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Influence of palladium films on hydrogen gas entry into iron: a study by electrochemical impedance spectroscopy
International Journal of Hydrogen Energy 25 (2000) 61±65
In¯uence of palladium ®lms on hydrogen gas entry into iron: a study by electrochemical impedance spectroscopy P. Bruzzoni*, R.M. Carranza, J.R. Collet Lacoste ComisioÂn Nacional de EnergõÂa AtoÂmica, Av. del Libertador 8250, 1429, Buenos Aires, Argentina
Abstract Covering the bare surface of a hydride forming metallic material with a thin ®lm of Pd is a method to accelerate hydriding, through avoiding the formation of a passive oxide ®lm and allowing hydrogen to diuse easily through the Pd ®lm. In this work we performed permeation experiments to study the hydrogen ingress rate into Pd-plated iron membranes. The experimental conditions are such that the Pd deposit and their interfaces (rather than the bulk iron substrate) determine the permeation results. Thus, it is possible to characterize the Pd ®lm behavior with respect to the passage of hydrogen. We used the electrochemical impedance spectroscopy technique, where the excitation signal was a modulated hydrogen pressure, and the response signal was the modulated hydrogen ¯ux, measured by an electrochemical method at the exit side of the permeation membrane. # 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Among the methods for hydrogen storage, the formation of metal hydrides is a promising alternative. One of the key properties of the hydride forming systems is the hydrogen exchange rate between the metallic (or hydride) phase and the gas phase. Most hydride forming metals and alloys are covered by a passive ®lm that hinders the passage of hydrogen through the metal±gas interface. A method to accelerate hydriding is to avoid the formation of the passive ®lm, by covering the bare surface of the hydride forming metal with a thin layer of palladium. Palladium is the optimal material for this purpose because of its large permeation coecient and the fact that Pd is a
* Corresponding author. Tel.: +54-11-4754-7287; fax: +5411-4754-7362. E-mail address: [email protected] (P. Bruzzoni)
relatively noble metal that is covered by an extremely thin passive ®lm. Palladium ®lms have been successfully used to accelerate hydriding of a zirconium base alloy at 200±4008C [1]. It is also well known the use of palladium ®lms to allow hydrogen entry into iron (which is a non hydride forming metal) by exposure to hydrogen gas, even at room temperature. This property has been used for hydrogen permeation experiments with gas phase charging and electrochemical detection [2±4]. In this work we performed permeation experiments to study the hydrogen ingress rate into Pd-plated iron membranes. Iron is not a hydride forming metal. Besides, the diusion coecient of hydrogen in pure and annealed iron is very high. In view of these properties, the use of thin iron membranes allows us to study the passage of hydrogen through the Pd ®lm and both the Pd/Fe and Pd/environment interfaces. The experimental conditions are such that the permeation results are mainly determined by the ®lm/interface properties and just slightly by the properties of the
0360-3199/00/$20.00 # 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 9 9 ) 0 0 0 1 0 - 5
62
P. Bruzzoni et al. / International Journal of Hydrogen Energy 25 (2000) 61±65
bulk iron substrate. The electrochemical impedance spectroscopy technique was applied in this work, with a modulated hydrogen pressure as the excitation signal [5], and a modulated hydrogen ¯ux, measured by an electrochemical method at the exit side of the iron membrane, as the response signal. 2. Experimental The passage of hydrogen through the Pd ®lm was investigated using the hydrogen permeation technique on iron membranes coated on both sides with an electroplated Pd ®lm. The permeation technique involves introduction of hydrogen at the entry side of the Pdplated membrane and detection of hydrogen at the exit side of the membrane. 2.1. Hydrogen introduction procedure The gas phase charging method was used for hydrogen introduction. Hydrogen gas was admitted into a charging chamber and put in contact with the Pd-plated entry side of the membrane. Hydrogen gas (99.999%) was supplied by a Packard hydrogen generator. Prior to the introduction of hydrogen, the charging chamber was rinsed repeatedly with nitrogen (99.99%) to eliminate oxygen. This rinsing procedure was repeated during the admission of hydrogen. 2.2. Hydrogen detection procedure The detection of hydrogen was carried out by the electrochemical method. The detection chamber was an electrochemical cell. The electrolyte was deaereated 0.1 N NaOH and was in contact with the Pd-plated exit side of the membrane, which acted as the working electrode. The counter electrode was a Pt wire. The reference electrode was Hg/HgO. The applied potential was 0.2 V vs. the normal hydrogen electrode in all the experiments. 2.3. Permeation membranes The permeation membranes were disks of 35 mm diameter. The exposed area for the permeation experiment was a circle of 20 mm diameter. Two dierent types of iron membranes were used for this work: thick and thin membranes. Thick membranes were ca 0.8 mm in thickness and were made from high purity iron with the following contents of impurities in mg/g: C 60, Si 50, Mn 9, P 18, S 30, Al < 30, N < 10, O 40 and B 17. This material was kindly supplied by Max Planck Institut fuÈr Eisenforschung GmbH, DuÈsseldorf. The membranes were annealed for 6 h at 8508C in a hydrogen atmosphere. These membranes were used to
