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CH3OH solution
Electrochimica Acta 47 (2002) 1983 /1988 www.elsevier.com/locate/electacta
Oxygen reduction at Pt and Pt70Ni30 in H2SO4/CH3OH solution J.-F. Drillet a,*, A. Ee a, J. Friedemann a, R. Ko¨tz b,1, B. Schnyder b, V.M. Schmidt a,1 a
Mannheim University of Applied Sciences, Institute of Environmental Engineering, D-68163 Mannheim, Germany b Paul Scherrer Institute, General Energy Department, CH-5232 Villigen, Switzerland Received 23 November 2001
Abstract The electrochemical oxygen reduction reaction (ORR) was studied at Pt and Pt alloyed with 30 atom% Ni in 1 M H2SO4 and in 1 M H2SO4/0.5 M CH3OH by means of rotating disc electrode. In pure sulphuric acid, the overpotential of ORR at 1 mA cm 2 is about 80 mV lower at Pt70Ni30 than at pure Pt. It was found that in methanol containing electrolyte solution the onset potential for oxygen reduction at PtNi is shifted to more positive potentials and the alloy catalyst has an 11 times higher limiting current density for oxygen reduction than Pt. Thus, PtNi as cathode catalyst should have a higher methanol tolerance for fuel cell applications. On the other hand, no significant differences in the methanol oxidation on both electrodes was found using cycling voltammetry, especially regarding the onset potential for methanol oxidation. During all the measurements no significant electrochemical activity loss was observed at Pt0.7Ni0.3. Ex-situ XPS investigations before and after the electrochemical experiments have revealed Pt enrichment in the first surface layers of the PtNi. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Platinum; Platinum /nickel alloy; Oxygen reduction reaction (ORR); Methanol tolerant cathode; Direct methanol fuel cell
1. Introduction The use of methanol as energy carrier and its direct electrochemical oxidation in a direct methanol fuel cell (DMFC) represents an important challenge for the polymer electrolyte fuel cell technology, since the complete system would be simpler without a reformer and further gas treatment steps. However, beside the difficulty in finding an anode catalyst with high activity for methanol oxidation, the methanol crossover through the polymer electrolyte leads to a mixed potential at the cathode, which results from the oxygen reduction reaction (ORR) and the methanol oxidation occurring simultaneously. This effect causes a negative potential shift at the cathode and a significant decrease of performance in a DMFC. This problem should be solved either by using electrolytes with lower methanol permeability or by developing new cathode catalysts. For the latter, two concepts can be identified. First, cathode catalysts in a DMFC should
* Corresponding author. E-mail address: [email protected] (J.-F. Drillet). 1 ISE member.
show a high methanol tolerance, that means that the oxygen reduction will not be affected by the adsorption and oxidation of methanol. Secondly, in order to shift the mixed potential to more positive potentials, the catalyst should show a higher exchange current density for oxygen reduction compared to Pt, which is the best cathode catalyst. In this case, it is assumed that the methanol oxidation at the cathode should not be affected significantly by alloying or is at least of minor influence. High methanol tolerance is reported in the literature for non-noble metal catalysts based on chalcogenides [1 /4] or metalloporphyrins [5]. These catalysts have already shown nearly the same current/potential behaviour in the absence as well as in the presence of methanol. However, in methanol free solution these materials did not attain the activity of Pt. Furthermore, the long time stability under fuel cell conditions have still to be improved. Some platinum-based binary alloys such as PtFe, PtCo, PtNi and PtCr exhibit a higher catalytic activity for the ORR in pure acid electrolytes than pure platinum [6 /10]. The enhanced electrocatalysis can be explained by an electronic factor, i.e. the change of the d-band vacancy in Pt upon alloying and/or by geometric
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 0 2 7 - 0
1984
J.-F. Drillet et al. / Electrochimica Acta 47 (2002) 1983 /1988
effects (Pt coordination number and Pt /Pt distance) (see [9] and references therein). Both effects should enhance the reaction rate for oxygen adsorption and breaking the O /O bond during the reduction reaction. For example, the lattice constant a0 in the cubic PtX alloy (X /Fe, Co, Ni) decreases with increasing alloying component X. In the case of PtNi, the maximum electrochemical activity for ORR has been found to occur at 30 atomic% Ni [9,10]. The purpose of this work is first to evaluate the activity of pure Pt and Pt0.7Ni0.3 for the oxygen reduction in pure 1 M H2SO4 electrolyte solution and secondly, to study the influence of methanol on the ORR. This aspect should be of great practical interest for fuel cell applications. The experiments were done under controlled mass transport conditions using rotating disk electrodes of the materials.
