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Electrochimica Acta 121 (2014) 421– 427

Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

in oxide - mesoporous carbon composites as platinum catalystupports for ethanol oxidation and oxygen reduction

d Ariful Hoque, Drew C. Higgins, Fathy M. Hassan, Ja-Yeon Choi,ark D. Pritzker, Zhongwei Chen ∗

epartment of Chemical Engineering, Waterloo Institute for Nanotechnology, Waterloo Institute of Sustainable Energy, University of Waterloo, 200niversity Ave. W, Waterloo, ON, N2L 3G1, Canada

r t i c l e i n f o

rticle history:eceived 7 November 2013eceived in revised form0 December 2013ccepted 12 December 2013vailable online 27 December 2013

a b s t r a c t

Unique tin oxide-mesoporous carbon (SnO2-CMK-3) composites have been synthesized as platinumnanoparticle electrocatalyst supports for low temperature fuel cell applications. In comparison withstate-of-the-art commercial carbon-supported platinum (Pt/C) and pure CMK-3-supported platinum(Pt/CMK-3), Pt/SnO2-CMK3 demonstrated improved Pt-mass and surface area based ethanol oxidationreaction (EOR) activity through half-cell electrochemical investigations, providing a 64.7 and 97.6 mVreduction in overpotential at 100 mA mg−1

Pt upon comparison to Pt/CMK-3 and commercial Pt/C. Further-

eywords:etal oxide-carbon composite

uel cellthanol oxidation reactionxygen reduction reaction

more, improvements to the oxygen reduction reaction (ORR) kinetics were observed, with Pt/SnO2-CMK3providing a kinetic current density of 3.40 mAcm−2 at an electrode potential of 0.9 V vs RHE. The improvedperformance of Pt/SnO2-CMK-3 for EOR and ORR was attributed to the beneficial impact of the supportproperties, along with potential interactions occurring between the support and catalyst particles. Com-plemented by extensive physicochemical characterization, these unique materials show high promise

pera

esoporous carbon for application in low tem

. Introduction

Proton exchange membrane fuel cells (PEMFCs) are considered promising alternative energy technology in order to decrease thextensive reliance on depleting global fossil fuel reserves. These lowemperature fuel cells, including direct ethanol fuel cells (DEFCs)nd hydrogen fuel cells, can efficiently convert chemical energy intolectrical energy with minimal process emissions and environmen-al benignity. Moreover, ethanol is a promising fuel alternative dueo its low toxicity and, abundant availability, along with its easierroduction from renewable sources [1]. The performance and mar-etability of PEMFCs can be improved by developing inexpensive,igh performance and durable component materials [2]. In partic-lar, electrocatalysts utilized to facilitate the electrode reactions

ncluding the cathodic oxygen reduction reaction (ORR) and thenodic ethanol oxidation reaction (EOR) significantly affect the per-ormance of PEMFCs. Developing high performance, inexpensivelectrocatalyst materials is of high importance in order to realizehe full potential of these systems.

Traditionally, platinum (Pt) or a Pt-based alloy (PtM) is theEMFC catalyst of choice due to its good catalytic activity andhemical stability. Unfortunately, due to the high cost and limited

∗ Corresponding author. Tel.: +1519 888 4567 ext 38664.E-mail address: zhwchen@uwaterloo.ca (Z. Chen).

013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.12.075

ture fuel cells.© 2014 Elsevier Ltd. All rights reserved.

global availability of this precious metal, careful electrocatalystdesign strategies must be applied in order to reduce the amountof platinum used in these materials. To this end, researchers haveinvestigated: i) Pt/PtM with deliberately controlled nanostructuressuch as nanotubes or nanowires [3,4] or ii) unique electrocata-lyst supports [5]. In the latter case, it has been well establishedthat the particular properties of the Pt-based catalyst support canhave a direct impact on the overall activity and stability. Thus, itis possible to tailor PEMFC performance by developing novel cat-alyst support materials with favorable physicochemical propertiesand catalyst-support interactions. Recently, much effort has beendevoted to developing novel catalyst supports, including nano-structured carbons (such as carbon nanotubes, carbon nano-fibersand mesoporous carbon) [6,7] and, transition metal oxides, carbidesand nitrides [8,9].

