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Evaluation of Pd electroless films ability to be used as H-permeable anodes
Electrochimica Acta 53 (2008) 8138–8143
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Evaluation of Pd electroless films ability to be used as H-permeable anodes M. Cristina F. Oliveira ∗ Centro de Química de Vila Real, Departamento de Química, Universidade de Trás-os-Montes e Alto Douro, Quinta dos Prados, Apartado 1013, Vila Real, Portugal
a r t i c l e
i n f o
Article history: Received 26 March 2008 Received in revised form 4 June 2008 Accepted 7 June 2008 Available online 22 June 2008 Keywords: Pd electroless Pd alloys H-permeable anodes Pd membranes Pd–P
a b s t r a c t The prospective ability of using thin Pd electroless films made of pure Pd and Pd–P alloys as hydrogen permeable anodes was evaluated by a comparative characterization of these materials. The morphology, structure and composition effect on their ability to adsorb and oxidize hydrogen in 0.1 M NaOH solution was investigated. The results revealed that pure Pd (prepared at low T) followed by Pd–P alloy (low P content) exhibit the highest activity to absorb and oxidize hydrogen. It was concluded that the Pd alloy amorphicity (brought by a low content of P) and the pure Pd core porosity (brought by a low deposition rate) are the main factors responsible for such behaviour. In acid medium, it was found that Pd alloy films hinder the hydrogen absorption. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The use of palladium-based membranes in hydrogen separation/purification devices is broadly known but in contrast, the use of this material as a hydrogen permeable anode has not been much explored. Its application is confined to some hydrogenation reactions [1] and quite few fuel cells. The applicability of hydrogenpermeable anodes in hydrogen-air fuel cells have been demonstrated in alkaline [2,3] and proton conducting fuel cells (PCFCs) [4,5], on which a thick commercial Pd foil (25 m) has been used as the Pd membrane. The employment of this material, even when a Pd-black film is deposited on it, presents several drawbacks: material cost, low hydrogen flux and mechanical instability due to hydrogen embrittlement. Reduction of the Pd thickness is an attractive approach to solve some of the above problems as it would increase the H2 flux and reduce the cost effectiveness of the membrane. According to the literature, Pd membranes can be made of pure palladium or can be made of a Pd alloy (e.g. Pd–Ag) [6]. Pd alloys present better mechanical properties than pure Pd because phase transitions from ␣ to -Pd hydrides rest below room temperature which means that hydrogen embrittlement can be avoided. Among the most used deposition methods to prepare Pd membranes, electroless plating has been the most popular one because of its high simplicity. Several reducing agents are available for electroless deposition, however hydrazine has been the most used one on the preparation of Pd electroless membranes [7–11]. Nonetheless, the
∗ Tel.: +351 259350286; fax: +351 259350480. E-mail address: [email protected]. 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.06.022
hypophosphite ion is less toxic and gives rise to the formation of a Pd alloy (phosphorous is inevitably incorporated in the metal bulk during the metal deposition process giving rise to a Pd–P alloy). Despite these apparent advantages, literature reports on the preparation of Pd-electroless membranes from a hypophosphite ion solution, are very scarce [12,13]. In this work, the prospective ability of thin Pd-electroless films made of pure Pd and Pd–P alloys, to be used as hydrogen-permeable anodes (exhibiting no pinholes and displaying a high ability to absorb and to oxidize hydrogen) will be evaluated by a comparative characterization of these materials. This characterization will be made in terms of analysing their structure, morphology, composition and electrochemical behaviour in alkaline medium within a potential domain suitable for hydrogen absorption and oxidation. The permeability of the prepared films will not be appraised because experiments will be performed under impermeable boundary conditions (smooth metal substrates will be used as the film support). A porous support was not employed at this stage in order to allow the roughness factor comparison of the different prepared materials. 2. Experimental 2.1. Preparation of Pd-electroless films Four different Pd-electroless membranes were prepared: two of pure Pd and two of a Pd–P alloy. Pure-Pd membranes were assigned as Pd (low T) and Pd (high T) because the plating conditions for their preparation differed mainly on the deposition temperature. Despite preparation of Pd (low T) membrane required a reducing
M.C.F. Oliveira / Electrochimica Acta 53 (2008) 8138–8143
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Table 1 Electroless baths composition and deposition conditions of Pd pure and Pd alloy films on Ni and Cu substrates, respectively Pd film Pd (low T) Pd (high T) Pd alloy (low P) Pd alloy (high P)
Reducing agent (mM) N2 H4 H2 PO2 −
27.0 12.0 1.0 10.0
Pd2+ (mM)
NH3 (in H2 O) at 25% (v/v) (cm3 )
EDTA (M)
Tdep (◦ C)
tdep (min)
Reference
28.0 20.0 10.0 10.0
30 23 11.2 8.8
0.11 0.11 0.01 0.29 M NH4 Cl
25 50 50 50
300 50 90 90
[10] [11] [12] [13]
