- Email: [email protected]
Pulsed electrodeposition of nickel hexacyanoferrate films for electrochemically switched ion exchange
Separation and Purification Technology 63 (2008) 407–414
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Pulsed electrodeposition of nickel hexacyanoferrate films for electrochemically switched ion exchange Xiaogang Hao a , Yongguo Li a , Mark Pritzker b,∗ a b
Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1
a r t i c l e
i n f o
Article history: Received 2 February 2008 Received in revised form 2 June 2008 Accepted 3 June 2008 Keywords: NiHCF Thin films Pulse electrodeposition Electrochemically switched ion exchange Cyclic voltammetry
a b s t r a c t Nickel hexacyanoferrate (NiHCF) thin films were prepared on graphite substrates by pulse electrodeposition and investigated for use as electrochemically switched ion exchange (ESIX) materials. The pulse waveforms investigated consisted of an applied potential during the on-period and zero current during the off-period. Cyclic voltammetry (CV) was used to study the effects of the pulse potential and off-time on the ion-exchange capacity of NiHCF film electrodes. The cycle life and regenerability of the film electrodes were also investigated. Experimental results show that the optimum pulse potential, on/off-period (ton /toff ) and duty cycle are 0.3 V (vs. SCE), 300 ms/300 ms and 50%, respectively. The deposited NiHCF is uniformly distributed over the surface of the graphite and yields thin film electrodes with high ion exchange capacity for K+ , good stability and regenerability. The films produced by pulse deposition were also found to have superior exchange capacity and stability to those produced by the more common CV method. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Electrochemically switched ion exchange (ESIX) is an environmentally benign method to separate ions via reversible electrochemical modulation of the matrix charge density [1–3]. Ion loading and unloading can be easily controlled by changing the redox states of ion exchange thin films formed on conductive substrates to separate ions from mixed solutions and regenerate the matrix. Since electrochemical rather than chemical potential modulation is the main driving force for exchange [4], the secondary waste created by chemical regeneration and associated rinse water is eliminated. Thus, ESIX has the potential to replace traditional ion exchange and so has recently garnered coinsiderable interest [5–10]. Nickel hexacyanoferrate (NiHCF, formula Ah Nik [Fe(CN)6 ]l ·mH2 O, h, k, l, m = stoichiometric numbers, A = alkali metal cation), an inorganic coordination compound with an open, zeolite-like structure, is an excellent candidate for ESIX of alkali cations because of its differing affinity for alkali cations (Cs+ > Rb+ > K+ > Na+ > Li+ ) [11]. NiHCF thin films have been prepared on conducting substrates by many methods, such as the oxidation of nickel in ferricyanide solutions [1,12,13], cathodic deposition from marginally stable electrolytes containing divalent
∗ Corresponding author. Tel.: +1 519 888 4567x32542; fax: +1 519 746 4979. E-mail address: [email protected] (M. Pritzker). 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.06.001
nickel and ferricyanide [3,4,14–17] or chemical deposition from mixtures of divalent nickel and ferrocyanide salts [5–7,18,19]. In previous studies [3,6], NiHCF films were deposited on graphite substrates by cyclic voltammetric (CV) electrodeposition and found to have excellent selectivity for Cs+ in concentrated K+ - and Na+ -containing solutions, but the deposited films were not very uniform and dense. Therefore, its adhesion to the substrate was not stable and the ion exchange capacity low. Pulsed electrodeposition has been applied to deposit metal [20], alloys [21–26] and semiconductor [27] thin films because of the small grain size and high deposit quality that can be achieved. The additional operating variables such as pulse waveform, on/off pulse time or duty cycle, applied and mean current density, offer effective ways to control macroscopic properties such as morphology [20–22], electronic properties through crystallinity and elemental composition [22,23], electrical conductivity [24,25], porosity and adhesion [26,27]. Few reports are available that demonstrate the growth of NiHCF films by pulsed electrodeposition. Most commonly, this technique involves the application of unipolar or bipolar current or potential waveforms. In the case of unipolar pulse deposition, a cathodic potential or current is applied during a portion of the pulse to deposit the film followed by a relaxation period in which no current is allowed to flow. In the case of bipolar pulses, the waveforms consist of a cathodic potential or current followed by an anodic potential or current where oxidation occurs. The objective of this study is to use unipolar pulse deposition to produce NiHCF films on
408
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
graphite substrate with high cation-exchange capacity and stability and to investigate the effects of some pulse parameters on the resulting ESIX performance of the film electrode.
3. Results and discussion
2. Experimental
Fig. 1 shows typical potential–time and current–time transients obtained during pulse deposition with an on-potential of 0.3 V, ontime of 0.3 s and off-time of 0.3 s over 2000 cycles (corresponding to 1200 s deposition time). The responses shown in this figure correspond to the first 15 s (Fig. 1a and b) and to the final 15 s of pulse deposition (Fig. 1c and d) of a NiHCF film. At the start of the process prior to the formation of any NiHCF film, the open circuit potential on the graphite electrode is approximately 0.74 V. As pulse deposition begins and the film starts to form, the open circuit potential measured during the off-time decreases rapidly from this value to approximately 0.55 V and then decreases more gradually over time as the film thickens (Fig. 1a). After 15 s of deposition, the open circuit potential during the off-time has reached 0.46 V. By the end of the 2000 cycles (∼1200 s elapsed time), only a small amount of deposition is occurring during the on-times. This is reflected by the fact that the maximum current reached during the final pulse cycles has decreased to −0.16 mA. At the same time, the open circuit potential during the off-time has decreased to approximately 0.34 V and is now decreasing only very slowly. This leaves an overpotential of only −0.04 V when a potential of 0.3 V is applied during the on-time. On the basis of the other experiments of this study, this trend where the open circuit potential during the off-time decreases and approaches the value of the applied potential during the on-time as the film thickens is always observed to occur during pulse deposition regardless of the applied on-potential. The formation of NiHCF thin films on the surface of graphite was confirmed by comparison of SEM images of these samples before and after pulse electrodeposition. Energy-dispersive X-ray spectroscopy (EDS) analysis also confirmed the presence of iron and nickel on the surface of the graphite substrate. Furthermore, XPS revealed the appearance of Fe2p peaks at 710 eV, Ni2p at 860 eV and N1s at 400 eV only after pulsed deposition. The K2p peak also appeared, providing evidence that the as-prepared NiHCF is a Kcontaining compound. This is not surprising since potential cycling of the NiHCF film was conducted in a K+ -containing solution before the XPS scan. The peak current–time transients recorded during electrochemical deposition of NiHCF thin films at different on-potentials during each pulse are shown in Fig. 2. Only the maximum current measured during each pulse cycle is plotted in this figure. The peak current sharply decreases within the first few seconds and then remains relatively steady during the remainder of the pulses. In pulse electrodeposition process, the peak current during the ontime corresponds to the reduction of Fe(CN)6 3− to Fe(CN)6 4− . The rapid fall of the peak current in the first few seconds may be due to the combined effects of double layer charging and transport limitations of reactants in the boundary layer at the electrode surface and through the NiHCF film as it forms. Double layer charging influences the electrode response at the start of each pulse due to the periodic change in electrode potential. If the double layer structure returns to the same structure at the start of each pulse as it had at the start of the previous ones, then double layer charging will have no effect on the decrease in peak currents observed in Fig. 2 at the outset of deposition. However, if there is a gradual and continual change in double layer structure from one pulse to the next, then double layer effects could play a role in the trends observed in Fig. 2. Such an effect will eventually disappear and thereafter the double layer will only play a role in the electrode response during the course of a given pulse. An appreciable non-zero peak current is still observed after 1200 s of pulse deposition (Fig. 1d). This behaviour differs from
2.1. Experiment instrument and reagents All reagents were analytical grade and all solutions prepared using Millipore water (18.2 M cm). The electrochemical experiments were performed using a VMP2 Potentiostat (Princeton, USA) controlled with EC-Lab software. A three-electrode system was used with a graphite-working electrode and a platinum sheet counter electrode. All reported potentials are referenced to a saturated calomel electrode (SCE). SEM images were obtained using an American KEVEX SIGAMA unit, while XPS scans were acquired with a VG Scientific ESCALab250i-XL unit. 2.2. Preparation of film electrode Graphite rods were chosen as substrates for the pulsed deposition experiments. A 4 mm diameter rod was first mechanically polished using SiC abrasive paper, then rinsed with deionized water and dried in air. A 4 cm2 effective surface area was exposed for deposition at room temperature under quiescent conditions. The pulsed deposition experiments were performed in freshly prepared 0.002 mol L−1 NiSO4 , 0.002 mol L−1 K3 Fe (CN)6 and 0.25 mol L−1 Na2 SO4 mixed solution. Pulsed electrodeposition involves the application of a periodic current density or potential waveform to the working electrode. In this study, each pulse consisted of an on-time ton when a cathodic potential was applied and an off- time toff when no current was allowed to flow. In particular, the potential waveforms consisted of on-potentials (or pulse potentials) Von ranging from 0.2 to 0.7 V followed by a zero current density during the off-time. The ontime was fixed as 0.3 s, while off-times of 0.1, 0.2, 0.3 and 0.4 s were used, thereby yielding duty cycles of 75%, 60%, 50% and 43%, respectively. The number of deposition pulse cycles to generate the films was varied from 1000 to 5000. Every experiment in this study was repeated at least two times and the results obtained found to be very reproducible. For comparison with the more common CV method, NiHCF deposits on graphite electrodes were also produced by cycling the electrode potential between 0 and 1200 mV at 25 mV s−1 in freshly prepared 0.002 mol L−1 NiSO4 , 0.002 mol L−1 K3 Fe(CN)6 , and 0.25 mol L−1 Na2 SO4 mixed solution. Deposition was continued for 12 cycles to produce the film. The film was then thoroughly rinsed with water and dried. 2.3. Performance experiments on NiHCF film electrode After film growth, each electrode was cycled 25 times in a 1 mol L−1 KNO3 solution between the potentials of 400 and 1200 mV at 25 mV s−1 and the capacity of NiHCF film electrodes was determined by measurement of the charge associated with oxidation and reduction. The samples were rinsed with deionized water and dried in air. Then the morphology and composition of the film were characterized by scanning electron microscopy (SEM) and Xray photoelectron spectroscopy (XPS). The regenerability of the film electrode was assessed by cycling a Cs-loaded film between the potentials of 400 and 1200 mV at 25 mV s−1 in a 1 mol L−1 KNO3 solution. The cycle life was assessed by carrying out 500 repetitive voltammetric cycles on a K-loaded film in 1 mol L−1 KNO3 at a scan rate of 25 mV s−1 .
