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Electrochemical preparation and characterization of electrodes modified with mixed hexacyanoferrates of nickel and palladium
www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 487 (2000) 57 – 65
Electrochemical preparation and characterization of electrodes modified with mixed hexacyanoferrates of nickel and palladium Pawel J. Kulesza a,*, Marcin A. Malik b, Roman Schmidt a, Anna Smolinska a, Krzysztof Miecznikowski a, Silvia Zamponi c, Andrzej Czerwinski a, Mario Berrettoni c, Roberto Marassi c a
b
Department of Chemistry, Uni6ersity of Warsaw, Pasteura 1, PL-02 -093 Warsaw, Poland Department of Metallurgy and Materials Engineering, Technical Uni6ersity of Czestochowa, Armii Krajowej 19, PL-42 -200 Czestochowa, Poland c Dipartimento di Scienze Chimiche, Uni6ersita di Camerino, 6ia S. Agostino 1, I-62032 Camerino, MC, Italy Received 9 August 1999; received in revised form 21 April 2000; accepted 24 April 2000
Abstract Mixed nickel/palladium hexacyanoferrates have been prepared both as thin films and bulk precipitates (powders) attached to electrode surfaces. The mixed material does not seem to be a simple mixture of hexacyanoferrates of nickel and palladium, and it shows unique voltammetric and electrochromic characteristics when compared with the respective single-metal hexacyanoferrates. Electrodeposition of a mixed film is achieved by potential cycling in the solution for modification containing nickel(II), palladium(II) and hexacyanoferrate(III). It comes from elemental analysis that, in general, the stoichiometric ratios of nickel to palladium in mixed metal hexacyanoferrate films reflect relative concentrations of Pd(II) and Ni(II) in the solutions for modification. In the case of the films that have been electrodeposited from the solutions containing palladium ions in amounts lower or comparable with those of nickel ions, the mechanism of film growth seems to involve formation of nickel hexacyanoferrate during negative potential scans followed by simultaneous insertion of palladium ions as countercations into the system. In such cases, palladium ions tend to substitute potassium countercations at interstitial positions in the electrodeposited nickel II II hexacyanoferrate microstructures. We have determined the following stoichiometric formula, K1.74 − 2y PdII y Ni1.13[Fe (CN)6] (where y B0.72) for such films. At higher molar fractions of palladium in solutions for modification, the formation of a mixed phase of nickel/palladium hexacyanoferrate (in which both nickel(II) and palladium(II) are nitrogen-coordinated within the cyanometallate lattice) is expected. This seems to be more probable than simple codeposition of separate palladium hexacyanoferrate and nickel hexacyanoferrate microstructures during the film growth. Mixed (composite) nickel/palladium hexacyanoferrate films show long-term stability as well as promising charge storage and transport capabilities during voltammetric potential cycling. Well-defined and reversible cyclic voltammetric responses have been obtained in lithium, sodium and potassium electrolytes. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Mixed metal hexacyanoferrates; Mechanism of electrodeposition; Mechanical attachment; Cyclic voltammetry; Formal potentials; Interstitial and lattice-substituted mixed phases; Visible spectra; Electrochromism; Alkali metal electrolytes; Charge storage
1. Introduction There has been growing interest in metal hexacyanoferrates as electroactive materials due to their interesting redox and spectrochemical properties, such as electrochromicity [1 – 4], capability to store counterca* Corresponding author. Tel.: +48-22-8220211 ext. 289; fax: +4822-8225996. E-mail address: [email protected] (P.J. Kulesza).
tions [5–8], ion-exchange selectivity [2,9–12] and ability to mediate (catalyze) electrochemical reactions [13–18]. Among the cyanometallate systems, nickel(II) hexacyanoferrate(III/II) (NiHCNFe) [7,13,16,19–22] can be considered as a model material for certain fundamental and applied studies for the following reasons. NiHCNFe films show well-defined and reproducible responses not only in supporting electrolytes containing hydrated K+, but also other alkali metal cations such as hydrated Li+, Na+, Rb+ or Cs+. Unlike Prussian
0022-0728/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 0 ) 0 0 1 5 6 - X
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Blue and many of its metal-substituted analogues [2– 6,12,23], both oxidized and reduced NiHCNFe structures seem to be fairly open and permit unimpeded transport of alkali metal cations of different sizes while providing charge balance during the system’s redox reaction. NiHCNFe films can be easily fabricated (on various electrode substrates) by electrodeposition through potential cycling [20,22]. Recently, it has been demonstrated that certain transition metal cations, such as Ag+, Tl+ and Cd2 + can be intercalated or inserted into the structures of metal hexacyanoferrates including NiHCNFe [24 – 26]. Our general view is that NiHCNFe could be a very good host matrix for various ions and other systems. Also, new metal and mixed metal hexacyanoferrates that form thin films on electrode surfaces have been reported [25,27–34]. The existence of different kinds of mixed phases or phase mixtures, e.g. with respect to mixed compositions of nitrogen-coordinated metal ions, interstitial ions or the oxidation states of iron, have been postulated. In the search for novel cyanometallate systems with unique physicochemical properties, we explore NiHCNFe as a host material into which Pd(II) is incorporated during electrodeposition. We have chosen palladium as a second, cyanometallate-forming, metal for the following reasons. PdHCNFe itself is deeply colored (purple) in its oxidized form. Pd(II) is a divalent ion, and it is likely to have a strong stabilizing effect on the NiHCNFe cation-exchange lattice. Indeed, stable and fairly thick films of composite hexacyanoferrate of nickel and palladium (Ni/PdHCNFe), that are capable of accumulating sizable amounts of charge, can be grown on common electrode substrates. Composite Ni/PdHCNFe shows different voltammetric behavior and distinct electrochromic characteristics in comparison to single metal (Ni or Pd) hexacyanoferrates. We address here the mechanistic aspects of fabrication of composite (mixed) films, their stoichiometry, voltammetric and electrochromic characteristics, as well as their behavior in various alkali metal electrolytes. We have established that, depending on the ratio of palladium to nickel ions in the solution for modification, Pd2 + can be intercalated at interstitial positions of NiHCNFe or a truly mixed phase of heteronuclear hexacyanoferrate of nickel and palladium (Ni/PdHCNFe) is formed. The latter possibility appears at higher ratios of Pd to Ni, and it seems to be more probable than simple codeposition of palladium hexacyanoferrate (PdHCNFe) together with NiHCNFe during the film growth.
2. Experimental All chemicals were analytical grade materials and were used as received. Solutions were prepared from
triply distilled water. Microcrystalline powders of Niand HCNFe, namely K2NiII[FeII(CN)6]*nH2O II III KNi [Fe (CN)6]*nH2O, were prepared by slow precipitation as reported earlier [35,36] except that the starting 40 mmol dm − 3 NiCl2 solution was prepared in 1 mol dm − 3 HCl. The latter step was dictated by the necessity of keeping the same experimental conditions (pH) during all preparative procedures. Palladium hexacyanoferrate (PdHCNFe) precipitates were prepared by mixing the following solutions: 10 cm3 of 40 mmol dm − 3 PdCl2 in 1 mol dm − 3 HCl, with 20 cm3 2 mol dm − 3 KCl, and 20 cm3 40 mmol dm − 3 K4[Fe(CN)6] (or K3[Fe(CN)6). Bulk samples of Ni/PdHCNFe were prepared by mixing 20 cm3 of 40 mmol dm − 3 NiCl2 in 1 mol dm − 3 HCl with 10 cm3 of 40 mmol dm − 3 PdCl2 in 1 mol dm − 3 HCl, 20 cm3 1 mol dm − 3 KCl, and 10 cm3 40 mmol dm − 3 K3[Fe(CN)6] (or K4[Fe(CN)6]). As a rule, after standing for 12 h, all precipitates were centrifuged twice, and they were washed each time with deionized water. The elemental composition and stoichiometry of Ni/ PdHCNFe films deposited on a large-surface area Ptelectrode (6 cm2) were determined, following their dissolution in concentrated H2SO4, using atomic absorption spectrometry (Solar 939, ATI Unicam, UK). For each determination, the films were prepared 3–5 times and, each time, they were dissolved in the same 2.0 cm3 of hot concentrated H2SO4. Later, the analytes were diluted to 10 cm3 before they were subjected to atomic absorption determinations. Both analytes and standards contained H2SO4 at the same level. Ni/ PdHCNFe films (deposited on graphite foil from Goodfellow, UK) were also examined using an energy dispersive X-ray (EDX) analysis probe (Leica Cambridge Model 360). Samples of microcrystalline precipitates of NiHCNFe, Ni/PdHCNFe and PdHCNFe (10–15 mg), which were analyzed using atomic absorption, were pretreated in hot solution (6 cm3) of concentrated 5:1 H2SO4 + H2O2 for 30 min. Following dissolution, they were diluted with deionized water to 100 cm3. Blanks and standard solutions were prepared in the same way. Ni/PdHCNFe films were typically fabricated on glassy carbon and platinum substrates. Electrodeposition was done in a solution containing 0.40 mol dm − 3 KCl+ 0.40 mol dm − 3 HCl+ 0.50 mmol dm − 3 K3[Fe(CN)6], and various amounts of PdCl2 and NiCl2 (total concentration of Pd2 + + Ni2 + , 1.0 mmol dm − 3). The procedure involved 15–80 full voltammetric cycles at 50 mV s − 1 in the potential range from 0.850 to 0 V. Loadings (in mol cm − 2) of metal hexacyanoferrate films on electrode surfaces were estimated upon determination of charges under the system’s voltammetric peaks (oxidation) recorded at a slow scan rate, 5 mV s − 1.