agent concentration that was approximately double than for deposition of Pd (high T), it was found that on using a lower hydrazine concentration solution (10 mM), a homogeneous Pd film was not regularly obtained. Concerning the Pd–P alloys, these membranes were assigned as Pd alloy (low P) and Pd alloy (high P) because the main difference on the electroless plating composition was the hypophosphite ion concentration, and therefore the P content of the alloy. The films were prepared using electroless plating bath compositions and plating conditions described in the literature (Table 1). Excepting Pd alloy (high P), all of these films have been reported for H2 purification/separation devices. Time deposition was monitored upon the metal substrate immersion into the plating solution. Pure Pd and Pd alloy films were deposited on smooth metal discs of nickel ( = 5 mm) and copper ( = 9.1 mm), respectively, which were previously inserted, under pressure, into a Teflon holder and polished to mirror finishing with successively finer grades of alumina, down to 0.3 m. This electrode assembly allowed deposition of Pd film just on one face of the disc and its removal by attack of acid (for electrothermal analysis) without damage of the electrode support. 2.2. Characterization of Pd-electroless films Structural analysis of the films was carried out in a Philips X’Pert diffractometer by X-ray diffraction using Cu K␣ radiation. The identification of crystalline phases was done using the JCPDS database cards. On the SEM/EDS analysis a Philips-FEI Quanta 400 microscope was employed. Atomic force microscopy was used for the topographic characterization of the Pd-black surface. The measurements were performed in a Nanoscope IIIa Multimode AFM Microscope (Digital Instruments, Veeco) in tapping mode using etched silicon probes (RTESP7 NanoprobeTM, Digital Instruments) with a resonance frequency of about 300 kHz. The electrochemical instrumentation consisted of an Autolab (model 100) potentiostat/galvanostat. Experiments were performed in one compartment cell with a Pt flag and a saturated calomel electrode (to which the potential is referred) as the counter and reference electrode, respectively. In the cyclic voltammetric experiments the potential was initially scanned from the anodic limit potential in the cathodic direction, at a scan rate of 10 mV s−1 . The solutions were deaerated with N2 and an oxygen-free nitrogen atmosphere was kept in the cell during the measurements. The typical hydrogen loading procedure consisted in applying a rather negative electrode potential (typically −1.3 or −1.4 V) for variable time in the 0.1 M NaOH solution. A fresh NaOH solution was prepared every day from high purity water (Millipore system) and 99.998% pure NaOH (Aldrich). Following the loading portion of the experiment, it was applied −0.10 V to discharge hydrogen incorporated in the Pd film. From the total amount of anodic charge collected, the H content in the metal was calculated assuming the discharge of absorbed hydrogen is the only electrolytic reaction at the working electrode and that hydrogen is quantitatively removed from the electrode. The amount of Pd in the film was determined by electrothermal atomisation after dissolving it in an HNO3 + HCl solution (1:1). Reproducible results were obtained from new prepared electrodes.
3. Results and discussion 3.1. Characterization of the morphology, composition and structure of Pd-electroless films Scanning electron microscopy was carried out to determine the Pd film thickness and assess the film morphology and uniformity. Fig. 1 shows that the thickness of the Pd-black film is extremely uniform (which is a characteristic of electroless deposition) and approximately the same on the different prepared materials (it varies from 2.2 to 2.8 m). Regarding pure-Pd films, spherical particles trapped on the surface were found on Pd (high T) film. Some of these were not well fixed to the surface, leaving holes behind it. The formation of these spherical particles is probably related to an insufficient stability of the deposition bath. It may be generated within the solution due to its self-decomposition, falling down on the growing film. Looking at the cross-section of the films it was observed that both Pd films were dense and compact at the surface, but a porous and spongy structure was found on Pd (low T) core. This remarkable structure seems to be the end result of a very low deposition rate. Concerning Pd alloys films, these also revealed to be dense and compact, either at the surface, either in the core. However, Pd alloy (low P) displays some small pinholes on the surface, leaving the copper substrate uncovered on some surfaces sites. EDS analysis showed that the high P content film contains about 9.5 at.% of P and the lower P content film contains about 4.0 at.% of P. Atomic force microscopy allowed looking in more detail the surface morphology of these films, particularly, in a region free of the high-dimension particles trapped on the surface (Fig. 2). It is clear that Pd (low T) presents the most irregular surface, displaying a wide range of particles dimension and the highest rms roughness (Rq). In contrast, Pd alloy (high P) shows Pd particles with the most uniform grain size and the smoothest surface. Curiously, the same trend has not been found on evaluating the roughness factor by electrochemical means [14] (Table 2). This discrepancy seems to reveal that the surface area is not only dependent on the average particles height (which determines the rms roughness), but is may also be ruled by the particles density on the surface. The roughness factor (rf) was not calculated for Pd alloy (high P) because it was found that PdO reduction peak overlapped with a non-identified cathodic peak at rather positive anodic limit potentials (whereas the anodic dissolution of P probably occurs). It is important to remark that despite electroless deposition yields a quite low surface roughness in contrast to others deposition
Table 2 Surface characterization parameters of Pd-electroless films (roughness factor, Rq, particles dimension) and palladium loading
Pd (low T) Pd (high T) Pd alloy (low P) Pd alloy (high P) a
Roughness factor
Rq (5 × 5 m)
∅ particles (nm)
Pd loading (mg cm−2 )
5.3 6.1 4.2 –
77 24 21 18
22–106 40–50 8–18 18–22
2.8 (0.52)a 2.6 (0.43)a 1.2 (0.28)a 2.2
These values rely on the real surface area of Pd films.
