Effects of saccharin and anions (SO4 , Cl ) on the ...

J Solid State Electrochem DOI 10.1007/s10008-014-2616-7

ORIGINAL PAPER

Effects of saccharin and anions (SO42−, Cl−) on the electrodeposition of Co–Ni alloys Liliana Altamirano-Garcia & Jorge Vazquez-Arenas & Mark Pritzker & Rosa Luna-Sánchez & Román Cabrera-Sierra

Received: 15 May 2014 / Revised: 14 August 2014 / Accepted: 17 August 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract The electrodeposition of Co–Ni alloys is investigated to examine the mechanistic effects of chloride, sulfate and saccharin on the resulting alloy composition, deposition current efficiency and partial current densities of the cathodic reactions. When deposition is carried out in chloride, sulfate or mixed sulfate-chloride solutions without saccharin, the influence of hydrogen evolution (HER) becomes dominant and deleterious at higher overpotentials, leading to very low metal deposition current efficiency and the formation of a hydroxide/oxide film on the substrate. This problem is significantly reduced when saccharin is added to the mixed sulfatechloride plating bath and to a lesser extent the sulfate-only solution. Although saccharin is ineffective in suppressing H+ reduction at low overpotentials, it is very effective at inhibiting H2O reduction at high overpotentials and enabling metal deposition to more easily occur. The system follows anomalous behavior at all current densities both in the absence and presence of saccharin, although it approaches normal behavior as the current density increases toward −1,000 A m −2 due to Co(II) reduction being mass transfer-controlled. L. Altamirano-Garcia : R. Luna-Sánchez Depto. Energía, Universidad Autónoma Metropolitana Azcapotzalco, Av. San Pablo, No. 180, 02200 Mexico, DF, Mexico J. Vazquez-Arenas (*) Depto. de Química, Universidad Autónoma Metropolitana Iztapalapa, San Rafael Atlixco No. 186, 09340 Mexico, DF, Mexico e-mail: [email protected] M. Pritzker Chemical Engineering Department, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada R. Cabrera-Sierra Depto. Ing. Química Industrial, Instituto Politécnico Nacional -ESIQIE, UPALM Ed. 7, Mexico 07738, DF, Mexico

Keywords Electrodeposition . Reaction mechanism . Co–Ni alloys . Electrolyte . Saccharin

Introduction The control of the composition of Co–Ni alloys formed by electrodeposition is challenging due to its anomalous behavior whereby the less noble metal (Co) deposits preferentially over the more noble metal (Ni) even when its bulk concentration in the plating solution is lower [1, 2]. Thus, control of its composition becomes critical in order to enhance specific properties. Another complicating factor for the effectiveness of electrodeposition as a method for producing Co–Ni alloys with controllable composition is the occurrence of the hydrogen evolution reaction (HER), which decreases the current efficiency of plating [3, 4]. In order to better control the composition and properties of Co–Ni alloy films, additives are typically added to the plating solution. The use of the organic additive saccharin on the deposition of alloys containing iron-group metals and alloys has been studied [5–11], although Co–Ni alloys have received much less attention than other systems. Although saccharin is often added as a brightener, it can also act to inhibit the HER during deposition due to adsorptive effects [8]. If the saccharin concentration in the bulk solution becomes high enough, it adsorbs on the electrode to such an extent that both metal deposition and the HER are inhibited and the cathode is effectively passivated [5]. However, to our knowledge, no mechanistic information has been reported on the behavior of saccharin during Co–Ni formation. This information is crucial to exploit the advantages of this reagent and determine the proper amounts to be added to increase the efficiency of electrodeposition and control deposit morphology. Thus, the primary objective of this investigation is to analyze the role of the electrolyte and saccharin on the

J Solid State Electrochem

electrodeposition of Co–Ni alloys. The plating baths consist of synthetic solutions that match the levels in leach liquors generated from spent Ni–Cd battery components [12, 13]. Previous experiments have shown that the composition of Co–Ni and other iron-group alloys produced by electrodeposition is strongly affected by whether the process is carried out in a sulfate or chloride electrolyte [14, 15]. Consequently, we investigate the influence of sulfate, chloride and mixed chloride-sulfate electrolytes on the composition of the resulting Co–Ni alloy coatings and current efficiency. Particular attention is also focused on the effect of saccharin on the electrodeposition of Co–Ni alloys from these baths, something which has not been investigated to date. As part of this study, the electrode response during galvanostatic deposition is examined in detail to gain further insight into the effects of the electrolyte components on the processes involved.

Aesar, 99.99 % purity). All the potential values reported herein correspond to the SHE scale. The rotating disk working electrode was a copper rod embedded in nylon that exposed a circular face with area 0.126 cm2 to the solution. Further details of the preparation of the working electrode have been reported in Refs. [5, 17–21]. The duration of plating at each current density was adjusted to ensure that a total of 1.5 C of charge was passed over the course of each experiment. Following deposition, the composition of the resulting alloy coatings was determined utilizing a high-dispersion inductively coupled plasma (ICP) spectrometer (Teledyne LeemanTM model Prodigy) for the dissolved cobalt and nickel content [5, 17–21]. With this information known, the partial current densities for Ni(II) reduction, Co(II) reduction, and HER could be determined.

