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Electrochimica Acta 56 (2011) 7610– 7614

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

lectrochemical oxidation of ethylene glycol on Pt-based catalysts in alkalineolutions and quantitative analysis of intermediate products

ohei Miyazakia,∗, Tomoki Matsumiyab, Takeshi Abea, Hiroki Kuratac, Tomokazu Fukutsukaa,azuo Kojimab, Zempachi Ogumia

Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, JapanGraduate School of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, JapanInstitute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

r t i c l e i n f o

rticle history:eceived 3 March 2011eceived in revised form 12 May 2011ccepted 25 June 2011vailable online 2 July 2011

a b s t r a c t

Electrocatalytic activities of Pt/C, Pt-Ru/C, and Pt-Ni/C for the oxidation of ethylene glycol in a basicsolution are evaluated by cyclic voltammetry and quasi-steady state polarization. Based on the resultsof Tafel slopes from quasi-steady state polarization, the catalytic activities for ethylene glycol oxidationare in the order of Pt-Ru/C > Pt-Ni/C > Pt/C. The analysis of intermediate products for ethylene glycoloxidation by higher performance liquid chromatograph (HPLC) demonstrates that the degree of ethylene

eywords:nion exchange membrane fuel cellst-based alloy catalyststhylene glycolirect alcohol

glycol oxidation is dependent on catalysts. Pt-Ru/C shows the highest current densities for ethylene glycoloxidation, but shows lower fuel utilization. On the other hand, Pt-Ni/C shows higher ability to cleavageC–C bonds, but is suffered from catalyst poisoning. To improve the tolerance for catalyst poisoning, weconstruct a novel Pt-Ni-SnO2/C catalyst, compare its catalytic activities, and evaluate the intermediates.Pt-Ni-SnO2/C shows superior catalytic activities for ethylene glycol oxidation, resulting in the highestdegree of complete electro-oxidation of ethylene glycol to CO2.

. Introduction

Direct oxidation of alcohols in polymer electrolyte fuel cells haseceived much attention over the last years. These direct alcoholuel cells (DAFCs) are expected as a potential alternative powerource for portable electric devices, since DAFCs have some advan-ages in terms of energy densities and easy transportation andandling, as compared with hydrogen fuel. Among direct alcohol

uels, methanol has been most widely studied. However, in an acidedium, there is an obstacle of sluggish kinetics in methanol oxi-

ation [1,2]. Therefore, alkaline direct methanol fuel cells, in whichethanol oxidation proceeds under basic conditions, have been

ntensely studied to realize fast kinetics of methanol oxidation.Our group has been focusing on ethylene glycol as a direct fuel

n anion-exchange membrane fuel cells (AEMFCs). Ethylene glycolas superior energy density (7.56 kWh dm−3) and higher boilingoint (471 K) than some typical alcohol fuels such as methanol andthanol [3]. The oxidation of ethylene glycol in alkaline medium

s faster than that in acid medium. And, surprisingly, ethylene gly-ol provides larger oxidation currents than methanol and polyolsuch as glycerol, erythritol, and xylitol in basic solutions [4]. Thus,

∗ Corresponding author. Fax: +81 75 383 2488.E-mail address: myzkohei@elech.kuic.kyoto-u.ac.jp (K. Miyazaki).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.06.078

© 2011 Elsevier Ltd. All rights reserved.

ethylene glycol is a promising fuel for AEMFCs, which overcomes aconventional direct methanol fuel cell (DMFC).

Ethylene glycol has the above-mentioned advantageous fea-tures as DAFCs’ fuel, but it has a crucial problem derived fromits inherent molecular structure. Ethylene glycol is a C2 moleculehaving a C–C bond in its structure. C–C bond is a stumbling blockfor electro-organic chemists since thus C–C bond is comparativelystrong and cannot be easily cleaved [5]. As well as ethanol oxida-tion, ordinal Pt catalyst cannot achieve the complete oxidation ofethylene glycol (as shown in Eq. (1)), and leaves some intermediateproducts (Scheme 1):

(CH2OH)2 + 10OH− → 2CO2 + 8H2O + 10e− (1)

Therefore, in order to increase the efficiency of fuel utilization,active catalysts, having sufficient ability to break C–C bond in ethy-lene glycol, are strongly required. As for ethanol oxidation, Adzicand co-workers [6] reported that Pt-Rh-SnO2 catalyst oxidizedethanol and facilitated its oxidation to CO2 at lower potentials.However, to our best knowledge, there is no available literatureconcerning the oxidation of ethylene glycol to CO2 from an electro-

catalytical view point.

