Ternary PtCoNi functional films prepared by electrodeposition: Magnetic and electrocatalytic properties

Ternary PtCoNi functional films prepared by electrodeposition: Magnetic and electrocatalytic properties

Electrochimica Acta 109 (2013) 187–194 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elect...

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Electrochimica Acta 109 (2013) 187–194

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Ternary PtCoNi functional films prepared by electrodeposition: Magnetic and electrocatalytic properties S. Grau, M. Montiel, E. Gómez, E. Vallés ∗ Electrodep, Departament de Química Física and Institut de Nanociència i Nanotecnologia (IN2UB), Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain

a r t i c l e

i n f o

Article history: Received 24 January 2013 Received in revised form 29 May 2013 Accepted 10 July 2013 Available online xxx Keywords: Ternary functional films Electrodeposition Platinum alloys Magnetic properties Catalytic properties

a b s t r a c t The possibility of preparing PtCoNi films by means electrodeposition has been tested. Simultaneous deposition of the three metals has been possible, over an initial platinum deposit. Control of Co:Ni ratio in solution and deposition potential have permitted to prepare nanometric PtCoNi films with different composition and morphology. Modification of the properties of the films as a consequence of a gradual substitution of Co by Ni has been analyzed. The presence of Ni into the deposits drastically affects the magnetic properties of the films, by controlling the morphology and crystalline structure of the deposits, because the partial substitution of Co by Ni favours the evolution from hcp Co phase to fcc Pt phase. Catalytic properties of the PtCoNi films respect to the oxygen reduction reaction were also clearly dependent on the Ni amount into the deposits: substitution of some Co by Ni improves (for low Ni content) or worsens (for increasing Ni content) the properties of PtCo films. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction The preparation of platinum alloys is object of interest due to their specific properties. Although platinum is a very expensive metal, some of its alloys have recently investigated for specific applications, especially consequence of their magnetic and electrocatalytic properties. The magnetic platinum alloys showing growing interest are those in which platinum is combined with ferromagnetic metals. There has recently been a growing interest in the miniaturization of hard magnetic materials and in their integration in micromechanical systems (MEMS) because this fact would enable the efficiency of these devices. The Fe–Pt and Co–Pt alloys are known to exhibit high coercivity, due to high magnetocrystalline anisotropy of the L10 FePt or CoPt phase, and good resistance to the corrosion [1–3]. These alloys can be used for magnetic recording or specialized permanent magnet applications, but to obtain the ordered L10 phase and the desired hard magnetic properties, an annealing of the samples is necessary, from the directly obtained fcc alloys [4,5]. In our group, the preparation of films and structures of CoPt alloy has been studied in order to test their magnetic properties; CoPt alloys have been studied due to their potential applications in MEMS, as a specific field into the general interest for sensing and actuating devices, as has been reported [6]. In our studies, platinum

∗ Corresponding author. Fax: +34 934031231. E-mail address: [email protected] (E. Vallés). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.07.097

rich deposits showing an hcp crystalline structure were deposited and analyzed in order to obtain deposits with the maximum coercivity without annealing of the samples [7,8]. The incorporation of nickel into these deposits to substitute partially the cobalt content can improve the durability of the deposit and minimize the environmental impact of the cobalt. On the other hand, platinum alloys have been investigated due to their catalytic properties. The oxygen reduction reaction (ORR) is a key step in cathodes of fuel cells due to its relatively high overpotential [9–11], and platinum is the material of choice for application in both acid and alkaline fuel cells. Alloying platinum with light transition metals that increase the value of Pt d-band vacancies per atom, such as Ti [12], V [13,14], Cr [14–17], Mn [18], Fe [19–21], Co [22–27], and Ni [28–30], have been found to exhibit higher electrocatalytic activities towards the ORR than platinum alone in low temperature fuel cells [12–30]. These alloys improve both the performance and the resistance to sintering and coalescence, and reduce the amount of platinum required for fuel cell applications [30–32]. Also, alloys of platinum with atoms that modify the oxophylicity of the particles seem to improve the kinetics on the ORR by impeding the formation of the oxide layer that competes with O2 adsorption [33]. One of the alloys studied for their catalytic properties respect to the ORR reaction is the PtCo one, although recent works have analyzed the better catalytic properties respect to ORR in basic media of PtCoNi films than PtCo ones, both prepared from chemical vapor deposition [28]. The possibility of preparing ternary alloys containing platinum, cobalt and nickel with variable composition can be interesting for

