Effective preparation of carbon nanotube-supported

Materials Chemistry and Physics99(2006)

80–87

Effective preparation of carbon nanotube-supported

Pt–Ru electrocatalysts

Chun-Ching Chien?,King-Tsai Jeng

Institute of Nuclear Energy Research(INER),P.O.Box3-19,1000Wenhua Road,Longtan,Taoyuan325,Taiwan

Received1June2005;received in revised form31August2005;accepted30September2005

Abstract

Carbon nanotube(CNT)-supported electrocatalysts,such as Pt/CNT and Pt–Ru/CNT,have been intensely studied recently for membrane fuel cell applications.These novel electrocatalysts were generally prepared by chemical reduction deposition using ethylene glycol(EG)as a reducing agent.However,due to different metal deposition conditions,CNT-supported binary and multi-component electrocatalysts cannot be prepared using EG alone with satisfactory results.In this study,an effective method for preparation of CNT-supported Pt–Ru alloy electrocatalysts has been developed.Suitable amounts of sodium hydrogen sul?te(NaHSO3)and calcium hydroxide aqueous solutions were employed as additives to EG forming a modi?ed reducing agent.This new approach was based on the use of more suitable Pt–Ru deposition environments created by the modi?ed EG near the isoelectric point(IEP)on the acid-oxidized CNT surfaces.It gave rise to enhanced formation of Pt–Ru/CNT with high metal deposition ef?ciency,excellent nanoparticle morphology and desired electrocatalyst composition.In addition,the prepared Pt–Ru/CNT exhibited high methanol electrooxidation activity in0.5M H2SO4aqueous solution better than that of a carbon black-supported commercial product.Overall, preparation of Pt–Ru/CNT using the modi?ed EG resulted in much better electrocatalyst formations than those prepared using a variety of common reducing agents.

?2005Elsevier B.V.All rights reserved.

Keywords:Alloys;Nanostructures;Electrochemical properties;Adsorption

1.Introduction

Membrane fuel cells,particularly direct methanol fuel cells (DMFCs),are regarded as potential mobile power sources due to high energy density,easy operation and simple fuel supply. However,DMFCs strongly depend on the use of Pt electrocat-alyst for effective oxygen reduction and Pt–Ru electrocatalyst for methanol fuel electrooxidation.The use of binary or multi-component electrocatalysts for methanol electrooxidation is to solve the catalyst poisoning problems caused by CO and other reaction intermediates under low temperature reaction condi-tions.Conventionally,highly conductive carbon blacks,such as Vulcan XC72(Cabot)and Shawinigan(Chevron),with high surface areas are used as supports for electrocatalysts to ensure large electro-reaction surfaces and good electronic conduction. Although such carbon black-supported electrocatalysts have exhibited moderate performances on DMFCs so far,new elec-?Corresponding author.Tel.:+88634711400x5038;fax:+88634711410.

E-mail address:ccchien@https://www.360docs.net/doc/a15083589.html,.tw(C.-C.Chien).trocatalyst carbon supports,such as carbon spheres[1],graphite nano?bers[2],carbon nanohorns[3]and carbon nanotubes [4–6],are actively being sought with attempts to signi?cantly improving fuel cell performances.

In particular,more and more carbon nanotubes(CNT)have been investigated[7–11]recently as advanced electrocatalyst supports due to their distinctive characteristics.Although several preparation methods are feasible,most CNT-supported electro-catalysts reported so far were prepared using chemical reduction deposition methods.A variety of common reducing agents,e.g. HCHO,HCOOH,NaBH4,NaH2PO2,N2H4,etc.,can be used to convert metal ions into metal particles and deposit onto CNT surfaces.However,Lordi et al.[12]were the?rst to report successful preparation of nanosized Pt particles supported on single-walled CNT as a heterogeneous catalyst using a polyol process.

At present,reductive preparation of CNT-supported electro-catalysts using ethylene glycol(EG)as a reducing agent seems to be the most popular approach[4–7].In such a polyol process, CNTs were generally?rst subjected to oxidation pre-treatment [12]using a strong acid,such as HNO3or mixture of HNO3

0254-0584/$–see front matter?2005Elsevier B.V.All rights reserved. doi:10.1016/j.matchemphys.2005.09.080

C.-C.Chien,K.-T.Jeng/Materials Chemistry and Physics99(2006)80–8781

and H2SO4,solution so as to remove impurities and generate suf?cient amounts of functional groups,such as OH,COOH, C O,etc.,on the surfaces.These surface functional groups have stronger attraction forces toward metal ions than bare carbon nanotube surfaces and some even have ion exchange capabilities,such as carboxylic acid groups.Therefore,they are believed to work as metal-anchoring sites in order to facilitate metal nuclei formation and electrocatalysts deposition.In fact, this process is somewhat similar to electroless deposition that requires some imperfect sites on the substrate surfaces to anchor and hold the reduced metals.

