Elsevier

Solid State Ionics

Volume 225, 4 October 2012, Pages 751-754
Solid State Ionics

Chaotic motion of ions in polymer gel electrolytes: First observations

https://doi.org/10.1016/j.ssi.2012.05.019Get rights and content

Abstract

For the first time, motion of the ions of polymer gel electrolytes separated by an irradiated porous membrane has been studied with a desktop set-up. Two polymer gel electrolytes, viz., lithium polymer gel electrolyte and magnesium gel electrolyte, have been synthesized and separated by a porous PET (polyethylene terephthalate) film. The I–V behavior of such a system has been found to exhibit chaotic motion. The chaotic motion of ions is influenced by parameters such as, pore size, pore shape, concentration of electrolytes, applied voltage etc. By choosing pore size as one of the parameters, chaotic motion has been trapped using a modified Sprott circuit. The value of Lyapunov exponent has confirmed the chaotic behavior of the electrochemical system.

Highlights

► For the first time the non linear dynamics of an electrochemical micro system has been reported. ► The dynamics has been captured by a desktop experiment. ► The different regimes of spiking and non-spiking chaotic motions are being presented. ► Sprott's circuit has been modified to capture the phenomena of ion transport through membrane. ► Attractor formation and Lyapunov exponent confirm the chaotic behavior of presently investigated system.

Introduction

The biological membrane channels in human and animal bodies, like mitochondrial channels, endoplasmic reticulum channels, nuclear pore complex and toxin channels, serve as pathways to metabolites, and the transport of macromolecules such as nucleic acids and proteins takes place through them. Many examples show that the artificial ion channels (ion tracks) in polymeric membranes exhibit the same behavior as biological ion channels in the presence of an external potential. Ion channels are created by high energy ion irradiation and subsequent etching. Ion channels have an interesting property of ion current fluctuation for a constant voltage across the membrane. Ion channels in PET membrane show ion selectivity and inhibition to the flow of divalent cations and protons. The geometry of their porous structure does not change along with the ion strength of the medium being filtered [1], [2], [3].

A membrane is an interface between two adjacent phases acting as a selective barrier, regulating the transport of substances between the two compartments. In the present study, irradiated polymeric membrane has been used as a separator for ion migration of polymer gel electrolytes in the presence of applied potential. The porous solid polymer membrane of PET, which has excellent mechanical strength and ability to mimic biological channels, has been used. Many other features of irradiated polymer membranes have been described earlier [4], [5], [6], [7].

In the present study, polymer gel electrolytes have been used because they are excellent substitutes for liquid electrolyte as they comprise of high dielectric constant plasticizers/solvents with different salts. Also, they possess properties of both liquid and solid electrolytes. PVdF-HFP polymer host matrix has been used with the plasticizers (ethylene carbonate (EC) and propylene carbonate (PC)) and two salts (lithium perchlorate and magnesium perchlorate) dissolved in a solvent tetrahydrofuran (THF). Plasticizers have been added to increase the fluidity of the electrolyte while the solvent is only a medium which facilitates the ion motion (Li+ and ClO4) and does not contribute to the conductivity value in any significant way. The polymer gel electrolytes having different salt content with same pH and concentration can flow through the porous PET membrane. Their current–voltage behavior has been investigated to understand the current fluctuations due to the ion exchange of the polymer gel electrolytes through the porous PET.

In general, an electrochemical system exhibits various types of dynamical behavior including bi-stability, periodicity, quasi-periodicity and chaos [8], [9]. Present system shows current fluctuation (non-linearity) when an external potential is applied. To study this non‐linearity in the current voltage behavior, the electrochemical system has been combined with an electronic circuit.

Chaotic motion is a complex behavior of dynamical systems whose time evolution of state shows sensitivity to initial conditions and has positive Lyapunov exponent. Earlier, many studies have been undertaken depicting the chaotic behavior in various experiments in different fields, e.g., mechanical, chemical, physical, as well as in social sciences [9], [10], [11], [12], [13], [14], [15], [16]. To understand the system dynamics, it has to be combined with an electronic circuit that will give its complete explanation. In this respect, many circuits such as the Chua's circuit and Sprott's circuit, which incorporate a nonlinear element, have demonstrated the presence of chaos. Our main aim in this paper is to capture the dynamical behavior of the ion conducting gel electrolytes passing through the porous PET membrane. For this purpose, Sprott's circuit has been used which acts as an oscillator and contains three successive integrators coupled with the nonlinear element G(x) [17]. Modification has been done in the circuit to capture the non linearity of an electrochemical system because the Sprott circuit was originally designed to trap the non-linearity of electronic circuits. It is difficult to analyze the nonlinearity in a micro system due to the complexities involved in the generally huge experimental set-up. A desktop set-up is being presented here to capture and analyze the non-linearity due to both, diffusion and drift ion motion, via a simple electronic circuit.

Section snippets

Sample preparation

Swift heavy ion irradiation of PET creates latent tracks in it and etching of these tracks breaks some of ester bonds (generating free carboxyl groups) and makes the porous membrane more hydrophilic. The dangling bonds respond to the external electric field and also introduce another ion–current rectification mechanism [2]. The cation selective nature of PET nanopores shows that there is a preferential direction of ion flow. The above is also indicated in its diode like I–V characteristic [2],

Results

In the present study, the I–V measurements have been done for PET films etched for different durations which results in the formation of pores of different sizes, Fig. 2. For the smaller pore size, Fig. 2(a), the current magnitude is in the order of μA whereas, for the larger pore size, the current magnitude increases to mA, Fig. 2(b). For 150 minutes etched film, a peak is observed for the applied voltage of ~ 22 V while for higher etching time (180 minutes), the peak appears for applied voltage

Discussion

It is well known that the polymer gel electrolytes conduct via both, the anions and the cations with their respective transference numbers varying depending on the ionic radii and the charge state. In the system under study, due to the favorable biasing, the anions (ClO4) in the Mg PGE compartment will start collecting at the porous PET membrane. This will further increase the field for more Li+ flow to the Mg PGE compartment. Due to their large radii and low mobility, only a few anions will

Conclusions

Our study, using porous PET membranes in an ion exchange system, is summarized below.

Two polymer gel electrolytes having different conductivities and different ions, when separated by a porous PET membrane, can show sustained ion current fluctuation even if the concentration and pH values are the same. The present study gives information about the effects of the etching time (pore size) of the irradiated PET film on the I–V behavior of the system. When integrated with modified Sprott's circuit,

Acknowledgements

We wish to thank the UGC, CSIR, DST (Govt. of India) and the University of Delhi for their financial support. Our thanks are due to Ms. Vinita Suyal for contribution in data analysis. The authors are also grateful to Dr. D. Fink and the operators at the Helmholtz Centre for Materials and Energy, Berlin, for irradiation of the samples. One of the authors, SR, thanks CSIR for the award of Senior Research Fellowship.

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