assess this new measurement technique with a wellde®ned material with negligible surface eects. Thin membranes were ca 0.1 mm in thickness and were made from pure iron (Goodfellow, 99.5%). They were annealed at 9008C for 1 h. Due to their low thickness, the passage of hydrogen through the bulk substrate was very fast. Therefore these membranes were suitable for studying surface eects (i.e. the eect of the palladium ®lm) on the hydrogen permeation. 2.4. Palladium plating procedures The iron membranes were electroplated with two dierent types of palladium ®lms: the `nano-Pd ®lm' and the ``micro-Pd ®lm''. The nano-Pd ®lm was obtained by electroplating from a dilute deaereated solution containing ca 2.5 mg of Pd in the form of Na2[Pd(NO2)4] and 150 ml of 0.1 M NaOH. The plating current was 0.1 mA cmÿ2. Prior to deposition the iron membrane was cleaned, electropolished, pickled in HCl (1:1), rinsed with water and immediately put in the prepolarized deposition cell containing 150 ml of 0.1 M NaOH to accomplish the cathodic reduction of the remnant oxide ®lm. After 400 s the palladium salt was added to begin electrodeposition. The electrodeposition time was 2400 s. This Pd ®lm could neither be detected by X-ray diraction nor observed by Scanning Electron Microscopy. However, the presence of Pd was detected by energy dispersive analysis of Xrays (EDAX) using a low energy (6 kV) electron beam. Also, a study by X-ray photoelectron spectroscopy (XPS) show that the Pd ®lm thickness was in the order of a few nm. The micro-Pd ®lm was obtained by a conventional electroplating procedure [6]. The plating solution contained 5 g of PdCl2, 240 g of Na3PO410H2O, 55 g of (NH4)3PO4, 3.5 g of benzoic acid and 1000 ml of H2O. The temperature was 608C, the plating time was 120 s and the plating current 2 mA cmÿ2. The resulting ®lm thickness was 1000 nm as determined by weight increase, assuming that the ®lm density is the same as that of bulk palladium. This ®lm was studied by XRay Diraction and Scanning Electron Microscopy. The X-Ray Diraction Diagram showed the presence of a-Pd, con®rming the ®lm thickness by the Rietveld method. The Scanning Electron Microscopy showed a continuous Pd ®lm of low roughness. In order to ensure a minimum amount of oxide at the Pd/Fe interface, this micro-Pd ®lm was applied on the nano-Pd ®lm. 2.5. Measurement technique Instead of the usual technique in which hydrogen ¯ux at the exit side is recorded after a step in charging conditions, the frequency response analysis technique
P. Bruzzoni et al. / International Journal of Hydrogen Energy 25 (2000) 61±65
was used in this work [5]. The general principle of this technique is to apply an oscillating (usually sinusoidal) excitation signal to the system and to measure a response signal of the same frequency. The complex ratio response signal/excitation signal is the so-called transfer function. By changing the frequency, the spectrum of transfer function as a function of frequency is obtained. For the case of this work, the excitation signal was an oscillating pressure superimposed to a steady hydrogen pressure in the charging chamber. This was achieved through a moving piston connected to the charging chamber, which was driven by a DC servo motor. The motor was powered by an electronic controller based in the feedback principle: it compares the voltage signal from the generator of the frequency response analyzer (desired pressure) with the output of a pressure transducer of the charging chamber. The controller delivers power to the motor according to the dierence between both signals. This system can operate at frequencies as high as 5 Hz. The amplitude of the pressure signal was limited in order to keep the mechanical stress in the iron membrane well under the yield point. The response signal was the hydrogen permeation current, which was measured as a potential drop through a resistance in series with the counter electrode. Before obtaining the frequency response spectrum, hydrogen was admitted into the chamber and the permeation transient was allowed to develop. The run began when the hydrogen current at the exit side reached a steady value. All measurements were carried out at 258C.