In particular, mesoporous carbon materials such as CMK-3 havebeen extensively studied as catalyst supports for PEMFCs due totheir unique structural properties including large surface areaswith mono-dispersed and well-ordered three-dimensional inter-connected mesopores. The excellent performance of mesoporouscarbon-supported Pt and Pt alloy catalysts towards PEMFC elec-trode reactions has been attributed to the good dispersion of

uniformly sized catalyst particles, high electrical conductivity andenhanced mass transfer throughout the catalyst layer due to thespecific pore network structure [10]. Recently, tin oxide (SnO2)-based supports have also attracted significant interest as electrode

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aterials [11] since these materials have been shown to be capa-le of enhancing reaction kinetics and improving contaminantolerance during operation. Particularly, SnO2-containing electro-atalysts can effectively split the C-C bond in ethanol at roomemperature in acidic solutions, facilitating the EOR at low poten-ials to form CO2 products as SnO2 modifies the electronic structuref metal sites. Moreover, Ota et al. [12] recently found that SnO2nfluenced the oxidation and reduction electrocatalytic behaviorf Pt by suppressing the adsorption/desorption of oxygen on plat-num and the formation/reduction of platinum oxides above 0.6 VRHE). The authors attributed this to the strong catalyst–supportnteractions between SnO2 and Pt, which might make Pt more resis-ant to corrosion. This favourable interaction was also expected toncrease the catalytic activity of these materials towards the oxygeneduction reaction.

In the present study, mesoporous CMK-3 was modified withnO2 to make composite SnO2-CMK-3 supports in order to combinehe benefits of each individual constituent previously highlighted.hese composites were used as supports for platinum nanopar-icles deposited by using ethylene glycol (EG) reduction method.his technique produced small, uniformly sized and well-dispersedlatinum catalyst nanoparticles which demonstrated favourableerformance towards the EOR and the ORR on the basis of half-celllectrochemical investigations. The physical properties of the cat-lysts were characterized by scanning electron microscopy (SEM),nergy dispersive x-ray analysis (EDX), x-ray diffraction (XRD) andransmission electron microscopy (TEM).

. Materials and Methods

.1. Synthesis of SBA-15

SBA-15 was synthesized from tetraethyl orthosilicate (TEOS,8%) as a silica source and pluronic® P123 (EO23PO70EO23) as aurfactant using a previously established procedure [13]. Briefly,.2 g of Pluronic® P123 were added to 60 mL of 2 M HCl solutionnd sonicated until P123 was completely dissolved. Then, 4.2 g ofEOS were added and the resulting mixture was stirred vigorouslyor 8 minutes. This solution was maintained at 38 ◦C for 24 hrs andubsequently at 100 ◦C for 24 hrs without stirring. The suspensionas then transferred into an autoclave and heated to 100 ◦C for

4 hrs. Finally, the solid product was collected by centrifugation,ashed and oven-dried overnight at 100 ◦C. The resulting powderas calcined at 550 ◦C in air for 6 hrs to obtain the SBA-15 product.

.2. Synthesis of CMK-3

Mesoporous carbon CMK-3 was prepared with sucrose as thearbon source, sodium hydroxide (NaOH), ethanol and sulfuric acidH2SO4, 98%) using a procedure described previously [14]. Briefly,

g of SBA-15 was added to a solution containing 1.25 g sucrose and.14 mL H2SO4 dissolved in 5 mL H2O. After being sonicated for 1r, the resulting mixture was dried in an oven at 100 ◦C for 6 hrsnd subsequently dried for another 6 hrs at 160 ◦C. Then, this solidontaining the partially carbonized organic precursor was addedo an aqueous solution consisting of 0.8 g sucrose, 0.09 mL H2SO4nd 5 mL H2O. Once again, after sonication for 1 hr, this resultingixture was dried in an oven at 100 ◦C and for 6 more hrs at 160 ◦C.

he solid was then carbonized at 900 ◦C under inert gas (Ar) for hrs. Finally, the carbon-silica composite so obtained was washed

ith 1 M NaOH in an ethanol-H2O solution (50:50 v/v) twice at

0 ◦C in order to dissolve the silica template. After silica removal,he carbon sample was washed with ethanol until its pH reached 7nd then dried at 100 ◦C for 4 hrs in the oven.