Here we report an effective C–C bond cleavage electrocatalystfor ethylene glycol, and discuss the intermediate products throughethylene glycol oxidation using HPLC.

dx.doi.org/10.1016/j.electacta.2011.06.078http://www.sciencedirect.com/science/journal/00134686http://www.elsevier.com/locate/electactamailto:myzkohei@elech.kuic.kyoto-u.ac.jpdx.doi.org/10.1016/j.electacta.2011.06.078

K. Miyazaki et al. / Electrochimica Acta 56 (2011) 7610– 7614 7611

2CO32−

CH2OH

CH2OHEthyelene

glycol

CHO

CH2OHGlyco l

aldehyd e

CHO

CHO

COO−

CH2OHGlycolate

COO−

CHOGlyoxylate

COO−

COO−

Oxalate

3OH−

2H2O + 2e−

2OH−

2H2O + 2e−

2OH−

2H2O + 2e−

2OH−

2H2O + 2e-

3OH−

2H2O + 2e−

3OH−

2H2O + 2e−

4OH−

2H2O + 2e−

tion of ethylene glycol in an alkaline solution.

2

2

(wub(GAalSsntpf

2

pCbserttadclf

sw((atgimp

0.0 0.2 0. 4 0.6 0.8

0

40

80

120

160

200

Cur

rent

den

sity

/ m

A c

m-2

Poten tia l / V (vs. RHE)

Pt/ C Pt-Ru /C Pt-Ni/C

-1.0 -0. 5 0. 0 0.5 1. 0 1.5

350

400

450

500

550

600

Pot

entia

l / m

V (v

s. R

HE

) Pt /C Pt- Ru/ C Pt-Ni/ C

(a)

(b)

Glyoxal

Scheme 1. Stepwise process of the oxida

. Experimental

.1. Catalysts preparation and characterization

Carbon-supported Pt, Pt-Ru (1:1), and Pt-Ni (1:1) alloy catalystsPt/C, Pt-Ru/C, and Pt-Ni/C, respectively), purchased from E-TEKith metal loading of 40 wt%, were used as received. The proceduresed in preparing SnO2 nanoparticles was similar to that describedy Jiang et al. [7]. In a mixture of 50 mL ethylene glycol and water1:50 by mole), 100 mg of hydrochloric acid (35%, Nacalai Tesque,R) and 200 mg of SnCl2·2H2O (Nacalai Tesque, GR) were dissolved.fter the solution was fluxed at 463 K for 50 min, we obtained

SnO2 colloid solution, whose clear color turned into slight yel-ow. Catalyst powder of 50 mg Pt-Ni/C was added into 10.8 g ofnO2 colloid solution (SnO2: 3.15 mg mL−1), and this mixture wastirred for 30 min and further sonicated for 30 min to deposit SnO2anoparticles on Pt-Ni/C. X-ray diffraction (Rigaku, RINT-2500),ransmission electron microscopy (Hitachi, H-9000NAR), and X-rayhotoelectron spectroscopy (ULVAC-PHI Model 5500) were usedor characterizing Pt-Ni-SnO2 catalysts.

.2. Electrochemical measurements and HPLC analysis

Evaluations of electrochemical activities of each catalyst wereerformed by using catalyst-coating glassy carbon disk electrodes.atalysts in 1-hexanol solutions were deposited on a glassy car-on disk electrode (Ø 6 mm) and fixed with Nafion ionomerolutions. Metal loading was uniformed at 25.5 �g cm−2. A three-lectrode electrochemical cell was equipped with Pt wire andeversible hydrogen electrode (RHE) as counter and reference elec-rodes, respectively. All potentials were referred to RHE throughouthis study. An aqueous electrolyte solution of 1 mol dm−3 KOHnd 1 mol dm−3 ethylene glycol was deaerated by Ar bubbling. Aisk electrode was rotated at 900 rpm in cyclic voltammetry andhronoamperometry to achieve the steady-state diffusion of ethy-ene glycol solutions. We defined currents after 800 s beginningrom a potential step as quasi-steady state currents.

In order to analyze the intermediate compounds, HPLC analy-is of electrolyte solutions was performed. A three-electrode cellas used. Catalyst ink, which contains catalyst, Nafion ionomer

5 wt% Nafion, Aldrich), and ethanol, was coated on carbon clothElectrochem, EC-CC1-060) to serve as a working electrode. Pt wirend RHE were used as counter and reference electrodes, respec-ively. A solution of 1 mol dm−3 KOH and 0.5 mol dm−3 ethylene

lycol was used. Potential holding was conducted at 0.50 V to obtainntermediates during the electro-oxidation of ethylene glycol. The

ethod and a set of equipments for HPLC analysis were describedreviously [8,9].

log (i / mA c m-2)

Fig. 1. (a) Cyclic voltammograms and (b) Tafel plots on Pt/C, Pt-Ru/C, and Pt-Ni/Ccatalysts in an aqueous solution of 1 mol dm−3 KOH and 1 mol dm−3 ethylene glycol.

