Indian Journal of Chemistry Vol. 44A, May 2005. pp. 913-923

Development of electrocatalysts for direct alcohol fuel cells

Luhua Jiang", Shuqin Song", Zhenhua Zhou"' b, Shiyou Yan", Huanqiao Li", Gongquan Sun", Bing ZhOU"·b & Qin Xin"·e.*

"Direct Alcohol Fuel Cell Laboratory. eState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Graduate School of the Chinese Academy of Sciences, CAS, China

bHeadwater NanoKinetix, 1501 New York Avenue, Lawrenceville, NJ 08648, USA Email: xinqin@dicp.ac.cn

Received 17 November 2004

In the present review, we summarize the recent progress in electrocatalysts for direct alcohol fuel cells, focussing on the research of electrocatalysts for both alcohol oxidation and oxygen reduction, which are crucial in the development of fuel cells. A modi fied EG (ethylene polyol) method to prepare well-dispersed nano-sized Pt-based electrocatalysts with high loadings is reported . By thi s method, a more active carbon supported PtRu catalyst for methanol oxidation reaction and a PtSn catalyst for ethanol oxidation reaction have been sy nthesized successfully. Furthermore, a methanol tolerant Pd-based catalyst for cathode oxygen reduction reaction has been developed. HRTEM and HR-EDS have been employed to characterize the microstructure and micro-components of the above electrocatalysts. Results show that the bimetallic e lectrocatalysts prepared by the modified EG method display uniform size and homogeneous components at nanometer scale.

IPC Code: Int. CI.7 B82B; C25B; HOIM8/00; B01123114; BOIJ23/40

Proton exchange membrane fuel cells (PEMFCs) are of interest as promising power sources for automobiles in stationary and portable applications because of their compactness, excellent ease of start­up and shut-down and high power densi ty '·5. On the other hand, hydrogen is not a primary fuel. And also, it may be hazardous during its production, storage and application. In order to simplify the PEMFC's system and avoid the hazard resulting from use of hydrogen as fuel, direct liquid-feed PEMFCs were developed6

in 1960's. Amongst the investigated possible liquid fuels7

. '6, methanol and ethanol are of high interest due to their relatively simple structure and high oxidation activity. Till recently, focus has been on methanol since it possesses the advantage of not requiring cleavage of the C-C bond. An excellent methanol oxidation catalyst is characterized by both dehydrogenation at low temperature and removal of the methanol oxidation residues such as CO-like species. In order to improve the activity of Pt, different promoters such as Ru 17.20, Sn21

.25 , W26.27

,

M028.29

, have been adopted to enhance the methanol oxidation reaction (MOR) activity of Pt and weaken CO-like species produced on Pt active sites. Till date, the PtRu alloy is considered to be most effective because of its bi-functional mechanism l8

. For ethanol, complete electro-oxidation is a little difficult since it requires the cleavage of C-C bond. Hence, a good

ethanol oxidation catalyst should be multifunctional including ability for dehydrogenation, removal of CO­like species and cleavage of C-C bond at relatively low temperature. This makes the ethanol electrocatalyst more important. In the case of cathode electrocatalysts, considering not only the electrocatalytic activity to oxygen reduction but also the tolerance to methanol crossover from anode to cathode, Fe, Co, Ni , V have been tried as additives to platinum3o

.33

. Some non-noble metal catalysts have also been prepared and used34

.37

. However, a new challenge appeared when the non-noble metals were used as the cathode. The non-noble metals would leach out in the acidic working environment. Moreover, novel carbon supports have also been attempted in order to modify and improve the Pt­based catalysts' activity to methanol oxidation and oxygen reduction38

-42.

In the present article, systematic studies on the electrocatalysts, favorable to MOR, ethanol oxidation reaction (EOR) and oxygen reduction reaction (ORR) will be reviewed.

