Journal of Electroanalytical Chemistry 628 (2009) 67–72

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Journal of Electroanalytical Chemistry

journal homepage: www.elsevier .com/locate / je lechem

Effect of the presence of ½CoðbpyÞ3�2þ on the electrochromic responses of films

of a redox active Ni–Al-layered double hydroxide

Debajyoti Mondal, Gilles Villemure *

Department of Chemistry, University of New Brunswick, P.O. Box 4400, Fredericton, New Brunswick, Canada E2B 5A3

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 September 2008Received in revised form 19 December 2008Accepted 13 January 2009Available online 22 January 2009

Keywords:HydrotalciteCyclic voltammetryUV–visible spectroscopyColouring reactionBleaching reactionNickel oxidesAnodic electrochromism

0022-0728/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.jelechem.2009.01.007

* Corresponding author. Tel.: +1 506 453 4998; faxE-mail address: gvd@unb.ca (G. Villemure).

The addition of ½CoðbpyÞ3�2þ to the electrolyte was found to improve the reversibility of the colour change

observed in films of a Ni–Al-LDH upon oxidation–reduction of nickel in the LDH layers. Sustained changesin relative transmittance between 95% in the reduced state and 75% in the oxidized state were obtainedfor a film subjected to continuous potential scanning in a pH 8 borate buffer containing 0.1 mM½CoðbpyÞ3�

2þ. This improvement in the electrochromic response of the film is attributed to mediation ofthe redox process in the LDH by the cobalt cations that resulted in the full reduction of all nickel sitesback to Ni(II) by the end of each individual scan. In the absence of the cobalt cation, the colour changeswere not fully reversible and a rapid darkening of the film was observed. Because the unaided electro-chemical reduction of nickel in LDH films is very slow, oxidized nickel sites remained in the films atthe end of each scan. This accumulation of Ni(III,IV) sites was confirmed by increases in the average nickelvalence in the films determined by iodometry, and by the appearance of reflection due to a Ni(III) oxy-hydroxide phase in the XRD pattern of the film. This was not found when the cycling was done in pres-ence ½CoðbpyÞ3�

2þ. Mediation of the reduction of nickel in the Ni–Al-LDH by the cobalt cation was veryefficient. The presence of 5 lM ½CoðbpyÞ3�

2þ was sufficient to cause a significant improvement of thebleaching process.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Layered double hydroxides (LDHs), also known as hydrotalcitesor anionic clays consist of MIIðOHÞ6 and MIIIðOHÞ6 edge-sharingoctahedrons forming sheets similar to those of brucite [1]. Net po-sitive charges on the layers are balanced by exchangeable anionsintercalated between the sheets [2–5]. Electrodes modified withfilms of Ni–Al-LDHs in particular, have attracted considerableattention [6–10]. Ni–Al-LDH films have been reported to catalyzethe electrochemical oxidation of primary alcohols and sugars[11,12]. They have been used as ion selective electrodes for poten-tiometric sensing of anions such as nitrate and chloride [13], andfor amperometric sensing of methanol, mannitol and glucose[14]. LDHs containing nickel are under consideration as alternativecathode materials for nickel batteries [15,16]. The structure of a-Ni(OH)2 is very similar to that of Ni–Al-LDH [6]. However, a-Ni(OH)2 is not stable in base. It is rapidly converted to b-Ni(OH)2

after only a few potential scans. One promising approach to stabi-lize the alpha phase of nickel hydroxide is to substitute some of theNi(II) in the layers by trivalent metals, such as aluminum, iron,chromium, etc. [15,16], in essence by converting a-Ni(OH)2 into anickel LDH.

ll rights reserved.

: +1 506 453 4981.