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the commented purposes in order to improve or modify the properties of the CoPt binary alloys. The electrodeposition can be an adequate method to prepare films or structures of the ternary alloy. The aim of the present work is to analyze the possibility of preparing, by means electrodeposition, ternary PtCoNi films with variable nickel content, because the partial substitution of cobalt by nickel can improve the durability of the deposit and minimize the environmental impact of the cobalt. Both the magnetic and electrocatalytic properties of these electrodeposited films will be determined. The introduction of different proportions of nickel into the PtCo alloys and the properties of the resulting films will be studied. Platinumrich deposits are especially desired because platinum-rich alloys present good performance to the ORR reaction and, simultaneously, can present hard-magnetic behaviour.

−1.0 V at room temperature. The electrode rotation speed was regulated by a Metrohm 628-10 unit. The rotation rate was 1500 rpm. Current densities were calculated in terms of electrochemically active surface area. In the manuscript, all potentials are referred to the Ag/AgCl. Electrodeposition and electrochemical measurements were carried out using a microcomputer-controlled potentiostat/galvanostat Autolab PGSTAT30 with GPES software. The composition of the deposits was analyzed by means both a Leica Stereoscan S-360 equipment or a X-ray Fluorescence Fischerscope® X-RAY XDAL® . A good concordance has been observed between the two determinations. The thickness of the films was measured using a Zygo NEW VIEW 100 white-light interferometer. The thicknesses of the deposits allow evaluating the deposition rate and the efficiency of the process. The efficiency of the process was calculated using Eq. (1):

2. Experimental ε= In order to analyze the possibility of nickel incorporation into PtCo deposits, different electrolytic solutions were prepared. The composition of the solutions were: [NH4 Cl] = 0.1 M, [H3 BO3 ] = 0.16 M, [Na2 PtCl6 ] = 1.2 × 10−3 M, −3 [CoCl2 ] + [NiCl2 ] = 2.5 × 10 M. The pH was maintained at 4.5 (initial pH of 4 was adjusted with a NaOH solution) and the [Co2+ ]:[Ni2+ ] ratio was varied (100:0, 90:10, 80:20, 50:50). Temperature of deposition was maintained at 29 ◦ C. All the reagents were of analytical grade. Solutions were prepared with distilled water treated with a Millipore Milli Q system. The electrochemical study of the electrodeposition process and the preparation of the deposits from the different solutions have been performed using a thermostatized cell with Ag/AgCl(s)/KCl (3 M) as electrode of reference, platinum spiral as counter electrode and Si/Ti(10 nm)/Au(100 nm) pieces, with exposed area of 0.25 cm2 , as working electrodes. Some deposits were also prepared over a glassy carbon rotating disc electrode (RDE) of 0.071 cm2 of area. Previous to each deposit, the Si/Ti/Au pieces were washed with ethanol and rinsed with Milli-Q water, whereas the glassy carbon electrode was polished with alumina 0.05 ␮m to obtain a mirror finish, and it was rinsed with Milli-Q water in an ultrasonic bath. The solution was de-aerated by argon bubbling before each experiment and maintained under argon atmosphere during it. Stirring of the solution was maintained during the electrodeposition to assure a constant composition throughout the deposits. The prepared samples were tested as electrocatalysts for the oxygen reduction reaction by means of linear sweep voltammetry (LSV) studies with a rotating disc electrode (RDE). These tests were recorded at 1 mV s−1 and carried out at 25 ◦ C in a conventional three-compartment electrochemical glass cell. An Ag/AgCl, KCl 3 M electrode and a platinum spiral as the reference and the counter electrodes were used. A 0.5 M KOH (Merck) solution in Milli-Q water was used as the electrolyte. In order to clean and activate the electrode surface a series of cyclic voltammetry (CV) experiments were done before the LSV experiments. Prior to each CV measurement the electrolyte was purged with argon for 30 min to deareate the system. Samples were cycled at 50 mV s−1 between −1.1 and 0 V until reproducible voltammograms were obtained. The electrochemically active surface area of each catalyst was calculated using the hydrogen adsorption charge from the cyclic voltammogram in 0.5 M H2 SO4 at 50 mV s−1 between −0.25 and 1.2 V. The charge density associated with a monolayer of hydrogen atoms adsorbed on polycrystalline platinum (210 ␮C cm−2 ) was assumed. The active surface area was calculated by integration of the area under hydrogen adsorption region and subtracting the double layer contribution. Before the LSV experiments the electrolyte was saturated with oxygen by bubbling high purity oxygen for 30 min. The polarization curves were obtained at 1 mV s−1 between 0 and

