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titanium dioxide nanofibers for glycerol electrooxidation in alkaline medium
Electrochemistry Communications 11 (2009) 2199–2202
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Palladium/titanium dioxide nanofibers for glycerol electrooxidation in alkaline medium Liang Su, Wenzhao Jia, Ashley Schempf, Yu Lei * Department of Chemical, Materials and Biomolecular Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 06269, USA
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 23 September 2009 Accepted 24 September 2009 Available online 27 September 2009 Keywords: Pd/TiO2 nanofibers Glycerol Electrooxidation Poisoning intermediate
a b s t r a c t Highly conductive palladium/titanium dioxide (Pd/TiO2) nanofibers have been successfully fabricated by electroless-plating Pd on the electrospun TiO2 nanofibers. The application of Pd/TiO2 nanofibers for electrooxidation of glycerol was demonstrated. The results showed that Pd/TiO2 nanofibers can greatly promote glycerol electrooxidation in alkaline medium, and both glycerol and KOH concentrations had an effect on the peak current density and the peak potential. The mechanism of desorption of poisoning intermediate was discussed by changing the upper potential limit. The application of the Pd/TiO2 nanofibers for electrooxidation of methanol, ethylene glycol, and 1,2-propanediol was also demonstrated. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Direct alcohol fuel cells (DAFCs), which convert chemical energy of various liquid hydroxyl derivatives of hydrocarbons into electrical energy [1], have been emerging as a new generation of compact power sources for portable electronic devices and electric-powered vehicles and thus attracting extensive attention in recent years [2]. Among all alcohols, the electrooxidation of methanol [3,4] and ethanol [5,6] has been intensively investigated. However, the high volatility of ethanol and the severe toxicity of methanol may cause serious problems in the practical application [3], potentially limiting their further commercialization. Recently, polyhydric alcohols such as ethylene glycol, propanediol and glycerol have been appearing as preferable alternatives to overcome the aforementioned shortcomings [7]. Among all the polyhydric alcohols, glycerol (the simplest triol) is promising since it can be massively produced by microbial fermentation [8]. Furthermore, there is a large amount of glycerol generated as by-product (10 wt.%) in biodiesel production [9]. Therefore, the application of glycerol in DAFCs is highly favorable in terms of safety, economic efficiency and environmental benefit. Various catalysts have been applied to convert chemical energy of alcohols to electrical energy. Platinum (Pt) based electrocatalyst is widely recognized as the most effective one for the electrooxidation of alcohols in acidic medium [10,11]. However, the high price of platinum impedes its extensive application. Recently, palladium * Corresponding author. Tel.: +1 860 486 4554. E-mail address: [email protected] (Y. Lei). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.09.030
(Pd) becomes a promising substitute for Pt because of its outstanding electrocatalytic ability in alcohol oxidation reaction and more abundant existence on the earth than Pt [12]. Shen et al. have shown that Pd/C promoted with oxides electrocatalysts were superior to Pt-based catalysts in alkaline media due to its higher activity and better poisoning tolerance [13,14]. Liu et al. have also shown the excellent catalytic performance of Pd on the electrooxidation of 1-propanol and 2-propanol [15]. Recently, Xu et al. successfully fabricated highly ordered Pd nanowire array which showed improved activity for ethanol oxidation compared to that of commercial E-TEK PtRu/C catalyst [16,17]. In this study, palladium/titanium dioxide (Pd/TiO2) nanofibers were fabricated by electroless-plating of Pd on the surface of electrospun TiO2 nanofibrous membrane, offering an excellent catalyst for electrooxidation of alcohols. The elecrocatalytic property of the Pd/TiO2 nanofibers modified glassy carbon electrode (GCE) was systematically investigated for glycerol electrooxidation in alkaline medium. The results demonstrated the potential applicability of the as-prepared nanostructured Pd/TiO2 electrocatalyst in DAFCs with polyhydric alcohols fuel.
2. Experimental TiO2 nanofibers were prepared by electrospinning using a similar method described elsewhere [18], and the electroless-plating of Pd on TiO2 nanofibers was carried out following our reported procedure [19]. Field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectrometer (EDX), and X-ray
L. Su et al. / Electrochemistry Communications 11 (2009) 2199–2202
diffraction (XRD) were applied to characterize the as-prepared samples. Cyclic voltammetry (CV) was carried out in a three-electrode system. A platinum wire and an Hg/HgO (MMO, 1.0 M KOH, 0.098 V vs. SHE) electrode were employed as the counter and the reference electrode, respectively. Pd/TiO2 nanofibrous membrane was fixed on the surface of GCE with nail oil and served as working electrode. All the solutions were purged with nitrogen before experiments.
