblyrelatedtothepresenceofdoublelayersassociatedwithhydroxypyrovateandoxalateions.Theresultsareconsistentwiththelow-frequencyerrorfittinganalysis(10-4),ACSimulink-Matlab fitting responds and the Kronig-Kramers transform test. ThetestedZn-AuandCu-Auelectrodesshowsimilarvoltammetrybehaviorasthegoldelectrode,aswitnessedbytheresultsofcycleanalysisandthescanrateanalysis.ThedischargechronoamperometrytestfurthershowsthattheZn-AuelectrodeandCu-Auhavehighercurrentdensitiesthanthegoldelectrodeatapotentialof-0.25Vvs.Ag/AgCl(5mAcm-2,4.5mAcm-2,and3mAcm-2respectively).

Keywords: Microgrids;Voltammetry;Oxidation;Gold;Glycerol;Impedance

Received: June07,2017; Accepted: June27,2017; Published: July03,2017

2017Vol. 3 No. 1: 5

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is liquidunderambientconditions.Therearedifferenttypesoffuels thatwere tested in the past as possible alternative fuelssuchas,hydrazine,organiccompounds,andformicacid[9-11].Nevertheless, there are technical and commercial necessitiesthat a fuelmust comply in order to be used in a fuel cell e.g.availability,transport,safety,andcost.Takingthisintoaccount,thequantityofpossiblefuelsisreducedtoafewtypes.

Electrochemical oxidation of glycerol has already beenconsideredinthepast,andspecialattentionwasgiventowardstheselectionofcatalystinalkalineandacidmedia.Thefollowingmaterialshavebeenreported:Pt[12],Pt-Ru[13],Au,Pd[14,15]and Ag [16]. In the case of gold there are several advantagesoverothercatalystusedforglyceroloxidation,e.g.ithasafastelectrocatalytic response, is stable in alkaline media, is moreabundant and it has a lower price than palladiumor platinum[17]. Although gold has been studied as a catalyst for glyceroloxidation,themechanismofthereaction isstill far frombeingunderstood. E.g. in the studyofUreta [18], themechanismofalcohol oxidation in alkaline media on a gold electrode wasinvestigated and it is suggested that the optimal oxidation foralcohols is at pH 11. Also, Shi [19] studied the kinetics of lowconcentrationofglycerolinalkalinemediausingvoltammetryonagoldfoil.Recently,Qi[20]studiedthegenerationoftartronadeduringglyceroloxidationinalkalinemedia,usinganAuCcoatedelectrode. From these studies were concluded that glyceroloxidationreactionatgoldelectrodebeginswithtwoprimary-OHgroups and the chemical reaction possibly decrease the over-oxidation of the secondary -OH and C-C bond cleavage. Also,Chornaja [21] proved that the glycerol oxidationpathways areprobablytwodependingonthestudiedcatalyst, leadingtotheformationof intermediatecompounds suchas tartronic, lactic,glycolic andoxalic ions.Marshall [22], studied the influenceofgold nanoparticles loading onAu/C electrode towards glyceroloxidation showing that the oxidation process is dependent onparticle load and size distribution. Kwon [23,24] studied themechanism of glycerol oxidation on polycrystalline gold andplatinumelectrodes,Kwon found that theglyceroloxidation isstrongly influencedbypH, ano catalyticactivitywasobservedinacidmedia.Padayachee[25]usedelectrochemicalimpedancespectroscopy (EIS)withanAu/MnO2/Carbonalloyelectrode tocompare resistances at different potentials. In his study wasdetermined that the lowest cell resistance is at values of 0 Vto 0.2V vs. Ag/AgCl implying an optimal potential for glyceroloxidation.However,noclearconclusionwasdrawnforthecircuitfittedforthereaction.

In order to build a glycerol oxidation technology viable like ina fuel cell, themechanism of the chemical reaction has to bestudied in more detail to understand the reaction pathwaysandthekinetics.Also,tomakethisapproachfeasible,stillmorestudiesneedtobedonetowardsdevelopmentandresearchofstableelectrodeswithlowcatalystload.

This research aims to yield to an approach for a possibledevelopment of a glycerol fuel cell by observing the glyceroloxidation mechanism and kinetics using a gold electrode inalkalinemedia.Besides,itaimstocompare the electrochemical behaviorofgoldcoatedmetalspreparedinsitu(Zn-AuandCu-Au)inordertoreducethegoldloadusedforglyceroloxidation

andforpracticalimplementation.Thepracticalimplementationis related to the use of the glycerol fuel in microgrids. Thesimulation of renewable energies (solar panel, wind turbinesto name a few) combine with batteries and fuel cell is undersimulation in our group in The Netherlands [3], however, thelarge-scaleapplicationgoesalongwithanunderstandingofthechemical, electrochemical and physical characteristics of thedifferenttechnologies.

Thispaperisorganizedasfollows:ExperimentalmethodsandtheEISfittingcircuitisexplained.Generalapproachusedinthispaperfordatainterpretationispresented.Experimentalresultstestingthegoldelectrodeisdividedinthreeparts.First,voltammetryisusedforcomparisonofcatalystforglyceroloxidation(Pt,Ag,Au,Cuandglassycarbon).Second, thecyclicvoltammetryanalysisof cycles and scan rate effect of glycerol oxidation in alkalineaqueoussolutionongoldelectrodeisshown.Andthird,theEISresultsforthestudyofthemechanismthattheglyceroloxidationreactionundertakeswhenusingthegoldelectrode.PracticaltestwiththegoldcoatedZnandCuelectrodesanalyzedwithcyclicvoltammetry and chronoamperometry (cycles and scan rateeffect) to observe the glycerol oxidation in alkaline media incomparisonwiththegoldelectrode.Lastly,theconclusionsarepresented.