Qd nVF 100 = 100 Q MQ

(1)

where Qd is the charge corresponding to the deposit formation, Q is the experimental circulated charge,  is the density of the Ptx Coy (x + y = 1) or the Ptx Coy Niz (x + y + z = 1) deposits estimated from the composition and the cell volume, V is the volume of the deposit obtained from the measure of the thickness, M is the molecular weight of the Ptx Coy or Ptx Coy Niz deposits and n is the total number of electrons to deposit these compounds. A Hitachi H-4100FE was used for the observation of the different samples. Magnetic properties of the deposits were characterized using a SQUID magnetometer at room-temperature. The magnetic field was applied parallel and perpendicular to the substrate. Structure of deposits were studied by means of a PANalytical X’Pert-PRO MRD diffractometer with parallel optical geometry and using Cu K␣ radiation ( = 0.1542 nm) and incident angle of 1◦ to avoid the response of the seed-layer. A 10–100◦ 2 range was used, with a step size of 0.05◦ and a counting time of 15 s by step. 3. Results and discussion The electrodeposition process from each solution was studied from voltammetric experiments on Si/Ti/Au substrates at 50 mV s−1 ; always a start potential at which no redox process takes place (0.5 V) was selected, scanning after to negative potentials. The voltammetric curves corresponding to the blank solution (NH4 Cl 0.1 M + H3 BO3 0.16 M, pH 4.5), Pt solution (NH4 Cl 0.1 M + H3 BO3 0.16 M + Na2 PtCl6 1.2 × 10−3 M, pH 4.5) and the different PtCoNi solutions were recorded, compared and analyzed. The ammonium chloride-boric acid medium selected (blank solution) does not affect the detection of the Pt deposition process. When the voltammetric study of the Pt solution was performed, the potential range of the start of platinum deposition from the selected bath was detected; also, the hydrogen processes over the first deposited platinum were identified. Different cathodic limits were used in order to better identification of each voltammetric peak. Bibliographic information and specific experiments were considered to identify the peaks. A cathodic limit of −1.0 V allows detecting all the redox processes corresponding to the system: Platinum deposition from the bath begins during the first reduction peak (R1 ), diffusion controlled. In the peak R2 , reduction continues and over the first deposited Pt, reduction of protons begins, previous to the main hydrogen evolution (R3 ) (Fig. 1) [34,35]. In the anodic scan, the oxidation of molecular hydrogen retained over the electrode appears (O1 ) [8,36], except when the anodic scan is recorded in stirring conditions (to detach the hydrogen), even after a hold in the R3 zone to favour the hydrogen evolution (Fig. 2). The platinum deposition can be detected from its superficial oxidation peak at potentials more positive than 500 mV (superficial

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Fig. 1. Cyclic voltammetries of the blank solution (no current) and the Pt solution at three cathodic limits (a) −1.0, (b) −0.75 and (c) −0.6 V. Anodic limit: 1 V. Stationary conditions: Si/Ti/Au substrate.

platinum oxidation – SPO-peak). In order to study and identify only the first peaks, in Fig. 3 shorter cathodic limit (−0.55 V) was used and the potential of the end of the scan was enlarged to 0.2 V; in this figure the peak of formation of superficial platinum oxides (SPO) and its corresponding reduction peak (reduction of the superficial platinum oxides – R SPO) are clearly seen and identified. As the appearance of the SPO peak reveals the formation of platinum deposit on the substrate, Fig. 4 shows that when the scan is reversed after the reduction peak R1 , platinum was already formed; also, a small loop corresponding to a nucleation and growth process was observed in the R1 peak. Then, platinum begins to deposit in the potentials corresponding to the first reduction peak R1 . After the identification of the different peaks appearing in the Pt solution voltammetry, the voltammetric curves corresponding to the PtCo and PtCoNi systems were analyzed. In the solution free of Ni(II) (PtCo solution) the voltammetric curves recorded show a similar profile than that detected for the Pt solution (Fig. 5, curve a).

