Novel methanol-tolerant Ir-S/C chalcogenide electrocatalysts for oxygen reduction in DMFC fuel cell

Particuology 9 (2011)155-160Contents lists available at ScienceDirectPARTICLOLOGYParticuologyEL SEVIERjournal homepage: www.elsevier.com/locate/particNovel methanol-tolerant Ir-S/C chalcogenide electrocatalysts for oxygenreduction in DMFC fuel cellJingyu Ma, Desheng Ai*, Xiaofeng Xie, Jianwei GuoInstitute ofNuclear & New Energy Technology. Tsinghua University, Beljing 00084, ChinaARTICLE INFOABSTRACTArticle history:Novel methanol-tolerant oxygen-reduction catalysts, iridium- sulphur (Ir-S) chalcogenides with difer-Received 2 December 2009ent Ir/S atomic ratios, were synthesized via a precipitation method using H2IrClg and Na25Oz as the Ir andReceived in revised form 23 April 2010S precursors. Powder X-ray difraction (XRD) and transmission electron microscopy (TEM) were used toAccepted 21 May 2010characterize the IrS1-x/C chalcogenide catalysts. Particle size ranging from 2.58nm though obvi-ous agglomeration was found on carbon support. However, these chalcogenide catalysts showed strongKeywords:catalytic activity towards the oxygen reduction reaction (ORR) and high methanol tolerance, stronglyDMFCsuggesting these novel catalysts as promising candidates for direct methanol fuel cell (DMFC) cathodelridiumSulphurapplications.Cathodic catalyst◎2011 Chinese Society of Particuology and Institute ofProcess Engineering. Chinese Academy ofOxygen reductionSciences. Published by Elsevier B.V. AIll rights reserved.Methanol tolerant1. IntroductionSavadogo, Lee, Mitsushima, Kamiya, & Ota, 2004; Raghuveer,Ferreira, & Manthiram, 2006), transition metal macrocyclic com-Direct methanol fuel cell (DMFC) is an ideal power source used pounds (Bashyam & Zelenay, 2006; Bogdanoff et al.. 2004; Bron,for portable power, particularly vehicular transport, due tothe high Fiechter, Hilgendoff, & Bogdanoff, 2002) and transition metalheating value of methanol and the associated benefts ofconvenient oxides (Kim, Ishihara, Mitsushima, Kamiya, & Ota, 2007; Liu,fuel management and low system complexity. Yet commercial suc-Ishihara, Mitsushima. Kamiya, & Ota, 2007) have been proposed ascess of DMFC has not been realized, mainly because of low fuelpotential catalysts for ORR, but their tolerance to methanol is stilleficiency and low power density. A major obstacle is ascribed to unclear. Some inspiring results of ruthenium (Ru)-based chalco-methanol (MeOH)crossover: MeOH migrates from the anode to the genides showed high ORR activity and high MeOH tolerance incathode by crossing the membrane. It directly reacts withO2 onthe acidic media, and a series of chalcogenide catalysts (RuxXy, X=S,cathode catalyst sites, thus leading to the cathode potential drop Se and Te) were synthesized and tested for the electroreduction ofand fuel eficiency decrease. Usually, under current membrane con-O2 (Cao, Wieckowski, Iukai, & Alonso-Vante. 