Gel polymer electrolytes based on PMMAIII. PMMA gels containing cadmium�

Jirı Vondrak a,*, Marie Sedlarıkova b, Jana Velicka a, Bretislav Klapste a,Vıtezslav Novak b, Jakub Reiter a

a Institute of Inorganic Chemistry, Czech Academy of Sciences, 250 68 Rez, Czech Republicb Institute of Electrotechnology, Technical University of Brno, 602 00 Brno, Czech Republic

Received 29 July 2002

Abstract

Gel polymer electrolyte containing cadmium perchlorate was prepared by polymerisation of methylmethacrylate with a solution

containing Cd perchlorate in propylene carbonate (PC). Electric conductivity, fundamental electrochemical properties and Cd

electrodeposition were investigated. The deposition of Cd was fairly homogenous.

# 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Gel polymer electrolytes; Polymethylmethacrylate; Cadmium gel electrolyte; Electrodeposition; Cadmium deposition

1. Introduction

In our previous papers [1,2] we have described the

preparation of gel polymer electrodes. These gels were

prepared by the addition of lithium perchlorate or

fluoroborate in propylene carbonate (PC) into the

methylmetacrylate monomer and subsequent polymer-

isation. A method of the estimation of transference

numbers was developed in mentioned paper. Surpris-

ingly, the transference number of lithium ion in the gel

was found rather low, in the range 0.2�/0.3. Hence, the

current is carried by anions mainly. A hypothesis was

created, according to which the extremely small lithium

ions are captured by the chains of PMMA while the

anions movement is not inhibited by them and rather

mobile in the liquid component of the gel. Similar

hypothesis was applied for the ionic motion in PEO

polymer electrolytes [3] in which the absence of liquid

between polymeric chains causes blocking of the anionic

movement, in contrary to gelous PMMA�/PC gels.

Therefore, we decided to prepare analogous gelscontaining a series of other cations than lithium, and

to compare their electric conductivities and other

properties. We prepared gels containing sodium, mag-

nesium and zinc.

This paper deals with the gels which contain cadmium

perchlorate as a main electroactive salt. Cadmium was

chosen as another divalent cation for comparison.

Moreover, metallic cadmium as well as cadmium saltsare sufficiently stable and easy to manipulate.

2. Materials and procedures

Anhydrous cadmium and magnesium perchlorates

were dissolved in anhydrous PC and this solutions

were used for gel preparation as 0.25 M stock solutions.

The preparation of the gel and conductivity measure-

ment were described earlier. A three-electrode system

was used for kinetic measurements. It consisted of a

PTFE base and a lid (see Fig. 1). Openings for workingand reference electrodes were drilled in the base; they

were equipped with adjusting screws, which were used

for the fixation of the working and reference electrodes.

�As papers No. I and II, previous contributions marked here as

Refs. [Electrochim. Acta 44 (1999) 3067; Electrochim. Acta 46 (2001)

2047] are considered.

* Corresponding author. Tel.: �/4202-6617-2198; fax: �/4202-2094-

1502.

E-mail address: vondrakj@iic.cas.cz (J.v.v. Vondrak).

Electrochimica Acta 48 (2003) 1001�/1004

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0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0013-4686(02)00813-7

Similar opening for the counter-electrode was drilled in

the lid. A shallow cavity created in the base served for

the accommodation of the sample electrolyte and the

electrolyte of the reference electrode.

Cadmium wire with the diameter 1 mm served as the

reference electrode. A slice of cadmium containing gelwas in contact with the reference electrode.

Copper foil, glassy carbon rod, or cadmium rod were

used as working electrodes.

The gel under investigation (a slice10�/10�/1 mm

approximately) was situated between working and

counter electrodes and it was in contact with the former

slice of electrolyte at the reference electrode.

The electrochemical measurements were performed attemperature 20�/22 8C using the potentiostat AUTO-

LAB. Impedance spectroscopy was evaluated in the

frequency range 0.0005�/10 000 Hz.

The morphology of cadmium deposits was observed

by scanning electron microscope.

