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Activation of nickel-anodes for alkaline fuel cells
Applied Surface Science 179 (2001) 251±256
Activation of nickel-anodes for alkaline fuel cells M. Schulze*, E. GuÈlzow, G. Steinhilber Institut fuÈr Technische Thermodynamik, Deutsches Zentrum fuÈr Luft- und Raumfahrt, Pfaffenwaldring 38-40, D-70569 Stuttgart, Germany
Abstract In alkaline fuel cells (AFC) electrodes containing porous nickel and PTFE can be used as anodes. A low-cost production technique yields passive, polymer covered electrodes, which have to be activated before being used in electrochemical devices. During this activation process the surface structure as well as the chemical composition and the oxidation state of the used materials are changed by a given electrochemical current. The electrodes are investigated in different states during this activation process with X-ray photoelectron spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and nitrogen adsorption. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Alkaline fuel cells; Nickel; Electrodes; X-ray photoelectron spectroscopy; Reduction of nickel oxide
1. Introduction A fuel cell is an electrochemical device used to convert the chemical energy of, e.g. hydrogen and oxygen directly into electric current with a high ef®ciency. In principle, the direction of the electrode reactions in water electrolysis is reversed in the fuel cell. The electrochemical fuel cell reactions take place at the three-phase interface between electrocatalyst, electrolyte solution and reaction gas. In an alkaline fuel cell a KOH solution is used as electrolyte. The liquid electrolyte can penetrate the porous electrode like the reaction gases. Therefore, in alkaline fuel cells three-phase interfaces are not only on the electrode surface but are also formed in the volume of the electrode. Consequently, the reaction zone expands into the depth and high reactive surface areas in porous
*
Corresponding author. Tel.: 49-711-6862-456; fax: 49-711-6862-747. E-mail address: [email protected] (M. Schulze).
electrodes can be used for the alkaline fuel cell reaction. At DLR low-temperature fuel cells are investigated focusing on the preparation of gas diffusion electrodes and membrane±electrode assemblies, electrochemical behavior, physical characterization, long-term behavior and degradation mechanisms. The preparation of the gas diffusion electrode based on Sauer [1] and Winsel [2] has been further developed for electrodes of different low-temperature fuel cell types [3], alkaline fuel cells (AFC) [4±7] and polymer electrolyte membrane fuel cells (PEFC) [8±12] as well as for other electrochemical applications [13]. The electrochemical reaction takes place in the three-phase border zone between catalyst surface, electrolyte and gas phase. The investigated anodes for AFC applications are a composite of a metal catalyst and polytetra¯uoroethylene (PTFE) supported by a metal web on the backside of the electrode. The metal web electrically conducts the electrode and guarantees mechanical stability. The PTFE is used as an organic binder to hold the catalyst particles
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 2 9 1 - 4
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M. Schulze et al. / Applied Surface Science 179 (2001) 251±256
together. Due to its hydrophobic character the PTFE is important for the gas transport in the electrode and yields a hydrophobic pore system that is not ¯ooded by the alkaline electrolyte so that the electrode can be supplied with the fuel gas from the backside. Porous nickel is used as catalyst, the initial material is a nickel±aluminum alloy containing 50 wt.% of each metal. In a KOH the catalyst is activated by dissolving the aluminum at 350 K and it contains less than 5% of aluminum after this procedure. During the activation process the nickel is loaded with 0.5±1.2 hydrogen atoms per nickel atom [14]. After this preparation of the catalyst, the powder is washed and dried. Due to the hydrogen in the metal and the reactivity of a metallic nickel surface the contact with oxygen during the drying process has to be avoided. The catalyst powder cannot be handled in the reactive state to produce the electrodes. Therefore, after drying the catalyst powder is passivated by controlled oxygen exposure under temperature control of the powder. During this preparation step the hydrogen is removed and the catalyst surface oxidized. Before using the electrode in fuel cells the oxide must be reduced. In this paper the activation step is investigated. 2. Experimental 2.1. Electrochemical experiments The electrodes were activated in an electrochemical half cell con®guration [5] consisting of the electrode in a holder which allows the gas supply from the backside of the electrode and nickel counter electrode and a Hg/HgO reference electrode in a tempered vessel containing 30% KOH. The same test set-up was used for the electrochemical characterization of fuel cell electrodes. The working electrodes have a geometric area of 6 cm2. They were removed from the test set-up to analyze their surfaces and separated into three samples for the different characterization methods applied. For each measurement a new electrode was used. The electrodes were characterized after different activation times at the standard current density of 5 mA/cm2 operating as hydrogen evolution electrodes. From the backside of the electrode hydrogen was supplied during the activation process.