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Paraffin-impregnated graphite substrates (diameter, 4 mm), which were prepared as described elsewhere [26], were also used as electrodes for cyclic voltammetric measurements. A small amount (a few mg) of the finely ground Ni/CoHCNFe precipitate was transferred and
59
distributed uniformly by gentle rubbing on the electrode surface. The resulting modified electrode was rinsed with distilled water. Tin oxide covered glass slides from PPG Industries, USA (‘Nesa’, 10–20 V/square) having a geometric area of 1.2 cm2 were used as optically transparent electrodes. To prepare fairly thick films suitable for spectroelectrochemical investigation, the film preparation procedure described above for Ni/PdHCNFe involved 70 full voltammetric potential cycles. Visible spectra were recorded using a Hewlett–Packard 8452 diode array spectrophotometer. Experiments were carried out at room temperature. All potentials were expressed versus the saturated calomel electrode (SCE) reference. Electrochemical measurements were done with a CH Instruments (Austin, TX) model 660 analyzer.
3. Results and discussion
3.1. Voltammetric identity of electrodeposited Ni/PdHCNFe films
Fig. 1. Electrodeposition of Ni/PdHCNFe films on glassy carbon by potential cycling in solutions containing: 0.40 mol dm − 3 KCl +0.40 mol dm − 3 HCl+0.50 mmol dm − 3 K3[Fe(CN)6, and (A) 0.50 mmol dm − 3 PdCl2 + 0.50 mmol dm − 3 NiCl2; (B) 0.33 mmol dm − 3 PdCl2 + 0.67 mmol dm − 3 NiCl2; (C) 0.67 mmol dm − 3 PdCl2 +0.33 mmol dm − 3 NiCl2. Scan rate: 50 mV s − 1. Electrode area: 0.20 cm2.
Fig. 2. Voltammetric responses of metal hexacyanoferrate films which were obtained by potential cycling in solutions containing: 0.40 mol dm − 3 KCl+0.40 mol dm − 3 HCl + 0.50 mmol dm − 3 K3[Fe(CN)6, and (a) 1 mmol dm − 3 NiCl2; (b) 0.33 mmol dm − 3 PdCl2 +0.67 mmol dm − 3 NiCl2; (c) 0.50 mmol dm − 3 PdCl2 + 0.50 mmol dm − 3 NiCl2; (d) 0.67 mmol dm − 3 PdCl2 + 0.33 mmol dm − 3 NiCl2; (e) 0.67 mmol dm − 3 PdCl2 +0.33 mmol dm − 3 NiCl2. Electrolyte: 1 mol dm − 3 KCl. Scan rate: 10 mV s − 1. Electrode (glassy carbon) area: 0.20 cm2.
Fig. 1 shows voltammetric data illustrating the growth of Ni/PdHCNFe films during potential cycling as described in Section 2. In order to get a better insight into the mechanism of the film growth, Ni/PdHCNFe films were grown from the mixtures for modification containing Ni(II) and Pd(II) at various ratios: (A) 1:1, (B) 2:1 and (C) 1:2. In all cases, the surface voltammetric peak currents grew following each negative potential scan that involved reduction of Fe(CN)36 − to Fe(CN)46 − . Although potentials of the growing peaks were comparable, their shapes were dependent on the amount of Pd(II), relative to Ni(II), in the mixture for modification. When the level of Pd(II) was low, the voltammetric pattern with two sets of overlapping peaks was obtained (Fig. 1(B)), and it resembled the behavior that had been previously observed during electrodeposition of simple NiHCNFe films [22]. In the modification mixtures containing larger amounts of Pd(II), single sets of voltammetric peaks were developed (Fig. 1(A and C)). Following electrodeposition in solutions containing Ni(II) and Pd(II) at various ratios, the resulting films were examined in potassium electrolyte (Fig. 2). For comparison, we also provide responses which were obtained following potential cycling in the mixtures for modification containing K3[Fe(CN)6], KCl, and a single metal ion, either Ni(II) (Fig. 2(a)) or Pd(II) (Fig. 2(e)). It is important to note that, in the absence of Ni(II) in the modification solution, barely any palladium hexacyanoferrate (PdHCNFe) film was produced on the glassy carbon (Fig. 2(e)). Curves b, c and d illustrate cyclic voltammograms of Ni/PdHCNFe films contain-
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ing various amounts of palladium relative to nickel in the composite microstructure. In view of the results shown in Figs. 1 and 2, the following observations can be made. At low amounts of nickel relative to palladium, the voltammetric behavior of Ni/PdHCNFe (Fig. 1(B), Fig. 2(b)) resembles that of simple NiHCNFe [22] with its characteristic two sets of peaks appearing in a potassium electrolyte (Fig. 2(a)). Upon increasing the amount of palladium in the mixture for modification, the resulting voltammetric responses are characterized by single sets of fairly broad peaks (Fig. 1(C), Fig. 2, curves c and d). Although their potentials become more positive with the increasing ratio of Pd to Ni in the system (Fig. 2, curves c and d), their values are still comparable with the potentials of the second, more positive, set of peaks of NiHCNFe (appearing at ca 0.59 V in Fig. 2(a)). An interesting issue is whether NiPdHCNFe is produced as a phase mixture or whether it exists in the form of a mixed phase. In the latter case, different kinds of mixed systems can be envisioned, including those with respect to the mixed composition of nitrogen-coordinated metal ions or with respect to the interstitial ions [25,30]. Having in mind a recent report [25], we have addressed the possibility of formation of a mixed phase by plotting the system’s formal (mid-peak) potentials as a function of the molar fraction of palladium ions, xPd (Fig. 4), where xPd =nPd/(nPd +nNi) and nPd, nNi stand for the numbers of moles of Pd2 + and Ni2 + ions, respectively. It is noteworthy that up to the values of xPd equal at least to 0.39 or even to 0.53, as approximated in Fig. 4, the formal potential of the mixed system’s redox reaction is practically unchanged, and it is approximately equal to the formal potential of the most positive redox reaction of NiHCNFe that is believed to involve K2Ni[FeII(CN)6]/KNi[FeIII(CN)6] species [22]. Such observations imply the interstitial incorporation of Pd2 + ions into NiHCNFe, presumably via substitution of K+ countercations in the structure, rather than through formation of a phase mixture of NiHCNFe and PdHCNFe. At higher molar fractions of palladium ions, the formal potential of Ni/PdHCNFe starts to depend linearly on xPd (Fig. 4). In view of the recent work discussing the behavior of mixed cadmium iron hexacyanoferrate [25], the formation of a mixed phase, in which Pd2 + and Ni2 + would occupy lattice positions, cannot be excluded.