3. Results

3.1. Resistivity of Cd containing gel

The specific resistivities at 20 8C are shown in Table 1

in comparison to the resistivity of other gels containing

equal concentration of anions. As we see, the conduc-

tivity of gel with a smaller Mg2� cation is lower than

that of gel containing larger Cd2� cation. Hence, the

mobility of Mg2� is lower than the mobility of Cd2�.Also the apparent activation energy, obtained from

Arrhenius plot at temperatures over 0 8C, is higher for

small Li� and Mg2� ions. Hence, the movement of

smallest ions is more difficult than the mobility of larger

ions as it was suggested in our previous papers.

In Fig. 2, the influence of temperature on the specific

resistivity r in the range between �/70 and �/68 8C is

plotted in coordinates of Arrhenius type (using decadiclogarithm in Fig. 2). At temperatures over 0 8C, the

conductivity was expressed approximately by the Ar-

rhenius formula

ln(r)�A=T�B

and the apparent activation energies E were evaluated

from the parameter A in temperature range over 0 8Cfrom the parameter A . Obtained values are given in

Table 1 as well. We see that the gels exhibiting lower

conductivity (or higher resistivity) possessed higher

apparent activation energy at the same time. Further

we see the non-linearity caused by the solidification of

the gels. The change in the nature of the impedance

below �/25 8C was analogical to that observed in the

system LiBF4�/TEA BF4 as described earlier [1] (Table2).

3.2. Voltammetry

The voltammogram of a glassy carbon electrode in

cadmium containing gel is shown in Fig. 3. We can see

the peak of metal deposition at potential close to �/0.2V versus Cd electrode, and the peak of its dissolution on

the anodic part of the voltammogram.

However, the current at the voltammetric experiments

has grown from one cycle to the following one. It may

Fig. 1. Three electrode vessel for the electrochemistry of gel electro-

lytes.

Table 1

Properties of gels containing Na, Li, Zn, Cd and Mg perchlorates

Salt r (V cm), 20 8C A B E (kJ mol�1)

Cd 52.8 1730 �/1.758 14.4

Na 23.1 1824 �/3.733 15.2

Li 48.4 2280 �/4.758 19.0

Zn 24.0 1977 �/4.251 16.4

Mg 62.8 2376 �/4.793 19.8 Fig. 2. The influence of temperature on specific resistivity of the gel (in

Arrhenius plot, decadic logarithm plotted on vertical axis).

J.v.v. Vondrak et al. / Electrochimica Acta 48 (2003) 1001�/10041002

be caused by the improvement of the contact or by

enlargement of real surface area of the working elec-

trode on which some cadmium remained from previous

cycles.

3.3. Impedance spectroscopy

The impedance of a gel in contact to glassy carbon

electrode was measured at two potential values at 0 mV

and at �/300 mV versus Cd�/Cd2�. The first of them is

the reversible potential of Cd deposition and no Cd

deposition should be expected at the other one. The

results are shown in Fig. 4 together with the simulations

by equivalent circuits.

An equivalence circuit consisting of resistances R andtwo constant phase elements Q (proportional to fre-

quency to an exponent n ) was chosen for the interpreta-

tion; this circuit is drawn in Fig. 4 as an insert. It is

described by a formula R1(Q1[R2Q2]R3) according to the

Boucamp notation.

At the potential �/0.3 V, where no cadmium deposi-

tion should occur, the equivalent circuit consists of a

series resistance R1�/382 V, in series to which is aparallel combination of a CPE element Q1�/2.15�/

10�6 S and proportional to v0.75 and another series

R2�/Q2 chain (R2�/16 800 V, Q2�/4.3�/10�5 S and

proportional to v0.45). The first resistance is due to the

gel electrolyte, and the first CPE corresponds to the

double layer impedance. The exponent (0.75) indicated

well known problems with an uneven surface of a solid

metallic electrode which were the original reason for the

introduction of CPE elements in the impedance spectro-

scopy. The second chain is difficult to explain. Appar-

ently, it is some background reaction of the electrolyte

itself. The element R3 was missing in this case.

