A core‐shell graphene nanosheets (GNS) and amorphous carbon composite (GNS@a‐C)was pre‐paredbyachlorinationmethodandusedasahighlyefficientcatalystsupportforoxygenreductionreaction.Herein,GNS as a shell,with excellent conductivity, high surface area, and corrosion re‐sistance, served as a protecting coating to alleviate the degradation of amorphous carbon core.Platinumnanoparticleswerehomogeneouslydepositedonthecarbonsupport(Pt/GNS@a‐C)andshowed a good catalytic activity and a higher electrochemical stability when compared with acommercialPt/Ccatalyst.ThemassactivityofPt/GNS@a‐Ccatalystwas0.121A/mg,whichwasalmosttwiceashighasthatofPt/C(0.064A/mg).Moreover,Pt/GNS@a‐Cretained51%ofitsinitialelectrochemicalspecificareaafter4000operatingcycleswhencomparedwithPt/C(33%).Thus,thepreparedcatalystfeaturedexcellentelectrochemicalstability,showingpromiseforapplicationinpolymerelectrolytemembranefuelcells.
Polymer electrolyte membrane fuel cells (PEMFCs) havebeen considered as an environmentally friendly solution forautomotive, backup, and residential power needs. However,PEMFCsareyettobeimplementedonacommercialscale.Oneof the most important factors preventing their commercialimplementation is the severe degradation of the traditionalPt/C catalyst employed in PEMFCs. For example, amorphouscarbon, which is a commonly used carbon support, is susc‐eptible to corrosive conditions [1] including highwater cont‐ent, low pH (< 1), relatively high temperatures (~50–90 °C),high potentials (~0.6–1.0 V), and high oxygen concentration.Thecorrosionofthecarbonsupportsubsequentlyleadstothedetachmentofnoblemetalnanoparticles (NPs) fromthe sup‐port,resultinginaggregationofnoblemetalPNs,andthenthe
catalytic activity decreases. Furthermore, oxidation of carboncanchangethesurfacehydrophobicityofthesupportthatcanhindergastransport[2].
Thefollowingtwostrategiescanbeusedtomitigatecarboncorrosion:(1)graphitizationofcarbonblackand(2)alternativeuseofamorestablecarbonsupport.Graphitizationofcarbonplaysan important role in improving theelectrochemical sta‐bility of the support [3]. Higher amounts of graphitic carbonleadtoreductionofdefectsitesonthecarbonstructurewherecarbonoxidationoccurs[4].Graphitizationcanbeachievedbyheating carbon materials in protective gas at a high temp‐erature(≥1600°C)[5,6].Graphitizationaffordsthefabricationofmaterialswithhighresistancetooxidationandcorrosionbutwith reduced numbers of surface oxygen‐containing groups.The latter will accordingly affect metal deposition on thegraphitizedcarbonsupport [7].The second strategy to allevi‐
ating carbon corrosion involves the use of carbon supportswithhigherstability.Recently,graphenenanosheets(GNS),astwo‐dimensional layers of sp2‐bonded carbon, have attractedconsiderable attention owing to their high surface area, re‐markable mechanical stiffness, excellent conductivity, andchemicalandelectrochemicalstabilities[8–10]forapplicationincatalystsupports[11,12].However,GNStendtoagglomerateor restack through van derWaals interactions [9,10], consid‐erablyloweringthesurfaceareaandlimitingpermeationoftheelectrolyte between the graphene layers, consequently de‐creasing the active surface area. These severely restrict theapplicationofGNS[13–15].ManyattemptshavebeenmadetoinhibittherestackingofGNSthroughsurfacefunctionalization[16–18], electrostatic stabilization [19], and synthesis of gra‐phenecompositesconsistingofsecondarybuildingblockssuchascarbonblack,carbonnanotube,andconductivenanoceram‐ic [13,14,20].Recently,ourgroup reporteda facilemethod topreparegraphenefroma‐Si1–xCxora‐Ti1–xCxnanofilmsusingachlorinationmethodundermildprocessingconditions[21,22].As reported, graphene can exist as graphene‐amorphous car‐bon (GNS@a‐C) core‐shell structures, which can afford in‐creased corrosion resistance for high electrochemical perfor‐mance[21].