The electrochromic properties of Ni(OH)2 and NiO films havebeen extensively investigated [17–23]. Electrochromic materialsare substances whose optical properties change reversibly whenthey are subjected to an electric field, typically going from a trans-parent state, known as the bleached state, to a coloured state [24–28]. These materials have potential applications for informationdisplay, light shutters, smart windows, variable-reflectance mir-rors, variable-emittance thermal radiators, etc. [29–32]. The col-ouring reactions in NiO and Ni(OH)2 films are similar involvingthe oxidation of Ni(OH)2 to Ni(III) oxyhydroxides [17,25]. In thecase of the oxides, this Ni(OH)2 is first formed in situ at the surfaceof the grains. A common method to prepared electrochromic NiOfilms is the thermal treatment of Ni(OH)2 films prepared by sol–gel process [18,19] or electro deposition [20–22,33,34]. The elec-trochromic properties of nickel oxides and hydroxide films are inmany ways complementary to those of WO3 films. Tungsten oxidefilms go from transparent to blue on reduction [17,18]. Nickeloxide or hydroxide films go from clear to brown–black on oxida-tion [19]. Tungsten oxide films adsorb more toward the red endof the visible spectrum, while NiO absorbs more blue light, produc-ing when combined a neutral grey coloured dark state that is desir-able for applications such as smart windows [23,25].

To our knowledge the electrochromic properties of nickel LDHshave not been studied. It has long been known that the oxidation ofNi–Al-LDH films was accompanied by a colour change from clear to

68 D. Mondal, G. Villemure / Journal of Electroanalytical Chemistry 628 (2009) 67–72

black [6]. However, this colour change is not easily reversed be-cause complete reduction of nickel in LDH films back to Ni(II) is dif-ficult [6,9]. Recently, we reported very large cathodic peak currentsfor ½CoðbpyÞ3�

2þ cations at electrodes modified with Ni–Al-LDHfilms when the switching potential used was high enough to oxi-dize the LDH Ni(II) sites [35]. This was attributed to mediation ofthe reduction of Ni(III) in the films by the ½CoðbpyÞ3�

2þ cations.We report here the effect of the addition of ½CoðbpyÞ3�

2þ to the elec-trolyte on the electrochromic response of Ni–Al-LDH films. Thepresence of even a small concentration of ½CoðbpyÞ3�

2þ was foundto improve greatly the rate and extend of bleaching of the filmsduring the cathodic scans, resulting in improved reversibility ofthe electrochromic process.

2. Experimental section

NiCl2 � 6H2O was obtained from Mallinckrodt. All other chemi-cals were from Aldrich and were used as received. [Co(bpy)3]Cl2

was prepared by reacting CoCl2 � 6H2O with excess bipyridyl inhot ethanol, as described by Nyholm and Burstall [36]. The Ni–Al-LDH was prepared by titration of a mixed solution of the metalions with NaOH, as described by Boclair and Braterman [37].Briefly, a solution 0.075 M in Ni(II) and 0.025 M in Al(III) was pre-pared by dissolving the metal chlorides in 250 mL of carbonate-free water. The solution was brought to boil and titrated with0.8 M NaOH under flowing nitrogen to approximately 92% of theend point required for the formation of a 3:1 LDH. To improve crys-tallinity, the LDH slurry was aged in an autoclave (Parr Instrument,Moline, Illinois) under argon at an initial pressure 100 psi and atemperature of 150 �C for 18 h [38]. The precipitate was collectedand the <0.2 lm size fraction separated by centrifugation [39].The solid was washed with distilled water and dried in a vacuumover silica gel for 2 days.

The layered structure of the LDH was confirmed by powder X-ray diffraction (XRD). The XRD patterns were obtained on a BrukerD8 X-ray diffractometer using Cu Ka radiation at 40 kV and 20 mAand a scanning rate of 0.04 2h s�1. Elemental analysis of the LDHpowder was done at the Research Productivity Council, New Bruns-wick, Canada. Inductively coupled plasma emission spectrometry(ICP) and ion chromatography were used to determine the metalsand chloride contents, respectively. The total carbon content of theLDH was determined as CO2 in a Leco induction furnace. Resultswere used to calculate a structural formula for the LDH,[Ni0.783Al0.217(OH)2][Cl0.135(CO3)0.04].

