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The mechanism of formic acid electrooxidation on iron tetrasulfophthalocyanine-modified platinum electrode
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Electrochemistry Communications 10 (2008) 131–135 www.elsevier.com/locate/elecom
The mechanism of formic acid electrooxidation on iron tetrasulfophthalocyanine-modified platinum electrode Zhonghua Zhang, Xiaochun Zhou, Changpeng Liu, Wei Xing
*
State Key Laboratory of Electro-analytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun, Jilin 130022, PR China Received 10 October 2007; received in revised form 26 October 2007; accepted 27 October 2007 Available online 4 November 2007
Abstract The mechanism of formic acid electrooxidation on iron tetrasulfophthalocyanine (FeTSPc) modified Pt electrode was investigated with electrochemical methods. It was found that a ‘‘third-body’’ effect of FeTSPc on Pt electrode predominates during the electrooxidation process based on unusual electrochemical results. The modification leads formic acid electrooxidation to take place through a desired direct pathway, in which the mechanism is proposed to be the gradual dehydrogenation of formic acid and the reaction of formate with hydroxyl species. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Formic acid electrooxidation; Electrochemical impedance spectroscopy; Iron tetrasulfophthalocyanine; Polycrystalline Pt electrode
1. Introduction In recent years, formic acid has been attracting great attentions as a promising alternative fuel for liquid fuel cells [1–3] due to its many advantages, such as higher electrochemical activity, no toxicity, less poisoning to Pt-based electrocatalysts [4] and lower crossover through Nafion membrane [5,6] than methanol. It is obvious that formic acid electrooxidation (FAEO) plays an important role in direct formic acid fuel cells (DFAFCs). Generally, FAEO on Pt-based catalysts proceeds through a dual path mechanism that involves a direct dehydrogenation pathway and an indirect dehydration pathway [7,8]. The strong adsorption of COad on Pt surface in the indirect pathway will occupy its active sites and lead to poisoning Pt catalysts. Therefore, the direct pathway is the desired one due to no strongly adsorbed intermediates and thus FAEO can proceed rapidly on the unoccupied active sites on Pt sur*
Corresponding author. Tel.: +86 431 85262223; fax: +86 431 85685653. E-mail address: [email protected] (W. Xing). 1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.10.031
face. Very recently, it was found that FAEO on iron tetrasulfophthalocyanine (FeTSPc) modified Pt can take place through the desired direct pathway [9]. The electrocatalytic activity of the modified Pt for FAEO has been enhanced dramatically to 10 times. In this communication, we elucidated the function of FeTSPc on the enhancement and the mechanism of FAEO on FeTSPc-modified platinum electrode. Unusual electrochemical impedance spectra (EIS) without pseudo-inductive behavior were obtained on the modified electrode, which validated that the function of FeTSPc was associated with a ‘‘third-body’’ effect. The mechanism was drawn out to be the gradual dehydrogenation of formic acid and the reaction of formate and hydroxyl species. 2. Experimental FeTSPc (Aldrich) and all other chemicals were of analytical grade and used as received. All solutions were prepared with Millipore water (18.2 MX cm) and deaerated with ultra-pure nitrogen gas. Electrochemical measurements were performed at ambient temperature with an
132
Z. Zhang et al. / Electrochemistry Communications 10 (2008) 131–135