At the potential of 0 V, i.e. at reversible potential of a

Cd/Cd2� couple, different values were found. The series

resistance R1�/432 V was slightly higher than that at �/

0.3 V. The CPE Q1 was closer to the capacity (8.53�/

10�7 S, proportional to v0.80). The resistance R2�/6820

V and the CPE Q2 (2.4�/10�5 S, proportional to v0.57)

indicate the occurrence of electrochemical reaction in an

Table 2

Components of equivalence circuit at two potential levels 0 and 0.3 V

Element 0 V 0.3 V

R1 382 432

Q1 2.15�/10�6 0.853�/10�7

n1 0.75 0.80

R2 16 800 6820

Q2 4.3�/10�5 2.4�/10�5

n2 0.80 0.57

R3 101 000

Resistances: in V, constant phase elements Q in S, exponents n are

dimensionless. Detailed explanation in text.

Fig. 3. Cyclic voltammetry of cadmium gel.

Fig. 4. Impedance spectrum of the Cd gel at 0 and �/0.3 V (vs. metallic

Cd/Cd2� reference electrode).

Fig. 5. SEM picture of the Cd layer formed from gel electrolyte.

J.v.v. Vondrak et al. / Electrochimica Acta 48 (2003) 1001�/1004 1003

uneven surface, perhaps controlled by diffusion. Parallel

to this last term, a resistance R3�/101 kV was found.

3.4. Deposition of metallic cadmium

The possibility of compact cadmium deposition on

copper substrate was checked and the results compared

with the deposition of cadmium on the same substrate

from the stock solution of pure Cd perchlorate in PC.

The results are shown in Figs. 5 and 6. The deposit was

rather uniform in gel electrolytes (Fig. 5). However,

strong passivity occurred soon after application ofcurrent, probably due to electrolyte depletion at the

electrode interface. On contrary, strongly irregular

deposits were formed from liquid electrolyte (see Fig.

6) the non-homogeneity of which was visible even by a

naked eye. In this way, it seemed similar to the non

homogenous deposition of lithium from aprotic solvent

which represents one of major problems occurring in

lithium secondary batteries.To our opinion, an almost non-conducting film is

formed on the copper surface in liquid electrolyte and

cadmium deposition is possible in the microscopic holes

in it. Therefore, aggregates of cadmium are formed

there. According to the SEM-EDAX analysis, the

surface between the aggregates contains just carbon

compound on copper without the components of the

electrolyte.

In the case of electrodeposition from the gel, the

situation seems different. The formation of the of thewell adhering film should be suppressed by the good

adhesion of the gel to metallic surface. Also some

chemical interaction may tend to the dissolution of the

film. We cannot exclude the presence of residual

unreacted oligomers and polymerisation catalysts in

the gel which can act as surfactants.

Another question has arisen by the increasing current

and peak charge in the course of cycling. We supposethat the fine grains in the deposit were formed in

consequent cycling followed by strong adhesion of the

gel to the newly formed particles thus increasing

physical surface area of the deposit. This process should

appear as an increase of total current in subsequent

cycles.

3.5. Applicability of the results

It is rather difficult to expect any widely spreadapplication of cadmium gels. Therefore, it is no need

to treat any environmental hazard brought by these gels.

As the only application it appears now just the use of gel

reference electrode in special electrochemical sensors

and other applications in electroanalytical chemistry.

Acknowledgements

This work was supported by the Grant Agency of theAcademy of Sciences of Czech Republic (Grant No.

A4032002), by the Grant Agency of Czech Republic

(Grant No. 104/02/0731) and by Ministry of Education

of Czech Republic (Grants No. ME-216/96 and CEZ J

22/98:2622 00010).

References

[1] J. Vondrak, M. Sedlarıkova, J. Reiter, T. Hodal, Electrochim. Acta

44 (1999) 3067.

[2] J. Vondrak, M. Sedlarıkova, J. Velicka, B. Klapste, V. Novak, J.

Reiter, Electrochim. Acta 46 (2001) 2047.

[3] N. Ogata, D. Kobunshii, O. Kodansha, in: P.J. Gellings, H.J.M.

Bouwemeester (Eds.), The CRC Handbook of solid State Electro-

chemistry, vol. 137 (1990), CRC Press, Boca Rotan, 1997, p. 217.

Fig. 6. SEM picture of the Cd layer formed from liquid electrolyte.

J.v.v. Vondrak et al. / Electrochimica Acta 48 (2003) 1001�/10041004