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Fig. 1. SEM images of as-deposited Pd-electroless films.
methodologies, like electrodeposition (rf ∼ = 10) [15] or chemical displacement reaction (rf = 30) [16], it may be enhanced by applying a thermal treatment to the electroless alloys [17]. Notwithstanding such procedure was not applied in this work as it is out of the main objectives of the present study.
As expected, the obtained amount of Pd on the prepared films (per geometric surface area) is higher than on anode materials prepared with Pd nanoparticles dispersed on a supporting material (0.08–1.0 mg cm−2 ) [18–20]. However, it is important to remark that an effective comparison should only rely on Pd loading determined
M.C.F. Oliveira / Electrochimica Acta 53 (2008) 8138–8143
Fig. 2. AFM surface topographies of Pd-electroless films at a magnification of 5m × 5 m.
per real surface area and unfortunately such data was not available in the literature data. Fig. 3 shows the XRD pattern of as-deposited Pd-electroless films. The sharpest peaks are ascribed to the substrate due to the thin thickness of the deposit. The XRD pattern exhibit clearly five diffraction peaks at about 2 = 40◦ , 47◦ , 68◦ , 82◦ and 86◦ which are indexed to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) facets of palladium. Contrasting to pure Pd films, the broadness of Pd peaks at the Pd alloy films reveals that these films are amorphous. In agreement with others electroless alloys [21], the degree of amorphicity of the alloy seems to increase as the phosphorus content of the Pd–P alloy increases as well.
3.2. Electrochemical characterization of the Pd-electroless films in alkaline medium Fig. 4 shows typical steady-state voltammograms obtained in 0.1 M NaOH for pure-Pd and Pd-alloys electroless films at 10 mV s−1 , which is a scan rate high enough to avoid non-electrochemical recombination reaction and low enough to allow diffusion of hydrogen from the bulk of the electrode to the surface electrode [22]. Comparison of charge density of peak I on the several prepared Pd films will allow evaluating, in a whole, the films ability to absorb and oxidize hydrogen. Concerning pure Pd films, it can be concluded that Pd (low T) film exhibits a higher ability to absorb and to oxidize hydrogen than Pd (high T) film. It was found that peak I charge is 2.7 times higher on Pd (low T) than on Pd (high T) (if charge density comparison relied on the real surface area, the difference ability would be even higher). On account of the different structure of Pd core on both films, it is suggested that the spongy structure observed on Pd (low T) film promotes hydrogen absorption (and consequently its oxidation) being responsible for the observed behaviour.
Fig. 3. XRD pattern of as-deposited Pd-electroless films.
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Fig. 5. Two typical current–time transient for the charge (at −1.4 V) and discharge (at −0.1 V) of Pd (high T) film in 0.1 M NaOH solution.
that Pd (low T) presents a better performance than Pd (high T) film. Comparing pure Pd and Pd alloys it can be concluded that pure Pd (low T) exhibits the highest activity to absorb and oxidize hydrogen, followed by Pd alloy (low P). The plateau observed in Fig. 6 corresponds, in terms of H/Pd ratio, to 1.10, 0.44 and 0.88 in Pd (low T), Pd (high T) or Pd alloy (high P) and Pd alloy (low P) films, respectively. These data allows concluding that, Pd (low T) and Pd alloy (low P) films present a better activity towards hydrogen absorption/oxidation than Pd films prepared by others methodologies (electrodeposition or laser pulsed deposition) [22,25,26]. The rather high H/Pd ratio for Pd (low T) Fig. 4. Cyclic voltammograms of Pd-electroless films in 0.1 M NaOH; 10 mV s−1 .