Fig. 2. Cyclic voltammetries of the Pt solution for a cathodic limit of −1.0 V. (a) Stationary conditions, (b) stationary conditions in the cathodic scan, hold of 30 s at −1 V and stirring conditions in the anodic scan. Si/Ti/Au substrate.

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Fig. 3. Cyclic voltammetry of the Pt solution for a cathodic limit of −0.55 V, with a start potential of 0.5 V and a final potential scan of 0.2 V. Si/Ti/Au substrate.

The electrodeposition process is significantly dependent on the pH of the solution and a strict control of the pH is necessary to attain reproducibility: a slight decrease of the pH significantly enhances the peak R2 . Cobalt codeposition occurs simultaneously to hydrogen evolution, in the potential zone of R3 : To corroborate the PtCo codepositon in this potential zone, holds in the reduction scan (at −1 V) were performed to accumulate the PtCo alloy; in these conditions an oxidation peak at around −0.3 V appears corresponding to some alloy oxidation and the peak corresponding to the superficial platinum oxidation increases (Fig. 5, curve b). The oxidation scan was performed in stirring conditions to avoid the molecular hydrogen oxidation. The oxidation of the PtCo is difficult to detect as corresponds to a noble platinum alloy. When Co(II) is partially substituted by Ni(II), no drastic changes in the voltammetric behaviour were observed (Fig. 6, curve a) which seems to reveal that Co and Ni were simultaneously deposited with platinum after the initial platinum formation. When the codeposition of the three metals is enhanced by means a hold at −1 V,

Fig. 4. Cyclic voltammetry of the Pt solution for two cathodic limits in the potential region of the reduction peak R1 . Si/Ti/Au substrate.

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Fig. 5. Cyclic voltammetries of the PtCo solution ([Co2+ ]:[Ni2+ ]) = 100:0. (a) Stationary conditions, (b) stationary conditions in the cathodic scan, hold of 30 s at −1 V and stirring conditions (from −1 to −0.5 V) in the anodic scan. Si/Ti/Au substrate.

Fig. 6. Cyclic voltammetries of the PtCoNi solution with [Co2+ ]:[Ni2+ ] = 50:50. (a) Stationary conditions, (b) stationary conditions in the cathodic scan, hold of 30 s at −1 V and stirring conditions (from −1 to −0.5 V) in the anodic scan. Si/Ti/Au substrate.

Fig. 7. Composition of the deposits obtained as a function of the applied potential with a deposition charge of 2 C cm−2 .

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some alloy oxidation is detected, now at −0.1 V (Fig. 6, curve b). The incorporation of nickel into the deposits leads to a less oxidizable alloy than in the case of the previous PtCo electrodeposited alloy. In all cases, the superficial oxidation of the first electrodeposited platinum is observed at the more positive potentials. From the voltammetric results, the simultaneous deposition of PtCo or PtCoNi from the selected bath seems occur, over an initial platinum deposition. Binary and ternary deposits were formed and analyzed. Deposits were prepared potentiostatically, by applying different potentials in the range −600 to −1100 mV. Stirring of the solution was maintained always during the deposition in order to assure a constant composition throughout the thickness of the

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deposits. In according to the behaviour detected in the voltammetric study, low negative deposition potentials led to deposits very rich in platinum; when the potential was gradually made more negative, Pt percentage decreased and the Co + Ni percentage increased (Fig. 7). The PtCo solution allows to obtain PtCo deposits with a variable percentage of platinum between 90 and 65 wt.%. The incorporation of Ni(II) to the solution leads to PtCoNi deposits in which cobalt and nickel simultaneously deposit with platinum. However the Co:Ni ratio in the ternary PtCoNi deposits obtained from the selected bath is drastically lower than those obtained from pure CoNi deposits in chloride medium [37]. This reveals that in the presence of platinum anomalous codeposition of Co and Ni is damped.

Fig. 8. FE-SEM images of (a and b) PtCo (28 wt.% Co) and (c and d) PtCoNi (14 wt.% Co, 14 wt.% Ni) alloys deposited on Si/Ti/Au pieces; and (e and f) PtCo (26 wt.% Co) and (g and h) PtCoNi (12 wt.% Co, 13 wt.% Ni) alloys deposited on glassy carbon RDE. Deposition charge is 2 C cm−2 for (a, c, e and g) and 8 C cm−2 for (b, d, f and h).