EG&G PARC potentiostat/galvanostat (model 273A), Perkin–Elmer Lock-in Amplifier (model 5210) and a conventional three-compartment electrochemical cell. All potentials in the communication were referred to Ag|AgCl|KCl (sat.) electrode. A smooth polycrystalline Pt electrode (4 mm in diameter) as working electrode was polished with 0.3 and 0.05 lm alumina, respectively, and then sonicated in Millipore water before test. The CV measurements were carried out at a sweep rate of 100 mV s1 after the Pt electrode was immersed for 5 min into 0.1 M HClO4 solution containing FeTSPc until stable curves were obtained. Prior to EIS measurement, the modified electrode was cleaned electrochemically by cycling the potential between 0.2 and 1.2 V at 100 mV s1 to obtain stable CV curves. After equilibration for 5 min at measurement potentials, EIS were recorded at 10 points per decade in the frequency range of 0.01 Hz–100 KHz. An 10 mV amplitude of the sinusoidal potential signal was fixed to minimize the signal noise. 3. Results and discussion Fig. 1a shows the CVs for polycrystalline Pt electrode in 0.1 M HClO4 solutions with and without 0.1 mg mL1 FeTSPc, respectively. The hydrogen adsorption/desorption area and Pt oxidation/reduction area are suppressed to a certain extent due to the adsorption of FeTSPc on Pt electrode. The coverage of FeTSPc on Pt electrode can be calculated as ca. 0.36 by the hydrogen desorption peak area, which means that FeTSPc occupies nearly 1/3 of Pt sites. Obvious differences in the CVs for FAEO can be found between the modified electrode and the bare Pt electrode as shown in Fig. 1b. There exist two anodic peaks at ca. 0.47 V on the modified electrode instead of four peaks (I, II, IV and V) on the bare electrode. The positions of the peaks on the modified electrode coincide with those of peak I and IV on the bare electrode. Peak I and II on bare electrode are attributed to the oxidation of adsorbed formate in direct pathway on the sites unblocked by intermediate COad and the oxidation of both COad in indirect path and formate in direct path, respectively [10,11]. This hints that the oxidation of formic acid on the modified Pt electrode takes place through direct pathway. Furthermore, the two anodic peaks with approximately equal size and similar shape on the modified electrode indicate that there is no hysteresis effect in the CV process, which implies little or no strongly adsorbed intermediates during FAEO on the modified electrode. Interestingly, the connatural ‘‘pseudoinductive behavior’’ associated with strongly adsorbed intermediates during FAEO on bare Pt electrodes [6,12– 14] disappeared in the EIS on the modified electrode as Fig. 2 showed, which substantiates the conclusion from the CV results that there is little or no strongly adsorbed intermediate, e.g. COad. It is evident that the modification of FeTSPc can prevent radically the poisoning species on Pt, which enhances the electrocatalytic activity for FAEO. Likewise, there is no
Fig. 1. The cyclic voltammograms for polycrystalline Pt electrode (a) in 0.1 M HClO4 solutions (b) 0.1 M HCOOH + 0.1 M HClO4 solutions with (dashed line) and without (solid line) 0.1 mg mL1 FeTSPc, respectively. Sweep rate: 100 mV s1.
poisonous species during FAEO on Pd electrode, where the auto-inhibition of the reaction is caused probably by C(OH)2 species [8]. The electrocatalytic activity of Pd could be enhanced significantly by submonolayer of inactive elements for the electrooxidation, such as Pb, Bi, Cd, which was explained by the ‘‘third-body’’ effect [12]. Generally, the ‘‘third-body’’ effect plays a little role on Pt-based electrodes, where co-catalytic effect predominates [13,14]. FeTSPc itself exhibits no catalytic activity for FAEO [9] and cannot possess co-catalytic effect. It is reasonable to attribute the enhancement for FAEO to the ‘‘third-body’’ effect from FeTSPc. It is observed that there are three characteristic loops as shown in Fig. 3 that is extracted from Fig. 2a at two potentials, 0.2 and 0.4 V, for clearly representing the EIS characteristics. Fig. 4 represents the dependence of those loops on potential. The capacitive loop one is magnified in the insets of Fig. 3 is an incomplete arc in high-frequency range of 100–20 KHz at all potentials. As Fig. 4a showed, loop one is independent of potential within the measure error
Z. Zhang et al. / Electrochemistry Communications 10 (2008) 131–135
Fig. 2. The Nyquist plots (panel a) and Bode plots (panel b) for 0.1 M formic acid electrooxidation on polycrystalline Pt electrode in 0.1 M HClO4 solution containing 0.1 mg mL1 FeTSPc at various potentials. f: 0.01 Hz–100 KHz.
