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Surface and electrochemical characterization of Ni–Zr intermetallic compounds
Intermetallics 8 (2000) 299±304
Surface and electrochemical characterization of Ni±Zr intermetallic compounds S. Sprianoa, F. Rosalbinoa, M. Bariccob, P.V. Morrab, E. Angelinia, C. Antonionea,*, J.-M. Sirec, P. Marcusc a Dipartimento di SMIC-Politecnico di Torino, C.so Duca degli Abruzzi, 24, 10129 Torino, Italy Dipartimento di Chimica IFM and INFM, UniversitaÁ di Torino, Via Giuria, 9, 10125 Torino, Italy c Laboratoire de Physico-Chimie des Surfaces, CNRS, ENSCP, Rue Curie, 11, 75005 Paris France
b
Received 6 April 1999; accepted 18 October 1999
Abstract In this paper the Ni±Zr system has been considered with the aim of investigating the role of composition and structure of an early±late transition metals system on the electrocatalytic activity, for the hydrogen evolution reaction. As a matter of fact, pure Ni and Zr show low activity, while their intermetallic compounds generate a higher catalytic eciency. Five alloys, with increasing Ni content starting from Ni33Zr67 up to Ni75Zr25, have been prepared and characterized. The alloy of composition (Ni0.55Mn0.30 V0.10Co0.05)2.1Zr has also been considered, in order to investigate the catalytic eciency related to a Laves structure. The thickness and composition of the surface oxides have been investigated and their eect on reducing the catalytic eciency of the as-prepared alloys has been discussed. The activity of the samples submitted to a surface activation treatment with hydro¯uoric acid, that removes the oxide layer and allows to evidence the properties of the compounds, has been observed to increase signi®cantly. The trend of the electrocatalytic eciency with the composition of the alloys is discussed considering a synergetic eect between Ni and Zr. The Laves phase appears slightly more active than the binary intermetallic compound. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics; Miscellaneous; B. Electrical resistance and other electrical properties; G. Catalysis
1. Introduction The electrocatalytic activity of an electrode with respect to the hydrogen evolution reaction is an important parameter both for its relevance to hydrogen evolution applications, as in fuel cells or electrodeposition, and for the theoretical study of adsorption/desorption mechanisms and kinetics of hydrogen on metals and alloys. Early-late transition metal alloys exhibit good electrocatalytic properties for the hydrogen evolution reaction and constitute an interesting research area in relation to the development of alternative materials to traditional electrodes. Ni and Zr pure elements show lower activity with respect to the noble metals and are on the opposite sides of the ``volcano curve'' [1], representing the electrocatalytic activity versus the metal±hydrogen * Corresponding author. Tel.: +39-11-5644608; fax: +39-115644699. E-mail address: [email protected] (S. Spriano).
bond energy. Their alloys, on the contrary, have an electronic structure which may generate higher catalytic activity [2]. This eect can be explained on the basis of the Brewer intermetallic bonding model, considering that when metals having empty or half-®lled vacant d-orbitals are alloyed with metals having internally paired d-electrons, not available for bonding in the pure metal, there arises an unusually strong intermetallic interaction [3]. Intermetallic compounds, belonging to the Ni2Zr type, with partial substitution of Ni by Mn, V or Co and of Zr by Ti, have also been recently proposed as possible hydrogen storage alloys for Ni-metal hydride batteries, in substitution of conventional (Mm)Ni5based alloys. To obtain a slight increase in the activity it has been also suggested to move the composition ratio from stoichiometric to (V,Ni)2+aZr [4,5]. These Zr based phases have a cubic Laves structure (C15) and show a high reversible capacity, a long electrochemical charge±discharge cycle life and high corrosion resistance [6]. On the other hand, they present a lower rate capability
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00105-3
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with respect to the conventional rare-earth based alloys, that is related to the surface reaction kinetics of hydrogenation or dehydrogenation. These Zr-based electrodes need a surface activation with chemical etching or charge±discharge cycling because of the low electrocatalytic activity of the asprepared material. Chemical etching with HF, Nh4F or KOH is fundamental for Ni±Zr based electrodes in order to dissolve the insulator oxide layer on the surface. This treatment may also promote the formation of nanocrystals of Ni dispersed at the surface of the electrode, due to selective dissolution of Zr [7]. As a matter of fact, the presence of chemically active sites together with an enhanced super®cial roughness improve the catalytic eciency of the alloys. Another method, that has been suggested as activation, is the annealing of the alloy at 200±300 C, in the presence of a low pressure of oxygen, in order to transform the monoclinic Zr oxide into the tetragonal one, which is more permeable [8]. The aim of this paper is the study of the role of composition on the electrocatalytic activity of binary Ni±Zr alloys, with composition ranging from Ni33Zr67 up to Ni75Zr25, corresponding to single intermetallic compounds or eutectic mixtures. The alloy of composition (Ni0.55Mn0.30V0.10Co0.05)2.1Zr has also been considered in order to investigate the catalytic eciency due to the Laves structure. A detrimental in¯uence of the presence of surface oxides on the catalytic eciency of the asprepared alloys has been evidenced. So the electrochemical properties of the alloys submitted to a surface activation treatment, by soft etching with HF solution, have been measured in order to evidence the real activity of the intermetallic compounds. 