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a

b

c

Fig. 9. Magnetization-magnetic field curves corresponding to (a) PtCo (28 wt.% Co), (b) PtCoNi (18 wt.% Co, 6 wt.% Ni) and (c) PtCoNi (14 wt.% Co, 14 wt.% Ni) deposits of 8 C cm−2 . Magnetic field applied parallel (continuous line) and perpendicular (pointed line) to the films.

PtCo and PtCoNi deposits with similar platinum percentages (around 70 wt.%) were characterized in order to analyze the influence of the partial substitution of cobalt by nickel. The morphology of PtCo and PtCoNi deposits obtained at the same deposition charge (2 and 8 C cm−2 ) was compared (Fig. 8). For the lower deposition charges, edged crystallites of hexagonal morphology were observed for the PtCo deposits, whereas the morphology changes to nodular one in the presence of Ni into the deposits. When the deposition charge was increased, deposits were more compact, but clear difference was maintained between PtCo and PtCoNi morphologies. The morphological change was better observed in the deposits prepared on glassy carbon substrates. The determination of the thickness of the deposits (in the 80–360 nm range) allows calculating both the deposition rate and the efficiency of the electrodeposition process, in according to the equation introduced in the experimental section (Table 1 and Eq. (1)). The efficiency was calculated by comparing the theoretical and the experimental charge. Moderate efficiency of the deposition process is observed (between 15% and 28%) as corresponds to significant hydrogen coevolution over platinum or platinum alloys. The growth rate was quantified in both cases, which allows determining the necessary time to electrodeposit a desired thickness of the films. The test bath allows preparing binary PtCo and ternary PtCoNi films of variable composition and morphology, controlled by both the [Co(II)]/[Ni(II)] ratio in the bath and the applied potential for the deposition. The magnetic and catalytic properties of platinumrich deposits were measured and the influence of the Ni presence into the deposits was analyzed. Fig. 9 shows the normalized magnetization-magnetic field applied curves for different deposits obtained. PtCo deposits with around 70–75 wt.% of Pt show high values of coercivity (1400 Oe) and a clear magnetic easy axis in the direction of the magnetic field applied, as is usual for thin films with a significant shape anisotropy.

The substitution of some cobalt by nickel into the deposits clearly leads to a drastic decrease of the coercivity of the films as the Ni percentage into the deposits increases. Possible changes in the crystalline structure of the alloys due to the presence of some nickel were analyzed by means X-ray diffraction. The X-ray diffractograms show (Fig. 10a) that PtCo deposits obtained from the selected bath present a main hcp CoPt phase, with the diffraction peaks drastically shifted to lower 2 values respect to those of pure hcp Co because platinum was incorporated into the crystalline phase of cobalt as it was detected from

a

b

Table 1 Growth rate and efficiency of the deposition processes, at the same deposition potential (−1100 mV). Deposits

[Co]:[Ni] ratio

Main phase

wt.% Coa

wt.% Ni

εb (%)

GRc

CoPt CoPtNi CoPtNi CoPtNi

100:0 90:10 80:20 50:50

hcp Co hcp Co fcc Pt fcc Pt

38 31 18 12

0 4 6 12

28 24 15 18

11 11 10 9

a b c

wt.% Pt = 100 − wt.% Co − wt.% Ni. Efficiency. Growth rate/␮m h−1 cm−2 .

Fig. 10. X-ray diffraction patterns of (a) PtCo (28 wt.% Co) and (b) PtCoNi (14 wt.% Co, 14 wt.% Ni) deposits with a deposition charge of 2 C cm−2 .

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a

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b

Fig. 11. Polarization curves recorded at 1 mV s−1 in O2 saturated 0.5 M KOH, at 1500 rpm, for Co- and Co/Ni-containing alloys deposited on Si/Ti/Au pieces (a) and glassy carbon electrodes (b). Current density is given in terms of electrochemically active surface area.

