铂_石墨烯氧还原电催化剂的共还原法制备及表征

[Article]物理化学学报(Wuli Huaxue Xuebao )Acta Phys.⁃Chim.Sin .2012,28(12),2879-2884December Received:July 5,2012;Revised:September 13,2012;Published on Web:September 25,2012.∗Corresponding author.Email:zfma@;Tel:+86-21-54742894.The project was supported by the National Natural Science Foundation of China (21073120,21176155)and Science and Technology Foundation of Shanghai Municipality,China (10JC1406900).国家自然科学基金(21073120,21176155)及上海市自然科学基金(10JC1406900)资助项目ⒸEditorial office of Acta Physico ⁃Chimica Sinicadoi:10.3866/PKU.WHXB 201209252铂/石墨烯氧还原电催化剂的共还原法制备及表征王万丽马紫峰*(上海交通大学化学工程系,上海200240)摘要:使用硼氢化钠共还原法制备40%(w )铂/石墨烯电催化剂用于氧还原反应.通过循环伏安测试发现,这种方法制备所得铂/石墨烯催化剂对氧还原反应活性较铂/碳催化剂差,但稳定性有所提高.在稳定性测试中,铂/石墨烯电催化性能衰减为50%,较铂/碳(79%)好.X 射线衍射(XRD)和透射电子显微镜(TEM)表征发现在铂/石墨烯催化剂中两者存在明显交互作用,这可能是阻止石墨烯再堆垛和防止铂颗粒团聚的主要原因.通过对单电池性能测试也发现铂/石墨烯催化剂更有利于电池长期稳定.关键词:石墨烯;共还原法;电催化剂;氧还原反应;质子交换膜燃料电池中图分类号:O646Synthesis and Characteristics of Pt/graphene by Co-ReductionMethod for Oxygen Reduction ReactionsWANG Wan-LiMA Zi-Feng *(Department of Chemical Engineering,Shanghai Jiao Tong University,Shanghai 200240,P .R.China )Abstract:40%(w )Pt/graphene composites were prepared by sodium borohydride chemical co-reduction,and were subsequently used as an electrocatalyst for oxygen reduction reactions.The electrocatalytic activity and stability was evaluated by cyclic voltammetry.The results indicated that the initial activity of Pt/graphene was lower than that of Pt/C due to the oxygen diffusion inhibition;however,the Pt/graphene showed superior durability characteristics.Degradation tests showed a 50%degradation of Pt/graphene,which was substantially less than that of Pt/C (79%).X-ray diffraction and transmission electron microscope results showed that the composite formed strong interactions between the platinum nanoparticles and the graphene supports.The graphene supports may also prevent the graphene sheets from folding or re-stacking,which would hinder platinum nanoparticles ʹaggregation.The performance of a single cell was also tested,confirming an improvement in durability.Key Words:Graphene;Co-reduction method;Electrocatalyst;Oxygen reduction reaction;Proton exchange membrane fuel cell1IntroductionGaphene has attracted great attention from researchers in both theoretical and applied chemistry in recent years.Its use has also been studied in capacitors,1,2lithium batteries,3-6and fuel cells 7-9because of its interesting properties,such as ultra-high surface area (there is a theoretical surface area of 2620m 2·g -1for an isolated graphene sheet),special quantum proper-ties 10-13and so on.Proton exchange membrane (PEM)fuel cells have been de-veloped as a promising energy technology because of their in-herent advantages,such as simplicity,viability,and quick start-up,which give them of great potential in almost any con-2879Acta Phys.⁃Chim.Sin.2012V ol.28ceivable application.14However,several problems still hinder its commercialization.One major problem is the corrosion of carbon support which results in the catalyst being degraded quickly.Much research has been done,however,to improve the durability of carbon support.It was found that carbon mate-rials with nanostructure can improve catalytic properties signif-icantly because of their special electronic properties.15Gra-phene has also been studied as one of the candidates with the most likely potentiality for its ultrahigh surface area and rela-tively high conductivity.16The Pt/graphene nanocomposite has been identified as a ma-terial possibly able to play an important part in the develop-ment of low temperature fuel cells.9,17The nano composite has a large surface area,a high electrochemical surface area (ECSA),and well dispersity of platinum particles.However, its performance in a single cell,particularly the durability of the catalyst,still needs to be further researched.In this study,Pt/graphene composite was co-reduced,charac-terized,and compared with Pt/C for oxygen reduction reaction. The electrochemical properties of Pt/graphene were discussed from several aspects,especially its durability.2Experimental2.1Material synthesisAll the chemicals used were analytical reagents,purchased from Sigma-Aldrich(USA).Prior to Pt/gaphene preparation,graphite oxide(GO)was synthesized by the modified Hummers method.18,19In brief,the graphite powder(1g)was stirred in concentrated sulfuric acid with potassium permanganate added gradually in a water bath till completely oxidized.The reaction was terminated by the ad-dition of a large amount of distilled water and30%(w)H2O2 solution,and then the mixture was centrifuged and washed sev-eral times till a natural pH value.The dried film was stored and dispersed in solvents as needed.The Pt/graphene was fabricated by a co-reduction process. 