Acta 121 (2014) 421– 427

2.3. Synthesis of SnO2-CMK-3

Initially, approximately 20 mL of 12 M HCl acid were poured intoa beaker containing the as-synthesized mesoporous carbon CMK-3and the contents were washed, filtered and dried in order to ensureremoval of impurities from CMK-3. Then, 100 mg CMK-3 were dis-persed in 250 mL DI H2O and 3 mL of 12 M HCl. Simultaneously, asolution was prepared by mixing 100 mg SnCl2.2H2O and 100 mLDI H2O and then added dropwise into the CMK-3 suspension whilebeing agitated with a magnetic stirrer. The magnetic stirring wasmaintained for 5 hrs at room temperature. Finally, the resultingSnO2-CMK-3 composite was filtered, washed and dried for 24 hrs at100 ◦C. After drying, the solid was annealed at 400 ◦C in the presenceof Ar to obtain the final SnO2-CMK-3 product.

2.4. Pt deposition on CMK-3 and SnO2-CMK-3

Pt deposition on pure CMK-3 and SnO2-CMK-3 composite sup-ports was carried out using the EG method [15,16]. For this method,20 mg CMK-3 or SnO2-CMK-3 composite were suspended in 15 mLEG and sonicated for 30 minutes to ensure the support materialswere well dispersed in the solvent. Subsequently, 1 mL hexachloro-platic (H2PtCl6.2H2O) acid in EG solution (5 mg Pt per mL EGsolution) corresponding to a platinum loading of 20 wt. % was addeddropwise and the contents were stirred for 15 minutes. Followingthis step, a few drops of 2.5 M NaOH solution in EG were addedto increase the solution pH above 9. The entire solution was thensubjected to reflux at 140 ◦C for 3 hrs to ensure complete Pt precur-sor reduction. After cooling, several drops of 2 M HCl were addedto reduce the pH of the solution to approximately 6. Finally, theresulting suspension was filtered to yield a solid product that wasthen washed thoroughly with DI H2O and dried overnight in theoven at 70 ◦C.

2.5. Physicochemical characterization

The structures of the SnO2-CMK-3 composite support and theprepared catalysts were characterized using SEM (JEOL JAX-840)operating at 10 kV and equipped with EDX for compositional analy-sis. XRD analysis was conducted using monochromatic Cu K� x-rays(0.154 nm wavelength) and an Inel XRG 3000 diffractometer. Themorphology of the catalysts was analyzed by TEM (JEOL 2010F).

2.6. Electrochemical measurements

The electrochemical measurements were conducted with aPINE instrument AFCBPI biopotentiostat and a conventional three-compartment electrochemical cell. A Pt wire and Ag/AgCl electrodewere used as counter and reference electrodes, respectively. Allpotentials reported herein have been converted to the reversiblehydrogen electrode (RHE) scale. The working electrode was a com-mercial glassy carbon rotating disk electrode having an apparentsurface area of 0.19635 cm2. The working electrode was prepared asfollows. Initially, 2 mg of the catalyst was dispersed ultrasonicallyin 950 �L isopropanol solution containing 50 �L Nafion solution(5 wt% Aldrich Co., USA). Then, 10 �L of the above solution wasdeposited onto the glassy carbon electrode and allowed to dry.The Pt mass loading calculated for each catalyst was 20 �g/cm2.Also, 20 wt. % platinum nanoparticles supported on carbon black(Pt/C, BASF) was used to compare the electrochemical activity ofthe as prepared catalysts. EOR was assessed by conducting a linearpotential scan between 0 to 1.5 V vs. RHE at a scan rate 10 mVs−1

in 0.5 M H2SO4 containing 0.5 M ethanol under nitrogen satura-tion. Chronoamperometry (CA) experiments were conducted in thesame electrolyte at an electrode potential of 0.84 V vs. RHE. Cyclicvoltammetry (CV) and ORR investigations were carried out in a