2. Experimental Pt and Pt0.7Ni0.3 disc electrodes with a diameter of 12 and 2 mm in thickness were manufactured by Hauner GmbH (Germany) with 99.95% purity. Pt and Ni pellets were melted together in a vacuum arc oven and then pressed and turned to the final diameter of 1 cm2 for both. The electrodes were polished with diamond paste with particle size of 3 mm. Before each experiment the electrodes were cleaned in a H2SO4/H2O2 solution for about 30 s. After rinsing with pure water they were inserted in a PTFE disc holder. The electrodes were then cycled at dE /dt/100 mV s 1 between the onset potentials of hydrogen and oxygen evolution until reproducible cyclic voltammograms (CVs) were obtained under N2 atmosphere.
After electrode activation the potential was stopped at E /0.9 V and a negative going potential sweep was started with dE /dt/5 mV s 1 to the final potential of E //0.1 V in oxygen saturated 1 M H2SO4. The rotating disc electrode (EG&E Instruments) measurements were performed up to a rotation speed of 4000 rpm. An Ag/AgCl electrode was used as reference. Methanol containing solutions (cMethanol /0.5 mol dm 3) were changed after each RDE experiment in order to maintain a constant methanol concentration in the electrolyte. All experiments were done at room temperature. Ex-situ X-ray photoelectron spectroscopy (XPS) measurements were carried out at the Paul Scherrer Institute (Switzerland) in order to determine the Pt /Ni atomic ratio of the electrode surface before and after the experiments. The XPS spectra were recorded with an ESCALAB 220i XL (VG Scientific) photoelectron spectrometer using monochromatic Al Ka radiation. The X-ray power was 200 W. The spectra were recorded in the constant analyzer mode with analyzer pass energies of 50 eV for the survey spectra and 20 eV for the high-resolution detail spectra. The composition of the samples was determined by quantitative analysis of the spectra using the cross sections of Scofield [11]. The base pressure in the analysis chamber during analysis was always better than 5 /10 9 mbar.
3. Results and discussion Fig. 1 shows CVs of Pt and Pt70Ni30 in 1 M H2SO4 at dE/dt/40 mV s 1. The Pt oxidation commences at about 600 mV versus Ag/AgCl, while the Pt alloy oxidation starts at about 700 mV. The oxygen evolution overpotential at PtNi is about 20 mV lower than at pure
Fig. 1. CVs in 1 M H2SO4; dE /dt 40 mV s 1; broken line: Pt; solid line: Pt /Ni.