However,EG is only a mild reducing agent requiring a long reaction time for involved reduction reactions to go to comple-tion even for the preparation of Pt/CNT.Its reducing strength, or power,may be strong enough for reduction of Pt ions but a little bit too weak for the reduction of Ru ions.In addition,in the preparation of Pt–Ru/CNT dif?culties arise that Pt and Ru ions cannot be reduced simultaneously at the same pH of the reac-tion solution with competitive reduction rates using EG alone. For example,a high pH value(pH>13)of the EG solution was reported to be favorable for Pt formation on CNT[4]but this is,in fact,not an acceptable condition for Ru formation.It was found in our experiments that once the pH of the EG solution is adjusted to a value higher then4undesirable precipitates,includ-ing Ru(OH)3,of Ru salts appear.The reduction reactions of such precipitates are dif?cult to proceed leading to large particle size formation,low metal deposition,non-uniform dispersion and undesirable electrocatalyst composition on CNT and,in turn, resulting in poor performance with respect to methanol elec-trooxidation.As a result,Pt–Ru/CNT cannot be readily prepared by the same polyol procedure as that of Pt/CNT with satisfactory results.

The shortcoming of EG to have a poor reducing power may be overcome by the use of more active reducing agents,such as sodium borohydride and formaldehyde,to enhance reduction reactions.However,our experience indicated that Pt–Ru/CNT prepared by reduction with conventional active reducing agents tends to form large agglomerates and incorporate impurities,e.g. boron oxides,leading to poor catalytic activity on methanol elec-trooxidation.Other feasible approaches to prepare Pt–Ru/CNT include electrodeposition[5,13],wet impregnation followed by chemical reduction[14],metal vapor deposition[15],etc.,but large particle formation and poor catalyst morphology are com-mon drawbacks.Innovative approaches are urgently needed to solve these problems.

In this study,an effective preparation method was sought using modi?ed reducing agents.The strategy was to enhance the reducing strength of EG and modify the reaction environments so that ligand-complexed Pt and Ru ions can be simultaneously reduced at the same low pH range,e.g.pH2–4,with competitive speci?c rates and without preferences in the formation of any speci?c metal or undesirable precipitates,and at the same time to maintain those good characteristics of EG as a reducing agent in electrocatalyst formation on CNT.To realize this idea,the fea-sibility was resorted to the concept of potential of zero charge (PZC)for a double layer structure at a CNT/solution interface or,more precisely,the isoelectric point(IEP)for a?ow system as in our case with reactions proceeding under high-speed stir-ring conditions.At such a speci?c point,no net charge occurs on the CNT surfaces and,therefore,Pt and Ru can be deposited at competitive speci?c rates due to similar speci?c adsorption rates for different ions.Thus,suitable additives were investigated as modi?cation agents to EG in order to bring about the occur-rence of IEP under proper deposition conditions.In addition, microwave irradiation,with fast and even heating capability, and ultrasound sonication were employed to enhance reductive reactions so that the metal deposition process can be completed in a shorter time with signi?cantly improved results in terms of electrocatalyst formation for preparation of Pt–Ru/CNT.The prepared Pt–Ru/CNT electrocatalysts were tested for methanol electrooxidation in0.5M H2SO4solution.Investigations on using a variety of reducing agents different from EG as well as using electrodeposition method for the preparation of CNT-supported Pt–Ru electrocatalysts were also carried out.The results were analyzed and compared.

2.Experimental

2.1.Preparation of CNT-supported Pt–Ru electrocatalysts

Multi-walled carbon nanotubes were obtained from Advance Nanopower Inc.,Taiwan having diameters of8–15nm and a high surface area of233m2g?1. The as-received carbon nanotubes were?rst oxidized in a hot solution,composed of8M HNO3and2M H2SO4,for several hours under re?uxing conditions to remove impurities and generate surface functional groups.Puri?cation of CNT surfaces prevents self-poisoning by foreign impurities while functional group generation enhances electrocatalyst formation.Examination on surfaces of acid-oxidized carbon nanotubes was carried out using a Fourier transform infrared (FT-IR)spectrometer(Bio-Rad,FTS-40)to ensure formation of desired surface functional groups.