3.2. Mechanism of growth and composition of films It is now accepted that metal hexacyanoferrate films are formed during negative potential scans when Fe(CN)36 − is reduced to Fe(CN)46 − , and the latter species reacts with a metal ion to form a sparingly soluble precipitate [22,23]. In the case of Ni/PdHCNFe, dynamics of the film growth decreased upon increasing
the amount of Pd(II), relative to Ni(II), in the mixture for modification (compare peak heights in Fig. 2). In the extreme case, we were unable to generate PdHCNFe film when no Ni(II) was present in the mixture for modification (Fig. 2(e)). Simple precipitation experiments clearly showed that nickel hexacyanoferrate(II) precipitate was formed much faster than that of palladium hexacyanoferrate(II). Therefore, during the fabrication of Ni/PdHCNFe films by potential cycling, the electrodeposition of NiHCNFe was presumably much more effective than that of PdHCNFe. In other words, generation of NiHCNFe microstructures acted as the ‘driving force’ for the formation of mixed Ni/PdHCNFe. This mechanism is likely to be operative for the composite films, which were electrodeposited from the solutions for modifications containing Pd2 + at concentrations not exceeding those of Ni2 + ions. Under such conditions, we expect the following reactions to occur during the electrodeposition by potential cycling in the mixture for modification. First, the reduction of hexacyanoferrate(III) proceeds during negative potential scans at potentials below 0.3 V: FeIII(CN)36 − + e − UFeII(CN)46 −
(1)
Hexacyanoferrate(II) ions, which are generated in the vicinity of the electrode surface, react primarily with Ni(II) rather than Pd(II). In view of the previous reports concerning preparation of NiHCNFe [16,22], the following reactions can be proposed: FeII(CN)46 − + Ni2 + U{NiII[FeII(CN)6]}2 −
(2)
II − FeII(CN)46 − + 3/2Ni2 + U{NiII 1.5[Fe (CN)6]}
(3)
To explain two sets of voltammetric peaks of NiHCNFe in potassium electrolyte (Fig. 2(a)), two predomiII nating forms, K2NiII[FeII(CN)6] and KNiII 1.5[Fe (CN)6], have been postulated [22]. The more positive set of peaks of NiHCNFe is usually attributed to K2NiII[FeII(CN)6], which is more strongly paired by potassium ions. Since Pd2 + ions are also present in the mixture for modification, they are expected to be inserted together with K+ into NiHCNFe. Having in mind electrostatic interactions, Pd2 + ions are more preferably attracted by{NiII[FeII(CN)6]}2 − than II − {NiII [Fe (CN) ]} . The fact that the formal (mid1.5 6 peak) potential of the peaks of composite Ni/PdHCNFe (Fig. 2) is closer to that characteristic of II K2NiII[FeII(CN)6] rather than KNiII 1.5[Fe (CN)6] [22], is in agreement with our hypothesis. Also, the data of Fig. 4, showing that the mid-peak potential of the composite film hardly changes with the palladium content up to the values of xPd equal to 0.4 to 0.5, are consistent with our view that Pd2 + (together with K+) ions are inserted into electrodeposited NiHCNFe as countercations at interstitial positions. The actual stoichiometry of electrodeposited composite film will be dependent on
P.J. Kulesza et al. / Journal of Electroanalytical Chemistry 487 (2000) 57–65
Fig. 3. Cyclic voltammogram of PdHCNFe film on glassy carbon. Preparation details in the text. Electrolyte: 1 mol dm − 3 KCl. Scan rate: 10 mV s − 1. Electrode area: 0.20 cm2.
relative concentrations of Pd2 + and K+, and the most likely reaction can be written, in general, as follows: {NiII[FeII(CN)6]}2 − +yPd2 + +2(1 −y)K+ II II UK2(1 − y)PdII y Ni [Fe (CN)6]
(4)
where y is a stoichiometric parameter (0By B 1). The Ni/PdHCNFe films, which were prepared as for Figs. 2 and 4 (and subsequently reduced at 0.0 V), were subjected to elemental analysis using atomic absorption according the procedure described Section 2. The respective dry films were also examined with the use of scanning electron microscopy equipped with an EDX probe. It is noteworthy that such elements as Fe, Ni and Pd were determined in all Ni/PdHCNFe films. Reference was made to the analytical data of simple NiHCNFe (Fig. 2(a)) and PdHCNFe (Fig. 3) films. Further, the relative amount of Pd was found to increase with the increasing content of Pd2 + ions in the mixture of modification. On the basis of elemental analysis (atomic absorption), we determined the empiriII cal stoichiometric formula, K1.74 − 2y PdII y Ni1.13II [Fe (CN)]6 (0ByB 0.72), for the composite Ni/PdHCNFe films electrodeposited from the solutions for modification containing amounts of Pd2 + equal or lower in comparison with Ni2 + ions. The empirical formula is in agreement with our theoretical description, K2(1 − y)PdII y NiII[FeII(CN)6]. Some excess of nickel content, relative to iron in the empirical stoichiometric formula, may reflect the fact that some KNi1.5[Fe(CN)6] is codeposited during potential cycling. Nevertheless, careful EDX examination of grains, from which the composite film is formed, implies the existence of a single-phase compound (containing Fe, Ni, and Pd) rather than a combination of separate single-metal microparticles of PdHCNFe or NiHCNFe. We have also correlated the molar fractions of palladium ions (xPd) in the composite films with the analogous molar fractions of Pd2 + in the solutions for modification (Fig. 5). The dependence is not linear, and it shows that much Pd-richer structures are produced when xPd in the solution for modification exceeds 0.5. Apparently, the mechanism of the formation of Ni/
61
PdHCNFe films changes because a simple incorporation of Pd2 + at interstitial positions cannot be a predominant factor when xPd \ 0.5 (in solution). Despite some differences in ionic radii (72 and 86 pm for Ni2 + and Pd2 + , respectively), it can be hypothesized that, in the solutions containing larger amounts of Pd2 + (in comparison with Ni2 + ), palladium ions can enter the system’s lattice positions to form a mixed heteronuclear cyanometallate lattice (hybrid phase) with respect to palladium and nickel ions as nitrogencoordinated metal ions. The fact that the mid-peak potentials of the composite system depend linearly on the molar fraction of Pd2 + in the film (xPd \0.5), supports this hypothesis. It follows from Fig. 2 that the peaks in the potential range 0.4–0.5 V tend to disappear upon increasing the amount of Pd2 + relative to Ni2 + in the mixture for modification (curves c and d). In view of the previous report [22], it can be concluded that, under such condiII tions, barely any KNiII 1.5[Fe (CN)6] is formed. In view of the data of Fig. 2(e), we also think that electrodeposition of sizable amounts of simple palladium hexacyanoferrate (PdHCNFe) is unlikely. In a separate experiment, we addressed the voltammetric characteristics of PdHCNFe film (Fig. 3) that was prepared as follows. First, the palladium layer was electrodeposited onto the glassy carbon surface from the solution of 10 mmol dm − 3 PdCl2 in 1 mol dm − 3 HCl for 120 s at − 1.1 V. In the next step, the electrode was subjected to potential cycling at 50 mV s − 1 in 1.25 mmol dm − 3 K3[FeIII(CN)6]+ 0.5 mol dm − 3 KCl within the potential range from 0.0 to 1.1 V. The resulting PdHCNFe exhibited voltammetric peaks at potentials approximately 0.8–0.9 V (Fig. 3). Since these peaks were ca 200 mV more positive than those of Ni/PdHCNFe (Fig. 2, curves b–d), it is plausible to assume that sizable amounts of simple PdHCNFe will not be formed during electrodeposition of composite Ni/PdHCNFe. Once more, we think that a mixed phase of Ni/PdHCNFe, rather than a simple mixture of NiHCNFe and PdHCNFe, predominates in the electrodeposited films.
3.3. Voltammetric characteristics in the presence of 6arious alkali metal cations We consider here typical Ni/PdHCNFe films that have been prepared as described in Section 2, but using solutions for modification containing equimolar amounts of Ni2 + and Pd2 + ions. On the basis of elemental analysis (atomic absorption), we have determined an empirical formula for such films, K0.30Pd0.72Ni1.13[FeII(CN)6]. In view of the results presented and discussed above (Figs. 2 and 4), palladium ions are expected to occupy interstitial positions in the metal hexacyanoferrate lattice. Fig. 6 shows voltammet-
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Fig. 4. The dependence of the formal (mid-peak) potential of the composite Ni/PdHCNFe film as a function of the molar fraction of palladium ions, xPd (xPd = nPd/(nPd +nNi), and nPd, nNi are numbers of moles of Pd2 + and Ni2 + in the film). The insets show cyclic voltammetric responses (scan rate: 5 mV s − 1) recorded for representative Ni/PdHCNFe films characterized by specific values of xPd. Electrode substrate: Pt (flag) of geometric area, 6 cm2.
ric responses of Ni/PdHCNFe films in supporting electrolytes containing various alkali metal cations: (a) LiCl, (b) NaCl, (c) KCl, and (d) CsCl at 1 mol dm − 3 concentration. It is noteworthy that well-defined voltammograms were obtained in the electrolytes containing lithium, sodium or potassium. The system’s formal (mid-peak) potentials move towards more positive values upon transfers from LiCl to NaCl and KCl. This behavior resembles characteristics of NiHCNFe film [19–22]. What is different — in comparison to NiHCNFe — is the distorted voltammetric behavior in cesium electrolyte (curve d). Apparently, the structure of a composite, Pd-containing, Ni/PdHCNFe is different, and it is probably less ‘open’ than that of NiHCNFe. It should be remembered that the ability of NiHCNFe to transport Cs+ ions during redox reactions is unique among metal hexacyanoferrates. Comparison of the dynamics of charge propagation in Ni/PdHCNFe versus NiHCNFe in various alkali metal supporting electrolytes will be the subject of our next communication. It is also noteworthy that composite Ni/PdHCNFe film shows very good stability on various electrode substrates including glassy carbon, platinum and SnO2coated glass. Using a typical film deposited on glassy carbon (as for Fig. 6) during potential cycling for 2 h
within a potential range from 0 to 0.9 V in 1 mol dm − 3 KCl (as for Fig. 6(c)), the loss of peak current on the level less than 5% was observed. For example, the dissolution of simple NiHCNFe film under the same experimental conditions was much faster. It is likely that the improved stability of Ni/PdHCNFe is due to the lower solubility product of the composite material. An alternative explanation may concern the mechanism of degradation: perhaps, during potential cycling, the material containing Pd2 + at interstices does not suffer reorganization in a manner analogous to single metal hexacyanoferrates [37,38]. Having in mind the good stability and the fact that sizable charges can be re-
Fig. 5. The dependence of the molar fractions of palladium ions (xPd) in the electrodeposited films on the respective values of xPd in the solutions for modifications. Other experimental details as for Fig. 4.