C~mparison of modified EG method with other preparation methods

Particle size, morphology and structure strongly influence the activity of the catalysts. Hence, the first step is to develop a simple preparation method to

914 INOlAN J CHEM, SEC A. MA Y 2005

control the particle size. Several preparation methods, including impregnation-reducti on method (formaldehyde was used as the reducing agent)4J, hydroperoxide oxidation decomposi tion method4446

,

and the modified EG (ethylene glycol) method4749,

were attempted in our lab. The detailed process of the moditied EG method is described in Appendix A. Table I summarizes the data of IEM and XRD characterization of carbon supported PtRu (20 wt. Pt%- 10 wt. Ru %) by the three different preparation methods. The characterization results of the commercial PtRu/C (Johnson Matthey I nc.) and PUC are also summarized in Table I for comparison. From Table I, it can be seen that PtRu/C catalyst prepared by the EG method exhibits a smaller mean particle size as compared to that of Pt/C catalyst. Also, the lattice parameter of PtRu/C is smaller as compared to that of PUc. The latter phenomenon suggests that there exists a strong interaction between Pt and Ru . While in the case of PtRu/C prepared by the impregnation-reduction method, the corresponding lattice parameter is similar to that of PUc. This means a weak interaction between Pt and Ru . Furthermore, PtRu/C prepared by hydroperoxide method has lattice parameter similar to that of the commercial PtRu/C. It can also be found from Table I that specific surface area of all the samples obtained from XRD is slightly higher than that from TEM . It is a fact that there is some difference between these two results, although, they present a similar trend .

These as-prepared carbon supported Pt-based alloys were adopted as the anode cata:ysts for direct methanol fuel cell (DMFC), respectively. The single DMFC test results are presented in Fig. I. The MEA preparation and operation conditions are the same as those described above except that the cell temperature was 75"C. The inset figure is the corresponding 1- V curves in the activation control region . Figure I shows that the single DMFC with PtRu/C (EG method) exhibits the

best performance in the entire current density range. In the case of PtRu/C by impregnation-reduction method. the cell performance is much inferior to the others. which may be attributed to both the weak interaction between Pt and Ru and a relatively bigger particle size (Table I). It can also be seen from Fig. 1, that in the acti vation-control region, the performance of the single DMFC with PtRu/C by hydroper xide method is similar to that of the commercial PtRu/C. Comparison of the three preparation methods shows that the DMFC exhibits the best cell performance when PtRu/C prepared by EG method was used. This can be attributed to the good di spersion of the catalysts and the stronger metal interaction in the catalysts. which is dependent on the preparation process. Modified EG method to control particle size and distribution

Figure 2 shows the TEM images of Pt/C with different metal loadings. Figure 3 shows clearly the

0.7

0.6

~ 0.5

! ~ 0.4

" u 0.3

0.2

-A-Irnpt'egnarion-reducrion -'Y-(, ,munn;ca1

50 100 150 200 250 300 350 400 450

Current Density (mAlcm2)

Fig . I--Compari son of cell performance of s ing le DMFC wi th PtRu/C as anode cata lysts by different methods. (PeRu = I: I. 20 wt Pt%-IO wt Ru%): 2.0 mg (Pt+Ru )/cm1

; emC1h"n,,\ = 1.0 mollL: Flow rate: 1.0 mLimin; Cathode: Pt/C (20 wt Pt%. John son Matthey Inc); 1.0 mg Pr/c m1

; Po, = 2.0 atm; Elec trolyle: a ti on®-

115 membrane].

Table I- The XRD and TEM characterization results o f PtRu/C prepared by different methods

XRD

PtRu/C sample Lattice parameter (A) Mean particle size Speci fic surface area (nm) (SXRD, m2/g)"

Commercial-JM 3.8900 2.4 136. 1 By impregnation-

3.9059 2.7 120.9 reduction method By hydroperoxide

3.8989 2.5 130.7 method By EG method 3.8830 1.9 171.9

"S XRD was calculated according to t1, ~ average particle diameter from XRD patterns "S'nAr was calculated according to the average particle diameter from TEM images

TEM

Mean particle size Specific surface area (11m) (STEM, m2/g)" 2.7 120.9

3.5 96.1

2.8 116.7

2.0 163.4

JIANG el al. : ELECTROCATA LYSTS FOR DIRECT ALCOHOL FUEL CELLS

,

50 nm

Fig. 2-TEM images of (a) IO % PtlC; (b) 20 % PI/C; (c) 30 % Pt/C; (d) 40 % PtlC; (e) 50 % PtlC; (f) 60 % Ptlc.

dependence of the mean particle s ize on the metal loadings. It can be see that the particle size increases with increasing metal loadings. When the metal loading increases from 10 % to 40%, the particle size does not increase prominently, but the distance between the neighbouring metal particles decreases. When the metal loading is 50 %, the mean particle size is increased to 4.3 nm abruptly. When the metal loading further increases to 60 %, the mean particle size increases slightly to 4.6 nm. Even so, the modified polyol process is a suitable method to prepare highly dispersive catalysts with high loadings.