10mg GO was exfoliated into10mL water by ultrasonication to form GO dispersion.Adding H2PtCl6·6H2O with stirring and adjusting its pH value to13by1mol·L-1potassium hydrox-ide,the dispersion was then reduced by0.1mol·L-1sodium bo-rohydride added dropwise at room temperature for4h.Pt/gra-phene powder was gathered by filtration through a mixed cellu-lose ester membrane filter(0.45μm pore size),and then it was dried in air.After grinding and sifting through a200-mesh sieve,the powder was roasted at200°C for1.5h.For comparison,40%(w)Pt/C was fabricated using the same procedure using Vulcan XC-72R(Cabot Corporation, USA)instead of graphene.Pure graphene powder was pre-pared in the same procedure without the platinum precursor. 2.2CharacterizationThe content of platinum in samples was analyzed by induc-tively coupled plasma mass spectrometer(ICP-MS,Agilent Technologies,USA).The powder X-ray diffraction(XRD)measurements of the samples were recorded on an X-ray pow-der diffractometer(D/max-2200/PC,Rigaku Corporation,Ja-pan)using Cu Kαradiation(λ=0.15406nm)with scattering an-gles(2θ)of20°-80°.Scanning electron microscope(SEM, S-4800,Hitachi Corporation,Japan)and energy-dispersive X-ray(EDX)spectroscopy(Inca Oxford,U.K.)attached to the SEM were used to confirm the deposition of Pt on the gra-phene.The catalyst features were characterized by a transmis-sion electron microscope(TEM,JEM-2010,Jeol,Japan).2.3Electrochemical measurementsThe electrochemical measurements were conducted in a three-electrode cell recorded by a potentiostat/galvanostat mod-el273(EG&G Princeton Applied Research,USA).Platinum mesh and saturated calomel electrode(SCE)were used as the counter and reference electrodes,respectively.0.5mol·L-1 H2SO4were employed as the electrolyte.The sample inks were prepared by mixing10mg sample powder with0.45mL deion-ized water,0.03mL isopropanol,and0.02mL Nafion®solu-tion(5%(w),Dupont Company,USA),followed by sonication in a water bath for10min.20μL ink was dispensed and dried in air on the glassy carbon electrode(φ=0.5cm).The electrode was immersed in de-aerated electrolyte and pretreated in cy-cles between-0.24and1V at a scan rate of100mV·s-1for 100cycles.The anodic corrosion was measured by anodic lin-ear sweep voltammetry from-0.05V(vs open circuit voltage) to1.76V at a scan rate of5mV·s-1.The cyclic voltammetry (CV)performance of supported platinum catalyst was mea-sured in a standard way between-0.24and0.96V at a scan rate of5mV·s-1.The electrochemical surface area was calcu-lated from the hydrogen adsorption-desorption peak of the CV profiles.20The accelerated durability was measured by cyclic voltammetry in the range of-0.24to0.96V for1000cycles at a scan rate of50mV·s-1.Oxygen reduction reaction(ORR) performance was evaluated by rotating disc electrode(RDE) technique.The polarization curves for oxygen reduction reac-tion were measured in an oxygen saturated electrolyte by scan-ning the potential from0.9to0.1V at a scan rate of5mV·s-1. All potentials were reported with respect to the normal hydro-gen electrode(NHE)scale.The gas diffusion layer for the cathode and the gas diffusion electrode(1.0mg·cm-2)for the anode were purchased from ElectroChem,Inc.The platinum loading at the anode side is relatively high,so the overpotential due to the anode half reac-tion can be neglected allowing the focus to be solely on the cathode side.21In this experiment,catalyst suspension was mixed by a supported platinum catalyst,Nafion(5%(w),Du-pont Company,USA),and isopropanol.The mass ratio of cata-lyst to Nafion was3:4.After being thoroughly dispersed,the suspension was brushed on the gas diffusion layer at the plati-num loading until0.4mg·cm-2using as the cathode electrode. Nafion117membrane(Dupont Company,USA)was pretreat-ed by boiling in5%(w)hydrogen peroxide for1h,followed