3. Results and discussion Typical SEM images of the as-prepared TiO2 nanofibers before and after the deposition of Pd are presented in Fig. 1a and b. The TiO2 nanofibers have an average diameter of 115 nm, while the Pd/TiO2 nanofibers have a larger diameter due to Pd-coating. The average thickness of the coated Pd layer is 176 nm. The coating of Pd on TiO2 nanofibers was further confirmed by EDX analysis. As shown in the inset of Fig. 1c, only Pd element was detected which can be attributed to the fact that the surface of TiO2 was fully covered by a Pd layer. The XRD pattern of the as-prepared sample is presented in Fig. 1c where all diffraction peaks can be assigned to the Pd face-centered-cubic (fcc) crystalline structure (JCPDS card 46-1043). In addition, the as-prepared Pd/TiO2 nanofibers showed excellent electric conductivity which is indispensable in the application of alcohol electrooxidation. Fig. 2a shows the CV of electrooxidation of 0.1 M glycerol in 0.5 M KOH solution on the Pd/TiO2 modified GCE in the scan range from 0.9 V to 0.5 V (vs. MMO). Compared to the CV in the absence of glycerol, a strong glycerol oxidation peak can be observed. Similar to the electrooxidation of other alcohols [5,15], the glycerol
elctrooxidation promoted by Pd in alkaline medium is characterized by two well-defined peaks in the forward and the backward scan, which are due to the electrooxidation of glycerol and the removal of the poisoning intermediates, respectively. As a comparison, there is only one obvious reduction peak at ca. 0.25 V observed in 0.5 M KOH, which is due to the reduction of palladium(II) oxide to Pd0 [20]. By contrast, TiO2 nanofibers modified GCE has no catalytic activity for glycerol electrooxidation in alkaline medium (inset of Fig. 2a). Fig. 2b presents the effect of KOH concentration on the glycerol electrooxidation. The cyclic voltammograms are recorded from 0.9 V to 0.5 V with 0.1 M glycerol, but only the forward scan peaks are presented. One can see that a higher KOH concentration results in a lower oxidation peak potential and an enhanced peak current density. However, both the negative shift of the oxidation peak potential and the enhancement of peak current density show the tendency to gradually level off with the increase of KOH concentration. The results indicate that glycerol electrooxidation is improved by the greater availability of OH-. As the simultaneous attack of hydroxyl groups at C1 and C3 carbon atoms is unfavorable [21], the major glycerol electrooxidation on Pd in alkaline media can be rationally described as follows in light of previous works [21–23].
Pd þ OH $ Pd—ðOHÞads þ e
ðIÞ
Pd þ C2 H3 ðOHÞ2 —CH2 ðOHÞ þ 3OH
$ Pd—½C2 H3 ðOHÞ2 —COads þ 3H2 O þ 3e
ðIIIÞ
! C2 H3 ðOHÞ2 —COOH þ 2Pd C2 H3 ðOHÞ2 —COOH þ OH ! C2 H3 ðOHÞ2 —COO þ H2 O
ðIVÞ
b 100 nm
1 µm
Intensity Pd (200)
500
Pd
0 1
20
20
3
keV
10
Pd (311)
30
Pd
1000
Pd (220)
Intensity (a.u.)
40
1 µm
Pd (111)
1 µm
c
ðIIÞ
Pd—ðOHÞads þ Pd—½C2 H3 ðOHÞ2 —COads
a
50
4
Pd (222)
2200
0 30
40
50
60
70
80
90
2 Theta (degree) Fig. 1. The SEM images of TiO2 nanofibers (a), Pd/TiO2 nanofibers (b), and XRD patterns of Pd/TiO2 nanofibers (c). The insets are high magnification SEM images and EDX analysis, respectively.
L. Su et al. / Electrochemistry Communications 11 (2009) 2199–2202
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Fig. 3. CVs of Pd/TiO2 nanofibers modified GCE. (a) 0.5 M KOH + 0.1 M glycerol with different Eupper. The inset shows the CVs in 0.5 M KOH with different Eupper. (b) 0.5 M KOH with or without 0.1 M various alcohols with a scan range from 0.9 V to 0.9 V.