2.2. XPS investigations The XPS characterization of the electrodes was performed in a standard UHV chamber equipped with an XP-spectrometer (spherical analyzer r 127 mm, 300 W X-ray tube with Al- and Mg-excitation) and a quadruple mass spectrometer. The residual pressure p in the analysis chamber was approximately 4 10 10 mbar. The composition of the residual pressure gas was veri®ed by the mass spectrometer. The investigated GDE were rinsed with distilled water after removal from the half cell con®guration and subsequently dried in the vacuum lock at 350 K for 8 h. The pressure increased by nearly two orders of magnitude after transferring the GDE sample into the UHV chamber. To investigate the GDE, XP spectra of the C 1s, F 1s, Ni 2p, Cu 2p and O 1s levels were recorded during depth pro®ling by alternating XPS measurements and ion etching. During depth pro®ling of GDE the etching period was increased after 10 cycles. The changes of the surface composition during the depth pro®ling are not only caused by the etching of the surface but also by the decomposition of the polymer induced by the ionizing irradiation [15]. The experimental conditions, i.e. drying temperature and time, X-ray doses and Ar ion doses during the ion etching, were kept constant for all samples. 2.3. Porosimetry The porosimetry and the speci®c surface were measured by adsorption of nitrogen [16]. For these measurements the samples were dried in vacuum at 400 K for 18 h. The nitrogen adsorption measurements were performed in a commercial system (Carlo Erba Sorptomatic Series 1800) and evaluated with the standard BET algorithms [17]. 3. Results After reactive mixing of the different components for the electrodes, i.e. PTFE, copper powder and the oxidized nickel catalyst, in a knife mile the powder is rolled to a self-carrying strip, which is rolled in a second step onto a metal web to form the electrode which then has a thickness of approximately 350 mm. The pore system and the speci®c surface of the AFC
M. Schulze et al. / Applied Surface Science 179 (2001) 251±256
253
Fig. 1. Speci®c surface determined after different activation times at 5 mA/cm2.
anodes are determined by the uncovered surface of the metallic catalyst. The PTFE has no measurable surface in the nitrogen adsorption measurements [6]. Because of the rolling process as one step of the electrode preparation the PTFE is smeared to a thin ®lm which covers the metallic catalyst and reduces the speci®c surface recorded in BET measurements. The electrodes must be activated before being used in fuel cells. The standard activation procedure
Fig. 2. XPS depth pro®les of a non-activated electrode and an activated electrode.
Fig. 3. C 1s spectra of a non-activated electrode, after activation times of 18 and 115 h at 5 mA/cm2 and after 135 h hydrogen exposure (currentless).
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M. Schulze et al. / Applied Surface Science 179 (2001) 251±256
applied is to reduce the nickel oxide in 30% KOH electrochemically by using the electrode as hydrogen generating electrode for an electrolyzer at a current density of 5 mA/cm2 for 18 h [18]. In addition to the electrochemical reduction conditions hydrogen is exposed to the electrodes from the backside. During the activation process the speci®c surface measured by the nitrogen adsorption increases with the activation time. The pore system of the nickel catalyst is re-exposed again. The speci®c surface does not increase linearly in time as shown in Fig. 1. The increasing speci®c surface clearly indicates that the PTFE ®lm covering the catalyst is removed during the activation process. Therefore, in the XP spectra after activation the nickel signals should be increased considerably and the PTFE signal decreased. The XP spectra and depth pro®le of the
activated electrodes do not differ signi®cantly from that of the non-activated electrode as shown in Fig. 2. The PTFE ®lm can also be seen quite clearly for both activated and non-activated electrode. At the beginning of depth pro®ling the nickel signal is suppressed for the activated as well as for the non-activated electrode, but the initial nickel signal is increased by the activation process by a factor of approximately 2 after 18 h at 5 mA/cm2. After the long-term activation experiments the initial nickel concentration is increased by the factor 5 (115 h at 5 mA/cm2) compared with the non-activated electrode. The initial nickel intensity shows qualitatively but not quantitatively the same behavior as the speci®c surface. Like the initial intensity of the nickel signal in the XP spectra the copper signal is also increased by activation, which is, however, additionally induced
Fig. 4. Ni 2p signal during depth pro®ling after different activations: (a) non-activated, (b) 4.5 h at 5 mA/cm2, (c) 8.5 h at 5 mA/cm2, (d) 18 h at 5 mA/cm2.