3. Results and discussion 3.1. Thick membrane The thick membranes were used to test the reliability of this new technique. The material (pure, annealed iron) is a well-known reference material [7,8]. If we assume that the impedances of both the entry and exit surfaces are negligible, then: . The hydrogen concentration in iron just beneath the entry surface cH will follow Sieverts's law, p cH=ks pH , where ks is the Sieverts constant and pH is the partial pressure of hydrogen. In this case a boundary condition of oscillating concentration is expected at the entry surface. . The only diusion medium will be the iron membrane. . Due to the applied anodic potential, the hydrogen concentration in iron just beneath the exit surface will approach zero.
63
We call this theoretical diusion system `concentration/single plate'. We de®ne a theoretical transfer function H=õÄ/pÄ, the ratio of the current density signal at the exit surface to the hydrogen pressure signal at the entry surface. H is a complex number whose mathematical expression is, H
aL S, sinh aL
where r pf a 1 j , D
j
p ÿ1,
where L is the membrane thickness, f is the frequency and D is the diusion coecient. S is a scale factor, in this case given by p ks DF pH S , 2L where F is the Faraday constant and pH is the continuous component of the pressure at the entry surface. The physical meaning of S is the slope of the steady state i vs p curve. Fig. 1 shows typical transfer function spectra (presented as Nyquist and Bode plots) of the thick iron membranes. There is a close ®t to the `concentration/ single plate' model. The obtained diusion coecient agrees very well with literature data (Table 1). This shows that, in this system, the time to achieve Sieverts's equilibrium at the entry surface is much shorter than the characteristic time constant (L 2/D ) for the diusion through the membrane. The size of the impedance diagram is half the ratio of steady state current/steady state hydrogen pressure, also in agreement with Sieverts's law. However, the diagram size is about 70% of the expected value, according to the permeation coecient value found in the literature [3]. This can be partially explained by the uncertainty in the area of the exposed membrane surface. 3.2. Thin membranes For thin membranes the situation is quite dierent: the characteristic time constant for diusion in the bulk iron is ca 50 times smaller in the thin membranes Table 1 diusion coecient of hydrogen in iron Author
D at 258C (cm2 sÿ1)
Riecke and Bohnenkamp [7] Kiuchi and McLellan [8] This work (from thick membranes)
0.954 10ÿ4 0.727 10ÿ4 1.02 10ÿ4
64
P. Bruzzoni et al. / International Journal of Hydrogen Energy 25 (2000) 61±65
Fig. 1. Nyquist and phase angle plots for the thick permeation membranes. System: H2/Pd/Fe (L = 877 mm)/Pd/0.1 M NaOH. (.) Experimental; (w) Concentration/single plate ®t (D = 1.02 10ÿ4 cm2 sÿ1). The normalization factor S is de®ned in the text.