despite tiny difference, which is clearly observed from the superposition of Bode plots (Fig. 2b) of loop one at all potentials. Therefore, loop one should be attributed to interfacial behavior between the modified electrode and the solution. The capacitive loop two in medium-frequency range of 20 KHz–10 Hz at all potentials in Fig. 3 is potential-dependent as shown in Fig. 4b, which indicates that it is associated with a Faradaic reaction. Herein, the Faradaic reaction is gradual dehydrogenation of FAEO (Eqs. (1) and (2)) [6,15]. It is evident from the potential-dependence of loop two diameter in Fig. 4b that charge-transfer resistance for the dehydrogenation decreases before 0.30 V and then increases with increasing potential. It can be drawn out that the dehydrogenation is driven mainly by potential before 0.3 V and influenced mainly by another factor after 0.3 V that will be discussed later. The capacitive loop three in low-frequency range of 10–0.01 Hz in Fig. 3 exhibits different behaviors as shown in Fig. 3a (before 0.3 V) and in the Fig. 3b (after 0.3 V). Loop three in Fig. 3a is small and depressed in the first quadrant while loop three in Fig. 3b is reversed into the second quadrant. The former is associated with the mass-transfer process of
133
Fig. 3. The typical EIS at two potentials, 0.2 V (panel a) and 0.4 V (panel b), reconstructed after extracted from Fig. 2.
formic acid because it varies with the rotating rate of the electrode. The latter is indicative of negative charge-transfer resistance, which is associated with the formation of passivation originating from the chemisorbed hydroxyl species on electrode surface [15–17]. As shown in Fig. 4c, the diameter of loop three decreases initially from 0.3 to 0.45 V, keeps constant in 0.45–0.50 V and then increases with increasing potential, which indicates that the reactive resistance also follows the same trend as that on Pt/C [6]. If there is no adsorbed intermediate reacting with hydroxyl species, the reactive resistance would increase continuously due to that the strong adsorption of hydroxyl species on Pt surface can occupy the sites of formic acid and its intermediate [6,15]. Obviously, the inconsistent trend obtained in EIS means that there exists another intermediate reacting with hydroxyl species. We have proved from the results of CV and EIS that COad is not the intermediate of FAEO on the modified electrode. It was discovered with in situ Fourier transform infrared spectroscopy in an attenuated total reflection configuration that formate is validated as a reactive intermediate in the direct pathway [18,19]. It is reasonable to propose that the reaction of formate with hydroxyl species leads to the initial decrease of Faraday
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Z. Zhang et al. / Electrochemistry Communications 10 (2008) 131–135
Fig. 4. Three characteristic impedance loops locating in the high, medium and low-frequency as a function of potential, respectively.
resistance (Eqs. (3) and (4)). When the potential reaches 0.45–0.5 V, the resistance arrives at the minimum while the CV current for FAEO at the crest (Fig. 1b), which indicates that the reaction of formate with OH species is in equilibrium. With further increasing potential, more OH species occupy so much sites that the reactive resistance increases and the CV current decreases rapidly. In addition, it can be deduced from the difference in the potential-dependence of loop three (Figs. 2 and 3) that the rate-determining step for FAEO transforms from the mass-transfer process to the reaction of formate with hydroxyl species at 0.30 V. According to the above discussion, a mechanism for FAEO on the modified electrode is as follows: Pt þ HCOOH Pt ðHCOOÞ þ Hþ þ e þ
Pt ðHCOOÞ ! Pt þ CO2 þ H þ e Pt þ H2 O ! Pt OH þ Hþ þ e
Pt OH þ Pt ðHCOOÞ ! 2Pt þ CO2 þ H2 O
FeTSPc on Pt electrode plays an important role in the enhancement effect on FAEO. The mechanism is proposed to be the gradual dehydrogenation of formic acid and the reaction of formate with hydroxyl species. The rate-determining step of FAEO on the modified electrode is potential-dependent, which can transform from the masstransfer process to the reaction of formate with hydroxyl species at a transition potential. Acknowledgements The authors are grateful for the financial sponsors of Nature Science Foundation of China (20433060, 20703043).