M. Schulze et al. / Applied Surface Science 179 (2001) 251±256
by changes of the copper distribution in the electrode described in more detail in a further paper to be published. Differences in the decomposition of the polymers caused by the ionizing radiation (X-ray and ion etching) during depth pro®ling can be used as indicator of changes in the PTFE induced by electrochemical stressing [15]. The analysis of the irradiation-induced PTFE decomposition of the standard activated electrodes does not show an indicator for a change of the PTFE compared with the non-activated electrode. In contrast, for the long-term activated electrodes the XPS spectra indicate a PTFE decomposition. The carbon signal which can be related to graphite has increased signi®cantly while the ¯uorine signal has decreased (Fig. 3). These facts indicate a damaging of the PTFE during the long-term activation as also observed in long-term fuel cell operation [15]. Due to the high electron beam voltage and the resolved penetration depth the PTFE ®lms cannot be imaged in the SEM measurements, which means that a direct study of the topologic of the PTFE ®lms by SEM is not possible. On the ®rst XPS spectra of depth pro®les of the nickel catalyst nickel oxide is observed. This surface nickel oxide layer can be formed during the transfer process from the KOH electrolyte into the vacuum system [19,20]. Fig. 4 depicts the measured Ni 2p signals for the non-activated and activated electrodes. These spectra clearly show that the nickel is already reduced after 4.5 h at a current density of 5 mA/cm2. 4. Discussion It is very surprising that the electrodes are reduced after a short activation time because the catalyst is almost entirely covered by the PTFE ®lm as observed in the XP spectra and in the speci®c surface area measurements. Perforations in the PTFE ®lm are suf®cient to allow the contact of the electrolyte to the nickel catalyst and therefore to reduce the nickel oxide. During the activation process the perforations in the PTFE ®lm are increased or new ones are formed. The perforation may be created by the generated hydrogen, which is formed on the nickel surface and separates the PTFE ®lm from the catalyst surface. Therefore, the initial nickel signal in the XP spectra of
255
the activated electrode is increased as observed. Also the speci®c surface is increased without total removal of the PTFE ®lm. In the XP spectra of the electrodes activated for different times, the nickel oxide is totally reduced, which seems to contradict the results of the speci®c surface measurements. From the XPS measurements it cannot be concluded whether the nickel is also reduced in the volume of the electrode, which is relevant for the electrochemical behavior. In the speci®c surface measurements the surface of the catalyst particles in the volume of the porous electrode determines the value recorded. From the speci®c surface measurements it can clearly be seen that at short activation times a main part of the catalyst is covered in a gas or electrolyte tight way by the PTFE ®lm. Therefore, it can be concluded that the activation process starts on the surface of the electrode and the activation zone expands in the depth with increasing activation time. There are two effects leading to this behavior. The electrolyte penetration into the electrode is hindered by the hydrophobic character of the PTFE. With increasing PTFE damaging and time the electrolyte and activation zone shift deeper into the electrode. The perforation in the PTFE ®lm can enhance the electrolyte penetration into the electrode. The electrochemical reduction is coupled to a charge transfer and the ion transport, where an electrical ®eld is necessary. The potential gradient maximum is on the interface between the electrolyte and the catalyst. Caused by the good electric conductivity in the electrode and the low penetration depth starting the activation the gradient is maximal on the electrode surface, which is why the nickel of the surface is reduced ®rst. After the reduction of the nickel has started and a ®rst metallic nickel surface in contact to the electrolyte has been formed, the overvoltage of the hydrogen generation decreases and so the further reduction of the nickel oxide in the volume of the electrode runs slower than at the beginning. The mechanism of the PTFE changes during the activation process is indistinct; two in principle different mechanisms can induce the changes of the PTFE. First the chemical decomposition of the PTFE and second a change of the mechanical distribution of the PTFE. A change of the chemical composition is indicated by the differences of the C 1s spectra of