2006; Lewera et al,ditions, platinum (Pt) is used as a catalyst. However, it only exhibits 2007; Liu,Zhang, & Hu, 2007; Schulenburg et al, 2006). It was foundlow oxygen reduction reaction (ORR) if methanol is present. Thus that addition of Se and S protected Ru from oxidation, as had beenit is of great interest to explore non-platinum catalysts with highsupported by earlier research (Alonso-Vante, Malakhov, Nikitenko,methanol tolerance for DMFC cathode.Savinova, & Kochubey, 2002; Bashyam & Zelenay, 2006; Schmidt,In recent years. researchers have paid great attention to findingPaulus, Gasteiger, Alonso-Vante, & Behm, 2000; Zaikovskii et al,novel electrocatalysts with high ORR activity and high methanol2006). It was found further (such as Alonso-Vante et al, 2002;tolerance. Non-platinum catalysts such as Ru-based chalcogenide Bashyam & Zelenay, 2006; Schmidt et al, 2000; Zaikovski et al,(Alonso-Vante & Tributsch, 1986; Tributsch et al, 2001). Pd- 2006) that Ru could be protected from oxidation by adding Se andbased alloys (Norskov, Rossmeisl, Logadottir, & Lindqvist, 2004;S in the chemical environment.Among the transition metals including Pt, iridium (Ir) is one ofthe most stable in acidic media (Rand & Woods, 1974). AlthoughIr has a much lower activity towards ORR than Pt, it is interest-Abbreviations: CV, cyclic voltammetry: DMFC, direct methanol fuel cell; GCE,ing to |H oxidation is also lowerglassycarbon electrode: RHE. reversible hydrogen electrode; ORR oxygenreductionreaction; RDE. rotating disk electrode.(Aramd中国煤化工1987: Bgoar2. vs●Corresponding author. Tel: +86 10 62771089; fax: +86 10 62771089.KhazovCnanamuthu & Ptrocelli,YHCNMHGinE-mail ddress: ai desh@tsinghuaedu.cn(D.Ai).d Lee, Zhang. and Zhang1674-2001/$ - see front matter。2011 Chinese Society of Particuology and Institute of Process Engineering. Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.dol:10.1016/j-partic.2010.05.0151561 Ma et al./ Prticuology 9(2011) 155-160(2007) sugsted that I-Se chalcogenide showed enhanced ORR of the surry was dispersed on a holey. amorphous carbon fim onactivity and high MeOH tolerance. It was also reported that the a Cu grid for analysis.novel IrxC01-x/C alloy electrocatalysts had strong ORR activity andhigh methanol tolerance. These results inspired us to explore Ir-2.3. Electrode preparation and electrochemical measurementsbased catalysts further.In this work, a class of carbon-supported IrS1_ x chalcogenideElectrochemical measurements were taken using a rotatingcatalysts was synthesized via the precipitation method usingglassy carbon disk-platinum ring electrode. The catalyst ink, pre-H2IrCl6 and NazSOz as the Ir and S precursors. The catalytic activi-pared according to Schmidt's report (Paulus, Schmidt, Gasteiger,ties towards ORR and MeOH tolerance were measured in an acidic& Behm, 2001). was applied manually to the glassy carbon disksolutionin the presence of methanol.Cyclic voltammetry and rotat-surface. For comparison, Pt/C (20%, Johnson Matthey) catalyst wasing disk electrode methods were used for the characterization ofprepared and coated on the electrode surface in the same way.the catalyst activity and the catalyzed ORR kinetics. These resultsA saturated Hg/Hg2Cl2 reference electrode was used, though allshowed that the novel Ir-S chalcogenide catalysts could be promis-potentials are reported with respect to the reversible hydrogening catalysts for DMFC cathodes.electrode (RHE). Pt wire was used as counter electrode. Aqueoussolutions of 0.5 M H2SO4' both with and without 0.5 M CH3OH2. Experimental(MeOH), were used as electrolytes. The electrode was dried in air toobtain a thin active catalyst layer. The Ir or Pt loading on the glassy2.1. Chemical synthesiscarbon electrode (GCE) was maintained at 0.200 mg/cm2. CyclicContrary to conventional