7 imica Acta 56 (2011) 7610– 7614

3

3

ocpoPoiotafipC

cFa

Pt/ C Pt-Ru/ C Pt- Ni/C0

20

40

60

80

100

Cur

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effi

cien

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%

Glycolate Format e Ot hers

F[

612 K. Miyazaki et al. / Electroch

. Results and discussion

.1. Ethylene glycol oxidation on Pt-based catalysts

Fig. 1a shows cyclic voltammograms (CVs) of ethylene glycolxidation. At the potentials of 0.30–0.60 V, the order of oxidativeurrent densities was found to be Pt-Ru/C > Pt-Ni/C > Pt/C. Tafellots calculated from the steady-state currents show the samerder as CVs for ethylene glycol oxidation (Fig. 1b). In particular,t-Ru/C showed the highest catalytic activities for ethylene glycolxidation. Generally, Ru easily forms oxygen-containing species onts surface. Such oxygen-containing species formed on the surfacef Ru are effective to suppress CO poisoning of Pt catalysts, andherefore Pt-Ru/C work efficiently for the oxidation of alcohols suchs methanol and ethanol [10,11]. Similarly, Pt-Ru/C was effectiveor the oxidation of ethylene glycol. In contrast, Pt-Ni/C had a slightncrease in catalytic activities for ethylene glycol oxidation, as com-ared with Pt/C. Therefore, Ni alloying is not sufficient to preventO poisoning of Pt catalysts.

In HPLC analysis, two intermediate products of formate and gly-

olate were detected for ethylene glycol oxidation in KOH solution.ig. 2 shows current efficiencies for formate and glycolate, obtainedfter 50 C passing at 500 mV. Glycolate is an intermediate product

Fig. 2. Current efficiencies of each intermediate on Pt/C, Pt-Ni/C, and Pt-Ru/C cata-lysts after 50 C passed at 500 mV.

ig. 3. (a) TEM image of SnO2 nanoparticle; (b) TEM image of Pt-Ni-SnO2/C catalysts; (c) FFT-filtered lattice image of Pt-Ni-SnO2/C using (2 0 0)SnO2 [red circles] and (1 1 1)Pt-Niblue circles].

K. Miyazaki et al. / Electrochimica Acta 56 (2011) 7610– 7614 7613

20 40 60 80

Pt-Ni-Sn O2/C

Pt-Ni/C

Pt-Ru /C

Inte

nsity

/ A

rb. u

nit

2θ / degre e

Pt/C

vt

(

t

(

saopanomtcsoaaooth

hitc

3

NmSf

-1.0 -0. 5 0. 0 0.5 1. 0 1.5

350

400

450

500

550

600

Pot

entia

l / m

V (v

s. R

HE

)

log (i / mA c m-2)

Pt/C Pt-Ni/C Pt-Ni-SnO2/C

0.0 0.2 0.4 0.6 0.80

40

80

120

160

200

Cur

rent

den

sity

/ m

A c

m-2

Poten tia l / V (vs. RHE)

Pt /C Pt-Ni/ C Pt-Ni- SnO2/C

(a)

(b)

Fig. 4. XRD patterns of Pt/C, Pt-Ru/C, Pt-Ni/C, and Pt-Ni-SnO2/C catalysts.

ia 4-electron oxidation, in which C–C bond remains, as given byhe following reaction:

CH2OH)2 + 5OH− → CH2OHCOO− + 4H2O + 4e− (2)Formate is generated via 6-electron oxidation, which involves

he C–C bond cleavage of ethylene glycol:

CH2OH)2 + 8OH− → 2HCOO− + 6H2O + 6e− (3)In Fig. 2, “Others” refer to the rest of electric charges con-

umed for ethylene glycol oxidation, and mainly corresponds to themount of CO2 production, which indicates the degree of completexidation of ethylene glycol. According to HPLC analysis, the mostroduced product was glycolate for all catalysts, which is in goodgreement with our previous report [8]. As compared with Pt/C, it isoted that Pt-Ru/C showed higher production of glycolate insteadf formate, whereas Pt-Ni/C showed the highest production of for-ate instead of glycolate. These results can be understood from the

wo aspects: the tolerance for poisoning and the ability of C–C bondleavage. Since formate is one of the species that initiate the poi-oning of Pt catalysts [1,12] and Pt-Ru/C exhibited smaller amountf formate, it is indicated that Pt-Ru/C catalyst had higher toler-nce for catalyst poisoning. However, Pt-Ru/C had poor catalyticctivity for C–C bond cleavage, which resulted in larger amountf glycolate production. In contrast, Pt-Ni/C showed larger amountf formate and smaller amount of glycolate. The former indicatedhat Pt-Ni/C had less tolerance for poisoning, but the latter showedigher catalytic activities for C–C bond cleavage.

In order to utilize effectively an organic fuel, catalysts must beighly capable to break C–C bonds and highly tolerant for poison-

ng. Therefore, we added SnO2 nanoparticles to Pt-Ni/C catalysto obtain a catalyst with both high catalytic activity for C–C bondleavage and concomitant tolerance for poisoning.

.2. Characterization of Pt-Ni-SnO2 catalysts

Fig. 3a–c shows TEM images of SnO2 nanoparticles and Pt-

i-SnO2 catalysts. Solely SnO2 nanoparticles, prepared by sol–gelethod, had a diameter of 10–15 nm (Fig. 3a). TEM image of Pt-Ni-

nO2 catalyst (Fig. 3b) shows that some particles co-existed havingringes with different d-spacing values. Therefore, we performed

Fig. 5. (a) Cyclic voltammograms and (b) Tafel plots of Pt/C, Pt-Ni/C and Pt-Ni-SnO2/C catalysts in an aqueous solution of 1 mol dm−3 KOH and 1 mol dm−3 ethyleneglycol.

the filtration of fast Fourier transform (FFT) using the (2 0 0) latticeof SnO2 and the (1 1 1) lattice of Pt-Ni alloy [13,14]. Two differentgroups of particles (Pt-Ni alloy particles and SnO2 nanoparticles)were clearly distinguished in Fig. 3c. Pt-Ni alloy particles existedadjacent to SnO2 nanoparticles, thus we can expect the significantinterplay between Pt-Ni alloy particles and SnO2 nanoparticles andtheir synergetic catalytic activities. XPS analysis showed that anapproximate atomic ratio of Pt/Sn was 5/4, and therefore the cat-alyst consisted of about 16 wt% SnO2 in Pt-Ni-SnO2/C. In addition,based on the results of XRD patterns (Fig. 4), we confirmed thateach catalyst had the same crystalline size (ca. 4 nm) and the addi-tion of SnO2 nanoparticles did not altered the crystal structure ofPt-Ni alloy.

3.3. Ethylene glycol oxidation on Pt-Ni-SnO2 catalysts

Electrocatalytic activities of the resultant Pt-Ni-SnO2/C werestudied by cyclic voltammograms and Tafel plots as shown in

Figs. 5 and 6. Pt-Ni-SnO2/C showed superior catalytic activitiesfor ethylene glycol oxidation to Pt-Ni/C and Pt/C. Therefore, it isconfirmed that the addition of SnO2 nanoparticles is effective toimprove the tolerance for catalyst poisoning of Pt-Ni/C. The HPLC

7614 K. Miyazaki et al. / Electrochimica

0

20

40

60

80

100

Glycola te For mate Others

Cur

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effi

cien

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%

Pt-Ni -SnO2/CPt-Ni/C

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acbutNCepiAaPemoN

[[[12] V. Grozovski, V. Climent, E. Herrero, J.M. Feliu, Phys. Chem. Chem. Phys. 12

ig. 6. Current efficiencies of each intermediate on Pt-Ni/C and Pt-Ni-SnO2/C cata-ysts after 50 C passed at 500 mV.

nalysis for intermediates for ethylene glycol oxidation show thathe amount of formate production decreased, and the degree ofomplete oxidation increased by Pt-Ni-SnO2/C. However, SnO2anoparticles did not reduce the formation of glycolate. Theseesults indicate that the addition of SnO2 nanoparticles had a spe-ific effect of improving the tolerance for catalyst poisoning.