J.-F. Drillet et al. / Electrochimica Acta 47 (2002) 1983 /1988
1985
Table 1 XPS analysis of PtNi after electrochemical measurements C 1s Atomic ratio (%) Surface Depth
44.1 /
Binding energy (eV) Surface 284.6 Depth /
O 1s
Pt 4f
Pt 4d 5/2
Ni 2p 3/2
18.5 /
/ /
33.1 76.6
4.4 23.3
532.35 /
71.1 71.1
314.5 314.5
852.5 852.5
Pt. The potential region for hydrogen adsorption and desorption are similar for both electrodes. No substantial differences of the electrode roughness could be observed between Pt and Pt70Ni30, since the currents for adsorption/desorption of hydrogen and oxygen in the CVs are comparable (see Fig. 1). The electrochemical active electrode surface obtained by integrating the hydrogen desorption peaks according to [12] leads to roughness factors of 1.86 and 1.97 for Pt and PtNi, respectively. For this estimation it was assumed that the adsorption/desorption on PtNi behaves like on Pt. The surface and bulk composition of the electrodes were analyzed by XPS after the electrochemical measurements. The electrodes were sputtered until a constant Pt/Ni atomic ratio was measured in order to determine the bulk atomic composition. The XPS results of the PtNi alloy are summarized Table 1. On the surface a Pt/Ni atomic ratio of 7.5 was found. This is not in accordance with the data reported by Toda et al. [8,9]. They found on sputtered PtNi films after the electrochemical treatment a skin of pure Pt on the surface and Ni could not be identified. On the other hand, the results in the present work were obtained with a bulk PtNi alloy disk. Furthermore, an alloy with a
Pt/Ni
Impurity
7.52 3.29
F, N, Ag
pure Pt surface layer should show the same CV as a bulk Pt electrode. This is not the case in the present work (see the CV of Pt and PtNi in Fig. 1). In the bulk (approx. 25 nm under the toplayer) the Pt/ Ni ratio was determined to be 3.29 (77/23). It should be noticed that the determined alloying rate differs from the value given by the manufacturer (70/30). The experimental results regarding the ORR in pure H2SO4 are summarized in Fig. 2. Pt70Ni30 shows a higher activity for the ORR than pure Pt. The onset potential for oxygen reduction is shifted to more positive potentials and the overpotential at a current density of 1 mA cm 2 is about 80 mV less compared to pure Pt. Toda et al. found a corresponding lower overpotential of 150 mV on sputtered Pt61Ni39 [8]. The mass transport corrected Tafel plots for oxygen reduction were obtained by means of Levich /Koutecky plots (1/j vs. 1/v 0.5) and are represented in Fig. 3. The corresponding Tafel slopes were 83 mV/decade for PtNi and 120 mV/decade for Pt. The methanol oxidation in unstirred solutions of 1 M H2SO4/0,5 M CH3OH can be seen in Fig. 4. The methanol containing electrolyte was previously purged with nitrogen in order to avoid oxygen contamination.
Fig. 2. O2 reduction in 1 M H2SO4; dE /dt 5 mV s 1; rotating speeds: 1000 (1), 2000 (2) and 3000 (3); broken line: Pt; solid line: PtNi.
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J.-F. Drillet et al. / Electrochimica Acta 47 (2002) 1983 /1988
Fig. 3. Mass transport corrected Tafel plots for ORR in 1 M H2SO4 (methanol free solution) at Pt and PtNi.
The onset potential for methanol oxidation at about 500 mV in the positive going potential scan is the same for Pt and PtNi. Additionally, the CVs of both electrodes exhibited no significant differences. It can be concluded that methanol oxidation is not significantly affected by Ni alloying. Furthermore, no significant dependence of the oxidation currents on the rotating speed of the electrode could be observed in the presence of methanol. This clearly indicates that the methanol oxidation is kinetically controlled. The formation of surface poisons such as CO, CHO, COH (see for example [13]) during methanol adsorption leads to a low electrochemical activity of Pt and PtNi. The electrochemical methanol oxidation on
the alloy was not studied in more detail but deserves further interest. Fig. 5 show the O2 reduction in oxygen saturated solution 1 M H2SO4/0.5 M CH3OH at rotation speeds up to 3000 rpm. It is obvious that in the presence of methanol the activity decrease for ORR is higher at pure Pt than at PtNi. The limiting current densities obtained at 50 mV versus Ag/AgCl and 3000 rpm are 4.28 mA cm 2 for PtNi and 0.39 mA cm 2 for pure Pt (see curves (3) in Fig. 5). In the potential range between 600 and 500 mV positive current of about 100 mA cm 2 is visible which can be assigned to methanol oxidation (compare Fig. 4). Therefore, the mass transport correction by means of Levich /Koutecky plot in H2SO4/0.5 M
Fig. 4. Methanol oxidation on Pt (broken line) and Pt70Ni30 (solid line) in1 M H2SO4/0.5 M CH3OH; dE /dt 5 mV s 1, unstirred solution.
