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Studies on the influence of palladium coatings on the electrochemical and structural properties of a metal hydride alloy
Surface & Coatings Technology 197 (2005) 215 – 222 www.elsevier.com/locate/surfcoat
Studies on the influence of palladium coatings on the electrochemical and structural properties of a metal hydride alloy Renato Canha Ambrosio1, Edson A. Ticianelli* Instituto de Quı´mica de Sa˜o Carlos-USP, Av. do Trabalhador Sa˜ocarlense, 400, Caixa Postal 780, CEP 13560-970, Sa˜o Carlos, SP, Brazil Received 25 March 2004; accepted in revised form 14 July 2004 Available online 11 September 2004
Abstract Electroless palladium deposition was made on the surface of LaNi4.7Sn0.3 metal hydride particles with the aim of studying the influence on the charge/discharge performance of this alloy. The palladium layer was produced by chemical replacement of a copper film deposited on the alloy and its structure was studied by X-ray absorption spectroscopy. The residual copper layer remaining on the alloy surface after palladium deposition was mainly composed of metallic nanometric particles. Ex situ X-ray absorption near-edge structure (XANES) at the Ni K edge indicated that the Pd/Cu layer may protect the Ni atoms against corrosion during the charge/discharge cycles. XANES also indicates that the palladium layer presents the same unit cell characteristics of the bulk Pd in a foil. The electrode prepared with the coated alloy presented lower charge/discharge overpotentials as well as lower charge transfer resistance as verified by electrochemical impedance spectroscopy. D 2004 Elsevier B.V. All rights reserved. Keywords: Palladium deposition; Electroless plating; XAS; Metal hydride; Discharge overpotential
1. Introduction Metal hydride electrodes have received increasing attention because of their widespread application as anode in alkaline rechargeable batteries. A metal hydride electrode consists of an alloy with high hydrogen storage capacity. In the charge cycle, electrochemically generated hydrogen atoms are stored in the bulk alloy, while in the discharge cycle, the stored hydrogen is oxidized on the alloy surface. One of the problems associated with the use of metal hydride electrodes is the performance degradation upon multicycling, which is related to the oxidation of the alloy’s elements. Thus, the microencapsulation with several metals
* Corresponding author. Tel.: +55 16 3373 9945; fax: +55 16 3373 9952. E-mail address: [email protected] (E.A. Ticianelli). 1 Present Address: Universidade Federal do Rio Grande do Norte, Departamento de Quı´mica, Av. Salgado Filho, s/n, CEP 59078-900, Natal, RN, Brazil. 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.07.087
have been studied as a way of protecting the alloy particles against corrosion [1–8]. Electroless palladium coatings are one of the methods tested for improving the charge/discharged characteristics of the metal hydride electrodes. Visintin et al. [8] and Barsellini et al. [9] studied the performance characteristics of Pd/coated mischmetal-based alloy electrodes. The Pd coating was found to be very effective for increasing both the discharge capacity and the rate capability, and for decreasing the number of galvanostatic cycles required for activating the alloy. The improved performance of the Pdcoated alloy over that of an uncoated alloy electrode was attributed to the catalytic effect of Pd on the charge-transfer step of the hydriding–dehydriding reactions occurring at the electrode surface. Geng [10] studied mischmetal-based alloy powders modified with palladium and nickel–palladium (Ni–Pd) coatings. The utilization efficiency of the discharge capacity of the alloy electrodes coated with Pd and 10 wt.% Ni–Pd increased. The high rate dischargeability was also significantly improved by coating with Pd and 10 wt.% Ni–Pd [10].
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The catalytic effect of Pd on hydrogen absorption in mechanically alloyed Mg2Ni, LaNi5 and FeTi was studied by Zaluski et al. [11]. The palladium treatment gave powders able to absorb hydrogen at room temperature with no need for any activation cycle. Also, hydrogen absorption characteristics for the nanocrystalline materials were enhanced, increasing the absorption rates, even at low temperatures. Metal hydride research using nanostructured additives (i.e. copper and palladium nanotubes) showed more negative electrode potentials during the discharge, compared to that of conventional systems resulting in an increase on the power density of the metal hydride battery [12]. Park et al. [13] performed in situ Pd deposition on Mg2Ni electrodes during the charge cycles to improve the electrode properties. The Pd ions were deposited continuously during the charge step with no additional treatment. The ionic conductivity of the modified electrolyte was similar to that of the original electrolyte, and the deposited Pd layer did not affect the hydrogen diffusion. The formation of a passive Mg(OH)2 layer was effectively retarded, and this enhanced the cyclic stability and rate capability of the nanocrystalline Mg2Ni electrode in the Pd-added electrolyte. As mentioned above, several investigations have shown that surface coatings improve the performance of metal hydride alloy electrodes. However, very little characterization work, other than scanning electron microscopy (SEM), has been done on the modified alloy powders. To obtain more information about the deposition process and the alloy protection mechanism, it is essential to elucidate the local structure of the deposited layer on a nanometric scale. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) are particularly useful techniques for providing such structural information in dispersed materials [7]. Despite of the beneficial effects arising from the palladium deposition on the metal hydride surface, up to now, no work has been published on the characterization of the structural properties of the modified material. This paper reports the results of electrochemical, electronic and structural studies of Pd coatings on a hydrogen storage alloy. The palladium deposition was performed by the replacement of a Cu-deposited layer on the surface of LaNi4.7Sn0.3 metal hydride alloy particles. The residual copper layer and the Ni atoms in the base alloy were characterized in terms of electronic structure using XANES. XANES measurements were also performed on the Pd K edge in order to characterize the properties of the palladium-deposited layer under in situ and ex situ conditions. Potentiodynamic polarization measurements and electrochemical impedance spectroscopy were used to characterize the charge storage capacity and the kinetics of the hydriding–dehydriding process on coated and uncoated alloy electrodes. Also, the charge storage capacity of the electrodes was monitored as a function of the charge/ discharge cycle number.