In our studies, we found that the particle size is sensitive to the water content in the solvent. By simply controlling the amount of water content in the solvent, particle sIze and distribution of

5.0

4.5

:~ 3.S (/)

OJ

~ 3.0 1\1

Q.

2.5

2.0

10 20 30 40 !So Metal Loadings (wt %)

Fig. 3--Metal particle size versus metal loadings.

915

60

916 INDIAN J CHEM, SEC A. MAY 2005

r··' '" "

I 1 I

- -Fig. 4--TEM images of 20% PtfC with different panicle sizes. [(a) d"" '"11=2.0 nm; (b) d"'et",=2 .6 nm; (c) dmcan=3 .0 nm.

electrocatalysts could be controlled finely at nanoscale. Figure 4 shows the TEM images of 20% PtJC with different particle sizes. The particle size of Pt/C increases from 2.0 nm to 3.0 nm with increasing water content.

The modified EG method is suitable for preparation of both monometallic and bimetallic high­dispersive electrocatalysts with high metal loadings . Employing this method, bimetallic electrocatalysts including PtRu, PtSn and PtPd have also been prepared50. 71 , 75. 76,

In the following studies, all the catalysts were prepared by the modified polyol method if there is no special mention.

Studies on micro-components of electrocatalysts for MOR, EOR and ORR

Electrocatalysts for MOR In the case of direct methanol fuel cells, as

compared to oxygen reduction, methanol oxidation accounts for the main activation loss because this process involves a 6-electron transfer per methanol molecule. Also the catalyst is self-poisoned by the adsorbed intermediate products such as COads when Pt alone is used. From the thermodynamics point of view, methanol electrooxidation is driven due to the negative Gibbs free energy change in the fuel cell. On the other hand, in the real operational conditions, its rate is obviously limited by the sluggish reaction kinetics. In order to speed up the anode reaction rate, it is necessary to develop an effective electrocatalyst with higher activity for methanol electrooxidation. Carbon (XC-nC, Cabot Corp.) supported PtRu, PtPd,