J.-F. Drillet et al. / Electrochimica Acta 47 (2002) 1983 /1988
1987
Fig. 5. Oxygen reduction in methanol containing electrolyte (1 M H2SO4/0.5 M CH3OH); dE /dt 5 mV s 1; rotating speeds: 1000 (1), 2000 (2) and 3000 (3); broken line: Pt; solid line: PtNi.
CH3OH (compare Fig. 3) and the interpretation of Tafel plots seems to be very complicated, since the recorded current/potential curves are a superposition of oxygen reduction and methanol oxidation, especially in the Tafel region. The polarisation curves for PtNi show a decrease of overpotential for ORR of about 90 mV at 1.5 mA cm 2 in comparison to Pt. This is about 60 mV less than that reported for sputtered Pt61Ni39 in 0.1 M HClO4 by Toda et al. [8,9]. This is probably due to a smaller Ni alloying ratio closed to 25% or/and to the different electrode preparation method. The presence of 0.5 M CH3OH in the electrolyte solution affects the ORR at Pt70Ni30. The limiting current density is decreasing from 6.2 down to 4.28 mA cm 2 and the potential values getting at 1.5 mA cm 2 are shifting from 478 to 448 mV. Nevertheless, Pt70Ni30 shows a better activity for the ORR in presence of methanol than Pt in pure sulphuric acid solution. Nevertheless, further investigations are necessary in order to confirm these first results and to get a deeper insight into the reaction pathways during oxygen reduction in methanol containing electrolyte. Specially, the amount of H2O2 as intermediate formed during oxygen reduction should be determined on Pt and PtNi. Other Pt-based catalysts such as PtCr, PtFe or PtCo as smooth bulk alloy electrodes and supported on carbon should be tested as cathode catalyst for the DMFC.
4. Conclusions The experimental findings can be summarized as follows:
. A significantly enhanced activity for ORR 1 M H2SO4 was observed at a PtNi alloy in accordance with results reported in the literature. . In pure 1 M H2SO4, PtNi with an analyzed surface composition of Pt88Ni12 (by XPS) exhibited about 80 mV less overpotential at j/1 mA cm 2 in comparison to Pt. The mass transport corrected Tafel slopes are 83 mV/decade for PtNi and 120 mV/decade for Pt. . The electrochemical methanol oxidation in oxygen free solution at Pt and PtNi is not diffusion controlled and shows no significant differences. . In methanol containing electrolyte the ORR at PtNi is shifted to more positive electrode potentials. This effect should lead to higher cell voltages if this material is used as a cathode catalyst in a DMFC. . At the PtNi electrode an 11 times higher limiting current density was recorded in 1 M H2SO4/0.5 M CH3OH.
Acknowledgements The authors thank Ms McKay from WPT, DaimlerChrysler in Mannheim for polishing the electrodes. Financial support from DaimlerChrysler AG is gratefully acknowledged.
References [1] N. Alonso, H. Tributsch, Nature 323 (1996) 431. [2] N. Alonso Vante, B. Schubert, H. Tributsch, A. Perrin, J. Catal. 112 (1988) 384.
1988
J.-F. Drillet et al. / Electrochimica Acta 47 (2002) 1983 /1988
[3] B. Schubert, N. Alonso Vante, E. Gocke, H. Tributsch, Ber. Bunsenges. Phys. Chem. 92 (1988) 1279. [4] T.J. Schmidt, U.A. Paulus, H.A. Gasteiger, N. Alonso Vante, R.J. Behm, J. Electrochem. Soc. 147 (2000) 2620. [5] R. Jiang, D. Chu, J. Electrochem. Soc. 147 (2000) 4605. [6] B.C. Beard, P.N. Ross, J. Electrochem. Soc. 137 (1990) 3368. [7] N. Neergat, A.K. Shukla, K.S. Gandhi, J. Appl. Electrochem. 31 (2001) 373. [8] T. Toda, H. Igarashi, M. Watanabe, J. Electrochem. Soc. 145 (1998) 4185.