M.A. Hoque et al. / Electrochimica Acta 121 (2014) 421– 427 423

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ig. 1. SEM images of (A) SnO2-CMK-3 (B) Pt/CMK-3 and (C) Pt/SnO2-CMK-3 (inset-E

.1 M HClO4 solution. Prior to each test, the electrode was cycledeveral times between 0.025 and 1.2 V to produce clean surfacesnd activate the catalysts. The CVs were obtained over a poten-ial range from 0.025 to 1.2 V vs. RHE at a scan rate of 50 mVs−1

nder nitrogen saturation and quiescent conditions. The ORR testsnvolved linear potential sweeps at a 5 mVs−1 scan rate between

and 1.2 V vs. RHE under O2 saturation and at electrode rota-ion speeds of 100, 400, 900 and 1600 rpm. Background currentsbtained under nitrogen saturation were removed to eliminateapacitance and redox contributions.

. Results and discussion

.1. Physicochemical characterization

Characterization of CMK-3, possessing a well ordered, meso-orous structure with a BET surface area of 1556 m2g−1 and averageore size of 3.2 nm is provided in our previous work [17]. Examina-ion of the surface morphologies of SnO2-CMK-3, Pt/CMK-3 andt/SnO2-CMK-3 by SEM at 10 keV yielded typical micrographshown in Fig. 1 (A), (B) and (C), respectively. Fig. 1 (A) presentshe general features of CMK-3-SnO2 that contains microsized par-icles similar to that of SnO2-free CMK-3 materials, indicating thathe incorporation of SnO2 does not have a significant impact onhe overall morphology. Pt particles are not visible on the surfacesf CMK-3 and CMK-3-SnO2 supports in Fig. 1 (B) and (C) due toheir small nanoparticle size. The elemental constituents of theeveloped electrocatalysts were determined by energy dispersive-ray (EDX). The resulting spectrum displayed in Fig. 1 (C-inserted)learly indicates the presence of C, O, Pt and Sn in the Pt/SnO2-CMK-

material.The XRD diffractograms of SnO2-CMK-3 support, Pt/SnO2-CMK-

, Pt/CMK-3 and Pt/C catalysts are shown in Fig. 1 (D). The patternsor SnO2-CMK-3 and Pt/SnO2-CMK-3 confirm the presence of annO2 phase in these composites [18–20]. The peak observed ata. 25o in all catalysts is attributed to the (002) graphitic planeeflection of carbon. The diffraction peak observed at ca. 39.5o in

he platinum-containing catalysts corresponds to the face-centeredubic phase of Pt (111) [21]. Due to the reduction conditions whichroduce a very small and highly disperse Pt, the other peaks cor-esponding to Pt i.e. Pt (200), Pt (220) and Pt (311), are difficult

) XRD diffractograms of SnO2-CMK-3, Pt/C, Pt/CMK-3 and Pt/SnO2-CMK-3 catalysts.

to distinguish in the Pt/CMK-3 and Pt/SnO2-CMK-3 electrocata-lysts. The mean particle size of the different catalyst materials hasbeen calculated from the Pt (111) peak by applying the Scherrerequation:

d = k�

B cos(1)

Here, d is the average nanoparticle size (nm), k is the shape factor(0.89), � is the x-ray wavelength (0.154 nm), B is the full-width-at-half-maximum (FWHM) of the peak (rad) and � is the Bragg angle indegrees. From this expression, the average platinum nanoparticlesizes based on the Pt (111) peak of Pt/C, Pt/CMK-3 and Pt/SnO2-CMK-3 are estimated to be 3.11, 2.01 and 2.13 nm, respectively.