Results Experimental

Linear potential scans

Electrochemical experiments were conducted at room temperature in solutions containing 1,000 mol m−3 Na2SO4 (99.3 % purity, Fisher) as supporting electrolyte and NiSO4 (99.3 % purity, Fisher), NiCl2 (98.1 % purity, J. T. Baker), CoSO4 (99.3 % purity, Fisher), CoCl2 (98.1 % purity, J. T. Baker) and sodium saccharinate salt (99 % purity, Cedrosa) at the concentrations given in Table 1. The pH in each bath was adjusted to be 3.0 by the addition of the appropriate amount of concentrated acid. The total dissolved Co(II) and Ni(II) concentrations in baths 1–5 were set to 18 and 400 mol m−3, respectively, to match the levels in solutions obtained by our group after leaching Ni–Cd battery components with H2SO4 and purifying the resulting leach liquors to remove cadmium [16]. All solutions were freshly prepared in deionized water (∼pH 6.8) and deoxygenated with N2 (Praxair, grade 4.8) prior to experiments. A conventional three-electrode rotating disk cell was used to conduct the electrochemical experiments at room temperature. The reference electrode for this cell was a saturated mercury/mercurous sulfate electrode (Hg/Hg2SO4, 0.64 V vs SHE), while the counter electrode was a graphite bar (Alfa

Figure 1 presents the linear voltammetry responses obtained on a copper substrate rotating at 1,000 rpm in solutions containing a supporting electrolyte (SE, 1,000 mol m−3 Na2SO4) and Ni(II) and Co(II) at the following concentrations: (a) 0 mol m−3 (SE), (b) 400 mol m −3 NiSO 4 + 18 mol m−3 CoSO4 (bath 1) and (c) 400 mol m−3 NiSO4 + 18 mol m−3 CoSO4 +24 mol m−3 saccharin (bath 2). As shown in previous studies on the Co–Ni system [18], the first process that occurs as the scan proceeds in the negative direction both in the absence and presence of the dissolved metals is H+ reduction at a potential of approximately −0.8 V (label I in Fig. 1). However, when saccharin is present as in bath 2, the response in this region of the scan differs and a steeper rise in the cathodic current is observed at potentials between −0.8 and −1.0 V. This potential region coincides closely with where saccharin has been observed to adsorb most strongly on Co– Fe–Ni [22] alloy and Ni [23] surfaces. One likely explanation is that the increase in current in this region can be attributed to metal deposition (region II, at approximately −0.95 V). Enough saccharin may adsorb on the surface to slow down the reduction of H+ or H2O and enhance metal deposition.

Table 1 Synthetic electrolytes simulating leach liquors generated from spent Ni-Cd batteries used to electrodeposit Co–Ni alloys at pH 3 Bath

[Na2SO4] (mol m−3)

[NiSO4] (mol m−3)

[NiCl2] (mol m−3)

[CoSO4] (mol m−3)

[CoCl2] (mol m−3)

Saccharin (mol m−3)

SE 1 2 3 4 5

1,000 1,000 1,000 0 0 0

0 400 400 0 300 300

0 0 0 400 100 100

0 18 18 0 18 18

0 0 0 18 0 0

0 0 24 24 24 0

J Solid State Electrochem 0 I II

-300

j (A m-2)

-600 -900 III

-1200

IV

Bath 1 Bath 2 SE

-1500

-1800 -1.7

-1.4

-1.1 -0.8 E / V vs SHE

-0.5

-0.2

Fig. 1 Voltammograms obtained on a copper disk electrode rotating at 1,000 rpm in the following baths described in Table 1: SE (red line), bath 1 (black line), bath 2 (blue line) at pH 3. Scan rate=100 mV s−1

As the scan continues to more negative potentials in baths 1 and 2, both H+ and H2O reduction presumably continue to occur along with alloy deposition. These side reactions not only reduce the current efficiency for metal deposition, but also raise the surface pH during the co-deposition of Co and Ni. Eventually, if the surface pH increases enough, surface hydroxides and oxides are formed along with the alloy at more negative potentials [24], as also reported for single cobalt deposition [25]. As shown previously from X-ray analysis and modeling [18], the rise in surface pH becomes high enough during the scan that nickel oxide or hydroxide forms on the electrode surface, leading to the appearance of a peak at approximately −1.2 V (labeled as III). Since the solution is being stirred when this scan is obtained, the onset of this peak is not attributed to limitations of the transport of an electroactive species to the electrode. Further support for this interpretation comes from the results of LSV scans in bath 2 conducted at 100 and 1,500 rpm (not included here) showing that this peak remains unchanged regardless of the electrode rotation speed. Instead, the formation of an oxide or hydroxide on the surface would be expected to have a passivating effect on the electrode and cause the sharp drop in current at potentials between −1.2 and −1.3 V observed in the scans for both baths 1 and 2 in Fig. 1. However, the oxide layer is not thick or protective enough to completely passivate the electrode and as much as −600 A m−2 current density can still flow. Once the potential becomes more negative than about −1.3 V, the current abruptly rises again, presumably due to breakdown of the passive oxide layer and reactivation of the surface due to water reduction [3, 18]. As shown previously [18, 3], the current densities that flow in this portion of the voltammograms far exceed the sum of the limiting current densities for the reduction of Co(II), Ni(II), and H+ reduction for the conditions of this study. Thus, this rapid rise in current labeled as IV is attributed to water reduction in the scans shown in Fig. 1.