other deposition baths [7]. Other small peaks were detected in the diffractogram, assigned to the presence of a cubic Pt3 Co phase. The structural study determines that although the cobalt content in the deposits is low (around a 28–32 wt.%) it is enough to control the crystalline structure of the alloy. The main hcp phase justifies the hexagonal morphology observed in the SEM pictures of the PtCo deposits. When cobalt was partially substituted by nickel in the alloy, the crystalline structure also changes. Fig. 10b shows that although the hcp CoPt phase is still present, a main fcc PtCoNi phase was clearly detected, assigned to a fcc platinum phase distorted by cobalt and nickel incorporation. This structural change is related also to the morphological change observed in the deposits (from hexagonal to nodular type) when cobalt is partially substituted by nickel. It seems that the presence of some nickel (cubic structure) favours the cubic phase of platinum, the main component of the alloy. The lower proportion of cobalt into the deposits difficult the predominance of the hcp phase. The change of the proportion of the crystalline phases in the PtCoNi, decreasing the more magnetically anisotropic hcp phase, justifies the decrease of the coercivity of the films as the nickel percentage into the deposits increases. This fact is observed in pure CoNi alloys: the substitution of some cobalt by nickel in electrodeposited films induces less anisotropic magnetic films, because the presence of nickel into CoNi films favours the formation of the fcc phase when deposition is performed at increasing more negative deposition potentials [38] instead the usual hcp phase for pure cobalt deposits [39]. Same effect seems occur for the ternary PtCoNi electrodeposited films. Although the partial substitution of Co by Ni in PtCoNi deposits by means electrodeposition has been possible, the nickel presence is not able to maintain the hcp crystalline phase induced by the cobalt in the PtCo deposits. The effect of the nickel is to favour a platinum fcc crystalline phase, leading to a gradual decrease of the coercivity of the films. The influence of the nickel presence in the deposits, controlling both morphology and crystalline structure of the films can also justify the values of efficiency observed in Table 1. For pure PtCo films of mainly hcp phase and edged morphology, the maximum value of efficiency is observed. The incorporation of very low percentages of nickel, maintaining the main crystalline phase and morphology, only slightly decreases the efficiency of the process, but when the nickel percentage increases, inducing main platinum fcc phase and nodular morphology, clear decreasing efficiency is

observed. This indicates that hydrogen evolution is probably more favoured for this last type of PtCoNi films. From these results, the catalytic activity of the films over other processes was studied. The behaviour of the PtCo and PtCoNi films as electrocatalysts in the oxygen reduction reaction (ORR) was investigated. Fig. 11a shows the current density data obtained from linear sweep voltammetry experiments for several pieces with PtCo and PtCoNi deposits. These pieces were stuck on a rotating electrode with colloidal silver, and the experiments were carried out at 1500 rpm and 1 mV s−1 (0.5 M KOH, room temperature). Under these experimental conditions, colloidal silver shows no significant activity on the ORR in comparison with PtCo and PtCoNi films. Samples with lower nickel content display similar performance on the ORR and they are the best of the series, displaying lower overpotential in the kinetic and mixed controlled region than CoPt alloys. Nevertheless, the performance of samples with the highest nickel content is rather poor, probably as a consequence of the morphological changes previously observed. It seems, then, that the moderate substitution of cobalt by nickel slightly improves the performance of the platinum rich PtCo films, whereas increasing nickel percentages into the deposits are not adequate. In order to corroborate these results, deposits were prepared under the same experimental conditions directly on a glassy carbon (GC) rotating disc electrode. Although PtCoNi deposits prepared over GC present some differences in the nickel content respect to those prepared on Si/Ti/Au at the same deposition potential, the performance on the ORR follows the same trend as the previous samples (Fig. 11b): Lower Ni content > without Ni > Higher Ni content. Although there are several recent studies relating the Co/Pt [40] or Ni/Pt [41] ratio to ORR activity, to the best of our knowledge, the relationship between Co/Ni/Pt ratio and catalytic activity has not discussed in literature in the range of compositions studied by us.

4. Conclusions Electrodeposition of ternary platinum-rich PtCoNi alloys with different nickel percentages has been possible from a moderately acidic chloride bath by varying the Ni(II) concentration in the solution and the deposition potential. Cobalt and nickel can be simultaneously electrodeposited with platinum, although simultaneous hydrogen evolution also takes place. This opens the possibility of using a simple method as electrodeposition to prepare