2. Experimental The LaNi4.7Sn0.3 base alloy was prepared from high purity metals by an arc melting technique under inert gas atmosphere at reduced pressure. Ingots were re-melted four times and then annealed for 72 h in vacuum at 950 8C to ensure good homogeneity. Energy dispersive X-ray analyses made at several spots have confirmed within the experimental error the nominal composition of the alloy. Alloy powders with a mean particle size less than 30 Am were obtained by crushing the ingots using a mechanical method. X-ray diffraction (XRD) patterns confirmed the homogeneity and the hexagonal CaCu5-type structure of the base alloy. Prior to the palladium deposition, the LaNi4.7Sn0.3 alloy powder was immersed in a beaker containing the activating solution (acidified SnCl2 and after acidified PdCl2) for 5 min. During the activation, Pd seeds are formed on the surface of the alloy. After the activation, the alloy powder was transferred to a separate beaker containing 50 ml of the copper deposition bath (10 g/l CuSO4, 30 g/l EDTA, pH=12–12.2 adjusted with 6 M KOH). To this bath, 2 ml of formaldehyde was added. The beaker was then kept for 1 h under stirring at 70 8C. After Cu plating, the solid was washed in distilled water and dried in a dissecator using silica gel at room temperature (~25 8C). The copper concentration on the material was obtained by atomic absorption technique and found to be 10% (w/w). The dry covered alloy powder was then placed in a 0.005 mol l 1 PdCl2 solution for 20–30 min. Since Pd is more noble than Cu, Cu is oxidized and goes into solution while Pd replaces Cu. From now on, this material is designated as LaNi4.7Sn0.3/Cu–Pd. Electrodes were prepared by pressing a mixture comprised of 0.050 g of the active powder, 0.050 g of Teflonized carbon black (Vulcan XC-72, with 33 weight percent polytetrafluoroethylene, PTFE) binder, on both sides of a nickel screen with a geometric area of 2 cm2. Electrochemical measurements were carried out in a three-electrode cell in 6 mol l 1 KOH, with a Pt mesh counter electrode and a Hg/HgO–KOH 6 mol l 1 reference electrode. Cycle-life tests were conducted by charging the electrode with a cathodic current of 10 mA (200 mA/g of the active material) and discharging with an anodic current of the same magnitude to a cutoff cell potential of 700 mV. Kinetic impedance measurements were made when the electrode charge storage capacity had reached the maximum, with the electrodes maintained at open-circuit potential, for several states of charge and temperatures. The electrochemical impedance spectra (EIS) were recorded in the frequency range of 10 kHz to 1 mHz, with an ac amplitude of 5 mV. X-ray absorption spectroscopic (XAS) measurements were conducted at the XAS beam line at the LNLS-National Synchrotron Light Source, Brazil. The radiation was monochromatized using Si(111) single crystal for the Ni and Cu K edges measurements, while a Si(220) single
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crystal was used for the Pd K edge. The data acquisition set up comprised three ionization chambers (incident I o, transmittance I t and reference I r detectors). The reference channel was primarily used for internal calibration of the edge position, using pure metal foils of Ni, Cu and Pd. Analyses were carried out in the transmittance mode at the Ni, Cu and Pd K edges under ex situ and in situ conditions. In situ measurements were performed using a home made spectroelectrochemical cell [14] and the working electrodes consisted of pellets formed with the dispersed catalysts agglutinated with the same configuration as for the electrochemical measurements. XANES spectra of metallic Cu, Pd, Cu2O and CuO were collected for comparison with those for the samples. Data analysis was conducted using the WinXas software [15]. XANES spectra were first corrected for the background absorption by fitting the pre-edge data (from 60 to 20 eV below the edge) to a linear formula, followed by extrapolation and subtraction from the data over the energy range of interest. Finally, the spectra were normalized taking as reference a second polynomial function obtained by fitting the data throughout the EXAFS oscillations. Next, the spectra were calibrated for the edge position, at the inflection point of the edge jump of the data from the reference channel, using the second derivative method. In all the XANES spectra presented here, the energy axis are related to energy of the metal foil K edge.
3. Results and discussion The morphology of the composite alloy powder (LaNi4.7Sn0.3/Cu–Pd) is shown in Fig. 1. The powder presents a composite morphology, in which smaller particles clusters with size of the order of 1 Am are attached to the
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Fig. 2. EDS line-mapping results for LaNi4.7Sn0.3/Cu–Pd performed at the line indicated in Fig. 1. The coordinates indicate the relative concentration and the abscissa the distance along the line. The elements and the emission line are indicated in the pictures. The top picture represents the topographic profile.
surface of large particles with a size of about 30 Am, although not uniformly. EDS line-mapping was performed in order to identify the distribution of La, Ni, Pd and Cu in the composite material. The results displaying the variation of the signal of these elements at the line shown in the micrograph (Fig. 1) are presented in Fig. 2. It can be seen that the nickel and lanthanum concentration profiles are the
Fig. 1. Electron micrograph of LaNi4.7Sn0.3/Cu–Pd alloy before cycling. The line indicates where the EDS line-mapping was performed.