o.8.------ ·------------,150

-.-PtlC

0.7 -"r- PtPdlC ___ .PtW/C 125 -4.-PtSnlC

~

~ 0.4

0.1

~~~~-L~-~~--~~~~~~~~O ~ 100 I~ 200 2~ 300 3~ 400 4~ soo ~

Current Density (rnA/em')

Fig . 5-Cell performance o f single DMFC with different anode catalysts. [Teell = 90°C. Anode: Pt-based catalysts prepared by different methods (Pt:M = 1:1. 20 wt Pt%), 1.33 mg Ptfcm~ bUI 2.0 mg Ptf cm! when PtfC was used as the anode catalysts: Cmclhallol = 1.0 moUL; fl ow rate: 1.0 mLimin. Cathode : Pt/C (20 WI PI%, Johnson Matthey Inc); 1.0 mg Ptfc m! ; P 02 = 2.0 aIm:

Electrolyte: Nafion®-115 membrane].

PtW and PtSn were prepared by the modified EG method as described above5o. Pt content in all the catalysts was 20%.

The single DMIFC test results with these as­prepared Pt-based catalysts as the anode catalysts are shown in Fig. 5. For comparison, Pt/C as the anode catalysts is also shown in Fig. 5. The cathode catalyst was PtJC (20%, Johnson Matthey Inc .). The geometric electrode area was 2.0x2.0 cm2

. The Pt loadings were 1.33 mg/cm2 for the anode and 1.0 mg/cm2 for the cathode. The membrane electrode assembly (MEA) preparation procedure has previously been reported in

JIANG el al.: ELECTROCATALYSTS FOR DlRECT ALCOHOL FUEL CELLS 917

detaiI 5). The single fuel cell test was calTied out at

90uC on an in-house apparatus with l.0 mollL aqueous methanol solution at a flow rate of 1.0 mLimin supplied to the anode and 2 atm oxygen fed to the cathode.

It can be seen from Fig. 5 that in the case of PtJC as the anode catalyst, single DMFC exhibits the poorest performance. The open circuit voltage (OCY) is only 0.56 Y, which is very less as compared to the theoretical value (1.18 V). Thi s is mainly due to the slow reaction kinetics and methanol crossover. It can also be seen from Fig. 3 that the maximum discharge current density is about 165 mA/cm2 and the peak power density is only 17.5 mW/cm2 at 120 mA/cm2

•

From these experimental results, it can be concluded that PtJC alone is not a suitable electrocatalyst for methanol electrooxidation. It is necessary to enhance the activity of Pt with other additives.

Figure 5 shows that Pd, Ru , Sn and W as the additive to Pt can improve the single DMFC performance to varying extents. Among them, PtPd/C presents only slight improvement in the DMFC performance in comparison to PtJc. The OCY and peak power density increase to about 0.58 Y and 27.9 mW/cm2 at 171 mA/cm2, respectively. From the open circuit voltage point of view, PtSn/C should give the highest value amongst all the investigated catalysts if their performance were evaluated under the same cond itions . However, it was observed that with PtSn as the anode, the cell voltage, decreases quickly along with increase in current density. This is probably due

to the poor electronic conductivity of PtSn/C catalyst, which results from the Sn oxides being in different valence states in the catalysts52

. In particular, in the higher CUITent density range, the ohmic effect becomes stronger, leading to faster cell vo ltage drop . On the one hand, W is considered to be capable of providing abundant O-containing species and the change from one valence state to another can accelerate the removal of the adsorbed intermediates such as COad s. On the other hand, according to the single DMFC test results , it shows inferior performance as compared to PtP.'i/C. The above results suggest that PtRu is the most effective anode electrocatalysts for direct methanol fuel cell s. This is in agreement with earlier studies reporting that PtRu is the most effective catalyst for methanol electrooxidation.

In order to clarify the relationship between the microstructure of PtRu/C catalyst and the corresponding performance, HRTEM and HR-EDS experiments were carried out with on-particles employing the PtRuiC prepared by impregnation­reduction method (denoted as PtRu-l) and 'modified polyol method (denoted as PtRu-2). Figure 6 shows the HRTEM images and HR-EDS analysis of PtRu-1 and PtRu-2. From the HRTEM images, it can be seen that the metal pmticles of PtRu-l are not uniform, and some of the small particles agglomerate into larger particles. For PtRu-2, the particles disperse on the support uniformly. PtRu-1 and PtRu-2 have a similm' mean particle size; 3.5 ± 2.5 nm and 2.5 ± 0 .5 nm respectively.

Fig. 6----TEM images of (a) PtRu/C-l and (b) PtRu-2.

918 lNDlAN J CHEM, SEC A, MA Y 2005