Compared to pure Pd films, the different ability to absorb and oxidize hydrogen on the two prepared Pd-alloy membranes is not so distinct. Pd alloy (low P) shows apparently a higher ability to absorb and oxidize hydrogen than the high P content alloy (the anodic charge is 1.9 times higher), but a reliable comparison is not much feasible when the real surface area of the surface film is unknown. Differences on the peak potentials of peak I on the two films are not fully understood but may be attributed to different locations of absorbed hydrogen, i.e. whether it is preferentially accumulated close to the electrode surface or in the Pd bulk. Additional experiments in the alkaline medium were also performed to evaluate quantitatively the films ability to absorb and to oxidize hydrogen. In such experiments the Pd films were loaded and unloaded by constant potential electrolysis. Fig. 5 presents typical charge and discharge curves for pure Pd-membranes in 0.1 M NaOH. Integration of these curves allowed plotting the anodic charge versus cathodic charge, both divided by the Pd load for each film (Fig. 6). The observed plateau reflects the saturation of the electrode with hydrogen which is consistent with the concomitant observation of evolved bubbles. The obtained results allow concluding that Pd alloy (low P) displays a higher ability to absorb and oxidize hydrogen than the high content P alloy. This result is in agreement with Paseka [23] and Lu et al. [24] data for Ni–P alloys. These authors have also found a higher amount of absorbed hydrogen on low P content alloys (3–5 at.%) than on electrodes with a P content of about 13–14 at.%. They both claimed that despite no electrocatalytic synergetic reaction occurs by the presence of the non-metallic element, the formation of an amorphous structure, to a certain low limit content of phosphorus, increases Pd ability to absorb hydrogen. Concerning pure Pd films, the obtained results (Fig. 6) reveal to be in good agreement with the voltammetric data, confirming
Fig. 6. Plot of Qa vs. Qc for Pd-electroless films in 0.1 M NaOH solution.
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does not depend on the cathodic limit potential and that its peak potential is exactly the same in a Ni pure substrate. So, it maybe suggested that this peak is due to the oxidation of the metal substrate through the small holes that were found on the film surface. Comparison of the anodic charge assigned to hydrogen oxidation (at 0.0 V) on both pure Pd films reinforces the conclusion that Pd (low T) displays a higher activity to absorb and oxidize hydrogen than Pd (high T). 4. Conclusions The overall results seem to reveal that the Pd amorphicity (brought by a low content of P) and the Pd core porosity (brought by a low deposition rate) are the main factors responsible for the high activity of the Pd-based electroless films to absorb and oxidize hydrogen in the alkaline medium. It is foreseen that these materials are suitable to be used in the future as hydrogen-permeable anodes in fuel cells. Acknowledgements I am grateful to Dr. Ana Viana (FCUL) for the AFM measurements and Dr. Nuno Martins (UTAD-UME) for the SEM/EDS analysis. References
Fig. 7. Cyclic voltammograms of Pd-electroless films in 0.1 M H2 SO4 ; 10 mV s−1 .
may be endorsed to H2 trapping within the porous structure of the film. 3.3. Electrochemical characterization of the Pd-electroless films in acid medium In order to compare Pd-electroless films activity in alkaline and acid medium, cyclic voltammograms were also recorded in sulphuric acid medium. Typical cyclic voltammograms for pure Pd and Pd alloy membranes are shown in Fig. 7. It is clearly revealed that Pd alloys do not absorb hydrogen in this medium, which is consistent with the formation of gas bubbles, observed from about −0.70 V. In order to evaluate whether this behaviour was exclusively related to the presence of HSO4 − ions, others electrolytes (HClO4 and HCl) were used, but the hydrogen absorption hindrance retained. The understanding of this peculiar behaviour is currently under investigation. Concerning Pd pure films, a small peak at about −0.30 V appears on Pd (high T). It was found that the charge of this peak