P.J. Kulesza et al. / Journal of Electroanalytical Chemistry 487 (2000) 57–65
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3.4. Comparison with mechanically attached precipitates
Fig. 6. Voltammetric responses of composite Ni/PdHCNFe film in electrolytes (1 mol dm − 3): (a) LiCl, (b) NaCl, (c) KCl and (d) CsCl. Other conditions as for Fig. 2.
Fig. 7. Voltammetric behavior of mechanically attached metal hexacyanoferrates: (a) NiHCNFe; (b) Ni/PdHCNFe; (c) PdHCNFe; and (d) NiHCNFe +PdHCNFe (physical mixture). Powders were introduced onto wax impregnated graphite electrode in the reduced forms. Electrolyte: 1 mol dm − 3 KCl. Scan rate: 10 mV s − 1. Electrode area: 0.20 cm2.
Fig. 8. Visible absorbance spectra of (A) NiHCNFe; (B) Ni/PdHCNFe; and (C) PdHCNFe in the (a) reduced and (b) oxidized forms. Precipitates were introduced onto glass slides as gels.
versibly accumulated as is evident from the system’s voltammetric peaks (Fig. 6), the composite material may be of importance in the construction of charge storage devices such as faradaic (redox) supercapacitors.
It has been established that metal hexacyanoferrates can not only be electrodeposited, but also mechanically attached to electrode surfaces by an abrasive method [25,26]. Fig. 7 shows cyclic voltammetric responses of the precipitates attached mechanically to a paraffinimpregnated graphite electrode: (a) NiHCNFe II II (K1.38NiII 1.31[Fe (CN)6]), (b) Ni/PdHCNFe (K1.74Pd0.60II II II Ni0.53[Fe (CN)6]), (c) PdHCNFe (K1.98Pd1.01[FeII(CN)6]) and (d) NiHCNFe admixed with PdHCNFe. The respective materials were prepared as described in Section 2, and their stoichiometries were determined using atomic absorption. It follows from comparison of curves d and b in Fig. 7 that the voltammetric characteristics of a mixture of NiHCNFe+ PdHCNFe is markedly different from that of Ni/PdHCNFe. Thus, Pd/NiHCNFe is not a simple mixture of NiHCNFe and PdHCNFe. On the other hand, the voltammetric response of NiHCNFe+ PdHCNFe (curve d) can be viewed in terms of superposition of the respective responses of NiHCNFe (curve a) and PdHCNFe (curve c). The voltammetric behavior of the mechanically attached material often differs from that characteristic of a respective film prepared by electrodeposition through potential cycling. Indeed, the voltammetric response of mechanically attached Ni/PdHCNFe precipitate (Fig. 7(b)) is somewhat different from the voltammetric pattern of electrodeposited Ni/PdHCNFe (Fig. 2(c)). Despite some ohmic effects, which may affect the cyclic voltammetry of mechanically attached films, it is apparent from Fig. 7 that Ni/PdHCNFe prepared by the abrasive method resembles PdHCNFe more than NiHCNFe. Indeed, the elemental analysis data (atomic absorption) shows the larger content of Pd (relative to Fe and Ni) in the mechanically attached Ni/PdHCNFe precipitates when compared with the respective electrodeposited films (remembering that both bulk samples and films were prepared from solutions containing equimolar amounts of nickel and palladium ions). We have also examined the visible spectra of the investigated metal hexacyanoferrate precipitates (introduced onto glass slides as gels). Fig. 8 summarizes spectra of (A) NiHCNFe, (B) Ni/PdHCNFe and (C) PdHCNFe in their (a) reduced and (b) oxidized states. In all cases, the spectra of the oxidized and reduced forms are different. Further, upon incorporation of palladium, the absorption band of the oxidized form tends to appear at higher wavelengths, and it resembles the response of oxidized PdHCNFe somewhat. Both oxidized Ni/PdHCNFe and PdHCNFe are deep red. We have also performed an in-situ spectroelectrochemical experiment using an optically transparent electrode covered with composite film that has been
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Fig. 9. The relationship between the visible spectrum of a composite Ni/PdHCNFe film (xPd =0.4) electrodeposited on an optically transparent electrode and the potential applied to the electrode: (a) 0.900; (b) 0.600; and (c) 0.300 V.
electrodeposited from the solution for modification (as described in Section 2) containing equimolar amounts of Ni2 + and Pd2 + ions. It follows from Fig. 9 that the system’s spectral response changes depending on the potential applied. What is important from the viewpoint of potential electrochromic applications is that the spectral difference (contrast) between the oxidized and reduced forms of composite Ni/PdHCNFe system is clearly marked.
4. Conclusions We demonstrate the possibility of preparation of thin films of composite (mixed) nickel/palladium hexacyanoferrates on electrode surfaces. The systems can be electrodeposited by potential cycling or fabricated by mechanical attachment through the abrasive method. On the basis of electron microscopic, voltammetric and spectrochemical experiments, we postulate that both electrodeposited and bulk (precipitated) composite NiHCNFe microstructures are not simple mixtures of nickel and palladium hexacyanoferrates. The relative concentrations of nickel(II) and palladium(II) ions in the solutions for modification determine the stoichiometric ratios of these ions in the electrochemically generated composite films. Higher ratios of palladium to nickel have been found in the analogous bulk Ni/ PdHCNFe precipitates. When the concentration of palladium ions in the solution for modification is lower or equal to that of nickel ions, the most likely mechanism of the formation (by potential cycling) of a composite film involves electrodeposition of NiHCNFe followed by incorporation of Pd2 + ions at the interstitial positions of NiHCNFe to form Ni/PdHCNFe. Composition of such Ni/PdHCNFe (reduced) films has been determined using atomic absorption, and it can be expressed in terms of an empirical stoichiometric forII II mula, K1.74 − 2y PdII y Ni1.13[Fe (CN)]6 (y B 0.72), consistent with the existence of a mixed phase with respect to interstitial ions. When, in the solution for modification,
the concentration of palladium(II) is higher than that of nickel(II), composite films, that contain relatively larger amounts of palladium (in comparison to nickel) ions, are electrodeposited. The structure of such films is probably complex, but our present results support the possibility of the formation of heteronuclear nickel/palladium hexacyanoferrate as a predominately mixed phase in which both nickel(II) and palladium(II) can act as nitrogen-coordinated metal ions in the cyanometallate three-dimensional cubic type lattice. In other words, the system may feature a heteronuclear cyanide-bridged network analogous to that recently proposed for hybrid nickel/cobalt hexacyanoferrate [39]. Composite Ni/PdHCNFe shows promising electrochromic properties. Good stability and favorable dynamics of charge transport during potential cycling in lithium, sodium and potassium salt electrolytes could be of importance in electrochemical applications including charge storage.
Acknowledgements This work was supported by the State Committee for Scientific Research (KBN), Poland under Grant 3T09A 09313 and by the Italian National Research Council, CNR (Rome, Italy). P.J.K. and M.A.M. appreciate travel grants from the University of Camerino. We appreciate the technical help of Laura Petetta (CIGA, University of Camerino) with the EDX analysis.
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P.J. Kulesza et al. / Journal of Electroanalytical Chemistry 487 (2000) 57–65 [17] J. Zhou, E. Wang, J. Electroanal. Chem. 331 (1992) 1029. [18] M.S. Lin, B.I. Jan, Electroanalysis 9 (1997) 340. [19] S. Sinha, B.D. Humphrey, A.B. Bocarsly, Inorg. Chem. 23 (1984) 203. [20] J. Joseph, H. Gomathi, G. Prabhakara Rao, Electrochim. Acta 36 (1991) 1537. [21] L.T. Kubota, Y. Gushikem, J. Electroanal. Chem. 362 (1993) 219. [22] J. Bacskai, K. Martinusz, E. Czirok, G. Inzelt, P.J. Kulesza, M.A. Malik, J. Electroanal. Chem. 385 (1995) 241. [23] (a) P.J. Kulesza, M. Faszynska, J. Electroanal. Chem. 252 (1988) 461. (b) P.J. Kulesza, M. Faszynska, Electrochim. Acta. 34 (1989) 1749. [24] P.J. Kulesza, T. Jedral, Z. Galus, Electrochim. Acta 34 (1989) 851. [25] N.F. Zakharchuk, N. Naumov, R. Stosser, F. Scholz, H. Mehner, J. Solid State Electrochem. 3 (1999) 264. [26] A. Dostal, B. Meyer, F. Scholtz, U. Schroder, M.A. Bond, F. Marken, S.J. Shaw, J. Phys. Chem. 99 (1995) 2096. [27] M. Jiang, Z. Zhao, J. Electroanal. Chem. 292 (1990) 281. [28] S. Bharathi, J. Joseph, D. Jeyakumar, G. Prabhakara Rao, J. Electroanal. Chem. 319 (1991) 341.