However, the particle size di stribution of PtRu-l , 1-6 nm, is much broader than that of PtRu-2 0.5-2.5 nm). The on-paJticle HR-EDS results clearly show that the distribution of Pt and Ru in the PtRu-1 sample is not unjfonn. For the laJ'ger pal1icles, the atorruc ratio of PtRu is 8.8: 1, while for the smaller particles, the corresponding value is I :8.0. This indicates that the larger particles are Pt-rich, while the smaller paJticles are Ru-rich. For PtRu-2, the atomic ratio of Pt to Ru is much more uniform. On larger particles, the atomic ratio of PtRu is 1.2: 1 and for smaller particles, the corresponding value is around 0.7: I, which is close to the nominal ratio.

According to bifunctional mechanism proposed by Watanabe53, the dehydrogenation of methanol occurs on the Pt acti ve sites as a result of producing CO-like species. The Ru species could decompose water at a lower potential than Pt to produce - OH species, which could react with the CO-like species on Pt active sites and detoxify them. On the premise of this mechanism, the model catalyst should be a good PtRu alloy (atomjc ratio PtlRu=l: I) with the two elements Illixing at atomic scale (Fig. 7).

On the basis of this model, Pt and Ru should be mixed at atomic scale in order to remove the CO-like species smoothly. From the above HRTEM and HR­EDS analysis, it is seen that PtRu-2 has uniform particle size (2 .S±O.Snm) with sharp and uniform distribution of Pt and Ru at atomk scale. This structure is favorable for removing CO-like species according to bifunctional mechanism. PtRu-l , which is characterized by broad particle size distribution (3.5±2.S nm) and uneven distlibution of Pt and Ru at atolllic scale, cannot eliminate the poison smoothly. The accumulated poisons will lead to Pt active sites deactivating gradually. The catalyst pelformaJlce as the anode of DMFCs (Fig. I) gave strong evidence regarding this.

Electrocatalysts for ethanol electrooxidation In order to extend the prac tical application of low­

temperature fuel cells and to facilitate their

G Ru

PI , CO

OH

Fig. 7-PtRu model catalyst fo r methanol e lectroox idation.

penetration into the transport market it is also desirable to increase the number of liquid fuels that can be employed in these devices. Amongst the possible fuels, ethanol is most promising because it is a n atur~t1 l y

available and renewable fuel , and offers positive impact on both economy and en vironment5~.55 . Moreover, ethanol has a higher energy density(8 .0 I kWh kg· l

) as compared to methanol (6.09 kWh kg· I).

Therefore, ethanol is more attractive and appears to fulfill most of requi rements of the fuel for low temperature fuel cells5

(i, 57 .

Compared to methanol electrooxidati on, ethanol electrooxidation seems to be a more complicated process because it involves a 12-electron transfer per ethanol molecule and cleavage of C-C bond. In order to speed up development of DEFCs, it is necessary and important to develop a novel electrocatalyst with higher activity for ethanol electrooxidation.

In our earlier studies49, we have reported the preparation of carbon supported PtPd, PtW, PtRu and PtSn by the modified EG method and evaluation as ethanol oxidation electrocatalysts. It was found that the PtSn system is more suitable for EOR than the other catalysts. In our research, the optimum ratio of PtSn was screened and the results show58 that Sn displays the highest promoting effect on Pt when the ratio of PtSn is 3: 1.

In the subsequent studies, efforts were made to control the composition of PtSnlC catalyst at atomic scale. We can now obtain uniform composition of PtSn catalyst by modifying the preparation conditions. Figure 8 shows the HRTEM image of Pt3Sn/C nanoparticles. It can be seen from Fig. 8 that the metal paJticles are uniform with a mean particle size of 3 nm. HR-EDS analysis was carried out at a random linear area and the result shows that the distribution of Pt is consistent with that of Sn. This indicates that the distribution of Pt and Sn is uniform in all of the particles.

However, the promoting mechanism of Sn is not clear. Our primary research indicated tin oxide to be the active component for ethanol electrooxi­dation49.52.59. Here our focus was to study the effects of the chemkal state of tin and the component of PtSn/C catalysts on the performance of DEFCs. For comparison, two PtSnlC catalysts with tin ox ide and PtSn alloy were prepared, respectively. For the fonner, tin oxide with a di ameter of 1 nm was prepared, first in EG solution (Fig. 9), and then platinum was reduced on the surface or near the tin oxide (denoted as PtSn- I).