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potential functional ternary PtCoNi films with variable properties as a function of the relative percentages of the metals. From the voltammetric results, oxidation of nickel-containing alloys takes place at more positive potentials than pure PtCo ones, which reveals lower tendency to its oxidation. Cobalt content can be gradually substituted by nickel one, maintaining the platinum percentage in solution. The nickel presence conditions the morphology and crystalline structure of the films, favouring deposits of fcc PtCoNi phase and nodular morphology. As a consequence, the nickel presence conditions the magnetic properties of the PtCo films, the hydrogen evolution reaction, and its performance for the ORR activity. Less anisotropic films were obtaining, showing lower coercivity. The coercivity of the films can be regulated as a function of the nickel percentage. The improvement of the performance of the Pt-rich PtCo films respect to the ORR activity when the films present low percentages of nickel is also a promising result. Acknowledgements This work was supported by contract CTQ2010-20726 (subprogram BQU) from the Comisión Interministerial de Ciencia y Tecnología (CICYT) and IMB-CNM (CSIC) NGG-258 project. The authors wish to thank the Serveis Cientificotècnics (Universitat de Barcelona) for the use of their equipment. M. Montiel acknowledges the Generalitat de Catalunya for a Beatriu de Pinós postdoctoral fellowship. References [1] J. Lyubina, B. Rellinghaus, O. Gutfleisch, M. Albrecht, Structure and magnetic properties of L10-ordered Fe–Pt alloys and nanoparticles Handbook of Magnetic Materials, 19, 2011, pp. 291–407. [2] A. Alam, B. Kraczek, D. Johnson, Structural, magnetic, and defect properties of Co–Pt-type magnetic-storage alloys: density-functional theory study of thermal processing effects, Physical Review B 82 (2010) 024435. [3] S.C. Chen, C.D. Chen, T.H. Sun, S.L. Ou, C.L. Shen, W.H. Su, Effect of Pt content on structure and magnetic properties of Fe100−x Ptx films deposited on thermally oxidized Si substrates by rapid thermal annealing, Vacuum 87 (2013) 205–208. [4] N. Yasui, A. Imada, T. Den, Electrodeposition of (0 0 1) oriented CoPt L1[sub 0] columns into anodic alumina films, Applied Physics Letters 83 (2003) 3347–3349. [5] I. Zana, G. Zangari, Co–Pt micromagnets by electrodeposition, Journal of Applied Physics 91 (2002) 7320–7322. [6] A. Nespoli, S. Besseghini, S. Pittaccio, E. Villa, S. Viscuso, The high potential of shape memory alloys in developing miniature mechanical devices: a review on shape memory alloy mini-actuators, Sensors and Actuators A: Physical 158 (2010) 149–160. [7] M. Cortés, E. Gómez, E. Vallés, Magnetic properties of nanocrystalline CoPt electrodeposited films. Influence of P incorporation, Journal of Solid State Electrochemistry 14 (2010) 2225–2233. [8] M. Cortés, A. Serrà, E. Gómez, E. Vallés, CoPt nanoscale structures with different geometry prepared by electrodeposition for modulation of their magnetic properties, Electrochimica Acta 56 (2011) 8232–8238. [9] F. Barbir, PEM Fuel Cells, Elsevier Academic Press, Burlington, MA, USA, 2005, ISBN 978-0-12-078142-3. [10] K.-L. Hsueh, E.R. Gonzalez, S. Srinivasan, Electrolyte effects on oxygen reduction kinetics at platinum: a rotating ring-disc electrode analysis, Electrochimica Acta 28 (1983) 691–697. [11] E. Yeager, Electrocatalysts for O2 reduction, Electrochimica Acta 29 (1984) 1527–1537. [12] B.C. Beard, P.N. Ross Jr., Characterization of a titanium-promoted supported platinum electrocatalyst, Journal of the Electrochemical Society 133 (1986) 1839–1845. [13] E. Antolini, R.R. Passos, E.A. Ticianelli, Electrocatalysis of oxygen reduction on a carbon supported platinum–vanadium alloy in polymer electrolyte fuel cells, Electrochimica Acta 48 (2002) 263–270. [14] H. Yano, M. Kataoka, H. Yamashita, H. Uchida, M. Watanabe, Oxygen reduction activity of carbon-supported Pt–M (M = V, Ni, Cr, Co, and Fe) alloys prepared by nanocapsule method, Langmuir 23 (2007) 6438–6445. [15] M. Min, J. Cho, K. Cho, H. Kim, Particle size and alloying effects of Pt-based alloy catalysts for fuel cell applications, Electrochimica Acta 45 (2000) 4211–4217.

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