The LDH films were deposited by the solvent evaporation meth-od on tin-doped indium oxide coated glass substrates (ITO) (Delta

Fig. 1. Transmittance versus potential curves obtained for Ni–Al-LDH films recorded in0.1 mM ½CoðbpyÞ3�

2þ (20 mV/s in 0.1 M borate buffer pH 8). Average transmittance betwereference (see text).

Technology, Stillwater, MN) [34]. The ITO substrates were cut into0.7 � 5 cm strips, cleaned by sonication in water and then in etha-nol. Twenty microliters aliquots of a 10 g/L suspension of the LDH,prepared by stirring the powder in water for 3 h, were spread over0.80 cm2 areas of the substrates and the water was allowed toevaporate in air overnight. Examination of the coated slides bySEM showed the films completely covered the ITO surfaces andwere uniform and featureless [40].

For the electrochromic studies, a 1 � 2 cm quartz cuvette pol-ished on all four sides was used as electrochemical cell. It wasplaced in the beam of a Hewlett Packard model 8452 diode arrayspectrophotometer. The ITO working electrodes, coated with theLDH films, were placed in the cuvette in such a way that the filmsurfaces were parallel to the 1 � 2 cm faces of the cuvette. AnAg|AgCl|3 M NaCl reference electrode (BAS MF-2063) and a Pt wirecounter electrode were placed in the cuvette, to the sides out of thepath of the beam. Changes in the UV–visible absorbance of the LDHfilms were recorded in situ during the electrochemical measure-ments. Unless otherwise stated, the reported transmittances arethe average transmittances of the films between 400 and500 nm, using the fully reduced as prepared film on the ITO sub-strate as reference. The electrochemical measurements were madewith a Princeton Applied Research PARC Model 273 potentiostat.The supporting electrolyte was a 0.1 M potassium borate bufferpH 8.0. All the solutions were degassed with N2 prior to the mea-surements and all measurements were made at room temperature.

The average oxidation state of Ni in the Ni–Al-LDH films afterpotential scans or potential steps was determined by iodometry[41]. The films were immersed in a solution containing 0.1 M aceticacid, 0.1 M potassium acetate and 0.1 M potassium iodide for15 min. The iodine formed was determined from the absorbanceof this solution at 352 nm (� = 12,000 M�1 cm�1) [41]. All solutionswere degassed with N2 and the measurements done under a nitro-gen atmosphere.

3. Results

Fig. 1A shows the transmittance of a Ni–Al-LDH film as a func-tion of the applied potential recorded in situ during a potentialscan. Prior to the scan, the fully reduced film was transparent.The transmittance remained constant near 100% until the potentialreached about 1.0 V where the onset of oxidation of Ni(II) in theLDH resulted in an immediate decrease in transmittance. Thisdarkening of the film during the anodic scan was not fully revers-ible. The transmittance barely returned to 70% by the end of thecathodic scan. This was expected. The complete electrochemical

situ during the first cyclic voltammetric scan in (A) absence and (B) presence ofen 400 and 500 nm using the fully reduced as prepared film on the ITO substrate as

Fig. 3. Transmittance of Ni–Al-LDH films subjected to potential steps in presence of(a) 0.0 lM, (b) 1.5 lM, (c) 5.0 lM, (d) 15 lM, (e) 50 lM and (f) 100 lM ½CoðbpyÞ3�

2þ

(Potential was stepped from 0.4 to 1.4 V at 10 s and from 1.4 to �0.5 V at 110 s).

Table 1Transmittance of Ni–Al-LDH films versus concentration of ½CoðbpyÞ3�

2þ for the firstbleaching step.

½CoðbpyÞ3�2þ (lM) Tcoloured

a (%) Tbleachedb(%) DT DOD

100 32 94 62 0.4750 30 90 60 0.4710 27 84 57 0.495.0 30 65 35 0.341.0 29 52 23 0.250 25 44 19 0.25

a Average transmittance of the films between 400 and 500 nm after 100 s ofoxidation at 1.4 V.

b Average transmittance of the films between 400 and 500 nm after 100 s ofoxidation at 1.4 V followed by 100 s of reduction at �0.5 V.