JIANG el a/ .: ELECTROCATALYSTS FOR DIRECT ALCOHOL FUEL CELLS 9 19

Fig. 8--HRTEM image of Pr.,Sn/C nanoparticles.

For the latter, the precursors of Pt and Sn were first mixed together, and then reduced in EG soluti on (denoted as PtSn-2).

The XRD patterns of PtSn-1 and PtSn-2 are shown in Fig. 10. It can be seen that apart from the diffraction peaks of Pt ( III), Pt (200), Pt (220) and Pt (3 11 ), diffraction peaks ofSn02 (101) and Sn02 (211) appear at around 34° and 52° respectively (PCPDF#41 1445) for PtSn-l. Both PtSn-1 and PtSn-2 have the same particle size of around 2.3 nm as calculated by Scherrer Formula60

. The lattice parameters of the PtSn-l and PtSn-2 are 3.928 and 3.946 A respectively according to Vegard's Law 61.

Compared to the lattice parameter of Pt, which is 3.923 A, the crystalline lattice of Pt in PtSn-2 sample is dilated prominently, while that of PtSn-l displays little dilation . In general, the dilation of crystalline lattice reflects the alloy degree of two metals. To clarify the microstructure of PtSn catalysts, HRTEM images of PtSn-] and PtSn-2 were studied (Fig. II ). In the HRTEM image of PtSn- J, Sn02 nanoparticles were found in the vicinity of Pt particles. The nominal 0.264 nm spacing of the Sn02 (\ 0 I) plane is indicated by the arrow in Fig. II (a) . The nominal 0.228 and 0.198 nm spacing of the Pt (Ill) and (200) planes, respectively, of the fcc lattice of typical faceted particles is also indicated by the arrows in Fig. 11 (a). In Fig. 9(b), only Pt (Ill) plane was found with spacing of 0.234 nm and no separate Sn02 phase was found near the Pt particles. Based on the above

...... ~

...... ""'0" . ~ "';. ....

' .

. .

Fig. 9--TEM image of tin ox ide nanopanicles.

20 30 40 50 60 70 2 theta (0)

80

Fig. 100XRD patterns of (a) PtSn-l and (b) PtSn-2 .

discussion, it can be concluded that partial Sn exists in tin oxide, partial Sn alloy with Pt in the PtSn-1 sample and most of Sn alloy with Pt in the PtSn-2 sample.

Cyclic voltammetry (CV) is a practical and simple method to characterize the electrocatalytic activity of catal ysts. Figure 12 shows the CV curves of PtS n electrode in 0.3M HCI04 + 1M EtOH solution. The initial electrooxidation potentials of ethanol (at the current of 0.2 mA) on PtSn-l and PtSn-2 are 0.033 V and 0.124 V, respectively. In the entire sweeping range, the ethanol electrooxidation current on PtSn-1 electrode is higher than that on PtSn-2 electrode. This indicates that PtSn- I is a better catalyst for ethanol than PtSn-2.

920 INDI AN J CHEM. SEC A. MA Y 2005

Fig. II - HRTEM images or (a) PtSn- 1 and (b) PtSn-2.

Figure 13 shows the /- V curves of DEFCs with PtSn-1 and PtSn-2 as the anode catalyst, respectively. It can be seen th at the DEFC with PtSn- 1 as anode catalyst shows much hi gher performance than that \-\lith PtSn-2 as anode. The max imum power density and the maximum discharge current of the DEFCs wi th PtSn- 1 and PiSn-2 as the anode catalyst are 8 1 (200 mA cnf") and 47 mW cm-2 (240 mA cm-\ respect ively. In our previous research52

, it was deduced that tin ox ide could offer oxygen-containing species at lower potenti al than platinum. The oxygen­con taining species could react with the CO-like poisons resulting from the ethanol electrooxidati on. Herein , according to the detailed research , PtSn-1 includes PtSn alloy and tin ox ide nanoparticles, while in PtSn-2 most of tin alloys with platinum as a result of dil ating the lattice parameter of Pt. In the study of MOR on PtSn catalysts. it is beli eved that the dilati on of lattice parameter of Pt inhibits the ability of Pt to adsorb methanol and dissociate C-H bonds6c

.

Similarly, the adsorption and dissoc iation of ethanol may be inhibited due to the complete alloying of Pt and Sn. Our previous research indicates that suitabl e dilation of Pt crystalline latti ce constant is favorabl e

50 to the ethanol adsorption . Ethanol molecules adsorbed on the active sites of Pt are dehydrogenated to produce CO-like species. For the PtSn-2 sampl e, the ethanol electrooxidation residues could not be removed from Pt active sites smoothl y since there is no oxygen-containing species around Pt active sites_ However. for PtSn-l the electrooxidation res idues