impregnation method, IrxS1. x/C cata-voltammogram (CV) data were recorded in the potential rangelysts were prepared via precipitation. The value ofx (-0.5- 1.0) inof0V to 1.0V versus RHE at a scan rate of 20 mV/s. The rotatingIrxS1_ x/C represents the chemical stoichiometry of Ir in the com-disk electrode (RDE) curves were obtained in the potential rangepound. Vulcan XC-72R carbon black (Carbot Corp.. SBET = 250m2/g)of 1.3V to Ov versus RHE with an applied scan rate of 5 mV/s. Allwas used as the support. Typical synthesis for 20% Iro.7So3/(Cmeasurements were carried out under ambient conditions.(atomic ratio of Ir:S=7:3, and total loading of Ir and S- 20%)catalyst was as fllws. First, a mixture of carbon black (80mg,3. Results and discussionpreviously treated under 600°C in nitrogen for 3h) and deionizedwater(30 mL)were ultrasonicated for 30min.3.11 mLof0.03123M 3.1. Physical characterizationchloroiridic acid (H2IrCl6) (AR, General Research Institute for Non-ferrous Metals) and 1.37 ml of 0.03043 M sodium sulite (Na2S03)Fig. 1 shows the XRD pattern of 20% IrxS1. x/C (with diferent(AR, Beijing Chemical Works)were added into the suspension. 1 MSe/Ir atomic ratios) catalysts as compared to that of 20% Ir/C cat-NaOH (AR, Beijing Chemical Works) solution was added into thisalyst. The first peaks located at about 25%, observed in all of themixture to adjust the pH> 12. And this mixture containing metalXRD patterns, can be assigned to the carbon support particles.slats was stirred constantly at 50°C for 10h. After that, 50mLof1M The IrS1. -x/C samples presented the characteristic peaks of face-sodium borohydride (NaBH)(AR, Beiing Chemical Reagent Corp,)centered-cubic (c) crystalline iridium. The characteristic peakswas added to reduce the metal ions completely, and the mixture at about 41°, 47*。, 69* and 83 correspond to the Ir(11 1). (200),was strred for another 12h. The resulting powders were filtered,(220). and(311)planes, respectively. No iridium oxide difactionwashed with deionized water, and dried overnight at 80°C Simpeaks were shown present in Fig. 1, indicating the complete reduc-larly, lr/C and lIrxS1 -x/C with different S/Ir atomic ratios were alsotion of H2IrCl6. The mean particle size of the Ir particles in lr/C andprepared by the same procedure. Table 1 gives the sample descrip-IrxS1-x/C samples were calculated using the following Scherrer'stions.equation (Radmilovic, Gasteiger, & Ross, 1995) with a full width at2.2. Physical characterization of the Ir -S/C chalcogenide catalyst1r(111)X-ray difaction (XRD) measurements were conducted onBruker D8 Advance X-ray dfactormeter with Cu Ka radiation(Aka1 = 1.5418A) between 20 and 90° at a step of 0.02*. And thePowder Diffraction File Databases from the International Centerfor Difaction Data was used as a reference to interpret the peakassignments of the XRD spectra.1r(200)Transmission electron microscope (TEM) images were recordedon a JEOL JEM-2011 electron microscope operated at 120kV. TheIr(220)Ir(311)C(002)IrS1-x/C sample was placed in a日vial containing ethanol, and thenwas ultrasonically agitated to form a homogeneous slurry. A drop6)5)Table 1Sample description.3)2)No.DescriptionS:Ir (atomic ratio)Abbreviation中国煤化工20wt%Ir19.6wt% Ir+0.4wt%S:9IoasSa/IC20MHCNMHG 7090:8IraxSo2/C18.6wts Ir+ 1.4WtxS3:7Ira7So3/C《llua (ueyree)18.0wt ir+2.0wtxs:6IrasSo4/CFig 1. XRD pttens