than in the thick ones. The surface impedance at the Pd ®lm and the Pd/Fe interface is no longer negligible. We report here two kinds of surface eects: the eect of aging and the eect of Pd ®lm thickness. 3.3. Eect of aging Fig. 2 shows transfer function spectra for thin iron membranes coated with the nano-Pd ®lm. The diagram of a fresh membrane shows good agreement with the `concentration-single plate' model. The obtained diusion coecient is 5.10ÿ5 cm2 sÿ1 at 258C. The dierence with the literature values may be due to: 1. The higher content of impurities of the thin membrane material; 2. An incipient surface eect. As new measurements on the same membrane are carried out, the Nyquist plot changes its shape from the spiral-type diagram, characteristic of a diusion process, to a semi-circular one. The phase angle in turn
shifts to higher absolute values for a given frequency. These changes of the spectra with exposure time in the measurement cell will be called `aging eect' in the following paragraphs. After prolonged aging the transfer function resembles that of an RC electrical network, suggesting that there is some component of the system that restricts hydrogen ¯ow in the same way as an electrical resistance, and some other component where hydrogen accumulates, playing the role of the capacitor. The reason for this change in properties is not clear yet. 3.4. Eect of the Pd ®lm thickness Fig. 3 shows initial transfer function spectra for thin iron membranes covered by the nano-Pd ®lm and the micro-Pd ®lm. The absolute value of the phase angle shifts to higher values for the micro-Pd ®lm, clearly showing a delay for hydrogen diusion with this thicker ®lm. This result suggests that the presence of non-covered areas (highly probable in the nano-Pd
Fig. 2. Aging eect. Nyquist plot and phase angle plot for the thin permeation membranes covered with the nano-Pd ®lm. System: H2/Pd/Fe (L = 100 mm)/Pd/0.1 M NaOH. (.) First spectrum; (w) Concentration/single plate ®t of ®rst spectrum (D = 5.05 10ÿ5 cm2 sÿ1); (Q) Spectrum after 6 days in cell. The normalization factor S is de®ned in the text.
P. Bruzzoni et al. / International Journal of Hydrogen Energy 25 (2000) 61±65
65
Fig. 3. Nyquist plot and phase angle plot for the thin permeation membranes covered with the nano- and micro-Pd ®lms (®rst spectrum). System: H2/Pd/Fe (L = 100 mm)/Pd/0.1 M NaOH. (.) Iron coated with the nano-Pd ®lm; (w) Concentration/single plate ®t for iron covered with the nano-Pd ®lm (D = 5.05 10ÿ5 cm2 sÿ1); (R) Iron coated with the micro-Pd ®lm. The normalization factor S is de®ned in the text.
®lm) does not play an important role in reducing or delaying the passage of hydrogen through the gas/ metal interface.
reasons still not clear during the exposure to hydrogen, increasing the overall delay in hydrogen permeation.
4. Conclusion
Acknowledgements
. The present new technique of hydrogen permeation combined with frequency response analysis and gas phase charging is adequate to study both bulk and surface eects. The characterization of a surface with respect to its ability to allow the passage of hydrogen may be of interest for the optimization of hydriding processes in hydrogen storage devices. . The use of thin membranes of annealed iron with fast response to hydrogen permeation provides a sensitive tool to study surface eects. . Thin Pd ®lms (in the nanometer range) are more eective for a fast passage of hydrogen than conventional (micrometer range) ones. The presence of non-covered areas in the thin ®lms does not seem to play an important role in reducing or delaying the passage of hydrogen. The Pd ®lm changes by
MarõÂ a Cristina Oviedo and co-workers are thanked for the XPS measurements. References [1] Domizzi G, Bruzzoni P. ComisioÂn Nacional de EnergõÂ a AtoÂmica, Buenos Aires, Argentina. Unpublished results. [2] Quick NR, Johnson HH. Acta Metall 1978;26:903. [3] Riecke E. Werkstoe und Korrosion 1981;32:66. [4] Bruzzoni P, Garavaglia R. Corr Sci 1992;33:1797. [5] Cummings DL, Blackburn DA. Met Trans 1985;16A:1013. [6] Machu W. In: Moderne Galvanotechnik. Weinheim: Verlag Chemie GmbH, 1954. p. 487. [7] Riecke E, Bohnenkamp K. Z Metallkde 1984;75:76. [8] Kiuchi K, McLellan RB. Acta Metall 1983;31:961.