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[29] S. Liu, H.L. Li, M. Jiang, P. Li, J. Electroanal. Chem. 426 (1997) 27. [30] S. Jayarama Reddy, A. Dostal, F. Scholz, J. Electroanal. Chem. 403 (1996) 209. [31] J. Joseph, H. Gomathi, G.P. Rao, J. Electroanal. Chem. 431 (1997) 231. [32] M.A. Malik, P.J. Kulesza, Electroanalysis 8 (1996) 113. [33] M. Jiang, X. Zhou, Z. Zhao, J. Electroanal. Chem. 287 (1990) 389. [34] C.-X. Cai, K.-H. Xue, Y.-M. Zhou, H. Yang, Talanta 44 (1997) 339. [35] P.J. Kulesza, Inorg. Chem. 29 (1990) 2395. [36] P.J. Kulesza, Z. Galus, J. Electroanal. Chem. 323 (1992) 261. [37] A. Roig, J. Navarro, J.J. Garcia, F. Vicente, Electrochim. Acta 39 (1994) 437. [38] T.R.I. Cataldi, R. Guascito, A.M. Salvi, J. Electroanal. Chem. 417 (1996) 83. [39] P.J. Kulesza, M.A. Malik, J. Skorek, K. Miecznikowski, S. Zamponi, M. Berrettoni, M. Giorgetti, R. Marassi, J. Electrochem. Soc. 146 (1999) 3757.
Electrochemical preparation and characterization of electrodes modified with mixed hexacyanoferrates of nickel and palladium Pawel J. Kulesza a,*, Marcin A. Malik b, Roman Schmidt a, Anna Smolinska a, Krzysztof Miecznikowski a, Silvia Zamponi c, Andrzej Czerwinski a, Mario Berrettoni c, Roberto Marassi c a
b
Department of Chemistry, Uni6ersity of Warsaw, Pasteura 1, PL-02 -093 Warsaw, Poland Department of Metallurgy and Materials Engineering, Technical Uni6ersity of Czestochowa, Armii Krajowej 19, PL-42 -200 Czestochowa, Poland c Dipartimento di Scienze Chimiche, Uni6ersita di Camerino, 6ia S. Agostino 1, I-62032 Camerino, MC, Italy Received 9 August 1999; received in revised form 21 April 2000; accepted 24 April 2000
Abstract Mixed nickel/palladium hexacyanoferrates have been prepared both as thin films and bulk precipitates (powders) attached to electrode surfaces. The mixed material does not seem to be a simple mixture of hexacyanoferrates of nickel and palladium, and it shows unique voltammetric and electrochromic characteristics when compared with the respective single-metal hexacyanoferrates. Electrodeposition of a mixed film is achieved by potential cycling in the solution for modification containing nickel(II), palladium(II) and hexacyanoferrate(III). It comes from elemental analysis that, in general, the stoichiometric ratios of nickel to palladium in mixed metal hexacyanoferrate films reflect relative concentrations of Pd(II) and Ni(II) in the solutions for modification. In the case of the films that have been electrodeposited from the solutions containing palladium ions in amounts lower or comparable with those of nickel ions, the mechanism of film growth seems to involve formation of nickel hexacyanoferrate during negative potential scans followed by simultaneous insertion of palladium ions as countercations into the system. In such cases, palladium ions tend to substitute potassium countercations at interstitial positions in the electrodeposited nickel II II hexacyanoferrate microstructures. We have determined the following stoichiometric formula, K1.74 − 2y PdII y Ni1.13[Fe (CN)6] (where y B0.72) for such films. At higher molar fractions of palladium in solutions for modification, the formation of a mixed phase of nickel/palladium hexacyanoferrate (in which both nickel(II) and palladium(II) are nitrogen-coordinated within the cyanometallate lattice) is expected. This seems to be more probable than simple codeposition of separate palladium hexacyanoferrate and nickel hexacyanoferrate microstructures during the film growth. Mixed (composite) nickel/palladium hexacyanoferrate films show long-term stability as well as promising charge storage and transport capabilities during voltammetric potential cycling. Well-defined and reversible cyclic voltammetric responses have been obtained in lithium, sodium and potassium electrolytes. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Mixed metal hexacyanoferrates; Mechanism of electrodeposition; Mechanical attachment; Cyclic voltammetry; Formal potentials; Interstitial and lattice-substituted mixed phases; Visible spectra; Electrochromism; Alkali metal electrolytes; Charge storage
1. Introduction There has been growing interest in metal hexacyanoferrates as electroactive materials due to their interesting redox and spectrochemical properties, such as electrochromicity [1 – 4], capability to store counterca* Corresponding author. Tel.: +48-22-8220211 ext. 289; fax: +4822-8225996. E-mail address: [email protected] (P.J. Kulesza).
tions [5–8], ion-exchange selectivity [2,9–12] and ability to mediate (catalyze) electrochemical reactions [13–18]. Among the cyanometallate systems, nickel(II) hexacyanoferrate(III/II) (NiHCNFe) [7,13,16,19–22] can be considered as a model material for certain fundamental and applied studies for the following reasons. NiHCNFe films show well-defined and reproducible responses not only in supporting electrolytes containing hydrated K+, but also other alkali metal cations such as hydrated Li+, Na+, Rb+ or Cs+. Unlike Prussian
0022-0728/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 0 ) 0 0 1 5 6 - X
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Blue and many of its metal-substituted analogues [2– 6,12,23], both oxidized and reduced NiHCNFe structures seem to be fairly open and permit unimpeded transport of alkali metal cations of different sizes while providing charge balance during the system’s redox reaction. NiHCNFe films can be easily fabricated (on various electrode substrates) by electrodeposition through potential cycling [20,22]. Recently, it has been demonstrated that certain transition metal cations, such as Ag+, Tl+ and Cd2 + can be intercalated or inserted into the structures of metal hexacyanoferrates including NiHCNFe [24 – 26]. Our general view is that NiHCNFe could be a very good host matrix for various ions and other systems. Also, new metal and mixed metal hexacyanoferrates that form thin films on electrode surfaces have been reported [25,27–34]. The existence of different kinds of mixed phases or phase mixtures, e.g. with respect to mixed compositions of nitrogen-coordinated metal ions, interstitial ions or the oxidation states of iron, have been postulated. In the search for novel cyanometallate systems with unique physicochemical properties, we explore NiHCNFe as a host material into which Pd(II) is incorporated during electrodeposition. We have chosen palladium as a second, cyanometallate-forming, metal for the following reasons. PdHCNFe itself is deeply colored (purple) in its oxidized form. Pd(II) is a divalent ion, and it is likely to have a strong stabilizing effect on the NiHCNFe cation-exchange lattice. Indeed, stable and fairly thick films of composite hexacyanoferrate of nickel and palladium (Ni/PdHCNFe), that are capable of accumulating sizable amounts of charge, can be grown on common electrode substrates. Composite Ni/PdHCNFe shows different voltammetric behavior and distinct electrochromic characteristics in comparison to single metal (Ni or Pd) hexacyanoferrates. We address here the mechanistic aspects of fabrication of composite (mixed) films, their stoichiometry, voltammetric and electrochromic characteristics, as well as their behavior in various alkali metal electrolytes. We have established that, depending on the ratio of palladium to nickel ions in the solution for modification, Pd2 + can be intercalated at interstitial positions of NiHCNFe or a truly mixed phase of heteronuclear hexacyanoferrate of nickel and palladium (Ni/PdHCNFe) is formed. The latter possibility appears at higher ratios of Pd to Ni, and it seems to be more probable than simple codeposition of palladium hexacyanoferrate (PdHCNFe) together with NiHCNFe during the film growth.