Experimental MethodsThereagents:Glycerol(C3H8O3)≥98%,H2SO4ACSreagent,37%,AuBr2 reagent ≥ 99% and NaOH reagent grade ≥ 98%, pelletswerepurchasedfromSigma-Aldrichwereusedwithno furtherpurification.

Theequipment:Cyclicvoltammetry-chronoamperometryexperi-mentswereperformedwith a PGSTAT101 compactunit fromthecompanyMetrohmAutolab.ThepHandtemperaturewererecordedwithaHannaHI9811-5pHmeter.Impedancespectros-copywasperformedwithaModel600ESeriesElectrochemicalAnalyzerfromthecompanyCHinstruments.

The EIS analysis: EIS analysis was done with the EIS fittingprocedureinwhichtheelementsareanalyzedwiththesoftwaremethodEQUIVCRTdevelopedbyBoukamp[26-28].Thismethodprovides information on the kinetics of the reaction and arevalidated by a linear Kronig-Kramers transform test methoddevelopedbythesameauthor[29].Inthispaper,acircuitfittingtestinSimulink-MatlabforaviewofACsignaloutputisused,inordertovalidatethecircuitfitted.

The Electrochemical set up: A three electrode ensemble wasusedforthisstudy.Variousworkingelectrodeswereused:Glassycarbon(GC),Au,AgandPt(Autolabelectrodes)andmetalssuchas Zn, Cu and carbon graphite (CG) 98% gradematerials fromSigma-Aldrich. The counter electrodewas carbongraphite andthe reference electrode Ag/AgCl. The electrode ensemblewasimmersedina50mlaqueouselectrolyte.

Electrochemicalcoating:Glassycarbon,Zn,andCu (area0.072cm2)were immersedfirst ina solutionof1MH2SO4 fora fewseconds to remove impurities, then rinsed with demi-waterand dried for an hour. Each of the electrodes was immersedseparately inanelectrochemical cellwith0.01MAuBr2 as the

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ofdifferentcatalystinthesameelectrolyte.Themeasurementsweredoneat100mV/sin1Mglycerolin1MNaOH.

The voltammogram for the catalytic effect of Pt for glyceroloxidationinalkalinemediainthepotentialwindowof-0.6Vto1Vvs.Ag/AgClisshowninFigure 2a.Thefirstpeak(-0.3Vand10mAcm-2) corresponds toPt reductionand the secondpeakcorrespondstoglyceroloxidation(around0.1Vand5mAcm-2)inaccordancewithresearchreportedbyAkinbayowa[13].Figure 2b and 2c showthevoltammetryofa silverandglassycarbonelectrode for glycerol oxidation in alkalinemedia respectively.Clearly, silver andaglassy carbonhave less catalyticeffect forglycerol oxidationwhich is in-linewith research performed bySong[31].Thevoltammetryforthecoppershowthreecurrentdensitypeaksatdifferentpotentialsat-0.3V,-0.1Vand0.1VvsAg/AgCl (Figure 2d). Thefirst two current density peaks (fromleft to right)mightcorrespond tocopperoxidation indifferentstates(Cu3+toCuandCu2+toCu)asstudiedbyRozali[32].Thethirdpeakmaybeinterpretedasaglyceroloxidation.However,itmayalsoberelatedtocorrosionasexplainedinliterature[33].The glycerol oxidation in alkaline aqueous solution on gold isshown inFigure 2e,wheretwocurrentdensitypeaksat -0.2Vand-0.1Vpotentialareobserved.Thefirstpeakcorrespondstoatypicalsuddenincreaseinoxidationuponreducinggoldfromitsoxidizedstateinalkalinemedia.

The second peak corresponds to the glycerol oxidation whichis presented in the literature by different authors [14-22]. Ingeneral, thecatalyticactivity forglyceroloxidation isobservedwith Pt, Cu, and Au electrodes. However, for the Ag and GCelectrodes,noclearevidenceofglyceroloxidation isobserved,whencomparingthecurrentdensitiesofthedifferentmetalsatapotentialof-0.2V.Theresultsshowthatthegoldelectrodehasthehighestvalue(11mAcm-2) followedbycopper(8mAcm-2)andtheplatinumelectrodehasthelowestcurrentdensity(4mAcm-2).Basedontheseobservations,wefocusedonthecatalyticactivityofgoldandgoldcoatedZnandCuelectrodesforglyceroloxidation,asfurtherreportedbelow.

Au electrode kineticsFigure 3a shows repeated voltammetry cycling (30 cycles) inthe potential window between -0.85 V to 0 V vs. Ag/AgCl atAuelectrode in1Mglycerol +1MNaOH.Aftereach cycle, anincreaseincurrentdensityisobserved.Thisincreaseincurrentdensitymayberelatedtochangesinthesurfaceoftheelectrode.The change in the surfacewas observed before and after thecyclingexperiment inFigure 3b.Eachcyclemayhaveaneffecton the electrode surface,which could affect the catalytic areafor the oxidation reaction, thus increasing the current densityduring theexperiment. Thechanges in the currentdensityarerelated also to losses in the faradic activity.Glycerol oxidationis affected by the increase in the surface area, as the glycerolionsapproachingtheelectrodeinteractwiththesurfacecreatinglarger spaces. The current started to increase at around -0.25V and the steady increase continues until all glycerol near thesurfaceoftheelectrodehasbeenoxidized.

Tounderstandmoreaboutthekineticsoftheglyceroloxidationin alkaline media at the gold electrode, the influence of the

400300

200100

0-100-200-300-400

-1.4-1.2 -1 -0.8- 0.6-0. 4-0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4E vs (Ag/AgCl)/V

j/mA

cm-2

Ideal Anode

Ipc

Epc

Epa

IpaIdeal Cathode

V = Epa + Epc at a given I

Illustrationofanidealcyclicvoltammetryofafuelcellsystem with reactions occurring at the cathode andanodeelectrode.