ð1Þ ð2Þ ð3Þ ð4Þ
4. Conclusions The FAEO mechanism on FeTSPc-modified Pt electrode was found to undergo a desired direct pathway, where poisonous intermediate, e.g. COad, was not formed on the modified electrode. The ‘‘third-body’’ effect of
References [1] C. Rice, R.I. Ha, R.I. Masel, P. Waszczuk, A. Wieckowski, T. Barnard, J. Power Sources 111 (2002) 83. [2] Y.M. Zhu, S.Y. Ha, R.I. Masel, J. Power Sources 130 (2004) 8. [3] S. Ha, B. Adams, R.I. Masel, J. Power Sources 128 (2004) 119. [4] O. Wolter, J. Willsau, J. Heitbaum, J. Electrochem. Soc. 132 (1985) 1635. [5] Y.W. Rhee, S.Y. Ha, R.I. Masel, J. Power Sources 117 (2003) 35. [6] X. Wang, J.-M. Hu, I.M. Hsing, J. Electroanal. Chem. 562 (2004) 73. [7] A. Capon, R. Parsons, J. Electroanal. Chem. 44 (1973) 239. [8] A. Capon, R. Parsons, J. Electroanal. Chem. 45 (1973) 205. [9] X. Zhou, W. Xing, C. Liu, T. Lu, Electrochem. Commun. 9 (2007) 1469.
Z. Zhang et al. / Electrochemistry Communications 10 (2008) 131–135 [10] H. Okamoto, W. Kon, Y. Mukouyama, J. Phys. Chem. B 108 (2004) 4432. [11] H. Okamoto, W. Kon, Y. Mukouyama, J. Phys. Chem. B 109 (2005) 15659. [12] R.R. Adzic, M.D. Spasojevic, A.R. Despic, J. Electroanal. Chem. 92 (1978) 31. [13] S. Daniele, S. Bergamin, Electrochem. Commun. 9 (2007) 1388. [14] S. Uhm, S.T. Chung, J. Lee, Electrochem. Commun. 9 (2007) 2027.
135
[15] W. Chen, J. Kim, S.H. Sun, S.W. Chen, Phys. Chem. Chem. Phys. 8 (2006) 2779. [16] R.E. Melnick, G.T.R. Palmore, J. Phys. Chem. B 105 (2001) 1012. [17] R.E. Melnick, G.T.R. Palmore, J. Phys. Chem. B 105 (2001) 9449. [18] G. Samjeske, M. Osawa, Angew. Chem.-Int. Ed. 44 (2005) 5694. [19] G. Samjeske, A. Miki, S. Ye, M. Osawa, J. Phys. Chem. B 110 (2006) 16559.
Electrochemistry Communications 10 (2008) 131–135 www.elsevier.com/locate/elecom
The mechanism of formic acid electrooxidation on iron tetrasulfophthalocyanine-modified platinum electrode Zhonghua Zhang, Xiaochun Zhou, Changpeng Liu, Wei Xing
*
State Key Laboratory of Electro-analytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun, Jilin 130022, PR China Received 10 October 2007; received in revised form 26 October 2007; accepted 27 October 2007 Available online 4 November 2007
Abstract The mechanism of formic acid electrooxidation on iron tetrasulfophthalocyanine (FeTSPc) modified Pt electrode was investigated with electrochemical methods. It was found that a ‘‘third-body’’ effect of FeTSPc on Pt electrode predominates during the electrooxidation process based on unusual electrochemical results. The modification leads formic acid electrooxidation to take place through a desired direct pathway, in which the mechanism is proposed to be the gradual dehydrogenation of formic acid and the reaction of formate with hydroxyl species. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Formic acid electrooxidation; Electrochemical impedance spectroscopy; Iron tetrasulfophthalocyanine; Polycrystalline Pt electrode
1. Introduction In recent years, formic acid has been attracting great attentions as a promising alternative fuel for liquid fuel cells [1–3] due to its many advantages, such as higher electrochemical activity, no toxicity, less poisoning to Pt-based electrocatalysts [4] and lower crossover through Nafion membrane [5,6] than methanol. It is obvious that formic acid electrooxidation (FAEO) plays an important role in direct formic acid fuel cells (DFAFCs). Generally, FAEO on Pt-based catalysts proceeds through a dual path mechanism that involves a direct dehydrogenation pathway and an indirect dehydration pathway [7,8]. The strong adsorption of COad on Pt surface in the indirect pathway will occupy its active sites and lead to poisoning Pt catalysts. Therefore, the direct pathway is the desired one due to no strongly adsorbed intermediates and thus FAEO can proceed rapidly on the unoccupied active sites on Pt sur*
Corresponding author. Tel.: +86 431 85262223; fax: +86 431 85685653. E-mail address: [email protected] (W. Xing). 1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.10.031