3.1. Transient curves during pulsed deposition
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
409
Fig. 1. (a and c) Potential–time and (b and d) current–time transients during pulse deposition of NiHCF from 0.002 mol L−1 NiSO4 , 0.002 mol L−1 K3 Fe (CN) 6 and 0.25 mol L−1 Na2 SO4 mixed solution. The pulse conditions include 0.3 V pulse potential, 0.3 s on-time and 0.3 s off-time.
what would be expected for a diffusion-controlled process in a quiescent solution when polarization occurs under DC conditions. This points at one of the advantages of pulsed electrolysis over DC electrolysis. The application of pulses provides a relaxation period during the off-time when the reactants Fe(CN)6 3− and Ni2+ for film formation can be replenished in the boundary layer region or within the film. Another contributing factor is that NiHCF films are sufficiently electronically and ionically conducting that they do not
completely passivate as they grow [4,28]. However, the resistance to the flow of current does gradually increase as the film thickens. Perhaps the most surprising observation concerning Fig. 2 is the effect of the pulse on-potential on the peak currents, particularly over the early part of polarization. One would expect the magnitude of the peak current due to the reduction of Fe(CN)6 3− to rise as the pulse on-potential becomes more negative. Yet, as the results indicate, the peak current is higher and decreases more gradually with time at 0.3 V than at 0.2 or 0.4 V. Even at later times when the magnitude of the peak current has levelled off, it remains higher at 0.3 V. Although the current is not a direct measure of the rate of NiHCF film formation, the trend observed in Fig. 2 may be related to the effect of potential on the rate of film formation, as will be discussed in Section 3.2.2. 3.2. Effect of pulse parameters on ion exchange capacity The ion-exchange capacity of NiHCF film is one of the key properties for its successful application as an ESIX material. CV is a useful tool for determining both the redox site capacity of electroactive materials and the changes in capacity after numerous repeated loading/unloading cycles. The capacity or relative electroactivity was explored straightforwardly by comparing the areas under the cyclic voltammograms obtained on NiHCF film electrodes previously prepared under different pulse deposition conditions, as discussed in the following sub-sections.
Fig. 2. The effect of pulse potential on variation of the peak currents reached during pulse deposition of NiHCF from 0.002 mol L−1 NiSO4 , 0.002 mol L−1 K3 Fe (CN) 6 and 0.25 mol L−1 Na2 SO4 mixed solution. The on-time is 0.3 s and the duty cycle is 50%. The inset is an enlargement of the initial 17 s of the current–time curves.
3.2.1. Effect of pulse off-time Fig. 3 shows the voltammograms of films that had been prepared using a pulse potential of 0.3 V and different pulse off-times. The voltammograms shown in Fig. 3 correspond to the 25th cycle
410
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
Fig. 3. Cyclic voltammograms obtained in 1 mol L−1 KNO3 showing the effect of the pulse off-time on the capacity of NiHCF films. The pulse conditions used to prepare the films include 0.3 V pulse potential, 0.3 s on-time and (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4 s off-times. The voltammograms were obtained at a 25 mV s−1 scan rate.
obtained after the film electrodes were immersed in 1 mol L−1 KNO3 solution. Each of the NiHCF films was produced by deposition for 2000 cycles with a constant pulse on-time of 0.3 s and different pulse off-times between 0.1 and 0.4 s. The positive currents in Fig. 3 correspond to the oxidation of the NiHCF film and K+ deintercalation from the matrix, while negative currents are due to the reduction of the film and K+ intercalation [4]. Fig. 3 shows that the variation of toff leads to quite different reversible charge densities and CV shapes. Similar to that shown in previous studies [14,18,29,30], potential cycling in the presence of a K+ -containing electrolyte leads to the appearance of two distinct current peaks during both the positive- and negative-going scans. Principal component analysis (PCA) of combined CV-EDS data in a previous study showed the direct link between the appearance of multiple peaks for K+ intercalation/deintercalation and non-stoichiometry of the NiHCF lattice [18]. Energy dispersive X-ray spectroscopy has shown that when the more negative anodic and cathodic peaks are large, the more positive anodic and cathodic peaks and the K+ content in the film tend to be small [14,30]. Conversely, in CVs obtained on films with high K+ content, the more negative peaks are found to be smaller than the more positive ones. As shown in Fig. 3, the charge associated with the more negative peaks rises relative to that of the more positive peaks as toff increases. This trend suggests that this change in pulse operating conditions tends to lower the potassium content in the film that results when it undergoes 25 potential scan cycles in the presence of 1 mol L−1 KNO3 . Another effect observed is a small shift of the cathodic peaks to lower potential with an increase in toff . For the purposes of this analysis, the average of the integrated charge during the anodic and cathodic scans obtained for the film immersed in 1 mol L−1 KNO3 was used as a measure of the ion exchange capacity of the films. As clearly evident in Fig. 3, the area under the voltammograms and ion exchange capacity increases as toff rises from 0.1 to 0.3 s, but then decreases with further increase to 0.4 s. In pulse electrodeposition, the growth of NiHCF films involves the reduction of Fe(CN)6 3− to Fe(CN)6 4− followed by reaction of Fe(CN)6 4− with Ni(II) on the electrode. The observed trend in the ion exchange capacity in Fig. 3 can be explained in terms of the role that the pulse off-time has on the concentration profiles of reactants in the vicinity of the electrode. The off-time provides a relaxation period to replenish the concentration of reactants Fe(CN)6 3− and Ni(II) after being depleted during the previous on-time. In this way,
the use of pulse electrolysis tends to reduce mass transfer limitations that can arise during DC electrolysis. Presumably, for the rate of consumption of Fe(CN)6 3− and Ni(II) that occurs due to the application of 0.3 V for 0.3 s during the on-time, an off-time of 0.1 s is insufficient to replenish these reactants and so the rate of Fe(CN)6 3− reduction and NiHCF film formation are limited during the subsequent on-time. If this is the case, one would expect a longer off-time to increase the amount of film that forms over the duration of 2000 pulse cycles, as observed for pulse off-times of 0.2 and 0.3 s in Fig. 3. When the off-time is made long enough, further increases in toff should no longer have any effect. However, as observed in Fig. 3, a rise in toff from 0.3 to 0.4 s causes a drop in the amount of film formed and the ion exchange capacity of the film. This effect is confirmed by the current–time transients obtained during pulse deposition when the films were formed. Consequently, at least another factor must be playing a role to explain the observed trends in Fig. 3. Although no net current flows during the off-time, it is possible that redox reactions still occur in the film. Monitoring of the current–time response during pulse deposition with an on-potential of 0.3 V, on-time of 0.3 s and off-times of 0.3 and 0.4 s showed that the electrode potential reached during the off-time over the course of 2000 pulse cycles is in the range of 0.3–0.5 V. Separate experiments on the Fe(CN)6 3−/4− system on a graphite electrode in the absence of a NiHCF film also indicated that the reduction of Fe(CN)6 3− to Fe(CN)6 4− is already occurring when the electrode potential is between 0.3 and 0.4 V. Thus, it is possible that during each pulse off-time, a redox reaction involving the reduction of Fe(CN)6 3− and the oxidation of some of the pre-existing NiHCF film can take place. The contention that NiHCF can be oxidized during the off-time is supported by an estimate of its Nernst potential under the conditions prevailing during pulse deposition. The Nernst potential is obtained by correcting the formal potential for the concentrations of the dissolved reactants and products of the electrode reaction. Although there is some uncertainty as to the precise reaction that occurs during NiHCF oxidation (for example, see [14] and [18] for detailed analysis of this aspect), there is no disagreement that it involves the release of the intercalating cation and that the stoichiometric ratio of the intercalating cation and electrons in the half-cell reaction is 1:1. Thus, correction of the formal potential to yield the Nernst potential is given by the amount RT/F ln[Cj ], where [Cj ] is the concentration of the intercalating cation in the solution. Since the solution under question in our pulse deposition experiments contains much more Na+ than K+ , then Na+ is considered to be the intercalating ion. The formal potential for NiHCF oxidation when Na+ is the intercalating ion has been reported to be 0.361 V SCE [31]. Thus, for our solution containing 0.25 mol/L Na2 SO4 , the Nernst potential for NiHCF reduction is estimated to be 0.343 V SCE. As shown in Fig. 1, the electrode potential during the off-time remains above this value over the course of the 2000 pulse cycles. Thus, on the basis of this estimation of the Nernst potential, one would expect NiHCF to be oxidized during the off-time period of the pulse cycles, as proposed. If this is the case, some of the cathodic current flowing during each on-time would be used to re-reduce the portion of the film oxidized during the previous off-time and so not contribute to the formation of new film. This would partially counteract the positive effect that pulse deposition has on replenishing the reactants Fe(CN)6 3− and Ni(II). Furthermore, as the duration of the off-time increases, the influence of such a redox process would tend to grow, whereas the effect of replenishing the reactants would level off. The net result would be a lowering of the ion exchange capacity if the off-time becomes long enough, as observed in Fig. 3. Consequently, on off-time toff of 0.3 s could turn out to be the optimum for the deposition of high capacity NiHCF films at a 0.3 s on-time and 0.3 V on-potential.