Fig. 2. CVs of Pd/TiO2 nanofibers (a) and TiO2 nanofibers (inset of a) modified GCE in 0.5 M KOH in the presence and absence of 0.1 M glycerol. (b) Effect of KOH concentration on 0.1 M glycerol electrooxidation with Pd/TiO2 nanofibers modified GCE. (c) Effect of glycerol concentration in 0.5 M KOH on its electrooxidation with Pd/TiO2 nanofibers modified GCE. (d) and (e) CVs of the forward and backward scans for 0.1 M glycerol, ethylene glycol, 1,2-propanediol, and methanol in 0.5 M KOH with Pd/TiO2 nanofibers modified GCE, respectively.
The extent of glycerol electrooxidation, namely the forward scan peak current density (If), is determined by the coverage of hydroxyl group and glycerol adsorbed on the Pd active sites, denoted by hPd(OH)ads and hPd(G)ads, respectively. The increase of KOH concentration will not only enhance hPd(OH)ads, but also greatly facilitate the adsorption of glycerol on the catalyst, which results in higher If and negative shift of peak potential. Fig. 2c shows the effect of glycerol concentration on the electrooxidation in 0.5 M KOH (the forward scan peaks only). The oxidation current initially increases with the glycerol concentration up to 0.5 M glycerol, which is attributed to the increase of hPd(G)ads, and then drops with further concentration increase because excessive adsorbed glycerol could block the access of OH- to the active sites on Pd, resulting in an insufficient coverage of (OH)ads and leading to a decrease of the peak current density accordingly. The performance of the Pd/TiO2 nanofibers for the electrooxidation of other alcohols was also investigated. Fig. 2d and e demonstrate the forward and backward scan peak regions for the electrooxidation of glycerol, methanol, ethylene glycol, and 1,2-propanediol, respectively. The well-defined forward and backward scan peaks can be observed for the examined alcohols and demonstrate the excellent performance of the Pd/TiO2 nanofibers. In addition, one can see that under the same molar concentration, glycerol electrooxidation showed the highest forward scan peak current density among all alcohols, indicating that glycerol is a promising candidate in DAFCs.
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L. Su et al. / Electrochemistry Communications 11 (2009) 2199–2202
Fig. 3a shows the effect of different Eupper values on glycerol electrooxidation and its inset presents the corresponding CVs in KOH solution without glycerol. The inset shows that the reduction peak during backward scan in KOH solution increases and shifts negatively with the increase of Eupper since more PdO and stronger Pd–O bond can be generated at higher potential [20]. For glycerol electrooxidation, the increase of Eupper results in insignificant change of If but dramatic diminution of the backward scan peak current density (Ib). In addition, a strong reduction peak corresponding to the reduction of PdO was also observed when Eupper of 0.9 V was applied. The results imply that the adsorbed posioning intermediate species can be effectively removed at higher potential during the forward scan, resulting in the sharp decrease of Ib and the increase of PdO reduction peak in the reverse scan accordingly. It is generally believed that the main poisoning species in the electrooxidation of alcohol is (CO)ads [3,6,7,21,24]. When Eupper is relatively low, the removal of the poisoning species probably happens in the reverse scan as follows. PdðCOÞads þ Pd—ðOHÞads þ 3OH $ 2Pd þ CO2 3 þ 2H2 O þ e
ðVÞ
However, when higher Eupper is applied, Pd(CO)ads can be likely oxidized to form PdO during forward scan through following equation. PdðCOÞads þ 6OH $ PdO þ CO2 3 þ 3H2 O þ 4e
ðVIÞ
Higher Eupper enhances the reaction extent of Eq. (VI). On one hand, with the removal of more poisoning intermediates at higher potential during forward scan, Ib decreases in the backward scan. Especially, as shown in Fig. 3b, no peak can be observed for methanol during backward scan which can be assigned to the complete removal of the poisoning intermediate at higher potential during forward scan. On the other hand, more PdO and stronger Pd–O bond generated at higher Eupper result in the enhanced reduction peak of PdO to Pd and the negative shift of the reduction peak potential during the backward scan. In addition to the major oxidation peak, another oxidation peak can be observed for glycerol, EG, and 1,2propanediol at higher potential. However, the removal of poisoning species or direct alcohol oxidation can not explain this oxidation peak as there is no such peak observed for methanol under the same condition (Fig. 3b). Therefore, this oxidation peak may be attributed to further oxidation of the remaining OH groups. 4. Conclusion Pd/TiO2 nanofibers have been prepared by electroless-plating of Pd on electrospun TiO2. The Pd/TiO2 nanofibers modified GCE