2. Experimental procedure The following alloys have been prepared by arc melting of pure elements (purity of Zr=99.9% and of Ni,Mn,Co,V=99.99%), under Ar atmosphere: Ni33Zr67, Ni36Zr64, Ni50Zr50, Ni64Zr36, Ni75Zr25, (Ni0.55Mn0.30V0.10 Co0.05)2.1Zr. Each ingot has been remelted several times in order to homogenise the composition and, in some cases, annealed for at least 2 h close to the melting temperature, in order to obtain the equilibrium structure. Pure elements, in form of rod or plate, have been used for comparison. Each ingot has been mechanically polished before characterization. Structural characterisation has been performed by powder x-ray diraction (XRD), using cobalt Ka incident radiation (l 0:17902 nm). Surface characterization has been carried out by a scanning electron microscope (SEM) and energy dispersion spectroscopy (EDS) using a Leica Stereoscan 420 microscope. The surface of the electrodes was analysed also by x-ray photoelectron spectroscopy (XPS) using a VG ESCALAB Mk2 with
Al K( (1486.6 eV) as an x-ray source. The pass energy of the hemispherical analyser was 50 eV for survey scans and 20 eV for the narrow energy regions of Ni 2p3/2, Zr 3d5/2,3/2, O1s and C1s. Electrodes of about 1.5 cm2 have been prepared by embedding the samples in acrylic resin. Electrochemical characterization has been achieved by means of cathodic polarization curves traced in oxygen free 1 M KOH at 25 C, in the potential range between the H2 reversible potential (ÿ1068 mV vs SCE) and ÿ2000 mV vs SCE . The experimental i±E data, corrected for the ohmic drop, have been ®tted by a statistical routine which provides the values of the exchange current density (i0) that can be reasonably taken as a measure of the catalytic eciency [1]. Cyclic voltammograms have also been recorded in the potential range ÿ1500 +500 mV vs SCE at a scan rate of 10 mV/s for 10 cycles, obtaining good results reproducibility after 5 or 6 cycles. Surface activation treatment has been performed by immersion in 0.01 M HF for 50 min at room temperature. After the chemical etching the samples have been washed in distilled water and dried in air. Electrochemical activation has been performed by cathodic polarization at ÿ2000 mV vs SCE for 10 min. 3. Results and discussion 3.1. As prepared alloys The x-ray diraction patterns of the as-prepared alloys are shown in Fig. 1. Diraction peaks of tetragonal ZrO2 are barely observed in the low angular range. The diraction pattern of a single intermetallic compound (NiZr2, NiZr or Ni3Zr) [9] has been found respectively for Ni33Zr67, Ni50Zr50 and Ni75Zr25 samples, in agreement with the phase diagram. In some
Fig. 1. X-ray diraction patterns of as-prepared (a) Ni75Zr25, (b) Ni64Zr36, (c) Ni50Zr50, (d) Ni36Zr64, (e) Ni33Zr67, (f) Laves phase. Cobalt K incident radiation (l 0:17902 mm) used.
S. Spriano et al. / Intermetallics 8 (2000) 299±304
cases the preferred orientation of surface crystals, due to the quenching in the arc furnace, changes the relative intensity of some diraction peaks. For the Ni36Zr64 composition an eutectic mixture of NiZr and NiZr2 has been found. In the corresponding x-ray diraction pattern, the diraction peaks of NiZr2 are more visible, owing to the higher quantity of this compound in the mixture, in agreement with the lever rule. The x-ray diraction pattern of the Ni64Zr36 composition is rather complicated. It can be indexed considering a mixture of Ni21Zr8 and Ni10Zr7 intermetallic compounds [9], according to the phase diagram, even if some diraction peaks belong to a cubic metastable Ni2Zr phase, which has been reported as oxygen stabilized [10]. The chemical nature of the surface of the dierent alloys has been investigated in order to evaluate the in¯uence of surface oxide layers on the electrocatalytic
301
properties. In the case of the as-prepared Ni50Zr50 alloy the XPS spectra (Fig. 2 ) show that nickel is essentially in the metallic state (Eb 852:8 eV), whereas zirconium is essentially in the oxide state. In fact the binding energy of Zr 3d5/2 is 182.8 eV, which is assigned to Zr4+ in zirconium oxide. In the O1s region, the main signal is observed at 530.8 eV, corresponding to O2ÿ in zirconium oxide, and a shoulder is visible at higher binding energy (surface hydroxyls and/or adsorbed H2O). The fact that the signal from metallic nickel located under the oxide layer is detected, indicates that the oxide layer is very thin (<50AÊ). Surface contamination by carboncontaining species is observed in the C1s region. The measured binding energy of the carbon signal (285.8 eV) indicates that there is no charging eect during the surface analysis, which is consistent with the fact that the surface oxide layer is very thin. There is no detectable
Fig. 2. XPS spectra of as-prepared (a) Ni33Zr67, (b) Ni50Zr50, (c) Ni64Zr36.