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M. Schulze et al. / Applied Surface Science 179 (2001) 251±256
non-activated and long term activated electrodes. The high local electric ®elds in the thin PTFE ®lm can induce the PTFE decomposition, especially in combination with a chemical attack by intermediates or ions in the electrolyte. The change of the structure of the PTFE ®lm is indicated by the increasing of the speci®c surface area measurements with nitrogen adsorption. If hydrogen is electrochemically formed or water is formed by the reduction of the NiO under the PTFE ®lm the local pressure increases and can be suf®cient to break the PTFE ®lm. In an experiment of currentless activation the speci®c surface is increased without reduction of the nickel oxide but with an increase of the initial nickel signal, which shows that the PTFE is also damaged under these conditions and perforations are formed (Fig. 3). The main difference in this experiment is that the nickel oxide is not reduced. Caused by the dissolution of aluminum residues (observed by EDX) a little amount of hydrogen is produced during the experiment. Local ¯uctuations of the concentration in the electrolyte and local inhomogeneity of the catalyst in the electrodes induce electric ®eld in the electrodes. Therefore, the same mechanisms can cause the PTFE changes in the experiment with and without current. 5. Conclusions During the activation process several processes take place in parallel. The two main processes are the perforation of the PTFE ®lm and the reduction of the nickel catalyst. The damaging of the PTFE ®lm is primarily effected by the activation time. This process can be observed by activation with hydrogen generation as well as in the experiment of currentless activation. This step is necessary to allow the gas and the electrolyte the contact to the nickel catalyst surface. The reduction of the nickel oxide strongly depends on the charging. In the experiment of currentless activation the nickel oxide is not reduced. The nickel oxide on the surface of the electrodes is reduced after a short time, but not in the volume of the electrode, and thus the activation zone shifts into the electrode
volume during the activation process. This behavior is clearly re¯ected in the increase of the speci®c surface and the electrochemical behavior of the electrodes. The observed different steps running during activation lead to changes of the structure and the composition of the electrode. The same process excludes that the reduction of the nickel oxide can take place in the fuel cell operation and these may effect the degradation of the electrodes in long-term fuel cell operation. References [1] H. Sauer, German Patent DE 2 941 774 (CI H01M4/88). [2] A. Winsel, German Patent DE 3 342 969 (CI C25B11/06). [3] K. Kordesch, G. Simader, Fuel Cells and Their Applications, VCH, Weinheim, 1996. [4] E. GuÈlzow, K. Bolwin, W. Schnurnberger, Dechema-Monographien Bd 117 (1989) 365. [5] E. GuÈlzow, B. Holzwarth, M. Schulze, N. Wagner, W. Schnurnberger, Dechema-Monographien Bd 125 (1992) 561. [6] E. GuÈlzow, et al., PTFE bonded gas diffusion electrodes for alkaline fuel cells, in: Proceedings of the 9th World Hydrogen Energy Conference 92, Paris, p. S1507. [7] S. Ur-Rahman, M.A. Al-Saleh, A.S. Al-Zakri, S. Gultekin, J. Appl. Electrochem. 27 (1997) 215. [8] E. GuÈlzow, et al., Fuel Cell Bull. 15 (1999) 8±12. [9] E. GuÈlzow, et al., J. Power Sources 86 (2000) 352. [10] D. Bevers, E. GuÈlzow, A. Helmbold, B. MuÈller, Innovative production technique for PEFC electrodes, in: Proceedings of the Fuel Cell Seminar 96, Orlando, p. 668. [11] A. Helmbold, German Patent DE 197 57 492 A 1 (1999). [12] D. Bevers, N. Wagner, German Patent DE 195 09 749 C2. [13] N. Wagner, German Patent DE 195 09 748. [14] J. Winsel, K. Verbrennung, F. Steiner, Verlag GmbH, Wiesbaden, 1962, p. 117. [15] M. Schulze, K. Bolwin, E. GuÈlzow, W. Schnurnberger, Fresenius J. Anal. Chem. 353 (1995) 778. [16] C. Erba, Microstructure Line 5 (1987) 1. [17] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Acadamic Press, London 1982. [18] K. RuÈhling, Untersuchungen an neuartigen PTFE-gebundenen Raney-Nickel- und Silberelektroden, Diploma work, University of Kassel, 1986. [19] M. Lorenz, M. Schulze, Characterization of electrochemical oxidized nickel surfaces, Fresenius J. Anal. Chem. 365 (1999) 154±157. [20] M. Schulze, R. Reiûner, M. Lorenz, W. Schnurnberger, Electrochim. Acta 44 (1999) 3969.
