Zeolitic inorganic–organic polymer electrolytes: synthesis, characterization and ionic conductivity of a material based on oligo(ethylene glycol) 600, (CH3)2SnCl2 and K4Fe(CN)6

Electrochimica Acta 46 (2001) 1587– 1594 www.elsevier.nl/locate/electacta

Zeolitic inorganic –organic polymer electrolytes: synthesis, characterization and ionic conductivity of a material based on oligo(ethylene glycol) 600, (CH3)2SnCl2 and K4Fe(CN)6 V. Di Noto a,*, M. Fauri b, M. Vittadello a, S. Lavina a, S. Biscazzo b a

Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Uni6ersita` di Pado6a, Via Loredan 4, I-35131 Padua, Italy b Dipartimento di Ingegneria Elettrica, Uni6ersita` di Pado6a, Via Gradenigo, 6 /a, I-35131 Padua, Italy Received 15 July 2000; received in revised form 30 October 2000

Abstract This paper reports the synthesis of a new Z-IOPE material based on poly(ethylene glycol) 600, (CH3)2SnCl2 and K4Fe(CN)6. This material was synthesized via a sol-gel transition. FIR and MIR spectroscopy studies together with detailed compositional data allowed us to propose a final structure for this Z-IOPE material. It was concluded that this compound is a mixed inorganic-organic network in which clusters formed by tin and iron complexes are bonded together by PEG 600 bridges. The conformation of polyethereal chains in the bulk material is of the TGT (T = trans, G= gauche) type. Impedance spectroscopy measurements revealed that the material has a conductivity of 4.77.10 − 5 S cm − 1 at 21.3°C. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Inorganic– organic polymer electrolytes; Sol-gel transition; Metallorganic tin complexes; Iron cyanometallate derivatives; Impedance spectroscopy

1. Introduction Classical polymer electrolytes consist of organic macromolecules [1–4] (usually polyethereal) doped with inorganic salts. These systems exhibit ionic conductivity in the amorphous regions but not in crystalline regions. With the goal of increasing the conductivity of these materials, inorganic species have been introduced into the polymer matrix, resulting in the (nano)composite class of polymer electrolytes (composite polymer electrolytes, CPE) [5–7]. Inorganic oxides in contact with polymeric chains produce the so-called ‘grain boundaries’ effect, i.e. polymer–ceramic interfaces with a high ionic conductivity [6,7] of about 10 − 5 S cm − 1. * Corresponding author. Tel.: + 39-049-8275229; fax: +39049-8275161. E-mail address: [email protected] (V. Di Noto).

In an alternative approach to augment conductivity, attempts have been made to obtain homogeneous monophasic materials by introducing inorganic atoms into the skeleton of polyethereal macromolecules. This approach has given rise to other two categories of inorganic– organic hybrids with promising ionic conductivities. The first class is formed by all those materials prepared by copolymerization of organic macromolecules with metal and non-metal alkoxides [8 – 11] (organically modified ceramics as polymer electrolytes, ORMOCERS-APE). They are three-dimensional networks in which organic macromolecules are bridged together by inorganic atoms like Al, Ti, Zr, Sn or Si. Their conductivity depends on the doping salts and on the size of coordination ‘nests’ present in the host material. The second class of inorganic– organic hybrids are the so-called zeolitic inorganic– organic polymer electrolytes (Z-IOPE) [12– 14]. This class ex-