shows excellent catalytic activity for the electrooxidation of glycerol as well as methanol, ethylene glycol, and 1,2-propanediol in alkaline solution. Both KOH and glycerol concentrations affect the glycerol electrooxidation in terms of peak potential and the forward and backward scan peak current densities. Among the examined alcohols with the same molar concentration, glycerol possesses the highest If, indicating that it is a potential candidate in fuel cell application. Moreover, higher potential can effectively remove the poisoning species during the forward scan. When higher Eupper is applied, another oxidation peak can also be observed in the CVs of glycerol, ethylene glycol, and 1,2-propanediol between 0.65 V and 0.75 V, suggesting the further oxidation of the remaining OH groups. Acknowledgments We greatly appreciate the funding from NSF and USGS. LS also thanks the partial support from UConn Center for Environmental Science and Engineering. Reference [1] B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. [2] K. Matsuoka, Y. Iriyama, T. Abe, M. Matsuoka, Z. Ogumi, J. Power Sources 150 (2005) 27. [3] K. Matsuoka, Y. Iriyama, T. Abe, M. Matsuoka, Z. Ogumi, Electrochim. Acta 51 (2005) 1085. [4] W.L. Holstein, H.D. Rosenfeld, J. Phys. Chem. B 109 (2005) 2176. [5] H.P. Liu, J.P. Ye, C.W. Xu, S.P. Jiang, Y.X. Tong, Electrochem. Commun. 9 (2007) 2334. [6] S.C.S. Lai, S.E.F. Kleyn, V. Rosca, M.T.M. Koper, J. Phys. Chem. C 112 (2008) 19080. [7] A.N. Grace, K. Pandian, Electrochem. Commun. 8 (2006) 1340. [8] Z.X. Wang, J. Zhuge, H.Y. Fang, B.A. Prior, Biotechnol. Adv. 19 (2001) 201. [9] M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C. Della Pina, Angew. Chem. Int. Ed. 46 (2007) 4434. [10] G.A. Camara, R.B. de Lima, T. Iwasita, Electrochem. Commun. 6 (2004) 812. [11] R. Ganesan, J.S. Lee, Angew. Chem. Int. Ed. 44 (2005) 6557. [12] O. Savadogo, K. Lee, K. Oishi, S. Mitsushima, N. Kamiya, K.I. Ota, Electrochem. Commun. 6 (2004) 105. [13] P.K. Shen, C.W. Xu, Electrochem. Commun. 8 (2006) 184. [14] C.W. Xu, P.K. Shen, Y.L. Liu, J. Power Sources 164 (2007) 527. [15] H.P. Liu, J.Q. Ye, C.W. Xu, S.P. Jiang, Y.X. Tong, J. Power Sources 177 (2008) 67. [16] C.W. Xu, H. Wang, P.K. Shen, S.P. Jiang, Adv. Mater. 19 (2007) 4256. [17] H. Wang, C.W. Xu, F.L. Cheng, S.P. Jiang, Electrochem. Commun. 9 (2007) 1212. [18] Y. Wang, W.Z. Jia, T. Strout, A. Schempf, H. Zhang, B.K. Li, J.H. Cui, Y. Lei, Electroanalysis 21 (2009) 1432. [19] L. Su, W.Z. Jia, A. Schempf, Y. Ding, Y. Lei, J. Phys. Chem. C 113 (2009) 16174. [20] M.-C. Jeong, C.H. Pyun, I.-H. Yeo, J. Electrochem. Soc. 140 (1993) 1986. [21] L. Roquet, E.M. Belgsir, J.M. Leger, C. Lamy, Electrochim. Acta 39 (1994) 2387. [22] A. Kahyaoglu, B. Benden, C. Lamy, Electrochim. Acta 29 (1984) 1489. [23] G. Yildiz, F. Kadirgan, J. Electrochem. Soc. 141 (1994) 725. [24] R. Mancharan, J.B. Goodenough, J. Mater. Chem. 2 (1992) 875.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Palladium/titanium dioxide nanofibers for glycerol electrooxidation in alkaline medium Liang Su, Wenzhao Jia, Ashley Schempf, Yu Lei * Department of Chemical, Materials and Biomolecular Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 06269, USA
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 23 September 2009 Accepted 24 September 2009 Available online 27 September 2009 Keywords: Pd/TiO2 nanofibers Glycerol Electrooxidation Poisoning intermediate