TEM images and the corresponding particle size distributionsare presented in Fig. 2 for Pt/C (A and B), Pt/CMK-3 (C andD) and Pt/SnO2-CMK-3 (E and F). It can be seen that the plat-inum nanoparticles are homogeneously deposited onto the CMK-3and SnO2-CMK-3 supports with uniform nanoparticle sizes. Basedon hundreds of particle measurements, the average platinumnanoparticle sizes of Pt/C, Pt/CMK-3 and Pt/SnO2-CMK-3 weredetermined to be 3.1 ± 0.6, 2.05 ± 0.5 and 2.1 ± 0.9 nm, respectively.These results are in good agreement with the average nanoparti-cle sizes calculated from XRD results. These results confirm that theCMK-3 and SnO2 modified CMK-3 supports have favourable surfaceproperties to facilitate the deposition of homogenously dispersed,uniformly sized platinum nanoparticles. The small particle sizesand excellent dispersion can most likely be attributed to the highsurface area of the CMK-3 supports, along with a high degree ofmesoporosity and minimal micropore content [22].

3.2. Electrochemically active surface area

Fig. 3 (A) presents steady state CV curves collected for Pt/C,Pt/CMK-3 and Pt/SnO2-CMK-3 collected after electrode activation.From each voltammogram, it is possible to calculate the electro-chemically active surface area (ECSA) using the following equation[23]:

ECSA = Q

0.21 × L(2)

Q denotes the charge density for hydrogen adsorption(mC cm−2) that can be determined by integrating the hydrogen

424 M.A. Hoque et al. / Electrochimica Acta 121 (2014) 421– 427

Fig. 2. TEM images and the corresponding platinum nanoparticle size distribution diagrams of Pt/C (A and B), Pt/CMK-3 (C and D) and Pt/SnO2-CMK-3 (E and F).

Fig. 3. (A) Cyclic voltammograms for Pt/C, Pt/CMK-3 and Pt/SnO2-CMK-3 catalysts in 0.1 M HClO4 at a scan rate of 50 mVs−1. Linear potential scans for ethanol oxidationusing Pt/C, Pt/CMK-3 and Pt/SnO2-CMK-3 catalysts expressed on the basis of (B) unit Pt mass and (C) unit Pt specific surface area in 0.5 M H2SO4 + 0.5 M CH3CH2OH solutionsat a scan rate of 10 mVs−1 (D) Transient current density curves for ethanol oxidation obtained at 0.84 V vs. RHE.

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Fig. 4. (A) Potential scans showing the ORR activities of Pt/C, Pt/CMK-3 and Pt/SnO2-CMK-3 catalysts obtained at a 5 mVs−1 scan rate and 1600 rpm rotation speed inan O2-saturated 0.1 M HClO4 solution, (B) ORR activities of Pt/SnO2-CMK-3 at the

M.A. Hoque et al. / Electroch

esorption region and dividing by the potential scan rate, Lepresents the mass loading of Pt on the electrode (mg cm−2)nd the factor 0.21 (mC cm−2) is due to the charge requiredo reduce a monolayer of hydrogen on Pt. ECSA values of 48.4,2.4 and 53.2 m2 g−1Pt are calculated for Pt/C, Pt/CMK-3 andt/SnO2-CMK-3, respectively. The increase in ECSA for Pt/CMK-3nd Pt/SnO2-CMK-3 can be attributed to smaller average plat-num nanoparticle sizes, along with improved accessibility due tohe ordered pore structure of the supports [24]. The CV curves oft/CMK-3 and Pt/SnO2-CMK-3 also show higher capacitive currentshan the conventional Pt/C catalyst. It is well known that electro-hemical double-layer capacitance is largely affected by the specificrea, pore structures and surface activity of the support materials.rdered mesoporous CMK-3 possess very high surface area, uni-

orm pore size and high pore volume that are responsible for thebserved increase in the double-layer capacitance.

.3. Ethanol oxidation reaction

The catalytic activities of the Pt/CMK-3 and Pt/SnO2-CMK-3aterials for the EOR are compared to that of state-of-the-art com-ercial Pt/C on the basis of unit mass platinum in Fig. 3 (B) and

n specific surface area platinum in Fig. 3 (C). The plots comparehe variation in the current densities for ethanol oxidation withotential in the presence of the various catalysts. For example,t a mass-based current density of 100 mA mg−1Pt, the potentialn the presence of Pt/SnO2-CMK-3 is shifted negatively by 64.7nd 97.6 mV in comparison to that measured for Pt/CMK-3 andt/C catalysts, respectively. This improvement in performance fort/SnO2-CMK-3 is consistent over the entire range of potentialsnvestigated in the scan. Pt/SnO2-CMK-3 also showed an enhancedctivity on the basis of surface area, with a negative potential shift of3.7 and 82.9 mV at a specific current density of 0.15 mA cm−2Pt inomparison to that measured on the Pt/CMK-3 and Pt/C catalysts,espectively. The results clearly highlight the enhanced catalyticctivities of Pt/SnO2-CMK-3 towards the EOR.