Activation of nickel-anodes for alkaline fuel cells M. Schulze*, E. GuÈlzow, G. Steinhilber Institut fuÈr Technische Thermodynamik, Deutsches Zentrum fuÈr Luft- und Raumfahrt, Pfaffenwaldring 38-40, D-70569 Stuttgart, Germany
Abstract In alkaline fuel cells (AFC) electrodes containing porous nickel and PTFE can be used as anodes. A low-cost production technique yields passive, polymer covered electrodes, which have to be activated before being used in electrochemical devices. During this activation process the surface structure as well as the chemical composition and the oxidation state of the used materials are changed by a given electrochemical current. The electrodes are investigated in different states during this activation process with X-ray photoelectron spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and nitrogen adsorption. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Alkaline fuel cells; Nickel; Electrodes; X-ray photoelectron spectroscopy; Reduction of nickel oxide
1. Introduction A fuel cell is an electrochemical device used to convert the chemical energy of, e.g. hydrogen and oxygen directly into electric current with a high ef®ciency. In principle, the direction of the electrode reactions in water electrolysis is reversed in the fuel cell. The electrochemical fuel cell reactions take place at the three-phase interface between electrocatalyst, electrolyte solution and reaction gas. In an alkaline fuel cell a KOH solution is used as electrolyte. The liquid electrolyte can penetrate the porous electrode like the reaction gases. Therefore, in alkaline fuel cells three-phase interfaces are not only on the electrode surface but are also formed in the volume of the electrode. Consequently, the reaction zone expands into the depth and high reactive surface areas in porous
*
Corresponding author. Tel.: 49-711-6862-456; fax: 49-711-6862-747. E-mail address: [email protected] (M. Schulze).
electrodes can be used for the alkaline fuel cell reaction. At DLR low-temperature fuel cells are investigated focusing on the preparation of gas diffusion electrodes and membrane±electrode assemblies, electrochemical behavior, physical characterization, long-term behavior and degradation mechanisms. The preparation of the gas diffusion electrode based on Sauer [1] and Winsel [2] has been further developed for electrodes of different low-temperature fuel cell types [3], alkaline fuel cells (AFC) [4±7] and polymer electrolyte membrane fuel cells (PEFC) [8±12] as well as for other electrochemical applications [13]. The electrochemical reaction takes place in the three-phase border zone between catalyst surface, electrolyte and gas phase. The investigated anodes for AFC applications are a composite of a metal catalyst and polytetra¯uoroethylene (PTFE) supported by a metal web on the backside of the electrode. The metal web electrically conducts the electrode and guarantees mechanical stability. The PTFE is used as an organic binder to hold the catalyst particles
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 2 9 1 - 4
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M. Schulze et al. / Applied Surface Science 179 (2001) 251±256
together. Due to its hydrophobic character the PTFE is important for the gas transport in the electrode and yields a hydrophobic pore system that is not ¯ooded by the alkaline electrolyte so that the electrode can be supplied with the fuel gas from the backside. Porous nickel is used as catalyst, the initial material is a nickel±aluminum alloy containing 50 wt.% of each metal. In a KOH the catalyst is activated by dissolving the aluminum at 350 K and it contains less than 5% of aluminum after this procedure. During the activation process the nickel is loaded with 0.5±1.2 hydrogen atoms per nickel atom [14]. After this preparation of the catalyst, the powder is washed and dried. Due to the hydrogen in the metal and the reactivity of a metallic nickel surface the contact with oxygen during the drying process has to be avoided. The catalyst powder cannot be handled in the reactive state to produce the electrodes. Therefore, after drying the catalyst powder is passivated by controlled oxygen exposure under temperature control of the powder. During this preparation step the hydrogen is removed and the catalyst surface oxidized. Before using the electrode in fuel cells the oxide must be reduced. In this paper the activation step is investigated. 2. Experimental 2.1. Electrochemical experiments The electrodes were activated in an electrochemical half cell con®guration [5] consisting of the electrode in a holder which allows the gas supply from the backside of the electrode and nickel counter electrode and a Hg/HgO reference electrode in a tempered vessel containing 30% KOH. The same test set-up was used for the electrochemical characterization of fuel cell electrodes. The working electrodes have a geometric area of 6 cm2. They were removed from the test set-up to analyze their surfaces and separated into three samples for the different characterization methods applied. For each measurement a new electrode was used. The electrodes were characterized after different activation times at the standard current density of 5 mA/cm2 operating as hydrogen evolution electrodes. From the backside of the electrode hydrogen was supplied during the activation process.