7

6

5

4

« 3 E -.~ 2

0

-1

9' \ /~'~ Y .! j- ",j \' !

£ ... /. -. //><' .. ~~'"

~// .... .. ... / i f/ ...... ..

- PtSn-1 ........ PtSn-2

-2 -0.2 0.0 0.2 0.4 O.S 0.8 1.0

Potential (y vs SeE)

Fig. 12-Cyclie vo ltamln::lgrams of PtSn electrodes ill 0.3 M HCI04 + I M EtOH solution at room temperature.

could react with the oxygen-conta ining spec ies res ulting from tin ox ide in the vicinity of Pt parti cles to free Pt active si tes . On the basis of the above di scussion, an ideal PtSn electrocatalyst for ethanol electrooxidation would be Pt alloyed with Sn to a suitable degree with Sn ex isting partially as oxide.

It is noteworthy that even if PtSn/C cataly . ts exhibit higher electrocatalytic activity for ethanol ox idation , majority of the oxidation products are still the species containing C-C bond. which will have an obv ious effect on the fu el ce1l 63

. It is cru cial and necessary to develop a novel catalyst or add a third element to modify the PtSn/C and PtRu/C to present higher specific dehydrogenation activity and C-O and

JIANG el al.: ELECTROCATAL YSTS FOR DIRECT ALCOHOL FUEL CELLS 92 1

0.9 90

\: a •

a PtSn-1 80 0.8 .. .... -*-........ '* b PtSn-2

0.7 *.... ....... . / '.. 70

'*'* / / \ '*:;* *~ 60 ~0.6 -.. *" ....... '*

b·.... / ' *-.... * .. '~* -.... \ 50 EO.s / .) ...... -.-.-.-...., '0 <I -I *, \ 40 ;;. 0.4 / .,II .... , -I' i ' * = * / / . -. "-.. I -. 1 • 30 U 0.3 *.. .... '* ;.~ - -I , I \.

20 0.2 / ', \ ' I \ 10 0.1 ' I *

0 0.0

0 40 80 120 160 200 240 280

Current Density (rnA em-:!)

Fig. I3--Performance of DEFCs wi th (a) PtSn- 1 and (b) PtSn-2 as the anode catalyst.

C-C bond cleavage during the ethanol oxidation process.

Methanol-tolerant electrocatalysts for oxygen reduction

As fa r as the oxygen reduction is concerned, Pt­based electrocatalyts are always used as the cathode catalysts in direct methanol fue l ce ll s. However, methanol crossover is one of the main obstacles for the development of direct methanol fuel cell. In order to avoid or reduce the effect of methanol crossover on the DMFC's cathode performance, many efforts have been made6

H>9 . Amongst them, one solution is to develop methanol-tolerant catalysts such as

I h I 'd '~-37 d'f' P macrocyc es or c a cogenl es' , or mo ly t catalyst by adding another metal such as Fe, Co, i and Pd to decrease the effect of permeated methanol from the anode on the Pt cathode catalysts50

. In the former case, although it is known that the methanol tolerant catalysts are good oxygen reduction electrocatalysts and inactive to methanol molecu les, their instability in acid media, especially at the operation temperatures rangi ng from 60-90"C, limits their wide application in DMFC In the latter case, PtFe seems to be promising70. However, there also ex ists the loss of Fe due to the ac idic environment in the long operation conditions.

A novel carbon su pported Pd-rich Pd)Pt t has been developed which shows 71 beller direct methanol fuel cell performance than PtiC The polarization curves for oxygen reduction reaction (ORR) in 0.5 moUL HCIO~ wi th or without methanol are shown in Fig. 14. In the case or aqueous HClO.) so lution in absence of methanol. Pd, Pt JC ex hibits the ORR performance comparable 10 thal of PtlC In the case of aqueous

160 ,.------------______ -,

120

200

e

-0- PtlCwlthoulmethlnol 1\ -6- Ptl\-I/C wIthout methlnol/ e - e - PtiC with methanol - .t.- PtWC with methanol e

I \ / e

/ . ",,~;a;l~'~t i """,""AS/,

i=Q:::Q :::Q=Q;;::Q-~ =1iI~"'~~;:::O =«=«-m-~-~~= - 3-6-6-6

400 600 800 1 000

Potential (mY vs. NHE)

1200

Fig . I4--Polarization curves for oxygen reducti on reacti on over Pd, PtI /C and PtlC in 0 2-sat urated O.S mo l/L HClO. in the presence or in the absence o f O. I mollL methanol at room temperature. [Potential sweep rate : S mY/s: Oxygen feeding ra te : S mL/min ; Rotation speed: 2S00 rpm] .

O.R 1211

0.7 __ Pt/C

100 ,--. 0.6 N

5

§05

... 80 ~

'" 5 .. ~

:g 004 60 ~ > ~ 0.3 '" .. U ,)0 Q ...

0.2 .. ~

0. 1 20 ~

o 50 100 ISO 200 250 JOO 350 ~OO

Current Density (mAlcm2)

Fig. I5-Performance o f Pd, Pt I/C and PtlC as DM FC cathode catal ysts respective ly. [Teel l = 7S°C. Anode : PtRu/C (20 wt Pt %- IO wt Ru %. John son Matthey Inc): 2 .0 mg (Pt+ Ru)/cmc: Cmethallol = 1.0 mollL; flow rate: 1.0 mL/min . Cathode: Pt/C or Pd, Pt, /C (20 wt metal 'Yo ): 10 mg metal/c m": P 0 2 = 2.0 atm: Elec trol yte: Nafio n®- IIS membrane].