2. Experimental All chemicals were analytical grade materials and were used as received. Solutions were prepared from
triply distilled water. Microcrystalline powders of Niand HCNFe, namely K2NiII[FeII(CN)6]*nH2O II III KNi [Fe (CN)6]*nH2O, were prepared by slow precipitation as reported earlier [35,36] except that the starting 40 mmol dm − 3 NiCl2 solution was prepared in 1 mol dm − 3 HCl. The latter step was dictated by the necessity of keeping the same experimental conditions (pH) during all preparative procedures. Palladium hexacyanoferrate (PdHCNFe) precipitates were prepared by mixing the following solutions: 10 cm3 of 40 mmol dm − 3 PdCl2 in 1 mol dm − 3 HCl, with 20 cm3 2 mol dm − 3 KCl, and 20 cm3 40 mmol dm − 3 K4[Fe(CN)6] (or K3[Fe(CN)6). Bulk samples of Ni/PdHCNFe were prepared by mixing 20 cm3 of 40 mmol dm − 3 NiCl2 in 1 mol dm − 3 HCl with 10 cm3 of 40 mmol dm − 3 PdCl2 in 1 mol dm − 3 HCl, 20 cm3 1 mol dm − 3 KCl, and 10 cm3 40 mmol dm − 3 K3[Fe(CN)6] (or K4[Fe(CN)6]). As a rule, after standing for 12 h, all precipitates were centrifuged twice, and they were washed each time with deionized water. The elemental composition and stoichiometry of Ni/ PdHCNFe films deposited on a large-surface area Ptelectrode (6 cm2) were determined, following their dissolution in concentrated H2SO4, using atomic absorption spectrometry (Solar 939, ATI Unicam, UK). For each determination, the films were prepared 3–5 times and, each time, they were dissolved in the same 2.0 cm3 of hot concentrated H2SO4. Later, the analytes were diluted to 10 cm3 before they were subjected to atomic absorption determinations. Both analytes and standards contained H2SO4 at the same level. Ni/ PdHCNFe films (deposited on graphite foil from Goodfellow, UK) were also examined using an energy dispersive X-ray (EDX) analysis probe (Leica Cambridge Model 360). Samples of microcrystalline precipitates of NiHCNFe, Ni/PdHCNFe and PdHCNFe (10–15 mg), which were analyzed using atomic absorption, were pretreated in hot solution (6 cm3) of concentrated 5:1 H2SO4 + H2O2 for 30 min. Following dissolution, they were diluted with deionized water to 100 cm3. Blanks and standard solutions were prepared in the same way. Ni/PdHCNFe films were typically fabricated on glassy carbon and platinum substrates. Electrodeposition was done in a solution containing 0.40 mol dm − 3 KCl+ 0.40 mol dm − 3 HCl+ 0.50 mmol dm − 3 K3[Fe(CN)6], and various amounts of PdCl2 and NiCl2 (total concentration of Pd2 + + Ni2 + , 1.0 mmol dm − 3). The procedure involved 15–80 full voltammetric cycles at 50 mV s − 1 in the potential range from 0.850 to 0 V. Loadings (in mol cm − 2) of metal hexacyanoferrate films on electrode surfaces were estimated upon determination of charges under the system’s voltammetric peaks (oxidation) recorded at a slow scan rate, 5 mV s − 1.
P.J. Kulesza et al. / Journal of Electroanalytical Chemistry 487 (2000) 57–65
Paraffin-impregnated graphite substrates (diameter, 4 mm), which were prepared as described elsewhere [26], were also used as electrodes for cyclic voltammetric measurements. A small amount (a few mg) of the finely ground Ni/CoHCNFe precipitate was transferred and
59
distributed uniformly by gentle rubbing on the electrode surface. The resulting modified electrode was rinsed with distilled water. Tin oxide covered glass slides from PPG Industries, USA (‘Nesa’, 10–20 V/square) having a geometric area of 1.2 cm2 were used as optically transparent electrodes. To prepare fairly thick films suitable for spectroelectrochemical investigation, the film preparation procedure described above for Ni/PdHCNFe involved 70 full voltammetric potential cycles. Visible spectra were recorded using a Hewlett–Packard 8452 diode array spectrophotometer. Experiments were carried out at room temperature. All potentials were expressed versus the saturated calomel electrode (SCE) reference. Electrochemical measurements were done with a CH Instruments (Austin, TX) model 660 analyzer.
3. Results and discussion
3.1. Voltammetric identity of electrodeposited Ni/PdHCNFe films
Fig. 1. Electrodeposition of Ni/PdHCNFe films on glassy carbon by potential cycling in solutions containing: 0.40 mol dm − 3 KCl +0.40 mol dm − 3 HCl+0.50 mmol dm − 3 K3[Fe(CN)6, and (A) 0.50 mmol dm − 3 PdCl2 + 0.50 mmol dm − 3 NiCl2; (B) 0.33 mmol dm − 3 PdCl2 + 0.67 mmol dm − 3 NiCl2; (C) 0.67 mmol dm − 3 PdCl2 +0.33 mmol dm − 3 NiCl2. Scan rate: 50 mV s − 1. Electrode area: 0.20 cm2.
Fig. 2. Voltammetric responses of metal hexacyanoferrate films which were obtained by potential cycling in solutions containing: 0.40 mol dm − 3 KCl+0.40 mol dm − 3 HCl + 0.50 mmol dm − 3 K3[Fe(CN)6, and (a) 1 mmol dm − 3 NiCl2; (b) 0.33 mmol dm − 3 PdCl2 +0.67 mmol dm − 3 NiCl2; (c) 0.50 mmol dm − 3 PdCl2 + 0.50 mmol dm − 3 NiCl2; (d) 0.67 mmol dm − 3 PdCl2 + 0.33 mmol dm − 3 NiCl2; (e) 0.67 mmol dm − 3 PdCl2 +0.33 mmol dm − 3 NiCl2. Electrolyte: 1 mol dm − 3 KCl. Scan rate: 10 mV s − 1. Electrode (glassy carbon) area: 0.20 cm2.
Fig. 1 shows voltammetric data illustrating the growth of Ni/PdHCNFe films during potential cycling as described in Section 2. In order to get a better insight into the mechanism of the film growth, Ni/PdHCNFe films were grown from the mixtures for modification containing Ni(II) and Pd(II) at various ratios: (A) 1:1, (B) 2:1 and (C) 1:2. In all cases, the surface voltammetric peak currents grew following each negative potential scan that involved reduction of Fe(CN)36 − to Fe(CN)46 − . Although potentials of the growing peaks were comparable, their shapes were dependent on the amount of Pd(II), relative to Ni(II), in the mixture for modification. When the level of Pd(II) was low, the voltammetric pattern with two sets of overlapping peaks was obtained (Fig. 1(B)), and it resembled the behavior that had been previously observed during electrodeposition of simple NiHCNFe films [22]. In the modification mixtures containing larger amounts of Pd(II), single sets of voltammetric peaks were developed (Fig. 1(A and C)). Following electrodeposition in solutions containing Ni(II) and Pd(II) at various ratios, the resulting films were examined in potassium electrolyte (Fig. 2). For comparison, we also provide responses which were obtained following potential cycling in the mixtures for modification containing K3[Fe(CN)6], KCl, and a single metal ion, either Ni(II) (Fig. 2(a)) or Pd(II) (Fig. 2(e)). It is important to note that, in the absence of Ni(II) in the modification solution, barely any palladium hexacyanoferrate (PdHCNFe) film was produced on the glassy carbon (Fig. 2(e)). Curves b, c and d illustrate cyclic voltammograms of Ni/PdHCNFe films contain-
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ing various amounts of palladium relative to nickel in the composite microstructure. In view of the results shown in Figs. 1 and 2, the following observations can be made. At low amounts of nickel relative to palladium, the voltammetric behavior of Ni/PdHCNFe (Fig. 1(B), Fig. 2(b)) resembles that of simple NiHCNFe [22] with its characteristic two sets of peaks appearing in a potassium electrolyte (Fig. 2(a)). Upon increasing the amount of palladium in the mixture for modification, the resulting voltammetric responses are characterized by single sets of fairly broad peaks (Fig. 1(C), Fig. 2, curves c and d). Although their potentials become more positive with the increasing ratio of Pd to Ni in the system (Fig. 2, curves c and d), their values are still comparable with the potentials of the second, more positive, set of peaks of NiHCNFe (appearing at ca 0.59 V in Fig. 2(a)). An interesting issue is whether NiPdHCNFe is produced as a phase mixture or whether it exists in the form of a mixed phase. In the latter case, different kinds of mixed systems can be envisioned, including those with respect to the mixed composition of nitrogen-coordinated metal ions or with respect to the interstitial ions [25,30]. Having in mind a recent report [25], we have addressed the possibility of formation of a mixed phase by plotting the system’s formal (mid-peak) potentials as a function of the molar fraction of palladium ions, xPd (Fig. 4), where xPd =nPd/(nPd +nNi) and nPd, nNi stand for the numbers of moles of Pd2 + and Ni2 + ions, respectively. It is noteworthy that up to the values of xPd equal at least to 0.39 or even to 0.53, as approximated in Fig. 4, the formal potential of the mixed system’s redox reaction is practically unchanged, and it is approximately equal to the formal potential of the most positive redox reaction of NiHCNFe that is believed to involve K2Ni[FeII(CN)6]/KNi[FeIII(CN)6] species [22]. Such observations imply the interstitial incorporation of Pd2 + ions into NiHCNFe, presumably via substitution of K+ countercations in the structure, rather than through formation of a phase mixture of NiHCNFe and PdHCNFe. At higher molar fractions of palladium ions, the formal potential of Ni/PdHCNFe starts to depend linearly on xPd (Fig. 4). In view of the recent work discussing the behavior of mixed cadmium iron hexacyanoferrate [25], the formation of a mixed phase, in which Pd2 + and Ni2 + would occupy lattice positions, cannot be excluded.