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satellite of the Ni 2p3/2 core level, which is usually found at 6 eV above the main peak for pure Ni, indicating a dierent electronic structure of Ni, in the NiZr compound, as compared to pure nickel. In fact the intensity and the position of this satellite are sensitive to the electronic structure of the alloy, and the observed result may indicate a reduced density of states at the Fermi level [11±13]. Comparing these results with those obtained on the other binary alloys it can be observed that for all compositions there is no detectable nickel oxide in the surface layer and that the thickness of the zirconium oxide layer decreases with increasing Ni content in the alloy. In fact the intensity of the nickel signal in Ni50Zr50 is about 2 times higher than that measured on the Ni33Zr67 alloy. This is probably due both to the higher Ni content in this alloy and to the fact that the oxide layer is thinner, as revealed by the presence of a signal from metallic zirconium at low binding energy (Zr 3d5/2 at 178.4 eV). In Fig. 3 the cathodic polarization curve of the Ni50Zr50 alloy is shown, as an example of the electrocatalytic behaviour of as-prepared electrodes. The i0 value, obtained by linear ®tting, is 4.55 10ÿ6 A/cm2. The electrocatalytic activity of all the as-prepared alloys is very low and increases slightly with the increase of Ni content (Fig. 4 ), as a consequence of the increase of the amount of the active element in the alloy and of the thinning of the surface oxide layer, as previously shown. Cyclic voltammetry performed on the as-prepared electrodes reveals the presence of Ni oxyhydroxide at the surface of the alloys with Ni content higher than 50% at., while the voltammetry of the Zr-rich alloys does not show any signal of redox events. As a conclusion, the microstructural, chemical and electrochemical analysis of the as-prepared alloys shows that the behaviour of these electrodes is aected by the presence of a Zr oxide layer and, consequently, in order to remove it and obtain information on the real activity
Fig. 4. i0 values of as-prepared and chemically activated Ni±Zr alloys as a function of composition.
of the intermetallic compounds, a surface pre-treatment has been performed. 3.2. Pre-treated alloys Pre-treatments have been performed by chemical etching with HF 0.01 M and by electrochemical activation at 2000 mV vs SCE, in order to chemically dissolve or to electrochemically reduce the oxide layer. HF dissolves ZrO2 and produces stable anion complexes like [ZrF6]2ÿ, [ZrF7]3ÿ, [ZrF8]4ÿ according to the following reaction: ZrO2 6HF () H2 ZrF6 2H2 O This diluted acid solution has been chosen because it causes only a slight variation in sample colour, from metallic silver to grey, as a consequence of the dissolution of zirconium oxide layer, while, with stronger solution, hydrogen embrittlement occurs with signi®cative changing in composition and surface morphology [17,14]. XRD patterns of all the alloys remain unchanged after soft etching and SEM pictures and EDS analysis of pre-treated samples show no change in surface morphology and composition with respect to asprepared alloys. It can be concluded that the etching reaction is limited to the very ®rst layers of the surface. Cyclic voltammograms show, for all the pre-treated samples, the reversible peak at +300 mV vs SCE due to the reaction [15] Ni OH2 () NiOOH H eÿ
Fig. 3. Cathodic polarization curves of as-prepared, electrochemically and chemically activated Ni50Zr50 alloy.
The intensity of the Ni oxyhydroxide peak progressively increases with the increasing of the Ni content in the alloy, as it can be observed in Fig. 5. This indicates that there is an enrichment in Ni at the surface, as a consequence of the dissolution of ZrO2 induced by the etching treatment, and that it is more eective for the Ni-rich alloys. The curves also show the increase of the negative current density, due to the hydrogen evolution reaction,
S. Spriano et al. / Intermetallics 8 (2000) 299±304
Fig. 5. Cyclic voltammograms recorded on chemically activated Ni± Zr alloys.
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appears to be the main factor in determining the electrocatalytic activity of the Ni±Zr alloys, as already evidenced in [3]. Although the alloys with higher Ni content show i0 values very closed to each other, the current density, j, obtained from the cathodic polarization curves at the overpotential ÿ250 mV progressively increases with the Ni content in the alloy, giving a higher electrocatalytic eciency for the Ni75Zr25 intermetallic compound, as evidenced in Table 2. No eect of the crystallographic structure of the intermetallic compounds can be evidenced from the present results. Analogously, the presence of a duplex microstructure in eutectic alloys does not signi®cantly in¯uence the electrocatalytic activity. 3.3. Laves phase
with the increasing of the Ni content, as evidenced in Table 1, which reports the cathodic current density values obtained at the potential E 1500 mV vs SCE. The increase of the hydrogen evolution reaction kinetic can be attributed to the presence of Ni at the surface of the samples, as a consequence of the ZrO2 dissolution induced by the chemical etching, the concentration of which increases with the increase of the Ni content in the alloy. Both the electrochemical and chemical pre-treatments in¯uence the electrocatalytic behaviour of the Ni±Zr electrodes: the exchange current density values increase, in the case of the Ni±Zr compound, of one order of magnitude by the cathodic activation and of two orders by the chemical one, as it can be observed from Fig. 3. The diluted HF etching has been used for the other alloys because it produces a stronger activation than the electrochemical polarisation without any evident hydrogen embrittlement, while samples treated with stronger etchings become so brittle that they are dicult to handle. In Fig. 4 the i0 values of all the pre-treated alloys are reported, as a function of composition. These values are related to the real structure and composition of the alloy and, consequently, they can be considered to discuss the electrocatalytic activity of the Ni±Zr intermetallic compounds. Comparing the alloys with parent elements it can be observed that for the intermetallic compounds a synergic eect between Ni and Zr occurs, leading to exchange current density values higher than for pure Ni. The increase of the activity with increasing Ni content in the alloy is evident. So, the composition of the compounds