Although analysis for the presence of metal hydrides was not conducted in the present study, one cannot rule out the possibility of their formation on the surface of the deposits during the linear potential scan and other electrochemical experiments due to the affinity of Co or Ni and nascent hydrogen. A number of previously reported electrochemical, chemical and microscopy analyses have provided evidence of the formation of metastable fcc hydrides of Ni (−0.80 V on glassy carbon) and Co (at applied current density of 100 A m−2 on annealed copper or approximately −0.86 V on quartz crystal coated with Pt) under conditions similar to those of the present study [25–30]. These metal hydrides were shown to be unstable under ordinary temperature and pressure and slowly release hydrogen over period of time ranging from 20 min to 40 h to form a martensitic Ni or Co phase. The influence of saccharin on the formation of Co–Ni alloys was further examined by carrying out linear scans in solutions containing two different concentrations of this additive (24 and 48 mol m−3), but with fixed Ni(II) and Co(II) concentrations. Although not shown here, the two scans show very similar features. Perhaps the most significant difference is the observation that the current density is slightly larger from −0.8 to −1.0 V (region I–II) at the higher saccharin concentration. This result is consistent with the trend observed in Fig. 1 that the current in this very same region is greater in bath 2 which contains 24 mol m−3 saccharin than in bath 1 in which saccharin is absent. To be sure, these findings provide evidence that this additive is not acting as a blocking agent that passivates the electrode surface at these concentrations. Nevertheless, only inferences regarding the influence of saccharin during Co–Ni deposition can be made on the basis of these linear potential scans. The possible effects of oxide formation and saccharin adsorption on Co–Ni deposition are further analyzed in the “Galvanostatic deposition” section. The addition of chloride ions is known to enhance the formation kinetics of Co–Ni alloys due to complexing effects that accelerate the discharge of both ions and enhance electrolyte conductivity [31, 19, 32]. Some evidence of the influence of chloride can be gleaned in Fig. 2 which presents the potential scans obtained in baths 3–5, all of which contain chloride. Unlike the solutions considered in Fig. 1, baths 3–5 contain no Na2SO4. As expected, comparison of these scans with those in Fig. 1 indicates that a lower overpotential is required to initiate the reduction processes in the presence of chloride. A peak appears in the scans for baths 3–5 (Fig. 2), but significant differences are evident in the three cases. The fastrising portion of the response is most prominent in the case of the chloride-only solution (bath 3), leading to the appearance of the peak at higher current densities As before, this oxide layer does not completely passivate the electrode and a current density of at least −600 A m−2 still flows. Interestingly, this minimum current density that the oxide permits is virtually

J Solid State Electrochem 0

100

I

II

90

-300

80

wt% Ni

j (A m-2)

-600 -900 -1200

III

-1500

-1800 -1.7

-1.4

-1.1

60

Bath 3 Bath 4 Bath 5

IV

-0.8

-0.5

70

50 40 -1100

-0.2

Bath 2 Bath 3 Bath 4 Bath 5 Bath 1 -900

-700

E / V vs SHE

Fig. 2 Voltammograms obtained on a copper disk electrode rotating at 1,000 rpm in the following baths described in Table 1: bath 3 (black line), bath 4 (red line), bath 5 (blue line) at pH 3. Scan rate=100 mV s−1

identical in baths 3 and 4 and very similar to that observed in baths 1 and 2 (Fig. 1) which contain no chloride. The effect of saccharin in the mixed sulfate-chloride electrolyte can be examined by comparing the electrode responses obtained in baths 4 and 5 in Fig. 2. Cathodic current begins to flow earlier during the scan when saccharin is absent (bath 5) than when it is present (bath 4). The influence of saccharin on the scans obtained in the mixed sulfate-chloride electrolyte appears to be complex. The presence of saccharin has a very large effect by shifting the peak as much as 0.45 V in the negative direction (compare with bath 5). On the other hand, when saccharin is added to the sulfate-only bath, the effect is small (baths 1 and 2 in Fig. 1). It is well known that chloride ions interact much more strongly with metal surfaces during deposition than do sulfate or water [33]. The difference in the effect of saccharin in the two types of baths suggests that the additive may interact with chloride in a way that significantly alters the electrode response during the scan.

Galvanostatic deposition

-500 j/Am

-300

-100

100

2

Fig. 3 Variation of weight percentage of nickel with applied current density in Co–Ni alloys formed on a copper disk electrode rotating at 1,000 rpm in the following baths described in Table 1: bath 1 (blue ex symbol), bath 2 (red filled circle), bath 3 (green filled upright triangle), bath 4 (black filled square), bath 5 (yellow filled diamond) at pH 3

and a rotation speed of 1,000 rpm using the Levich equation and literature values for the diffusion coefficients of the electroactive species [32]. Thus, as the current is increased further, it is distributed to Ni deposition and H2 evolution and enriches the coating in Ni. Also, as evident from Fig. 3, the system follows anomalous behavior at all current densities in every bath, although it approaches normal behavior as the current density increases toward −1,000 A m−2. A comparison of the variation of the alloy deposition current efficiency with current density in baths 1–5 shown in Fig. 4 reveals a few trends. The mixed sulfate-chloride solution containing saccharin (bath 4) can maintain current efficiencies above ∼90 % over a very wide range of current densities, which is better performance than that achieved in a sulfate-only solution containing saccharin (bath 2), a chlorideonly solution containing saccharin (bath 3), a mixed sulfatechloride solution containing no additive (bath 5) or a sulfateonly solution containing no additive (bath 1). The current efficiencies in all baths rise sharply to levels exceeding 80 % as the current density is raised from −50 to −300 A m−2. Once 100