Fig. 3 (D) presents the data from chronoamperometry exper-ments conducted at a potential of 0.84 V vs. RHE in a solutionontaining 0.5 M H2SO4 and 0.5 M ethanol under nitrogen satura-ion to investigate the stability of the three catalysts during ethanolxidation. In all three cases, the current densities drop over time,ut eventually stabilize. However, the highest stable current den-ity is obtained for the Pt/SnO2-CMK-3 catalyst. The current densityf 50.5 mA mg−1Pt obtained with this catalyst after 1000s is about.8 times higher than the value of 27.5 mA mg−1Pt for the Pt/CMK-3atalyst and 3.1 times higher than the value of 16.1 mA mg−1Pt onhe Pt/C catalyst.

Clearly, activity enhancements are observed when usingrdered mesoporous CMK-3 based supports in comparison to com-ercial Pt/C, whereby it is well known that the physical properties

f the catalyst support materials will have a strong impact on theesultant electrochemical performance. These results are interest-ng, considering that the average platinum nanoparticle sizes fort/CMK-3 and Pt/SnO2-CMK-3 are ca. 30% smaller than those oft/C. Generally larger ensembles of platinum are required in ordero adsorb and oxidize ethanol, with poisonous intermediates andy products bond much stronger on smaller nanoparticles due tohe presence of low-coordinated surface atoms [25,26], therebylocking EOR active sites. The beneficial impact of CMK-3 basedupports is thus apparent, most likely due to the highly mesoporoustructure that can facilitate mass transport through the catalystayer to the active platinum nanoparticles. These findings are sup-

orted by Song et al., 22 who highlighted significant improvements

n electrochemical activity when using high surface area carbonupports with well ordered, mesoporous structures (i.e. CMK-3).his improvement was most prolific in the case of liquid based

same conditions as above for rotation speeds of 100, 400, 900 and 1600 rpm, inset:Koutecky-Levich plots of three different catalysts at various rotation speeds.

reactants (ethanol) owing to the ease of mass transport to catalystparticles, thereby increasing the overall utilization and activity, andoffering reasonable explanation for the enhancements observed inthe present study.

The beneficial impact of SnO2 incorporation is also apparent,evidenced by the marked increase in EOR activity for Pt/SnO2-CMK-3 in comparison to Pt/CMK-3. It has been established thatthe presence of Sn or SnO2 species can provide EOR activityenhancements [27,28]. The exact nature of the effect of SnO2species remains elusive; however is commonly attributed to twodifferent effects. The first involves variations of the electronicproperties of the platinum nanoparticles due to the charge trans-fer occurring between two different metallic species in contact[28]. These changes can modulate the adsorption and catalyticproperties of the platinum nanoparticles, thereby influencingEOR activity. The second proposed mechanism of performanceenhancement is attributed to a synergistic effect arising due tothe presence of adsorbed hydroxide species provided by SnO2.These species can react with, or remove adsorbed intermedi-ate and byproduct contaminants [29], exposing fresh catalyticsites for the EOR. While the contributions of these effects in thepresent materials remains unknown, the performance enhance-ments provided by SnO2-CMK-3 supports are compelling, andelucidating the nature of the support impact will be a focus of futureinvestigations.

3.4. Oxygen Reduction Reaction (ORR)

Fig. 4 (A) compares the ORR activities of Pt/C, Pt/CMK-3

and Pt/SnO2-CMK-3 catalysts at an electrode rotation speed of1600 rpm. Among the catalysts investigated, Pt/SnO2-CMK-3showed the highest ORR activity, followed by Pt/CMK-3 whichshowed slightly improved performance than commercial Pt/C.

