Preparation and characterization of Pt-WO3/C catalysts for direct ethanol fuel cells

RARE METALSol.29,No.3,Jm20l0,p.255DOI:10.1007/s12598-010-00440Preparation and characterization of pt-WO3/c catalysts for direct ethanolfuel cellsWU Feng, b, LIU Yanhong, and WU Chuan, bO The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg e er 2000 TTSchool of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, C/Received 4 May 2009: received in revised form 7 September 2009; accepted 16 SeptemlAbstractThree co-impregnation/chemical reduction methods in acidic solutions of pH l, including ethylene glycol (EG), NaBH4, and HCOOH,were compared for Pt-WO3/C catalysts. Pt-WOy/C catalysts containing 10 wt and 20 wt %o platinum per carbon were prepared by the threemethods: their morphology and electrocatalytic activities were characterized. The 20 wt. Pt-WO3/C catalyst prepared by theco-impregnation/EG method presented the optimal dispersion with an average particle size of 4. 6 nm and subsequently the best electrocatalytic activity, and so, it was further characterized Its anodic peak current density for ethanol oxidation from linear sweep voltammetry (Lsv)is 7.9 mA cm, which is 1. 4 and 5.2 times as high as those of the catalysts prepared by co-impregnation/NaBH4 and co-impregnationHCOOH reduction methods, 2.1 times as high as that of the 10 wt %Pt-WOy/C catalyst prepared by co-impregnation/EG method, respectimeKeywords: direct ethanol fuel cells; catalysts; preparation method; characterization; ethanol electrooxidation1 Introductionthe favorable electronic effect [15] as well as PtSn, but alsothe prominent hydrogen spillover [16-17]. However, theFuel cells have been hailed as important power sources preparation of highly dispersed supported Pt-WOx systemfor the future becaf their renewable and environ- has little been investigated. Gotz et al [6 synthesizedment-friendly characteristics. Direct ethanol fuel cells Pt-Ru-WO, /C catalyst with an average particle size of 1-2DEFCs) have recently received increased attention because nm by Bonnemann method. Viswanathan et al. [9-10]re-of the nontoxicity, renewability, zero green-house contribu- ported the results of Pt-WO3 supported on carbon nanotubestion, high-power density [1], and low crossover of ethanol (CNTs) by thermal decomposition method. The averagethrough membrane [2]. Based on the electrolyte membrane particle sizes were 5 and 10 nm, respectively, larger thanused, DEFCs can be divided into two types: acid and alka- those of Pt and PtRu. Bock et al. [18 prepared Pt-WOy/Cline electrolyte DEFCs. A considerable amount of effort has and PtRu/C catalysts using impregnation/NaBH4 reductionbeen devoted to acid DEFCs [3]. The development of acid method in an acidic solution of ph <1, which had particleDEFCs lags that of hydrogen and methanol fuel cells by a size distributions of 6-15 and 2.5-5 nm, respectively. Nothuge margin because of the difficulty of a cleavage of C-c only the thermal decomposition and NaBH4 reductionbond and a delivery of 12 electrons per ethanol molecule to methods were not favorable to control particle sizes, but alsoform CO2 [4]. New Pt-based catalysts with narrow size dis- the particle size control of Pt-WO3 were more difficult thantution, high dispersion, and high activity need to be in- PtRu. And that Bonnemann method was complex.vestigated for the fast anodic kineticsChan et al. [19] prepared Pt and WO3 colloidal ethylenePt-WO, catalysts have shown excellent performance not glycol (EG) solutions, separately, and mixed them with car-only for the electrooxidations of reformate gas [5-6] and bon slurry to obtain Pt-WO3/C catalyst with an average parmethanol [7-10], separately, but also for the electrooxidation ticle size of 1.9 nm. In this method, the formations of Pt andof ethanol [11-12]. Moreover, they presented not only the wO3 were completely independent. Yet the electrocatalyticbifunctional mechanism [13-14] as well as PtSn and PtRu, activity of Pt-WO3/C catalyst was affected by the interaction中国煤化工Corresponding author: WU ChuanE-mail: chuanwuabit. edu. cnCNMHG256RARE METALS. Vol 29. No. 3. Jun 2010of pt and wosouIrceThis study adopted co-impregnation/chemical (EG,The electrocatalytic activities of 10 wt % and 20 wt%NaBH4, and HCOOh)reduction methods in acidic solutions Pt-WO3/C catalysts were tested by cyclic voltammetry(Cv)of ph l to obtain Pt-WO3 /C catalysts, and compared their and linear sweep voltammetry (Lsv) on a CH1660C poten-morphology and activity. A highly active Pt-WO3/C catalyst iostat using a conventional three-electrode system at roomwith an average particle size of 4.6 nm was achieved by temperature in an electrolyte solution of I mol-L-co-impregnation/EG method in acidic solution of pH 1 CH3,OH and 0.5 molL H2SO4, separately. Cv wascompared with the other two methods. In this method, the carried out at a scan rate of 0.05 V's and in a potentialformations of Pt and wO, were in the same solution and range of 0-1. 