Infrared characterization of electrochromic nickel hydroxide prepared by homogeneous chemical precipitation

180

TI1hl SolM Films. 229 (1993) 180- 186

Infrared characterization of electrochromic nickel hydroxide prepared by homogeneous chemical precipitation R . M . T o r r e s i , M . V. V / t z q u e z * , A . G o r e n s t e i n

a n d S. I. C 6 r d o b a

de Torresi

Departamento de Fisica Aplicada, Instituto de Fisica "'Gleb Wataghin", UNICAMP, CP 6165. 13081 Campinas (SP) (Brazil) (Received October 29, 1992; accepted December 23, 1992)

Abstract

Two different methods for preparing electrochromic nickel hydroxide films are tested: thermal hydrolysis of [Ni(NH3)6]2 + complex and thermal decomposition of a mixture of urea and NiSO4. Both methods lead to adherent and homogeneous films presenting good etectrochromic properties. Thermogravimetric analysis shows that Ni(OH)2 prepared from urea decomposition has intercalated water molecules, while hydroxides prepared from ammonia complexes present//-like structure without hydration. Ex-situ infrared spectroscopy was used to characterize virgin, bleached and colored states of the films under different experimental conditions. IR spectra of the colored state show the diminution of the 3650 cm- ~ band related to free OH- stretching vibrations. The disappearance of this band is associated with the oxidation process and also with the physicochemical nature of alkaline cations incorporated in the oxide matrix during the coloration process.

1. Introduction

The different structural characteristics of nickel (II) hydroxides and their oxidation products have been the subject of a considerable number of studies [1-4]. Four structures have been clearly established: ~-Ni(OH)2. Hydrated hydroxide with a stacking of brucite-type layers along the c axis which are oriented in random fashion and separated by intercalar water molecules [5]. The presence of adsorbed species which are retained from the mother solution during precipitation has been reported in several studies [1-6]. fl(II)Ni(OH)2. Hydroxide with a lamellar structure. It crystallizes in the hexagonal system with a = 3.126 A and c =4.605 ,~. This "well-crystallized" structure is isomorphous with brucite, Mg(OH)2 [1]. fl(III)NiOOH. Structure closely related to that of /3(II) phase from which it derives, due to deintercalation of one proton and one electron. It crystallizes in the hexagonal system with a = 2.82 ~ and c = 4.85 A [2]. 7-NiOOH. This phase has a lamellar structure with layers of the same composition of /3(III) phase but variable quantities of water molecules and alkaline metal ions are intercalated between the layers, leading to an interlamellar spacing of about 7 ,~. The unit cell is tripled with a = 2.82 A and c = 21 • [2].

*On leave from INFIQC, Dto. de Fisicoquimica, Fac. de Ciencias Quimicas, Univ. Nacional de Crrdoba, 5016 Crrdoba, Argentina.

0040-6090/93/$6.00

All these phases are involved in the reactions of nickel oxide electrodes (NOE). Such modifications are summarized in the well-known Bode diagram [7]. The structures described above represent extreme situations.It is possible to obtain other phases with intermediate characteristics, particularly depending on the preparation method. Moreover, the several chemical reactions that occur during the aging of the electrode must be taken into account [8-11]. On the other hand, nickel oxide has been considered an interesting material for electrochromic devices, and a great variety of preparation modes leading to different results has been proposed: cathodic electrodeposition [ 12], electrochemical cycling [13], magnetron sputtering [14], electron-beam reactive evaporation [15] and high temperature or anodic oxidation [16]. In the present study, nickel hydroxide thin films were prepared using two different homogeneous chemical precipitation methods; one based on Merlin's method [ 17] and the other on the thermal decomposition of urea-NiSO4 mixtures [18]. These methods give a well-defined /3 or ~ nickel hydroxide structure, respectively. The aim of this paper is not to develop a new electrochromic material prepared by a new method, but to study the electrochromic phenomena of Ni(OH) 2 thin films with well-defined structure in order to reveal the role played by these different crystallographic forms in the definition of the chemical identity of both bleached and colored states. Infrared spectroscopy has been extensively used in order to obtain structural information about Ni(OH)2 and its related compounds [19-24].