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
Fig. 4. Cyclic voltammograms obtained in 1 mol L−1 KNO3 showing the effect of the pulse potential on the capacity of NiHCF films. The pulse conditions used to prepare the films include 0.3 s on-time, 0.3 s off-time and (a) 0.2, (b) 0.3, (c) 0.4, (d) 0.5 and (e) 0.7 V pulse potentials. The voltammograms were obtained at a 25 mV s−1 scan rate.
3.2.2. Effect of pulse potential The pulse potential is one of the main operating parameters of this technique. Therefore, it is important to choose the appropriate potential to produce high quality films. Fig. 4 shows the CV curves obtained after 25 cycles in 1 mol L−1 KNO3 of a series of NiHCF films that were previously formed using different pulse potentials of 0.2, 0.3, 0.4, 0.5 and 0.7 V, while maintaining the duty cycle, on-time and duration constant at 50%, 0.3 s and 2000 cycles, respectively. As can be seen from Fig. 4, the pulse potential has large influence on the ion exchange capacity. For a pulse with ton = 0.3 s and toff = 0.3 s, the film electrode prepared with a 0.3 V pulse potential during the on-time exhibits a larger CV area and ion exchange capacity than the films produced at the other potentials. This observation is consistent with the result in Section 3.1 and Fig. 2 that the on-time peak current measured during film formation is higher at a pulse potential of 0.3 V than at 0.2 or 0.4 V. Judging from the different shapes of the CVs in Fig. 4, pulse potential also appears to have some influence on the film composition and structure. The effect of pulse potential appears to be complex. As stated earlier, film formation requires the reduction of Fe(CN)6 3− followed by the reaction of Fe(CN)6 4− and Ni(II). As the pulse potential becomes more negative, one would expect the rate of reduction of Fe(CN)6 3− and film formation to increase. Thus, the finding that the apparent exchange capacity of the film produced at a pulse potential at 0.3 V is greater than that produced at 0.4, 0.5 or 0.7 V is wholly consistent with this expectation. In view of this, the observation from Fig. 4 that the capacity of the film formed at a pulse potential of 0.3 V is higher than that of one produced at a more negative potential of 0.2 V is surprising. Also unexpected is the finding that the capacity obtained at the most negative pulse potential is lower than at any of the other potentials studied. Further research is required to gain more insight into this aspect of the effect of pulse potential on film formation. Some comments are also worth noting regarding the results obtained at a pulse potential of 0.7 V which suggest some differences in film formation from that at the lower pulse potentials. Based on the results obtained at the lower potentials, it is somewhat surprising that a NiHCF film can actually form at such a high potential. However, examination of the current–time transients shows that the open circuit potential reached during the off-time after about 1000 s of deposition has leveled off at approximately 0.72 V,
411
Fig. 5. The effect of the number of pulse deposition cycles on the ion-exchange capacity of NiHCF films produced at 0.3 V pulse potential, 0.3 s on-time and 0.3 s off-time.
much higher than the values reached at the corresponding stage of deposition when lower pulse potentials are applied. Also, some differences are evident in the CV response in Fig. 4 for the film formed at a pulse potential of 0.7 V. For one thing, only a single peak appears during the anodic portion of the scan. Although two cathodic peaks occur, the more positive peak is much smaller than the other and, in fact, appears only as a shoulder. The more negative peak is also shifted significantly in the cathodic direction relative to those observed at the other pulse potentials. This leads to a larger separation (Ep ) between the anodic and the cathodic peak potentials in the CV. Large Ep values have been attributed to diffusion limitations inside the dense film during the cation-exchange process [7]. Difficulties associated with the diffusion of K+ into and out of the dense microstructure of the electrode may limit the rates of oxidation and reduction of the NiHCF compound. This suggests that the film produced on graphite at a pulse potential of 0.7 V is more densely packed than the films produced at the other potentials investigated in this study.
3.2.3. Effect of pulse deposition time Fig. 5 shows the variation in the ion-exchange capacity as the number of pulse deposition cycles is raised. It is observed that the capacity rises with an increase in the number of cycles, suggesting that the ion-exchange capacity can be increased by lengthening the duration. The increase of the ion-exchange capacity is nearly linear over the first 1500 cycles, but slows down thereafter. Films have been prepared with capacities as high as 27.5 mC cm−2 (corresponding to 430 nm thickness) after 4500 cycles. Of course, as the film becomes thicker, the resistance to Fe(CN)6 3− reduction and NiHCF formation increases and film growth slows down. It is worth noting that this capacity achieved by pulse deposition exceeds the levels that have previously been reported for thin film NiHCF electrodes. Capacities of about 5 mC cm−2 are typical of anodically prepared films [12], whereas capacities in the range 10–15 mC cm−2 have been obtained for cathodically deposited films [4]. Recently, a small improvement to values of 16–17 mC cm−2 has been reported for NiHCF nanotube electrodes fabricated using porous alumina templates [9]. Perhaps the most success has been achieved using three-dimensional porous composite electrodes such as those consisting of SiO2 , graphite and NiHCF [7], which have considerably higher capacity than the thin film electrodes presented in the current and other studies.
412
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
Fig. 6. (a) Cyclic voltammogram obtained in 1 mol L−1 KNO3 for film that has only been in the K-form. Cyclic voltammogram for film originally in Cs-form during (b) 25th and (c) 1st cycle after immersion in 1 mol L−1 KNO3 . The scan rate is 25 mV s−1 . Fig. 7. Cyclic voltammograms of NiHCF films in 1 mol L−1 KNO3 obtained over 500 repeated potential cycles. The scan rate is 25 mV s−1 .
3.3. Electrochemical regeneration of NiHCF thin film In ESIX, the loading/unloading of metal ions on a NiHCF thin film can be easily achieved electrochemically by controlling the transition between reduced and oxidized states of NiHCF. In this way, a metal ion can be selectively removed from an aqueous solution. However, an important engineering aspect to consider is the ability to regenerate these films back to their original form and determine whether this leads to a loss in ion exchange capacity. To assess the ability to regenerate the films produced by pulse deposition, we conducted an experiment similar to that described previously [4]. First, a graphite substrate with a freshly prepared NiHCF coating was converted to Cs-form by immersing it in a 1 mol L−1 CsNO3 solution and subjecting its potential to 25 scan cycles between 400 and 1200 mV. Analysis of the film by EDS showed that Cs was the only alkali cation that could be detected, confirming that this procedure was able to completely convert the film to Cs-form. The film electrode in Cs-form was then immersed in a 1 mol L−1 KNO3 solution containing no Cs+ and converted back to K-form by cycling its potential between 400 and 1200 mV for 25 times. Fig. 6 shows the voltammograms of a film originally in the Cs-form during its regeneration back to the K-form in this way. The CVs for the regenerated film shown in Fig. 6 correspond to the 1st (curve c) and 25th (curve b) cycles obtained after immersion in the KNO3 solution. Also shown in this figure is the corresponding CV for a film that has only been in the K-form (curve a). After only one regeneration cycle, the response resembles the CV of the film that has only been in the K-form, indicating that regeneration occurs fairly readily. Furthermore, virtually all of the Cs+ has been removed from the film by the 25th cycle. When regeneration is extended beyond 25 scan cycles, the CV of the resulting film appears even closer in shape to that of curve a than what appears in Fig. 6. After regeneration for 25 cycles, the films formed by pulse deposition retain about 90% of the original ion-exchange capacity of the K-form film. Although not shown here, the rise in the ion exchange capacity of the regenerated NiHCF film with increase in the number of voltammetric cycles occurs without loss of good redox reversibility, as has been shown previously for films formed by the conventional CV method [3,4]. 3.4. Cycle life of NiHCF thin film The cycle life is another important property of ESIX films. The application of voltammetric cycles mimics the situation of repeated redox switching in an ESIX process and provides a measure of cycle
stability. The change in CVs obtained on NiHCF films produced under the optimum pulse deposition conditions determined in the previous sections were investigated in 1 mol L−1 KNO3 by scanning at 25 mV s−1 over 500 cycles. Fig. 7 presents the responses obtained over this range of cycles. Integration of the CV areas indicates that the NiHCF films lose only 10% of their capacity over the first 250 cycles and then virtually no more over the remaining 250 cycles. Comparison of these results with those of previously reported NiHCF films produced by the CV method shows that pulsed deposition significantly improves redox cycling stability and capacity for cation uptake and elution [6]. Also note that the two distinct cathodic peaks and two distinct anodic peaks disappear and only one broad peak remains with increase of the cycle number. This may be caused by structural modifications in the NiHCF film with cycling [28]. 3.5. Comparison of pulse and CV methods As discussed above, the optimum operating conditions for pulse deposition of NiHCF film electrodes with high cation-exchange capacity and good stability involve a pulse potential of 0.3 V and ton /toff period = 0.3/0.3 s. A film electrode synthesized on graphite under these conditions for 2000 cycles (total duration of 20 min including both ton and toff ) was compared to a film electrode obtained by the more traditional CV method. To provide a similar total duration for deposition, the CV method involved 12 consecutive linear potential scan cycles between 0 and 1200 mV at a rate of 25 mV s−1 (total duration of 19.2 min). The capacities of the two films produced in this way were then determined by obtaining their cyclic voltammograms in 1 mol L−1 KNO3 at a scan rate of 25 mV s−1 . The results shown in Fig. 8 indicate that the cation-exchange capacity of the NiHCF electrode prepared by pulse electrodeposition (curve a), as determined from the area under the redox peaks, is approximately three times higher than that of the electrode produced by CV electrodeposition (curve b). Fig. 9 shows the change in normalized ion exchange capacity of NiHCF films produced by the two deposition methods as they undergo 500 voltammetric cycles in 1 mol L−1 KNO3 solution. The capacity values in each case are normalized by their corresponding initial capacities, as measured in the 10th voltammetric cycle, to partly account for differences in the initial states of the two films. The preparation method of NiHCF is known to affect
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
413
systems so produced are suitable for electrochemically controlled ion separation processes. Experiments showed the ESIX performance of pulse deposited NiHCF film electrodes to have superior capacity and stability than film electrodes formed by the more common CV method. Acknowledgments Professor Hao gratefully acknowledges support by the Shanxi Scholarship Council of China to spend a half year at the University of Waterloo. This research was financially supported in part by the National Natural Science Foundation of China (no. 20676089) and Natural Science Foundation of Shanxi Province (no. 2007011029). References
Fig. 8. Cyclic voltammograms obtained in 1 mol L−1 KNO3 of NiHCF films prepared on graphite substrates by (a) pulsed electrodeposition and (b) CV electrodeposition. The scan rate is 25 mV s−1 .