[9] T. Toda, H. Igarashi, H. Uchida, M. Watanabe, J. Electrochem. Soc. 146 (1999) 3750. [10] M.T. Paffet, J.G. Beery, S. Gottesfeld, J. Electrochem. Soc. 135 (1988) 1431. [11] J.H. Scofield, J. Electron. Spectrosc. Relat. Phenom. 8 (1976) 129. [12] T. Biegler, D.A.J. Rand, R. Woods, J. Elektroanal. Chem. 29 (1971) 267. [13] A. Hamnett, in: A. Wieckowski (Ed.), Interfacial Electrochemistry */Theory, Experiment and Application, Marcel Dekker, New York, Basel, 1999, p. 843.
Oxygen reduction at Pt and Pt70Ni30 in H2SO4/CH3OH solution J.-F. Drillet a,*, A. Ee a, J. Friedemann a, R. Ko¨tz b,1, B. Schnyder b, V.M. Schmidt a,1 a
Mannheim University of Applied Sciences, Institute of Environmental Engineering, D-68163 Mannheim, Germany b Paul Scherrer Institute, General Energy Department, CH-5232 Villigen, Switzerland Received 23 November 2001
Abstract The electrochemical oxygen reduction reaction (ORR) was studied at Pt and Pt alloyed with 30 atom% Ni in 1 M H2SO4 and in 1 M H2SO4/0.5 M CH3OH by means of rotating disc electrode. In pure sulphuric acid, the overpotential of ORR at 1 mA cm 2 is about 80 mV lower at Pt70Ni30 than at pure Pt. It was found that in methanol containing electrolyte solution the onset potential for oxygen reduction at PtNi is shifted to more positive potentials and the alloy catalyst has an 11 times higher limiting current density for oxygen reduction than Pt. Thus, PtNi as cathode catalyst should have a higher methanol tolerance for fuel cell applications. On the other hand, no significant differences in the methanol oxidation on both electrodes was found using cycling voltammetry, especially regarding the onset potential for methanol oxidation. During all the measurements no significant electrochemical activity loss was observed at Pt0.7Ni0.3. Ex-situ XPS investigations before and after the electrochemical experiments have revealed Pt enrichment in the first surface layers of the PtNi. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Platinum; Platinum /nickel alloy; Oxygen reduction reaction (ORR); Methanol tolerant cathode; Direct methanol fuel cell
1. Introduction The use of methanol as energy carrier and its direct electrochemical oxidation in a direct methanol fuel cell (DMFC) represents an important challenge for the polymer electrolyte fuel cell technology, since the complete system would be simpler without a reformer and further gas treatment steps. However, beside the difficulty in finding an anode catalyst with high activity for methanol oxidation, the methanol crossover through the polymer electrolyte leads to a mixed potential at the cathode, which results from the oxygen reduction reaction (ORR) and the methanol oxidation occurring simultaneously. This effect causes a negative potential shift at the cathode and a significant decrease of performance in a DMFC. This problem should be solved either by using electrolytes with lower methanol permeability or by developing new cathode catalysts. For the latter, two concepts can be identified. First, cathode catalysts in a DMFC should
* Corresponding author. E-mail address: [email protected] (J.-F. Drillet). 1 ISE member.
show a high methanol tolerance, that means that the oxygen reduction will not be affected by the adsorption and oxidation of methanol. Secondly, in order to shift the mixed potential to more positive potentials, the catalyst should show a higher exchange current density for oxygen reduction compared to Pt, which is the best cathode catalyst. In this case, it is assumed that the methanol oxidation at the cathode should not be affected significantly by alloying or is at least of minor influence. High methanol tolerance is reported in the literature for non-noble metal catalysts based on chalcogenides [1 /4] or metalloporphyrins [5]. These catalysts have already shown nearly the same current/potential behaviour in the absence as well as in the presence of methanol. However, in methanol free solution these materials did not attain the activity of Pt. Furthermore, the long time stability under fuel cell conditions have still to be improved. Some platinum-based binary alloys such as PtFe, PtCo, PtNi and PtCr exhibit a higher catalytic activity for the ORR in pure acid electrolytes than pure platinum [6 /10]. The enhanced electrocatalysis can be explained by an electronic factor, i.e. the change of the d-band vacancy in Pt upon alloying and/or by geometric
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 0 2 7 - 0
1984
J.-F. Drillet et al. / Electrochimica Acta 47 (2002) 1983 /1988
effects (Pt coordination number and Pt /Pt distance) (see [9] and references therein). Both effects should enhance the reaction rate for oxygen adsorption and breaking the O /O bond during the reduction reaction. For example, the lattice constant a0 in the cubic PtX alloy (X /Fe, Co, Ni) decreases with increasing alloying component X. In the case of PtNi, the maximum electrochemical activity for ORR has been found to occur at 30 atomic% Ni [9,10]. The purpose of this work is first to evaluate the activity of pure Pt and Pt0.7Ni0.3 for the oxygen reduction in pure 1 M H2SO4 electrolyte solution and secondly, to study the influence of methanol on the ORR. This aspect should be of great practical interest for fuel cell applications. The experiments were done under controlled mass transport conditions using rotating disk electrodes of the materials.