:~i 1993--- Elsevier Sequoia. All rights reserved

R. M. Torresi et al. / IR characterization o f electrochromic Ni(OH) 2

In this work structural changes produced by aging and/or voltammetric cycling in different alkaline media were studied using e x - s i t u diffuse-reflection IR spectroscopy. In addition, electrochromic measurements were carried out in order to analyze the electrochromic properties of Ni(OH)2 films prepared by the two methods studied here.

2. Experimental Homogeneous and transparent nickel hydroxide films (film thickness approx. 300 nm) were obtained using the following methods: Method 1. For the synthesis, 2 ml of 0.1M NiSO4 solution were added to 10 ml of 2M NH4OH solution in a glass tube to form the complex. Then the solution was heated for 15 minutes at 90 °C to promote complex decomposition. Method 2. In this procedure, 1.5 g of urea was dissolved in 10 ml of 0.1M NiSO4 solution in a glass tube and heated for 15 min at 90 °C in order to form Ni(OH)2. Reactions involved in the different methods are:

181

and detecting the transmitted intensity using a silicon photodetector. Thermogravimetric analyses were performed with carefully rinsed Ni(OH)2[A] or Ni(OH)2[U] dried powders under nitrogen flux at a heating rate of 10 °C min-1 using a General V2.2A DuPont 9900 thermal analysis equipment. Infrared spectroscopy experiments were carried out in a JASCO IR-700 spectrophotometer with wavenumber accuracy of _+4 cm -~ and 2 cm-I sampling step (in the 5000 to 2000 cm- l range), and _+2 cm- 1 and 1 cm-~ respectively (in the 2000 to 400 cm-t range). The SnO2/Ni(OH)2 electrodes were placed into a DR81 diffuse reflectance attachment and experiments were performed in the 1000-4000 cm-i range. A normalized spectrum was obtained by dividing the SnO2/Ni(OH)2 spectrum (R) by that of the SnO2 substrate taken as a reference (Ro). Each set of experiments was performed with a fresh sample: for this reason, in all figures the IR spectrum of virgin film is always shown.

3. Results and discussion 3. I. Electrochromic response

Method 1

Ni 2+ + 6NH4OH

, [Ni(NH3)6] 2+ -b 6H20

[Ni(NHa)612++2H20

(1)

# ;Ni(OH)2+6NH3+2H + (2)

Method 2

NH2CONH 2 + 3H20 Ni 2+ +2NHaOH

~' , CO 2 + 2NH4OH

(3)

* ,Ni(OH)2+2NH3+2H +

(4)

In both methods, films were deposited onto glasses covered with SnO2 conducting films (resistivity= 16 f~/I-q). These substrates were used for both spectroelectrochemical and IR spectroscopy experiments. The substrate was suspended in the solution during the thermal decomposition. Afterwards the electrode was taken out and carefully rinsed with purified water for subsequent characterization. The Ni(OH)2 precipitated by the ammonium complex and by urea decomposition will be referred to here as Ni(OH)2[A] and Ni(OH)2[U] respectively. After deposition, working electrodes (A = 2.2 cm 2) were placed in electrochemical cells containing different 0.1M alkaline solutions as electrolytes: LiOH, KOH or CsOH. A platinum sheet was used as the counter-electrode and all potentials were referred to the saturated calomel electrode (SCE). Electrochemical measurements were performed under potentiodynamic conditions. The voltammograms were recorded simultaneously with the change of monochromatic transmittance by illuminating the electrode with an H e - N e laser (2 = 632.8 nm)