D. Mondal, G. Villemure / Journal of Electroanalytical Chemistry 628 (2009) 67–72 69

reduction of nickel in LDHs back to Ni(II) is known to be slow andto require long reduction times [6,9]. Fig. 1B shows the effect ofadding ½CoðbpyÞ3�

2þ to the electrolyte solution. The darkening ofthe film during the anodic scan was not affected very much. Theminimum transmittance observed at the beginning of the cathodicscan, 52%, was only slightly larger than the minimum transmit-tance of 48% observed in the absence of the cobalt cation. The col-our change was however, much more reversible. By the end of thescan, the transmittance had returned to 98%. Two distinct stepswere also seen in the transmittance curve during the cathodic scan.The first began near 1.0 V, the potential at which electrochemicalreduction of the nickel sites began. The second occurred around0.0 V, the potential at which electrochemical reduction of½CoðbpyÞ3�

3þ began. The current potential curve obtained in pres-ence of the cobalt cation was similar to what we previously re-ported [35]. In particular, a very large cathodic peak wasobserved for the ½CoðbpyÞ3�

3þ=2þ couple. The cathodic peak currentat �0.22 V was 210 lA. The corresponding anodic peak currentwas less than 5 lA and cannot be seen on the current scale usedin Fig. 1B.

Fig. 2 shows variations in the transmittance of Ni–Al-LDH filmsrecorded during 40 continuous potential scans. In the absence ofthe cobalt cation, the colour changes in each individual scan werenot fully reversible and a rapid darkening was observed. At thestart of the 20th scan for example, the transmittance of the filmwas down to 9%, and it decreased to 6% on oxidation. In the pres-ence ½CoðbpyÞ3�

2þ, the colour change was much more reversiblewith the transmittance in the 20th scan varying between 95% inthe reduced state and 75% in the oxidized state.

Fig. 3 shows the transmittance of Ni–Al-LDH films recordedin situ during potential steps in electrolyte solutions containing dif-ferent concentrations of ½CoðbpyÞ3�

2þ. Starting at time t = 10 s, uponthe application of a positive potential of 1.4 V, the transmittance ofall films decreased rapidly as the nickel sites were oxidized. Thisdarkening of the film was not affected very much by the concentra-tion of ½CoðbpyÞ3�

2þ used. The minimum transmittances after 100 sof oxidation were similar for all films, varying from 25% in absenceof the cation, to 32% in presence of 0.1 mM ½CoðbpyÞ3�

2þ (see Table1). The colouring response times were also similar for all films,with 90% of the transmittance changes occurring in the first45 ± 5 s of oxidation. The colouration efficiency [33] determinedfrom the slopes of the optical density change, DOD, versus chargeplots was approximately 30 cm2/C in absence of ½CoðbpyÞ3�

2þ, andslightly smaller, 20–25 cm2/C in presence of the cation. These val-

Fig. 2. Transmittance of Ni–Al-LDH films during continuous potential cycling in (a)absence and (b) presence of 0.1 mM ½CoðbpyÞ3�

2þ (20 mV/s between �0.5 and 1.4 Vvs. Ag/AgCl in 0.1 M borate buffers pH 8).

ues are comparable to previous reports for nickel hydroxide andoxide films [20,33,42,43]. Note, the DOD versus charge plots wereslightly curved with their slopes decreasing with time after thestep. This was likely due to contribution to the anodic charges fromoxidation of water at the high oxidation potential used, 1.4 V.