Figure 1

electrolyte. AuBr2 was selected for electroplating as the sub-products of the electrodeposition are more environmentallyfriendlythancommongoldcyanateelectrodeposition[30].Thecathode material used was carbon graphite and no referenceelectrodewasused.Then,apotentialof2.0Vwasfixed(lettingthecurrentfree)inthecellfor30sinordertodevelopalayerofgoldaswitnessedbyachangeincolorontheelectrode.Duringtheexperiment, any smell of brominewasnotdetected.Aftertheelectrodeplatingmethod, theelectrodeswere rinsedwithdemi-wateranddriedduringthenight.

General Approach Anidealfuelcellsystemmaybeinterpretedbymeansofcyclicvoltammetry,takinganodeandcathodereactionsseparated. Ifthetworeactionsarecombinedinoneplottheymaybedepictedas shown in Figure 1. In a good glycerol fuel cell, glycerol isoxidizedattheanode,andthecurrentdensity,therefore,shouldstart to rise at a potential as negative as possible. The valueof this potential is generally determined by the catalyst, sidereactions,andcellresistance.Meanwhile,atthecathodeoxygenisreducedandthecurrentdensityshouldstarttoincreaseatapotentialaspositiveaspossible.Themaxpotentialofthesystemisprovidedbythedifferenceintheglyceroloxidationandoxygenreductionpotentials and the current by the steady state ratesofthesereactionsattheanodeandatthecathoderespectively.Thisispossiblebytakingintoaccountthelowestpossibleohmicresistance, which is determined by cell geometry, impuritiesand reaction kinetics. This research is nevertheless focusedonanalyzinggoldasananodeelectrodeforglyceroloxidationanditspossibleuseinafuelcell.Asliteratureshowedthatgoldisarelativegoodelectrode,is lesscorrosiveandstableatdifferentpH.Consequently,we looked for the lowestohmic resistancesandlowcostsubstratesforgoldcoating.

Results and Discussion Analysis of catalyst for glycerol oxidationIn order to gain insight into the glycerol oxidation, cyclicvoltammetryisusedonthedifferentcatalystasshowninFigure 2.Thegoalwastogetaquickcomparisonoftheelectrochemistry

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scanrate is studied inFigure 4a.Thecurrentdensitygraduallyincreasedwiththescanrates.Thecorrelationbetweenthescanrateandthecurrentdensityat-0.15VvsAg/AgClwaslinearatascanratecoefficientof1/5(Ivsv1/5)andnotat1/2acoefficient(Figure 4b).Otherscanratecoefficientsweretested(Figure S1) but no linearity was observed. This possibly suggests that theglyceroloxidationongoldelectrodeisnottotallycontrolledbydiffusion and does not follow Levich behavior. Also, with theincreaseofthescanratethepotentialwasnegativelyshifted.ThevariedpotentialmayrepresentanalterationinthegoldsurfaceduetotheinteractionwithglycerolandOHions[34].However,no clear evidencewas observed in the voltammogram for theformation of oxidase compounds. EIS studies could elucidatedetailsaboutthemechanismoftheglyceroloxidationatthegoldelectrodeandfindcloserelatedoxidizedcompounds.

Au electrode mechanism for glycerol oxidation in alkaline media and EIS analysisEIS analysiswas performed in different steps: first, a plausiblereaction pathway was taken from literature and a possiblereaction model was created. Second, an expected circuit wasdrawnbasedontheassumptionofthemodel forthereaction.Third, the circuitwas comparedwith the lab results using the

EQUIVCRTsoftwareandfourththefittedcircuitswereanalyzedwith the Kramers-kroning transform and AC Simulink-Matlabcircuitsignaloutputtovalidatethedataobtained.

Reactionpathways:Wang[12],explainedthatglyceroloxidationinanalkalineaqueoussolutionhastwopathwaysdependingonthecatalyst.ThefirstreactionpathwasderivedforPtinwhich,glycericacidisthemainglyceroloxidationproductwithtartronic,lactic,glycolicandoxalicacidsasby-products(Figure 5a).Glycolicandoxalicacidsareformedduringtheoxidationoftartronicacid.Inthesamestudy,itwasshowedthatthereactionproductsoftartronicacidoxidationwithdioxygeninthepresenceofthe5.7%Pt-2.4%Bi/C catalyst aremesoxalic and oxalic acids.However,theformationofglycolicacidwasnotdetected.Fortheglyceroloxidationona1%Au/Ccatalyst,theauthorsfoundthatthereisanotherpathwayforthereactioninwhichglycolateandoxalateionsmaybe createdonly by thepresenceof hydroxypyrovate(with the according to intermediate dihydroxyacetone). Oneplausibleexplanationforthetwoglyceroloxidationpathsisthebehavioroftheglycerolmoleculeinaqueoussolutionasshownin literature[35]. It isassumedthatglyceroloxidationwilltakeplacewheretheelectrongoestowardstheelectrode.Inaqueoussolution, this happens at the electrode-electrolyte interface.Thiscanbeconsideredasacharacteristicofasurfacephase,in

Cyclicvoltammetryofa)Ptb)Agc)GCd)Cuande)Auelectrodeon1MC3H8O3+1MNaOHelectrolyte,100mV/sandAg/AgClreferenceelectrode.