The electrochromic behavior of the Ni(OH)2[A] and Ni(OH)2[U] electrodes was compared by following the change of transmittance during potentiodynamic cycling. The stabilized response for both samples is shown in Fig. 1, where it can be seen that the transmittance change is quite similar, indicating that the thickness of the optically active material is almost the same. During the first cycle (not shown in Fig. 1), in order to color completely the nickel hydroxide film prepared by method 2 (Ni(OH)2[U]), it is necessary to polarize at high anodic potentials. On the other hand, the asgrown films transmitted ca. 75% of the incident light but, after the first redox cycle, transmittance in the reduced state falls to ca. 55% in the cathodic limit and it remains stabilized at this value in the subsequent cycles. Taking into account that Ni + 3 compounds are black-colored, the loss of transmittance after the first cycle indicates that it is not possible to reduce all the material oxidized during the first anodic scan and a certain amount of oxidized material remains even at a bleaching potential value. The electrochromic efficiency parameter (r/) was determined from the charge density and the optical density (OD) according to [25]: ~/-

A(OD) Q

(5)

In Fig. 2 the change in the optical density with the electric charge (taken from the voltammogram) is shown for both Ni(OH)dA] and Ni(OH):[U] elec-

182

R. M. Torresi et al. / IR characterization o f electrochromic Ni(OH)2 I

I

I

I

Ni(III) electro-oxidation process is much more important than for the Ni(OH)2[A] electrodes. The r/values obtained with these electrodes are similar to those obtained for NiOx electrochromic samples (40 cm 2 C-I)[26] and other transition metal oxides like CoOx (approx. 50 cm 2 C-1) [27]. The differences observed in the electrochromic processes provide evidence that these preparation methods lead to nickel hydroxides with different structures. Electrochromic parameters like coloration efficiency and contrast depend strongly on the physicochemical characteristics of the materials. On the other hand, the overpotential of the oxygen evolution reaction seems to be reduced on Ni(OH)2[U] electrodes. This is additional evidence that methods 1 and 2 give different types of nickel hydroxides, and it seems that oxygen evolution is a very sensitive reaction to structural changes of the catalysts [28].

t~~

J.5 N I

1.0

--

0.5

--

H

E

u

0 -0.5

---

75

5O I-

o~ 25

3.2. T h e r m o g r a v i m e t r i c a n a l y s & o

I

1

t

-0.75-050-025

I

0

025 0.50 075

E/V(SCE)

Fig. 1. E vs. j and E vs. % T potentiodynamic profiles obtained for ( - - ) SnO~_/Ni(OH)2[A] and ( . . . . ) SnO2/Ni(OH)2[U] electrodes. Electrolytic solution: 0.1 M KOH, v = 5 mVs-~.

I

I

I

1.5

>-

Z hi (:3 .J

In Fig. 3, T G A curves of both Ni(OH)2[A ] (unbroken line) and Ni(OH)2[U] (dashed line) are shown. From the former, just one clearly defined thermal decomposition process appears, corresponding to a loss of mass of approx. 15.5%, starting at approximately 250°C. This process corresponds to the dehydroxilation of Ni(OH)2, in agreement with previously reported results by Le Bihan and Figlarz [29] for wellcrystallized /~-Ni(OH)2. This sample does not show intercalated water molecules, in contrast to the observation for Ni(OH)z[U] powders. The T G A curve for this latter sample shows two main losses of mass: the first, which lies between 30 and 200 °C, corresponds to the removal of adsorbed water and a proportion of the

U

0.5 0

'

o

I

l

[

I 0

0.025

0.050

0.075

O. I

Q / C c m -2

Fig. 2. Optical density vs. electric charge for ( O ) SnO2/Ni(OH)2[A ] and ( 0 ) SnO2/Ni(OH)2[U] electrodes. Electrochromic efficiency q in cm 2 C-~ is also indicated.

trodes. The electrochromic efficiencies (q) are calculated from the slope in the linear regions and they are also depicted in Fig. 2. Ni(OH)e[A] electrodes show quite similar q values for both anodic and cathodic scans, indicating a reasonable reversibility of the electrochromic process in these electrodes. On the contrary, for Ni(OH)z[U] electrodes, the electrochromic efficiency during the anodic scan is approximately half that in the cathodic one, indicating that in this case the oxygen evolution reaction occurring in parallel with the N i ( I I ) -

-I 5.5"/.

!

-21,5%

80

\

\

\

\

70

60 k

50 0

200 400 TEMPERATURE/°C

600

Fig. 3. T G A curves of Ni(OH)z[A ] (initial weight = 2.7260 mg) ( and Ni(OH)2[U] (initial weight = 5.5590 mg) ( - - - ) .