a b s t r a c t Highly conductive palladium/titanium dioxide (Pd/TiO2) nanofibers have been successfully fabricated by electroless-plating Pd on the electrospun TiO2 nanofibers. The application of Pd/TiO2 nanofibers for electrooxidation of glycerol was demonstrated. The results showed that Pd/TiO2 nanofibers can greatly promote glycerol electrooxidation in alkaline medium, and both glycerol and KOH concentrations had an effect on the peak current density and the peak potential. The mechanism of desorption of poisoning intermediate was discussed by changing the upper potential limit. The application of the Pd/TiO2 nanofibers for electrooxidation of methanol, ethylene glycol, and 1,2-propanediol was also demonstrated. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Direct alcohol fuel cells (DAFCs), which convert chemical energy of various liquid hydroxyl derivatives of hydrocarbons into electrical energy [1], have been emerging as a new generation of compact power sources for portable electronic devices and electric-powered vehicles and thus attracting extensive attention in recent years [2]. Among all alcohols, the electrooxidation of methanol [3,4] and ethanol [5,6] has been intensively investigated. However, the high volatility of ethanol and the severe toxicity of methanol may cause serious problems in the practical application [3], potentially limiting their further commercialization. Recently, polyhydric alcohols such as ethylene glycol, propanediol and glycerol have been appearing as preferable alternatives to overcome the aforementioned shortcomings [7]. Among all the polyhydric alcohols, glycerol (the simplest triol) is promising since it can be massively produced by microbial fermentation [8]. Furthermore, there is a large amount of glycerol generated as by-product (10 wt.%) in biodiesel production [9]. Therefore, the application of glycerol in DAFCs is highly favorable in terms of safety, economic efficiency and environmental benefit. Various catalysts have been applied to convert chemical energy of alcohols to electrical energy. Platinum (Pt) based electrocatalyst is widely recognized as the most effective one for the electrooxidation of alcohols in acidic medium [10,11]. However, the high price of platinum impedes its extensive application. Recently, palladium * Corresponding author. Tel.: +1 860 486 4554. E-mail address: [email protected] (Y. Lei). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.09.030
(Pd) becomes a promising substitute for Pt because of its outstanding electrocatalytic ability in alcohol oxidation reaction and more abundant existence on the earth than Pt [12]. Shen et al. have shown that Pd/C promoted with oxides electrocatalysts were superior to Pt-based catalysts in alkaline media due to its higher activity and better poisoning tolerance [13,14]. Liu et al. have also shown the excellent catalytic performance of Pd on the electrooxidation of 1-propanol and 2-propanol [15]. Recently, Xu et al. successfully fabricated highly ordered Pd nanowire array which showed improved activity for ethanol oxidation compared to that of commercial E-TEK PtRu/C catalyst [16,17]. In this study, palladium/titanium dioxide (Pd/TiO2) nanofibers were fabricated by electroless-plating of Pd on the surface of electrospun TiO2 nanofibrous membrane, offering an excellent catalyst for electrooxidation of alcohols. The elecrocatalytic property of the Pd/TiO2 nanofibers modified glassy carbon electrode (GCE) was systematically investigated for glycerol electrooxidation in alkaline medium. The results demonstrated the potential applicability of the as-prepared nanostructured Pd/TiO2 electrocatalyst in DAFCs with polyhydric alcohols fuel.
2. Experimental TiO2 nanofibers were prepared by electrospinning using a similar method described elsewhere [18], and the electroless-plating of Pd on TiO2 nanofibers was carried out following our reported procedure [19]. Field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectrometer (EDX), and X-ray
L. Su et al. / Electrochemistry Communications 11 (2009) 2199–2202
diffraction (XRD) were applied to characterize the as-prepared samples. Cyclic voltammetry (CV) was carried out in a three-electrode system. A platinum wire and an Hg/HgO (MMO, 1.0 M KOH, 0.098 V vs. SHE) electrode were employed as the counter and the reference electrode, respectively. Pd/TiO2 nanofibrous membrane was fixed on the surface of GCE with nail oil and served as working electrode. All the solutions were purged with nitrogen before experiments.