In¯uence of palladium ®lms on hydrogen gas entry into iron: a study by electrochemical impedance spectroscopy P. Bruzzoni*, R.M. Carranza, J.R. Collet Lacoste ComisioÂn Nacional de EnergõÂa AtoÂmica, Av. del Libertador 8250, 1429, Buenos Aires, Argentina
Abstract Covering the bare surface of a hydride forming metallic material with a thin ®lm of Pd is a method to accelerate hydriding, through avoiding the formation of a passive oxide ®lm and allowing hydrogen to diuse easily through the Pd ®lm. In this work we performed permeation experiments to study the hydrogen ingress rate into Pd-plated iron membranes. The experimental conditions are such that the Pd deposit and their interfaces (rather than the bulk iron substrate) determine the permeation results. Thus, it is possible to characterize the Pd ®lm behavior with respect to the passage of hydrogen. We used the electrochemical impedance spectroscopy technique, where the excitation signal was a modulated hydrogen pressure, and the response signal was the modulated hydrogen ¯ux, measured by an electrochemical method at the exit side of the permeation membrane. # 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Among the methods for hydrogen storage, the formation of metal hydrides is a promising alternative. One of the key properties of the hydride forming systems is the hydrogen exchange rate between the metallic (or hydride) phase and the gas phase. Most hydride forming metals and alloys are covered by a passive ®lm that hinders the passage of hydrogen through the metal±gas interface. A method to accelerate hydriding is to avoid the formation of the passive ®lm, by covering the bare surface of the hydride forming metal with a thin layer of palladium. Palladium is the optimal material for this purpose because of its large permeation coecient and the fact that Pd is a
* Corresponding author. Tel.: +54-11-4754-7287; fax: +5411-4754-7362. E-mail address: [email protected] (P. Bruzzoni)
relatively noble metal that is covered by an extremely thin passive ®lm. Palladium ®lms have been successfully used to accelerate hydriding of a zirconium base alloy at 200±4008C [1]. It is also well known the use of palladium ®lms to allow hydrogen entry into iron (which is a non hydride forming metal) by exposure to hydrogen gas, even at room temperature. This property has been used for hydrogen permeation experiments with gas phase charging and electrochemical detection [2±4]. In this work we performed permeation experiments to study the hydrogen ingress rate into Pd-plated iron membranes. Iron is not a hydride forming metal. Besides, the diusion coecient of hydrogen in pure and annealed iron is very high. In view of these properties, the use of thin iron membranes allows us to study the passage of hydrogen through the Pd ®lm and both the Pd/Fe and Pd/environment interfaces. The experimental conditions are such that the permeation results are mainly determined by the ®lm/interface properties and just slightly by the properties of the
0360-3199/00/$20.00 # 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 9 9 ) 0 0 0 1 0 - 5
62
P. Bruzzoni et al. / International Journal of Hydrogen Energy 25 (2000) 61±65
bulk iron substrate. The electrochemical impedance spectroscopy technique was applied in this work, with a modulated hydrogen pressure as the excitation signal [5], and a modulated hydrogen ¯ux, measured by an electrochemical method at the exit side of the iron membrane, as the response signal. 2. Experimental The passage of hydrogen through the Pd ®lm was investigated using the hydrogen permeation technique on iron membranes coated on both sides with an electroplated Pd ®lm. The permeation technique involves introduction of hydrogen at the entry side of the Pdplated membrane and detection of hydrogen at the exit side of the membrane. 2.1. Hydrogen introduction procedure The gas phase charging method was used for hydrogen introduction. Hydrogen gas was admitted into a charging chamber and put in contact with the Pd-plated entry side of the membrane. Hydrogen gas (99.999%) was supplied by a Packard hydrogen generator. Prior to the introduction of hydrogen, the charging chamber was rinsed repeatedly with nitrogen (99.99%) to eliminate oxygen. This rinsing procedure was repeated during the admission of hydrogen. 2.2. Hydrogen detection procedure The detection of hydrogen was carried out by the electrochemical method. The detection chamber was an electrochemical cell. The electrolyte was deaereated 0.1 N NaOH and was in contact with the Pd-plated exit side of the membrane, which acted as the working electrode. The counter electrode was a Pt wire. The reference electrode was Hg/HgO. The applied potential was 0.2 V vs. the normal hydrogen electrode in all the experiments. 2.3. Permeation membranes The permeation membranes were disks of 35 mm diameter. The exposed area for the permeation experiment was a circle of 20 mm diameter. Two dierent types of iron membranes were used for this work: thick and thin membranes. Thick membranes were ca 0.8 mm in thickness and were made from high purity iron with the following contents of impurities in mg/g: C 60, Si 50, Mn 9, P 18, S 30, Al < 30, N < 10, O 40 and B 17. This material was kindly supplied by Max Planck Institut fuÈr Eisenforschung GmbH, DuÈsseldorf. The membranes were annealed for 6 h at 8508C in a hydrogen atmosphere. These membranes were used to