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articularly, the kinetic current densities at a potential of 0.9 Vs RHE are 1.80, 2.10 and 3.40 mA cm−2 for Pt/C, Pt/CMK-3,t/SnO2-CMK-3, respectively. Fig. 4 (B) shows the ORR activityeasurements obtained for Pt/SnO2-CMK-3 when rotating the

isk electrode at speeds of 100, 400, 900 and 1600 rpm. Thisata was used to assemble Koutecky-Levich plots for Pt/SnO2-MK-3 which is shown in Fig. 4 (B-inset) along with Pt/CMK-3nd Pt/C for comparison. These plots clearly indicates thathe ORR follows a predominantly 4-electron pathway, with aetailed description regarding the calculation of the overall elec-ron transfer number for the ORR described previously in theiterature [30].

Most notably, a significant shift in onset potential (ca. 26 mV)or Pt/SnO2-CMK-3 was observed in comparison with Pt/CMK-3nd Pt/C. Shao et al. [31] recently highlighted that the mass activityf platinum catalysts was highest for nanoparticle sizes of 2.2 nmhich can potentially contribute to the high activity observederein (ca. 2.1 nm particle size for Pt/SnO2-CMK-3), although doesot explain the significant shift in activity upon comparison tot/CMK-3 with similar particle size. This leads us to believe that theresence of SnO2 provides a beneficial impact, most likely modu-

ating the electronic and adsorption properties of platinum in ordero impart improved ORR activity. These improvements are consis-ent with results of previous investigations [32]. Furthermore, inur previous work, we highlighted that SnO2 could be embeddednto the pores of CMK-3, with some deposition on the outer surface17]. In the present work, the amount of Sn precursor utilized waseduced significantly, whereby based on the electrochemical dataresented, the presence of SnO2 did not hamper the 3D microstruc-ure of CMK-3 and could furthermore enhance the reaction kinetics.t has also been reported that the migration and coalescence oflatinum particles can be mitigated on SnO2-containing catalystupports [33] that could potentially provide important stabilitynhancements. Upon comparison of Pt/CMK-3 with Pt/C, it is clearhat an enhancement in ORR current is observed in the mixedinetic-diffusion control potential region. This could likely ariseue to the high surface area and ordered mesoporous structure ofMK-3 that can facilitate effective reactant access and enhance theRR kinetics in this potential range. This leads to the conclusion

hat the ordered mesoporous structure of CMK-3 coupled with theeneficial Pt-SnO2 interactions can improve the ORR performancef Pt/SnO2-CMK3 catalyst materials.

. Conclusions

SnO2-CMK-3 composites were developed as platinum nanopar-icle supports for PEMFC applications. Platinum nanoparticles wereeposited by the EG method which was found to produce small (ca..1 nm), uniformly sized and well-dispersed catalyst nanoparticles.he structural and physical properties of these unique catalyst sup-orts were found to result in excellent activity towards the EORnd ORR. Specifically, for mass based EOR activity, at a current of00 mAmgPt

−1 a negative potential shift of 97.6 mV was observedor Pt/SnO2-CMK-3 in comparison with commercial state-of-the-rt Pt/C catalyst. Moreover, at an EOR specific-activity based currentensity of 0.15 mAcm−2, a negative potential shift of 82.9 mV wasbserved for Pt/SnO2-CMK-3 in comparison with Pt/C, indicat-ng the practicality of this catalyst for fuel cell applications. Inerms of ORR activity, Pt/SnO2-CMK-3 showed a 1.9 times higherinetic current density at 0.9 V vs RHE compared to state-of-the-rt Pt/C catalyst. While elucidation of the exact nature of the

atalyst-support interactions and stability testing are still undernvestigation, the reported SnO2-CMK-3 composites are presenteds promising catalyst support materials for low temperature fuelells.

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Acta 121 (2014) 421– 427

Acknowledgement

This work was funded by the Natural Sciences and Engineer-ing Research Council of Canada (NSERC), the Waterloo Institute forNanotechnology and the University of Waterloo. TEM images wereobtained at the Canadian Center for Electron Microscopy (CCEM)located at McMaster University.

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