Reagent grade reducing agents,i.e.ethylene glycol and NaHSO3solution, were obtained from Merck and electrocatalyst precursor salts,i.e.H2PtCl6·6H2O and RuCl3,were purchased from Alfa Aesar.They were used as received with-out further puri?cation.The procedure employed for reduction deposition of Pt–Ru electrocatalysts on pre-treated CNT is illustrated in Fig.1.It is similar to those reported previously by other groups[9,16]using a microwave irradia-tion heating method to signi?cantly shorten the reaction time.However,proper modi?cation of the EG reducing agent has been conducted as a main focus to suit the reaction condition for Pt–Ru/CNT formation.As an example,in the preparation of20wt.%Pt–10wt.%Ru/CNT,1.65g of acid-oxidized CNT was added to50ml of EG.The above mixture was sonicated for10min followed by high-speed stirring,using a high-speed stirrer(Heidolph,Silent Crusher M),for 30min to form a homogeneous paste so that EG was able to completely cover the carbon nanotube surfaces.Then,1.264g of H2PtCl6·6H2O and0.506g of RuCl3as Pt–Ru electrocatalyst precursors were dissolved in10ml EG together with aqueous solution of1ml of1M NaHSO3and about0.1ml of1000ppm Ca(OH)2.The?nal pH was adjusted to about2–4.After this,the solution was slowly added to the prepared CNT/EG paste and subjected to high-speed stirring for at least30min for complexed metal ions to adsorb onto the surfaces of car-bon nanotubes followed by formation of Pt–Ru electrocatalyst through chemical reduction reaction.The resultant mixture was then heated at70–140?C using a microwave heater for10–120min for the reduction reaction to go to com-pletion.In the preparation of Pt–Ru/CNT,the amounts of precursor salts,i.e. H2PtCl6·6H2O and RuCl3,were used with an atomic ratio(Pt:Ru)of about1:1.

After reduction reaction,the reacted mixture was?ltered and the collected carbon nanotubes were washed and rinsed with a suf?cient amount of Millipore water(resistivity>16.0M cm).The?ltrate and wash wastewater were analyzed using an inductively coupled plasma–optical emission spectroscope(ICP–OES) (Jobin Yvon,Ultima-2)to determine the total amounts of un-reacted metal ions and washed out metal ions and particles.Then,the actual amounts of metals supported on CNTs were calculated.Finally,the electrocatalyst-loaded carbon

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Fig.1.A schematic?owchart illustrates the procedure for preparation of CNT-supported Pt–Ru electrocatalysts using the modi?ed EG.

nanotubes were dried in an oven at100?C under vacuum condition for sev-eral hours.Preparations of CNT-supported electrocatalysts using other reducing agents were conducted in a similar manner.

2.2.Physical characterization and electrochemical investigation

on methanol electrooxidation

Physical characterization of the prepared Pt–Ru/CNT electrocatlysts was conducted using a transmission electron microscope(TEM)(JEOL,JEM2010 operating at200kV)for morphology and particle size analyses and a scanning electron microscope(SEM)(JEOL,JSM-T330A operating at20kV)with energy dispersive spectrocopy(EDS)capability for elemental analyses.Electrochem-ical investigations on the prepared Pt–Ru/CNT electrocatalysts with respect to methanol electrooxidation were carried out using cyclic voltammetry.For all the electrochemical tests,the same amount of electrocatalyst(0.5mg)was used each time.Each sample was mounted on a glassy carbon electrode(0.196cm2) and?xed with0.1ml of5wt.%Na?on solution.A three-electrode system was employed with Pt as a counter electrode and Ag/AgCl as a reference electrode. Methanol electrooxidation experiments were performed at room temperature in 0.5M H2SO4aqueous solution containing1.0M CH3OH.