Herein, for the first time,we report a core‐shell GNS@a‐Ccompositepreparedbyachlorinationmethodanduseditasasupport of Pt NPs for oxygen reduction reaction (ORR). Thecore‐shell structure not only inhibits the restacking of GNSmentioned earlier, but also restricts the corrosion of theamorphouscarboncoreundertheharshoperatingconditionstypically used in PEMFCs. Comparison between the currentPt/GNS@a‐Ccatalystand thecommercialPt/Ccatalyst showsthattheformerhasagoodcatalyticactivityandaremarkablyhighstability.
Silicon carbide (SiC) samples (where the shell and core isamorphousandcrystallineSiC,respectively)withNano‐shells/films bought fromKaierNanoCo. andused as receivedwereplaced inahorizontalhot‐wall tubular flowreactoroperatingatambientpressures.Then, thereactorwasheated to800 °Cunder pure He and subsequently exposed to a He/Cl2 atmo‐spherefor1h.Thereactionwasstoppedbyflushingthereac‐torwithpureHegasat800 °C for1h toremoveresidualCl2andby‐products.Thefurnacewascooledto25°CunderpureHe,andthentheGNS@a‐Cwasobtained.
MorphologiesofthesupportandcatalystwereanalyzedonaJEOL2100high‐resolutiontransmissionelectronmicroscope(HRTEM), operating at 10 kV. Raman spectroscopywas perf‐ormed on a Renishaw using Ar ion laser with an excitationwavelength of 514.5 nm. X‐ray diffraction (XRD)was perfor‐medon aRigakuX‐ray diffractometer equippedwith a CuKαradiation source.XRDpatternswere collectedusing a stepof0.01° and a count time of 2 s per step within a 2θ range of10°–90°.
Electrochemicalstudieswereconductedusingacomputer‐controlled Autolab PGSTAT 30 potentiostat (Eco Chemie B.V,Holland)withathree‐electrodecellsetup.Asaturatedcalomelelectrode was used as the reference electrode and platinumwirewasusedasthecounterelectrode.However,inthispaper,all potentials are expressedon the scaleof the reversible hy‐drogen electrode (RHE). The electrolyte solution (0.1 mol/LHClO4)waspurgedusinghigh‐purityN2for30minpriortoanyelectrochemicalmeasurements. The sample (3mg)was disp‐ersed in the stock solution (1000 L) that was prepared bymixing isopropanol (600L)withpurewater(380L)and5wt%Nafionionomersolution(20L;DuPontCo.,Ltd.).Then,theformedinkwascoatedonamirror‐polishedglassycarbondisk electrode as a working electrode. The electrochemicalerosion(ECE)ofthesupportwasassessedataconstantpoten‐tialof1.2Vasa functionoftimetovarytheECErates.Cyclicvoltammogramsintherangeof0–1.2Vwererecordedperiod‐icallybeforeandaftertheECEtestataconstantscanrateof50mV/s. An electrochemical‐accelerated durability test (ADT)wasconductedbycyclicvoltammetry(CV)analysisperformedat 0.6–1.2 V for 4000 cycles. CVs were recorded before andafterADTfrom0to1.2Vatascanrateof50mV/s.Finally,theORR activity of the catalystswas assessed in anO2‐saturated0.1mol/LHClO4solutionona rotatingdiskelectrodesystem.Polarization curves were obtained at room temperature at ascan rate of 10mV/s and a rotation rate of 1600 r/min, rec‐ordedfrom1.1to0.2V.
Asobserved inFig.1(a),afterchlorination, theamorphouscarbonNPswerecoveredwithafewlayersofGNS,resultingina core‐shell GNS@a‐C architecture. Crystal residues of β‐SiCwerenotobservedafterchlorination,indicatingthenear‐com‐plete transformation of SiC into nanoporous carbide‐ derivedcarbon (CDC) matrix and graphene shells (Fig. 1(b)). Fig. 2showstheRamanspectrumofthefinalsiliconcarbide‐derivedcarbon (SiC‐CDC).Theoccurrenceofa2Dpeakand two rela‐tivelybroadDandGpeaksat1320and1580cm−1indicatesthe
presenceoforderedgraphiticdomainsinthea‐Si1–xCxnetwork[23,24], thereby confirming the successful transformation ofa‐Si1–xCx nanoshell on the crystalline SiC NPs into graphenenanoshell. The D band was associated with the presence ofdefectsandstagingdisorderofgrapheneandamorphouscar‐bon,andtheGbandcouldbeusedtoinvestigatethedegreeofgraphitization.Asnoted in the followingdiscussion, thepres‐enceofdefectsongrapheneisconducivetotheadsorptionofPtNPs on the support. Therefore, a‐Si1–xCx nanoshellswere suc‐cessfullyconvertedintographeneonsurfacesofβ‐SiCNPsbychlorination, while the β‐SiC core was converted into amor‐phouscarbon.