for Ir/C and ItS, ./C calyst: (1) tr/C. (2) losc, (3)17.1 wt% lr+2.9wt%S:5IrasSas/CIrosSo2/C, (4) Iraz5a3/C, (5)rosSo4/C and (6) lrasSos/C.1 Mo el(urticuoloxv9(211) 155-16057Table2Structural parmeters of Ir(C and I51-C alytsCatalystIr(220) peak position(')Latie parameter Ge (A)I-lr dstanced(A)Particle size by XRD(nm)rIC69.1673.85342.72485.855428IrasSo1/C69.1962704320IresSo2/C692841 82552705025Iro7Sos/C69 4013 82292.703224IosSic69.5123.81492.697527IrosSos/Cthe nano-clusters, nanoparticles of 2-4 nm can be clearly distin-guished. Though uneven distribution of particles on carbon supportwas determined, the collective behavior of these particles in XRDcharacterization can be used to evaluate their effective catalyticbehavior. While it is evident that the synthesis procedure needsfurther improvement, it is yet beyond the scope of this paper.3.2. Cycic voltammetric charaterizationFig, 3(a) shows the complete cyclic voltammograms (CVs) forIr/C and lrxS1_ x/C catalysts in N-saturated 0.5M H2SO4 solutionat 25 C. The hydrogen adsorption/desorption (Hayd) peaks can beobserved in the potential range between 0.01V and 0.2V versusRHE. However, the Iro.zS0.3/C catalyst showed a decreasing Ha/dpeak compared to that of Ir/(C, suggesting that the electrochemi-cal active surface may be decreased. The Hsja peaks decreased withFig 2. TEM micrograph of Ira>Sos/C Chalogenide powder.increasing S amount. Since the mean Ir particle sizes in Ir/C andIrxSμ_ x/C was quite close and hydrogen was not adsorbed on thehalf maximum (FWHM) of the Ir (22 0) dffaction line.S surface, we suggest that the effective Ir surfaces are those siteswhich are not completely shielded by S additions._0.9AKa1L=;(1B29 cosOmax'33. Electrocatalytic activity towards ORRwhere L is the average size of metal crystallites, λka1 is the X-raywavelength Cu-Ka radiation (Aka1 = 1.5418 A), Omax is the angleRDE tests were used to evaluate the ecrocatalytie activitiesvalue ofr(220)peak, and B2 is the FWHM ofthe l(220)iff for the ORR on IrxS_ x/C catalysts, as shown by the ORR polariza-tion line. The structural parameters obtained by XRD analysis are tion curves in Fig. 4, obtained at 5mV/s and 2000rpm, for Ir/C andsummarized in Table 2. Oobviously. all these samples exhibit the IrSs, x/C catalysts in O2-saturated 0.5M H2SO4 solutinat25 C.same particle size of around 2.5 nm.The Ir/C and IrxS1_x/C catalysts showed well-defined limiting cur-For the morphology of the synthesized IrSμ_ x/C powder, TEMrents below 0.2V, suggesting good electrocatalytic activity for themeasurements were crred out. as shown in Fig. 2. indicat- ORR. AlthelrSIS-./C chalcogenide catalysts showed enhancedORRing that the powder consists of 20-50 nm nano-clusters. Within activities over that of the pure Ir catalyst. In terms of mass activity..a|bIr/Cr,S。/C2Ir,S./C官-Io。S8。JC-lr。S。JCIt/IC1- -6S。/C-Io。S。/C.2一-IoS。fC一lrg。fClr,S。.JC中国煤化工08YHC NMH G。0.ENvs. RHEFig 3. Cyclic voltammograms for: (2) Ira>So:/C and Ir/C catalysts in 0.5M H2SO, withou MeOH and (b) Ira>SaJ/C. lr/C and PIC (E-TEK)catalyss in0.5MH;SO, with 0.5M1581 Ma et al/ Particuologv 9(2011) 155- 1601.0-0009-u 0.8 keIr/C30.7lr。S。fCIr(三-0.5: los。JC .山0.6一→loS。IClr,S。.JClrgS。J/C.5 t- lrg,S。