3.Results and discussion

3.1.Oxidative treatment of carbon nanotube surfaces using

a strong acid solution

The carbon nanotubes used in this study were prepared by cat-alytic gas phase growth method having a three-dimensional,tan-gled structure with a tube diameter of about8–15nm as shown in Fig.2.The small tube diameter,in fact,favors electrocata-lyst nanoparticle formation when these metal nanoparticles

are

Fig.2.TEM image of as-received CNT.

well dispersed.In addition,the surface area is surprisingly high (233m2g?1),comparable to that of Vulcan XC72at250m2g?1, very suitable for electrode fabrication.This carbon nanotube material is considered to have several advantages over conven-tional carbon blacks.These include:(i)having more de?ned crystalline structure with higher conductivity,(ii)containing lit-tle impurities,such as metals and sul?des,and thus eliminating potential poisoning effects to electrocatalysts,and(iii)possess-ing three-dimensional structure and thus favoring the?ow of reactant and providing a large reaction zone when fabricated into electrodes.The carbon nanotubes are also chemically stable and resistant to thermal decomposition up to more than300?C.Due to these distinctive characteristics,this carbon nanotube material is very suitable for use as a new electrocatalyst support.

A recent report by Han et al.[17]indicated that a stronger nitric acid solution generates more functional groups on CNT surfaces than a weaker one and results in better electrocatalyst formation.Therefore,a concentrated nitric acid,with addition of sulfuric acid,solution was employed in this study for oxidation. The result on generation of surface functional groups obtained from FT-IR spectroscopy is shown in Fig.3.It is clear that several types of functional groups,particularly carbonyl and hydroxyl groups,have been generated on acid-oxidized carbon nanotube surfaces as expected.In addition,it can be seen that the longer the acid treatment time,the more amounts of surface functional groups were generated as evidenced by the FT-IR spectrographs. It was reported[12]that the dominant functional group is car-boxylic group generated by strong acid treatment.High density of surface functional groups indeed ensures that high loading of electrocatalyst can be supported on CNT surfaces,which is favorable for use in DMFCs.

It was found[18]that the oxidized CNT also induced a zeta potential(ζ)in a solution,such as an ethanol solu-tion,more negative than that of a pristine one due to the presences of acidic functional groups.These functional groups on oxidized CNT dissociate in the solvent and con-sequently impart negative charges on the CNT surfaces, which should be favorable for adsorption of multicharged metal complexes,such as Pt(EG)a(Cl?)b(HSO3?)c(4?b?c)+and

C.-C.Chien,K.-T.Jeng/Materials Chemistry and Physics99(2006)80–87

83

Fig.3.FT-IR spectrographs for generation of functional groups on strong acid-oxidized CNT surfaces with different treatment time:(a)1h,(b)3h,and(c) 6h.

Ru(EG)l(Cl?)m(HSO3?)n(3?m?n)+with their chloride ligands partially denuded.The accompanying large electrical repulsive forces between nanotubes prevent them from tangling and form-ing agglomeration.But,in reality the negatively charged CNT surfaces are not ideal for formation of Pt–Ru electrocatalysts in an unmodi?ed EG solution due to different speci?c adsorption rates of these two metal ions and,in turn,different speci?c depo-sition rates between Pt and Ru.Thus,proper adjustment of the CNT surface charges using the modi?ed EG to provide a suitable reaction condition is necessary in order to prepare Pt–Ru/CNT with excellent formation and ideal electrocatalyst composition as will be discussed later.

It should also be noted that the strong acid oxidation has partially changed the surface structures of carbon nanotubes in a way of“forced corrosion.”Although this oxidative surface treatment process is helpful for generations of surface functional groups and deposition of electrocatalyst metal,it may affect the conductivity and chemical stability of the carbon nanotubes if over-treated.Thus,cares should be taken in this respect.A treat-ment time of3–6h was found to be acceptable.No obvious increases in functional groups formation on oxidized CNT sur-faces using a longer treatment time.

3.2.Preparations of Pt–Ru/CNT using a modi?ed reducing agent

The main component of the modi?ed reducing agent is still ethylene glycol.In general,the function of EG is twofold:(a) to work as a chelating agent for complexation of metal ions in the reduction process,and(b)to serve as a reducing agent to convert metal ions into metal or alloy nanoparticles.In the preparation of Pt–Ru/CNT,the EG solution(usually contains a small amount of water)was generally adjusted to a suitable pH value,which favors formation of EG(ligand)–Pt(ion)and EG (ligand)–Ru(ion)complexes.Under this condition,the coordi-nation mechanism of ethylene glycol to metal ions is through the non-participating electron pair on oxygen and does not involve acid dissociation of alcohol groups.The formation of such com-plexes prevents too many Pt and Ru ions from sticking together. Thus,this results in the formation of small nanosized electrocata-lyst particles with uniform distribution when reduction reactions are carried out at suitable temperatures.