On the other hand, Fig. 3 shows that bleaching of the films whenthe potential was stepped back down to �0.5 V at t = 110 s wasvery dependent on the concentration of ½CoðbpyÞ3�

2þ used. Varia-tions in the transmittance of the films during the bleaching stepare summarized in Table 1. In the absence of the cation, the trans-mittance of the film after 100 s of reduction was 44% comparedwith 94% in the presence of 100 lM ½CoðbpyÞ3�

2þ. The presence ofas little as 5 lM ½CoðbpyÞ3�

2þ resulted in significant increases inboth DT and DOD obtained after 100 s of reduction. Table 1 alsoshows that the DT and DOD obtained in presence of 10, 50 or100 lM ½CoðbpyÞ3�

2þ were similar. A 10 lM concentration of thecobalt cation was sufficient to ensure almost complete bleachingof the films after 100 s of reduction. Higher concentrations of thecation only decreased the time required for this complete bleach-ing to occur.

Fig. 4 shows the powder X-ray diffraction patterns of Ni–Al-LDHfilms before and after potential cycling. A freshly prepared film thathad not been subjected to potential cycling (curve a) gave only twopeaks at 7.9 and 3.97 Å, attributed to the [003] and [006] reflectionof the LDH. The XRD pattern of a film that had been cycled 20 timesin a borate buffer pH 8 containing 0.1 mM ½CoðbpyÞ3�

2þ (curve c) wasvery similar, with the same two reflections observed at essentially

Fig. 4. XRD pattern of Ni–Al-LDH films as prepared film (a) after 20 cycles in aborate buffer (b) and after 20 cycles in borate buffer containing 0.1 mM ½CoðbpyÞ3�

2þ

(c) (20 mV/s between �0.5 and 1.4 V).

70 D. Mondal, G. Villemure / Journal of Electroanalytical Chemistry 628 (2009) 67–72

the same positions. However, the XRD pattern of a film that hadbeen subjected to the same 20 potential scans in the blank boratebuffer was different (curve b). In addition to the [003] and [006]reflections of the LDH, an additional peak at 12.4� 2h, and a verysmall refection at 25� 2h, corresponding to lattice spacings of 7.14and 3.6 Å are seen. These reflections can be taken as evidence ofthe presence of c-NiOOH in the film [44]. The reflection near 28�2h was due to the ITO substrate.

Table 2Nickel valences in Ni–Al-LDH films.

Films In absence of ½CoðbpyÞ3�2þ 0.1 mM ½CoðbpyÞ3�

2þ

Ni valencea Transmittance(%)

Ni valencea Transmittance(%)

20 Potentialcyclesb

2.5+ 9 2.0 95

Oxidizedc 2.4+ 25 2.3 32Reducedd 2.3+ 44 2.0 94

a Determine by iodometry after the electrochemical measurements (see text).b Film subjected to 20 potential cycles between �0.5 and 1.4 V (20 mV/s in borate

buffer pH 8).c After 100 s of oxidation at 1.4 V (see Fig. 3).d After 100 s of oxidation at 1.4 V followed by 100 s of reduction at �0.5 V.

Fig. 5. First and twentieth scans in the CV of Ni–Al-LDH films in (A) presence of 0.1 mM

Table 2 gives the average nickel valences in LDH films deter-mined by iodometry [41]. The average oxidation state of nickel ina film scanned 20 times in the blank borate buffer containing no½CoðbpyÞ3�

2þ was 2.5+. When the cycling was done in presence of0.1 mM ½CoðbpyÞ3�

2þ, the same measurement gave a nickel valenceof 2.0+. The average valence of nickel in LDH films after 100 s ofoxidation at 1.4 V in presence or absence of the cobalt cation weresimilar, 2.4 and 2.3+, respectively. However, in the absence of½CoðbpyÞ3�

2þ, 100 s of reduction at �0.5 V resulted in only a smalldecrease of the nickel valence to 2.3+. In the electrolyte containing0.1 mM ½CoðbpyÞ3�

2þ, 100 s of reduction at �0.5 V was sufficient tofully reduce all nickel sites in the film back to Ni(II).