Figure 2

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30

25

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150100500

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Au electrode Before Au Electrode After

a b30 Cycles

1 Cycle

3 mm

E vs (Ag/AgCl)/V

E vs (Ag/AgCl)/V

j/mA

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-2

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-2

CyclicvoltammetryatanAuelectrodeon1MC3H8O3+1MNaOHelectrolyte,100mV/sandAg/AgClreferenceelectrodea)Glyceroloxidationregionbetween-0.85Vand0Vshowing30cyclesandb)Auelectrodebeforeandaftertheexperiment.

Figure 3

a)Cyclicvoltammetry,scanrateeffectofanAuelectrodespectrabetween-0.8and0Vb)Currentdensityat-0.15Vinrelationwiththescanrate(Ivs.v1/5)Allexperimentsused1MC3H8O3+1MNaOHelectrolyteandAg/AgClreferenceelectrode.

Figure 4

whichthereistheexistenceofanelectricfield,whichiscausedbythe separationof charges that are in thebulkphases in contact,depending on catalyst the rate of the reactionmay be differentand this can contribute to thedifferent paths observedwith thedifferentcatalyst.

Expected circuitByusingtheglyceroloxidationreactionpathwayforgold(Figure 5a),anequivalentcircuitwasderived.Undertheassumptionthateachreactionstepmayberepresentedbyasub-analogelectricalelement.First,weconsideredthechemicalreactionthatisshownin Figure 5b inwhich diffusion is followed by physisorption inthe outer Helmholtz plane (IHP). Chemisorption processes arepresented in parallel to the double layer capacitance in theinner Helmholtz plane (IHP) and various species are capturedclosetotheelectrode(oxidation-desorptionsteps).Theconceptofadiffusioneffectatthebeginningofthereactionisrelatedtofaradiccurrentsthatcouldoccur.Thisassumptionisbasedontheformation of a thinner Nernst diffusion layer, or with a smaller

similar contribution of the absorbed species that are in theoverallelectrode impedance.Theparallelconnections in thecircuit are associated with the resistances provided by thereactions thatoccurduringphysisorptionandchemisorptionwhicharerelatedtotheoxidationanddesorptionofions.

Fitted circuitThefittingofthecircuitwasbasedontheassumptionthatoneparticularcircuitmayfit(ormimic)alldatathatwasfoundintheimpedancemeasurements.Figure 6aisthecircuitexpectedforEISfittingfromdatainliterature.Theimpedancedatawascollected at three different potentials -0.25 V,-0.15 V and-0.05VvsAg/AgClthatare inrelationwiththevoltammetryof gold for glycerol oxidation. After testing over 20 circuitsin the software EQUIVCRTdevelopedbyBoukamp [26], onecircuitseemedtofitwellthepotentialsstudiedfortheglyceroloxidationinalkalinemediaatthegoldelectrode(Figure 6b).AllspectrahadanX2valuebelow10-4,butalthoughtheseX2 values are large, the circuit mimics reasonably the presented

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a b

Pt

Au

Reaction representation of glycerol oxidation in alkaline media a) Pt and Au catalystchemical oxidation pathways [12] b) Reactionmodel for glycerol oxidation in alkalinemediaonAuelectrode,basedondatatakingfromliterature.

Figure 5

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Correlationbetweencurrentdensityat-0.15Vandascanrate(vx)withx=1/5,1/3/3,1/2.5,1/2,1/1.66,1/1.42on1MC3H8O3+1MNaOHelectrolyteandaAg/AgClreferenceelectrode.

Figure S1

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a

b

Equivalentcircuits fortheglyceroloxidation inalkalinemediaatanAuelectrodea)Circuit expected for EIS fitting from data in literature and b) Circuit fittedwith EISsoftwareEQUIVCRT.

Figure 6

dataandisinlinewithourcomprehensionofthereaction,thereforethefittedresultswere furtheranalyzed. Inprincipal, someof theresistanceandthediffusionelementaremerged,thismightbeduetothelowinfluenceinthefrequencystudiedordotoextremelownoiseinthesystem.Adiffusionelement(Warburg)isobservedinthe circuit in parallelwith thedouble layer capacitance, and it istreatedasaCPE(constantphaseelement(Q))withavalueofn=0.5withacapacitorandaresistorinparallel.

Spectra analysisTheimpedanceplotatdifferentpotentials(-0.05V,-0.15Vand-0.25VvsAg/AgCl)wasplottedwiththerealandfittedvalues,inthevoltammetryregionfortheoxidationofglycerol inalkalinemediawiththegoldelectrodeareshowninFigure 7a.ThelowestresistanceisobservedintheEISexperimentforthepotentialof-0.05Vvs.Ag/AgClwhichwasalsorelatedtothelargestcurrentin the reaction voltammogram (30 mA cm-2) when comparedwiththeotheranalyzedpotentials.

Inordertoverifythereliabilityofthedata,therelativeresidualsare plottedinFigure 7b.ThedatawereobtainedbyEquation(1):

( ) ( )( ) ( )

re,i re,i re

re,i im,i im

= X – X wi / X wi and

= X – X wi / X wi

∆

∆

(1)

Here, Xre,iandXim,iaretherealandimaginarypartsoftheimpedanceatadatapoint(i),Xre(wi),Xim(wi)aretherealandimaginarypartsofthemodelingfunctionforwi.The|X(wi)|isthevectorlengthexpressedintheabsolutevalueofthemodelingfunction(Xcanalsorepresentthestatisticalbehaviorofthecorrelationforfitteddatainthecircuit).Theresidualsshowatrendthatisclosertovaluesbetween0-1% for theerror in thedatafitting, showingthatdatainthefittingcorrelationhadalowerrorpercentageinalltheappliedpotentials.