ECE test was conducted to investigate the stability ofGNS@a‐C.Theresultswereconsistentwiththatfromourpre‐vious work [21]. Fig. 1(c) and (d) present the CV curves ofGNS@a‐C and commercial Vulcan XC‐72 carbon, respectively.Thepotentialwindowbetween0.3and0.8Visanindicatorofcapacitive current, which depends on the electrochemicallyaccessible area for diffusion of the electrolyte to the internalmicropores of the carbon matrix. Relative to XC‐72 carbonblack, GNS@a‐C shows a considerably higher electric double‐layer capacitance, indicative of a higher specific surface areaand greater accessibility to the electrolyte and charged ions.For both samples, the peaks between 0.6 and 0.8 Vwere at‐tributed to the oxidation–reduction of graphene that becamemoreprominentwithincreasingECEtreatmenttimesupto24h.Theincreaseinthepeakintensitycanbeascribedtothepres‐enceofsurfacedefectsthataggravatedthecorrosionofcarbon.After24h,theoxidationpeakintensityofXC‐72increasedwithtime, whereas that of GNS@a‐C decreased. This finding sug‐
gests that GNS@a‐C features higher corrosion resistance andimproved electrochemical stability owing to the presence ofgraphenelayersthatpreventscorrosionofamorphouscarbon.
PowderXRDanalysis(Fig.3)wasconductedtocomparethestructures of GNS@a‐C, XC‐72, commercial Pt/C, and Pt/GNS@a‐C.Thepeakat24.5°wasattributedtothe(002)carbonplaneofXC‐72.Incontrast,the(002)peakofGNS@a‐Cshiftedtoalowerangle(21.5°)relativetothatofXC‐72whichcouldbeattributedtothesynergeticeffectbetweengrapheneandamor‐phous carbon. Following chlorination, distinct crystalline SiCresiduesorcrystallinegraphitewasnotobserved,indicatinganear‐complete transformation of β‐SiC into carbon includingamorphous carbon and graphene. The peaks at 2θ= 30°–90°were indexed to Pt crystals with face‐centered cubic (fcc)structures.Thepeaksat39.7°,46.5°,67.7°,and81.4°wereas‐signed to the (111), (200), (220), and (311) planes of Pt, re‐spectively.
Fig. 4 shows themicrostructuresofPt/GNS@a‐CandPt/Ccatalysts.ThelatticeplanesofPt/GNS@a‐Cwithspacingof0.35and0.22nmwereattributedtographeneandPt(111),respec‐tively.Additionally, Fig. 4(a) and (c) (inset) show theparticlesize histograms of the catalyst samples as determined from
TEM images. Commercial Pt/C catalyst sample (Fig. 4(c) and(d))featuredPtNPswithanaveragesizeof3.2nm;someareasfeaturingaggregatedPtNPswereadditionallypresent,reveal‐ingtheinhomogeneousdistributionofthecatalystparticles.Incontrast, Pt NPs were well dispersed on GNS (Pt/GNS@a‐C)withanaveragediameterof2.8nmandaverynarrowparticlesizedistribution.ThesurfacedefectsonGNSservedasanchor‐ingsitesforthePtprecursortopreventaggregationofPtNPs.Smallercatalystparticlesarebelievedtodisplaybettercatalyticactivities relative to larger catalyst particles introduced at agivencontent.