assess this new measurement technique with a wellde®ned material with negligible surface eects. Thin membranes were ca 0.1 mm in thickness and were made from pure iron (Goodfellow, 99.5%). They were annealed at 9008C for 1 h. Due to their low thickness, the passage of hydrogen through the bulk substrate was very fast. Therefore these membranes were suitable for studying surface eects (i.e. the eect of the palladium ®lm) on the hydrogen permeation. 2.4. Palladium plating procedures The iron membranes were electroplated with two dierent types of palladium ®lms: the `nano-Pd ®lm' and the ``micro-Pd ®lm''. The nano-Pd ®lm was obtained by electroplating from a dilute deaereated solution containing ca 2.5 mg of Pd in the form of Na2[Pd(NO2)4] and 150 ml of 0.1 M NaOH. The plating current was 0.1 mA cmÿ2. Prior to deposition the iron membrane was cleaned, electropolished, pickled in HCl (1:1), rinsed with water and immediately put in the prepolarized deposition cell containing 150 ml of 0.1 M NaOH to accomplish the cathodic reduction of the remnant oxide ®lm. After 400 s the palladium salt was added to begin electrodeposition. The electrodeposition time was 2400 s. This Pd ®lm could neither be detected by X-ray diraction nor observed by Scanning Electron Microscopy. However, the presence of Pd was detected by energy dispersive analysis of Xrays (EDAX) using a low energy (6 kV) electron beam. Also, a study by X-ray photoelectron spectroscopy (XPS) show that the Pd ®lm thickness was in the order of a few nm. The micro-Pd ®lm was obtained by a conventional electroplating procedure [6]. The plating solution contained 5 g of PdCl2, 240 g of Na3PO410H2O, 55 g of (NH4)3PO4, 3.5 g of benzoic acid and 1000 ml of H2O. The temperature was 608C, the plating time was 120 s and the plating current 2 mA cmÿ2. The resulting ®lm thickness was 1000 nm as determined by weight increase, assuming that the ®lm density is the same as that of bulk palladium. This ®lm was studied by XRay Diraction and Scanning Electron Microscopy. The X-Ray Diraction Diagram showed the presence of a-Pd, con®rming the ®lm thickness by the Rietveld method. The Scanning Electron Microscopy showed a continuous Pd ®lm of low roughness. In order to ensure a minimum amount of oxide at the Pd/Fe interface, this micro-Pd ®lm was applied on the nano-Pd ®lm. 2.5. Measurement technique Instead of the usual technique in which hydrogen ¯ux at the exit side is recorded after a step in charging conditions, the frequency response analysis technique
P. Bruzzoni et al. / International Journal of Hydrogen Energy 25 (2000) 61±65
was used in this work [5]. The general principle of this technique is to apply an oscillating (usually sinusoidal) excitation signal to the system and to measure a response signal of the same frequency. The complex ratio response signal/excitation signal is the so-called transfer function. By changing the frequency, the spectrum of transfer function as a function of frequency is obtained. For the case of this work, the excitation signal was an oscillating pressure superimposed to a steady hydrogen pressure in the charging chamber. This was achieved through a moving piston connected to the charging chamber, which was driven by a DC servo motor. The motor was powered by an electronic controller based in the feedback principle: it compares the voltage signal from the generator of the frequency response analyzer (desired pressure) with the output of a pressure transducer of the charging chamber. The controller delivers power to the motor according to the dierence between both signals. This system can operate at frequencies as high as 5 Hz. The amplitude of the pressure signal was limited in order to keep the mechanical stress in the iron membrane well under the yield point. The response signal was the hydrogen permeation current, which was measured as a potential drop through a resistance in series with the counter electrode. Before obtaining the frequency response spectrum, hydrogen was admitted into the chamber and the permeation transient was allowed to develop. The run began when the hydrogen current at the exit side reached a steady value. All measurements were carried out at 258C.