Table 1showsthevaluesoftheelementscollectedfromthefittedcircuit(resistors,capacitors,andQelement),withanincreaseofthepotentialthefittedelementschangeproportionally.Thefirstresistance(R)isthelargestresistanceduetothedistanceoftheparticlesthatneedtotraveltotheelectrodedouble layer.Thefirstcapacitor(C)representsthedoublelayercapacitance,whichisapproximatelythesameatallthepotentials,togetherwiththeresistanceinparallel(R1)thatisconnectedtothesecondsetofresistances-capacitorthatareinseries(R2C1andR3C2).InallthecasesQhasannvalueofcloselyto0.5indicatingthepresenceoftheWelement.

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a)Impedancespectrareal(*)andfitteddata()atdifferentnegativepotentialstogetherwiththesteady-statecurrent fromAuCVexperimentson1MC3H8O3+1MNaOHelectrolyte100mV/sandanAg/AgClreferenceelectrode.b)fitted(*)andreal()equivalentcircuitfrequencyerror.

Figure 7

Data reliabilityInorder to validate the circuitmodeled. The circuitfittedmaybetestedusingtheK-KtransformbythemethoddescribedbyBoukamp [29]. Figure 8 shows that the datawas transformedaccordingtoK-Ktheorythusconfirmingthatthedataisreliable.The capacitors and the resistors presented in this studied arebasedonfittingandtheiractualpresenceinthereactionarenotascertained.However,basedonthelow-frequencyspectrait ispossibletoobservethepresenceofcapacitors.Thiscouldimplythattheprocessesareslow.Also,theanalysisoftheresidualerrorsupportsthetheoryofthepresenceofthecapacitivestructuresinthereaction.Furthermore,inordertovalidatethedatafurtherSimulink-MatlabwasusedandtheSimulinkcircuitfittingisshownin Figure 9.ThefittedcircuitisconnectedinserieswithregularACcircuitelementsavailableintheSimulinkpackage,thelogicofthesystemisgivenbythescopeunit.Thesignalfromthecircuitindicates that thecircuithasa logical frequencyoutput,and itmaybeusedasanelectronicsystem.

Gold coated metals proof of concept Although the analysis with the gold electrode showed goodresultsforglyceroloxidationinalkalinemedia,currentdensitiesand potentials are still low for a realistic scenario in a fuelcell construction. In order to offer an alternative for the goldelectrode, this section presents first, a gold coating methodforglassycarbonandthevoltammetrytestwhich iscomparedwithgoldinalkalineandinacidmediatoobservethechangesincurrentdensityandpotential in1Mglycerolaqueoussolution.Second,twogoldcoatedmetalsaretested(Zn-AuandaCu-Au)inordertocomparetheirelectrochemicalbehavioragainstpuregold.ZnorCumaybesubjecttocorrosionbutnocorrosionwasobservedinthecurrentexperiments.

Gold Coating effect: Glassy carbon (GC) was used as materialto deposit gold with the method described in Experimentak Methods, the GC electrode without gold is shown in Figure 10a. A clear layer of gold was deposited (Figure 10b).Thegold deposition corresponds to the classical electrochemical-

V (V) R (Ω) C (F) R1(Ω) R2(Ω) C1 R3(Ω) Q (F) n C3 (F) X2

-0.05 3.81E+00 6.51E-08 1.66E+01 4.08E+02 3.23E-04 1.36E+03 5.65E-05 4.34E-01 2.09E-06 4.83E-04-0.15 3.68E+00 7.36E-08 1.69E+01 1.20E+02 2.22E-04 1.95E+04 1.08E-05 4.02E-01 1.40E-06 1.45E-04-0.25 3.54E+00 7.97E-08 1.61E+01 2.14E+04 1.62E-04 6.05E+04 4.74E-06 4.78E-01 1.31E-06 1.17E-04

Table 1ValuesobtainedforthecircuitfittedfortheImpedancespectraof1MC3H8O3+1MNaOH.

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K-K transform analysis for experimental data ofimpedance measurements of Au electrode in1 M C3H8O3+1 M NaOH and Ag/AgCl referenceelectrode.

Figure 8

nucleation phenomenon in which the gold ions are travelingthrough the electrolyte in a diffusion control behaviour [36].After the electrode was prepared, two cyclic voltammogramsweremeasured onewith glycerol in acidicmedia (Figure 10c)and one with glycerol in alkaline media (Figure 10d). Bothvoltammogramswerecomparedwiththegoldelectrode.

TheCVofGC-AuelectrodecomparedwiththegoldelectrodeinacidmediaisshowninFigure 10c.Whenthepotentialincreasesinthepositivedirectionofthescanafirstcurrentdensitypeakwas observed, possibly, corresponding to gold oxidation, thenthe potential changed and the gold reduction current densitypeakwasobserved.Thisbehavioristypicalforagoldelectrodein acidmedia [17]. In both theGC-Au and the gold electrode,cathodic and anodic current density peaks were observed,this isan indicationthatgoldwaspresentontheGCelectrodeshowing similar behavior than the gold electrode. However, itwasnotobservedaclearevidenceofglyceroloxidationduringtheexperiment.Also,theGC-Aushowedlowercurrentdensitiesthanthegoldelectrode,thereasonsforthisaremayberelated

to the size of the gold layer, the GC-Au stability and possiblyintrinsicresistancesbetweenthematerials[37].

Figure 10d shows theCVofgoldandGC-Auelectrodes in1Mglycerolin1MNaOH.ItwasobservedthattheGC-Auelectrodehadasimilarbehaviorthanthegoldelectrodeshowingtwocurrentdensitypeaks,(oneforthegoldreductionatapproximately0Vand one for glycerol oxidation at approximately 0.2 V vs. Ag/AgCl). The comparison showed that similar voltammogramsmay be achieved by using the gold coating method however,lowercurrentdensitieswereobservedwhenusingGCelectrodeasa substrate forgolddeposition incomparisonwith thegoldelectrode.