The (Ni0.55Mn0.30V0.10Co0.05)2.1Zr alloy has been considered in order to evaluate the eect of crystallographic structure on the activity of the cubic Laves phase with respect to that of the binary Ni64Zr36 alloy. The alloy contains a single phase with the same structure as the cubic Laves phase (Ni0.5Mn0.5)2Zr (C15), as shown in Fig. 1. The lattice parameter of the cubic phase is 7.05 AÊ, a value slightly lower than that of (Ni0.5Mn0.5)2Zr (7.07AÊ), owing to the presence of additional elements in the alloy. Analogously to what has been previously reported about binary pre-treated alloys, the microstructure, surface morphology and composition of the alloy corresponding to the Laves phase do not change signi®catively after the surface treatment with HF, as revealed by XRD and SEM/EDS analysis. This is in agreement with what reported in literature, for the activation of the Laves phases by chemical etching, that is fundamental for the application of Zr based alloys for Ni-metal hydride batteries. In fact the etching reaction is limited to the very ®rst layers of the surface and involves a surface enrichment of Ni, in the metallic state, due to the dissolution of V, Mn and Zr oxides [16]. Similary to the binary alloys, the i0 value of the Laves phase, obtained by linear ®tting of cathodic polarization curves, is rather low (2.74 10ÿ6 A/cm2) for the as-prepared electrode. After chemical activation with HF a surface layer rich in Ni is produced increasing the electrocatalytic activity of the electrode, as previously noted for binary Ni±Zr compounds. The cathodic polarization
Table 1 Cathodic current density values evaluated from the cyclic voltammetries recorded on Ni±Zr alloys pre-treated in 0.01 MHF solution
Table 2 Current density values evaluated from the cathodic polarization curves recorded on Ni±Zr alloys pre-treated in 0.01 MHF solution
Alloy
j at E=ÿ1500 mV (mA/cm2)
Alloy
j at =ÿ250 mV (mA/cm2)
Ni33Zr67 Ni50Zr50 Ni75Zr25
ÿ0.81 ÿ1.83 ÿ4.53
Ni50Zr50 Ni64Zr36 Ni75Zr25
ÿ15.73 ÿ17.93 ÿ18.42
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Fig. 6. Cathodic polarization curves recorded on chemically activated Ni64Zr36 alloy and Laves phase.
curves performed on Laves phase and Ni64Zr36 alloy, pre-treated in HF solution, show that the former exhibits a current density higher than the latter, as it can be observed in Fig. 6, giving a i0 value of 1.16 10ÿ3 A/cm2. This is interesting considering that, as previously noted, the Zr-based Laves phases have also good charge and discharge property. As a matter of fact, the binary ZrV2, ZrCr2, ZrMn2 Laves compounds can be highly hydrogenated, but give too much stable hydrides, while the binary Ni±Zr intermetallic compounds that originate less stable hydrides, as Ni21Zr8 and Ni7Zr2, show a low charge capacity for hydrogen [17]. In this case the partial substitution of Ni by V, Co, Mn, that are smaller atoms and present a higher electronegativity dierence with hydrogen with respect to Ni, involves a contraction of the crystallographic lattice of the alloy with an increase of the hydrogen diusion in the bulk and stabilizes the absorbed hydrogen on the surface [18,19]. This substitution has also the consequence of destabilising the bond inside the intermetallic phase, that in fact is quite brittle and is often used in powder form mixed with Ni or Cu [20]. In order to obtain ecient electrodes with good charge±discharge properties, it has been suggested to mix a Laves intermetallic phase with a Ni±Zr compound [17]. Present results show that both phases can be activated in the same way, using HF solution, and that they also present interesting surface properties for electrocatalytic applications. 4. Conclusions The study of the surface properties of Ni±Zr compounds, as electrocatalytic active materials for the hydrogen evolution reaction, shows that the eciency of the as-prepared alloys is reduced by the presence of a Zr oxide layer, while, after activation by HF etching, a consistent increase of the exchange current density is
obtained. The etching produces the dissolution of Zr oxide and an enrichment in Ni at the very ®rst layers of the surface. The trend of the electrocatalytic activity of the dierent intermetallic compounds evidences that there is an increase in the activity increasing the Ni content in the phase, while the crystallographic structure or the presence of a duplex microstructure does not in¯uence signi®catively the eciency of the electrodes. The Laves phase can be activated in the same way and, after treatment, it appears slightly more active than the binary intermetallic compound, as a consequence of its crystallographic structure. A mixture of a Laves intermetallic phase with a Ni±Zr compound, activated by chemical etching, is then a promising material in order to obtain quite ecient electrodes, with good charge±discharge and electrocatalytic properties. Acknowledgements The present work was performed for the National Research Program ``Leghe e composti intermetallici'' ®nancially supported by the Italian ``Ministero dell0 UniversitaÁ della Ricerca Scienti®ca e Tecnologica'' References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Trasatti S. J Electroanal Chem 1972;39:173. JaksÆ ic MM. Mat Chem and Phys 1989;22:1. JaksÏ ic MM. J Mol Cat 1986;38:161. ZuÈttel A, Meli F, Chartouni D, Schlapbach L, Lichtenberg F, Friedrich B. J Alloys and Comp 1996;239:175. Kim DM, Lee SM, Jung JH, Jang KJ, Lee J. J Electrochem Soc 1998;145:93. Chen J, Dou SX, Liu HK. J Alloys and Comp 1997;256:40. Baricco M, Minichilli F, Angelini E, Spriano S, Rosalbino F, Spinelli P, Antonione C. Mat Sci Forum 1997;235±238:911. Jung JH, Lee KY, Lee JY. J Alloys and Comp 1995;226:166. Villars PV, Calvert LD. Pearson's handbook of crystallographic data for intermetallic phases. Ohio: American Society for Metals, Metals Park, 1985. Nash C, Jayanth CS. Bull Alloy Phase Diag 1984;5:144. Hillebrecht FU, Fuggle JC, Bennett PA, Zolnierek Z, Freiburg Ch. Phys Rev 1983;B27:2179. Espinos JP, Fernandez A, Gonzalez-Elipse AR, Munuera G. Surface Sci 1991;251/252:1012. Marcus P, Hinnen C. Surface Sci 1997;392:134. James WJ, Custead WJ, Straumanis ME. J Phys Chem 1960;64:286. Kreysa G, Hakansson B. J Electroanal Chem 1986;201:61. Gao X, Song D, Zhang Y, Zhou Z, Zhang W, Wang M, Shen P. J Alloys and Comp 1995;229:268. Joubert J-M, Latroche M, Percheron-Guegan A, Bouet J. J Alloys and Comp 1996;240:219. Nakano H, Wakao S. J Alloys and Comp 1995;231:587. Iwakura C, Kasuga H, Kim I, Inoue H, Matsuoka M. Electrochimica Acta 1996;41:2691. ZuÈttel A, Meli F, Schlapbach L. J Alloys and Comp 1994;206:31.