The TEM images of Pt–Ru/CNT electrocatalysts prepared using different EG reducing agents are compared.Fig.4(a) shows the one prepared using modi?ed EG while Fig.4(b) reveals that in more magni?ed details.One can clearly see that the modi?ed EG prepared Pt–Ru/CNT has uniform particle dis-persion and small particle sizes.On the other hand,the use of unmodi?ed EG resulted in discrete and more aggregate parti-cles as shown in Fig.4(c).By comparing with the result obtained using NaBH4as a reducing agent as shown in Fig.4(d),it can be seen that a strong reducing agent may not result in good catalyst formation on CNT.When the reducing agent is too strong,reduc-tion of metal salt or ion proceeds rapidly and the metal tends to form locally with large agglomerates.Fig.5shows the particle size distribution of Pt–Ru/CNT prepared using the modi?ed EG. It can be seen that the particle size is,indeed,very small and nar-rowly centered at about3nm with an average of about3.3nm. This is,in fact,just about the ideal size of Pt–Ru electrocatalyst for methanol electrooxidation[19].

The best reaction temperature was found to be around 110–140?C and the reaction time was dramatically reduced from6to8h to about30min using the modi?ed EG with microwave irradiation heating.This heating method has advan-tages over conventional conductive heating in that it is more uniform and effective.Therefore,suitable reaction tempera-tures can be reached within a much shorter time.In general, microwave irradiation heating can increase the reaction kinetics by1–2orders of magnitude over conventional heating methods within a short reaction time improving electrocatalyst nanopar-ticle formation.The metal deposition ef?ciency(η),i.e.the percentage of the total amount of metal ions originally exist in the reaction solution that deposit onto the CNT support surfaces, was found to be more than90%for Pt–Ru/CNT.In general,a loading of10–50wt.%Pt–Ru in a form of alloy on CNT can be readily prepared without much difference in particle size distri-bution using the modi?ed EG in conjunction with microwave irradiation heating.

In addition to particle morphology,the composition of the prepared Pt–Ru electrocatalyst is also of vital importance to have excellent electrocatalytic activity and poison-resistance in methanol electrooxidation.The effectiveness of the modi?ed EG on forming the right electrocatalyst composition in the prepara-tion of20wt.%Pt–10wt.%Ru/CNT(Pt:Ru=1:1)was examined by the elemental analyses using EDS and ICP.Fig.6(a)shows the EDS graph of the prepared Pt–Ru/CNT using the modi?ed EG and Fig.6(b)shows the one using unmodi?ed EG with much more pronounced Ru signal for the https://www.360docs.net/doc/a15083589.html,paring these two graphs to that of a standard taken using the same amount of a commercial20wt.%Pt–10wt.%Ru/C electrocatalyst(Johnson Matthey)as shown in Fig.6(c),it can be seen that the use of mod-i?ed EG gave rise to a much better electrocatalyst composition pattern close to that of the commercial product.The intensities of EDS signals of the prepared Pt–Ru/CNT relative to those of

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Fig.4.TEM images of prepared Pt/CNT using different reducing agents:(a)modi?ed EG,(b)modi?ed EG (with magni?ed details),(c)unmodi?ed EG,and (d)NaBH 4.

the commercial standard were used to estimate the amounts of Pt and Ru deposited on CNT surfaces.It turned out that these semi-quantitative results are comparable to those obtained from the more accurate ICP tests as shown in Table 1.It can be seen that from the ICP tests the Pt–Ru/CNT electrocatalyst prepared using the modi?ed EG has an atomic ratio (Pt:Ru)of 1:0.93very close to the ideal ratio of 1:1.On the other hand,when using unmod-i?ed EG,the resultant electrocatalyst is overly rich in Pt and has a poor atomic ratio of 1:0.37.Thus,the use of modi?ed EG indeed improved the reduction reaction of ruthenium ion and,in turn,enhanced the Pt–Ru formation on acid-treated CNT.