0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 0 ) 0 0 7 5 7 - X

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hibits the following properties [13,14]: (a) the organic macromolecules are linked to one another by bridging inorganic clusters; (b) the inorganic clusters are formed by the aggregation of two or more inorganic coordination complexes; (c) the inorganic clusters can be either negatively or positively charged. These systems are generally prepared by sol-gel transition and show high ionic conductivity. The first representative example of a Z-IOPE material was obtained from K2PdCl4, K3[Fe(CN)6] and polyethylene glycol 600 (PEG 600) [12]. A Z-IOPE network based on PEG 600, K4[Fe(CN)6] and SnCl4 has also been prepared through sol-gel transition [13]. In this Z-IOPE material, clusters having a net charge of 4 − are formed by three tin octahedra linked by two [Fe(CN)6]4 − bridging anions and are bonded together through PEG 600 bridges. This system conducts ionically and exhibits a conductivity of 3.7 × 10 − 5 S cm − 1 at 25°C. This report describes the preparation of a new type of tin Z-IOPE material with the formula [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n + 2On + 1)Kl]. This material was prepared by using PEG 600, K4[Fe(CN)6] and (CH3)2SnCl2 following the reaction procedure previously proposed in our laboratory [12–14]. We examined the structure of the system by infrared (FT-IR) spectroscopy and the conductivity at room temperature by impedance spectroscopy (IS).

2. Experimental

2.1. Reagents (CH3)2SnCl2 (Merck) and K4Fe(CN)6·3H2O (Alfa) were used as received. PEG of molecular weight 600 (Aldrich, ACS grade) was further dehydrated by standard methods and sealed under argon on 4 A activated molecular sieves to avoid contamination by moisture. All transfer and handling operations of final products were carried out in an argon atmosphere.

2.2. Instruments and methods Mid-infrared (MIR) and far-infrared (FIR) FT-IR spectra were collected by means of a Nicolet 5SxC spectrophotometer operating with a triglycine sulphate (TGS) detector at a nominal resolution of 4 cm − 1, recording 900 scans; measurements were carried out using as windows of a sealed cell KBr (for MIR) and polyethylene (for FIR). The FT-IR spectrum was decomposed by Gaussian or Lorentzian functions using a computer program elaborated in our laboratory and based on the principle of Pitha and Jones [15]. In general at the beginning the spectrum was smoothed by applying a binomial smoothing operation, then normal-

ized and finally corrected for the baseline. Baseline was determined by fitting the background of the spectrum with polynomial functions [15,16]. Particularly, before fitting, the number of bands and their starting parameters were determined as reported by Pelica`n et al. [16] by analyzing the properties of the first four derivatives of the spectrum. The analyses of Sn, Fe and K were performed by inductively coupled plasma atomic emission spectrometry (ICP-AES) by using the method of standard additions. The emission lines were: u(Sn) = 189.926 nm, u(Fe)= 259.940 nm, u(K)=766.490 nm. A Spectroflame Modula sequential and simultaneous ICP-AES spectrometer equipped with a capillary cross-flow nebulizer was used (Spectro Analytical, Kleve, Germany). Analytical determinations were performed using a plasma power of 1.2 kW, a radiofrequency generator of 27.12 MHz and an argon gas flow with nebulizer, auxiliary and coolant set at 1, 0.5 and 14 l min − 1, respectively. Sample dissolution for the ICP-AES determinations was carried out by microwave digestion of the material using a CEM MDS-2100 microwave system. Mineralization was performed by treating 115 mg of the product with a solution consisting of 54% (v/v) HNO3 (70%), 31% (v/v) HCl (37%) and 15% (v/v) H2O. AC impedance measurements were performed between 20 Hz and 1 MHz by means of a computer-controlled HP4284A precision LCR meter. The sample in the form of a pellet was placed in a home-made Teflon cell between two cylindrical gold-plated stainless steel electrodes. The electrode– electrolyte contact surface and the distance between electrodes were measured using a micrometer. No corrections for thermal expansion of the cell were carried out. The temperature was measured with an accuracy greater than 90.1°C.