Fig. 5A shows the 1st and 20th scans in the CV of a Ni–Al-LDHfilm recorded in an electrolyte solution containing 0.1 mM½CoðbpyÞ3�

2þ. The voltammogram was observed to evolve withthe number of scans. In the initial scan, one large broad oxidationpeak at 1.34 V, and a large reduction peak at 0.70 V were observed,along with a second large reduction peak at �0.22 V, attributed toreduction of the cobalt cation [35]. By the 20th scan however, theoxidation wave had split into two distinct oxidation peaks at 1.03and 1.30 V, and both the nickel and cobalt reduction peaks weresmaller and had shifted to higher potentials 0.76 and �0.05 V,respectively. In the absence of the cobalt cation no splitting ofthe nickel oxidation peak was observed and the nickel reductionpeak was actually larger in the 20th scan than in the 1st scan(Fig. 5B).

4. Discussion

The improved reversibility of the colouring–bleaching processin Ni–Al-LDH films, demonstrated by the results of Figs. 1–3, is inaccord with what was expected based on our recent report onthe CV of the ½CoðbpyÞ3�

2þ at electrodes modified with Ni–Al-LDHfilms [35]. As shown in Fig. 1B, a very large cathodic peak was ob-tained for the cobalt cation, 210 lA compared with a correspond-ing anodic peak of less than 5 lA. In a previous report, weproposed that this was due to charge transfer between½CoðbpyÞ3�

2þ and oxidized Ni(III,IV) sites in the LDH layers thatregenerated the oxidized form of the couple [35]. In other words,the cobalt bipyridyl cations acted as charge shuttles mediatingthe reduction of the LDH nickel sites. Because of the low conductiv-ity of reduced Ni–Al-LDH films, the full electrochemical reductionof all nickel sites in LDH films is difficult and requires long reduc-tion times [6,9] This slow reduction explains the incompletebleaching of the film during the cathodic half scan in Fig. 1A, andthe rapid drop in the transmittance of a film subjected tocontinuous potential cycling (Fig. 2, curve a). Because, the unaided

½CoðbpyÞ3�2þ and (B) absence of ½CoðbpyÞ3�

2þ (20 mV/s in 0.1 M borate buffer pH 8).

D. Mondal, G. Villemure / Journal of Electroanalytical Chemistry 628 (2009) 67–72 71

reduction of LDH films was slow oxidized Ni(III,IV) sites remainedin the films at the end of each scan. In other words, Ni(III,IV) sitestended to accumulate in films subjected to continuous potentialcycling. This is supported by the iodometry measurements in Table2. The average nickel oxidation state in a film scanned 20 times inthe blank borate buffer was 2.5+, compared with 2.0+ for a filmscanned in an electrolyte containing 0.1 mM ½CoðbpyÞ3�

2þ. This fullreduction of the nickel sites explains why, in presence of½CoðbpyÞ3�

2þ, the transmittance of the film returned to nearly100% at the end of each scan.

The shape of the transmittance versus potential curve in Fig. 1Bprovides good evidence for the occurrence of this mediated reduc-tion of the LDH nickel sites. The second step seen in the transmit-tance versus potential curve during the cathodic scancorresponded to the potential at which electrochemical reductionof the cobalt cation began. Similarly, the lack of changes in theXRD pattern of a film cycled in presence of 0.1 mM ½CoðbpyÞ3�

2þ

compared with what was found for a film cycled in the blank elec-trolyte (Fig. 4) is consistent with mediation of the reduction of thenickel sites by the cobalt cations.

Corrigan [45] reported that the addition of small amounts of½FeðCNÞ3�

3� to the electrolyte improved the durability of electro-chromic Ni(OH)2 films. He proposed that the ferricyanide ionsacted as redox mediators, allowing the reduction of nickel in partsof the materials that had become electrically isolated because ofmechanical deterioration of films during the cycling and couldtherefore not be reduced electrochemically [45]. This is similar towhat is proposed here except that electrical insulation is believedto arise from reduction the nickel at the base of the films, near thesurface of the ITO substrates [6,7,9] rather than from mechanicaldamage. In fact, we have found that the voltammetric responseof ferricyanide ions was also greatly enhanced at electrodes mod-ified with redox active Ni–Al-LDH films [6,7]. Ferricyanide anionshowever, can be intercalated in the positively charge LDHs[6,7,46]. This is not the case for metal bipyridyl cations [35]. Giventhis, the efficiency of the mediation of the reduction of nickel inLDHs by ½CoðbpyÞ3�

2þ is surprising. Fig. 3 and Table 2 show that5 lM ½CoðbpyÞ3�

2þ was sufficient to measurably improve bleachingof the films.