[1] H. Inoue, T. Abe, C. Iwakura, Chem. Commun. (1996) 55. [2] P. Cabot, E. Guezala, J. Calpe, M. García, J. Casado, J. Electrochem. Soc. 147 (2000) 43. [3] P. Cabot, E. Guezala, J. Casado, J. New Mater. Electrochem. Syst. 2 (1999) 253. [4] S. Yamaguchi, T. Shishido, H. Yugami, S. Yamamoto, S. Hara, Solid State Ionics 162/163 (2003) 291. [5] S. Yamaguchi, S. Yamamoto, T. Shishido, M. Omori, A. Okubo, J. Power Sources 129 (2004) 4. [6] S. Adhikari, S. Fernando, Ind. Eng. Chem. Res. 45 (2006) 875. [7] H.-B. Zhao, K. Pflanz, J.-H. Gu, A.-W. Li, N. Stroh, H. Brunner, G.-X. Xiong, J. Membr. Sci. 142 (1998) 147. [8] G.B. Sun, K. Hidajat, S. Kawi, J. Membr. Sci. 284 (2006) 110. [9] A. Li, W. Liang, R. Hughes, Catal. Today 56 (2000) 45. [10] B. Nair, J. Choi, M. Harold, J. Membr. Sci. 288 (2007) 67. [11] D. Tanaka, M. Tanco, S. Niwa, Y. Wakui, F. Mizukami, T. Namba, T. Suzuki, J. Membr. Sci. 247 (2005) 21. [12] D. Tanaka, M. Tanco, T. Negase, J. Okasaki, Y. Wakui, F. Mizukami, T. Suzuki, Adv. Mater. 18 (2006) 630. [13] E. Robertis, A. Fundo, A. Motheo, L. Abrantes, J. Braz. Chem. Soc. 16 (2005) 103. [14] A.N. Correia, L. Mascaro, S. Machado, L. Avaca, Electrochim. Acta 42 (1997) 493. [15] A. Czerwinski, R. Marassi, S. Zamponi, J. Electroanal. Chem. 316 (1991) 211. [16] M.C. Oliveira, A. Viana, Int. J. Hydrogen Energy 32 (2007) 3100. [17] A.M. Fundo, L.M. Abrantes, J. Electroanal. Chem. 600 (2007) 63. [18] D. Yuan, C. Xu, Y. Liu, S. Tan, X. Wang, Z. Wei, P. Shen, Electrochem. Commun. 9 (2007) 2473. [19] J. Pascual, S. Cigarroa, O. Feria, J. Power Sources 172 (2007) 229. [20] J. Liu, J. Ye, S. Jiang, Y. Tong, Electrochem. Commun. 9 (2007) 2334. [21] M.C. Oliveira, J. Appl. Catal. 329 (2007) 7. [22] A. Czerwinski, I. Kiersztyn, M. Grden, J. Electroanal. Chem. 492 (2000) 128. [23] I. Paseka, Electrochim. Acta 40 (1995) 1633. [24] G. Lu, P. Evan, G. Zangari, J. Electrochem. Soc. 150 (2003) A551. [25] J. Paillier, L. Roué, J. Electrochem. Soc. 152 (2005) E1. [26] M. Baldauf, D. Kolb, Electrochim. Acta 38 (1993) 2145.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Evaluation of Pd electroless films ability to be used as H-permeable anodes M. Cristina F. Oliveira ∗ Centro de Química de Vila Real, Departamento de Química, Universidade de Trás-os-Montes e Alto Douro, Quinta dos Prados, Apartado 1013, Vila Real, Portugal
a r t i c l e
i n f o
Article history: Received 26 March 2008 Received in revised form 4 June 2008 Accepted 7 June 2008 Available online 22 June 2008 Keywords: Pd electroless Pd alloys H-permeable anodes Pd membranes Pd–P
a b s t r a c t The prospective ability of using thin Pd electroless films made of pure Pd and Pd–P alloys as hydrogen permeable anodes was evaluated by a comparative characterization of these materials. The morphology, structure and composition effect on their ability to adsorb and oxidize hydrogen in 0.1 M NaOH solution was investigated. The results revealed that pure Pd (prepared at low T) followed by Pd–P alloy (low P content) exhibit the highest activity to absorb and oxidize hydrogen. It was concluded that the Pd alloy amorphicity (brought by a low content of P) and the pure Pd core porosity (brought by a low deposition rate) are the main factors responsible for such behaviour. In acid medium, it was found that Pd alloy films hinder the hydrogen absorption. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The use of palladium-based membranes in hydrogen separation/purification devices is broadly known but in contrast, the use of this material as a hydrogen permeable anode has not been much explored. Its application is confined to some hydrogenation reactions [1] and quite few fuel cells. The applicability of hydrogenpermeable anodes in hydrogen-air fuel cells have been demonstrated in alkaline [2,3] and proton conducting fuel cells (PCFCs) [4,5], on which a thick commercial Pd foil (25 m) has been used as the Pd membrane. The employment of this material, even when a Pd-black film is deposited on it, presents several drawbacks: material cost, low hydrogen flux and mechanical instability due to hydrogen embrittlement. Reduction of the Pd thickness is an attractive approach to solve some of the above problems as it would increase the H2 flux and reduce the cost effectiveness of the membrane. According to the literature, Pd membranes can be made of pure palladium or can be made of a Pd alloy (e.g. Pd–Ag) [6]. Pd alloys present better mechanical properties than pure Pd because phase transitions from ␣ to -Pd hydrides rest below room temperature which means that hydrogen embrittlement can be avoided. Among the most used deposition methods to prepare Pd membranes, electroless plating has been the most popular one because of its high simplicity. Several reducing agents are available for electroless deposition, however hydrazine has been the most used one on the preparation of Pd electroless membranes [7–11]. Nonetheless, the
∗ Tel.: +351 259350286; fax: +351 259350480. E-mail address: [email protected]. 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.06.022