2.3. Synthesis of the Z-IOPE The electrolytic complex was prepared by combining two separate solutions (A and B). Solution A (transparent), was prepared by dissolving 0.3603 g (1.64 × 10 − 3 mol) of (CH3)2SnCl2 in 2.27 g of water, followed by the addition of 1.81 g of PEG 600. Solution B (transparent yellow) was obtained by dissolving 0.2970 g (7.0 × 10 − 4 mol) K4Fe(CN).63H2O in 6.00 g of bidistilled water followed by 2.01 g of PEG 600. Mixing of solutions A and B resulted in the immediate formation of a yellow gel, which slowly eliminated water. This mixture was washed with water and filtered under an argon atmosphere. Further traces of water were eliminated by drying the material at 50°C and 10 − 5 mbar for 1 week. The homogeneous yellow material thus obtained exhibited the typical consistency of lubricant grease.

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Analytical data and molar ratios n Sn/n PEG, n K/n PEG and n CN/n PEG for the polymer are reported in Table 1.

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very different from the Z-IOPE prepared using SnCl4 [13].

3.2. FT-IR in6estigations

3. Results and discussion

3.1. Reaction obser6ations As previously reported for the Z-IOPE generated using SnCl4 [13], the material described in the present study was prepared through a sol-gel transition by the following reactions: (Mixture A +Mixture B) ¡ [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n + 2On + 1)Kl]gel ¡ −H2O [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n + 2On + 1)Kl]plastic where Mixture A =(CH3)2SnCl2 +aH2O+bPEG and Mixture B =K4Fe(CN)6 +a%H2O+b%PEG. A comparison of the molar ratios n Sn/n PEG, n Fe/ PEG n , n K/n PEG and n CN/n PEG in the reaction mixture and final product (Table 1) indicates that while n Sn/ n PEG, n Fe/n PEG and n CN/n PEG all show a decrease in the final product by 27.4%, 20.6% and 12.8%, respectively, n K/n PEG increases. These observations strongly suggest that tin and iron complexes are released in the liquid solution (consisting mainly of water) to the point of obtaining a mixture of metal complexes favouring gel structure formation. In this case water in the mixture acts as a catalyst for the inorganic condensation reaction. This behaviour is quite different from that already observed for the analogous system prepared using SnCl4 [13], whose molar ratios in the reagent mixture and final product showed little variation. Therefore, the analytical data reported in Table 1 suggest that the material proposed here is structurally

The structure of the prepared material was investigated by FT-IR spectroscopy. The MIR and FIR spectra of the material are shown in Fig. 1. An accurate analysis of the MIR spectrum (Fig. 1a) clearly indicates that this region is a composite resulting from the superposition of the PEG 600 spectrum and the peaks typical of the cyanide groups of cyanometallate complexes. The spectrum of PEG 600 component in the material is easily distinguished and registered in the 400– 1600 and 2500– 4000 cm − 1 ranges. In order to gain a detailed understanding of the conformation of PEG 600 chains and of the interactions between PEG 600 and the ions, the spectral decomposition of these regions by Gaussian functions was carried out as described in the experimental section after correction of the spectrum for background (Fig. 2). The results of such spectral resolution showed that in the material the peak parameters of the PEG 600 overlapping bands in terms of full width at half maximum (FWHM) and peak positions are quite coincident with those of the PEG400/(LiCl)x polymer electrolyte [17]. It was demonstrated that in this later system the polyethereal chains assume a TGT (T = trans, G = gauche) helical structure with internal rotation angles of ~(O–CH2) =~(CH2 –O)$191.5° (trans) and ~( CH2 –CH2) $60° (gauche) [17]. Therefore, on the basis of this findings and of previous IR investigations [3,13,17], the PEG 600 spectral features in [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n+2On+1)Kl] were assigned on the basis of a structural polyethereal chain model with TGT conformation (Table 2). On the other hand, in accordance with Ref. [17], possible salt– polyether chain interactions were detected by an accurate analysis of the 990– 1150 cm − 1 spectral range. Specifically, the bands at 1111 (E1), 1040

Table 1 Microanalytical data and molar ratios for the [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n+2On+1)Kl] complex Microanalytical data