HCI04 solution in presence of methanol, it is interesting to find that for Pt/C, the peak for methanol oxida ti on is so big that its activity to ORR is decreased significantly in the potential range from :'i00-900 m V. However, for Pd3Pt I/C , the meth anol oxidation peak does not appear in the ORR polarization curve. This suggests that PdJPt I/C can ex hibit a ORR se lec ti vi ty superior to Pt/C in the presence of methanol. Accordingly, in th e fue l ce ll operation mode, when Pd3Pt tiC is used as the cathode catalyst, the effect of methanol crossover on the cathode performance wi II be reduced at least by some

922 INDIAN J CHEM, SEC A, MA Y 2005

extent in comparison with PUc. The single DMFC tests with PdJPt l/C or Pt/C as the cathode catalysts were carried out and compared. Single fuel cell test results (Fig. 15) shows that in the entire current density range, single DMFC with Pd3Pt/C shows better performance than that wi th Pt/C as the cathode catalysts. The single fuel cell test results are in good agreement with the RDE results. The improved ORR activity on Pd3Pt/C may be due to the inactivity to methanol oxidation reaction but similar activity to oxygen reduction reaction . In conclusion, palladium­rich Pd3Pt i /C catalyst enhances the performance of the DMFC cathode for its selective ORR activity in presence of methanol and may be an alternative methanol tolerant cathode in DMFCs. The relationship between the microstructure of PdPt catalyst and the corresponding performance is under investigation.

Conclusions A convenient and environmental friendly modified

polyol process has been developed to provide catalysts with controllable nano-sized particle size, sharp distribution and good dispersion even at high metal loadings. Various carbon supported Pt-based electrocatalysts for both alcohol oxidation and oxygen reduction have been prepared employing this process and these exhibit excellent performance. HRTEM and HR-EDS analyses indicate that the PtRu/C, PtSn/C and PtPd/C synthesized by this method have uniform components. It is noteworthy that PtSn presents higher activity for ethanol electrooxidation than pure Pt. Furthermore, PtSnOx catalyst shows much higher activity for ethanol electrooxidation than PtSn alloy. Pd3Pt exhibits comparable electrocatalytic ability for oxygen reduction to Pt alone, and is insensitive to methanol oxidation, i.e., it is a good methanol-tolerant cathode catalyst. Moreover, HRTEM is a useful tool to study the microstructure and micro-components of electrocatalysts at nanometer scale.

Appendix A

Modified polyol method

A convenient, environment-friendly and low-cost modified polyol method has been developed to prepare nano-sized and highly dispersed carbon supported Pt-based electrocatalysts47

..18.72.74. The typical modified polyol preparation process for carbon supported Pt-based electrocatalysts is described concisely as follows: The required amount of H2PtCI6 '6H20, or its mixture with the corresponding compound of the modifier, such as

RuCl)'3H20, SnCl2'2H20, PdCl2, etc ., and carbon were added to the mixture of EG and deionized water in an appropriate ratio to form a homogeneous slurry by sonicating and mechanical stirring The slurry was heated to 130"C and kept at this temperature in an oil bath for 3 h. After cooling to room temperature, the black solid sample was filtered, washed and dried at 80°C for 10 h in a vacuum oven to obtain the corresponding electrocatalysts. Considering that the preparation parameters such as pH, reduction temperature, the rati o between EG:water ratio. etc., have an obvious effect on the catalyst's particle size, di spersion and composition, and consequently their activity. Sun et at. have optimized thi s method and have given in detail the effect of the operational parameters on the physico-chemical properties of the as-prepared catalysts50. 75. 76.

Acknowledgement This work was financially supported by Innovation

Foundation of Chinese Academy of Science (K2003D2), National Natural Science Foundation of China (Grant No. 20 1.73060) , Hi-Tech Research and Development Program of China (2003AASI7040) and Knowledge Innovation Program of the Chinese Academy of Sciences (KGCX2-SW -31 0) . Hundred of Elites Program of Chinese Academy of Sciences is also acknowledged by BZ.

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