2 V vS NHE. LSv was carried out at a scankely simultaneousrate of 0.001 V s and in a potential range of 0.4-1.0V vsNHE. Pt wire served as the counter electrode. A saturated2. Experimentalcalomel electrode (SCE)served as the reference electrodeThe working electrode was prepared by loading 15 uL of theChloroplatinic acid hydrate H2PtCl6 6H2O and ammo- catalyst ink on a 3 mm glassy carbon electrode uniformly.nium paratungstate hydrate(APT)Ns HO were The catalyst ink was prepared by mixing 5 mg of the cata-adopted as the platinum and tungsten precursors, respec- lyst, I mL of ethanol, and 50 uL of 5 wt Nafion solutiontively. Carbon black Vulcan XC-72R( Cabot Corp. )was ultrasonically. The area of thecarbon electrode wasused as the carbon support. EG, NaBH4, and HCOOh were 0.07065 cm. The loading of Pt-WO3/C on the electrode wasselected as the reducing agents, separately. All of the mate- 1.0 mgcmrials were analytically pure, except that APT was chemically3. Results and discussionPt-WO3/C catalysts containing 10 wt% and 20 wt%platinum per carbon were prepared by three different3.1. TEM analysisco-impregnation/chemical(EG, NaBH4, and HCOOH)re-shows the tEm images of 20 wt% Pt-wO3/Cduction methods, separately. All of the methods were carried catalysts prepared by the three methods, separately. Theout according to the process described by previous work in- Pt-WO3 nanoparticles prepared by co-impregnation/EGvolving NaBH4 [18]. The preparation procedure is briefly method were loosely assembled and uniformly dispersed ondescribed below, taking EG as an example. A mixed aque. the carbon support with a narrow diameter distribution ofous solution of H2PtCl and APt was slowly added into 2.9-6.1 nm and a mean diameter of 4.6 nm, as shown in Figscarbon slurry at 80C under stirring for I h. Then, the slurry 1(a) and 1(d)at different magnifications. As shown in Figwas dried in air at 100@C for 4 h. After that, the mixture was 1(b), the Pt-WO3/C catalyst prepared by co-impregnation/cooled and ground. EG was added to the mixture, and the Nabha reduction method presents more agglomerates with apH value of the suspension was adjusted below 1 using 0.5 wide particle size distribution of 1. 5-10 nm and less uniformmol L H SO4 under stirring. Subsequently, the suspension dispersion, and a large amount of blank carbon black withwas refluxed to form Pt-WO3/C catalyst. Finally, the sus- out supported Pt-WO3 particles, consistent with the resultspension was filtered, washed, and then vacuum dried at reported by Bock et al.[18]. Although some of the Pt-WO80°Cfor8h.nanoparticles prepared by co-impregnation/NaBH4 reduc-The morphology of 20 wt. Pt-WOy/C catalysts wastion method have a diameter distribution of 1.5-5.5 nmquired by transmission electron microscopy(TEM)using a smaller than that of the Pt-WO3 nanoparticles prepared byJEOL TEM 2010 high-resolution transmission electron mi- co-impregnation/EG method, the others of the Pt-WOcroscope. The chemical states of platinum and tungsten in nanoparticles prepared by co-impregnation/NaBH4 reduc-20 wt Pt-WOyC catalyst prepared by co-impregnation/ tion method were agglomerated very badly to 10 nm. TheEG method were confirmed by X-ray photoelectron spec- Pt-WO3 particles prepared by co-impregnation/HCOOH retroscopy(XPS)using a VG ESCALAB MK II X-ray photo- duction method present the worst agglomeration and woelectric spequipped with an Al Ka(e=1486.6shown in Fig. 1(c). The small nanoparticlev)radiation source. C Is binding energy was referenced at have a mean diameter of 1. 5 nm, the big particles are large284.6eV for the charge correct. The crystalline structures of to 50 nm, and a large amount of carbon black is blank withPt and WO3 in 20 wt Pt-WO3/C catalyst prepared by out supported Pt particles. The Pt-WO3/C catalyst preparedcO-impregnation/EG method were defined by X-ray diffrac- by co-impregnation/EG method demonstrates the most nar-tion(XRD)using a Rigaku D/ax 2000 X-ray diffractome- row size distrib中国煤化工 due to the modter equipped with a Cu Ka (=0. 