Molar ratios (%)

Sn Fe K N C H a b

3.29 0.73 2.38 1.20 48.73 8.85

n Sn/n PEG n Fe/n PEG n K/n PEG n CN/n PEG

Reagentsa

Product

D (%)b

0.2576 0.1105 0.4419 0.6629

0.1869 0.0877 0.4717 0.5783

−27.4 −20.6 +6.7 −12.8

Calculated from the weight composition of the reaction mixture used for the synthesis. D (%)= [(n i/n PEG)product−(n i/n PEG)reagents]×100/(n i/n PEG)reagents; i =Sn, Fe, K, CN.

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stretching [21]. The two peaks at 529 and 455 cm − 1 were attributed to the w a(Sn– C) and w s(Sn– C) vibrational modes, respectively [22,23]. The intensity peaking at 517 cm − 1 was associated with the w(Sn–O) mode [23]. It should be noted that the presence of the stretching vibrations w a(Sn– N) and w s(Sn– N) attributed to the peaks at 388 and 329 cm − 1, respectively [23], are indicative of the fact that Sn atoms are also coordinated by nitrogen ligands of cyanometallate complexes. The two modes registered at 299 and 243 cm − 1 were correlatively assigned to the w a(Sn– Cl) and w s(Sn–Cl) vibrations [22,24], respectively. The full assignments of FIR and MIR spectra for the polymer are reported in Table 2. In summary, these results are in accordance with Ref. [13] and demonstrate that (a) the CN bridges are bonded to Sn and Fe atoms and (b) PEG 600 bridges with TGT conformation link together Sn atoms. In accordance with the literature [12– 14], the spectral decomposition of the

Fig. 1. FT-IR absorbance spectra of the [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n + 2On + 1)Kl] Z-IOPE: (a) MIR; (b) FIR.

(E1) and 997 (E1) cm − 1 corresponding to the stretching vibrations of the polyethereal C–O groups were registered at lower frequencies and with higher intensities with respect to the peaks of the same vibrations of pristine PEG 600 [13,17]. These observations indicate that K+ in the bulk material is coordinated preferentially by the oxygen atoms of polyethereal chains. Further important structural information was gained through an exhaustive analysis of the CN stretching vibration region. In this spectral zone four peaks were measured at 2049, 2068, 2094 and 2143 cm − 1. The band centered at 2049 cm − 1 was attributed to the w(–CN)t stretching vibration of the terminal cyanide group of cyanometallate complexes [12–14], whilst the intensities at 2068, 2094 and 2143 cm − 1 were assigned to three types of structurally different bridging cyanide groups [12–14,18–20]. An accurate examination of the FIR spectrum depicted in Fig. 1b provides essential information regarding the structure of the material. The peaks measured at 571 and 584 cm − 1 were assigned to the w(Fe–CN)

Fig. 2. Decomposition of the MIR FT-IR spectrum for [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n + 2On + 1)Kl] Z-IOPE. The decomposition was performed adopting Gaussian functions. The ranges investigated are 2500– 3700 (a) and 970– 1560 cm − 1 (b).

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Table 2 FT-IR band assignments of the [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n+2On+1)Kl] complexa Species

Wavenumber (cm−1) Observedb

A2

3641 (m) 3337 (s) 2870 (s)

2883

2049 (w)

1350 (m) Â 1323 (vw)Ì Å

1344

1250 (w) 1070 (vs sh) 953 (m)

1264 1087 964

[17] [17] [17]

w(–CN)b

[12–14,18–20]

w(–CN)t

[12–14]

sr(CH2) (100)

[17]

w(CH2) (100)

[17]

t(CH2) (81) w(CO) (94) r(CH2) (49), tCH2) (18)

[17] [17] [17]

w(Fe–CN)

[21]

l(CCO) (89), r(CH2) (33)

[17]

548 (vw) Â Ì 529 (vw) Å

w a(Sn–C)