3. Results and discussion 3.1. Thick membrane The thick membranes were used to test the reliability of this new technique. The material (pure, annealed iron) is a well-known reference material [7,8]. If we assume that the impedances of both the entry and exit surfaces are negligible, then: . The hydrogen concentration in iron just beneath the entry surface cH will follow Sieverts's law, p cH=ks pH , where ks is the Sieverts constant and pH is the partial pressure of hydrogen. In this case a boundary condition of oscillating concentration is expected at the entry surface. . The only diusion medium will be the iron membrane. . Due to the applied anodic potential, the hydrogen concentration in iron just beneath the exit surface will approach zero.
63
We call this theoretical diusion system `concentration/single plate'. We de®ne a theoretical transfer function H=õÄ/pÄ, the ratio of the current density signal at the exit surface to the hydrogen pressure signal at the entry surface. H is a complex number whose mathematical expression is, H
aL S, sinh aL
where r pf a 1 j , D
j
p ÿ1,
where L is the membrane thickness, f is the frequency and D is the diusion coecient. S is a scale factor, in this case given by p ks DF pH S , 2L where F is the Faraday constant and pH is the continuous component of the pressure at the entry surface. The physical meaning of S is the slope of the steady state i vs p curve. Fig. 1 shows typical transfer function spectra (presented as Nyquist and Bode plots) of the thick iron membranes. There is a close ®t to the `concentration/ single plate' model. The obtained diusion coecient agrees very well with literature data (Table 1). This shows that, in this system, the time to achieve Sieverts's equilibrium at the entry surface is much shorter than the characteristic time constant (L 2/D ) for the diusion through the membrane. The size of the impedance diagram is half the ratio of steady state current/steady state hydrogen pressure, also in agreement with Sieverts's law. However, the diagram size is about 70% of the expected value, according to the permeation coecient value found in the literature [3]. This can be partially explained by the uncertainty in the area of the exposed membrane surface. 3.2. Thin membranes For thin membranes the situation is quite dierent: the characteristic time constant for diusion in the bulk iron is ca 50 times smaller in the thin membranes Table 1 diusion coecient of hydrogen in iron Author
D at 258C (cm2 sÿ1)
Riecke and Bohnenkamp [7] Kiuchi and McLellan [8] This work (from thick membranes)
0.954 10ÿ4 0.727 10ÿ4 1.02 10ÿ4
64
P. Bruzzoni et al. / International Journal of Hydrogen Energy 25 (2000) 61±65
Fig. 1. Nyquist and phase angle plots for the thick permeation membranes. System: H2/Pd/Fe (L = 877 mm)/Pd/0.1 M NaOH. (.) Experimental; (w) Concentration/single plate ®t (D = 1.02 10ÿ4 cm2 sÿ1). The normalization factor S is de®ned in the text.