face. Very recently, it was found that FAEO on iron tetrasulfophthalocyanine (FeTSPc) modified Pt can take place through the desired direct pathway [9]. The electrocatalytic activity of the modified Pt for FAEO has been enhanced dramatically to 10 times. In this communication, we elucidated the function of FeTSPc on the enhancement and the mechanism of FAEO on FeTSPc-modified platinum electrode. Unusual electrochemical impedance spectra (EIS) without pseudo-inductive behavior were obtained on the modified electrode, which validated that the function of FeTSPc was associated with a ‘‘third-body’’ effect. The mechanism was drawn out to be the gradual dehydrogenation of formic acid and the reaction of formate and hydroxyl species. 2. Experimental FeTSPc (Aldrich) and all other chemicals were of analytical grade and used as received. All solutions were prepared with Millipore water (18.2 MX cm) and deaerated with ultra-pure nitrogen gas. Electrochemical measurements were performed at ambient temperature with an
132
Z. Zhang et al. / Electrochemistry Communications 10 (2008) 131–135
EG&G PARC potentiostat/galvanostat (model 273A), Perkin–Elmer Lock-in Amplifier (model 5210) and a conventional three-compartment electrochemical cell. All potentials in the communication were referred to Ag|AgCl|KCl (sat.) electrode. A smooth polycrystalline Pt electrode (4 mm in diameter) as working electrode was polished with 0.3 and 0.05 lm alumina, respectively, and then sonicated in Millipore water before test. The CV measurements were carried out at a sweep rate of 100 mV s1 after the Pt electrode was immersed for 5 min into 0.1 M HClO4 solution containing FeTSPc until stable curves were obtained. Prior to EIS measurement, the modified electrode was cleaned electrochemically by cycling the potential between 0.2 and 1.2 V at 100 mV s1 to obtain stable CV curves. After equilibration for 5 min at measurement potentials, EIS were recorded at 10 points per decade in the frequency range of 0.01 Hz–100 KHz. An 10 mV amplitude of the sinusoidal potential signal was fixed to minimize the signal noise. 3. Results and discussion Fig. 1a shows the CVs for polycrystalline Pt electrode in 0.1 M HClO4 solutions with and without 0.1 mg mL1 FeTSPc, respectively. The hydrogen adsorption/desorption area and Pt oxidation/reduction area are suppressed to a certain extent due to the adsorption of FeTSPc on Pt electrode. The coverage of FeTSPc on Pt electrode can be calculated as ca. 0.36 by the hydrogen desorption peak area, which means that FeTSPc occupies nearly 1/3 of Pt sites. Obvious differences in the CVs for FAEO can be found between the modified electrode and the bare Pt electrode as shown in Fig. 1b. There exist two anodic peaks at ca. 0.47 V on the modified electrode instead of four peaks (I, II, IV and V) on the bare electrode. The positions of the peaks on the modified electrode coincide with those of peak I and IV on the bare electrode. Peak I and II on bare electrode are attributed to the oxidation of adsorbed formate in direct pathway on the sites unblocked by intermediate COad and the oxidation of both COad in indirect path and formate in direct path, respectively [10,11]. This hints that the oxidation of formic acid on the modified Pt electrode takes place through direct pathway. Furthermore, the two anodic peaks with approximately equal size and similar shape on the modified electrode indicate that there is no hysteresis effect in the CV process, which implies little or no strongly adsorbed intermediates during FAEO on the modified electrode. Interestingly, the connatural ‘‘pseudoinductive behavior’’ associated with strongly adsorbed intermediates during FAEO on bare Pt electrodes [6,12– 14] disappeared in the EIS on the modified electrode as Fig. 2 showed, which substantiates the conclusion from the CV results that there is little or no strongly adsorbed intermediate, e.g. COad. It is evident that the modification of FeTSPc can prevent radically the poisoning species on Pt, which enhances the electrocatalytic activity for FAEO. Likewise, there is no
Fig. 1. The cyclic voltammograms for polycrystalline Pt electrode (a) in 0.1 M HClO4 solutions (b) 0.1 M HCOOH + 0.1 M HClO4 solutions with (dashed line) and without (solid line) 0.1 mg mL1 FeTSPc, respectively. Sweep rate: 100 mV s1.