by treating in1mol·L-1sulfuric acid.After each step the Na-2880WANG Wan-Li et al .:Synthesis and Characteristics of Pt/graphene by Co-Reduction Method for Oxygen Reductions No.12fion membrane was boiled in deionized water for 30min.The prepared anode and cathode were positioned on the both sides of the membrane and hot pressed at 2tons and 110°C for 4min to form the membrane electrode assembly (MEA).The PEM fuel cell was tested in a fuel cell testing unit with an active electrode area of 1.5cm×1.5cm.The cell was operat-ed at atmospheric pressure.The oxygen and hydrogen flow rate was 200mL ·min -1.The temperature of the cell was 50°C.The current -voltage curves were collected after an activation process at a constant operation voltage of 0.6V .3Results and discussion3.1Material propertiesThe amounts of platinum loaded on Pt/C and Pt/graphene were determined by ICP-MS showing that the contents of plati-num were 39.62%(w )and 40.73%(w ),respectively.XRD pat-terns of Pt/graphene and graphene are shown in Fig.1,which contain typical platinum diffraction peaks and a broadening C (002)peak.The peaks at the 2θof 39.86°,46.25°,and 67.69°are assigned to (111),(200),and (220)facets of the face-cen-tered cubic structure of platinum respectively,which are in agreement with the standard card of platinum (JCPDS No.4-802).The crystallite size is calculated from Scherrer equa-tion as following:D =K λβcos θwhere,D is the crystallite size;K is the Scherrer constant (K =0.89);βis the peak width at half height;θis the angle of dif-fraction;λis the wavelength of X-ray (λ=0.154056).The crys-tallite size of platinum is calculated as 7.7nm,bigger than that of Pt/C (6.8nm).The calculated lattice constants of platinum supported by graphene and carbon are a =b =c =0.3912nm and a =b =c =0.3938nm,respectively.The change between these val-ues shows clearly that a stronger interaction exists when plati-num particles are deposited onto graphene.22On the other hand,carbon diffraction peaks of Pt/graphene composite are broad-ened and weakened compared to that of pure graphene,which suggests that there might be a certain interaction between plati-num particles and graphene support.It may be caused by plati-num particles embedding into the spaces between graphene lay-ers and leading to graphene lattices growing disorderly.8Theseresults also confirm that the platinum precursor and graphite oxide are reduced to the composite of Pt/graphene.The features of pure graphene and Pt/graphene composite are shown in Fig.2.The monolayer graphene sheets are found in TEM images,supporting the platinum particles (black dots).The outstretched wrinkles of graphene sheets are clearly ob-served,which are obviously different from those of Vulcan XC-72.This might be attributed to the dispersion of platinum particles which also hinders the migration of the platinum parti-cles.17The particle size of platinum observed is about 8nm which agrees well with the results from XRD.The multi-crys-tal diffraction rings of platinum can be assigned to the (111),(200),and (220)facets of platinum in the selected area electron diffraction (SAED)pattern.However,Pt/graphene has a partic-ular orientation which can be confirmed by the bright spots on the diffraction rings.This might be caused by the surface groups on the dispersed graphite oxide (epoxy and hydroxyl groups),acting as anchoring sites for platinum particles to de-posit on.The platinum particles could only have adhered to the top or the bottom of the graphene sheets.The graphene sheets are reduced with wrinkles emerging in succession,leading to parts of the platinum particles changing their facet directions on the graphene sheets.23,24This reducing process might have contributed to the morphology of the platinum particles ʹdeposi-tion.The deposition of platinum particles is also confirmed by SEM images (Fig.3).From the top view of pure graphene (Fig.3(a)),the irregular wrinkles (bright lines)could be ob-served clearly on the surface,meanwhile,no crack could be found,which proving that the graphene is compact and flat.This suggests that it is both impermeable to gas and water resis-tant.The lamellar structure can be found even it is grinded to a powder.Fig.3(b)indicates that the platinum particles (bright dots)are deposited on the graphene