2.2. XPS investigations The XPS characterization of the electrodes was performed in a standard UHV chamber equipped with an XP-spectrometer (spherical analyzer r 127 mm, 300 W X-ray tube with Al- and Mg-excitation) and a quadruple mass spectrometer. The residual pressure p in the analysis chamber was approximately 4 10 10 mbar. The composition of the residual pressure gas was veri®ed by the mass spectrometer. The investigated GDE were rinsed with distilled water after removal from the half cell con®guration and subsequently dried in the vacuum lock at 350 K for 8 h. The pressure increased by nearly two orders of magnitude after transferring the GDE sample into the UHV chamber. To investigate the GDE, XP spectra of the C 1s, F 1s, Ni 2p, Cu 2p and O 1s levels were recorded during depth pro®ling by alternating XPS measurements and ion etching. During depth pro®ling of GDE the etching period was increased after 10 cycles. The changes of the surface composition during the depth pro®ling are not only caused by the etching of the surface but also by the decomposition of the polymer induced by the ionizing irradiation [15]. The experimental conditions, i.e. drying temperature and time, X-ray doses and Ar ion doses during the ion etching, were kept constant for all samples. 2.3. Porosimetry The porosimetry and the speci®c surface were measured by adsorption of nitrogen [16]. For these measurements the samples were dried in vacuum at 400 K for 18 h. The nitrogen adsorption measurements were performed in a commercial system (Carlo Erba Sorptomatic Series 1800) and evaluated with the standard BET algorithms [17]. 3. Results After reactive mixing of the different components for the electrodes, i.e. PTFE, copper powder and the oxidized nickel catalyst, in a knife mile the powder is rolled to a self-carrying strip, which is rolled in a second step onto a metal web to form the electrode which then has a thickness of approximately 350 mm. The pore system and the speci®c surface of the AFC
M. Schulze et al. / Applied Surface Science 179 (2001) 251±256
253
Fig. 1. Speci®c surface determined after different activation times at 5 mA/cm2.
anodes are determined by the uncovered surface of the metallic catalyst. The PTFE has no measurable surface in the nitrogen adsorption measurements [6]. Because of the rolling process as one step of the electrode preparation the PTFE is smeared to a thin ®lm which covers the metallic catalyst and reduces the speci®c surface recorded in BET measurements. The electrodes must be activated before being used in fuel cells. The standard activation procedure
Fig. 2. XPS depth pro®les of a non-activated electrode and an activated electrode.
Fig. 3. C 1s spectra of a non-activated electrode, after activation times of 18 and 115 h at 5 mA/cm2 and after 135 h hydrogen exposure (currentless).
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M. Schulze et al. / Applied Surface Science 179 (2001) 251±256
applied is to reduce the nickel oxide in 30% KOH electrochemically by using the electrode as hydrogen generating electrode for an electrolyzer at a current density of 5 mA/cm2 for 18 h [18]. In addition to the electrochemical reduction conditions hydrogen is exposed to the electrodes from the backside. During the activation process the speci®c surface measured by the nitrogen adsorption increases with the activation time. The pore system of the nickel catalyst is re-exposed again. The speci®c surface does not increase linearly in time as shown in Fig. 1. The increasing speci®c surface clearly indicates that the PTFE ®lm covering the catalyst is removed during the activation process. Therefore, in the XP spectra after activation the nickel signals should be increased considerably and the PTFE signal decreased. The XP spectra and depth pro®le of the
activated electrodes do not differ signi®cantly from that of the non-activated electrode as shown in Fig. 2. The PTFE ®lm can also be seen quite clearly for both activated and non-activated electrode. At the beginning of depth pro®ling the nickel signal is suppressed for the activated as well as for the non-activated electrode, but the initial nickel signal is increased by the activation process by a factor of approximately 2 after 18 h at 5 mA/cm2. After the long-term activation experiments the initial nickel concentration is increased by the factor 5 (115 h at 5 mA/cm2) compared with the non-activated electrode. The initial nickel intensity shows qualitatively but not quantitatively the same behavior as the speci®c surface. Like the initial intensity of the nickel signal in the XP spectra the copper signal is also increased by activation, which is, however, additionally induced
Fig. 4. Ni 2p signal during depth pro®ling after different activations: (a) non-activated, (b) 4.5 h at 5 mA/cm2, (c) 8.5 h at 5 mA/cm2, (d) 18 h at 5 mA/cm2.