[22,23]

517 (m) Â Ì 511 (vw) Å

w(Sn–O)

[23]

476 (w) Â 469 (w) Ì 455 (m) Å

w s(Sn–C)

[22,23]

388 329 299 243

w a(Sn–N) w s(Sn–N) w a(Sn–Cl) w s(Sn–Cl)

[23] [23] [22,24] [22,24]

w a(CH2) (101, 18°) w s(CH2) (100, −5°) sr(CH2) (100, −132°) w(CH2) (95, 53°) w(CH2) (107, −129°) t(CH2) (73, −40°) t(CH2) (87, 116°) w(CO) (37, −27°), r(CH2) (29, −66°) w(CO) (81, 136°), w(CC) (21) w(CO) (36, −102°), r(CH2) (35, 70°), w(CC) (17) r(CH2) (35, 70°), w(CC) (27), w(CO) (14,167°)

[17] [17] [17] [17,22] [17] [17] [17] [17] [17] [17] [17]

556

533

(w) (w) (w) (w)

2938 (sh) 2845 (vs) 1485 (w) 1439 (sh) 1361 (sh) 1298 (m) 1223 (w) 1140 (vw) 1111 (vs) 1040 (w) 997 (vw) 887 847 833 818

a

w a(OH) w s(OH) w s(CH2) (100)

1470

584 (vw) 571 (vw)

E1

References

Calculatedc

2143 (vw)Â 2094 (m) Ì 2068 (s) Å 1470 (m) Â Ì 1456 (m) Å

Assignment and potential energy distributionc,d

(w) (w) (w) (w)

Â Ã Ì Ã Å

2940 2873 1471 1401 1353 1286 1234 1142 1112 1060 941 847

529 (vw)

524

359 (vw) Â 366 (vw) Ì Å

366

151 (m)

164

Ár(CH ) (58, −149°) 2 Í Äw(CO) (39, −9°)

[17]

l(CCO) (43, −142°), l(COO) (21), r(CH2) (17, 152°) [17] Ál(CCO) (42, −73°), Íl (COO) (38) Ä

€i(CC) (42), €i(CO) (38, −161°), l(CCO) (18, 16°)

[17] [17]

PEG 600 in the polymer electrolytic complex was assumed under the D(4p/7) symmetry group. Relative intensities are reported in parentheses; vs, strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder. c The normal coordinate analysis was derived from the references reported in the last column. d w, stretching; l, bending; t, twisting; r, rocking; sr, scissoring; €i, internal rotation; a, antisymmetric mode; s, symmetric mode. b

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is of the terminal type (  1/6) and the remaining five are bridging cyanides ( 5/6). The analytical data reported in Table 1 yield the following molar ratios: fM = (n Sn + n Fe)/N=0.215 4/20; fPEG = n PEG/N = 0.784516/20; fSn = n Sn/NM = 0.68050.7 3/4; and fFe = n Fe/NM = 0.319 1/4; where N =n Sn + n Fe + n PEG, NM = n Sn + n Fe and n Sn, n Fe and n PEG are the moles of Sn, Fe, and PEG 600, respectively in a unit sample mass. The fM and fPEG data clearly indicate that each 20 moles of components in the material contain 4 moles of metals (Sn + Fe) and 16 moles of PEG 600. The fSn and fFe demonstrate that of the 4 moles of metals, 3 moles are tin atoms and 1 is iron. Finally, the analytical data and the spectroscopic results point out the presence of Sn – O, Sn– N, Fe– C and Sn – Cl bonds in the system and prompt us to propose the structure depicted in Fig. 4. This network material is of the Z-IOPE type and is structurally very different from the Z-IOPE material already reported, which was prepared using SnCl4 [13] instead of (CH3)2SnCl2.

3.3. Impedance studies

Fig. 3. Decomposition of the MIR FT-IR spectrum for [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n + 2On + 1)Kl] Z-IOPE by Lorentzian functions. The analysis was performed in the 1850–2200 cm − 1 range.