hypophosphite ion is less toxic and gives rise to the formation of a Pd alloy (phosphorous is inevitably incorporated in the metal bulk during the metal deposition process giving rise to a Pd–P alloy). Despite these apparent advantages, literature reports on the preparation of Pd-electroless membranes from a hypophosphite ion solution, are very scarce [12,13]. In this work, the prospective ability of thin Pd-electroless films made of pure Pd and Pd–P alloys, to be used as hydrogen-permeable anodes (exhibiting no pinholes and displaying a high ability to absorb and to oxidize hydrogen) will be evaluated by a comparative characterization of these materials. This characterization will be made in terms of analysing their structure, morphology, composition and electrochemical behaviour in alkaline medium within a potential domain suitable for hydrogen absorption and oxidation. The permeability of the prepared films will not be appraised because experiments will be performed under impermeable boundary conditions (smooth metal substrates will be used as the film support). A porous support was not employed at this stage in order to allow the roughness factor comparison of the different prepared materials. 2. Experimental 2.1. Preparation of Pd-electroless films Four different Pd-electroless membranes were prepared: two of pure Pd and two of a Pd–P alloy. Pure-Pd membranes were assigned as Pd (low T) and Pd (high T) because the plating conditions for their preparation differed mainly on the deposition temperature. Despite preparation of Pd (low T) membrane required a reducing
M.C.F. Oliveira / Electrochimica Acta 53 (2008) 8138–8143
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Table 1 Electroless baths composition and deposition conditions of Pd pure and Pd alloy films on Ni and Cu substrates, respectively Pd film Pd (low T) Pd (high T) Pd alloy (low P) Pd alloy (high P)
Reducing agent (mM) N2 H4 H2 PO2 −
27.0 12.0 1.0 10.0
Pd2+ (mM)
NH3 (in H2 O) at 25% (v/v) (cm3 )
EDTA (M)
Tdep (◦ C)
tdep (min)
Reference
28.0 20.0 10.0 10.0
30 23 11.2 8.8
0.11 0.11 0.01 0.29 M NH4 Cl
25 50 50 50
300 50 90 90
[10] [11] [12] [13]
agent concentration that was approximately double than for deposition of Pd (high T), it was found that on using a lower hydrazine concentration solution (10 mM), a homogeneous Pd film was not regularly obtained. Concerning the Pd–P alloys, these membranes were assigned as Pd alloy (low P) and Pd alloy (high P) because the main difference on the electroless plating composition was the hypophosphite ion concentration, and therefore the P content of the alloy. The films were prepared using electroless plating bath compositions and plating conditions described in the literature (Table 1). Excepting Pd alloy (high P), all of these films have been reported for H2 purification/separation devices. Time deposition was monitored upon the metal substrate immersion into the plating solution. Pure Pd and Pd alloy films were deposited on smooth metal discs of nickel ( = 5 mm) and copper ( = 9.1 mm), respectively, which were previously inserted, under pressure, into a Teflon holder and polished to mirror finishing with successively finer grades of alumina, down to 0.3 m. This electrode assembly allowed deposition of Pd film just on one face of the disc and its removal by attack of acid (for electrothermal analysis) without damage of the electrode support. 2.2. Characterization of Pd-electroless films Structural analysis of the films was carried out in a Philips X’Pert diffractometer by X-ray diffraction using Cu K␣ radiation. The identification of crystalline phases was done using the JCPDS database cards. On the SEM/EDS analysis a Philips-FEI Quanta 400 microscope was employed. Atomic force microscopy was used for the topographic characterization of the Pd-black surface. The measurements were performed in a Nanoscope IIIa Multimode AFM Microscope (Digital Instruments, Veeco) in tapping mode using etched silicon probes (RTESP7 NanoprobeTM, Digital Instruments) with a resonance frequency of about 300 kHz. The electrochemical instrumentation consisted of an Autolab (model 100) potentiostat/galvanostat. Experiments were performed in one compartment cell with a Pt flag and a saturated calomel electrode (to which the potential is referred) as the counter and reference electrode, respectively. In the cyclic voltammetric experiments the potential was initially scanned from the anodic limit potential in the cathodic direction, at a scan rate of 10 mV s−1 . The solutions were deaerated with N2 and an oxygen-free nitrogen atmosphere was kept in the cell during the measurements. The typical hydrogen loading procedure consisted in applying a rather negative electrode potential (typically −1.3 or −1.4 V) for variable time in the 0.1 M NaOH solution. A fresh NaOH solution was prepared every day from high purity water (Millipore system) and 99.998% pure NaOH (Aldrich). Following the loading portion of the experiment, it was applied −0.10 V to discharge hydrogen incorporated in the Pd film. From the total amount of anodic charge collected, the H content in the metal was calculated assuming the discharge of absorbed hydrogen is the only electrolytic reaction at the working electrode and that hydrogen is quantitatively removed from the electrode. The amount of Pd in the film was determined by electrothermal atomisation after dissolving it in an HNO3 + HCl solution (1:1). Reproducible results were obtained from new prepared electrodes.