Surface and electrochemical characterization of Ni±Zr intermetallic compounds S. Sprianoa, F. Rosalbinoa, M. Bariccob, P.V. Morrab, E. Angelinia, C. Antonionea,*, J.-M. Sirec, P. Marcusc a Dipartimento di SMIC-Politecnico di Torino, C.so Duca degli Abruzzi, 24, 10129 Torino, Italy Dipartimento di Chimica IFM and INFM, UniversitaÁ di Torino, Via Giuria, 9, 10125 Torino, Italy c Laboratoire de Physico-Chimie des Surfaces, CNRS, ENSCP, Rue Curie, 11, 75005 Paris France
b
Received 6 April 1999; accepted 18 October 1999
Abstract In this paper the Ni±Zr system has been considered with the aim of investigating the role of composition and structure of an early±late transition metals system on the electrocatalytic activity, for the hydrogen evolution reaction. As a matter of fact, pure Ni and Zr show low activity, while their intermetallic compounds generate a higher catalytic eciency. Five alloys, with increasing Ni content starting from Ni33Zr67 up to Ni75Zr25, have been prepared and characterized. The alloy of composition (Ni0.55Mn0.30 V0.10Co0.05)2.1Zr has also been considered, in order to investigate the catalytic eciency related to a Laves structure. The thickness and composition of the surface oxides have been investigated and their eect on reducing the catalytic eciency of the as-prepared alloys has been discussed. The activity of the samples submitted to a surface activation treatment with hydro¯uoric acid, that removes the oxide layer and allows to evidence the properties of the compounds, has been observed to increase signi®cantly. The trend of the electrocatalytic eciency with the composition of the alloys is discussed considering a synergetic eect between Ni and Zr. The Laves phase appears slightly more active than the binary intermetallic compound. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics; Miscellaneous; B. Electrical resistance and other electrical properties; G. Catalysis
1. Introduction The electrocatalytic activity of an electrode with respect to the hydrogen evolution reaction is an important parameter both for its relevance to hydrogen evolution applications, as in fuel cells or electrodeposition, and for the theoretical study of adsorption/desorption mechanisms and kinetics of hydrogen on metals and alloys. Early-late transition metal alloys exhibit good electrocatalytic properties for the hydrogen evolution reaction and constitute an interesting research area in relation to the development of alternative materials to traditional electrodes. Ni and Zr pure elements show lower activity with respect to the noble metals and are on the opposite sides of the ``volcano curve'' [1], representing the electrocatalytic activity versus the metal±hydrogen * Corresponding author. Tel.: +39-11-5644608; fax: +39-115644699. E-mail address: [email protected] (S. Spriano).
bond energy. Their alloys, on the contrary, have an electronic structure which may generate higher catalytic activity [2]. This eect can be explained on the basis of the Brewer intermetallic bonding model, considering that when metals having empty or half-®lled vacant d-orbitals are alloyed with metals having internally paired d-electrons, not available for bonding in the pure metal, there arises an unusually strong intermetallic interaction [3]. Intermetallic compounds, belonging to the Ni2Zr type, with partial substitution of Ni by Mn, V or Co and of Zr by Ti, have also been recently proposed as possible hydrogen storage alloys for Ni-metal hydride batteries, in substitution of conventional (Mm)Ni5based alloys. To obtain a slight increase in the activity it has been also suggested to move the composition ratio from stoichiometric to (V,Ni)2+aZr [4,5]. These Zr based phases have a cubic Laves structure (C15) and show a high reversible capacity, a long electrochemical charge±discharge cycle life and high corrosion resistance [6]. On the other hand, they present a lower rate capability
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00105-3
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with respect to the conventional rare-earth based alloys, that is related to the surface reaction kinetics of hydrogenation or dehydrogenation. These Zr-based electrodes need a surface activation with chemical etching or charge±discharge cycling because of the low electrocatalytic activity of the asprepared material. Chemical etching with HF, Nh4F or KOH is fundamental for Ni±Zr based electrodes in order to dissolve the insulator oxide layer on the surface. This treatment may also promote the formation of nanocrystals of Ni dispersed at the surface of the electrode, due to selective dissolution of Zr [7]. As a matter of fact, the presence of chemically active sites together with an enhanced super®cial roughness improve the catalytic eciency of the alloys. Another method, that has been suggested as activation, is the annealing of the alloy at 200±300 C, in the presence of a low pressure of oxygen, in order to transform the monoclinic Zr oxide into the tetragonal one, which is more permeable [8]. The aim of this paper is the study of the role of composition on the electrocatalytic activity of binary Ni±Zr alloys, with composition ranging from Ni33Zr67 up to Ni75Zr25, corresponding to single intermetallic compounds or eutectic mixtures. The alloy of composition (Ni0.55Mn0.30V0.10Co0.05)2.1Zr has also been considered in order to investigate the catalytic eciency due to the Laves structure. A detrimental in¯uence of the presence of surface oxides on the catalytic eciency of the asprepared alloys has been evidenced. So the electrochemical properties of the alloys submitted to a surface activation treatment, by soft etching with HF solution, have been measured in order to evidence the real activity of the intermetallic compounds. 