Fig. 9. Normalized charge capacity as a function of cycle number for NiHCF film electrode prepared by (a) pulsed electrodeposition and (b) CV electrodeposition.
the redox cycling stability of the electroactive materials, as is also demonstrated in Fig. 9. A better performance is observed for the film electrode prepared by pulse electrodeposition, which is able to retain about 87% of its initial capacity after 500 cycles. On the other hand, the loss of capacity of NiHCF films formed by CV electrodeposition is significant and only 58% of the initial capacity remains. On the basis of these results, NiHCF film electrodes produced on graphite substrates by pulse electrodeposition appear to have higher stability than those produced by CV, leading to significantly improved cycling stability. 4. Conclusions Electrodeposition of NiHCF thin films has been carried out using a pulsed method with the objective to improve the capacity and the stability of the film electrodes. The pulse waveforms investigated consist of an applied electrode potential during the on-period followed by an off-period in which the system was allowed to relax under open circuit conditions. The following optimum pulse variables to produce NiHCF film electrodes with enhanced capacity and stability have been obtained: 0.3 V pulse potential, 0.3 s on-time and 0.3 s off-time. It has been demonstrated that the film-electrode
[1] S.D. Rassat, J.H. Sukamto, R.J. Orth, M.A. Lilga, R.T. Hallen, Development of an electrically switched ion exchange process for selective ion separations, Sep. Purif. Technol. 15 (1999) 207–222. [2] Y.H. Lin, X.L. Cui, Electrosynthesis, characterization, and application of novel hybrid materials based on carbon nanotube–polyaniline–nickel hexacyanoferrate nanocomposites, J. Mater. Chem. 16 (2006) 585–592. [3] X.G. Hao, J.X. Guo, Z.L. Zhang, S.B. Liu, Y.P. Sun, Electrochemically switched ion exchange properties of electrodeposited nickel hexacyanoferrate thin films, J. Chem. Ind. Eng. (China) 56 (2005) 2380–2386. [4] K.M. Jeerage, D.T. Schwartz, Characterization of cathodically deposited nickel hexacyanoferrate for electrochemically switched ion exchange, Sep. Sci. Technol. 35 (2000) 2375–2392. [5] X.G. Hao, J.X. Guo, S.B. Liu, Y.P. Sun, Electrochemically switched ion exchange performances of capillary deposited nickel hexacyanoferrate thin films, Trans. Nonferrous Met. Soc. China 16 (2006) 556–561. [6] M. Zhang, X.G. Hao, X.L. Ma, S.B. Liu, C.M. Fan, Y.P. Sun, Ion exchange performances of nickel hexacyanoferrate thin films on graphite substrate, Rare Metal Mater. Eng. 35 (2006) 249–253. [7] J. Reyes-Gomez, J.A. Medina, K.M. Jeerage, W.A. Steen, D.T. Schwartz, High capacity SiO2 -graphite composite electrodes with chemically incorporated metal MHCFs for electrochemically switched alkaline cation exchange, J. Electrochem. Soc. 151 (2004) D87–D92. [8] X.L. Cui, M.H. Engelhard, Y.H. Lin, Preparation, characterization and anion exchange properties of polypyrrole/carbon nanotube nanocomposites, J. Nanosci. Nanotechnol. 6 (2006) 547–553. [9] W. Chen, X.H. Xia, Highly stable nickel hexacyanoferrate nanotubes for electrically switched ion exchange, Adv. Funct. Mater. 17 (2007) 2943–2948. [10] Y.H. Lin, X.L. Cui, J. Bontha, Electrically controlled anion exchange based on polypyrrole and carbon nanotubes nanocomposite for perchlorate removal, Environ. Sci. Technol. 40 (2006) 4004–4009. [11] L.F. Schneemeyer, S.E. Spengler, D.W. Murphy, Ion selectivity in nickel hexacyanoferrate films on electrode surfaces, Inorg. Chem. 24 (1985) 3044–3046. [12] M.A. Lilga, R.J. Orth, J.P.H. Sukamto, S.M. Haight, D.T. Schwartz, Metal ion separations using electrically switched ion exchange, Sep. Purif. Technol. 11 (1997) 147–158. [13] M.A. Lilga, R.J. Orth, J.P.H. Sukamto, S.D. Rassat, J.D. Genders, R. Gopal, Cesium separation using electrically switched ion exchange, Sep. Purif. Technol. 24 (2001) 451–466. [14] K.M. Jerrage, W.A. Steen, D.T. Schwartz, Correlating nanoscale structure with ion intercalation in electrodeposited nickel hexacyanoferrate thin films, Chem. Mater. 14 (2002) 530–535. [15] K.M. Jerrage, W.A. Steen, D.T. Schwartz, Charge-density-dependent partitioning of Cs+ and K+ into nickel hexacyanoferrate matrixes, Langmuir 18 (2002) 3620–3625. [16] W.A. Steen, K.M. Jerrage, D.T. Schwartz, Raman spectroscopy of redox activity in cathodically electrodeposited nickel hexacyanoferrate thin films, Appl. Spectrosc. 56 (2002) 1021–1029. [17] W.A. Steen, S.W. Han, Q.M. Yu, R.A. Gordon, J.O. Cross, E.A. Stern, G.T. Seidler, K.M. Jeerage, D.T. Schwartz, Structure of cathodically deposited nickel hexacyanoferrate thin films using XRD and EXAFS, Langmuir 18 (2002) 7714–7721. [18] X.G. Hao, D.T. Schwartz, Tuning intercalation sites in nickel hexacyanoferrate using lattice nonstoichiometry, Chem. Mater. 17 (2005) 5831–5836. [19] W.A. Steen, D.T. Schwartz, A route to diverse combinatorial libraries of electroactive nickel hexacyanoferrate, Chem. Mater. 15 (2003) 2449–2453. [20] K.H. Kim, J.D. Lee, J.J. Lee, B.Y. Ahn, H.S. Kim, Y.W. Shin, Growth of ␣-Fe films on n-Si(1 1 1) substrate by pulsed electrodeposition in a non-aqueous solution, Thin Solid Films 483 (2005) 74–78. [21] V. Richoux, S. Diliberto, C. Boulanger, J.M. Lecuire, Pulsed electrodeposition of bismuth telluride films: influence of pulse parameters over nucleation and morphology, Electrochim. Acta 52 (2007) 3053–3060. [22] I. Baskaran, T.S.N. Sankara Narayanan, A. Stephen, Pulsed electrodeposition of nanocrystalline Cu–Ni alloy films and evaluation of their characteristic properties, Mater. Lett. 60 (2006) 1990–1995.
414
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
[23] R.K. Sharma, G. Singh, A.C. Rastogi, Pulsed electrodeposition of CdTe thin films: effect of pulse parameters over structure, stoichiometry and optical absorption, Sol. Energy Mater. Sol. Cells 82 (2004) 201–215. [24] A.V. Kokate, M.R. Asabe, S.B. Shelake, P.P. Hankare, B.K. Chougule, Structural, optical and electrical studies on pulse electrodeposited CdIn2 S4 thin films, Phys. B 390 (2007) 84–90. [25] D.D. Shivagan, P.M. Shirage, N.V. Desai, L.A. Ekal, S.H. Pawar, Structural, optical and electrical studies on pulse electrodeposited CdIn2 S4 thin films, Mater. Res. Bull. 36 (2001) 607–615. [26] S.Z. Tan, D.S. Yuan, Y.L. Liu, Study on the pulse electrodeposition of La-Ni-Co alloy from dimethylsulfoxide, Mater. Lett. 60 (2006) 2055–2058. [27] S.Y. Cheng, Y.Q. Chen, Y.J. He, G.N. Chen, The structure and properties of SnS thin films prepared by pulse electro-deposition, Mater. Lett. 61 (2007) 1408–1412.