2. Experimental Pt and Pt0.7Ni0.3 disc electrodes with a diameter of 12 and 2 mm in thickness were manufactured by Hauner GmbH (Germany) with 99.95% purity. Pt and Ni pellets were melted together in a vacuum arc oven and then pressed and turned to the final diameter of 1 cm2 for both. The electrodes were polished with diamond paste with particle size of 3 mm. Before each experiment the electrodes were cleaned in a H2SO4/H2O2 solution for about 30 s. After rinsing with pure water they were inserted in a PTFE disc holder. The electrodes were then cycled at dE /dt/100 mV s 1 between the onset potentials of hydrogen and oxygen evolution until reproducible cyclic voltammograms (CVs) were obtained under N2 atmosphere.
After electrode activation the potential was stopped at E /0.9 V and a negative going potential sweep was started with dE /dt/5 mV s 1 to the final potential of E //0.1 V in oxygen saturated 1 M H2SO4. The rotating disc electrode (EG&E Instruments) measurements were performed up to a rotation speed of 4000 rpm. An Ag/AgCl electrode was used as reference. Methanol containing solutions (cMethanol /0.5 mol dm 3) were changed after each RDE experiment in order to maintain a constant methanol concentration in the electrolyte. All experiments were done at room temperature. Ex-situ X-ray photoelectron spectroscopy (XPS) measurements were carried out at the Paul Scherrer Institute (Switzerland) in order to determine the Pt /Ni atomic ratio of the electrode surface before and after the experiments. The XPS spectra were recorded with an ESCALAB 220i XL (VG Scientific) photoelectron spectrometer using monochromatic Al Ka radiation. The X-ray power was 200 W. The spectra were recorded in the constant analyzer mode with analyzer pass energies of 50 eV for the survey spectra and 20 eV for the high-resolution detail spectra. The composition of the samples was determined by quantitative analysis of the spectra using the cross sections of Scofield [11]. The base pressure in the analysis chamber during analysis was always better than 5 /10 9 mbar.
3. Results and discussion Fig. 1 shows CVs of Pt and Pt70Ni30 in 1 M H2SO4 at dE/dt/40 mV s 1. The Pt oxidation commences at about 600 mV versus Ag/AgCl, while the Pt alloy oxidation starts at about 700 mV. The oxygen evolution overpotential at PtNi is about 20 mV lower than at pure
Fig. 1. CVs in 1 M H2SO4; dE /dt 40 mV s 1; broken line: Pt; solid line: Pt /Ni.