poisonous species during FAEO on Pd electrode, where the auto-inhibition of the reaction is caused probably by C(OH)2 species [8]. The electrocatalytic activity of Pd could be enhanced significantly by submonolayer of inactive elements for the electrooxidation, such as Pb, Bi, Cd, which was explained by the ‘‘third-body’’ effect [12]. Generally, the ‘‘third-body’’ effect plays a little role on Pt-based electrodes, where co-catalytic effect predominates [13,14]. FeTSPc itself exhibits no catalytic activity for FAEO [9] and cannot possess co-catalytic effect. It is reasonable to attribute the enhancement for FAEO to the ‘‘third-body’’ effect from FeTSPc. It is observed that there are three characteristic loops as shown in Fig. 3 that is extracted from Fig. 2a at two potentials, 0.2 and 0.4 V, for clearly representing the EIS characteristics. Fig. 4 represents the dependence of those loops on potential. The capacitive loop one is magnified in the insets of Fig. 3 is an incomplete arc in high-frequency range of 100–20 KHz at all potentials. As Fig. 4a showed, loop one is independent of potential within the measure error
Z. Zhang et al. / Electrochemistry Communications 10 (2008) 131–135
Fig. 2. The Nyquist plots (panel a) and Bode plots (panel b) for 0.1 M formic acid electrooxidation on polycrystalline Pt electrode in 0.1 M HClO4 solution containing 0.1 mg mL1 FeTSPc at various potentials. f: 0.01 Hz–100 KHz.
despite tiny difference, which is clearly observed from the superposition of Bode plots (Fig. 2b) of loop one at all potentials. Therefore, loop one should be attributed to interfacial behavior between the modified electrode and the solution. The capacitive loop two in medium-frequency range of 20 KHz–10 Hz at all potentials in Fig. 3 is potential-dependent as shown in Fig. 4b, which indicates that it is associated with a Faradaic reaction. Herein, the Faradaic reaction is gradual dehydrogenation of FAEO (Eqs. (1) and (2)) [6,15]. It is evident from the potential-dependence of loop two diameter in Fig. 4b that charge-transfer resistance for the dehydrogenation decreases before 0.30 V and then increases with increasing potential. It can be drawn out that the dehydrogenation is driven mainly by potential before 0.3 V and influenced mainly by another factor after 0.3 V that will be discussed later. The capacitive loop three in low-frequency range of 10–0.01 Hz in Fig. 3 exhibits different behaviors as shown in Fig. 3a (before 0.3 V) and in the Fig. 3b (after 0.3 V). Loop three in Fig. 3a is small and depressed in the first quadrant while loop three in Fig. 3b is reversed into the second quadrant. The former is associated with the mass-transfer process of
133
Fig. 3. The typical EIS at two potentials, 0.2 V (panel a) and 0.4 V (panel b), reconstructed after extracted from Fig. 2.
formic acid because it varies with the rotating rate of the electrode. The latter is indicative of negative charge-transfer resistance, which is associated with the formation of passivation originating from the chemisorbed hydroxyl species on electrode surface [15–17]. As shown in Fig. 4c, the diameter of loop three decreases initially from 0.3 to 0.45 V, keeps constant in 0.45–0.50 V and then increases with increasing potential, which indicates that the reactive resistance also follows the same trend as that on Pt/C [6]. If there is no adsorbed intermediate reacting with hydroxyl species, the reactive resistance would increase continuously due to that the strong adsorption of hydroxyl species on Pt surface can occupy the sites of formic acid and its intermediate [6,15]. Obviously, the inconsistent trend obtained in EIS means that there exists another intermediate reacting with hydroxyl species. We have proved from the results of CV and EIS that COad is not the intermediate of FAEO on the modified electrode. It was discovered with in situ Fourier transform infrared spectroscopy in an attenuated total reflection configuration that formate is validated as a reactive intermediate in the direct pathway [18,19]. It is reasonable to propose that the reaction of formate with hydroxyl species leads to the initial decrease of Faraday
134
Z. Zhang et al. / Electrochemistry Communications 10 (2008) 131–135
Fig. 4. Three characteristic impedance loops locating in the high, medium and low-frequency as a function of potential, respectively.