It is believed that the main contributive factor in promot-ing formation of uniform Pt–Ru nanoparticles comes from

the

Fig.5.Particle size distribution of CNT-supported Pt–Ru prepared at 110?C using the modi?ed EG.The average size is 3.3nm.

synergic effect of EG with the modi?cation agents,i.e.NaHSO 3and Ca(OH)2.Thus,it is necessary to explore the roles of NaHSO 3and Ca(OH)2in the modi?ed reducing agent that lead to such enhanced performances.The former is commonly employed as a reducing agent in preparation of Pt–Ru electro-catalysts [20,21]using carbon black supports with ?ne particle formation and therefore it was adopted into this modi?ed polyol process to prepare Pt–Ru/CNT electrocatalysts.In addition to enhancing the reducing power of EG,SO 3is a stronger ligand than Cl.Thus,it is able to form more stable complex ions with Pt 4+and Ru 3+leading to better dispersion and,in turn,narrower nanoparticle formation.In particular,the use of NaHSO 3sub-stantially promotes the conversion ef?ciency of Ru 3+to Ru metal 2–3times higher than before at a lower pH value of 2–4.This is indeed a signi?cant improvement in reductive preparation of Pt–Ru electrocatalysts supported on CNT using sul?te species as a modi?cation agent for EG.It should be mentioned that at such low pH values the capability of EG in the complexation of metal ions becomes relatively weak.The introduction of NaHSO 3as a modi?cation agent into the reaction solution provides a timely assistance in this respect.Thus,the dual role of NaHSO 3as a second chelating agent and as an enhancing reduction agent is of special importance.

However,the pH of the deposition solution,in fact,did play a key role in the proper formation of Pt–Ru/CNT.Zhao and Gao reported [22]that the zeta potential of multi-walled car-

C.-C.Chien,K.-T.Jeng/Materials Chemistry and Physics99(2006)80–8785 Table1

Comparison of elemental compositions of CNT-supported Pt–Ru prepared using modi?ed and unmodi?ed EG

Reducing agent Composition measured by EDS Composition measured by ICP

Weight percent a(wt.%)Atomic ratio(Pt:Ru)Weight percent(wt.%)Atomic ratio(Pt:Ru) Modi?ed EG Pt(18.97),Ru(8.63)1:0.88Pt(19.43),Ru(9.36)1:0.93

Unmodi?ed EG Pt(9.19),Ru(1.61)1:0.34Pt(10.07),Ru(2.05)1:0.37

a Calculated from the EDS signal intensities relative to those of a20wt.%Pt–10wt.%Ru/C

standard.

https://www.360docs.net/doc/a15083589.html,parison of EDS graphs:(a)Pt–Ru/CNT prepared using modi?ed EG,(b)Pt–Ru/CNT prepared using unmodi?ed EG,and(c)Pt–Ru/C obtained from Johnson Matthey.The use of modi?ed EG resulted in more pronounced Ru signal and better electrocatalyst composition close to that of the commercial product.

bon nanotubes in an ethanol solution is a function of the pH and increases its negative magnitude as the pH value of ethanol solution increases.They also found that the adsorption of a copolymer-based dispersant on the CNT surfaces shifts the zeta potential toward a positive direction.As the zeta potential varies from a positive to a negative value over a pH range,there exists an IEP at a speci?c pH.Since there is no net surface charge at the IEP,the adsorption of different ligand-complexed ions on the CNT surfaces will occur at almost the same speci?c rates.Thus, the composition of the?nal electrocatalyst product will depend mainly on the composition of the starting reactant,which can be readily controlled as desired.We employed this unique char-acteristic of CNT/solution interfaces and were able to make the preparation of electrocatalysts,particularly Pt–Ru,more effec-tive.Thus,the calcium hydroxide aqueous solution was used as a mild pH adjuster or a solution conditioner in conjunction with the use of NaHSO3in the modi?cation of EG.

In this case,the dispersants were the EG and NaHSO3com-plexed Pt and Ru ions.By carefully adjusted the metal deposition solution with a suitable amount of Ca(OH)2aqueous solution to a pH value of about2–4,close to where the IEP takes place and precipitation of undesired Ru salts can be avoided,a suit-able Pt–Ru deposition environment on the CNT surfaces was created.This naturally resulted in signi?cantly improved elec-trocatalyst formation.The use of a dilute NaOH aqueous solution to adjust the pH was also found to bring about similar results in electrocatalyst deposition.From the composition of the pre-pared Pt–Ru/CNT as shown in Table1,it evidences that the deposition of different metals on CNT can be properly regu-lated to almost the same speci?c rates using the modi?ed EG. It also indicates that the pH value where the IEP occurs with adsorption of ligand-complexed Pt and Ru ions on CNT is close to2–4.Thus,with the novel concept of having competitive spe-ci?c adsorption rates of different complexed metal ions around the IEP,the modi?ed EG is also expected to be applicable to preparations of a variety of multi-component electrocatalysts, e.g.Pt–Ru–Ir,Pt–Ru–Rh,Pt–Ru–Ir–Rh,etc.,with desired com-positions,which will have enhanced electrocatalytic activities and poisoning resistances for fuel cell applications.