R. M. Torresi et al. / IR characterization o f electrochromic Ni(OH) 2

intercalated water molecules. Beyond 200 °C, the remaining water molecules are removed from the interslab space simultaneously with the process of the dehydroxilation of Ni(OH)2, leading to the formation of NiO. The theoretical loss of mass for this process is 19.4%, while the experimental value is approx. 21.5%. These results show that the main difference between samples prepared by methods 1 and 2 is the absence of adsorbed and intercalated water molecules in the case of Ni(OH)2[A] samples, confirming the presence of well-crystallized fl-Ni(OH)2. In contrast, electrodes prepared by urea decomposition present a more open structure allowing the incorporation of water molecules in the hydroxide matrix.

3.3. Infrared spectroscopic analysis 3.3.1. Virgin Ni(On)2 samples Infrared spectroscopic studies were carried out with Ni(OH)2 films deposited onto SnO2-covered glasses according to the procedures described in the experimental section. In Fig. 4 the relative diffuse reflectance of as-grown nickel hydroxide films is shown. In Fig. 4(a) the IR spectrum of an Ni(OH)2[A] electrode is depicted. A narrow band at 3650 cm- ~, corresponding to the stretching vibration v(OH) characteristic of the fl(II) phase, can be seen. On the other hand, the spectrum of the hydroxide prepared by the second method (Ni(OH)2[U], Fig. 4(b)) shows the presence of a small band at 3650 c m - i and broad bands in the region of 3450-3100 cm -~ and 1650-1200 cm -~. Most of these

183

bands can be assigned to residual urea, and the last signal near 1100 cm-1 to adsorbed SO]- ions [8]. The multiplicity of bands can be considerably reduced if the electrode is carefully rinsed with water before the spectroscopic measurements (Fig. 4(c)). In this case there remain the bands corresponding to v(OH) at 3650 c m - ' , a broad band at 3350 cm-~ from the stretching vibration of water molecules or free OH species and a small signal at 1650 cm- ~corresponding to the bending mode of intercalated water molecules, according to thermogravimetric analysis (Fig. 3). In this study, all IR spectra of Ni(OH)2[U] electrodes were taken with carefully rinsed samples to avoid interference from the urea bands. The IR spectra of both films indicate that the Ni(OH)2[AI] can be assumed to be a fl(II) phase and the Ni(OH)2[U] has a structure intermediate between the "well-crystallized" fl(II) phase and the disordered ~-type hydroxides [1].

3.3.2. Open-circuit behavior The open-circuit behavior of these electrodes was studied by comparing IR spectra of as-grown films with the spectra taken after 30 rain at open circuit in contact with different alkaline solutions. Figures 5 and 6 show the IR spectra obtained with different solutions for Ni(OH)2[A] and Ni(OH)2[U] respectively. The former shows that IR spectra are not substantially modified when the solution is changed

H20 . 4

LiOH

_-

¢ o.''

u.I ¢,.) Z

hi U Z

~, I °j

W .J W hi

!

t

t

i ill

~jF-

I

'

I

'

I

,J W

- ' - - ~,' N ~

I

I

5000

,'

/ /

!

I

I

/

a

W

:5800

I

I1 I

I

I

I

1500

I000

I

4000

WAVENUMBER/cm-i Fig. 4. IR spectra of as-grown Ni(OH)2 thin films: (a) Ni(OH)2[A], (b) Ni(OH)2[U] and (c) Ni(OH)2[U] rinsed with distilled water.

N~j//I/CsOH

KOH

3000

I

,,

,

I

2000 4000 3000 W A V E N U M B E R / cm -t

,

1

2000

Fig. 5. IR spectra of Ni(OH)2[A ] films in open-circuit conditions: ( ) a s - g r o w n film a n d ( . . . . ) after 30 m i n in solution.

R. M. Torresi et al. / IR characterization o f electrochrornic Ni(OH)2

184

/ 1 1 . .M i I

H20

I--

rr

I

I

'

....

I

'

I

'

I

3

I

I

I

3000

I

I

20C0

"

4000

/I

I

.... ~....