3. Results and discussion Typical SEM images of the as-prepared TiO2 nanofibers before and after the deposition of Pd are presented in Fig. 1a and b. The TiO2 nanofibers have an average diameter of 115 nm, while the Pd/TiO2 nanofibers have a larger diameter due to Pd-coating. The average thickness of the coated Pd layer is 176 nm. The coating of Pd on TiO2 nanofibers was further confirmed by EDX analysis. As shown in the inset of Fig. 1c, only Pd element was detected which can be attributed to the fact that the surface of TiO2 was fully covered by a Pd layer. The XRD pattern of the as-prepared sample is presented in Fig. 1c where all diffraction peaks can be assigned to the Pd face-centered-cubic (fcc) crystalline structure (JCPDS card 46-1043). In addition, the as-prepared Pd/TiO2 nanofibers showed excellent electric conductivity which is indispensable in the application of alcohol electrooxidation. Fig. 2a shows the CV of electrooxidation of 0.1 M glycerol in 0.5 M KOH solution on the Pd/TiO2 modified GCE in the scan range from 0.9 V to 0.5 V (vs. MMO). Compared to the CV in the absence of glycerol, a strong glycerol oxidation peak can be observed. Similar to the electrooxidation of other alcohols [5,15], the glycerol
elctrooxidation promoted by Pd in alkaline medium is characterized by two well-defined peaks in the forward and the backward scan, which are due to the electrooxidation of glycerol and the removal of the poisoning intermediates, respectively. As a comparison, there is only one obvious reduction peak at ca. 0.25 V observed in 0.5 M KOH, which is due to the reduction of palladium(II) oxide to Pd0 [20]. By contrast, TiO2 nanofibers modified GCE has no catalytic activity for glycerol electrooxidation in alkaline medium (inset of Fig. 2a). Fig. 2b presents the effect of KOH concentration on the glycerol electrooxidation. The cyclic voltammograms are recorded from 0.9 V to 0.5 V with 0.1 M glycerol, but only the forward scan peaks are presented. One can see that a higher KOH concentration results in a lower oxidation peak potential and an enhanced peak current density. However, both the negative shift of the oxidation peak potential and the enhancement of peak current density show the tendency to gradually level off with the increase of KOH concentration. The results indicate that glycerol electrooxidation is improved by the greater availability of OH-. As the simultaneous attack of hydroxyl groups at C1 and C3 carbon atoms is unfavorable [21], the major glycerol electrooxidation on Pd in alkaline media can be rationally described as follows in light of previous works [21–23].
Pd þ OH $ Pd—ðOHÞads þ e
ðIÞ
Pd þ C2 H3 ðOHÞ2 —CH2 ðOHÞ þ 3OH
$ Pd—½C2 H3 ðOHÞ2 —COads þ 3H2 O þ 3e
ðIIIÞ
! C2 H3 ðOHÞ2 —COOH þ 2Pd C2 H3 ðOHÞ2 —COOH þ OH ! C2 H3 ðOHÞ2 —COO þ H2 O
ðIVÞ
b 100 nm
1 µm
Intensity Pd (200)
500
Pd
0 1
20
20
3
keV
10
Pd (311)
30
Pd
1000
Pd (220)
Intensity (a.u.)
40
1 µm
Pd (111)
1 µm
c
ðIIÞ
Pd—ðOHÞads þ Pd—½C2 H3 ðOHÞ2 —COads
a
50
4
Pd (222)
2200
0 30
40
50
60
70
80
90
2 Theta (degree) Fig. 1. The SEM images of TiO2 nanofibers (a), Pd/TiO2 nanofibers (b), and XRD patterns of Pd/TiO2 nanofibers (c). The insets are high magnification SEM images and EDX analysis, respectively.
L. Su et al. / Electrochemistry Communications 11 (2009) 2199–2202
2201
Fig. 3. CVs of Pd/TiO2 nanofibers modified GCE. (a) 0.5 M KOH + 0.1 M glycerol with different Eupper. The inset shows the CVs in 0.5 M KOH with different Eupper. (b) 0.5 M KOH with or without 0.1 M various alcohols with a scan range from 0.9 V to 0.9 V.