In this study, we explored the Pt-based catalysts with highctivities that realize complete electro-oxidation of ethylene gly-ol to CO2. Pt-Ni/C was found to possess catalytic activities for C–Cond cleavage of ethylene glycol. According to the previous studiessing in situ infrared spectroscopy, bulk Ni catalysts have abilityo decompose C2 molecules into CO2 [15]. Therefore, the alloyingi element is reasonably an effective way to C–C bond cleavage of2 fuels in fuel cells. However, Pt-Ni/C is not efficient to oxidizethylene glycol completely. Another stumbling block is catalystoisoning. In order to improve the tolerance for catalyst poison-

ng, we attempted to add SnO2 nanoparticles to Pt-Ni/C catalysts.s a result, the Pt-Ni-SnO2/C catalyst showed higher electrocat-lytic activities for the complete oxidation of ethylene glycol thant/C, Pt-Ni/C, and Pt-Ru/C. When we consider the effects of each

lement of Pt-Ni-SnO2/C catalysts, it was found that every elementight play each different role cooperatively; the dehydrogenation

f C–H bonds by Pt, the cleavage of C–C bonds in ethylene glycol byi, and the suppression of the poisoning of Pt catalysts by SnO2.

[[[

Acta 56 (2011) 7610– 7614

4. Conclusions

Electrocatalytic activities of Pt/C, Pt-Ru/C, and Pt-Ni/C for theoxidation of ethylene glycol were evaluated in basic solutions bycyclic voltammetry and quasi-steady state polarization. The cat-alytic activities were in the order of Pt-Ru/C > Pt-Ni/C > Pt/C. Theanalysis of intermediates for ethylene glycol oxidation by HPLCdemonstrated that the degree of complete oxidation of ethyleneglycol depended on the catalysts. As a result, the catalysts showsome disadvantages such as bad fuel utilization, catalyst poison-ing, etc. To solve these problems, we constructed Pt-Ni-SnO2/Ccatalyst and evaluated its catalytic activities and intermediates. Pt-Ni-SnO2/C showed superior catalytic activities for ethylene glycoloxidation and increased the degree of complete electro-oxidationof ethylene glycol. Therefore, we concluded that Pt is active fordehydrogenation, Ni is for cleavage of C–C bond, and SnO2 is forthe suppression of catalyst poisoning.

We believe these results provide a guideline to construct activecatalysts that break C–C bonds in C2 molecules and utilize ethyleneglycol as DAFCs’ fuel effectively.

References

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[2] E.J. Cairns, W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), Handbook of FuelCells—Fundamentals, Technology and Applications, vol. 1, Wiley, England, 2003(Chapter 17).

[3] Z. Ogumi, K. Miyazaki, in: J. Garche, C.K. Dyer, P. Moseley, Z. Ogumi, D. Rand,B. Scrosati (Eds.), Encyclopedia of Electrochemical Power Sources, Elsevier,Netherland, 2009.

[4] K. Matsuoka, M. Inaba, Y. Iriyama, T. Abe, Z. Ogumi, M. Matsuoka, Fuel Cells 2(2002) 35.

[5] O.A. Petrii, in: W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), Handbook of FuelCells-Fundamentals, Technology and Applications, vol. 2, Wiley, England, 2003(Chapter 45).

[6] A. Kowal, M. Li, M. Shao, K. Sasaki, M.B. Vukmirovic, J. Zhang, N.S. Marinkovic,P. Liu, A.I. Frenkel, R.R. Adzic, Nat. Mater. 8 (2009) 325.

[7] L. Jiang, G. Sun, Z. Zhou, S. Sun, Q. Wang, S. Yan, H. Li, J. Tian, J. Guo, B. Zhou, Q.Xin, J. Phys. Chem. B 109 (2005) 8774.

[8] K. Matsuoka, Y. Iriyama, T. Abe, M. Matsuoka, Z. Ogumi, Electrochim. Acta 51(2005) 1085.

[9] K. Miyazaki, H. Ishihara, K. Matsuoka, Y. Iriyama, K. Kikuchi, Y. Uchimoto, T.Abe, Z. Ogumi, Electrochim. Acta 52 (2007) 3582.

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(2010) 8822.13] ICDD #46-1088.14] J. Wu, A. Gross, H. Yang, Nano Lett. 11 (2011) 798.15] S.C. Chang, Y. Ho, M.J. Weaver, J. Am. Chem. Soc. 113 (1991) 9506.

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Electrochemical oxidation of ethylene glycol on Pt-based catalysts in alkaline solutions and quantitative analysis of inte...1 Introduction2 Experimental2.1 Catalysts preparation and characterization2.2 Electrochemical measurements and HPLC analysis

3 Results and discussion3.1 Ethylene glycol oxidation on Pt-based catalysts3.2 Characterization of Pt-Ni-SnO2 catalysts3.3 Ethylene glycol oxidation on Pt-Ni-SnO2 catalysts

4 ConclusionsReferences

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