2. Experimental procedure The following alloys have been prepared by arc melting of pure elements (purity of Zr=99.9% and of Ni,Mn,Co,V=99.99%), under Ar atmosphere: Ni33Zr67, Ni36Zr64, Ni50Zr50, Ni64Zr36, Ni75Zr25, (Ni0.55Mn0.30V0.10 Co0.05)2.1Zr. Each ingot has been remelted several times in order to homogenise the composition and, in some cases, annealed for at least 2 h close to the melting temperature, in order to obtain the equilibrium structure. Pure elements, in form of rod or plate, have been used for comparison. Each ingot has been mechanically polished before characterization. Structural characterisation has been performed by powder x-ray diraction (XRD), using cobalt Ka incident radiation (l 0:17902 nm). Surface characterization has been carried out by a scanning electron microscope (SEM) and energy dispersion spectroscopy (EDS) using a Leica Stereoscan 420 microscope. The surface of the electrodes was analysed also by x-ray photoelectron spectroscopy (XPS) using a VG ESCALAB Mk2 with
Al K( (1486.6 eV) as an x-ray source. The pass energy of the hemispherical analyser was 50 eV for survey scans and 20 eV for the narrow energy regions of Ni 2p3/2, Zr 3d5/2,3/2, O1s and C1s. Electrodes of about 1.5 cm2 have been prepared by embedding the samples in acrylic resin. Electrochemical characterization has been achieved by means of cathodic polarization curves traced in oxygen free 1 M KOH at 25 C, in the potential range between the H2 reversible potential (ÿ1068 mV vs SCE) and ÿ2000 mV vs SCE . The experimental i±E data, corrected for the ohmic drop, have been ®tted by a statistical routine which provides the values of the exchange current density (i0) that can be reasonably taken as a measure of the catalytic eciency [1]. Cyclic voltammograms have also been recorded in the potential range ÿ1500 +500 mV vs SCE at a scan rate of 10 mV/s for 10 cycles, obtaining good results reproducibility after 5 or 6 cycles. Surface activation treatment has been performed by immersion in 0.01 M HF for 50 min at room temperature. After the chemical etching the samples have been washed in distilled water and dried in air. Electrochemical activation has been performed by cathodic polarization at ÿ2000 mV vs SCE for 10 min. 3. Results and discussion 3.1. As prepared alloys The x-ray diraction patterns of the as-prepared alloys are shown in Fig. 1. Diraction peaks of tetragonal ZrO2 are barely observed in the low angular range. The diraction pattern of a single intermetallic compound (NiZr2, NiZr or Ni3Zr) [9] has been found respectively for Ni33Zr67, Ni50Zr50 and Ni75Zr25 samples, in agreement with the phase diagram. In some
Fig. 1. X-ray diraction patterns of as-prepared (a) Ni75Zr25, (b) Ni64Zr36, (c) Ni50Zr50, (d) Ni36Zr64, (e) Ni33Zr67, (f) Laves phase. Cobalt K incident radiation (l 0:17902 mm) used.
S. Spriano et al. / Intermetallics 8 (2000) 299±304
cases the preferred orientation of surface crystals, due to the quenching in the arc furnace, changes the relative intensity of some diraction peaks. For the Ni36Zr64 composition an eutectic mixture of NiZr and NiZr2 has been found. In the corresponding x-ray diraction pattern, the diraction peaks of NiZr2 are more visible, owing to the higher quantity of this compound in the mixture, in agreement with the lever rule. The x-ray diraction pattern of the Ni64Zr36 composition is rather complicated. It can be indexed considering a mixture of Ni21Zr8 and Ni10Zr7 intermetallic compounds [9], according to the phase diagram, even if some diraction peaks belong to a cubic metastable Ni2Zr phase, which has been reported as oxygen stabilized [10]. The chemical nature of the surface of the dierent alloys has been investigated in order to evaluate the in¯uence of surface oxide layers on the electrocatalytic
301
properties. In the case of the as-prepared Ni50Zr50 alloy the XPS spectra (Fig. 2 ) show that nickel is essentially in the metallic state (Eb 852:8 eV), whereas zirconium is essentially in the oxide state. In fact the binding energy of Zr 3d5/2 is 182.8 eV, which is assigned to Zr4+ in zirconium oxide. In the O1s region, the main signal is observed at 530.8 eV, corresponding to O2ÿ in zirconium oxide, and a shoulder is visible at higher binding energy (surface hydroxyls and/or adsorbed H2O). The fact that the signal from metallic nickel located under the oxide layer is detected, indicates that the oxide layer is very thin (<50AÊ). Surface contamination by carboncontaining species is observed in the C1s region. The measured binding energy of the carbon signal (285.8 eV) indicates that there is no charging eect during the surface analysis, which is consistent with the fact that the surface oxide layer is very thin. There is no detectable
Fig. 2. XPS spectra of as-prepared (a) Ni33Zr67, (b) Ni50Zr50, (c) Ni64Zr36.