80

current efficiency %

In order to more fully investigate Co–Ni deposition in the presence of saccharin, chronopotentiometry experiments on copper disk substrates rotating at 1,000 rpm were carried out in baths 1–5 over a range of current densities from −50 to −1,000 A m−2. Figures 3 and 4 show the variation of the alloy content and the current efficiencies with applied current density obtained in baths 1–5. As expected and commonly observed [18], the Ni content in the alloy films formed in all baths increases as the applied current density is raised. Since the Co(II) concentration is very low, its reduction rate becomes strongly influenced by mass transport even when relatively small currents are applied. This is reflected by the very large differences in the limiting current densities for Co(II) reduction and Ni(II) reduction that are estimated to be −179 and −3,716.1 A m−2, respectively, at these bath compositions

60

40

Bath 1 Bath 2 Bath 3

20

Bath 4 Bath 5

0 -1100

-900

-700

-500 j/Am

-300

-100

100

2

Fig. 4 Variation of deposition current efficiency with applied current density in Co–Ni alloys formed on a copper disk electrode rotating at 1,000 rpm in the following baths described in Table 1: bath 1 (blue ex symbol), bath 2 (red filled circle), bath 3 (green filled upright triangle), bath 4 (black filled square), bath 5 (yellow filled diamond) at pH 3


Effect of saccharin in sulfate-only solution A comparison of the results obtained in baths 1 and 2 indicates that the presence of saccharin tends to promote more anomalous Co–Ni electrodeposition from sulfate-only solutions. Although earlier studies have found saccharin to have a small influence on the composition of other electrodeposited irongroup alloy films [8, 10, 34, 20, 35, 36], anomalous behavior was observed in some cases [10, 34, 36] and more normal behavior in others [8, 20, 35, 37]. It is difficult to make definite conclusions regarding these differences since the experimental conditions used for these studies (e.g., identity of iron-group metals and anions in electrolyte, metal ion concentration, saccharin concentration) vary widely. Whereas the current density has very little effect on alloy composition depending on whether saccharin is present or absent, the trend observed with the corresponding current efficiency is more complex (Fig. 4). More insight into the behavior of the system in baths 1 and 2 can be gained by examining the variation of the partial current densities for Co deposition (jCo), Ni deposition (jNi), and H2 evolution (jH 2 ) with the steady state potential measured at each applied current density. These partial current densities can be readily calculated using Faraday’s law as follows: jNi ¼

mNi F zNi M Ni t

ð1Þ

jCo ¼

mCo F zCo M Co t

ð2Þ

Thus, jCo and jNi are determined independently from each other and directly from the ICP measurements, whereas jH 2 is only indirectly obtained using Eq. (3) with knowledge of j, jCo, and jNi. As will be discussed later, any saccharin that is reduced would do so at potentials similar to where H2O reduction also occurs. Given that the amount of H2O present is effectively unlimited, the current associated with saccharin electroreduction is considered to be negligible relative to jH 2 in Eq. (3). The variation of the total current density and partial current density in baths 1 and 2 is presented in Fig. 5a and b, respectively. At the lowest applied current density of −50 A m−2 in this study (Fig. 5a), jH 2 has a magnitude of ∼20 A m−2 and exceeds that of jCo and jNi, consistent with the conclusion reached in previous studies and the “Linear potential scans” section that H+ reduction is the first electrode reaction to occur during cathodic scans. The steady state electrode potential of −0.59 V is close to where faradaic current is first observed to occur during the scan obtained in bath 1 (Fig. 1). However, if cathodic polarization is conducted at a higher current density of −100 or −200 A m−2, the additional current is utilized almost exclusively by metal deposition and jH 2 remains essentially constant between −20 and −30 A m−2. This value a

j ( A m- 2 (

cathodic current densities larger than approximately −600 A m−2 are applied, larger differences in the current efficiencies among the baths are observed. The current efficiencies obtained in bath 1, 3 and 5 in particular and bath 2 to a lesser extent decrease significantly at these higher current densities.

-1200

b

jCo jNi jH2 Total current density

-1000

ð3Þ

where mi is the mass of species i (Ni, Co) deposited (g) determined from ICP, F is the Faraday constant (96,485.5 C mol−1), zi is the number of electrons transferred (2) to metal I in the electrochemical reaction, Mi is the molecular weight of metal i (g mol−1) and t is the electrolysis time (s). It should be noted that jH 2 is obtained from the difference between the overall applied current density and the sum of the partial current densities jCo and jNi and so is a combination of the current associated with the reduction of both H+ and H2O.

-800

j ( A m- 2 (

jH2 ¼ j− jNi − jCo

-600

-400 -200 0

-1.6

-1.4

-1.2

-1

-0.8

-0.6

E / V vs SHE

Fig. 5 Variation of total and partial current densities jCo, jNi, and jH 2 with steady state potential calculated respectively with Eqs. (1–3) for Co–Ni alloys formed on a copper disk electrode rotating at 1,000 rpm in the following baths described in Table 1: a bath 1 and b bath 2 at pH 3