Fig.5 showsCVcurvesofbothcatalysts recordedat roomtemperature. All voltammograms display a two‐peak reduc‐tion‐oxidation/adsorption‐desorptionfeature.Thefirstpeakat0.04–0.3VwasattributedtotheadsorptionanddesorptionofhydrogenonPt,andthesecondpeakat0.5–1.2VwasascribedtotheoxidationandreductionofPtmetal.Theelectrochemicalspecificarea(ECSA)ofthecatalystsampleswascalculatedbymeasuring the charge collected within the hydrogen adsorp‐tion–desorption region following double‐layer correction andassumingavalueof210μC/cm2foradsorptionontoahydro‐genmonolayer[25,26].AsdeterminedinFig.5(a),theECSAofPt/GNS@a‐C (90.1m2/g) was higher than that of Pt/C (79.2m2/g).BasedonthepolarizationcurvesforORRofthesecata‐lystsinFig.5(b),Pt/GNS@a‐Cfeaturedasimilarhalf‐wavepo‐tential(0.83V)tothatofPt/C(0.81V).Diffusion‐limitingcur‐rents were obtained in the potential region below 0.6 V,whereasamixedkinetic‐diffusioncontrolregionwasobservedbetween0.7and0.9V.ThekineticcurrentwascalculatedfromtheORRpolarization curve at 0.9V vs.RHE according to theKoutecky‐Levichequation[27].ThemassactivityofPt/GNS@a‐C (0.121 A/mg) was nearly twice as high as that of Pt/C(0.064 A/mg), demonstrating the higher ORR activity of
Pt/GNS@a‐CcomparedwiththatofPt/C.The improvedECSAandORRactivitycouldbeattributed to theoptimizeddisper‐sion and size distribution of Pt NPs, and the good electricalconductivityofGNS.
AsshowninFig.6(a)and(b),bothcatalystsdisplayedade‐crease in thehydrogen adsorption region followingADT.TheretainedECSA,asnormalizedwiththeinitialECSA,wasplottedas a function of cycle number in Fig. 6(c). After 4000 cycles,53% of the initial ECSA of Pt/GNS@a‐C was maintained,whereas only 35%of the initial ECSAwaspreserved inPt/C,therebydemonstrating that Pt deposited onGNS@a‐C is con‐siderablymorestablethanthatdepositedoncarbonunderthesametestingconditions.TheORRactivitiesofPt/GNS@a‐CandPt/CbeforeandafterADTare shown inFig.6(d).After4000cycles, Pt/C displayed 91mV negative shift of the half‐ wavepotential.Incontrast,Pt/GNS@a‐Conlydisplayeda31mVneg‐ative shift. The mass activity of Pt/C changed from 0.064 to0.010A/mg,correspondingtoadecreaseof84.4%.Incontrast,themassactivityofPt/GNS@a‐Cchangedbyonly66.7%.
Tofurthersubstantiatethedifferenceindegradationbetw‐eenPt/GNS@a‐CandPt/C,bothcatalystswereinvestigatedbyHRTEMafterADT.Fig.7showstheTEMimagesofthecatalystsandtheassociatedPtsizehistograms.Slightagglomerationofthe Pt NPs was observed for Pt/GNS@a‐C that displayed anincreased average particle size from 2.8 to 4.8 nm followingADT(Fig.7(a)and(c)).Bycontrast,moresevereagglomerationofthePtNPswasobservedforPt/CafterADT(Fig.7(b)),andthemean size increased to 5.2 nm after the potential cyclingtest (Fig. 7(d)). The high stability of Pt/GNS@a‐C relative tothatofPt/Cdemonstratesthatthegraphenelayeriseffectivein
inhibiting the migration and aggregation of Pt NPs, and in‐creasingtheresistanceofthesupporttoelectrochemicalcorro‐sion.However,forPt/C,themigrationandagglomerationofPtNPson thesurfaceofcarbonblackarepredominantlycausedbythecorrosionofcarbonsupportsandtheOstwaldripeningofPtNPsthatlowerstheelectrochemicalactivityofthecatalyst[27].
4. Conclusions
Core‐shell GNS@a‐C composites were successfully synthe‐sizedbyachlorinationmethodandusedasthesupportforPtnanoparticles.ThePt/GNS@a‐CcatalystdisplayedsignificantlyenhancedactivityandstabilityrelativetothecommercialPt/Ccatalyst. The improved activity was attributed to the goodconductivity of graphene that was also effective in inhibitingmigrationandaggregationofPtnanoparticlesbycoveringthe
amorphouscarboncore,aswellasprotectingthecarbonfromchemical and electrochemical corrosion. The present findingsdemonstrated the GNS@a‐C composite as a highly efficientcatalyst support possesses great potential application in fuelcellsandotherindustrialfields.