sheets dispersedly.The gra-phene wrinkles are still formed but less than that of pure gra-phene film,which might be caused by preventing restacking of the graphene sheets from van der Waals forces when platinum particles deposited onto.The cross section of the film is a lay-er-by-layer structure as shown in the insert in Fig.3(a).The lay-ers are stacked together closely;they are separated however from each other with only few layers stacking while platinum particles are deposited on the sheets as shown in the insert in Fig.3(b).The layer-by-layer structure is still maintainedwhileFig.1XRD patterns of Pt/C,Pt/graphene,andgrapheneFig.2TEM images of (a)graphene and (b)Pt/grapheneInsert is the SAED pattern of Pt/graphene.2881Acta Phys.⁃Chim.Sin .2012V ol.28the spaces between sheets are increased,suggesting the parti-cles are acting as holders between layers hence resulting in a less layer-stacking film with a larger surface area,which is to its application ʹs advantage.3.2Electrochemical properties and durabilityThe electrochemical properties were characterized by cyclic voltammetry in 0.5mol ·L -1sulphuric acid system.Prepared Pt/graphene and Pt/C catalysts exhibit the typical Pt peaks of hy-drogen under-potential deposition and oxidation of hydrogen around 0-0.3V in Fig.4(a,b).These peaks are similar to the published work 25which indicates that the Pt-oxides formation and reduction appear at the same potentials for Pt/graphene and Pt/C.The peaks also indicate that the platinum particles are active when supported by graphene,agreeing well with the refer-ence.26The electrochemical surface area (ECSA)is calculated according to the equation:20ECSA =Q H[Pt]⋅210where,Q H is the peak area of hydrogen adsorption-desorption,and [Pt]is the platinum loading.ECSA is calculated as 30.2and 28.0m 2·g -1for Pt/graphene and Pt/C,respectively.It indi-cates that the numbers of platinum active sites of Pt/graphene and Pt/C are in the same order of magnitude.The ECSA value of Pt/C agrees well with the published data.27It suggests that the number of active sites is not affected when the feature of catalyst support changes into layer structure.The oxygen reduction reaction behavior was investigated by RDE method.Fig.5(a)shows the result of ORR polarization ofPt/graphene.The variation of the ORR current density (i )changes significantly with the RDE rotating speed (ω)in the diffusion region.The relationship between i -1vs ω-1/2can be ex-pressed by the Koutecky-Levich equation:281i =1i k +1i di d =K ω1/2=0.62nFD 23O 2ν-16C O 2ω1/2where,i is the total ORR current density,i k is the kinetic cur-rent density (in the activation region),i d is the diffusion limited current density (in the high ORR reduction potential)where the current is a plateau and its value changes with the RDE ro-tating speed (ω)(Fig.5(b)).K is the Levich ʹs slope which con-tains the following parameters:n the number of electron trans-ferred in ORR,F the Faraday constant,C O 2the bulk concentra-tion of oxygen (1.03×10-3mol ·L -1),D O2the diffusion coeffi-cient of oxygen in the bulk solution (2.1×10-5cm 2·s -1),and v the kinematic viscosity of the solution (1.07×10-2cm 2·s -1).29Fig.3SEM images of (a)graphene and (b,c)Pt/grapheneInserts are the cross sections of (a)graphene and (b)Pt/graphene.Fig.4Cyclic voltammetry curves of (a)Pt/graphene and (b)Pt/C Fig.5(a)ORR polarization curves and (b)Koutecky-Levichplots of Pt/graphene2882WANG Wan-Li et al.:Synthesis and Characteristics of Pt/graphene by Co-Reduction Method for Oxygen Reductions No.12The calculated n values of the Pt/graphene and Pt/C are3.86and3.90,respectively.This result indicates that the ORR onthe Pt/graphene electro-catalyst proceeds via a four-electrontransfer process.The kinetic current density i k is obtained byextrapolation of the Koutecky-Levich plots toω-1/2to zero.Thevalues are listed in Table1.It is found that i k value of Pt/gra-phene is lower than that of Pt/C.This is an indication that theORR kinetic rate on Pt/graphene might be lower than that of Pt/C,even the reaction pathway is the same on both electrodes.The low kinetic rate on Pt/graphene might be due to the inhibi-tion of the oxygen diffusion on its surface.Grapheneʹs layerstructure that platinum particles embedded into it might im-pede the access of the