is very close to the limiting current density for H+ reduction estimated by the Levich equation to be approximately −27.1 A m−2 at pH 3. Thus, H+ reduction appears to rapidly reach mass transport-limiting conditions at low overpotentials. When the total current density is raised to between −200 and −300 A m−2, jH 2 rises in magnitude once again, signifying that H2O reduction is beginning to occur. The onset of this reaction is accompanied by a small jump in the polarization of the cathode from −0.68 to −0.82 V (note flattening in this region of the curves in Fig. 5a). Once H2O reduction is occurring, the resistance to charge transfer appears to drop and both jNi and jH 2 rise more steeply with respect to the steady state potential when the deposition is carried out at current densities between −300 and −600 A m−2. Interestingly, Co(II) reduction does not respond similarly and jCo remains constant between approximately −60 and −70 A m−2 regardless of the potential during deposition. A dramatic change in the response of the system occurs when the applied current density is raised from −600 to −800 A m−2. This involves a large jump in the polarization and flattening of the current-potential curves from −0.89 to −1.34 V. Examination of the cathodic scan for bath 1 in Fig. 1 reveals that this potential range encompasses the region containing peak III attributed to the formation of an oxide layer in a previous study [17]. It occurs only under conditions where H2O reduction which generates OH− ions at the cathode surface is already occurring. At an applied current density of −600 A m−2 and a steady state potential of −0.89 V where the oxide film appears to first affect the electrode response, jH 2 is measured to be approximately −151 A m−2. Assuming that H+ reduction is proceeding at its limiting level of −27.1 A m−2, the current density for H2O reduction at this critical point corresponds to −124.3 A m−2. Consequently, the generation of OH− at a rate of 1.29×10−3 mol m−2 s−1 is sufficient to cause a metal oxide film to occur during cathodic polarization in bath 1. Unlike the response during the LSV scan in bath 1 (Fig. 1), the current does not decrease in magnitude in the potential region between −0.89 and −1.34 V. This difference is also consistent with expectations if an oxide layer has formed on the electrode surface during polarization. Not only will the rates of the cathodic reactions be slowed by such a layer but they will also become much less sensitive to the electrode potential. Over the limited time span (∼4.5 s) that the potential is between −0.89 and −1.34 V during the scan, the reactions will not be able to keep pace and so the current will drop. On the other hand, when enough time is allowed for steady state to be reached, this limitation is removed. Another feature of the curves in Fig. 5a consistent with the formation of an oxide layer is that jCo, jNi and jH 2 each remain relatively constant in the potential region between −0.89 and −1.34 V, suggesting that the rate of each of these reactions is strongly affected (but not completely controlled) by mass transport through the oxide layer. If Ni(II) and Co(II) reduction were entirely mass

transfer-controlled, then the ratios of their currents should match the ratios of their concentrations. When a current density of −900 A m−2 is applied, two significant changes in the steady state response occur. Metal deposition effectively ceases and the extent of H2O reduction jumps sharply so that it consumes virtually all of the applied current. The observation that an increase in the current density from −800 to −900 A m−2 occurs without any change in the steady state potential suggests that these changes in the cathodic reactions are associated with some sort of structural change to the oxide layer that rapidly reactivates the electrode. This presumably corresponds to the IV region in the LSV scan in Fig. 1 just after the electrode is reactivated. Unfortunately, this structural change prevents any metal deposition from occurring. The cessation of metal deposition also underscores the important consequence that the oxide layer appears to have on metal deposition in this additive-free bath. With a further increase in current density to −1,000 A m−2, H2O reduction continues to be dominant, with a significant increase in polarization to −1.74 V. The steady state current-potential curves obtained in bath 2 are presented in Fig. 5b. Comparison of this response with that shown in Fig. 5a reveals that the effect of saccharin on Co–Ni deposition changes depending on the current and potential. At low current densities of −50 and −100 A m−2, the polarization is very similar to that obtained in the absence of the additive, but the distribution of the partial current densities is altered. The deposition of both metals is inhibited while the HER is enhanced in the presence of saccharin, particularly at −50 A m−2. The magnitude of jH 2 is between −30 and −40 A m−2, suggesting that H+ reduction has reached mass transfer limiting conditions. However, when the applied current densities is raised to −200 A m−2 and above, some differences in the electrode response from that observed in the absence of saccharin are exhibited. The polarization of the working electrode is higher by ∼0.1 V at each applied current density up to and including −700 A m−2 in the presence of saccharin than in its absence. jCo remains constant over a wide range of currents at intermediate and high overpotentials, as in the additive-free bath, but at levels between −80 and −100 A m−2, about a 40 % increase in the presence of saccharin. At the same time, jNi decreases in magnitude by ∼10 % until a current density of −600 A m−2 is reached, whereupon it becomes much larger than observed in the additive-free solution (bath 1). Perhaps the most striking effect of saccharin is the inhibition of jH 2 once current densities of −400 A m−2 are applied. In particular, the HER is strongly suppressed at potentials between −0.9 and −1.1 V which are reached when current densities between −400 and −600 A m−2 are applied. These effects on Co and Ni deposition and the HER at potentials from approximately −0.8 to −1.1 V (current densities from approximately −200 to −600 A m −2 ) to the