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same, as expected since these are the elements composing the base alloy. On the other hand, the concentration profiles of copper and palladium follow the particle morphology, indicating that the electrolessly deposited layers cover the base alloy particle, with the excess forming segregated clusters of Cu/Pd (white particles). XANES results at the Ni K edge for LaNi4.7Sn0.3 as well as for LaNi4.7Sn0.3/Cu–Pd are shown in Fig. 3, for uncycled and cycled electrodes. For comparison, the NiO spectrum is also included in Fig. 3. Generally, absorptions at the Ni K edge are due to excitations of 1s electrons to electronic states above the Fermi level [16]. Because of the selection rule, only transitions into empty p states are dipole allowed. For 3d transition metal such as Ni and Co, theoretical calculations, performed for the AB5-type metal hydride structure, show that there is mixing of p and d states and as a result, transitions into the empty p-like part of these mixed p–d states can occur [16,17]. This is responsible for the appearance of the pre-edge peak at about 3.0 eV for Ni (Figs. 3 and 4). At energies above the 1sY3d excitations, the principal rising edge occurs, which can include transitions to higher energy bound states such as 1sY4d, 1sYnp [18]. Multiple scattering features and other resonance transition features may also occur in this energy region. In Fig. 3, it is observed that both the uncoated and the coated alloys show some oxidation of the nickel atoms after 100 charge/discharge cycles, as seen from the intensities of the edge-hump around 20 eV. This must be one of the causes of performance degradation after multicycling, as previously reported [7,19–21]. Compared to the coated alloy, a decrease on the pre-edge peak is seen for the uncoated material after cycling, and this may indicate that
Fig. 3. XANES spectra in the transmission mode at Ni K edge for uncycled and cycled (100 cycles) LaNi4.7Sn0.3 and LaNi4.7Sn0.3/Cu–Pd alloy electrodes.
Fig. 4. XANES spectra at the Cu K edge for Cu-foil, Cu2O, CuO and copper residue after Pd deposit.
Ni atoms are somewhat less oxidized due to the presence of the Cu/Pd layer. Taking into account that the palladium deposition does not eliminate all the copper from the alloy as seen by the EDS results, this element was also analyzed by XANES. Fig. 4 presents the results at the Cu K edge for reference copper compounds and the uncycled Cu layer at the LaNi4.7Sn0.3/Cu–Pd material. For Cu oxides, an energy shift is observed on going from Cu2O (+1) to CuO (+2). It is widely recognized that a single well-defined peak at 5 to 5 eV is the fingerprint of Cu(I) species. This peak is due to the dipole-allowed 1sY4p electron transition of Cu(I). On the contrary, Cu(II) species exhibit a very weak absorption at about 5 to 0 eV, attributed to the dipole-forbidden 1sY3d electronic transition, a shoulder at about 0–9 eV, and a intense peak at about 9–25 eV, both due to 1sY4p transitions [22,23]. All these features are apparent in the reference spectra of Cu2O and CuO. An analysis of the XANES spectrum for the uncycled LaNi4.7Sn0.3/Cu–Pd (Fig. 4) reveals that the copper layer presents all the features of the metallic Cu-foil, although the slightly higher intensity of the absorption of the pre-edge feature indicates the presence of some Cu+. Although, with lower intensity, the shape of the oscillations above 10 eV is characteristic of copper in a face-centered cubic (fcc) packing structure as in the reference foil. Based on these results, it is presumed that the residual copper layer has a nanometric metallic structure with some Cu+ formed by the surface oxidation. Fig. 5 presents the Pd K edge XANES results obtained for ex situ and the in situ charged LaNi4.7Sn0.3/Cu–Pd sample, and for a Pd foil standard. The Pd K near-edge spectrum contains structure corresponding to the l=1 (plike) projected density of final states [24,25] with the Pd
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Fig. 5. XANES spectra at the Pd K edge for Pd-foil, and for the ex situ and in situ charged LaNi4.7Sn0.3/Cu–Pd alloy.
5p band contributing to the observed structure in (A). Peak (B) corresponds to the transition from the l=1 projection of the tails of the 4f orbitals from adjacent Pd atoms. The shape of the absorption at the Pd K edge, particularly feature A, reflects the extent of 4d–4p hybridization [25]. The XANES spectra for the ex situ and in situ measurements present essentially the same features as the metal foil, indicating that the palladium layer presents the same structure of the bulk metal. No edge energy shift is observed, compared to the Pd foil. However, the multiple scattering resonance feature C seems to be shifted to lower energies for the in situ sample. For similar clusters with different interatomic distances, the rule (E r E b)R 2=constant is valid, where E r is the energy of the resonance and E b is the energy of a bound state at threshold [26]. Therefore, the energy position of the multiple-scattering resonance can be related to the bond length. Taking into account the fact that hydriding exerts an indirect influence on the band structure of Pd by increasing the metal lattice constant [27], the shift of the multiple-scattering resonance feature (C) confirms the occurrence of the increase on an Pd–Pd bond length upon hydriding. All materials were characterized in terms of electrochemical and charge/discharge properties. Potentiodynamic polarization curves of the non-coated, copper-coated and palladium-coated materials are shown in Fig. 6. From the rest potential (ca. 0.9 V) to 0 V, the uncoated alloy exhibits a capacitive behavior while the copper-coated material presents two oxidation peaks at ca. 0.34 V (peak A) and 0.07 V (peak B). The electrochemical behavior of copper-coated electrode does not differ significantly from that of the bulk metal [28]. On the other hand, for the Pdcoated material it is observed an increase on the magnitude
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of the capacitive currents and the presence of a third anodic peak (C) at a higher potential, in addition to the two peaks related to the Cu layer. The anodic peaks (A) and (B) shown by the two coated samples must correspond to the formation of Cu(I) and Cu(II) oxide/hydroxide layer, respectively, while peak (C) must correspond to the oxidation of the palladium layer. The higher capacitive currents presented by the palladium containing sample is indicative of a large active surface area. The presence of the copper oxidation peaks on the palladium-containing sample clearly confirm that not all the copper atoms are replaced by palladium during the surface coating. The equilibrium potential measured for an uncycled electrode prepared with the palladium plated alloy was found to be around 0.43 V vs. Hg/HgO, which is a result of the equilibrium potential of metallic palladium on the same media [29]. On the other hand, for a non-coated electrode before any charge/discharge cycle, the open circuit potential was of the order of 0.20 V vs. Hg/ HgO, consistent with the presence of a nickel oxi– hydroxide layer [7]. Fig. 7 shows the charge and discharge potential vs. time profiles recorded for the three electrodes at the 10th cycle. In each case, well-defined charge/discharge plateaus are observed. For the LaNi4.7Sn0.3/Cu–Pd electrode, the discharge potential presents lower values (i.e. more negative) implying in smaller discharge overpotentials or higher specific power for the battery. Consistently, the coated electrode also presented smaller charging overpotentials. These improvements was also observed by other authors and was attributed to the catalytic properties of metallic palladium layer for the hydrogen oxidation reaction [8,9],
Fig. 6. Potentiodynamic polarization behavior of uncoated LaNi4.7Sn0.3 alloy and for the copper-coated (formaldehyde reduced) material and palladium-coated after copper oxidation. Scan rate 1 mV s 1.