M. Schulze et al. / Applied Surface Science 179 (2001) 251±256
by changes of the copper distribution in the electrode described in more detail in a further paper to be published. Differences in the decomposition of the polymers caused by the ionizing radiation (X-ray and ion etching) during depth pro®ling can be used as indicator of changes in the PTFE induced by electrochemical stressing [15]. The analysis of the irradiation-induced PTFE decomposition of the standard activated electrodes does not show an indicator for a change of the PTFE compared with the non-activated electrode. In contrast, for the long-term activated electrodes the XPS spectra indicate a PTFE decomposition. The carbon signal which can be related to graphite has increased signi®cantly while the ¯uorine signal has decreased (Fig. 3). These facts indicate a damaging of the PTFE during the long-term activation as also observed in long-term fuel cell operation [15]. Due to the high electron beam voltage and the resolved penetration depth the PTFE ®lms cannot be imaged in the SEM measurements, which means that a direct study of the topologic of the PTFE ®lms by SEM is not possible. On the ®rst XPS spectra of depth pro®les of the nickel catalyst nickel oxide is observed. This surface nickel oxide layer can be formed during the transfer process from the KOH electrolyte into the vacuum system [19,20]. Fig. 4 depicts the measured Ni 2p signals for the non-activated and activated electrodes. These spectra clearly show that the nickel is already reduced after 4.5 h at a current density of 5 mA/cm2. 4. Discussion It is very surprising that the electrodes are reduced after a short activation time because the catalyst is almost entirely covered by the PTFE ®lm as observed in the XP spectra and in the speci®c surface area measurements. Perforations in the PTFE ®lm are suf®cient to allow the contact of the electrolyte to the nickel catalyst and therefore to reduce the nickel oxide. During the activation process the perforations in the PTFE ®lm are increased or new ones are formed. The perforation may be created by the generated hydrogen, which is formed on the nickel surface and separates the PTFE ®lm from the catalyst surface. Therefore, the initial nickel signal in the XP spectra of
255
the activated electrode is increased as observed. Also the speci®c surface is increased without total removal of the PTFE ®lm. In the XP spectra of the electrodes activated for different times, the nickel oxide is totally reduced, which seems to contradict the results of the speci®c surface measurements. From the XPS measurements it cannot be concluded whether the nickel is also reduced in the volume of the electrode, which is relevant for the electrochemical behavior. In the speci®c surface measurements the surface of the catalyst particles in the volume of the porous electrode determines the value recorded. From the speci®c surface measurements it can clearly be seen that at short activation times a main part of the catalyst is covered in a gas or electrolyte tight way by the PTFE ®lm. Therefore, it can be concluded that the activation process starts on the surface of the electrode and the activation zone expands in the depth with increasing activation time. There are two effects leading to this behavior. The electrolyte penetration into the electrode is hindered by the hydrophobic character of the PTFE. With increasing PTFE damaging and time the electrolyte and activation zone shift deeper into the electrode. The perforation in the PTFE ®lm can enhance the electrolyte penetration into the electrode. The electrochemical reduction is coupled to a charge transfer and the ion transport, where an electrical ®eld is necessary. The potential gradient maximum is on the interface between the electrolyte and the catalyst. Caused by the good electric conductivity in the electrode and the low penetration depth starting the activation the gradient is maximal on the electrode surface, which is why the nickel of the surface is reduced ®rst. After the reduction of the nickel has started and a ®rst metallic nickel surface in contact to the electrolyte has been formed, the overvoltage of the hydrogen generation decreases and so the further reduction of the nickel oxide in the volume of the electrode runs slower than at the beginning. The mechanism of the PTFE changes during the activation process is indistinct; two in principle different mechanisms can induce the changes of the PTFE. First the chemical decomposition of the PTFE and second a change of the mechanical distribution of the PTFE. A change of the chemical composition is indicated by the differences of the C 1s spectra of