Fig. 2. CVs of Pd/TiO2 nanofibers (a) and TiO2 nanofibers (inset of a) modified GCE in 0.5 M KOH in the presence and absence of 0.1 M glycerol. (b) Effect of KOH concentration on 0.1 M glycerol electrooxidation with Pd/TiO2 nanofibers modified GCE. (c) Effect of glycerol concentration in 0.5 M KOH on its electrooxidation with Pd/TiO2 nanofibers modified GCE. (d) and (e) CVs of the forward and backward scans for 0.1 M glycerol, ethylene glycol, 1,2-propanediol, and methanol in 0.5 M KOH with Pd/TiO2 nanofibers modified GCE, respectively.
The extent of glycerol electrooxidation, namely the forward scan peak current density (If), is determined by the coverage of hydroxyl group and glycerol adsorbed on the Pd active sites, denoted by hPd(OH)ads and hPd(G)ads, respectively. The increase of KOH concentration will not only enhance hPd(OH)ads, but also greatly facilitate the adsorption of glycerol on the catalyst, which results in higher If and negative shift of peak potential. Fig. 2c shows the effect of glycerol concentration on the electrooxidation in 0.5 M KOH (the forward scan peaks only). The oxidation current initially increases with the glycerol concentration up to 0.5 M glycerol, which is attributed to the increase of hPd(G)ads, and then drops with further concentration increase because excessive adsorbed glycerol could block the access of OH- to the active sites on Pd, resulting in an insufficient coverage of (OH)ads and leading to a decrease of the peak current density accordingly. The performance of the Pd/TiO2 nanofibers for the electrooxidation of other alcohols was also investigated. Fig. 2d and e demonstrate the forward and backward scan peak regions for the electrooxidation of glycerol, methanol, ethylene glycol, and 1,2-propanediol, respectively. The well-defined forward and backward scan peaks can be observed for the examined alcohols and demonstrate the excellent performance of the Pd/TiO2 nanofibers. In addition, one can see that under the same molar concentration, glycerol electrooxidation showed the highest forward scan peak current density among all alcohols, indicating that glycerol is a promising candidate in DAFCs.
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L. Su et al. / Electrochemistry Communications 11 (2009) 2199–2202
Fig. 3a shows the effect of different Eupper values on glycerol electrooxidation and its inset presents the corresponding CVs in KOH solution without glycerol. The inset shows that the reduction peak during backward scan in KOH solution increases and shifts negatively with the increase of Eupper since more PdO and stronger Pd–O bond can be generated at higher potential [20]. For glycerol electrooxidation, the increase of Eupper results in insignificant change of If but dramatic diminution of the backward scan peak current density (Ib). In addition, a strong reduction peak corresponding to the reduction of PdO was also observed when Eupper of 0.9 V was applied. The results imply that the adsorbed posioning intermediate species can be effectively removed at higher potential during the forward scan, resulting in the sharp decrease of Ib and the increase of PdO reduction peak in the reverse scan accordingly. It is generally believed that the main poisoning species in the electrooxidation of alcohol is (CO)ads [3,6,7,21,24]. When Eupper is relatively low, the removal of the poisoning species probably happens in the reverse scan as follows. PdðCOÞads þ Pd—ðOHÞads þ 3OH $ 2Pd þ CO2 3 þ 2H2 O þ e
ðVÞ
However, when higher Eupper is applied, Pd(CO)ads can be likely oxidized to form PdO during forward scan through following equation. PdðCOÞads þ 6OH $ PdO þ CO2 3 þ 3H2 O þ 4e
ðVIÞ