2000 4000

~

/ ,"/

." ,'

Ko.

3000

2000 4000

3000

2000

WAVENUMBER/crn- i CsOH

\/ 4000

3000

~.o.

/

~

KOH

.

. . . . .

......'''"

I ~

4000

i

n~

~

L~j ne

L i OH

#

~-- o,,

,*

I

3000

,

I

Fig. 7. IR spectra of cycled Ni(OH)2[A ] films in different electrolytic solutions: ( ) a s - g r o w n film, ( . . . . ) bleached state and ( . . . . ) colored state.

2000

WAVENUMBER/crn -I

Fig. 6. IR spectra of N i ( O H ) 2 [ U ] films in open-circuit conditions: ( ) a s - g r o w n film and ( . . . . ) after 30 min in solution.

(H20, LiOH, K O H and CsOH), and in all cases the 3650 c m - ] absorption band corresponding to "free" O H - groups is observed. In the case of nickel hydroxide prepared from urea solution, IR spectra (Fig. 6) corresponding to LiOH, K O H and H20 do not present changes when compared with the spectrum of the virgin hydroxide: however, in the case of CsOH solution, the increase of the 3350 cm-~ band clearly shows the incorporation of water molecules. This fact could probably be explained by taking into account the strong specific adsorption of Cs ions [30] which would lead to incorporation of the electrolyte into the more open structure of Ni(OH)2[U] electrodes. 3.3.3. Study of bleached and colored states The SnO2/Ni(OH) 2 electrodes were cycled at a scan rate of 5 mV s - l in the - 0 . 6 5 to + 0 . 6 V potential range. Three IR spectra were recorded; one at the virgin state, another at the bleached state ( - 0.65V) and a third at the colored state (0.6V). Figure 7 shows the IR spectra obtained for Ni(OH)z[A] electrodes in 0.1M LiOH, K O H and CsOH electrolytic solutions. It can be seen that in all cases the spectra corresponding to the virgin and bleached states are very similar to those already observed under open-circuit conditions (Fig. 5), showing that in the reduced state there is not a great incorporation of the electrolyte. IR spectra obtained for the colored states (oxidized film) in LiOH and K O H electrolytes show a diminution of the 3650 cm-~ narrow band. This band completely disappears when the electrolytic solution is CsOH. As the oxidized film has a higher conductivity than the reduced one, IR bands are weaker because a smaller amount of radiation penetrates the N i O O H

layer [31, 32]. Nevertheless, the shape of the IR spectra is different for LiOH, K O H or CsOH electrolytes, indicating the influence of the alkaline cation in the colored state. The diminution of the 3650 c m - ] band is also expected because the oxidized form NiOOH has fewer free O H - groups than the reduced Ni(OH)2 form. However, the formation of Ni--=-O bonds and the increase of conductivity during oxidation are quite insufficient to explain the changes observed in the IR spectra when different alkaline electrolytes are used. Considering that cation incorporation takes place during the oxidation process, and assuming that the physicochemical behavior of these cations in the oxide matrix is approximately the same as for aqueous solutions, the IR spectra shown in Fig. 7 could be explained as follows: Li + and K + ions are surrounded by water molecules belonging to their solvation sphere; in contrast, Cs + ions are surrounded by coordinated water. That is to say that water molecules near Li + or K ÷ ions are oriented in the ion's force field [30, 33]: thus these water molecules are less capable of forming hydrogen bonds with the free O H - groups. On the other hand, coordinated water is not strongly influenced by ion's force field, allowing these molecules to form hydrogen bonds. That is why the narrow v(OH) band completely disappears when the electrolyte is CsOH, because all water molecules inside the film form hydrogen bonds with the free O H - groups. Another explanation could be found on the basis of the Lewis acid strength (L) of the cation, as was previously pointed out by Campet et al. [34]. A measure of the Lewis acid strength L is obtained from the empirical relationship [35, 36]: L = Z _ 7.7)(7. + 8.0

(6)

rz

where Z is the charge number of the atomic nucleus, r is the empirical ionic radius [37] of the element in the oxidation state Z, and Xz is the electronegativity of the elements in valence states according to the values calcu-

R. M. Torresi et al. / IR characterization o f electrochromic Ni(OH) 2 # Z

....