3. Results and discussion 3.1. Characterization of the morphology, composition and structure of Pd-electroless films Scanning electron microscopy was carried out to determine the Pd film thickness and assess the film morphology and uniformity. Fig. 1 shows that the thickness of the Pd-black film is extremely uniform (which is a characteristic of electroless deposition) and approximately the same on the different prepared materials (it varies from 2.2 to 2.8 m). Regarding pure-Pd films, spherical particles trapped on the surface were found on Pd (high T) film. Some of these were not well fixed to the surface, leaving holes behind it. The formation of these spherical particles is probably related to an insufficient stability of the deposition bath. It may be generated within the solution due to its self-decomposition, falling down on the growing film. Looking at the cross-section of the films it was observed that both Pd films were dense and compact at the surface, but a porous and spongy structure was found on Pd (low T) core. This remarkable structure seems to be the end result of a very low deposition rate. Concerning Pd alloys films, these also revealed to be dense and compact, either at the surface, either in the core. However, Pd alloy (low P) displays some small pinholes on the surface, leaving the copper substrate uncovered on some surfaces sites. EDS analysis showed that the high P content film contains about 9.5 at.% of P and the lower P content film contains about 4.0 at.% of P. Atomic force microscopy allowed looking in more detail the surface morphology of these films, particularly, in a region free of the high-dimension particles trapped on the surface (Fig. 2). It is clear that Pd (low T) presents the most irregular surface, displaying a wide range of particles dimension and the highest rms roughness (Rq). In contrast, Pd alloy (high P) shows Pd particles with the most uniform grain size and the smoothest surface. Curiously, the same trend has not been found on evaluating the roughness factor by electrochemical means [14] (Table 2). This discrepancy seems to reveal that the surface area is not only dependent on the average particles height (which determines the rms roughness), but is may also be ruled by the particles density on the surface. The roughness factor (rf) was not calculated for Pd alloy (high P) because it was found that PdO reduction peak overlapped with a non-identified cathodic peak at rather positive anodic limit potentials (whereas the anodic dissolution of P probably occurs). It is important to remark that despite electroless deposition yields a quite low surface roughness in contrast to others deposition
Table 2 Surface characterization parameters of Pd-electroless films (roughness factor, Rq, particles dimension) and palladium loading
Pd (low T) Pd (high T) Pd alloy (low P) Pd alloy (high P) a
Roughness factor
Rq (5 × 5 m)
∅ particles (nm)
Pd loading (mg cm−2 )
5.3 6.1 4.2 –
77 24 21 18
22–106 40–50 8–18 18–22
2.8 (0.52)a 2.6 (0.43)a 1.2 (0.28)a 2.2
These values rely on the real surface area of Pd films.
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Fig. 1. SEM images of as-deposited Pd-electroless films.
methodologies, like electrodeposition (rf ∼ = 10) [15] or chemical displacement reaction (rf = 30) [16], it may be enhanced by applying a thermal treatment to the electroless alloys [17]. Notwithstanding such procedure was not applied in this work as it is out of the main objectives of the present study.
As expected, the obtained amount of Pd on the prepared films (per geometric surface area) is higher than on anode materials prepared with Pd nanoparticles dispersed on a supporting material (0.08–1.0 mg cm−2 ) [18–20]. However, it is important to remark that an effective comparison should only rely on Pd loading determined
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Fig. 2. AFM surface topographies of Pd-electroless films at a magnification of 5m × 5 m.
per real surface area and unfortunately such data was not available in the literature data. Fig. 3 shows the XRD pattern of as-deposited Pd-electroless films. The sharpest peaks are ascribed to the substrate due to the thin thickness of the deposit. The XRD pattern exhibit clearly five diffraction peaks at about 2 = 40◦ , 47◦ , 68◦ , 82◦ and 86◦ which are indexed to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) facets of palladium. Contrasting to pure Pd films, the broadness of Pd peaks at the Pd alloy films reveals that these films are amorphous. In agreement with others electroless alloys [21], the degree of amorphicity of the alloy seems to increase as the phosphorus content of the Pd–P alloy increases as well.
3.2. Electrochemical characterization of the Pd-electroless films in alkaline medium Fig. 4 shows typical steady-state voltammograms obtained in 0.1 M NaOH for pure-Pd and Pd-alloys electroless films at 10 mV s−1 , which is a scan rate high enough to avoid non-electrochemical recombination reaction and low enough to allow diffusion of hydrogen from the bulk of the electrode to the surface electrode [22]. Comparison of charge density of peak I on the several prepared Pd films will allow evaluating, in a whole, the films ability to absorb and oxidize hydrogen. Concerning pure Pd films, it can be concluded that Pd (low T) film exhibits a higher ability to absorb and to oxidize hydrogen than Pd (high T) film. It was found that peak I charge is 2.7 times higher on Pd (low T) than on Pd (high T) (if charge density comparison relied on the real surface area, the difference ability would be even higher). On account of the different structure of Pd core on both films, it is suggested that the spongy structure observed on Pd (low T) film promotes hydrogen absorption (and consequently its oxidation) being responsible for the observed behaviour.
Fig. 3. XRD pattern of as-deposited Pd-electroless films.