Higher Eupper enhances the reaction extent of Eq. (VI). On one hand, with the removal of more poisoning intermediates at higher potential during forward scan, Ib decreases in the backward scan. Especially, as shown in Fig. 3b, no peak can be observed for methanol during backward scan which can be assigned to the complete removal of the poisoning intermediate at higher potential during forward scan. On the other hand, more PdO and stronger Pd–O bond generated at higher Eupper result in the enhanced reduction peak of PdO to Pd and the negative shift of the reduction peak potential during the backward scan. In addition to the major oxidation peak, another oxidation peak can be observed for glycerol, EG, and 1,2propanediol at higher potential. However, the removal of poisoning species or direct alcohol oxidation can not explain this oxidation peak as there is no such peak observed for methanol under the same condition (Fig. 3b). Therefore, this oxidation peak may be attributed to further oxidation of the remaining OH groups. 4. Conclusion Pd/TiO2 nanofibers have been prepared by electroless-plating of Pd on electrospun TiO2. The Pd/TiO2 nanofibers modified GCE
shows excellent catalytic activity for the electrooxidation of glycerol as well as methanol, ethylene glycol, and 1,2-propanediol in alkaline solution. Both KOH and glycerol concentrations affect the glycerol electrooxidation in terms of peak potential and the forward and backward scan peak current densities. Among the examined alcohols with the same molar concentration, glycerol possesses the highest If, indicating that it is a potential candidate in fuel cell application. Moreover, higher potential can effectively remove the poisoning species during the forward scan. When higher Eupper is applied, another oxidation peak can also be observed in the CVs of glycerol, ethylene glycol, and 1,2-propanediol between 0.65 V and 0.75 V, suggesting the further oxidation of the remaining OH groups. Acknowledgments We greatly appreciate the funding from NSF and USGS. LS also thanks the partial support from UConn Center for Environmental Science and Engineering. Reference [1] B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. [2] K. Matsuoka, Y. Iriyama, T. Abe, M. Matsuoka, Z. Ogumi, J. Power Sources 150 (2005) 27. [3] K. Matsuoka, Y. Iriyama, T. Abe, M. Matsuoka, Z. Ogumi, Electrochim. Acta 51 (2005) 1085. [4] W.L. Holstein, H.D. Rosenfeld, J. Phys. Chem. B 109 (2005) 2176. [5] H.P. Liu, J.P. Ye, C.W. Xu, S.P. Jiang, Y.X. Tong, Electrochem. Commun. 9 (2007) 2334. [6] S.C.S. Lai, S.E.F. Kleyn, V. Rosca, M.T.M. Koper, J. Phys. Chem. C 112 (2008) 19080. [7] A.N. Grace, K. Pandian, Electrochem. Commun. 8 (2006) 1340. [8] Z.X. Wang, J. Zhuge, H.Y. Fang, B.A. Prior, Biotechnol. Adv. 19 (2001) 201. [9] M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C. Della Pina, Angew. Chem. Int. Ed. 46 (2007) 4434. [10] G.A. Camara, R.B. de Lima, T. Iwasita, Electrochem. Commun. 6 (2004) 812. [11] R. Ganesan, J.S. Lee, Angew. Chem. Int. Ed. 44 (2005) 6557. [12] O. Savadogo, K. Lee, K. Oishi, S. Mitsushima, N. Kamiya, K.I. Ota, Electrochem. Commun. 6 (2004) 105. [13] P.K. Shen, C.W. Xu, Electrochem. Commun. 8 (2006) 184. [14] C.W. Xu, P.K. Shen, Y.L. Liu, J. Power Sources 164 (2007) 527. [15] H.P. Liu, J.Q. Ye, C.W. Xu, S.P. Jiang, Y.X. Tong, J. Power Sources 177 (2008) 67. [16] C.W. Xu, H. Wang, P.K. Shen, S.P. Jiang, Adv. Mater. 19 (2007) 4256. [17] H. Wang, C.W. Xu, F.L. Cheng, S.P. Jiang, Electrochem. Commun. 9 (2007) 1212. [18] Y. Wang, W.Z. Jia, T. Strout, A. Schempf, H. Zhang, B.K. Li, J.H. Cui, Y. Lei, Electroanalysis 21 (2009) 1432. [19] L. Su, W.Z. Jia, A. Schempf, Y. Ding, Y. Lei, J. Phys. Chem. C 113 (2009) 16174. [20] M.-C. Jeong, C.H. Pyun, I.-H. Yeo, J. Electrochem. Soc. 140 (1993) 1986. [21] L. Roquet, E.M. Belgsir, J.M. Leger, C. Lamy, Electrochim. Acta 39 (1994) 2387. [22] A. Kahyaoglu, B. Benden, C. Lamy, Electrochim. Acta 29 (1984) 1489. [23] G. Yildiz, F. Kadirgan, J. Electrochem. Soc. 141 (1994) 725. [24] R. Mancharan, J.B. Goodenough, J. Mater. Chem. 2 (1992) 875.