Inbothacidandalkalinemedia,theCVbehaviorofgoldelectrodewas observed when using GC-Au electrode. Furthermore, itis suggested that using GC as a substrate material for goldplating has an effect on the current density, potential, andthe capacitive plateau during voltammetry. This effectmay beduetotheelectrochemicalnatureofGC,whichmayaffectthenormalbehaviorofthegoldelectrodeforglyceroloxidation[18],similarobservationsweredonebyOthman[38],inwhichAu-PVCelectrodewas used for glycerol oxidation and itwas observedthat the gold coated material can mimic the electrochemicalbehaviorofgold.

The Zn-Au electrodeBased on the observations done with the GC electrode. Itis plausible that changing the substrate material for goldelectrodepositionmayaffectthecurrentdensityandpotentialinthevoltammetryofglycerolinalkalinemedia.AZn-AuelectrodewaspreparedusingthesameprocedurefortheGC-Auelectrode.TheelectrodewascheckedvisuallybeforeandafterbeenintheaqueoussolutionofAuBr2anditwasobservedachangeincoloronthesurfaceofthezincelectrode,thechangemayberelatedtozincoxidation[39]andalso,toadisplacementofzincionsforgoldions[40].Inordertoobserveifgoldispresentedonthesurfaceof the zincelectrodevoltammetrywasused. Furthermore, theeffect of cyclic voltammetry cycling is presented in Figure 11.Thirty cyclesare shownata scan rateof100mV/s, the shapeof the voltammetry is similar to the gold electrode implying,thatpossiblyagoldlayerwascreatedonthesurfaceofthezincelectrodeandthatglyceroloxidationmaybepossiblyachievedsimilarly than with gold. After each cycle in the voltammetry,the current density increased, having a similar effect than theGC-Au electrode. This observation also suggests that the Zn-Auelectrodehas a layerof golddeepenough to continue theglyceroloxidationduringeachcycle.

TounderstandmoreaboutthekineticsoftheglyceroloxidationinalkalinemediaatZn-Auelectrode,theinfluenceofscanrateis studied in Figure 12a. Similar to the GC-Au electrode, thecurrentdensitygraduallyincreasedwiththescanrates,andthecorrelation between the scan rate and the current density at-0.15V is linearatascanratecoefficientof1/5 (Ivs.v1/5)andnotat1/2(Figure 12b).Otherscanratesexponentialcoefficientsweretestedbutnotlinearitywasobserved.ThisobservationmaysuggestthattheglyceroloxidationusingtheZn-Auelectrodeisnot totally controlled by diffusion, as in the casewith theGC-

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Simulink circuit fitting

Signal from circuit

Simulink-MatlabtestwithACsignaloutputforcircuitfittedforglyceroloxidationinalkalinemediaonagoldelectrode.

Figure 9

Auelectrode.Also,withtheincreaseofscanratethepotentialis negatively shifted. The varied potentialmay represent bothan alteration in the Zn-Au surface due to the interactionwithglycerolandOH-ions[34],andtheinteractionbetweengoldandzinc that shifted the potential to negative values, this possiblyrelated to the negative electrochemical potential of zinc andpossiblesidereactionofzincinalkalinemediaformingzincoxideandzinchydroxidebothwithlargernegativepotentialthanzincasshowninEquation2.However,theactualpresenceofthesereactionswasnotcharacterizedindetailinthisstudy.

2+

2+

Reaction E°/VAu + e Au +1.8Zn + 2e Zn –0.761

−

−

( )( )

2– –4

–2

–2

8

Zn OH + 2e Zn + 4OH –1.199

Zn OH + 2e Zn + 2OH –1.249

ZnO + H O + 2e Zn +

2OH –1.260

−

−

−

(2)

The Cu-Au electrodeTheCu-AuelectrodewasstudiedusingthesameconditionsfortheGC-AuelectrodeandtheZn-Auelectrode.TheelectrodewascheckedvisuallybeforeandafterbeingintheaqueoussolutionofAuBr2thecoloronthesurfaceofthecopperelectrodechanged.The changemay be possibly related to copper oxidation [41].Also,itmayresultfromdisplacementofcopperionsforthemorenoblegold ions[40].Theeffectofcyclicvoltammetrycycling ispresentedinFigure 13.Theshapeofthevoltammetryissimilarto thegoldelectrode theGC-AuandZn-Auelectrode, showingthatagoldlayerwascreatedonthesurfaceofthecopperandthatsimilarshapecanbeobservedinthevoltammetryofglyceroloxidationascomparedwiththeAuelectrode.Aftereachcycleinthevoltammetry,thecurrentdensity increasedsimilarlytotheGC-AuandtheZn-Auelectrodes.Thisobservationalsosuggeststhat the Cu-Au electrode had enough gold to continue theconstantcycling.

TheinfluenceofscanrateisshowninFigure 14a.Itcanbeseenthatthecurrentdensitygraduallyincreasedwiththescanrates.

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a)GCelectrodewithoutAucoatingb)GCelectrodewithAucoatingandcyclicvoltammetryofgoldelectrodeandGC-Auelectrodeonc)Acidmedia(1MC3H8O3+1MH2SO4)d)Alkalinemedia(1MC3H8O3+1MNaOH).Bothat100mV/sandAg/AgClreferenceelectrode.

Figure 10

CyclicvoltammetrycycleseffectatZn-Auelectrodeon1MC3H8O3+1MNaOHelectrolyte,100mV/sandAg/AgClreferenceelectrode.

Figure 11

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a)ScanrateeffectonCyclicVoltammetryofZn-Auelectrodeb)Currentdensityat-0.15Vrelationwithscanrate(Ivsv1/5),on1MC3H8O3+1MNaOHelectrolyteandAg/AgClreferenceelectrode.