adsorption of saccharin on the electrode. Kwon and Gewirth presented evidence from surface enhanced Raman scattering spectroscopy (SERS) that saccharin can adsorb onto Ni surfaces at all potentials during cathodic polarization from approximately −0.5 to −1.0 V whereupon it desorbs [23]. This potential range from −0.8 to −1.1 V is also in excellent agreement with the region where Brankovic et al. found saccharin to adsorb on Co–Fe–Ni alloy surfaces [22]. Furthermore, on the basis of double-layer capacitance measurements, Brankovic et al. observed that saccharin most strongly adsorbs and forms condensed films on the substrate at potentials between −0.9 and −1.1 V. As observed in Fig. 5b, the curve for the total current density flattens out signifying an increase in polarization at precisely these same potentials. Such an effect would be expected if saccharin is forming a condensed film on the electrode under these conditions. Another important observation reported by Brankovic et al. is that the current efficiency increases at the very potentials where saccharin adsorbs most strongly, presumably due to the suppression of the HER. A comparison of Fig. 5a, b confirms similar behavior for Co–Ni deposition at these potentials. At the same time, metal deposition is at the very least uninhibited and in fact may be enhanced under these conditions. The increase in polarization between −1.0 and −1.1 V observed in Fig. 5b is not likely due to oxide layer formation since it is not accompanied with the large rise in jH 2 , as observed in bath 1. Given that the partial currents show no inhibition of the HER at very low overpotentials where H+ reduction is dominant, the additive appears to act primarily on H2O reduction. This conclusion is reasonable when one considers the structure(s) believed to form when saccharin adsorbs on iron-group metal surfaces. Evidence from SERS, IR, and NMR studies on Ni surfaces indicates that structures in which the carbonyl oxygen and imide ring nitrogen are bound to the metal and the benzene ring is oriented toward the solution can form [38, 23]. One would therefore expect the adsorption of saccharin to increase the hydrophobicity of the cathode surface which in turn should impede H2O reduction in particular. On the whole, our results indicate that Co–Ni deposition exhibits somewhat more anomalous behavior in the presence of saccharin. Saccharin forms both soluble and solid complexes with divalent iron-group metal ions. A factor contributing to the increased anomalous behavior could be the differences in the ability of Co(II) and Ni(II) to form these saccharinate complexes. It is known that Co(II) forms such complexes more readily than Ni(II) [39, 40]. This may favor the adsorption of Co(II)-containing complexes on the electrode and ultimately the deposition rate of cobalt. The appearance of peak III in the scan for bath 2 in Fig. 1 indicates that an oxide layer forms on the cathode surface when it is sufficiently polarized despite the presence of saccharin. This is confirmed in the steady state current-potential

curves for bath 2 when a current density of −800 A m−2 is applied. As in the additive-free case, the rise in jNi and jH 2 with potential becomes steeper as the current at which the oxide layer forms is approached. However, other features of the response suggest that the oxide layer differs in some respects and is less disruptive to metal deposition than in the additive-free bath. For one thing, higher currents and more cathodic potentials are required in order for the oxide layer to form when saccharin is present. The regions in the j–E curves for bath 2 in Fig. 5b where the oxide layer is present are less flat and prominent than they are for bath 1 (Fig. 5a). The presence of the oxide layer also appears to have a less polarizing effect in bath 2. Assuming that H+ reduction is proceeding under mass transfer limiting conditions, the generation rate of OH− due to H2O reduction at the onset of the formation of the oxide layer is estimated to be 1.76×10−3 mol m−2 s−1, which is about 36 % higher than the value obtained in the absence of saccharin. This suggests that it is more difficult for the oxide layer to form when the solution contains the additive. Interestingly, the reactivation of the oxide layer at −900 A m−2 occurs at the same potential (−1.35 V) as it does in bath 1. However, unlike the previous case, this does not lead to the termination of metal deposition and to domination by the HER. In fact, at −1,000 A m−2, jNi reaches a higher value than at any other current and the electrode is much less polarized than in the additive-free solution. Effect of saccharin in chloride-only solution Bath 3 is unique among the solutions in this study in that it is sulfate-free. As shown in Fig. 3, the Ni content in the coatings produced in bath 3 is slightly, but consistently, lower than that in bath 2 at virtually all current densities. The current efficiencies rise steeply to ∼90 % as the current density is raised from −50 to −200 A m−2. Over this range, the current efficiencies are larger in the chloride-only solution than in the corresponding sulfate-only solution (bath 2), similar to the effect reported by Horkans for Fe-Ni co-deposition over a similar current range [21]. This effect has usually been ascribed to the general ability of chloride to enhance metal deposition by forming an ion bridge between the metal ion and electrode. The study by Horkans did not involve current densities larger than −200 A m−2. However, when the applied current for the Co– Ni system in this study is raised above −200 A m−2, the behavior changes and the current efficiency declines as the applied current density is increased. The drop-off becomes catastrophic when current densities reach −1,000 A m−2. Examination of the steady state current-potential curves obtained in bath 3 shows that the electrochemical reactions are affected differently in the chloride-only solution (Fig. 6). The working electrode in bath 3 is more polarized than in baths 1 and 2 at all current densities up to and including −800 A m−2, a somewhat surprising result given that metal

J Solid State Electrochem -1200


-1000

j (A m-2)

-800 -600

-400 -200 0 -1.6

-1.4

-1.2

-1 E / V vs SHE

-0.8

-0.6

Fig. 6 Variation of total and partial current densities jCo, jNi, and jH 2 with steady state potential calculated respectively with Eqs. (1–3) for Co–Ni alloys formed on a copper disk electrode rotating at 1,000 rpm in bath 3 at pH 3

deposition tends to be accelerated when conducted in chloride solutions rather than sulfate solutions. The most likely explanation for the increased polarization in bath 3 is that it does not contain a supporting electrolyte, unlike baths 1 and 2. Of the three reactions, Co(II) reduction is affected the least relative to that observed in the sulfate solution (bath 2) up to current densities of ∼−900 A m−2. On the other hand, a change in the behavior of Ni(II) and the HER is observed when the steady state potential during deposition reaches approximately −1.0 V (current density approximately −400 A m−2). At more positive potentials, jNi is close in magnitude to that observed in bath 2, whereas jH 2 remains between −25 and −35 A m−2, suggesting that the HER proceeds principally as H+ reduction. However, when the potential becomes more negative than approximately −1.0 V, Ni(II) reduction is suppressed, while the HER is enhanced, suggesting that H2O reduction becomes the dominant source of H2 evolution at approximately −1.0 V. These effects lead to the somewhat lower Ni content in the coatings (Fig. 3) and the significant decline in current efficiency (Fig. 4) observed in the case of bath 3. The change in behavior at −1.0 V is consistent with the findings reported by Kwon and Gewirth [23] who showed from SERS analysis that saccharin desorbs from Ni electrodes in a chloride-only electrolyte at this same potential. As discussed in the “Effect of saccharin in sulfate-only solution” section and in previous studies [22, 38], one of the particular effects of the presence of saccharin on the electrode surface is to suppress H2O reduction. Thus, it is not surprising that H2O reduction would be enhanced and the current efficiency drop sharply at potentials more negative than −1.0 V where saccharin no longer adsorbs on the cathode. Other factors may make saccharin less effective in the chloride-only solution than in the sulfate-only solution at high overpotentials. From detailed analysis of the reaction products of saccharin reduction during Ni deposition, Mockute et al. [41] found that the presence of Cl− in the plating bath affects the manner in which the additive interacts with the electrode