Using the modi?ed reducing agent,the metal deposition ef?-ciency for Pt–Ru/CNT was signi?cantly improved from about 40to>90%.A loading of10–50wt.%Pt–Ru on CNT with an atomic ratio of close to1:1can be readily obtained.Finally, it should be mentioned that the introduction of water into the modi?ed EG reducing agent through the addition of aqueous solutions of NaHSO3and Ca(OH)2also affected the formation of electrocatalysts.In an earlier study,Zhou et al.[8]found that the amount of water added to EG signi?cantly affects the par-ticle sizes of CNT-supported electrocatalysts.Li et al.[4]also reported that the higher water content in EG,the larger Pt particle size on CNT.The presence of water in EG may favor the forma-tion of complex salts and buffer the Pt–Ru particle formation in a slower rate.Its existence in the EG solution is also helpful for pH adjustment.Therefore,water should be used as an integral part of the additive for the modi?ed EG reducing agent.How-ever,the water content in the modi?ed reducing agent should be properly controlled so as to obtain the best performance and prevent the catalyst particles from aggregation.In this study, a water content of1–2vol.%seemed to give quite satisfactory results with small particle formations.

3.3.Electrocatalytic activity of prepared Pt–Ru/CNT on methanol electrooxidation

It is interesting to know if the prepared Pt–Ru/CNT exhibits better performance with respect to methanol electrooxidation as another evidence for better electrocatalyst formation using the modi?ed reducing agent.Fig.7illustrates the methanol elec-trooxidation currents obtained from cyclic voltammograms with

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Fig.7.Cyclic voltammograms involving methanol electrooxidation on a glassy carbon electrode in0.5M H2SO4solution containing1.0M CH3OH.The elec-trocatalysts used were(a)an in-house prepared Pt–Ru/CNT and(b)a commercial Pt–Ru/C with a catalyst loading of2.5mg cm?2for each.The experiments were carried out at room temperature with a sweep rate of10mV s?1. electrocatalyst?xed on a glassy carbon electrode using5wt.% Na?on solution at a loading of about2.5mg cm?2.It clearly shows better performance for prepared Pt–Ru/CNT than a com-mercial Pt–Ru/C electrocatalyst(20wt.%platinum–10wt.% ruthenium on carbon black)obtained from Johnson Matthey. This can be attributed to several factors,including(a)the pre-pared Pt–Ru/CNT electrocatalyst possesses perfect nanoparticle size and ideal composition that enhances its electrocatalytic activity,(b)the functional groups formed on the CNT surfaces result in high hydrophilicity that creates better electrochemical reaction environments on the electrode,and(c)the high elec-tronic conductivity of the CNT lowers the resistance in methanol electrooxidation.More importantly,this result proves that the use of modi?ed EG reducing agent is effective in preparation of Pt–Ru/CNT with high catalytic activity for methanol electrooxi-dation.Thus,the prepared Pt–Ru/CNT can be applied to DMFCs as an anode electrocatalyst for methanol electrooxidation.

https://www.360docs.net/doc/a15083589.html,parisons on carbon nanotube-supported Pt–Ru electrocatalysts prepared using various reducing agents and electrodeposition method

While there are several feasible reducing agents and prepara-tion methods that can be used to prepare CNT-supported Pt–Ru electrocatalysts,it is interesting to investigate which approach gives the best results.In general,to serve as an electrocatalyst, the CNT-supported Pt–Ru electrocatalyst particle must be uni-formly dispersed with a narrow particle size distribution(e.g. 2–5nm)as a prerequisite in addition to having the right elec-trocatalyst composition.Results obtained on using a variety of reducing agents as well as electrodeposition method are shown in Table2.It can be seen that Pt–Ru/CNT prepared by chem-ical reduction using modi?ed EG are much better than those obtained using unmodi?ed EG taking into accounts on ef?-ciency of catalyst particle formation,particle uniformity and distribution,as well as methanol electrooxidation for Pt–Ru. The enhanced metal deposition and methanol electrooxida-tion performance using the modi?ed EG are clearly caused by the improved reduction of Ru3+to Ru in the modi?ed EG with the aid of modi?cation agents with enhanced reaction conditions.