Figure 12

CyclicvoltammetrycycleseffectatCu-Auelectrodeon1MC3H8O3+1MNaOHelectrolyte,100mV/sandAg/AgClreferenceelectrode.

Figure 13

Thecorrelationbetweenthescanrateand thecurrentdensitywasmeasuredatapotentialof -0.15Vvs.Ag/AgCland itwasfoundtobelinearatascanratecoefficientof1/5(Ivs.v1/5)andnot at 1/2 (Figure 14b). These observations suggest that theglycerol oxidation at Cu-Au electrode is not totally controlled

by diffusion. Also,with the increaseof scan rate the potentialisnegativelyshiftedsimilarlytotheZn-Auelectrode.Thevariedpotentialpossibly representsbothanalteration in the copper-gold surface due to the interactionwith glycerol andOH- ionsproduceduring the voltammetry [34]. The voltammetryof the

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a)ScanrateeffectonCyclicVoltammetryofCu-Auelectrodeb)Currentdensityat-0.15Vinrelationwithscanrate(Ivs.v1/5),on1MC3H8O3+1MNaOHelectrolyteandAg-AgClreferenceelectrode.

Figure 14

Cu-Aushowsachangeinthevoltammetryshapetowardsmorenegativepotential.Thismaybeexplainedbytheelectrochemicalpotential of copper and other copper ions in alkaline solution(copperoxideandcopperhydroxide)whicharemorenegativethangoldinalkalinesolution(Equation3).

(3)

Comparison of Au, Zn-Au and Cu-Au electrodeThebehaviorofAu,Zn-Au,andCu-AuelectrodesiscomparedinFigure 15a, thecomparisonhasbeendoneusingthe idealplotforananodematerialinaglycerolfuelcell(Figure 1),usingtheregionbetween-0.8Vand0inthecyclicvoltammetryforglyceroloxidation in alkaline aqueous solution. Both Zn-Au and Cu-AuhavesimilarbehaviorthantheAuelectrodeinasolutionof1Mglyceroland1MNaOHat100mV/sandAg/AgClasareferenceelectrode.Whenanalyzingthecurrentdensitiesatapotentialof-0.15V(Figure 15b), thelargestcurrentdensityobservedisfromtheZn-Auelectrodewithavalueof7mAcm-2.Thelowestcurrentdensityobserved is fromtheAuelectrodewith0.7mAcm-2 at apotentialof-0.45V.Foralltheelectrodesstudiedthecurrentdensity decreaseswhile the potential goes to negative values.This behavior is lower in the Zn-Au electrode and the Cu-AuelectrodewhencomparewiththeAuelectrode.Theexperimentshowed that changing themetal for golddepositionmayhavean effect on currentdensity andpotential and that Zn-Au andCu-Auelectrodesarepossiblematerialstobeusedforglyceroloxidationinalkalinemedia.

Table 2 shows a list of different catalyst for glycerol oxidationcomparedwiththecatalystusedinourresearch.Theexperimentalconditions show that different electrolyte compositions are

used for the study of the glycerol oxidation. Furthermore, thecomparison of the performance of the catalyst has not beenstudied indetail toourknowledge.E.g. thePt,andPdcatalysthavedifferentalkalinemedia(NaOHorKOH),temperatureandpH.Nevertheless,intheanalysisofdifferentauthorssuggestthattheglyceroloxidationisacomplexreactionthatisinfluencedbyphysical, chemical and electrochemical conditions and findingtherightsetofcharacteristicsisstillachallengetoresearch.

Proof of concept: Chronoamperometry discharge analysis A prototype system was built to observe the dischargeperformance of the anode electrodes studied in this paper.Copperandzincfoilsof20cm2withelectrodepositgoldwereusedasanodes forglyceroloxidationusingtheprocessdescribed inExperimentalMethods.Thecathodewascarbongraphitefortheoxygenreductionreaction.Thegraphical representationof thecellisshowninFigure 16a.Thetwomaterialswereimmersedinasolutionof1Mglyceroland1MNaOH.Twodifferentpotentialsweretestedfor900secondsineachcell(-0.05Vand-0.15V),thevalueswereselectedbaseintheresultsofEISandCVanalysis,takingintoaccountthelowestresistanceobserved.Theresultsare shown in Figure 16. Overall, it is observed that when theloadpotentialisincreasedthecurrentdensitydecreases.Inthecaseofthegoldcell(Figure 16 b),at-0.05Vthecurrentdensityaveragedischargeisequalto0.07mAcm-2andat-0.15Visequalto0.01mAcm-2.InthecaseoftheZn-Auelectrode(Figure 16c),thecurrentdensitychangesfrom1.5mAcm-2 (at -0.05V)to1mAcm-2(at-0.15V)andfortheCu-Auelectrode(Figure 16d)thecurrentdensityaveragedischargeis3mAcm-2(at-0.05V)and1.5mAcm-2(at-0.15V).

The difference between the cells current densities is possiblyrelatedtoresistancethatcanbecreatedbetweentheelectrodes.

( )

2+ –

– –2 2

2

Reaction E°/VAu + e Au +1.8Cu O + H O +

2e 2Cu + 2OH – 0.360

Cu O

H + 2

– – e Cu + 2OH –0.222

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CyclicvoltammetryofAu,Zn-Au,andCu-Au in1MC3H8O3+1MNaOHelectrolyte,100mV/sandAg-AgClreferenceelectrodea) Zoom in voltammetry areabetween -0.8V to0V andb) Comparisonof negativepotentialswithcurrentdensity.