by promoting adsorption via its carbonyl group and hindering adsorption via its sulfonyl group. The ability of Cl− and saccharinate to form anionic complexes with both Ni(II) and Co(II) and adsorb onto the metal surface during electrodeposition is not likely to be a significant factor in this study since the electrode potential during electrodeposition is usually more negative than the potential-of-zero charge of polycrystalline copper estimated to be −0.64 V vs SHE at pH 3.2 [42]. The formation of both chloride and saccharinate complexes of Ni(II) and Co(II) also tends to lower the concentration and/ or the net charge of cations bound to H2O ligands (i.e., 2+ Ni(H2O)62+ (H2O)2+ 6 , Co(H2O)6 ) and the ion-pairs NiSO4 − and CoSO4. Also, since Cl and C7H4NO3S− are negatively charged, the Ni(II) and Co(II) complexes will either be anionic or less positively charged than the Ni(II) and Co(II) species present in the sulfate-only or mixed sulfate-chloride solutions. This would have the effect of raising the energy barrier, between the negative working electrode and the anionic or less positively charged Ni(II) and Co(II) species during cathodic polarization particularly at high overpotentials and tend to slow metal deposition relative to that occurring in sulfateonly solutions. Since H2O reduction is not similarly affected, the current efficiency particularly at high overpotentials should be reduced, as observed in bath 3 (Fig. 4). As with baths 1 and 2, the features in the steady state j-E curve in Fig. 6 from −1.1 to −1.2 V are consistent with the presence of an oxide layer, in agreement with the drop in current following peak III in the potential scans for bath 3 in Fig. 2. As in the other cases, the oxide layer occurs only when H2O reduction is also occurring and is the dominant HER. The generation rate of OH− due to H2O reduction at the onset of oxide layer formation which appears to occur at a current density of approximately −600 A m−2 on the basis of Fig. 6 is estimated to be 1.91×10−3 mol m−2 s−1 and is close to the value obtained in bath 2. Once the steady state potential reaches approximately −1.2 V, the electrode is reactivated, but H2O reduction becomes dominant and metal deposition ceases almost entirely when a current density of −1,000 A m−2 is applied, similar to the situation in the additive-free solution (bath 1). Effect of saccharin in mixed sulfate-chloride solutions The variation in alloy content with applied current density obtained in the mixed sulfate-chloride bath 4 is compared to that of the other baths in Fig. 3. The trend observed in bath 4 is similar to that in the other solutions since the total metal concentrations in bath 4 are the same and Co(II) is present at much lower levels than Ni(II). Although the Ni content in the coating is somewhat higher than in the other saccharincontaining solutions at current densities up to −300 A m−2, the effect is not large. Thus, the dissolved Ni(II) and Co(II) concentrations in these solutions appears to play a larger role


in determining the alloy content than does the type of electrolyte or the presence or absence of saccharin. On the other hand, bath 4 stands out above the other solutions with regard to the current efficiency. A higher current efficiency is attained in bath 4 than in any other bath at current densities above −50 A m−2 (Fig. 4), reaching ∼80 % at −100 A m−2 and ∼90 % at −200 Å m−2. Particularly noteworthy is its ability to maintain the current efficiency close to and above 90 % at all current densities up to and including −1,000 A m−2. The significant improvement achieved in bath 4 becomes clearer upon examination of the steady state current-potential curve obtained during Co–Ni deposition (Fig. 7a) which exhibits 3 distinct regions at low overpotentials (−0.55 to −0.95 V), intermediate overpotentials (−0.95 to −1.2 V), and high overpotentials (−1.2 to −1.36 V). In the low overpotential region, jCo remains approximately the same as in the other saccharin-containing baths 2 and 3, whereas jNi is significantly higher (from ∼15 to 50 % more depending on the current). This rise in jNi comes at the expense of jH 2 which never exceeds −25 A m−2 over this current range, suggesting that the HER proceeds predominantly via H+ reduction.