15406 nm) radiation erate reducibilityHCNMHGsolvent egWu F et aL., Preparation and characterization of Pt-WO3/C catalysts for direct ethanol fuel cells(b)都40 nm(d)等Fig. 1. TEM images of 20 wt %Pt-WOy/C catalysts prepared by the three different co-impregnation/chemical reduction methods(a, d)co-impregnation/EG; (b)co-impregnation/NaBH4; (c)co-impregnation/HCOOH2(a) and 2(b), respectively. The anodic current densities3. 2. Evaluation of electrocatalytic activitiesduring the positive potential sweep decease according toThe electrocatalytic activities of 20 wt. Pt-WO3/C those of the catalysts prepared by co-impregnationEG>catalysts prepared by the three methods for ethanol oxida- co-impregnation/NaBH4 >co-impregnation/HCOOH fromtion were investigated by CV and LSV, as shown in Figs. both the CV and lsv curves. The anodic peak current den-8}(b)HCOOHHCOOHNaBHNaBH,EG0.00.2040.60.81.01.2Potential/V vs NHEPotential /V vs NHEFig. 2. CV (a and lsv(b)curves of ethanol electrooxidation on 20 wt %o Pt-wOyc catalysts prepared by differentcO-impregnation/chemical reduction methods. Electrolyte solution 0.5 molL H2soperature25°Cand scan rates of 0.05 and 0.001 V's for CV and lsv, respectively中国煤化工CNMHG258RARE METALS, Vol 29, No 3, Jun 2010Table 1. Particle sizes and electrocatalytic activities of 20 wt %o Pt-wO3 /C catalysts prepared by different co-impregnation/ chemi-cal reduction methodsPt-WOy/C catalyst prepared by different methodsParticle size/Anodic peak current density /(mA' cm)nmo-impregnation/EG15-102Co-impregnation/HCOOH1.5,-50ity at peak potential of 1.0 V vs. nhe during the positive tion/EG method was further characterized by XPs and XRDpotential sweep of Cv curve from the Pt-WOy/C catalyst separately. XPS was employed as a bulk method for the suf-prepared by co-impregnation/EG method is 32.6 mAcm1.4 and 2.9 times as high as those of the other two catalysts8|(a)prepared by co-impregnation/NaBH4 and co-impregnation/HCOOH reduction methods, respectively, as shown in TableLSV is a process more close to steady state, better reflecting the activities of catalysts. The anodic peak current2density at a peak potential of 0.9 V vs. NHE of LSV curve10wt%from the Pt-WO3/C catalyst prepared by co-impregnation-20wt%EG method is 7.9 mA cm, 1.4 and 5.2 times as high as040.50.60.70.80.91.0those of the other two catalysts prepared by co-impregnation/Potential/V vs NHENaBH4 and co-impregnation/HCOOH reduction methods,respectively, as shown in Table 1. The Pt-WO3/C catalyst(b)prepared by co-impregnation/EG method exhibits the bestelectrocatalytic activity for ethanol oxidation. Table I sum4marizes the particle sizes and electrocatalytic activities of 20wt% Pt-WO3/C catalysts prepared by the three methodshe narrowest size distribution and highest dispersion explain the best electrocatalytic activity of the Pt-wO3/C cata10wt%lyst prepared by the co-impregnation/EG method020wt%Additionally, the electrocatalytic activity of each 20 wt40.50.60.70.80.91.0Pt-WO3/C catalyst for ethanol oxidation was compared withPotential /V vs NHEthat of each 10 wt. Pt-WO3/C catalyst by LsV, respecrely. Figs. 3(a-c) show the LSv curves of the catalystsprepared by co-impregnation/EG, co-impregnation/NaBH4,and co-impregnation/HCOOH methods, rescept for the Pt-WOy/C catalyst prepared by co-impregnation/EG method, the other two catalysts containing 20 wt%platinum per carbon exhibit decreased current densitiescompared with those containing 10 wt. platinum per10 wt%bon. The anodic peak current density of the Pt-WO3/C cata-20wt%lyst prepared by co-impregnation/EG method containing 20040.50.60.70.8091.0wt% platinum per carbon is 2. 1 times as high as that of thePotential/Vys, NHEcatalyst containing 10 wt %o platinum per carbon, as shownFig. 3. LSV curves of ethanol electrooxidation on 10 wt%2. The co-impregnation/EG method is more suitand 20 wt. Pt-wO /C catalysts prepared by the three differ-he preparation of high-platinum-content catalysts. ent co-impregnation/chemical reduction methods:(a)EG: (b)33. XPS and XRd investigationNaBH4;(c)HCOOH. Electrolyte solution 0.5 mol. H2SO4+I molL CH,CThe 20 wt. Pt-WOyC catalyst prepared by co-impregnaYHa中国煤化工 nd temperatureCNMHGWu F et aL., Preparation and characterization of Pt-WO3/C catalysts for direct ethanol fuel cellsTable 2. Electrocatalytic activities from LSV curves of 10 wt. and 20 wt Pt-wOy /C catalysts prepared by differentcO-impregnation/chemical reduction methodsAnodic peak current density /(mA' cmontent of platinum /wt.Co-impregnation/EGCo-impregnation/NaBhCo-impregnation/ HCOOHficiently small Pt-WO3 nanoparticles prepared by and 37. 