J.-F. Drillet et al. / Electrochimica Acta 47 (2002) 1983 /1988
1985
Table 1 XPS analysis of PtNi after electrochemical measurements C 1s Atomic ratio (%) Surface Depth
44.1 /
Binding energy (eV) Surface 284.6 Depth /
O 1s
Pt 4f
Pt 4d 5/2
Ni 2p 3/2
18.5 /
/ /
33.1 76.6
4.4 23.3
532.35 /
71.1 71.1
314.5 314.5
852.5 852.5
Pt. The potential region for hydrogen adsorption and desorption are similar for both electrodes. No substantial differences of the electrode roughness could be observed between Pt and Pt70Ni30, since the currents for adsorption/desorption of hydrogen and oxygen in the CVs are comparable (see Fig. 1). The electrochemical active electrode surface obtained by integrating the hydrogen desorption peaks according to [12] leads to roughness factors of 1.86 and 1.97 for Pt and PtNi, respectively. For this estimation it was assumed that the adsorption/desorption on PtNi behaves like on Pt. The surface and bulk composition of the electrodes were analyzed by XPS after the electrochemical measurements. The electrodes were sputtered until a constant Pt/Ni atomic ratio was measured in order to determine the bulk atomic composition. The XPS results of the PtNi alloy are summarized Table 1. On the surface a Pt/Ni atomic ratio of 7.5 was found. This is not in accordance with the data reported by Toda et al. [8,9]. They found on sputtered PtNi films after the electrochemical treatment a skin of pure Pt on the surface and Ni could not be identified. On the other hand, the results in the present work were obtained with a bulk PtNi alloy disk. Furthermore, an alloy with a
Pt/Ni
Impurity
7.52 3.29
F, N, Ag
pure Pt surface layer should show the same CV as a bulk Pt electrode. This is not the case in the present work (see the CV of Pt and PtNi in Fig. 1). In the bulk (approx. 25 nm under the toplayer) the Pt/ Ni ratio was determined to be 3.29 (77/23). It should be noticed that the determined alloying rate differs from the value given by the manufacturer (70/30). The experimental results regarding the ORR in pure H2SO4 are summarized in Fig. 2. Pt70Ni30 shows a higher activity for the ORR than pure Pt. The onset potential for oxygen reduction is shifted to more positive potentials and the overpotential at a current density of 1 mA cm 2 is about 80 mV less compared to pure Pt. Toda et al. found a corresponding lower overpotential of 150 mV on sputtered Pt61Ni39 [8]. The mass transport corrected Tafel plots for oxygen reduction were obtained by means of Levich /Koutecky plots (1/j vs. 1/v 0.5) and are represented in Fig. 3. The corresponding Tafel slopes were 83 mV/decade for PtNi and 120 mV/decade for Pt. The methanol oxidation in unstirred solutions of 1 M H2SO4/0,5 M CH3OH can be seen in Fig. 4. The methanol containing electrolyte was previously purged with nitrogen in order to avoid oxygen contamination.
Fig. 2. O2 reduction in 1 M H2SO4; dE /dt 5 mV s 1; rotating speeds: 1000 (1), 2000 (2) and 3000 (3); broken line: Pt; solid line: PtNi.
1986
J.-F. Drillet et al. / Electrochimica Acta 47 (2002) 1983 /1988
Fig. 3. Mass transport corrected Tafel plots for ORR in 1 M H2SO4 (methanol free solution) at Pt and PtNi.
The onset potential for methanol oxidation at about 500 mV in the positive going potential scan is the same for Pt and PtNi. Additionally, the CVs of both electrodes exhibited no significant differences. It can be concluded that methanol oxidation is not significantly affected by Ni alloying. Furthermore, no significant dependence of the oxidation currents on the rotating speed of the electrode could be observed in the presence of methanol. This clearly indicates that the methanol oxidation is kinetically controlled. The formation of surface poisons such as CO, CHO, COH (see for example [13]) during methanol adsorption leads to a low electrochemical activity of Pt and PtNi. The electrochemical methanol oxidation on
the alloy was not studied in more detail but deserves further interest. Fig. 5 show the O2 reduction in oxygen saturated solution 1 M H2SO4/0.5 M CH3OH at rotation speeds up to 3000 rpm. It is obvious that in the presence of methanol the activity decrease for ORR is higher at pure Pt than at PtNi. The limiting current densities obtained at 50 mV versus Ag/AgCl and 3000 rpm are 4.28 mA cm 2 for PtNi and 0.39 mA cm 2 for pure Pt (see curves (3) in Fig. 5). In the potential range between 600 and 500 mV positive current of about 100 mA cm 2 is visible which can be assigned to methanol oxidation (compare Fig. 4). Therefore, the mass transport correction by means of Levich /Koutecky plot in H2SO4/0.5 M
Fig. 4. Methanol oxidation on Pt (broken line) and Pt70Ni30 (solid line) in1 M H2SO4/0.5 M CH3OH; dE /dt 5 mV s 1, unstirred solution.