[28] N.R. de Tacconi, K. Rajeshwar, R.O. Lezna, Metal hexacyanoferrates: electrosynthesis, in situ characterization, and applications, Chem. Mater. 15 (2003) 3046–3062. [29] J. Bácskai, K. Martinusz, E. Czirók, G. Inzelt, P.J. Kulesza, M.A. Malik, Polynuclear nickel hexacyanoferrates: monitoring of film growth and hydrated countercation flux/storage during redox reactions, J. Electroanal. Chem. 385 (1995) 241–248. [30] S. Zamponi, M. Berretoni, P.J. Kulesza, K. Miecznikowski, M.A. Malik, O. Makowski, R. Marassi, Influence of experimental conditions on electrochemical behavior of Prussian blue type nickel hexacyanoferrate film, Electrochim. Acta 48 (2003) 4261–4269. [31] F. Scholz, A. Dostal, The formal potentials of solid metal hexacyanometalates, Angew. Chem. Int. Ed. 34 (1995) 2685–2687.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Pulsed electrodeposition of nickel hexacyanoferrate films for electrochemically switched ion exchange Xiaogang Hao a , Yongguo Li a , Mark Pritzker b,∗ a b
Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1
a r t i c l e
i n f o
Article history: Received 2 February 2008 Received in revised form 2 June 2008 Accepted 3 June 2008 Keywords: NiHCF Thin films Pulse electrodeposition Electrochemically switched ion exchange Cyclic voltammetry
a b s t r a c t Nickel hexacyanoferrate (NiHCF) thin films were prepared on graphite substrates by pulse electrodeposition and investigated for use as electrochemically switched ion exchange (ESIX) materials. The pulse waveforms investigated consisted of an applied potential during the on-period and zero current during the off-period. Cyclic voltammetry (CV) was used to study the effects of the pulse potential and off-time on the ion-exchange capacity of NiHCF film electrodes. The cycle life and regenerability of the film electrodes were also investigated. Experimental results show that the optimum pulse potential, on/off-period (ton /toff ) and duty cycle are 0.3 V (vs. SCE), 300 ms/300 ms and 50%, respectively. The deposited NiHCF is uniformly distributed over the surface of the graphite and yields thin film electrodes with high ion exchange capacity for K+ , good stability and regenerability. The films produced by pulse deposition were also found to have superior exchange capacity and stability to those produced by the more common CV method. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Electrochemically switched ion exchange (ESIX) is an environmentally benign method to separate ions via reversible electrochemical modulation of the matrix charge density [1–3]. Ion loading and unloading can be easily controlled by changing the redox states of ion exchange thin films formed on conductive substrates to separate ions from mixed solutions and regenerate the matrix. Since electrochemical rather than chemical potential modulation is the main driving force for exchange [4], the secondary waste created by chemical regeneration and associated rinse water is eliminated. Thus, ESIX has the potential to replace traditional ion exchange and so has recently garnered coinsiderable interest [5–10]. Nickel hexacyanoferrate (NiHCF, formula Ah Nik [Fe(CN)6 ]l ·mH2 O, h, k, l, m = stoichiometric numbers, A = alkali metal cation), an inorganic coordination compound with an open, zeolite-like structure, is an excellent candidate for ESIX of alkali cations because of its differing affinity for alkali cations (Cs+ > Rb+ > K+ > Na+ > Li+ ) [11]. NiHCF thin films have been prepared on conducting substrates by many methods, such as the oxidation of nickel in ferricyanide solutions [1,12,13], cathodic deposition from marginally stable electrolytes containing divalent
∗ Corresponding author. Tel.: +1 519 888 4567x32542; fax: +1 519 746 4979. E-mail address: [email protected] (M. Pritzker). 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.06.001
nickel and ferricyanide [3,4,14–17] or chemical deposition from mixtures of divalent nickel and ferrocyanide salts [5–7,18,19]. In previous studies [3,6], NiHCF films were deposited on graphite substrates by cyclic voltammetric (CV) electrodeposition and found to have excellent selectivity for Cs+ in concentrated K+ - and Na+ -containing solutions, but the deposited films were not very uniform and dense. Therefore, its adhesion to the substrate was not stable and the ion exchange capacity low. Pulsed electrodeposition has been applied to deposit metal [20], alloys [21–26] and semiconductor [27] thin films because of the small grain size and high deposit quality that can be achieved. The additional operating variables such as pulse waveform, on/off pulse time or duty cycle, applied and mean current density, offer effective ways to control macroscopic properties such as morphology [20–22], electronic properties through crystallinity and elemental composition [22,23], electrical conductivity [24,25], porosity and adhesion [26,27]. Few reports are available that demonstrate the growth of NiHCF films by pulsed electrodeposition. Most commonly, this technique involves the application of unipolar or bipolar current or potential waveforms. In the case of unipolar pulse deposition, a cathodic potential or current is applied during a portion of the pulse to deposit the film followed by a relaxation period in which no current is allowed to flow. In the case of bipolar pulses, the waveforms consist of a cathodic potential or current followed by an anodic potential or current where oxidation occurs. The objective of this study is to use unipolar pulse deposition to produce NiHCF films on
408
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
graphite substrate with high cation-exchange capacity and stability and to investigate the effects of some pulse parameters on the resulting ESIX performance of the film electrode.
3. Results and discussion
2. Experimental
Fig. 1 shows typical potential–time and current–time transients obtained during pulse deposition with an on-potential of 0.3 V, ontime of 0.3 s and off-time of 0.3 s over 2000 cycles (corresponding to 1200 s deposition time). The responses shown in this figure correspond to the first 15 s (Fig. 1a and b) and to the final 15 s of pulse deposition (Fig. 1c and d) of a NiHCF film. At the start of the process prior to the formation of any NiHCF film, the open circuit potential on the graphite electrode is approximately 0.74 V. As pulse deposition begins and the film starts to form, the open circuit potential measured during the off-time decreases rapidly from this value to approximately 0.55 V and then decreases more gradually over time as the film thickens (Fig. 1a). After 15 s of deposition, the open circuit potential during the off-time has reached 0.46 V. By the end of the 2000 cycles (∼1200 s elapsed time), only a small amount of deposition is occurring during the on-times. This is reflected by the fact that the maximum current reached during the final pulse cycles has decreased to −0.16 mA. At the same time, the open circuit potential during the off-time has decreased to approximately 0.34 V and is now decreasing only very slowly. This leaves an overpotential of only −0.04 V when a potential of 0.3 V is applied during the on-time. On the basis of the other experiments of this study, this trend where the open circuit potential during the off-time decreases and approaches the value of the applied potential during the on-time as the film thickens is always observed to occur during pulse deposition regardless of the applied on-potential. The formation of NiHCF thin films on the surface of graphite was confirmed by comparison of SEM images of these samples before and after pulse electrodeposition. Energy-dispersive X-ray spectroscopy (EDS) analysis also confirmed the presence of iron and nickel on the surface of the graphite substrate. Furthermore, XPS revealed the appearance of Fe2p peaks at 710 eV, Ni2p at 860 eV and N1s at 400 eV only after pulsed deposition. The K2p peak also appeared, providing evidence that the as-prepared NiHCF is a Kcontaining compound. This is not surprising since potential cycling of the NiHCF film was conducted in a K+ -containing solution before the XPS scan. The peak current–time transients recorded during electrochemical deposition of NiHCF thin films at different on-potentials during each pulse are shown in Fig. 2. Only the maximum current measured during each pulse cycle is plotted in this figure. The peak current sharply decreases within the first few seconds and then remains relatively steady during the remainder of the pulses. In pulse electrodeposition process, the peak current during the ontime corresponds to the reduction of Fe(CN)6 3− to Fe(CN)6 4− . The rapid fall of the peak current in the first few seconds may be due to the combined effects of double layer charging and transport limitations of reactants in the boundary layer at the electrode surface and through the NiHCF film as it forms. Double layer charging influences the electrode response at the start of each pulse due to the periodic change in electrode potential. If the double layer structure returns to the same structure at the start of each pulse as it had at the start of the previous ones, then double layer charging will have no effect on the decrease in peak currents observed in Fig. 2 at the outset of deposition. However, if there is a gradual and continual change in double layer structure from one pulse to the next, then double layer effects could play a role in the trends observed in Fig. 2. Such an effect will eventually disappear and thereafter the double layer will only play a role in the electrode response during the course of a given pulse. An appreciable non-zero peak current is still observed after 1200 s of pulse deposition (Fig. 1d). This behaviour differs from
2.1. Experiment instrument and reagents All reagents were analytical grade and all solutions prepared using Millipore water (18.2 M cm). The electrochemical experiments were performed using a VMP2 Potentiostat (Princeton, USA) controlled with EC-Lab software. A three-electrode system was used with a graphite-working electrode and a platinum sheet counter electrode. All reported potentials are referenced to a saturated calomel electrode (SCE). SEM images were obtained using an American KEVEX SIGAMA unit, while XPS scans were acquired with a VG Scientific ESCALab250i-XL unit. 2.2. Preparation of film electrode Graphite rods were chosen as substrates for the pulsed deposition experiments. A 4 mm diameter rod was first mechanically polished using SiC abrasive paper, then rinsed with deionized water and dried in air. A 4 cm2 effective surface area was exposed for deposition at room temperature under quiescent conditions. The pulsed deposition experiments were performed in freshly prepared 0.002 mol L−1 NiSO4 , 0.002 mol L−1 K3 Fe (CN)6 and 0.25 mol L−1 Na2 SO4 mixed solution. Pulsed electrodeposition involves the application of a periodic current density or potential waveform to the working electrode. In this study, each pulse consisted of an on-time ton when a cathodic potential was applied and an off- time toff when no current was allowed to flow. In particular, the potential waveforms consisted of on-potentials (or pulse potentials) Von ranging from 0.2 to 0.7 V followed by a zero current density during the off-time. The ontime was fixed as 0.3 s, while off-times of 0.1, 0.2, 0.3 and 0.4 s were used, thereby yielding duty cycles of 75%, 60%, 50% and 43%, respectively. The number of deposition pulse cycles to generate the films was varied from 1000 to 5000. Every experiment in this study was repeated at least two times and the results obtained found to be very reproducible. For comparison with the more common CV method, NiHCF deposits on graphite electrodes were also produced by cycling the electrode potential between 0 and 1200 mV at 25 mV s−1 in freshly prepared 0.002 mol L−1 NiSO4 , 0.002 mol L−1 K3 Fe(CN)6 , and 0.25 mol L−1 Na2 SO4 mixed solution. Deposition was continued for 12 cycles to produce the film. The film was then thoroughly rinsed with water and dried. 2.3. Performance experiments on NiHCF film electrode After film growth, each electrode was cycled 25 times in a 1 mol L−1 KNO3 solution between the potentials of 400 and 1200 mV at 25 mV s−1 and the capacity of NiHCF film electrodes was determined by measurement of the charge associated with oxidation and reduction. The samples were rinsed with deionized water and dried in air. Then the morphology and composition of the film were characterized by scanning electron microscopy (SEM) and Xray photoelectron spectroscopy (XPS). The regenerability of the film electrode was assessed by cycling a Cs-loaded film between the potentials of 400 and 1200 mV at 25 mV s−1 in a 1 mol L−1 KNO3 solution. The cycle life was assessed by carrying out 500 repetitive voltammetric cycles on a K-loaded film in 1 mol L−1 KNO3 at a scan rate of 25 mV s−1 .