1 ev, respectively, revealing the presence of littleco-impregnation/EG method to obtain the compositional in- amount of wO3. wO2 might be concomitant [2 1-22] but notformation [20]. Fig. 4 shows the regional Pt 4f and w 4f deconvoluted due to the weakness of the W 4f peaks. TheXPS spectra of the catalyst, separately. The Pt 4f XPS spec- atomic ratio of tungsten to platinum is 1/13, lower than therum in Fig. 4(a)shows two intense peaks attributed to Pt design.4f7/2 and Pt 4fs/2, which could be deconvoluted into two pairsThe lowered W/Pt atomic ratios were also reported byof doublets. The more intense doublets centered at binding Bock et al. [18] and Chan et al. [191, who preparedenergies of 71.2 and 74.5 eV reveal the presence of a great Pt-WOC catalysts by impregnation/NaBH4 reductionamount of metallic Pt(O). The less intense doublets centered method and mixing of Pt and wO3 colloidal solutions genat binding energies of 73.0 and 76.3 eV reveal the presence erated from EG, respectively. Yet the W/Pt atomic ratio inof little amount of bivalent Pt(i in the form of Pto or Pt-Ru-WOj,/C catalyst prepared by Bonnemann method wasPt(OH)2. The relative contents of Pt(O) and Pt(lD)are 71.8% almost the same as the design, reported by gotz et al.[6]and 28.2% estimated from their peak area, respectively. The Control of tungsten content in Pt-WO3 system prepared byW4f XPS spectrum in Fig 4(b) shows two weak peaks at- liquid-phase chemical reduction methods needs to be furthertributed to W 4f7/ and w 4fsn with binding energies of 35.3 studied(a)Binding energy/eⅤBinding energy /evig. 4. XPS spectra of 20 wt%Pt-WOy/C catalyst prepared by co-impregnation/EG method: (a)Pt 4f;(b)w 4f.XRD was conducted to obtain the structural informationPtDfor the bulk of Pt-WO3/C catalyst, as shown in Fig. 5. Thefirst broad peak located at the 20 value of about 25 is attributed to the carbon support that possesses graphite structure. The five other peaks located at the 20 values of 39.90P(200)46.4°,67.8°,81.6°,and85.9° are assigned to pt(l).PtC(002)P(20P1(222(200), Pt (220), Pt (311), and Pt(222), respectively, which isPt(311indicative of the face centered cubic structure of pt. diffrac-tion peaks of tungsten are not found, which is indicative ofthe amorphous structure of wo4. ConclusionsFig. 5. XRD patYHa0/ pt-wo. C catalyst preparedThe co-impregnation/EG method in acidic solution of ph by co-impregnatie中国煤化工CNMHGRARE METALS, Vol 29, No 3, Jun 2010I is more effective for Pt-WOyC catalysts. The Pt-WO3/C [9] Rajesh B, Karthik V,Karthikeyan S, Thampi KR, Bonardcatalyst prepared by the co-impregnation/EG method in. M, and Viswanathan B, Pt-WO3 supported on carbonidic solutions of pH I presents high dispersion, a narrownanotubes as possible anodes for direct methanol fuel cells,size distribution of 2.9-6.1 nm. Furthermore it displays 1.4Fuel,2002,81(17:2177and 5.2 times as high as the electrocatalytic activity for [10 Rajesh B, Ravindranathan Thampi K, Bonard J.M., Xanethanol oxidation of the other two catalysts prepared byco-impregnation/NaBH4 and co-impregnation/HCOOH re-nanotubes generated from template carbonization of polyphenyl acetylene as the supported for electrooxidation ofduction methods, respectively. Also, the co-impregnationmethanol, J. Phys. Chem. B, 2003, 107(12): 2701EG method in acidic solution of pH I presents suitability [11] Tanaka S, Umeda M, Ojima H, Usui Y, Kimura O,andfor the preparation of high-platinum-content catalysts com-Uchida I, Preparation and evaluation of a multi-componentpared with the other two methods. Additionally, furthercatalyst by using a co-sputtering system for anodic oxidationwork is necessary to investigate the control of tungsten conof ethanol, J. Power Sources, 2005, 152(1): 34tent in the Pt-WO3/C system.[12] Liao S.J., Linkov V, and Petrik L, Anodic oxidation ofethanol on inorganic membrane-based electrodes, Appl. CatalAcknowledgementsA,2004,258(2):183[13] Pereira L G.S., Santos F.R., Pereira M.E., Paganin V.A., andThis work was financially supported by the National Basic Research and Development Program of China (NoPEMFC anodes, Electrochim. 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