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Fig. 7. Charge and discharge profiles for the 10th cycle for the LaNi4.7Sn0.3 and LaNi4.7Sn0.3 Pd-coated alloy electrode. Discharge current=200 mA g 1.
but the possibility of being due to higher surface area of this sample can not be disregarded. Fig. 8 shows electrochemical impedance spectra for the activated LaNi4.7Sn0.3 and LaNi4.7Sn0.3/Cu–Pd alloys, obtained at several temperatures for fully charged electrodes. Fig. 9 shows the corresponding results for the same alloys in the fully charged, 50% discharged and fully discharged states. Firstly, it should be noted that these impedance responses are qualitatively similar to the results presented for other AB5-type metal hydride alloys [19– 21,30]. The results show only one important arc with magnitude and characteristic frequency dependent on the temperature, electrode composition and state of charge.
Previous work has shown that this feature is related to the kinetics of the charge transfer step of the hydriding– dehydriding processes [19–21,29]. According to this, the exchange current density (i o) for the charge transfer step was evaluated from the radius of this arc (R ct), using the Butler-Volmer equation in the limit of low overpotentials. These measurements were conducted at several temperatures in order to obtain the apparent activation energy of the electrode charge/discharge process. This parameter was obtained from the slope of the ln i o vs. 1/T plots for both electrodes (not shown here). Values of i o resulted 96 and 140 mA g 1 for the LaNi4.7Sn0.3 and LaNi4.7Sn0.3/ Cu–Pd metal hydride electrodes at 25 8C and 100% state of charge, while the apparent activation energies were 37 and 47 kJ mol 1, respectively. This is a somewhat surprising result, since it would be expected that the presence of Pd should lead to an increase of reaction kinetics [8,9] or a decrease of the activation energy for the HOR. In this sense, the higher exchange current density observed for the Cu/Pd-coated material must be just a consequence of the higher surface area arising from the nanometric palladium deposits. For the LaNi4.7Sn0.3 alloy electrode, it is seen that there is no significant differences between the impedance features for the fully charged and the 50% discharged electrodes. On the other hand, there is a reduction of the charge transfer resistance for the fully discharged electrode, as observed previously for many other metal hydride systems [19,30]. In the case of the Cu/Pd-coated alloy electrode, the charge transfer resistance is essentially independent of the state of charge. For the fully discharged LaNi4.7Sn0.3/Cu–Pd electrode, an increase of the imaginary component at the lower frequencies is observed. This feature has been assigned to a capacitive behavior of a fully discharged alloy [19]. It should be noted that this phenomenon is only observed for the condition in which the amount of absorbed H is small, that is, at low state of charge.
Fig. 8. Nyquist plots at several temperatures for (a) LaNi4.7Sn0.3 (10th cycle) and (b) LaNi4.7Sn0.3/Cu–Pd (4th cycle) fully charged electrodes.
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Fig. 9. Nyquist plots for (a) LaNi4.7Sn0.3 and (b) LaNi4.7Sn0.3/Cu–Pd electrodes at several states of charge at 298 K.
Fig. 10 presents plots of the discharge capacity normalized by the maximum value as a function of the charge/discharge cycle number plots for the palladiumcoated and uncoated alloy. The main difference between the cycling behavior of the original and the Cu/Pdcovered alloy electrode is the lower normalized capacity decay with cycling for the treated alloy. The discharge capacity for the palladium-coated materials was found to be 237 mA h g 1 and for the as prepared alloy and 278 mA h g 1. The lower values for discharge capacity of the Cu/Pd-coated alloy electrode can be attributed to the lower hydrogen absorption capacity per gram of the palladium clusters relatively for the capacity of the metal
hydride alloy [25,27], and to the fact that Cu does not absorbs hydrogen.
4. Conclusions Palladium deposition was carried out on the surface of a metal hydride alloy, by mean of copper replacement. The film microstructure was characterized by SEM, EDS and XANES techniques. The electrochemical behavior of the prepared electrodes was studied with impedance spectroscopy, potentiodynamic measurements and galvanostatic charge/discharge cycles. Based on these results, it is concluded that the electroless plated copper/palladiun acts as a protective layer for the Ni atoms on the LaNi4.7Sn0.3 active alloy. The palladium-coated electrode also presented lower discharge overpotentials, which is important to improve the battery performance. Microencapsulation by copper/palladium provides complete coverage of the intermetallic alloy powder surface that helps preventing corrosion or oxidation of both the active material (LaNi4.7Sn0.3 alloy) and the catalyst (Ni clusters). Also, the copper/palladium deposits was found to increase the cycle life of the metal hydride electrode. The main impedance features of the metal hydride electrodes were related to the charge transfer step of the hydriding–dehydriding processes. It was found that the catalytic activity of charge/discharge is improved with Cu/ Pd deposition, a factor exclusively related to a higher active area of the coated layer, since the values of the apparent activation energy are higher for the coated sample.
Acknowledgments
Fig. 10. Cycle performance of LaNi4.7Sn0.3 with and without the palladium treatment.