3.1. Transient curves during pulsed deposition
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
409
Fig. 1. (a and c) Potential–time and (b and d) current–time transients during pulse deposition of NiHCF from 0.002 mol L−1 NiSO4 , 0.002 mol L−1 K3 Fe (CN) 6 and 0.25 mol L−1 Na2 SO4 mixed solution. The pulse conditions include 0.3 V pulse potential, 0.3 s on-time and 0.3 s off-time.
what would be expected for a diffusion-controlled process in a quiescent solution when polarization occurs under DC conditions. This points at one of the advantages of pulsed electrolysis over DC electrolysis. The application of pulses provides a relaxation period during the off-time when the reactants Fe(CN)6 3− and Ni2+ for film formation can be replenished in the boundary layer region or within the film. Another contributing factor is that NiHCF films are sufficiently electronically and ionically conducting that they do not
completely passivate as they grow [4,28]. However, the resistance to the flow of current does gradually increase as the film thickens. Perhaps the most surprising observation concerning Fig. 2 is the effect of the pulse on-potential on the peak currents, particularly over the early part of polarization. One would expect the magnitude of the peak current due to the reduction of Fe(CN)6 3− to rise as the pulse on-potential becomes more negative. Yet, as the results indicate, the peak current is higher and decreases more gradually with time at 0.3 V than at 0.2 or 0.4 V. Even at later times when the magnitude of the peak current has levelled off, it remains higher at 0.3 V. Although the current is not a direct measure of the rate of NiHCF film formation, the trend observed in Fig. 2 may be related to the effect of potential on the rate of film formation, as will be discussed in Section 3.2.2. 3.2. Effect of pulse parameters on ion exchange capacity The ion-exchange capacity of NiHCF film is one of the key properties for its successful application as an ESIX material. CV is a useful tool for determining both the redox site capacity of electroactive materials and the changes in capacity after numerous repeated loading/unloading cycles. The capacity or relative electroactivity was explored straightforwardly by comparing the areas under the cyclic voltammograms obtained on NiHCF film electrodes previously prepared under different pulse deposition conditions, as discussed in the following sub-sections.
Fig. 2. The effect of pulse potential on variation of the peak currents reached during pulse deposition of NiHCF from 0.002 mol L−1 NiSO4 , 0.002 mol L−1 K3 Fe (CN) 6 and 0.25 mol L−1 Na2 SO4 mixed solution. The on-time is 0.3 s and the duty cycle is 50%. The inset is an enlargement of the initial 17 s of the current–time curves.
3.2.1. Effect of pulse off-time Fig. 3 shows the voltammograms of films that had been prepared using a pulse potential of 0.3 V and different pulse off-times. The voltammograms shown in Fig. 3 correspond to the 25th cycle
410
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
Fig. 3. Cyclic voltammograms obtained in 1 mol L−1 KNO3 showing the effect of the pulse off-time on the capacity of NiHCF films. The pulse conditions used to prepare the films include 0.3 V pulse potential, 0.3 s on-time and (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4 s off-times. The voltammograms were obtained at a 25 mV s−1 scan rate.
obtained after the film electrodes were immersed in 1 mol L−1 KNO3 solution. Each of the NiHCF films was produced by deposition for 2000 cycles with a constant pulse on-time of 0.3 s and different pulse off-times between 0.1 and 0.4 s. The positive currents in Fig. 3 correspond to the oxidation of the NiHCF film and K+ deintercalation from the matrix, while negative currents are due to the reduction of the film and K+ intercalation [4]. Fig. 3 shows that the variation of toff leads to quite different reversible charge densities and CV shapes. Similar to that shown in previous studies [14,18,29,30], potential cycling in the presence of a K+ -containing electrolyte leads to the appearance of two distinct current peaks during both the positive- and negative-going scans. Principal component analysis (PCA) of combined CV-EDS data in a previous study showed the direct link between the appearance of multiple peaks for K+ intercalation/deintercalation and non-stoichiometry of the NiHCF lattice [18]. Energy dispersive X-ray spectroscopy has shown that when the more negative anodic and cathodic peaks are large, the more positive anodic and cathodic peaks and the K+ content in the film tend to be small [14,30]. Conversely, in CVs obtained on films with high K+ content, the more negative peaks are found to be smaller than the more positive ones. As shown in Fig. 3, the charge associated with the more negative peaks rises relative to that of the more positive peaks as toff increases. This trend suggests that this change in pulse operating conditions tends to lower the potassium content in the film that results when it undergoes 25 potential scan cycles in the presence of 1 mol L−1 KNO3 . Another effect observed is a small shift of the cathodic peaks to lower potential with an increase in toff . For the purposes of this analysis, the average of the integrated charge during the anodic and cathodic scans obtained for the film immersed in 1 mol L−1 KNO3 was used as a measure of the ion exchange capacity of the films. As clearly evident in Fig. 3, the area under the voltammograms and ion exchange capacity increases as toff rises from 0.1 to 0.3 s, but then decreases with further increase to 0.4 s. In pulse electrodeposition, the growth of NiHCF films involves the reduction of Fe(CN)6 3− to Fe(CN)6 4− followed by reaction of Fe(CN)6 4− with Ni(II) on the electrode. The observed trend in the ion exchange capacity in Fig. 3 can be explained in terms of the role that the pulse off-time has on the concentration profiles of reactants in the vicinity of the electrode. The off-time provides a relaxation period to replenish the concentration of reactants Fe(CN)6 3− and Ni(II) after being depleted during the previous on-time. In this way,
the use of pulse electrolysis tends to reduce mass transfer limitations that can arise during DC electrolysis. Presumably, for the rate of consumption of Fe(CN)6 3− and Ni(II) that occurs due to the application of 0.3 V for 0.3 s during the on-time, an off-time of 0.1 s is insufficient to replenish these reactants and so the rate of Fe(CN)6 3− reduction and NiHCF film formation are limited during the subsequent on-time. If this is the case, one would expect a longer off-time to increase the amount of film that forms over the duration of 2000 pulse cycles, as observed for pulse off-times of 0.2 and 0.3 s in Fig. 3. When the off-time is made long enough, further increases in toff should no longer have any effect. However, as observed in Fig. 3, a rise in toff from 0.3 to 0.4 s causes a drop in the amount of film formed and the ion exchange capacity of the film. This effect is confirmed by the current–time transients obtained during pulse deposition when the films were formed. Consequently, at least another factor must be playing a role to explain the observed trends in Fig. 3. Although no net current flows during the off-time, it is possible that redox reactions still occur in the film. Monitoring of the current–time response during pulse deposition with an on-potential of 0.3 V, on-time of 0.3 s and off-times of 0.3 and 0.4 s showed that the electrode potential reached during the off-time over the course of 2000 pulse cycles is in the range of 0.3–0.5 V. Separate experiments on the Fe(CN)6 3−/4− system on a graphite electrode in the absence of a NiHCF film also indicated that the reduction of Fe(CN)6 3− to Fe(CN)6 4− is already occurring when the electrode potential is between 0.3 and 0.4 V. Thus, it is possible that during each pulse off-time, a redox reaction involving the reduction of Fe(CN)6 3− and the oxidation of some of the pre-existing NiHCF film can take place. The contention that NiHCF can be oxidized during the off-time is supported by an estimate of its Nernst potential under the conditions prevailing during pulse deposition. The Nernst potential is obtained by correcting the formal potential for the concentrations of the dissolved reactants and products of the electrode reaction. Although there is some uncertainty as to the precise reaction that occurs during NiHCF oxidation (for example, see [14] and [18] for detailed analysis of this aspect), there is no disagreement that it involves the release of the intercalating cation and that the stoichiometric ratio of the intercalating cation and electrons in the half-cell reaction is 1:1. Thus, correction of the formal potential to yield the Nernst potential is given by the amount RT/F ln[Cj ], where [Cj ] is the concentration of the intercalating cation in the solution. Since the solution under question in our pulse deposition experiments contains much more Na+ than K+ , then Na+ is considered to be the intercalating ion. The formal potential for NiHCF oxidation when Na+ is the intercalating ion has been reported to be 0.361 V SCE [31]. Thus, for our solution containing 0.25 mol/L Na2 SO4 , the Nernst potential for NiHCF reduction is estimated to be 0.343 V SCE. As shown in Fig. 1, the electrode potential during the off-time remains above this value over the course of the 2000 pulse cycles. Thus, on the basis of this estimation of the Nernst potential, one would expect NiHCF to be oxidized during the off-time period of the pulse cycles, as proposed. If this is the case, some of the cathodic current flowing during each on-time would be used to re-reduce the portion of the film oxidized during the previous off-time and so not contribute to the formation of new film. This would partially counteract the positive effect that pulse deposition has on replenishing the reactants Fe(CN)6 3− and Ni(II). Furthermore, as the duration of the off-time increases, the influence of such a redox process would tend to grow, whereas the effect of replenishing the reactants would level off. The net result would be a lowering of the ion exchange capacity if the off-time becomes long enough, as observed in Fig. 3. Consequently, on off-time toff of 0.3 s could turn out to be the optimum for the deposition of high capacity NiHCF films at a 0.3 s on-time and 0.3 V on-potential.