An unexpected result is the changes seen in the CV of LDH filmscycled in presence the ½CoðbpyÞ3�

2þ. It is tempting to suggest thatthe two nickel oxidation peaks seen in the 20th scan in Fig. 5Awere due to oxidation of Ni(II) to Ni(III) and Ni(IV). This has beenproposed to account for the two oxidation peaks seen in the CV ofNiO films [19]. This would not explain however, why the splittingof the nickel oxidation wave was not observed in absence of½CoðbpyÞ3�

2þ (Fig. 5B). Nor would it account for the decrease seenin the cobalt cation reduction peak. As discussed above, the largecobalt cathodic peak at Ni–Al-LDH electrodes is attributed tomediation of the reduction of the LDH nickel sites by½CoðbpyÞ3�

2þ. This regenerated the oxidized form of the couple thatcould be reduced again multiple time in any given scan [35]. Asmaller ½CoðbpyÞ3�

3þ reduction peak in the 20th scan suggests thatthe mediation of the reduction of the Ni sites was less efficient inthe 20th scan. Yet, Fig. 2 shows that the beaching process was asefficient in the 20th scan than in the 1st scan with the transmit-tances of the film returning to P95% at the end of every scan. Infact, the major effect of continuous potential cycling was a lessefficient colouring process. In the 1st scan the minimum transmit-tance reached 45% compared with only 75% in the 20th scan. Thisis consistent with the decrease in the size of the voltammetricwaves with the number of scans. For example, integration of thepeaks in Fig. 5 shows that the charge passing in the 20th scanwas only about one quarter of the charge passing in the 1st scan.Apparently, fewer of the LDH nickel sites were oxidized in the20th scan than in the 1st scan.

Finally, preliminary measurements were also made in a morebasic 0.1 M KOH electrolyte. Most publications on electrochromicnickel oxide and hydroxide films used highly basic electrolytes.Indications are that in 0.1 M KOH, DOD similar to those obtainedhere could be obtained with thinner films containing less LDH.The oxidation of nickel in Ni–Al-LDH films is easier at higher pH.The redox process however, also appeared to be less reversible athigher pH. Even in presence of 0.1 mM ½CoðbpyÞ3�

2þ the relativetransmittance did not returned anywhere near 100% after the ini-tial oxidation of the LDH films.

5. Conclusion

The addition of even a small concentration of ½CoðbpyÞ3�2þ to the

electrolyte solution was found to improve the reversibility of thecolour change in electrochromic Ni–Al-LDH films. This was attrib-uted to mediation of the reduction of the Ni sites in the films by thecobalt cation. The unaided electrochemical reduction of nickel inLDH films is much slower than the electrochemical oxidation ofnickel. Thus, in absence of the mediator, oxidized nickel sites accu-mulated in the LDH films when they were subjected to continuouspotential cycling, or multiple potential steps, resulting in rapiddarkening of the films. Mediation by the cobalt cation ensured thatcomplete reduction of nickel in the LDH back to Ni(II) occurred.This mediation was surprisingly efficient. Despite the fact thatthe positively charged cobalt ions were not adsorbed by the LDHfilms, the presence of as little as 5 lM ½CoðbpyÞ3�

2þ in the electro-lyte solution was sufficient to cause a significant improvement inthe bleaching process.

Acknowledgments

We thank Dr. D. Hall (Microscopy and Microanalysis facility) forthe SEM measurements, Dr. V. Reddy (Department of Geology) forthe XRD measurements and Mr. R. Kean (Research and ProductivityCouncil, Fredericton NB) for the elemental analysis of the LDH.Financial support was provided by the Natural Science and Engi-neering Research Council of Canada and the University of NewBrunswick.

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