The authors wish to thank FAPESP and the LNLSNational Synchrotron Light Laboratory, Brazil for technical and financial support.
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Studies on the influence of palladium coatings on the electrochemical and structural properties of a metal hydride alloy Renato Canha Ambrosio1, Edson A. Ticianelli* Instituto de Quı´mica de Sa˜o Carlos-USP, Av. do Trabalhador Sa˜ocarlense, 400, Caixa Postal 780, CEP 13560-970, Sa˜o Carlos, SP, Brazil Received 25 March 2004; accepted in revised form 14 July 2004 Available online 11 September 2004
Abstract Electroless palladium deposition was made on the surface of LaNi4.7Sn0.3 metal hydride particles with the aim of studying the influence on the charge/discharge performance of this alloy. The palladium layer was produced by chemical replacement of a copper film deposited on the alloy and its structure was studied by X-ray absorption spectroscopy. The residual copper layer remaining on the alloy surface after palladium deposition was mainly composed of metallic nanometric particles. Ex situ X-ray absorption near-edge structure (XANES) at the Ni K edge indicated that the Pd/Cu layer may protect the Ni atoms against corrosion during the charge/discharge cycles. XANES also indicates that the palladium layer presents the same unit cell characteristics of the bulk Pd in a foil. The electrode prepared with the coated alloy presented lower charge/discharge overpotentials as well as lower charge transfer resistance as verified by electrochemical impedance spectroscopy. D 2004 Elsevier B.V. All rights reserved. Keywords: Palladium deposition; Electroless plating; XAS; Metal hydride; Discharge overpotential
1. Introduction Metal hydride electrodes have received increasing attention because of their widespread application as anode in alkaline rechargeable batteries. A metal hydride electrode consists of an alloy with high hydrogen storage capacity. In the charge cycle, electrochemically generated hydrogen atoms are stored in the bulk alloy, while in the discharge cycle, the stored hydrogen is oxidized on the alloy surface. One of the problems associated with the use of metal hydride electrodes is the performance degradation upon multicycling, which is related to the oxidation of the alloy’s elements. Thus, the microencapsulation with several metals
* Corresponding author. Tel.: +55 16 3373 9945; fax: +55 16 3373 9952. E-mail address: [email protected] (E.A. Ticianelli). 1 Present Address: Universidade Federal do Rio Grande do Norte, Departamento de Quı´mica, Av. Salgado Filho, s/n, CEP 59078-900, Natal, RN, Brazil. 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.07.087
have been studied as a way of protecting the alloy particles against corrosion [1–8]. Electroless palladium coatings are one of the methods tested for improving the charge/discharged characteristics of the metal hydride electrodes. Visintin et al. [8] and Barsellini et al. [9] studied the performance characteristics of Pd/coated mischmetal-based alloy electrodes. The Pd coating was found to be very effective for increasing both the discharge capacity and the rate capability, and for decreasing the number of galvanostatic cycles required for activating the alloy. The improved performance of the Pdcoated alloy over that of an uncoated alloy electrode was attributed to the catalytic effect of Pd on the charge-transfer step of the hydriding–dehydriding reactions occurring at the electrode surface. Geng [10] studied mischmetal-based alloy powders modified with palladium and nickel–palladium (Ni–Pd) coatings. The utilization efficiency of the discharge capacity of the alloy electrodes coated with Pd and 10 wt.% Ni–Pd increased. The high rate dischargeability was also significantly improved by coating with Pd and 10 wt.% Ni–Pd [10].
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The catalytic effect of Pd on hydrogen absorption in mechanically alloyed Mg2Ni, LaNi5 and FeTi was studied by Zaluski et al. [11]. The palladium treatment gave powders able to absorb hydrogen at room temperature with no need for any activation cycle. Also, hydrogen absorption characteristics for the nanocrystalline materials were enhanced, increasing the absorption rates, even at low temperatures. Metal hydride research using nanostructured additives (i.e. copper and palladium nanotubes) showed more negative electrode potentials during the discharge, compared to that of conventional systems resulting in an increase on the power density of the metal hydride battery [12]. Park et al. [13] performed in situ Pd deposition on Mg2Ni electrodes during the charge cycles to improve the electrode properties. The Pd ions were deposited continuously during the charge step with no additional treatment. The ionic conductivity of the modified electrolyte was similar to that of the original electrolyte, and the deposited Pd layer did not affect the hydrogen diffusion. The formation of a passive Mg(OH)2 layer was effectively retarded, and this enhanced the cyclic stability and rate capability of the nanocrystalline Mg2Ni electrode in the Pd-added electrolyte. As mentioned above, several investigations have shown that surface coatings improve the performance of metal hydride alloy electrodes. However, very little characterization work, other than scanning electron microscopy (SEM), has been done on the modified alloy powders. To obtain more information about the deposition process and the alloy protection mechanism, it is essential to elucidate the local structure of the deposited layer on a nanometric scale. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) are particularly useful techniques for providing such structural information in dispersed materials [7]. Despite of the beneficial effects arising from the palladium deposition on the metal hydride surface, up to now, no work has been published on the characterization of the structural properties of the modified material. This paper reports the results of electrochemical, electronic and structural studies of Pd coatings on a hydrogen storage alloy. The palladium deposition was performed by the replacement of a Cu-deposited layer on the surface of LaNi4.7Sn0.3 metal hydride alloy particles. The residual copper layer and the Ni atoms in the base alloy were characterized in terms of electronic structure using XANES. XANES measurements were also performed on the Pd K edge in order to characterize the properties of the palladium-deposited layer under in situ and ex situ conditions. Potentiodynamic polarization measurements and electrochemical impedance spectroscopy were used to characterize the charge storage capacity and the kinetics of the hydriding–dehydriding process on coated and uncoated alloy electrodes. Also, the charge storage capacity of the electrodes was monitored as a function of the charge/ discharge cycle number.