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
Fig. 4. Cyclic voltammograms obtained in 1 mol L−1 KNO3 showing the effect of the pulse potential on the capacity of NiHCF films. The pulse conditions used to prepare the films include 0.3 s on-time, 0.3 s off-time and (a) 0.2, (b) 0.3, (c) 0.4, (d) 0.5 and (e) 0.7 V pulse potentials. The voltammograms were obtained at a 25 mV s−1 scan rate.
3.2.2. Effect of pulse potential The pulse potential is one of the main operating parameters of this technique. Therefore, it is important to choose the appropriate potential to produce high quality films. Fig. 4 shows the CV curves obtained after 25 cycles in 1 mol L−1 KNO3 of a series of NiHCF films that were previously formed using different pulse potentials of 0.2, 0.3, 0.4, 0.5 and 0.7 V, while maintaining the duty cycle, on-time and duration constant at 50%, 0.3 s and 2000 cycles, respectively. As can be seen from Fig. 4, the pulse potential has large influence on the ion exchange capacity. For a pulse with ton = 0.3 s and toff = 0.3 s, the film electrode prepared with a 0.3 V pulse potential during the on-time exhibits a larger CV area and ion exchange capacity than the films produced at the other potentials. This observation is consistent with the result in Section 3.1 and Fig. 2 that the on-time peak current measured during film formation is higher at a pulse potential of 0.3 V than at 0.2 or 0.4 V. Judging from the different shapes of the CVs in Fig. 4, pulse potential also appears to have some influence on the film composition and structure. The effect of pulse potential appears to be complex. As stated earlier, film formation requires the reduction of Fe(CN)6 3− followed by the reaction of Fe(CN)6 4− and Ni(II). As the pulse potential becomes more negative, one would expect the rate of reduction of Fe(CN)6 3− and film formation to increase. Thus, the finding that the apparent exchange capacity of the film produced at a pulse potential at 0.3 V is greater than that produced at 0.4, 0.5 or 0.7 V is wholly consistent with this expectation. In view of this, the observation from Fig. 4 that the capacity of the film formed at a pulse potential of 0.3 V is higher than that of one produced at a more negative potential of 0.2 V is surprising. Also unexpected is the finding that the capacity obtained at the most negative pulse potential is lower than at any of the other potentials studied. Further research is required to gain more insight into this aspect of the effect of pulse potential on film formation. Some comments are also worth noting regarding the results obtained at a pulse potential of 0.7 V which suggest some differences in film formation from that at the lower pulse potentials. Based on the results obtained at the lower potentials, it is somewhat surprising that a NiHCF film can actually form at such a high potential. However, examination of the current–time transients shows that the open circuit potential reached during the off-time after about 1000 s of deposition has leveled off at approximately 0.72 V,
411
Fig. 5. The effect of the number of pulse deposition cycles on the ion-exchange capacity of NiHCF films produced at 0.3 V pulse potential, 0.3 s on-time and 0.3 s off-time.
much higher than the values reached at the corresponding stage of deposition when lower pulse potentials are applied. Also, some differences are evident in the CV response in Fig. 4 for the film formed at a pulse potential of 0.7 V. For one thing, only a single peak appears during the anodic portion of the scan. Although two cathodic peaks occur, the more positive peak is much smaller than the other and, in fact, appears only as a shoulder. The more negative peak is also shifted significantly in the cathodic direction relative to those observed at the other pulse potentials. This leads to a larger separation (Ep ) between the anodic and the cathodic peak potentials in the CV. Large Ep values have been attributed to diffusion limitations inside the dense film during the cation-exchange process [7]. Difficulties associated with the diffusion of K+ into and out of the dense microstructure of the electrode may limit the rates of oxidation and reduction of the NiHCF compound. This suggests that the film produced on graphite at a pulse potential of 0.7 V is more densely packed than the films produced at the other potentials investigated in this study.
3.2.3. Effect of pulse deposition time Fig. 5 shows the variation in the ion-exchange capacity as the number of pulse deposition cycles is raised. It is observed that the capacity rises with an increase in the number of cycles, suggesting that the ion-exchange capacity can be increased by lengthening the duration. The increase of the ion-exchange capacity is nearly linear over the first 1500 cycles, but slows down thereafter. Films have been prepared with capacities as high as 27.5 mC cm−2 (corresponding to 430 nm thickness) after 4500 cycles. Of course, as the film becomes thicker, the resistance to Fe(CN)6 3− reduction and NiHCF formation increases and film growth slows down. It is worth noting that this capacity achieved by pulse deposition exceeds the levels that have previously been reported for thin film NiHCF electrodes. Capacities of about 5 mC cm−2 are typical of anodically prepared films [12], whereas capacities in the range 10–15 mC cm−2 have been obtained for cathodically deposited films [4]. Recently, a small improvement to values of 16–17 mC cm−2 has been reported for NiHCF nanotube electrodes fabricated using porous alumina templates [9]. Perhaps the most success has been achieved using three-dimensional porous composite electrodes such as those consisting of SiO2 , graphite and NiHCF [7], which have considerably higher capacity than the thin film electrodes presented in the current and other studies.
412
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
Fig. 6. (a) Cyclic voltammogram obtained in 1 mol L−1 KNO3 for film that has only been in the K-form. Cyclic voltammogram for film originally in Cs-form during (b) 25th and (c) 1st cycle after immersion in 1 mol L−1 KNO3 . The scan rate is 25 mV s−1 . Fig. 7. Cyclic voltammograms of NiHCF films in 1 mol L−1 KNO3 obtained over 500 repeated potential cycles. The scan rate is 25 mV s−1 .
3.3. Electrochemical regeneration of NiHCF thin film In ESIX, the loading/unloading of metal ions on a NiHCF thin film can be easily achieved electrochemically by controlling the transition between reduced and oxidized states of NiHCF. In this way, a metal ion can be selectively removed from an aqueous solution. However, an important engineering aspect to consider is the ability to regenerate these films back to their original form and determine whether this leads to a loss in ion exchange capacity. To assess the ability to regenerate the films produced by pulse deposition, we conducted an experiment similar to that described previously [4]. First, a graphite substrate with a freshly prepared NiHCF coating was converted to Cs-form by immersing it in a 1 mol L−1 CsNO3 solution and subjecting its potential to 25 scan cycles between 400 and 1200 mV. Analysis of the film by EDS showed that Cs was the only alkali cation that could be detected, confirming that this procedure was able to completely convert the film to Cs-form. The film electrode in Cs-form was then immersed in a 1 mol L−1 KNO3 solution containing no Cs+ and converted back to K-form by cycling its potential between 400 and 1200 mV for 25 times. Fig. 6 shows the voltammograms of a film originally in the Cs-form during its regeneration back to the K-form in this way. The CVs for the regenerated film shown in Fig. 6 correspond to the 1st (curve c) and 25th (curve b) cycles obtained after immersion in the KNO3 solution. Also shown in this figure is the corresponding CV for a film that has only been in the K-form (curve a). After only one regeneration cycle, the response resembles the CV of the film that has only been in the K-form, indicating that regeneration occurs fairly readily. Furthermore, virtually all of the Cs+ has been removed from the film by the 25th cycle. When regeneration is extended beyond 25 scan cycles, the CV of the resulting film appears even closer in shape to that of curve a than what appears in Fig. 6. After regeneration for 25 cycles, the films formed by pulse deposition retain about 90% of the original ion-exchange capacity of the K-form film. Although not shown here, the rise in the ion exchange capacity of the regenerated NiHCF film with increase in the number of voltammetric cycles occurs without loss of good redox reversibility, as has been shown previously for films formed by the conventional CV method [3,4]. 3.4. Cycle life of NiHCF thin film The cycle life is another important property of ESIX films. The application of voltammetric cycles mimics the situation of repeated redox switching in an ESIX process and provides a measure of cycle
stability. The change in CVs obtained on NiHCF films produced under the optimum pulse deposition conditions determined in the previous sections were investigated in 1 mol L−1 KNO3 by scanning at 25 mV s−1 over 500 cycles. Fig. 7 presents the responses obtained over this range of cycles. Integration of the CV areas indicates that the NiHCF films lose only 10% of their capacity over the first 250 cycles and then virtually no more over the remaining 250 cycles. Comparison of these results with those of previously reported NiHCF films produced by the CV method shows that pulsed deposition significantly improves redox cycling stability and capacity for cation uptake and elution [6]. Also note that the two distinct cathodic peaks and two distinct anodic peaks disappear and only one broad peak remains with increase of the cycle number. This may be caused by structural modifications in the NiHCF film with cycling [28]. 3.5. Comparison of pulse and CV methods As discussed above, the optimum operating conditions for pulse deposition of NiHCF film electrodes with high cation-exchange capacity and good stability involve a pulse potential of 0.3 V and ton /toff period = 0.3/0.3 s. A film electrode synthesized on graphite under these conditions for 2000 cycles (total duration of 20 min including both ton and toff ) was compared to a film electrode obtained by the more traditional CV method. To provide a similar total duration for deposition, the CV method involved 12 consecutive linear potential scan cycles between 0 and 1200 mV at a rate of 25 mV s−1 (total duration of 19.2 min). The capacities of the two films produced in this way were then determined by obtaining their cyclic voltammograms in 1 mol L−1 KNO3 at a scan rate of 25 mV s−1 . The results shown in Fig. 8 indicate that the cation-exchange capacity of the NiHCF electrode prepared by pulse electrodeposition (curve a), as determined from the area under the redox peaks, is approximately three times higher than that of the electrode produced by CV electrodeposition (curve b). Fig. 9 shows the change in normalized ion exchange capacity of NiHCF films produced by the two deposition methods as they undergo 500 voltammetric cycles in 1 mol L−1 KNO3 solution. The capacity values in each case are normalized by their corresponding initial capacities, as measured in the 10th voltammetric cycle, to partly account for differences in the initial states of the two films. The preparation method of NiHCF is known to affect
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
413
systems so produced are suitable for electrochemically controlled ion separation processes. Experiments showed the ESIX performance of pulse deposited NiHCF film electrodes to have superior capacity and stability than film electrodes formed by the more common CV method. Acknowledgments Professor Hao gratefully acknowledges support by the Shanxi Scholarship Council of China to spend a half year at the University of Waterloo. This research was financially supported in part by the National Natural Science Foundation of China (no. 20676089) and Natural Science Foundation of Shanxi Province (no. 2007011029). References
Fig. 8. Cyclic voltammograms obtained in 1 mol L−1 KNO3 of NiHCF films prepared on graphite substrates by (a) pulsed electrodeposition and (b) CV electrodeposition. The scan rate is 25 mV s−1 .