resistance (Eqs. (3) and (4)). When the potential reaches 0.45–0.5 V, the resistance arrives at the minimum while the CV current for FAEO at the crest (Fig. 1b), which indicates that the reaction of formate with OH species is in equilibrium. With further increasing potential, more OH species occupy so much sites that the reactive resistance increases and the CV current decreases rapidly. In addition, it can be deduced from the difference in the potential-dependence of loop three (Figs. 2 and 3) that the rate-determining step for FAEO transforms from the mass-transfer process to the reaction of formate with hydroxyl species at 0.30 V. According to the above discussion, a mechanism for FAEO on the modified electrode is as follows: Pt þ HCOOH Pt ðHCOOÞ þ Hþ þ e þ
Pt ðHCOOÞ ! Pt þ CO2 þ H þ e Pt þ H2 O ! Pt OH þ Hþ þ e
Pt OH þ Pt ðHCOOÞ ! 2Pt þ CO2 þ H2 O
FeTSPc on Pt electrode plays an important role in the enhancement effect on FAEO. The mechanism is proposed to be the gradual dehydrogenation of formic acid and the reaction of formate with hydroxyl species. The rate-determining step of FAEO on the modified electrode is potential-dependent, which can transform from the masstransfer process to the reaction of formate with hydroxyl species at a transition potential. Acknowledgements The authors are grateful for the financial sponsors of Nature Science Foundation of China (20433060, 20703043).
ð1Þ ð2Þ ð3Þ ð4Þ
4. Conclusions The FAEO mechanism on FeTSPc-modified Pt electrode was found to undergo a desired direct pathway, where poisonous intermediate, e.g. COad, was not formed on the modified electrode. The ‘‘third-body’’ effect of
References [1] C. Rice, R.I. Ha, R.I. Masel, P. Waszczuk, A. Wieckowski, T. Barnard, J. Power Sources 111 (2002) 83. [2] Y.M. Zhu, S.Y. Ha, R.I. Masel, J. Power Sources 130 (2004) 8. [3] S. Ha, B. Adams, R.I. Masel, J. Power Sources 128 (2004) 119. [4] O. Wolter, J. Willsau, J. Heitbaum, J. Electrochem. Soc. 132 (1985) 1635. [5] Y.W. Rhee, S.Y. Ha, R.I. Masel, J. Power Sources 117 (2003) 35. [6] X. Wang, J.-M. Hu, I.M. Hsing, J. Electroanal. Chem. 562 (2004) 73. [7] A. Capon, R. Parsons, J. Electroanal. Chem. 44 (1973) 239. [8] A. Capon, R. Parsons, J. Electroanal. Chem. 45 (1973) 205. [9] X. Zhou, W. Xing, C. Liu, T. Lu, Electrochem. Commun. 9 (2007) 1469.
Z. Zhang et al. / Electrochemistry Communications 10 (2008) 131–135 [10] H. Okamoto, W. Kon, Y. Mukouyama, J. Phys. Chem. B 108 (2004) 4432. [11] H. Okamoto, W. Kon, Y. Mukouyama, J. Phys. Chem. B 109 (2005) 15659. [12] R.R. Adzic, M.D. Spasojevic, A.R. Despic, J. Electroanal. Chem. 92 (1978) 31. [13] S. Daniele, S. Bergamin, Electrochem. Commun. 9 (2007) 1388. [14] S. Uhm, S.T. Chung, J. Lee, Electrochem. Commun. 9 (2007) 2027.
135
[15] W. Chen, J. Kim, S.H. Sun, S.W. Chen, Phys. Chem. Chem. Phys. 8 (2006) 2779. [16] R.E. Melnick, G.T.R. Palmore, J. Phys. Chem. B 105 (2001) 1012. [17] R.E. Melnick, G.T.R. Palmore, J. Phys. Chem. B 105 (2001) 9449. [18] G. Samjeske, M. Osawa, Angew. Chem.-Int. Ed. 44 (2005) 5694. [19] G. Samjeske, A. Miki, S. Ye, M. Osawa, J. Phys. Chem. B 110 (2006) 16559.