256
M. Schulze et al. / Applied Surface Science 179 (2001) 251±256
non-activated and long term activated electrodes. The high local electric ®elds in the thin PTFE ®lm can induce the PTFE decomposition, especially in combination with a chemical attack by intermediates or ions in the electrolyte. The change of the structure of the PTFE ®lm is indicated by the increasing of the speci®c surface area measurements with nitrogen adsorption. If hydrogen is electrochemically formed or water is formed by the reduction of the NiO under the PTFE ®lm the local pressure increases and can be suf®cient to break the PTFE ®lm. In an experiment of currentless activation the speci®c surface is increased without reduction of the nickel oxide but with an increase of the initial nickel signal, which shows that the PTFE is also damaged under these conditions and perforations are formed (Fig. 3). The main difference in this experiment is that the nickel oxide is not reduced. Caused by the dissolution of aluminum residues (observed by EDX) a little amount of hydrogen is produced during the experiment. Local ¯uctuations of the concentration in the electrolyte and local inhomogeneity of the catalyst in the electrodes induce electric ®eld in the electrodes. Therefore, the same mechanisms can cause the PTFE changes in the experiment with and without current. 5. Conclusions During the activation process several processes take place in parallel. The two main processes are the perforation of the PTFE ®lm and the reduction of the nickel catalyst. The damaging of the PTFE ®lm is primarily effected by the activation time. This process can be observed by activation with hydrogen generation as well as in the experiment of currentless activation. This step is necessary to allow the gas and the electrolyte the contact to the nickel catalyst surface. The reduction of the nickel oxide strongly depends on the charging. In the experiment of currentless activation the nickel oxide is not reduced. The nickel oxide on the surface of the electrodes is reduced after a short time, but not in the volume of the electrode, and thus the activation zone shifts into the electrode
volume during the activation process. This behavior is clearly re¯ected in the increase of the speci®c surface and the electrochemical behavior of the electrodes. The observed different steps running during activation lead to changes of the structure and the composition of the electrode. The same process excludes that the reduction of the nickel oxide can take place in the fuel cell operation and these may effect the degradation of the electrodes in long-term fuel cell operation. References [1] H. Sauer, German Patent DE 2 941 774 (CI H01M4/88). [2] A. Winsel, German Patent DE 3 342 969 (CI C25B11/06). [3] K. Kordesch, G. Simader, Fuel Cells and Their Applications, VCH, Weinheim, 1996. [4] E. GuÈlzow, K. Bolwin, W. Schnurnberger, Dechema-Monographien Bd 117 (1989) 365. [5] E. GuÈlzow, B. Holzwarth, M. Schulze, N. Wagner, W. Schnurnberger, Dechema-Monographien Bd 125 (1992) 561. [6] E. GuÈlzow, et al., PTFE bonded gas diffusion electrodes for alkaline fuel cells, in: Proceedings of the 9th World Hydrogen Energy Conference 92, Paris, p. S1507. [7] S. Ur-Rahman, M.A. Al-Saleh, A.S. Al-Zakri, S. Gultekin, J. Appl. Electrochem. 27 (1997) 215. [8] E. GuÈlzow, et al., Fuel Cell Bull. 15 (1999) 8±12. [9] E. GuÈlzow, et al., J. Power Sources 86 (2000) 352. [10] D. Bevers, E. GuÈlzow, A. Helmbold, B. MuÈller, Innovative production technique for PEFC electrodes, in: Proceedings of the Fuel Cell Seminar 96, Orlando, p. 668. [11] A. Helmbold, German Patent DE 197 57 492 A 1 (1999). [12] D. Bevers, N. Wagner, German Patent DE 195 09 749 C2. [13] N. Wagner, German Patent DE 195 09 748. [14] J. Winsel, K. Verbrennung, F. Steiner, Verlag GmbH, Wiesbaden, 1962, p. 117. [15] M. Schulze, K. Bolwin, E. GuÈlzow, W. Schnurnberger, Fresenius J. Anal. Chem. 353 (1995) 778. [16] C. Erba, Microstructure Line 5 (1987) 1. [17] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Acadamic Press, London 1982. [18] K. RuÈhling, Untersuchungen an neuartigen PTFE-gebundenen Raney-Nickel- und Silberelektroden, Diploma work, University of Kassel, 1986. [19] M. Lorenz, M. Schulze, Characterization of electrochemical oxidized nickel surfaces, Fresenius J. Anal. Chem. 365 (1999) 154±157. [20] M. Schulze, R. Reiûner, M. Lorenz, W. Schnurnberger, Electrochim. Acta 44 (1999) 3969.