than in the thick ones. The surface impedance at the Pd ®lm and the Pd/Fe interface is no longer negligible. We report here two kinds of surface eects: the eect of aging and the eect of Pd ®lm thickness. 3.3. Eect of aging Fig. 2 shows transfer function spectra for thin iron membranes coated with the nano-Pd ®lm. The diagram of a fresh membrane shows good agreement with the `concentration-single plate' model. The obtained diusion coecient is 5.10ÿ5 cm2 sÿ1 at 258C. The dierence with the literature values may be due to: 1. The higher content of impurities of the thin membrane material; 2. An incipient surface eect. As new measurements on the same membrane are carried out, the Nyquist plot changes its shape from the spiral-type diagram, characteristic of a diusion process, to a semi-circular one. The phase angle in turn
shifts to higher absolute values for a given frequency. These changes of the spectra with exposure time in the measurement cell will be called `aging eect' in the following paragraphs. After prolonged aging the transfer function resembles that of an RC electrical network, suggesting that there is some component of the system that restricts hydrogen ¯ow in the same way as an electrical resistance, and some other component where hydrogen accumulates, playing the role of the capacitor. The reason for this change in properties is not clear yet. 3.4. Eect of the Pd ®lm thickness Fig. 3 shows initial transfer function spectra for thin iron membranes covered by the nano-Pd ®lm and the micro-Pd ®lm. The absolute value of the phase angle shifts to higher values for the micro-Pd ®lm, clearly showing a delay for hydrogen diusion with this thicker ®lm. This result suggests that the presence of non-covered areas (highly probable in the nano-Pd
Fig. 2. Aging eect. Nyquist plot and phase angle plot for the thin permeation membranes covered with the nano-Pd ®lm. System: H2/Pd/Fe (L = 100 mm)/Pd/0.1 M NaOH. (.) First spectrum; (w) Concentration/single plate ®t of ®rst spectrum (D = 5.05 10ÿ5 cm2 sÿ1); (Q) Spectrum after 6 days in cell. The normalization factor S is de®ned in the text.
P. Bruzzoni et al. / International Journal of Hydrogen Energy 25 (2000) 61±65
65
Fig. 3. Nyquist plot and phase angle plot for the thin permeation membranes covered with the nano- and micro-Pd ®lms (®rst spectrum). System: H2/Pd/Fe (L = 100 mm)/Pd/0.1 M NaOH. (.) Iron coated with the nano-Pd ®lm; (w) Concentration/single plate ®t for iron covered with the nano-Pd ®lm (D = 5.05 10ÿ5 cm2 sÿ1); (R) Iron coated with the micro-Pd ®lm. The normalization factor S is de®ned in the text.
®lm) does not play an important role in reducing or delaying the passage of hydrogen through the gas/ metal interface.
reasons still not clear during the exposure to hydrogen, increasing the overall delay in hydrogen permeation.
4. Conclusion
Acknowledgements
. The present new technique of hydrogen permeation combined with frequency response analysis and gas phase charging is adequate to study both bulk and surface eects. The characterization of a surface with respect to its ability to allow the passage of hydrogen may be of interest for the optimization of hydriding processes in hydrogen storage devices. . The use of thin membranes of annealed iron with fast response to hydrogen permeation provides a sensitive tool to study surface eects. . Thin Pd ®lms (in the nanometer range) are more eective for a fast passage of hydrogen than conventional (micrometer range) ones. The presence of non-covered areas in the thin ®lms does not seem to play an important role in reducing or delaying the passage of hydrogen. The Pd ®lm changes by
MarõÂ a Cristina Oviedo and co-workers are thanked for the XPS measurements. References [1] Domizzi G, Bruzzoni P. ComisioÂn Nacional de EnergõÂ a AtoÂmica, Buenos Aires, Argentina. Unpublished results. [2] Quick NR, Johnson HH. Acta Metall 1978;26:903. [3] Riecke E. Werkstoe und Korrosion 1981;32:66. [4] Bruzzoni P, Garavaglia R. Corr Sci 1992;33:1797. [5] Cummings DL, Blackburn DA. Met Trans 1985;16A:1013. [6] Machu W. In: Moderne Galvanotechnik. Weinheim: Verlag Chemie GmbH, 1954. p. 487. [7] Riecke E, Bohnenkamp K. Z Metallkde 1984;75:76. [8] Kiuchi K, McLellan RB. Acta Metall 1983;31:961.