platinum active sites to oxygen.Howev-er,the carbonʹs spherical structure encourages the oxygen diffu-sion.Tafel slopes(b)and current density in activation regionof Pt/graphene and Pt/C are listed in Table1.The larger Tafelslope and the lower current density of Pt/graphene than thoseof Pt/C also prove the less activity of Pt/graphene for ORR,al-though Pt/graphene has almost the same platinum active sitesof hydrogen adsorption-desorption.The durabilities of both Pt/graphene and Pt/C were investi-gated under cyclic voltammetry for1000cycles in0.5mol·L-1sulphuric acid medium.The electrochemical surface area wascalculated every hundred cycles.Fig.6shows the normalizedECSA of Pt/graphene and Pt/C variation as a function of cy-cling number.Obvious degradations in ECSA values are foundas30.20m2·g-1before cycles and15.05m2·g-1after cycles of Pt/graphene while28.00m2·g-1before cycles and5.88m2·g-1 after cycles of Pt/C.The degradation of homemade Pt/gra-phene was slightly less than that in reference17where the de-crease of activity for ORR was about50%.17Meanwhile,thedecrease of Pt/C was79%,which was much bigger than that ofPt/graphene.Therefore,platinum on graphene was much morestable than that on carbon under the test condition.The degra-dation is mainly due to carbon corrosion.As to Pt/C,the car-bon support is spherical which encourages the aggregation of platinum particles,leading to a decrease in the platinum sur-face area.However,the better durability of Pt/graphene could be attributed to the grapheneʹs interesting electronic properties that there is an interaction between Pt particles and the gra-phene surface,this hindering the metallic phase coalescence.30 Moreover,it is more difficult for platinum particles to aggre-gate because the available particles are the surrounding ones in two dimensions,while the particles up or below are separated by graphene layers.The performance and durability of Pt/graphene in PEM fuel cell were also investigated.Fig.7(a)shows the performance in a single cell that the open circuit voltage based on Pt/graphene of0.975V and the maximum power density of158.65mW·cm-2at0.317V are observed.The initial performance based on Pt/graphene is poorer than that of Pt/C,which might be due to the inhibition of oxygen diffusion on its catalyst surface as de-scribed above.However,a higher stability is observed in Fig.7 (b).The decrease of voltage at400mA·cm-2as a function of time based on Pt/graphene is less than that of Pt/C,which might be caused by the greater durability of the Pt/grapheneTable1Electron transfer number,dynamic current density,exchange current density,Tafel slope,and current density ofPt/graphene and Pt/C at850mVPt/graphenePt/Cn3.863.90i k/(mA·cm-2)8.76611.387i0/(mA·cm-2)4.15×10-37.89×10-3b/(mV·dec-1)152123i850mV/(mA·cm-2)1.401.83Fig.6Normalized electrochemical surface area degradation ofPt/graphene and Pt/C Fig.7(a)Single cell performance and(b)stability ofPt/graphene and Pt/C2883Acta Phys.⁃Chim.Sin.2012V ol.28catalyst.This result indicates that graphene might be more suit-able for PEM fuel cell application than carbon.4ConclusionsThe40%(w)Pt/graphene composite prepared by sodium bo-rohydride chemical co-reduction was introduced as the electro-catalyst for oxygen reduction reaction.The electro catalytic ac-tivity and stability were evaluated.The results show that the layer structure of graphene maintains after reduction and the platinum particles with a particle size of8nm are dispersed on-to graphene.The activity for ORR based on Pt/graphene is low-er than that of Pt/C;however,better stability is observed in degradation test that the decrease of Pt/graphene is50%,which is less than that of Pt/C(79%).On both electrodes,the ORR proceeds via a four-electron process.The performance of a sin-gle cell is also tested.The improvement in durability is con-firmed by the delayed degradation of cell performance based on Pt/graphene.It might therefore be assumed that the gra-pheneʹs layer structure hinders the aggregation of platinum par-ticles.Meanwhile,the platinum particles act as holders,which against the folding of graphene sheets.This result might be use-ful for the design of the catalyst with carbon support to im-prove the long-term performance in 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