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Fig. 5. Two typical current–time transient for the charge (at −1.4 V) and discharge (at −0.1 V) of Pd (high T) film in 0.1 M NaOH solution.
that Pd (low T) presents a better performance than Pd (high T) film. Comparing pure Pd and Pd alloys it can be concluded that pure Pd (low T) exhibits the highest activity to absorb and oxidize hydrogen, followed by Pd alloy (low P). The plateau observed in Fig. 6 corresponds, in terms of H/Pd ratio, to 1.10, 0.44 and 0.88 in Pd (low T), Pd (high T) or Pd alloy (high P) and Pd alloy (low P) films, respectively. These data allows concluding that, Pd (low T) and Pd alloy (low P) films present a better activity towards hydrogen absorption/oxidation than Pd films prepared by others methodologies (electrodeposition or laser pulsed deposition) [22,25,26]. The rather high H/Pd ratio for Pd (low T) Fig. 4. Cyclic voltammograms of Pd-electroless films in 0.1 M NaOH; 10 mV s−1 .
Compared to pure Pd films, the different ability to absorb and oxidize hydrogen on the two prepared Pd-alloy membranes is not so distinct. Pd alloy (low P) shows apparently a higher ability to absorb and oxidize hydrogen than the high P content alloy (the anodic charge is 1.9 times higher), but a reliable comparison is not much feasible when the real surface area of the surface film is unknown. Differences on the peak potentials of peak I on the two films are not fully understood but may be attributed to different locations of absorbed hydrogen, i.e. whether it is preferentially accumulated close to the electrode surface or in the Pd bulk. Additional experiments in the alkaline medium were also performed to evaluate quantitatively the films ability to absorb and to oxidize hydrogen. In such experiments the Pd films were loaded and unloaded by constant potential electrolysis. Fig. 5 presents typical charge and discharge curves for pure Pd-membranes in 0.1 M NaOH. Integration of these curves allowed plotting the anodic charge versus cathodic charge, both divided by the Pd load for each film (Fig. 6). The observed plateau reflects the saturation of the electrode with hydrogen which is consistent with the concomitant observation of evolved bubbles. The obtained results allow concluding that Pd alloy (low P) displays a higher ability to absorb and oxidize hydrogen than the high content P alloy. This result is in agreement with Paseka [23] and Lu et al. [24] data for Ni–P alloys. These authors have also found a higher amount of absorbed hydrogen on low P content alloys (3–5 at.%) than on electrodes with a P content of about 13–14 at.%. They both claimed that despite no electrocatalytic synergetic reaction occurs by the presence of the non-metallic element, the formation of an amorphous structure, to a certain low limit content of phosphorus, increases Pd ability to absorb hydrogen. Concerning pure Pd films, the obtained results (Fig. 6) reveal to be in good agreement with the voltammetric data, confirming
Fig. 6. Plot of Qa vs. Qc for Pd-electroless films in 0.1 M NaOH solution.
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does not depend on the cathodic limit potential and that its peak potential is exactly the same in a Ni pure substrate. So, it maybe suggested that this peak is due to the oxidation of the metal substrate through the small holes that were found on the film surface. Comparison of the anodic charge assigned to hydrogen oxidation (at 0.0 V) on both pure Pd films reinforces the conclusion that Pd (low T) displays a higher activity to absorb and oxidize hydrogen than Pd (high T). 4. Conclusions The overall results seem to reveal that the Pd amorphicity (brought by a low content of P) and the Pd core porosity (brought by a low deposition rate) are the main factors responsible for the high activity of the Pd-based electroless films to absorb and oxidize hydrogen in the alkaline medium. It is foreseen that these materials are suitable to be used in the future as hydrogen-permeable anodes in fuel cells. Acknowledgements I am grateful to Dr. Ana Viana (FCUL) for the AFM measurements and Dr. Nuno Martins (UTAD-UME) for the SEM/EDS analysis. References
Fig. 7. Cyclic voltammograms of Pd-electroless films in 0.1 M H2 SO4 ; 10 mV s−1 .
may be endorsed to H2 trapping within the porous structure of the film. 3.3. Electrochemical characterization of the Pd-electroless films in acid medium In order to compare Pd-electroless films activity in alkaline and acid medium, cyclic voltammograms were also recorded in sulphuric acid medium. Typical cyclic voltammograms for pure Pd and Pd alloy membranes are shown in Fig. 7. It is clearly revealed that Pd alloys do not absorb hydrogen in this medium, which is consistent with the formation of gas bubbles, observed from about −0.70 V. In order to evaluate whether this behaviour was exclusively related to the presence of HSO4 − ions, others electrolytes (HClO4 and HCl) were used, but the hydrogen absorption hindrance retained. The understanding of this peculiar behaviour is currently under investigation. Concerning Pd pure films, a small peak at about −0.30 V appears on Pd (high T). It was found that the charge of this peak
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