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satellite of the Ni 2p3/2 core level, which is usually found at 6 eV above the main peak for pure Ni, indicating a dierent electronic structure of Ni, in the NiZr compound, as compared to pure nickel. In fact the intensity and the position of this satellite are sensitive to the electronic structure of the alloy, and the observed result may indicate a reduced density of states at the Fermi level [11±13]. Comparing these results with those obtained on the other binary alloys it can be observed that for all compositions there is no detectable nickel oxide in the surface layer and that the thickness of the zirconium oxide layer decreases with increasing Ni content in the alloy. In fact the intensity of the nickel signal in Ni50Zr50 is about 2 times higher than that measured on the Ni33Zr67 alloy. This is probably due both to the higher Ni content in this alloy and to the fact that the oxide layer is thinner, as revealed by the presence of a signal from metallic zirconium at low binding energy (Zr 3d5/2 at 178.4 eV). In Fig. 3 the cathodic polarization curve of the Ni50Zr50 alloy is shown, as an example of the electrocatalytic behaviour of as-prepared electrodes. The i0 value, obtained by linear ®tting, is 4.55 10ÿ6 A/cm2. The electrocatalytic activity of all the as-prepared alloys is very low and increases slightly with the increase of Ni content (Fig. 4 ), as a consequence of the increase of the amount of the active element in the alloy and of the thinning of the surface oxide layer, as previously shown. Cyclic voltammetry performed on the as-prepared electrodes reveals the presence of Ni oxyhydroxide at the surface of the alloys with Ni content higher than 50% at., while the voltammetry of the Zr-rich alloys does not show any signal of redox events. As a conclusion, the microstructural, chemical and electrochemical analysis of the as-prepared alloys shows that the behaviour of these electrodes is aected by the presence of a Zr oxide layer and, consequently, in order to remove it and obtain information on the real activity
Fig. 4. i0 values of as-prepared and chemically activated Ni±Zr alloys as a function of composition.
of the intermetallic compounds, a surface pre-treatment has been performed. 3.2. Pre-treated alloys Pre-treatments have been performed by chemical etching with HF 0.01 M and by electrochemical activation at 2000 mV vs SCE, in order to chemically dissolve or to electrochemically reduce the oxide layer. HF dissolves ZrO2 and produces stable anion complexes like [ZrF6]2ÿ, [ZrF7]3ÿ, [ZrF8]4ÿ according to the following reaction: ZrO2 6HF () H2 ZrF6 2H2 O This diluted acid solution has been chosen because it causes only a slight variation in sample colour, from metallic silver to grey, as a consequence of the dissolution of zirconium oxide layer, while, with stronger solution, hydrogen embrittlement occurs with signi®cative changing in composition and surface morphology [17,14]. XRD patterns of all the alloys remain unchanged after soft etching and SEM pictures and EDS analysis of pre-treated samples show no change in surface morphology and composition with respect to asprepared alloys. It can be concluded that the etching reaction is limited to the very ®rst layers of the surface. Cyclic voltammograms show, for all the pre-treated samples, the reversible peak at +300 mV vs SCE due to the reaction [15] Ni OH2 () NiOOH H eÿ
Fig. 3. Cathodic polarization curves of as-prepared, electrochemically and chemically activated Ni50Zr50 alloy.
The intensity of the Ni oxyhydroxide peak progressively increases with the increasing of the Ni content in the alloy, as it can be observed in Fig. 5. This indicates that there is an enrichment in Ni at the surface, as a consequence of the dissolution of ZrO2 induced by the etching treatment, and that it is more eective for the Ni-rich alloys. The curves also show the increase of the negative current density, due to the hydrogen evolution reaction,
S. Spriano et al. / Intermetallics 8 (2000) 299±304
Fig. 5. Cyclic voltammograms recorded on chemically activated Ni± Zr alloys.