1850–2200 cm − 1 region by the Lorentzian function [15] (Fig. 3) allowed us to semiquantitatively determine the concentration of bridging and terminal cyanide groups. The peak positions, full width at half maxima and band areas thus determined are summarized in Table 3 together with the percentage of each type of structurally distinct CN group (Ri (%)). The fractional amount of bridging, R CNb, and terminal, R CNt, cyanide groups are 0.80 and 0.20, respectively, suggesting that, among the six CN groups in the prepared material, one

The conductivity of [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n + 2On + 1)Kl] Z-IOPE was examined by impedance spectroscopy (IS) in the 20 Hz– 1 MHz frequency range and at 21.3°C. The measured Nyquist plot is reported in Fig. 5. A semicircular arc is observed at higher frequencies and a spike is present at lower frequencies. Data simulations were carried out using the EQUIVCRT program [25] and by adopting different equivalent circuit models. Results indicated that the impedance spectrum is well fitted if an equivalent circuit constituted by a parallel impedance in series with a constant phase element impedance is used [26,13]. The parallel impedance is constituted by a resistance in parallel with a capacitance. The capacitance of the parallel circuit was 1.41 × 10 − 10 F. The n value [25] of the CPE in series with the parallel circuit resulted 0.93, thus indicating that the straight line detected at low frequencies

Table 3 Band parameters for the w(CN) FT-IR vibrational modes Band

wi (cm−1)

Ai

w(–CN)b

2143 2094 2068

w(–CN)t

2049

FWHMi

Ri (%) b

R CNi

0.8607 8.1756 9.9337

21.00 37.61 27.90

3.61 34.26 41.62

R

4.8944

43.25

20.51

a

c

R CN teo

=0.80

:5/6

R CNt =0.20

:1/6

CNb

Ai and FWHMi are the band area and the full width at half-maximum of the peak centered at wi, respectively. Ri (%)=Ai×100/4i = 1 Ai. c R CNb =(A2143+A2094+A2068)/4i = 1 Ai ; R CNt = A2049/4i = 1 Ai.

a

b

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Fig. 4. Structural model proposed for the [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n + 2On + 1)Kl] Z-IOPE.

could be associated with a blocking electrode impedance, or space charge capacitance [26]. The bulk resistance so determined was used to obtain the conductivity at room temperature of the [Fex Sny -

(CH3)2y (CN)z Cl6 (C2n H4n + 2On + 1)Kl] Z-IOPE complex. Finally, owing to its conductivity of ca. 4.77 × 10 − 5 S cm − 1 at 21.3°C the Z-IOPE material described here can be considered a good ionic conductor.

4. Conclusions This paper describes the synthesis of a Z-IOPE system with the formula [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n + 2On + 1)Kl] by means of a sol-gel process. On the basis of the analytical data and FT-IR spectroscopic studies it was concluded that this polymer electrolyte is a mixed inorganic-organic network in which charged clusters formed by three tin complexes and one cyanometallate anion complex are bonded together by PEG 600 bridges. MIR FT-IR investigations allowed us to conclude that the PEG 600 chains in the bulk Z-IOPE material are present in a TGT conformation. The impedance spectroscopy studies carried out at room temperature demonstrated that the Z-IOPE material conducts ionically. Finally, its conductivity of ca. 4.77 × 10 − 5 S cm − 1 at 21.3°C classifies this Z-IOPE as a good polymer electrolyte.

Fig. 5. Nyquist plot obtained for the [Fex Sny (CH3)2y (CN)z Cl6 (C2n H4n + 2On + 1)Kl] Z-IOPE. The measurement was carried out at 21.3°C and in the frequency range from 20 Hz to 1 MHz.

Acknowledgements The authors express their gratitude to Mr. Claudio Comaron for skilful technical assistance.

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