3.2. Mechanism of growth and composition of films It is now accepted that metal hexacyanoferrate films are formed during negative potential scans when Fe(CN)36 − is reduced to Fe(CN)46 − , and the latter species reacts with a metal ion to form a sparingly soluble precipitate [22,23]. In the case of Ni/PdHCNFe, dynamics of the film growth decreased upon increasing
the amount of Pd(II), relative to Ni(II), in the mixture for modification (compare peak heights in Fig. 2). In the extreme case, we were unable to generate PdHCNFe film when no Ni(II) was present in the mixture for modification (Fig. 2(e)). Simple precipitation experiments clearly showed that nickel hexacyanoferrate(II) precipitate was formed much faster than that of palladium hexacyanoferrate(II). Therefore, during the fabrication of Ni/PdHCNFe films by potential cycling, the electrodeposition of NiHCNFe was presumably much more effective than that of PdHCNFe. In other words, generation of NiHCNFe microstructures acted as the ‘driving force’ for the formation of mixed Ni/PdHCNFe. This mechanism is likely to be operative for the composite films, which were electrodeposited from the solutions for modifications containing Pd2 + at concentrations not exceeding those of Ni2 + ions. Under such conditions, we expect the following reactions to occur during the electrodeposition by potential cycling in the mixture for modification. First, the reduction of hexacyanoferrate(III) proceeds during negative potential scans at potentials below 0.3 V: FeIII(CN)36 − + e − UFeII(CN)46 −
(1)
Hexacyanoferrate(II) ions, which are generated in the vicinity of the electrode surface, react primarily with Ni(II) rather than Pd(II). In view of the previous reports concerning preparation of NiHCNFe [16,22], the following reactions can be proposed: FeII(CN)46 − + Ni2 + U{NiII[FeII(CN)6]}2 −
(2)
II − FeII(CN)46 − + 3/2Ni2 + U{NiII 1.5[Fe (CN)6]}
(3)
To explain two sets of voltammetric peaks of NiHCNFe in potassium electrolyte (Fig. 2(a)), two predomiII nating forms, K2NiII[FeII(CN)6] and KNiII 1.5[Fe (CN)6], have been postulated [22]. The more positive set of peaks of NiHCNFe is usually attributed to K2NiII[FeII(CN)6], which is more strongly paired by potassium ions. Since Pd2 + ions are also present in the mixture for modification, they are expected to be inserted together with K+ into NiHCNFe. Having in mind electrostatic interactions, Pd2 + ions are more preferably attracted by{NiII[FeII(CN)6]}2 − than II − {NiII [Fe (CN) ]} . The fact that the formal (mid1.5 6 peak) potential of the peaks of composite Ni/PdHCNFe (Fig. 2) is closer to that characteristic of II K2NiII[FeII(CN)6] rather than KNiII 1.5[Fe (CN)6] [22], is in agreement with our hypothesis. Also, the data of Fig. 4, showing that the mid-peak potential of the composite film hardly changes with the palladium content up to the values of xPd equal to 0.4 to 0.5, are consistent with our view that Pd2 + (together with K+) ions are inserted into electrodeposited NiHCNFe as countercations at interstitial positions. The actual stoichiometry of electrodeposited composite film will be dependent on
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Fig. 3. Cyclic voltammogram of PdHCNFe film on glassy carbon. Preparation details in the text. Electrolyte: 1 mol dm − 3 KCl. Scan rate: 10 mV s − 1. Electrode area: 0.20 cm2.
relative concentrations of Pd2 + and K+, and the most likely reaction can be written, in general, as follows: {NiII[FeII(CN)6]}2 − +yPd2 + +2(1 −y)K+ II II UK2(1 − y)PdII y Ni [Fe (CN)6]
(4)
where y is a stoichiometric parameter (0By B 1). The Ni/PdHCNFe films, which were prepared as for Figs. 2 and 4 (and subsequently reduced at 0.0 V), were subjected to elemental analysis using atomic absorption according the procedure described Section 2. The respective dry films were also examined with the use of scanning electron microscopy equipped with an EDX probe. It is noteworthy that such elements as Fe, Ni and Pd were determined in all Ni/PdHCNFe films. Reference was made to the analytical data of simple NiHCNFe (Fig. 2(a)) and PdHCNFe (Fig. 3) films. Further, the relative amount of Pd was found to increase with the increasing content of Pd2 + ions in the mixture of modification. On the basis of elemental analysis (atomic absorption), we determined the empiriII cal stoichiometric formula, K1.74 − 2y PdII y Ni1.13II [Fe (CN)]6 (0ByB 0.72), for the composite Ni/PdHCNFe films electrodeposited from the solutions for modification containing amounts of Pd2 + equal or lower in comparison with Ni2 + ions. The empirical formula is in agreement with our theoretical description, K2(1 − y)PdII y NiII[FeII(CN)6]. Some excess of nickel content, relative to iron in the empirical stoichiometric formula, may reflect the fact that some KNi1.5[Fe(CN)6] is codeposited during potential cycling. Nevertheless, careful EDX examination of grains, from which the composite film is formed, implies the existence of a single-phase compound (containing Fe, Ni, and Pd) rather than a combination of separate single-metal microparticles of PdHCNFe or NiHCNFe. We have also correlated the molar fractions of palladium ions (xPd) in the composite films with the analogous molar fractions of Pd2 + in the solutions for modification (Fig. 5). The dependence is not linear, and it shows that much Pd-richer structures are produced when xPd in the solution for modification exceeds 0.5. Apparently, the mechanism of the formation of Ni/
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PdHCNFe films changes because a simple incorporation of Pd2 + at interstitial positions cannot be a predominant factor when xPd \ 0.5 (in solution). Despite some differences in ionic radii (72 and 86 pm for Ni2 + and Pd2 + , respectively), it can be hypothesized that, in the solutions containing larger amounts of Pd2 + (in comparison with Ni2 + ), palladium ions can enter the system’s lattice positions to form a mixed heteronuclear cyanometallate lattice (hybrid phase) with respect to palladium and nickel ions as nitrogencoordinated metal ions. The fact that the mid-peak potentials of the composite system depend linearly on the molar fraction of Pd2 + in the film (xPd \0.5), supports this hypothesis. It follows from Fig. 2 that the peaks in the potential range 0.4–0.5 V tend to disappear upon increasing the amount of Pd2 + relative to Ni2 + in the mixture for modification (curves c and d). In view of the previous report [22], it can be concluded that, under such condiII tions, barely any KNiII 1.5[Fe (CN)6] is formed. In view of the data of Fig. 2(e), we also think that electrodeposition of sizable amounts of simple palladium hexacyanoferrate (PdHCNFe) is unlikely. In a separate experiment, we addressed the voltammetric characteristics of PdHCNFe film (Fig. 3) that was prepared as follows. First, the palladium layer was electrodeposited onto the glassy carbon surface from the solution of 10 mmol dm − 3 PdCl2 in 1 mol dm − 3 HCl for 120 s at − 1.1 V. In the next step, the electrode was subjected to potential cycling at 50 mV s − 1 in 1.25 mmol dm − 3 K3[FeIII(CN)6]+ 0.5 mol dm − 3 KCl within the potential range from 0.0 to 1.1 V. The resulting PdHCNFe exhibited voltammetric peaks at potentials approximately 0.8–0.9 V (Fig. 3). Since these peaks were ca 200 mV more positive than those of Ni/PdHCNFe (Fig. 2, curves b–d), it is plausible to assume that sizable amounts of simple PdHCNFe will not be formed during electrodeposition of composite Ni/PdHCNFe. Once more, we think that a mixed phase of Ni/PdHCNFe, rather than a simple mixture of NiHCNFe and PdHCNFe, predominates in the electrodeposited films.