J.-F. Drillet et al. / Electrochimica Acta 47 (2002) 1983 /1988
1987
Fig. 5. Oxygen reduction in methanol containing electrolyte (1 M H2SO4/0.5 M CH3OH); dE /dt 5 mV s 1; rotating speeds: 1000 (1), 2000 (2) and 3000 (3); broken line: Pt; solid line: PtNi.
CH3OH (compare Fig. 3) and the interpretation of Tafel plots seems to be very complicated, since the recorded current/potential curves are a superposition of oxygen reduction and methanol oxidation, especially in the Tafel region. The polarisation curves for PtNi show a decrease of overpotential for ORR of about 90 mV at 1.5 mA cm 2 in comparison to Pt. This is about 60 mV less than that reported for sputtered Pt61Ni39 in 0.1 M HClO4 by Toda et al. [8,9]. This is probably due to a smaller Ni alloying ratio closed to 25% or/and to the different electrode preparation method. The presence of 0.5 M CH3OH in the electrolyte solution affects the ORR at Pt70Ni30. The limiting current density is decreasing from 6.2 down to 4.28 mA cm 2 and the potential values getting at 1.5 mA cm 2 are shifting from 478 to 448 mV. Nevertheless, Pt70Ni30 shows a better activity for the ORR in presence of methanol than Pt in pure sulphuric acid solution. Nevertheless, further investigations are necessary in order to confirm these first results and to get a deeper insight into the reaction pathways during oxygen reduction in methanol containing electrolyte. Specially, the amount of H2O2 as intermediate formed during oxygen reduction should be determined on Pt and PtNi. Other Pt-based catalysts such as PtCr, PtFe or PtCo as smooth bulk alloy electrodes and supported on carbon should be tested as cathode catalyst for the DMFC.
4. Conclusions The experimental findings can be summarized as follows:
. A significantly enhanced activity for ORR 1 M H2SO4 was observed at a PtNi alloy in accordance with results reported in the literature. . In pure 1 M H2SO4, PtNi with an analyzed surface composition of Pt88Ni12 (by XPS) exhibited about 80 mV less overpotential at j/1 mA cm 2 in comparison to Pt. The mass transport corrected Tafel slopes are 83 mV/decade for PtNi and 120 mV/decade for Pt. . The electrochemical methanol oxidation in oxygen free solution at Pt and PtNi is not diffusion controlled and shows no significant differences. . In methanol containing electrolyte the ORR at PtNi is shifted to more positive electrode potentials. This effect should lead to higher cell voltages if this material is used as a cathode catalyst in a DMFC. . At the PtNi electrode an 11 times higher limiting current density was recorded in 1 M H2SO4/0.5 M CH3OH.
Acknowledgements The authors thank Ms McKay from WPT, DaimlerChrysler in Mannheim for polishing the electrodes. Financial support from DaimlerChrysler AG is gratefully acknowledged.
References [1] N. Alonso, H. Tributsch, Nature 323 (1996) 431. [2] N. Alonso Vante, B. Schubert, H. Tributsch, A. Perrin, J. Catal. 112 (1988) 384.
1988
J.-F. Drillet et al. / Electrochimica Acta 47 (2002) 1983 /1988
[3] B. Schubert, N. Alonso Vante, E. Gocke, H. Tributsch, Ber. Bunsenges. Phys. Chem. 92 (1988) 1279. [4] T.J. Schmidt, U.A. Paulus, H.A. Gasteiger, N. Alonso Vante, R.J. Behm, J. Electrochem. Soc. 147 (2000) 2620. [5] R. Jiang, D. Chu, J. Electrochem. Soc. 147 (2000) 4605. [6] B.C. Beard, P.N. Ross, J. Electrochem. Soc. 137 (1990) 3368. [7] N. Neergat, A.K. Shukla, K.S. Gandhi, J. Appl. Electrochem. 31 (2001) 373. [8] T. Toda, H. Igarashi, M. Watanabe, J. Electrochem. Soc. 145 (1998) 4185.
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