2. Experimental The LaNi4.7Sn0.3 base alloy was prepared from high purity metals by an arc melting technique under inert gas atmosphere at reduced pressure. Ingots were re-melted four times and then annealed for 72 h in vacuum at 950 8C to ensure good homogeneity. Energy dispersive X-ray analyses made at several spots have confirmed within the experimental error the nominal composition of the alloy. Alloy powders with a mean particle size less than 30 Am were obtained by crushing the ingots using a mechanical method. X-ray diffraction (XRD) patterns confirmed the homogeneity and the hexagonal CaCu5-type structure of the base alloy. Prior to the palladium deposition, the LaNi4.7Sn0.3 alloy powder was immersed in a beaker containing the activating solution (acidified SnCl2 and after acidified PdCl2) for 5 min. During the activation, Pd seeds are formed on the surface of the alloy. After the activation, the alloy powder was transferred to a separate beaker containing 50 ml of the copper deposition bath (10 g/l CuSO4, 30 g/l EDTA, pH=12–12.2 adjusted with 6 M KOH). To this bath, 2 ml of formaldehyde was added. The beaker was then kept for 1 h under stirring at 70 8C. After Cu plating, the solid was washed in distilled water and dried in a dissecator using silica gel at room temperature (~25 8C). The copper concentration on the material was obtained by atomic absorption technique and found to be 10% (w/w). The dry covered alloy powder was then placed in a 0.005 mol l 1 PdCl2 solution for 20–30 min. Since Pd is more noble than Cu, Cu is oxidized and goes into solution while Pd replaces Cu. From now on, this material is designated as LaNi4.7Sn0.3/Cu–Pd. Electrodes were prepared by pressing a mixture comprised of 0.050 g of the active powder, 0.050 g of Teflonized carbon black (Vulcan XC-72, with 33 weight percent polytetrafluoroethylene, PTFE) binder, on both sides of a nickel screen with a geometric area of 2 cm2. Electrochemical measurements were carried out in a three-electrode cell in 6 mol l 1 KOH, with a Pt mesh counter electrode and a Hg/HgO–KOH 6 mol l 1 reference electrode. Cycle-life tests were conducted by charging the electrode with a cathodic current of 10 mA (200 mA/g of the active material) and discharging with an anodic current of the same magnitude to a cutoff cell potential of 700 mV. Kinetic impedance measurements were made when the electrode charge storage capacity had reached the maximum, with the electrodes maintained at open-circuit potential, for several states of charge and temperatures. The electrochemical impedance spectra (EIS) were recorded in the frequency range of 10 kHz to 1 mHz, with an ac amplitude of 5 mV. X-ray absorption spectroscopic (XAS) measurements were conducted at the XAS beam line at the LNLS-National Synchrotron Light Source, Brazil. The radiation was monochromatized using Si(111) single crystal for the Ni and Cu K edges measurements, while a Si(220) single
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crystal was used for the Pd K edge. The data acquisition set up comprised three ionization chambers (incident I o, transmittance I t and reference I r detectors). The reference channel was primarily used for internal calibration of the edge position, using pure metal foils of Ni, Cu and Pd. Analyses were carried out in the transmittance mode at the Ni, Cu and Pd K edges under ex situ and in situ conditions. In situ measurements were performed using a home made spectroelectrochemical cell [14] and the working electrodes consisted of pellets formed with the dispersed catalysts agglutinated with the same configuration as for the electrochemical measurements. XANES spectra of metallic Cu, Pd, Cu2O and CuO were collected for comparison with those for the samples. Data analysis was conducted using the WinXas software [15]. XANES spectra were first corrected for the background absorption by fitting the pre-edge data (from 60 to 20 eV below the edge) to a linear formula, followed by extrapolation and subtraction from the data over the energy range of interest. Finally, the spectra were normalized taking as reference a second polynomial function obtained by fitting the data throughout the EXAFS oscillations. Next, the spectra were calibrated for the edge position, at the inflection point of the edge jump of the data from the reference channel, using the second derivative method. In all the XANES spectra presented here, the energy axis are related to energy of the metal foil K edge.
3. Results and discussion The morphology of the composite alloy powder (LaNi4.7Sn0.3/Cu–Pd) is shown in Fig. 1. The powder presents a composite morphology, in which smaller particles clusters with size of the order of 1 Am are attached to the
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Fig. 2. EDS line-mapping results for LaNi4.7Sn0.3/Cu–Pd performed at the line indicated in Fig. 1. The coordinates indicate the relative concentration and the abscissa the distance along the line. The elements and the emission line are indicated in the pictures. The top picture represents the topographic profile.
surface of large particles with a size of about 30 Am, although not uniformly. EDS line-mapping was performed in order to identify the distribution of La, Ni, Pd and Cu in the composite material. The results displaying the variation of the signal of these elements at the line shown in the micrograph (Fig. 1) are presented in Fig. 2. It can be seen that the nickel and lanthanum concentration profiles are the
Fig. 1. Electron micrograph of LaNi4.7Sn0.3/Cu–Pd alloy before cycling. The line indicates where the EDS line-mapping was performed.