The use of H2SO3–H2O2followed by hydrogen reduction exhibited a similar effect to that of using the modi?ed EG but only with moderate enhancement in Pt–Ru/CNT formation and poorer results in particle morphology.Formation of undesir-able ruthenium salt precipitates in the process was one of the main concerns.Formic acid and formaldehyde tend to form silver–mirror reactions and are unable to generate desirable par-ticle sizes due to strong reducing powers and lack of suitable selectivities on substrate surfaces.It was also reported[23]that large particle sizes in the range of5–10nm,instead of desired 2–5nm,were obtained for Pt/CNT using HCHO as a reduc-ing agent.The problem of NaBH4,on the other hand,is that its reducing power is too strong leading to formation of large electrocatalyst clusters or agglomerates as illustrated earlier in Fig.4(d).Although Raney nickel has good reducing capability, the resultant Pt–Ru/CNT tends to collapse due to dissolution of nickel in acidic environments.The metal particles obtained by electrodeposition have sizes in the range of micrometers, which are too large for electrocatalyst applications.The control of proper electrocatalyst composition was also a big problem. Judging from these results,the use of modi?ed EG is much more advantageous than those using conventional reducing agents and

Table2

Comparison on preparation results of Pt–Ru/CNT by chemical reduction deposition using various reducing agents and by electrodeposition

Preparation method Deposition capability a Pt–Ru particle morphology b Performance on CH3OH electrooxidation c

Size Density Dispersion

Modi?ed EG

Unmodi?ed EG× ×

H2SO3–H2O2/H2

HCOOH × ××

HCHO × ××

NaBH4 ×

Raney Ni

Electrodeposition × ×

:good, :fair,×:poor.

a Deposition capability:good for>90%,fair for90–50%,and poor for<50%of total catalyst metal precipitated on CNT support.

b Pt–Ru particle morphology is compared to a commercial Pt–Ru/C(20wt.%Pt–10wt.%Ru)electrocatalyst obtained from Johnson Matthey;particle size:good for2–10nm,fair for10–50nm,and poor for>50nm.

c Performance of Pt–Ru on CH3OH electrooxidation is base

d on cyclic voltammetry tests in0.5M H2SO4aqueous solution containing1.0M CH3OH.

C.-C.Chien,K.-T.Jeng/Materials Chemistry and Physics99(2006)80–8787

electrodepsotion method as a good approach for preparation of Pt–Ru/CNT.

4.Conclusions

This study has successfully developed an innovative method for preparation of Pt–Ru/CNT electrocatalysts using an EG-based modi?ed reducing agent.Sodium hydrogen sul?te and calcium hydroxide were found to be effective EG modi?ers, which are particularly helpful for enhanced reduction of com-plexed Pt and Ru ions to form the desired binary Pt–Ru/CNT electrocatalyst.This process depended on the creation of a better metal deposition environment by the modi?ed EG near the IEP on the acid-treated CNT surfaces.The adsorption of different ligand-complexed ions onto CNT surfaces could be properly reg-ulated using the modi?ed EG to have competitive speci?c rates. Satisfactory preparation results were obtained at a pH value of 2–4for the deposition solution and with a reaction temperature of110–140?C,in conjunction with the use of microwave irradia-tion heating,for a substantially shortened reaction time of about 30min.It gave rise to formation of Pt–Ru/CNT with uniform particle dispersion,narrow particle size distribution and desired composition.The metal deposition ef?ciency for Pt–Ru/CNT was signi?cantly increased from about40%to more than90% and the average particle diameter was about3.3nm.In addition, the prepared Pt–Ru/CNT using the modi?ed EG exhibited bet-ter catalytic performance on methanol electrooxidation than that of a commercial product obtained form Johnson Matthey at the same electrocatalyst loading.Overall,the Pt–Ru/CNT prepared using the innovative modi?ed reducing agent gave much bet-ter results than those prepared using common reducing agents and by electrodeposition.These were compared in terms of easiness and completeness of metal deposition,metal particle morphology and composition as well as methanol electrooxida-tion performance.

Acknowledgements

This work is supported by National Science Council(NSC)of Taiwan under National Nanotechnology Development Projects. One of the authors,Dr.K.T.Jeng,also appreciates the?nancial support from Institute of Nuclear Energy Research(INER)for the performance of this work.

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