Figure 15

Experimental conditionsCatalyst Supported material Temperature Co pH Main electrolyte Observations

Pt MCN(mesoporouscarbonnitrides)9 60 Neutral 0.3MGlycerol

Thereisadecreaseofcatalyticactivityafterreusingofthecatalystalsotheexperimentswere

performedinabasefreeenvironment.

Pt-Ru MWCNT(multi-walledcarbonnanotubes)10 20-90 Alkaline 0.7MGlycerol+2M

KOH SuperiorcatalyticactivitythanPd-MWCNT

MWCNT(multi-walledcarbonnanotubes)10 20-90 Alkaline 0.7MGlycerol+2M

KOHItisobservedhighercurrentdensitiesat

temperaturesof80Co

Pd CCE(Carbonceramicelectrode)11 25 Alkaline 0.5mGlycerol+0.3

mNaOHTheconcentrationofNaOHinfluencesthecurrent

densityofglyceroloxidation

AuCarbon17 50-60 Alkaline

0.1MGlycerol+0.1NaOH+0.1MTartronicAcid

TheaddedtartronateimprovedtheglyceroloxidationduetoselectivityofAufortartronate

ionsathightemperature

MnO2-Carbon22 60 Alkaline 0.5MGlycerol+1M

KOHMnO2gavesupportforgoldnanoparticlesand

betterelectrodestability This Research

Au

AuGCZnCu

23 Alkaline 1MGlycerol+1MNaOH

HighcatalyticactivitySimilarcatalyticactivitythangoldHighercurrentdensitiesareobservedandpoten-tiallyshiftedtonegativevaluesHighercurrentdensitiesareobservedandpoten-tiallyshiftedtonegativevalues

Table 2 Comparisonofdifferentcatalystforglyceroloxidationfromliteraturewiththepresentstudy.

In thethreecells testedgoldwasobservedtohavethe lowestcurrent output at the given potentials, followed by Zn-Au andthehighestcurrentdensitywasobservedintheCu-Auelectrode.However,inthecaseofCu-Auattheendofthelastdischarge,itisobservedadoubleline,indicatingconductivityproblemswiththe cell, this couldbedue to surface changes in theelectrodeafterthedischargeandpossiblesurfaceoxidation(Cu2Oand/orCu(OH)2).Furtherexperimentationneedstobedoneinordertoknowtheexactmaximumcapacityoftheelectrodeandthegoldbonding.Figure 16eshowstheanalysisofathreeelectrodefuelcellconnectedinseries,thetestwasperformedusingdifferent

current densities in order to observe the discharge potentialresponseandthestabilityofthecathodeelectrodeforaperiodof300s.Whilethecurrentdensityincreasesthepotentialofthecellreducesconstantly.At0mAcm-2,thevoltageofthecellisstableat0.7V.However,thepotentialchangesduringthetimewhenaloadisnotapplied.Thisbehaviorsuggeststhatthestabilityofthecarbonelectrodeischangingduringthe300sexperiment.

Conclusions Ouranalysisshowsthatgoldhasthehighestcurrentdensityforglyceroloxidationinalkalinemediawhencomparedwithanother

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e

Dischargechronoamperometryat-0.05Vand-0.15VusingCarbongraphiteasacathodeelectrode,C3H8O3+1MNaOHelectrolyteanda)representationofthefuelcellb)Auelectrodec)Zn-Auelectroded)Cu-Auelectrodeande)Dischargepotentialcurveofathreeseriesconnectedcellswithcarbongraphiteascathodeelectrode.

Figure 16

catalystatapotentialof-0.2V(Pt,Ag,GCandCu).Itisfurtherobserved that gold surface can change in cyclic voltammetry.Thecurrentdensityshowslinearbehaviorwiththescanrateatanexponentialvalueof1/5andnotwiththesquareroot.Oneequivalent circuitfitted thedatawell atpotentialsof -0.05V,-0.15 V and -0.25 V vs. Ag/AgCl, with resistors and aWarburgelementinparallelwiththedoublelayercapacitance.Elementsare related to the possible presence of hydroxypyrovate andoxalateions,ourresultswereconsistentwiththelow-frequencyerrorfittinganalysis (10-4),ACSimulink-Matlabfitting respondsand theKronig-Kramers transformtest.TheZn-Auand theCu-Au electrodes showed voltammetry behavior similar to thegoldelectrode incyclicvoltammetryat thedifferentscanrate.ThedischargechronoamperometrytestshowedthattheZn-AuandCu-Auelectrodehadhighercurrentdensitiesthanthegoldelectrodeatapotentialof-0.25Vvs.Ag/AgCl(5mAcm-2,4.5mA18cm-2,and3mAcm-2respectively).Au,Zn-Au,andCu-Auarepossiblecandidates forglyceroloxidationtechnologiesandthe

goldcoatedmetalsarepossiblysuitableelectrodesforbuildingafuelcell. Inthisstudy,theairelectrodesused inthefuelcelltestwerelimitedinstabilityandinournextstudy,wewilladdressstableairelectrodesforourglycerolfuelcellapproach.

Upon success in prototyping,we aim to possibly use fuel cellsasbackupsystemsinmicrogridsorasanalternativetoreplacediesel generators in some situations. Although, more R&D isneededtosetthisinsighttoo.Inthemeanwhile,ourgrouphasstartedsimulationswiththeseproposeglycerolfuelcellinordertoanticipateonthisscenarios.

Acknowledgment ThisworkwassponsoredbytheSTW-I-Care11854projectandDr Ten BV, in The Netherlands as part of its ongoing fuel celldevelopmentresearchanddemonstrationofstoragesystemsforsmartgridintegration.AlsospecialthankstoMSc.MiheerVaidyaandtheDelftUniversityofTechnologyinTheNetherlands.

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