_s ~

..., ..'/°

,.,..,~-'""

t.lJ

>

..

! / l%w'~ R~.~ I o.2o ..%:" • ....

LU _J U. UJ n,.

..... ~

\/

..--'"........--""

..-

.........."

KOH

LiOH

CsOH

a..

1

4000

3000

I

~,

I

,

I

:t,

,

2000 4000 :3000 2000 4000 3000 WAVENUMBER/cm -I

I

2000

Fig. 8. IR spectra of cycled Ni(OH)2[U ] films in different electrolytic solutions. ( ) as-grown film, ( . . . . ) bleached state and ( . . . . ) colored state.

lated by Zhang [35]. So, considering eqn. (6) and calculating the Lewis acid strength for the different cations used here, the following L values are obtained: Li ÷, 1.974; K ÷, 1.526; Cs ÷, 1.483. It is obvious that a more acid donor element such as Li ÷ will polarize electronic charge from the O H group towards itself, thereby preventing these groups from forming hydrogen bonds with water molecules. This behavior does not occur for Cs ÷ ions having the lowest Lewis acid strength L. These facts agree with previous results obtained with the quartz crystal microbalance technique [12], showing the incorporation of solvated alkaline cations during the nickel hydroxide electrode charging process. Other authors [8] have already pointed out the contamination of fl(III)-NiOOH phase by the 7-NiOOH phase when cell overcharge is achieved. This phenomenon also implies the incorporation of alkaline cations and water molecules during oxidation of fl(II)-Ni(OH)2 electrodes. Figure 8 shows the IR spectra obtained for Ni(OH)2[U] electrodes in 0.1M LiOH, KOH and CsOH electrolytic solutions. The bleached-state behavior is again similar to that observed in open-circuit conditions. In the colored state, complete disappearance of the v(OH) band is observed for K ÷ and Cs ÷ ions. The influence of the different alkaline cations is not so evident because of the fact that the virgin structure is open and badly crystallized. In any case, the small influence of the nature of the alkaline cations can be explained on the same basis presented in the case of Ni(OH)2[A] electrodes.

4. Conclusions Both thermal hydrolysis of [Ni(NH3)6] 2+ complex and thermal decomposition of urea-NiSO4 solutions lead to the precipitation of homogeneous and transparent Ni(OH)2 films with interesting electrochromic properties. The electrochromic efficiency of Ni(OH) 2 films

185

prepared by the two methods here presented is of the same order as those obtained with NiOx films prepared by other methods. Infrared analysis of the virgin thin films allows the distinguishing of different structures for Ni(OH)2[A ] and Ni(OH)2[U ], the former corresponding to wellcrystallized fl-Ni(OH)2 and the latter to an intermediate phase between fl-Ni(OH)2 and the completely disordered ~-Ni(OH)2. The influence of the electrolytic solution was studied under open-circuit conditions and after electrochemical cycling. IR spectra obtained for the colored (oxidized) state show a great incorporation of water molecules and alkaline cations, especially in the more disordered structure Ni(OH)2[U]. The fl-Ni(OH)2 phase also shows, when colored, water and ion incorporation which could be associated with a contamination of fl(III) phase by 7-NiOOH phase. The role played by alkaline cations in the film is determined by the chemical nature of the ions and their ability to orient water molecules of their solvation or coordination spheres. This is also related to the Lewis acid strength of the cations in the sense that a more acidic donor element would polarize electronic charge from the OH groups towards itself, thereby screening them to form hydrogen bonds with water molecules of the neighbor environment.

Acknowledgments Authors wish to thank Dr. C. U. Davanzo (IQ/ UNICAMP) and Dr. F. C. Nart (USP, S~o Carlos) for careful reading of the manuscript and helpful discussions concerning IR spectrum analysis. Financial support from FAPESP, FINEP and CNPq is also acknowledged.

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R. M. Torresi et al. / IR characterization o f electrochromic Ni(OH) 2

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