JC-PVC.4 L0.20.0.60.8-11.0-00E/Vvs. RHElogQ, /mA cm3Fig4. Linear scan voltammograms for Ir/C and Ir,S. ./C catalysts in O-saturated0.5M H2SO4 at 25 C potental scan rate of 5mV/s. electrode roration speed ofFig 5. Tafel plots for the ORR on lr/C, Ir,S; sIC and P/C caralysts in O2-saturated05M HsSO,at 25 C potential scan rateorsmVis and etree Ttinratre2000rpm.2000 rpm.based on the metal loading, the Ira7o.s/C catalyst demonstratedthe highest activity towards ORR among these samples.The loading of S plays a dual role in the ORR activities of thewithout MeOHIryS1_ x/C catalysts by modication of the Ir active sites. When thesI 0.5 M MeOHamount was in a moderate range, the S displayed a positive effecton ORR activity, suggesting that only those modified Ir surfaces arethe active centers for ORR. While the S amount increased further,however, some Ir active sites were inhibited, hence decreasing theORR activity.According to the rotating disk electrode theory (Paulus et al,2001), the current density (i) at each electrode potential(E),、shown 占2in Fig. 4, should contain two contributions: the kinetic currentdensity (in) and the difusion-limited current density (ia). The rela-tionship between these current densities can be expressed as(2ig-iAccording to electrode kinetic theory (Paulus et al, 2001), theItr,S。/CPVCkinetic current density can be expressed as2.3RT，Fig 6. Comparison of mass acivities of In>Sa3/C and Pr/C0.7V versusη=a-nF log(i),(3RHE in )-sturatd 0sMHsSO4 with and without MCOH.where nis the overpotential, a is a constant, ik an exchange currentdensity, Rthe gas constant, T the temperature. a the electron trans-value ofig than other IrxS1-x/C and Ir/C catalysts, suggesting that itfer coffcient, n the electron transfer number in the determiningexhibits much higher activities than others.step of the ORR, F the Faraday constant.Fig.5 shows the mass transfer corrected Tafel plots for the ORR3.4. Methanol toleranceon lr/C IrxS_ x/C and Pt/C catalysts in O2-saturated 0.5 M H2SO4solution at 25 C. As shown in Table 3. the average Tafel slopesFig. 6 compares the specific kinetic current densities ofof lr/C and IrxS -x/C were about 130 mV/dec and the value of onIro.7So.3/C and Pr/C catalysts at 0.7V versus RHE in 0.5M H2SO4obtained from these Tafel slopes was about 0.5. Table 3 also shows under saturated 02 with and without MeOH. In the absence ofother kinetic parameters for the ORR on lr/C and IrxS1_ x/C.In terms methanol, the Pt/C Catalyst shows aspecific activity about 8.5 timesof kinetic current density, Iro.7So.3/C catalyst has a much higherlarger than that of the Io.>So3/C catalyst. However, in the presenceTable 3Kineic parameters for oxygen reduction on lr/C and Ir,S1. s/C catalysts in 0.5M H2SO,at 25 C中国煤化工IrICIrogSu/CIrosSoz/CMYHCNMHG/CIrasSoesICOnset potential(V)0.8630.9620.9640.9730.9680.958Tafel slope (mV/dec)137.4133.3125.1126.6130.927.8i at 0.7V (mA/cm2)0.04360.04720.04870.05270.04800.0471601 Ma et alraricologov 9(2011) 155-160Schulenburg. H Hilgendorf, M. Dorbandt, L, Radnik J Bogdanoff. P. Fiechter,S. Zaikovskil, V. L Nagabhushana, K s. Kriventsov, V. V. Loponow, K N,et al. (2006). Oxygen reduction at carbon supported ruthenium-selenium cata-Cherepanova. S. V. Kvon. R . et al. (2006). Synthesis and structuralcharacterization of Se- modifed carbon-supported Ru nanoparticles for theoxygen reduction reaction. Jourmal of Physical and Chemistry B. 110(13).ibutsch, H. Bron, M. Hilgendorf, M Schulenburg. H6881-6890.fuel clls Joumnal Ofplied Elctrochemistry, 31(7) 739 -748.中国煤化工MYHCNMHG