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same, as expected since these are the elements composing the base alloy. On the other hand, the concentration profiles of copper and palladium follow the particle morphology, indicating that the electrolessly deposited layers cover the base alloy particle, with the excess forming segregated clusters of Cu/Pd (white particles). XANES results at the Ni K edge for LaNi4.7Sn0.3 as well as for LaNi4.7Sn0.3/Cu–Pd are shown in Fig. 3, for uncycled and cycled electrodes. For comparison, the NiO spectrum is also included in Fig. 3. Generally, absorptions at the Ni K edge are due to excitations of 1s electrons to electronic states above the Fermi level [16]. Because of the selection rule, only transitions into empty p states are dipole allowed. For 3d transition metal such as Ni and Co, theoretical calculations, performed for the AB5-type metal hydride structure, show that there is mixing of p and d states and as a result, transitions into the empty p-like part of these mixed p–d states can occur [16,17]. This is responsible for the appearance of the pre-edge peak at about 3.0 eV for Ni (Figs. 3 and 4). At energies above the 1sY3d excitations, the principal rising edge occurs, which can include transitions to higher energy bound states such as 1sY4d, 1sYnp [18]. Multiple scattering features and other resonance transition features may also occur in this energy region. In Fig. 3, it is observed that both the uncoated and the coated alloys show some oxidation of the nickel atoms after 100 charge/discharge cycles, as seen from the intensities of the edge-hump around 20 eV. This must be one of the causes of performance degradation after multicycling, as previously reported [7,19–21]. Compared to the coated alloy, a decrease on the pre-edge peak is seen for the uncoated material after cycling, and this may indicate that
Fig. 3. XANES spectra in the transmission mode at Ni K edge for uncycled and cycled (100 cycles) LaNi4.7Sn0.3 and LaNi4.7Sn0.3/Cu–Pd alloy electrodes.
Fig. 4. XANES spectra at the Cu K edge for Cu-foil, Cu2O, CuO and copper residue after Pd deposit.
Ni atoms are somewhat less oxidized due to the presence of the Cu/Pd layer. Taking into account that the palladium deposition does not eliminate all the copper from the alloy as seen by the EDS results, this element was also analyzed by XANES. Fig. 4 presents the results at the Cu K edge for reference copper compounds and the uncycled Cu layer at the LaNi4.7Sn0.3/Cu–Pd material. For Cu oxides, an energy shift is observed on going from Cu2O (+1) to CuO (+2). It is widely recognized that a single well-defined peak at 5 to 5 eV is the fingerprint of Cu(I) species. This peak is due to the dipole-allowed 1sY4p electron transition of Cu(I). On the contrary, Cu(II) species exhibit a very weak absorption at about 5 to 0 eV, attributed to the dipole-forbidden 1sY3d electronic transition, a shoulder at about 0–9 eV, and a intense peak at about 9–25 eV, both due to 1sY4p transitions [22,23]. All these features are apparent in the reference spectra of Cu2O and CuO. An analysis of the XANES spectrum for the uncycled LaNi4.7Sn0.3/Cu–Pd (Fig. 4) reveals that the copper layer presents all the features of the metallic Cu-foil, although the slightly higher intensity of the absorption of the pre-edge feature indicates the presence of some Cu+. Although, with lower intensity, the shape of the oscillations above 10 eV is characteristic of copper in a face-centered cubic (fcc) packing structure as in the reference foil. Based on these results, it is presumed that the residual copper layer has a nanometric metallic structure with some Cu+ formed by the surface oxidation. Fig. 5 presents the Pd K edge XANES results obtained for ex situ and the in situ charged LaNi4.7Sn0.3/Cu–Pd sample, and for a Pd foil standard. The Pd K near-edge spectrum contains structure corresponding to the l=1 (plike) projected density of final states [24,25] with the Pd
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Fig. 5. XANES spectra at the Pd K edge for Pd-foil, and for the ex situ and in situ charged LaNi4.7Sn0.3/Cu–Pd alloy.
5p band contributing to the observed structure in (A). Peak (B) corresponds to the transition from the l=1 projection of the tails of the 4f orbitals from adjacent Pd atoms. The shape of the absorption at the Pd K edge, particularly feature A, reflects the extent of 4d–4p hybridization [25]. The XANES spectra for the ex situ and in situ measurements present essentially the same features as the metal foil, indicating that the palladium layer presents the same structure of the bulk metal. No edge energy shift is observed, compared to the Pd foil. However, the multiple scattering resonance feature C seems to be shifted to lower energies for the in situ sample. For similar clusters with different interatomic distances, the rule (E r E b)R 2=constant is valid, where E r is the energy of the resonance and E b is the energy of a bound state at threshold [26]. Therefore, the energy position of the multiple-scattering resonance can be related to the bond length. Taking into account the fact that hydriding exerts an indirect influence on the band structure of Pd by increasing the metal lattice constant [27], the shift of the multiple-scattering resonance feature (C) confirms the occurrence of the increase on an Pd–Pd bond length upon hydriding. All materials were characterized in terms of electrochemical and charge/discharge properties. Potentiodynamic polarization curves of the non-coated, copper-coated and palladium-coated materials are shown in Fig. 6. From the rest potential (ca. 0.9 V) to 0 V, the uncoated alloy exhibits a capacitive behavior while the copper-coated material presents two oxidation peaks at ca. 0.34 V (peak A) and 0.07 V (peak B). The electrochemical behavior of copper-coated electrode does not differ significantly from that of the bulk metal [28]. On the other hand, for the Pdcoated material it is observed an increase on the magnitude
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of the capacitive currents and the presence of a third anodic peak (C) at a higher potential, in addition to the two peaks related to the Cu layer. The anodic peaks (A) and (B) shown by the two coated samples must correspond to the formation of Cu(I) and Cu(II) oxide/hydroxide layer, respectively, while peak (C) must correspond to the oxidation of the palladium layer. The higher capacitive currents presented by the palladium containing sample is indicative of a large active surface area. The presence of the copper oxidation peaks on the palladium-containing sample clearly confirm that not all the copper atoms are replaced by palladium during the surface coating. The equilibrium potential measured for an uncycled electrode prepared with the palladium plated alloy was found to be around 0.43 V vs. Hg/HgO, which is a result of the equilibrium potential of metallic palladium on the same media [29]. On the other hand, for a non-coated electrode before any charge/discharge cycle, the open circuit potential was of the order of 0.20 V vs. Hg/ HgO, consistent with the presence of a nickel oxi– hydroxide layer [7]. Fig. 7 shows the charge and discharge potential vs. time profiles recorded for the three electrodes at the 10th cycle. In each case, well-defined charge/discharge plateaus are observed. For the LaNi4.7Sn0.3/Cu–Pd electrode, the discharge potential presents lower values (i.e. more negative) implying in smaller discharge overpotentials or higher specific power for the battery. Consistently, the coated electrode also presented smaller charging overpotentials. These improvements was also observed by other authors and was attributed to the catalytic properties of metallic palladium layer for the hydrogen oxidation reaction [8,9],
Fig. 6. Potentiodynamic polarization behavior of uncoated LaNi4.7Sn0.3 alloy and for the copper-coated (formaldehyde reduced) material and palladium-coated after copper oxidation. Scan rate 1 mV s 1.