Fig. 9. Normalized charge capacity as a function of cycle number for NiHCF film electrode prepared by (a) pulsed electrodeposition and (b) CV electrodeposition.
the redox cycling stability of the electroactive materials, as is also demonstrated in Fig. 9. A better performance is observed for the film electrode prepared by pulse electrodeposition, which is able to retain about 87% of its initial capacity after 500 cycles. On the other hand, the loss of capacity of NiHCF films formed by CV electrodeposition is significant and only 58% of the initial capacity remains. On the basis of these results, NiHCF film electrodes produced on graphite substrates by pulse electrodeposition appear to have higher stability than those produced by CV, leading to significantly improved cycling stability. 4. Conclusions Electrodeposition of NiHCF thin films has been carried out using a pulsed method with the objective to improve the capacity and the stability of the film electrodes. The pulse waveforms investigated consist of an applied electrode potential during the on-period followed by an off-period in which the system was allowed to relax under open circuit conditions. The following optimum pulse variables to produce NiHCF film electrodes with enhanced capacity and stability have been obtained: 0.3 V pulse potential, 0.3 s on-time and 0.3 s off-time. It has been demonstrated that the film-electrode
[1] S.D. Rassat, J.H. Sukamto, R.J. Orth, M.A. Lilga, R.T. Hallen, Development of an electrically switched ion exchange process for selective ion separations, Sep. Purif. Technol. 15 (1999) 207–222. [2] Y.H. Lin, X.L. Cui, Electrosynthesis, characterization, and application of novel hybrid materials based on carbon nanotube–polyaniline–nickel hexacyanoferrate nanocomposites, J. Mater. Chem. 16 (2006) 585–592. [3] X.G. Hao, J.X. Guo, Z.L. Zhang, S.B. Liu, Y.P. Sun, Electrochemically switched ion exchange properties of electrodeposited nickel hexacyanoferrate thin films, J. Chem. Ind. Eng. (China) 56 (2005) 2380–2386. [4] K.M. Jeerage, D.T. Schwartz, Characterization of cathodically deposited nickel hexacyanoferrate for electrochemically switched ion exchange, Sep. Sci. Technol. 35 (2000) 2375–2392. [5] X.G. Hao, J.X. Guo, S.B. Liu, Y.P. Sun, Electrochemically switched ion exchange performances of capillary deposited nickel hexacyanoferrate thin films, Trans. Nonferrous Met. Soc. China 16 (2006) 556–561. [6] M. Zhang, X.G. Hao, X.L. Ma, S.B. Liu, C.M. Fan, Y.P. Sun, Ion exchange performances of nickel hexacyanoferrate thin films on graphite substrate, Rare Metal Mater. Eng. 35 (2006) 249–253. [7] J. Reyes-Gomez, J.A. Medina, K.M. Jeerage, W.A. Steen, D.T. Schwartz, High capacity SiO2 -graphite composite electrodes with chemically incorporated metal MHCFs for electrochemically switched alkaline cation exchange, J. Electrochem. Soc. 151 (2004) D87–D92. [8] X.L. Cui, M.H. Engelhard, Y.H. Lin, Preparation, characterization and anion exchange properties of polypyrrole/carbon nanotube nanocomposites, J. Nanosci. Nanotechnol. 6 (2006) 547–553. [9] W. Chen, X.H. Xia, Highly stable nickel hexacyanoferrate nanotubes for electrically switched ion exchange, Adv. Funct. Mater. 17 (2007) 2943–2948. [10] Y.H. Lin, X.L. Cui, J. Bontha, Electrically controlled anion exchange based on polypyrrole and carbon nanotubes nanocomposite for perchlorate removal, Environ. Sci. Technol. 40 (2006) 4004–4009. [11] L.F. Schneemeyer, S.E. Spengler, D.W. Murphy, Ion selectivity in nickel hexacyanoferrate films on electrode surfaces, Inorg. Chem. 24 (1985) 3044–3046. [12] M.A. Lilga, R.J. Orth, J.P.H. Sukamto, S.M. Haight, D.T. Schwartz, Metal ion separations using electrically switched ion exchange, Sep. Purif. Technol. 11 (1997) 147–158. [13] M.A. Lilga, R.J. Orth, J.P.H. Sukamto, S.D. Rassat, J.D. Genders, R. Gopal, Cesium separation using electrically switched ion exchange, Sep. Purif. Technol. 24 (2001) 451–466. [14] K.M. Jerrage, W.A. Steen, D.T. Schwartz, Correlating nanoscale structure with ion intercalation in electrodeposited nickel hexacyanoferrate thin films, Chem. Mater. 14 (2002) 530–535. [15] K.M. Jerrage, W.A. Steen, D.T. Schwartz, Charge-density-dependent partitioning of Cs+ and K+ into nickel hexacyanoferrate matrixes, Langmuir 18 (2002) 3620–3625. [16] W.A. Steen, K.M. Jerrage, D.T. Schwartz, Raman spectroscopy of redox activity in cathodically electrodeposited nickel hexacyanoferrate thin films, Appl. Spectrosc. 56 (2002) 1021–1029. [17] W.A. Steen, S.W. Han, Q.M. Yu, R.A. Gordon, J.O. Cross, E.A. Stern, G.T. Seidler, K.M. Jeerage, D.T. Schwartz, Structure of cathodically deposited nickel hexacyanoferrate thin films using XRD and EXAFS, Langmuir 18 (2002) 7714–7721. [18] X.G. Hao, D.T. Schwartz, Tuning intercalation sites in nickel hexacyanoferrate using lattice nonstoichiometry, Chem. Mater. 17 (2005) 5831–5836. [19] W.A. Steen, D.T. Schwartz, A route to diverse combinatorial libraries of electroactive nickel hexacyanoferrate, Chem. Mater. 15 (2003) 2449–2453. [20] K.H. Kim, J.D. Lee, J.J. Lee, B.Y. Ahn, H.S. Kim, Y.W. Shin, Growth of ␣-Fe films on n-Si(1 1 1) substrate by pulsed electrodeposition in a non-aqueous solution, Thin Solid Films 483 (2005) 74–78. [21] V. Richoux, S. Diliberto, C. Boulanger, J.M. Lecuire, Pulsed electrodeposition of bismuth telluride films: influence of pulse parameters over nucleation and morphology, Electrochim. Acta 52 (2007) 3053–3060. [22] I. Baskaran, T.S.N. Sankara Narayanan, A. Stephen, Pulsed electrodeposition of nanocrystalline Cu–Ni alloy films and evaluation of their characteristic properties, Mater. Lett. 60 (2006) 1990–1995.
414
X. Hao et al. / Separation and Purification Technology 63 (2008) 407–414
[23] R.K. Sharma, G. Singh, A.C. Rastogi, Pulsed electrodeposition of CdTe thin films: effect of pulse parameters over structure, stoichiometry and optical absorption, Sol. Energy Mater. Sol. Cells 82 (2004) 201–215. [24] A.V. Kokate, M.R. Asabe, S.B. Shelake, P.P. Hankare, B.K. Chougule, Structural, optical and electrical studies on pulse electrodeposited CdIn2 S4 thin films, Phys. B 390 (2007) 84–90. [25] D.D. Shivagan, P.M. Shirage, N.V. Desai, L.A. Ekal, S.H. Pawar, Structural, optical and electrical studies on pulse electrodeposited CdIn2 S4 thin films, Mater. Res. Bull. 36 (2001) 607–615. [26] S.Z. Tan, D.S. Yuan, Y.L. Liu, Study on the pulse electrodeposition of La-Ni-Co alloy from dimethylsulfoxide, Mater. Lett. 60 (2006) 2055–2058. [27] S.Y. Cheng, Y.Q. Chen, Y.J. He, G.N. Chen, The structure and properties of SnS thin films prepared by pulse electro-deposition, Mater. Lett. 61 (2007) 1408–1412.
[28] N.R. de Tacconi, K. Rajeshwar, R.O. Lezna, Metal hexacyanoferrates: electrosynthesis, in situ characterization, and applications, Chem. Mater. 15 (2003) 3046–3062. [29] J. Bácskai, K. Martinusz, E. Czirók, G. Inzelt, P.J. Kulesza, M.A. Malik, Polynuclear nickel hexacyanoferrates: monitoring of film growth and hydrated countercation flux/storage during redox reactions, J. Electroanal. Chem. 385 (1995) 241–248. [30] S. Zamponi, M. Berretoni, P.J. Kulesza, K. Miecznikowski, M.A. Malik, O. Makowski, R. Marassi, Influence of experimental conditions on electrochemical behavior of Prussian blue type nickel hexacyanoferrate film, Electrochim. Acta 48 (2003) 4261–4269. [31] F. Scholz, A. Dostal, The formal potentials of solid metal hexacyanometalates, Angew. Chem. Int. Ed. 34 (1995) 2685–2687.