-1200 -1000


a

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-1

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E / V vs SHE

-1200

b


j (A m-2)

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-1.4

-1.2

-1

-0.8

-0.6

E / V vs SHE

Fig. 7 Variation of total and partial current densities jCo, jNi, and jH 2 with steady state potential calculated respectively with Eqs. (1–3) for Co–Ni alloys formed on a copper disk electrode rotating at 1,000 rpm in the following baths described in Table 1: a bath 4 and b bath 5 at pH 3

The intermediate overpotential region is marked by a significant increase in electrode polarization. Although such behavior is observed in the other baths, some differences are evident in this case. Very little H2O reduction is occurring in this region and neither Ni(II) nor Co(II) reduction is suppressed. Thus, it is most reasonable to conclude that the effects in this intermediate region can be attributed to an adsorbed saccharin film. Such a conclusion is supported by the results of Brankovic et al. [22] showing that saccharin forms a condensed film on Co–Fe–Ni alloy surfaces at these potentials. The most significant difference in the behavior of the system in bath 4 is observed in the high overpotential region. As in the previous baths, when enough current is applied, the polarizing effects that dominate in the intermediate region are overcome to reactivate the electrode. However, unlike the previous cases, it is not water reduction that consumes the majority of the additional current applied in the high overpotential region. Instead, the water reduction remains suppressed and jH 2 never exceeds −100 A m−2 even at current densities as high as −1,000 A m−2. On the other hand, Ni(II) reduction in particular and Co(II) reduction to a lesser extent rise significantly and reach levels not reached in the other baths. No evidence of desorption of saccharin at high overpotentials is observed on the basis of these electrode responses and alloy composition, unlike the situation in the chloride-only bath 3. Bath 5 is a mixed sulfate-chloride solution without any saccharin. Comparison of the deposit composition, current efficiency and partial current densities in baths 4 and 5 should enable the role of saccharin in this mixed electrolyte to be more clearly delineated. Coatings produced in the absence of saccharin in the mixed sulfate-chloride solution are richer in Ni than those generated in its presence (bath 4) at all current densities of −200 A m−2 and above, a similar trend to that observed in the sulfate-only solutions (baths 1 and 2) in Fig. 3. In the case of current efficiency, the behavior of Co–Ni deposition in bath 5 up to approximately −400 A m−2 differs markedly from that observed at higher current densities (Fig. 4). Similar current efficiencies in baths 4 and 5 are attained at lower currents. However, when the current density is raised to −500 A m−2 and above, the current efficiency drops to 40–50 % which is well below that maintained in bath 4. Further evidence of this abrupt change in behavior comes from the steady state j-E curves obtained in bath 5 (Fig. 7b). At potentials more positive than ∼−1.0 V, the partial currents are approximately the same as in bath 4. However, once the potentials becomes more negative than approximately −1.0 V, both jCo and jNi are reduced approximately in half to approximately −35 and approximately −180 A m−2, respectively, and jH 2 rises sharply to close to ∼−300 A m−2, reflecting that a significant amount of H2O reduction is now occurring. This rapid onset of H2O reduction occurs when jCo


and jNi are still relatively low and well below their mass transport-limiting levels. Thus, it is not necessary that Co(II), Ni(II), and H+ reduction all become rate-limited before the rate of H2O reduction becomes significant. Although jNi rises with a further increase in the applied current, it always remains below −400 A m−2 even at the highest current. No significant change in jCo is measured, on the other hand, so that it remains between −30 and −40 A m−2. However, only jH 2 rises significantly with further rise in current so that H2O reduction becomes the dominant cathodic reaction at high overpotentials. This behavior contrasts strongly with that observed in bath 4 where H2O reduction is strongly suppressed and metal deposition is enhanced at intermediate overpotentials. Such a difference is consistent with that observed in the sulfate-only solutions (baths 1 and 2) and findings from previous studies that saccharin interacts most strongly with the working electrode at potentials close to −1.0 V, acts primarily to suppress H2O reduction and in fact may facilitate metal deposition. Another consistent trend evident from the differences observed in baths 4 and 5 is the effect of saccharin on Co(II) reduction. As observed in the sulfate-only solutions, Co(II) reduction is consistently enhanced by the presence of saccharin in the mixed sulfatechloride bath, particularly at intermediate and high overpotentials. The results shown in Figs. 3 and 4 are promising from a practical point of view. By carrying out electrodeposition in solutions with the same composition as bath 4, one can achieve high current efficiencies that are largely independent of the applied current density over a wide range from approximately −200 to −1,000 A m−2. The choice of the current density can then be used to control the composition of the resulting coating. As indicated in Fig. 3, any desired alloy composition between ∼70 and 90 wt% Ni can be obtained by selection of the appropriate current density between −200 and −1,000 A m−2.

Conclusions This study focused on the effects of sulfate, chloride and saccharin on the electrodeposition of Co–Ni alloy films in synthetic solutions with metal concentrations similar to that produced by leaching of spent Ni–Cd battery components. The electrode responses during galvanostatic deposition provide evidence supporting findings from previous studies that saccharin adsorbs on the surface at potentials of approximately −1.0 V. From analysis of the amount and composition of deposited metal, the main effect of this adsorption is to inhibit H2O reduction at higher overpotentials, thereby facilitating metal deposition and widening the potential and current

ranges over which the electrodeposition of Co–Ni alloys can be effectively accomplished. Regardless of the nature of the electrolyte and the presence or absence of saccharin, deposition follows anomalous behavior at all current densities in every bath, although it approaches normal behavior as the current density increases toward −1,000 A m−2 and Co(II) reduction becomes mass transfercontrolled. The presence of saccharin tends to lower the Ni content and promote more anomalous behavior to a relatively small extent that remains essentially constant across a wide range of current densities from −100 to −1,000 A m−2. Higher deposition current efficiencies are achieved in a mixed sulfatechloride solution containing saccharin than in a sulfate-only solution containing saccharin, a chloride-only solution containing saccharin or a sulfate-only solution containing no additive. Differences in the effect of the current density on the current efficiency are evident at intermediate-to-high overpotentials depending on the bath composition. Acknowledgments The authors are indebted to the CONACyT (Mexico) Grants No. 2012–183230 and 205416–2013 for financial support to carry out this research.

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