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Fig. 7. Charge and discharge profiles for the 10th cycle for the LaNi4.7Sn0.3 and LaNi4.7Sn0.3 Pd-coated alloy electrode. Discharge current=200 mA g 1.
but the possibility of being due to higher surface area of this sample can not be disregarded. Fig. 8 shows electrochemical impedance spectra for the activated LaNi4.7Sn0.3 and LaNi4.7Sn0.3/Cu–Pd alloys, obtained at several temperatures for fully charged electrodes. Fig. 9 shows the corresponding results for the same alloys in the fully charged, 50% discharged and fully discharged states. Firstly, it should be noted that these impedance responses are qualitatively similar to the results presented for other AB5-type metal hydride alloys [19– 21,30]. The results show only one important arc with magnitude and characteristic frequency dependent on the temperature, electrode composition and state of charge.
Previous work has shown that this feature is related to the kinetics of the charge transfer step of the hydriding– dehydriding processes [19–21,29]. According to this, the exchange current density (i o) for the charge transfer step was evaluated from the radius of this arc (R ct), using the Butler-Volmer equation in the limit of low overpotentials. These measurements were conducted at several temperatures in order to obtain the apparent activation energy of the electrode charge/discharge process. This parameter was obtained from the slope of the ln i o vs. 1/T plots for both electrodes (not shown here). Values of i o resulted 96 and 140 mA g 1 for the LaNi4.7Sn0.3 and LaNi4.7Sn0.3/ Cu–Pd metal hydride electrodes at 25 8C and 100% state of charge, while the apparent activation energies were 37 and 47 kJ mol 1, respectively. This is a somewhat surprising result, since it would be expected that the presence of Pd should lead to an increase of reaction kinetics [8,9] or a decrease of the activation energy for the HOR. In this sense, the higher exchange current density observed for the Cu/Pd-coated material must be just a consequence of the higher surface area arising from the nanometric palladium deposits. For the LaNi4.7Sn0.3 alloy electrode, it is seen that there is no significant differences between the impedance features for the fully charged and the 50% discharged electrodes. On the other hand, there is a reduction of the charge transfer resistance for the fully discharged electrode, as observed previously for many other metal hydride systems [19,30]. In the case of the Cu/Pd-coated alloy electrode, the charge transfer resistance is essentially independent of the state of charge. For the fully discharged LaNi4.7Sn0.3/Cu–Pd electrode, an increase of the imaginary component at the lower frequencies is observed. This feature has been assigned to a capacitive behavior of a fully discharged alloy [19]. It should be noted that this phenomenon is only observed for the condition in which the amount of absorbed H is small, that is, at low state of charge.
Fig. 8. Nyquist plots at several temperatures for (a) LaNi4.7Sn0.3 (10th cycle) and (b) LaNi4.7Sn0.3/Cu–Pd (4th cycle) fully charged electrodes.
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Fig. 9. Nyquist plots for (a) LaNi4.7Sn0.3 and (b) LaNi4.7Sn0.3/Cu–Pd electrodes at several states of charge at 298 K.
Fig. 10 presents plots of the discharge capacity normalized by the maximum value as a function of the charge/discharge cycle number plots for the palladiumcoated and uncoated alloy. The main difference between the cycling behavior of the original and the Cu/Pdcovered alloy electrode is the lower normalized capacity decay with cycling for the treated alloy. The discharge capacity for the palladium-coated materials was found to be 237 mA h g 1 and for the as prepared alloy and 278 mA h g 1. The lower values for discharge capacity of the Cu/Pd-coated alloy electrode can be attributed to the lower hydrogen absorption capacity per gram of the palladium clusters relatively for the capacity of the metal
hydride alloy [25,27], and to the fact that Cu does not absorbs hydrogen.
4. Conclusions Palladium deposition was carried out on the surface of a metal hydride alloy, by mean of copper replacement. The film microstructure was characterized by SEM, EDS and XANES techniques. The electrochemical behavior of the prepared electrodes was studied with impedance spectroscopy, potentiodynamic measurements and galvanostatic charge/discharge cycles. Based on these results, it is concluded that the electroless plated copper/palladiun acts as a protective layer for the Ni atoms on the LaNi4.7Sn0.3 active alloy. The palladium-coated electrode also presented lower discharge overpotentials, which is important to improve the battery performance. Microencapsulation by copper/palladium provides complete coverage of the intermetallic alloy powder surface that helps preventing corrosion or oxidation of both the active material (LaNi4.7Sn0.3 alloy) and the catalyst (Ni clusters). Also, the copper/palladium deposits was found to increase the cycle life of the metal hydride electrode. The main impedance features of the metal hydride electrodes were related to the charge transfer step of the hydriding–dehydriding processes. It was found that the catalytic activity of charge/discharge is improved with Cu/ Pd deposition, a factor exclusively related to a higher active area of the coated layer, since the values of the apparent activation energy are higher for the coated sample.
Acknowledgments
Fig. 10. Cycle performance of LaNi4.7Sn0.3 with and without the palladium treatment.
The authors wish to thank FAPESP and the LNLSNational Synchrotron Light Laboratory, Brazil for technical and financial support.
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