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appears to be the main factor in determining the electrocatalytic activity of the Ni±Zr alloys, as already evidenced in [3]. Although the alloys with higher Ni content show i0 values very closed to each other, the current density, j, obtained from the cathodic polarization curves at the overpotential ÿ250 mV progressively increases with the Ni content in the alloy, giving a higher electrocatalytic eciency for the Ni75Zr25 intermetallic compound, as evidenced in Table 2. No eect of the crystallographic structure of the intermetallic compounds can be evidenced from the present results. Analogously, the presence of a duplex microstructure in eutectic alloys does not signi®cantly in¯uence the electrocatalytic activity. 3.3. Laves phase
with the increasing of the Ni content, as evidenced in Table 1, which reports the cathodic current density values obtained at the potential E 1500 mV vs SCE. The increase of the hydrogen evolution reaction kinetic can be attributed to the presence of Ni at the surface of the samples, as a consequence of the ZrO2 dissolution induced by the chemical etching, the concentration of which increases with the increase of the Ni content in the alloy. Both the electrochemical and chemical pre-treatments in¯uence the electrocatalytic behaviour of the Ni±Zr electrodes: the exchange current density values increase, in the case of the Ni±Zr compound, of one order of magnitude by the cathodic activation and of two orders by the chemical one, as it can be observed from Fig. 3. The diluted HF etching has been used for the other alloys because it produces a stronger activation than the electrochemical polarisation without any evident hydrogen embrittlement, while samples treated with stronger etchings become so brittle that they are dicult to handle. In Fig. 4 the i0 values of all the pre-treated alloys are reported, as a function of composition. These values are related to the real structure and composition of the alloy and, consequently, they can be considered to discuss the electrocatalytic activity of the Ni±Zr intermetallic compounds. Comparing the alloys with parent elements it can be observed that for the intermetallic compounds a synergic eect between Ni and Zr occurs, leading to exchange current density values higher than for pure Ni. The increase of the activity with increasing Ni content in the alloy is evident. So, the composition of the compounds
The (Ni0.55Mn0.30V0.10Co0.05)2.1Zr alloy has been considered in order to evaluate the eect of crystallographic structure on the activity of the cubic Laves phase with respect to that of the binary Ni64Zr36 alloy. The alloy contains a single phase with the same structure as the cubic Laves phase (Ni0.5Mn0.5)2Zr (C15), as shown in Fig. 1. The lattice parameter of the cubic phase is 7.05 AÊ, a value slightly lower than that of (Ni0.5Mn0.5)2Zr (7.07AÊ), owing to the presence of additional elements in the alloy. Analogously to what has been previously reported about binary pre-treated alloys, the microstructure, surface morphology and composition of the alloy corresponding to the Laves phase do not change signi®catively after the surface treatment with HF, as revealed by XRD and SEM/EDS analysis. This is in agreement with what reported in literature, for the activation of the Laves phases by chemical etching, that is fundamental for the application of Zr based alloys for Ni-metal hydride batteries. In fact the etching reaction is limited to the very ®rst layers of the surface and involves a surface enrichment of Ni, in the metallic state, due to the dissolution of V, Mn and Zr oxides [16]. Similary to the binary alloys, the i0 value of the Laves phase, obtained by linear ®tting of cathodic polarization curves, is rather low (2.74 10ÿ6 A/cm2) for the as-prepared electrode. After chemical activation with HF a surface layer rich in Ni is produced increasing the electrocatalytic activity of the electrode, as previously noted for binary Ni±Zr compounds. The cathodic polarization
Table 1 Cathodic current density values evaluated from the cyclic voltammetries recorded on Ni±Zr alloys pre-treated in 0.01 MHF solution
Table 2 Current density values evaluated from the cathodic polarization curves recorded on Ni±Zr alloys pre-treated in 0.01 MHF solution
Alloy
j at E=ÿ1500 mV (mA/cm2)
Alloy
j at =ÿ250 mV (mA/cm2)
Ni33Zr67 Ni50Zr50 Ni75Zr25
ÿ0.81 ÿ1.83 ÿ4.53
Ni50Zr50 Ni64Zr36 Ni75Zr25
ÿ15.73 ÿ17.93 ÿ18.42
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Fig. 6. Cathodic polarization curves recorded on chemically activated Ni64Zr36 alloy and Laves phase.
curves performed on Laves phase and Ni64Zr36 alloy, pre-treated in HF solution, show that the former exhibits a current density higher than the latter, as it can be observed in Fig. 6, giving a i0 value of 1.16 10ÿ3 A/cm2. This is interesting considering that, as previously noted, the Zr-based Laves phases have also good charge and discharge property. As a matter of fact, the binary ZrV2, ZrCr2, ZrMn2 Laves compounds can be highly hydrogenated, but give too much stable hydrides, while the binary Ni±Zr intermetallic compounds that originate less stable hydrides, as Ni21Zr8 and Ni7Zr2, show a low charge capacity for hydrogen [17]. In this case the partial substitution of Ni by V, Co, Mn, that are smaller atoms and present a higher electronegativity dierence with hydrogen with respect to Ni, involves a contraction of the crystallographic lattice of the alloy with an increase of the hydrogen diusion in the bulk and stabilizes the absorbed hydrogen on the surface [18,19]. This substitution has also the consequence of destabilising the bond inside the intermetallic phase, that in fact is quite brittle and is often used in powder form mixed with Ni or Cu [20]. In order to obtain ecient electrodes with good charge±discharge properties, it has been suggested to mix a Laves intermetallic phase with a Ni±Zr compound [17]. Present results show that both phases can be activated in the same way, using HF solution, and that they also present interesting surface properties for electrocatalytic applications. 4. Conclusions The study of the surface properties of Ni±Zr compounds, as electrocatalytic active materials for the hydrogen evolution reaction, shows that the eciency of the as-prepared alloys is reduced by the presence of a Zr oxide layer, while, after activation by HF etching, a consistent increase of the exchange current density is
obtained. The etching produces the dissolution of Zr oxide and an enrichment in Ni at the very ®rst layers of the surface. The trend of the electrocatalytic activity of the dierent intermetallic compounds evidences that there is an increase in the activity increasing the Ni content in the phase, while the crystallographic structure or the presence of a duplex microstructure does not in¯uence signi®catively the eciency of the electrodes. The Laves phase can be activated in the same way and, after treatment, it appears slightly more active than the binary intermetallic compound, as a consequence of its crystallographic structure. A mixture of a Laves intermetallic phase with a Ni±Zr compound, activated by chemical etching, is then a promising material in order to obtain quite ecient electrodes, with good charge±discharge and electrocatalytic properties. Acknowledgements The present work was performed for the National Research Program ``Leghe e composti intermetallici'' ®nancially supported by the Italian ``Ministero dell0 UniversitaÁ della Ricerca Scienti®ca e Tecnologica'' References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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