3.3. Voltammetric characteristics in the presence of 6arious alkali metal cations We consider here typical Ni/PdHCNFe films that have been prepared as described in Section 2, but using solutions for modification containing equimolar amounts of Ni2 + and Pd2 + ions. On the basis of elemental analysis (atomic absorption), we have determined an empirical formula for such films, K0.30Pd0.72Ni1.13[FeII(CN)6]. In view of the results presented and discussed above (Figs. 2 and 4), palladium ions are expected to occupy interstitial positions in the metal hexacyanoferrate lattice. Fig. 6 shows voltammet-
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Fig. 4. The dependence of the formal (mid-peak) potential of the composite Ni/PdHCNFe film as a function of the molar fraction of palladium ions, xPd (xPd = nPd/(nPd +nNi), and nPd, nNi are numbers of moles of Pd2 + and Ni2 + in the film). The insets show cyclic voltammetric responses (scan rate: 5 mV s − 1) recorded for representative Ni/PdHCNFe films characterized by specific values of xPd. Electrode substrate: Pt (flag) of geometric area, 6 cm2.
ric responses of Ni/PdHCNFe films in supporting electrolytes containing various alkali metal cations: (a) LiCl, (b) NaCl, (c) KCl, and (d) CsCl at 1 mol dm − 3 concentration. It is noteworthy that well-defined voltammograms were obtained in the electrolytes containing lithium, sodium or potassium. The system’s formal (mid-peak) potentials move towards more positive values upon transfers from LiCl to NaCl and KCl. This behavior resembles characteristics of NiHCNFe film [19–22]. What is different — in comparison to NiHCNFe — is the distorted voltammetric behavior in cesium electrolyte (curve d). Apparently, the structure of a composite, Pd-containing, Ni/PdHCNFe is different, and it is probably less ‘open’ than that of NiHCNFe. It should be remembered that the ability of NiHCNFe to transport Cs+ ions during redox reactions is unique among metal hexacyanoferrates. Comparison of the dynamics of charge propagation in Ni/PdHCNFe versus NiHCNFe in various alkali metal supporting electrolytes will be the subject of our next communication. It is also noteworthy that composite Ni/PdHCNFe film shows very good stability on various electrode substrates including glassy carbon, platinum and SnO2coated glass. Using a typical film deposited on glassy carbon (as for Fig. 6) during potential cycling for 2 h
within a potential range from 0 to 0.9 V in 1 mol dm − 3 KCl (as for Fig. 6(c)), the loss of peak current on the level less than 5% was observed. For example, the dissolution of simple NiHCNFe film under the same experimental conditions was much faster. It is likely that the improved stability of Ni/PdHCNFe is due to the lower solubility product of the composite material. An alternative explanation may concern the mechanism of degradation: perhaps, during potential cycling, the material containing Pd2 + at interstices does not suffer reorganization in a manner analogous to single metal hexacyanoferrates [37,38]. Having in mind the good stability and the fact that sizable charges can be re-
Fig. 5. The dependence of the molar fractions of palladium ions (xPd) in the electrodeposited films on the respective values of xPd in the solutions for modifications. Other experimental details as for Fig. 4.
P.J. Kulesza et al. / Journal of Electroanalytical Chemistry 487 (2000) 57–65
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3.4. Comparison with mechanically attached precipitates
Fig. 6. Voltammetric responses of composite Ni/PdHCNFe film in electrolytes (1 mol dm − 3): (a) LiCl, (b) NaCl, (c) KCl and (d) CsCl. Other conditions as for Fig. 2.
Fig. 7. Voltammetric behavior of mechanically attached metal hexacyanoferrates: (a) NiHCNFe; (b) Ni/PdHCNFe; (c) PdHCNFe; and (d) NiHCNFe +PdHCNFe (physical mixture). Powders were introduced onto wax impregnated graphite electrode in the reduced forms. Electrolyte: 1 mol dm − 3 KCl. Scan rate: 10 mV s − 1. Electrode area: 0.20 cm2.
Fig. 8. Visible absorbance spectra of (A) NiHCNFe; (B) Ni/PdHCNFe; and (C) PdHCNFe in the (a) reduced and (b) oxidized forms. Precipitates were introduced onto glass slides as gels.
versibly accumulated as is evident from the system’s voltammetric peaks (Fig. 6), the composite material may be of importance in the construction of charge storage devices such as faradaic (redox) supercapacitors.
It has been established that metal hexacyanoferrates can not only be electrodeposited, but also mechanically attached to electrode surfaces by an abrasive method [25,26]. Fig. 7 shows cyclic voltammetric responses of the precipitates attached mechanically to a paraffinimpregnated graphite electrode: (a) NiHCNFe II II (K1.38NiII 1.31[Fe (CN)6]), (b) Ni/PdHCNFe (K1.74Pd0.60II II II Ni0.53[Fe (CN)6]), (c) PdHCNFe (K1.98Pd1.01[FeII(CN)6]) and (d) NiHCNFe admixed with PdHCNFe. The respective materials were prepared as described in Section 2, and their stoichiometries were determined using atomic absorption. It follows from comparison of curves d and b in Fig. 7 that the voltammetric characteristics of a mixture of NiHCNFe+ PdHCNFe is markedly different from that of Ni/PdHCNFe. Thus, Pd/NiHCNFe is not a simple mixture of NiHCNFe and PdHCNFe. On the other hand, the voltammetric response of NiHCNFe+ PdHCNFe (curve d) can be viewed in terms of superposition of the respective responses of NiHCNFe (curve a) and PdHCNFe (curve c). The voltammetric behavior of the mechanically attached material often differs from that characteristic of a respective film prepared by electrodeposition through potential cycling. Indeed, the voltammetric response of mechanically attached Ni/PdHCNFe precipitate (Fig. 7(b)) is somewhat different from the voltammetric pattern of electrodeposited Ni/PdHCNFe (Fig. 2(c)). Despite some ohmic effects, which may affect the cyclic voltammetry of mechanically attached films, it is apparent from Fig. 7 that Ni/PdHCNFe prepared by the abrasive method resembles PdHCNFe more than NiHCNFe. Indeed, the elemental analysis data (atomic absorption) shows the larger content of Pd (relative to Fe and Ni) in the mechanically attached Ni/PdHCNFe precipitates when compared with the respective electrodeposited films (remembering that both bulk samples and films were prepared from solutions containing equimolar amounts of nickel and palladium ions). We have also examined the visible spectra of the investigated metal hexacyanoferrate precipitates (introduced onto glass slides as gels). Fig. 8 summarizes spectra of (A) NiHCNFe, (B) Ni/PdHCNFe and (C) PdHCNFe in their (a) reduced and (b) oxidized states. In all cases, the spectra of the oxidized and reduced forms are different. Further, upon incorporation of palladium, the absorption band of the oxidized form tends to appear at higher wavelengths, and it resembles the response of oxidized PdHCNFe somewhat. Both oxidized Ni/PdHCNFe and PdHCNFe are deep red. We have also performed an in-situ spectroelectrochemical experiment using an optically transparent electrode covered with composite film that has been
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Fig. 9. The relationship between the visible spectrum of a composite Ni/PdHCNFe film (xPd =0.4) electrodeposited on an optically transparent electrode and the potential applied to the electrode: (a) 0.900; (b) 0.600; and (c) 0.300 V.
electrodeposited from the solution for modification (as described in Section 2) containing equimolar amounts of Ni2 + and Pd2 + ions. It follows from Fig. 9 that the system’s spectral response changes depending on the potential applied. What is important from the viewpoint of potential electrochromic applications is that the spectral difference (contrast) between the oxidized and reduced forms of composite Ni/PdHCNFe system is clearly marked.
4. Conclusions We demonstrate the possibility of preparation of thin films of composite (mixed) nickel/palladium hexacyanoferrates on electrode surfaces. The systems can be electrodeposited by potential cycling or fabricated by mechanical attachment through the abrasive method. On the basis of electron microscopic, voltammetric and spectrochemical experiments, we postulate that both electrodeposited and bulk (precipitated) composite NiHCNFe microstructures are not simple mixtures of nickel and palladium hexacyanoferrates. The relative concentrations of nickel(II) and palladium(II) ions in the solutions for modification determine the stoichiometric ratios of these ions in the electrochemically generated composite films. Higher ratios of palladium to nickel have been found in the analogous bulk Ni/ PdHCNFe precipitates. When the concentration of palladium ions in the solution for modification is lower or equal to that of nickel ions, the most likely mechanism of the formation (by potential cycling) of a composite film involves electrodeposition of NiHCNFe followed by incorporation of Pd2 + ions at the interstitial positions of NiHCNFe to form Ni/PdHCNFe. Composition of such Ni/PdHCNFe (reduced) films has been determined using atomic absorption, and it can be expressed in terms of an empirical stoichiometric forII II mula, K1.74 − 2y PdII y Ni1.13[Fe (CN)]6 (y B 0.72), consistent with the existence of a mixed phase with respect to interstitial ions. When, in the solution for modification,
the concentration of palladium(II) is higher than that of nickel(II), composite films, that contain relatively larger amounts of palladium (in comparison to nickel) ions, are electrodeposited. The structure of such films is probably complex, but our present results support the possibility of the formation of heteronuclear nickel/palladium hexacyanoferrate as a predominately mixed phase in which both nickel(II) and palladium(II) can act as nitrogen-coordinated metal ions in the cyanometallate three-dimensional cubic type lattice. In other words, the system may feature a heteronuclear cyanide-bridged network analogous to that recently proposed for hybrid nickel/cobalt hexacyanoferrate [39]. Composite Ni/PdHCNFe shows promising electrochromic properties. Good stability and favorable dynamics of charge transport during potential cycling in lithium, sodium and potassium salt electrolytes could be of importance in electrochemical applications including charge storage.
Acknowledgements This work was supported by the State Committee for Scientific Research (KBN), Poland under Grant 3T09A 09313 and by the Italian National Research Council, CNR (Rome, Italy). P.J.K. and M.A.M. appreciate travel grants from the University of Camerino. We appreciate the technical help of Laura Petetta (CIGA, University of Camerino) with the EDX analysis.
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