Ionic conductivity of a PMMA based gel polymer electrolyte and its performance in solid state electrochemical cells

Gel polymer electrolytes are known to be very suitable for many electrochemical devices. They have been extensively studied for lithium based electrochemical cells. But, due to several drawbacks of lithium based cells such as safety issues, handling problems and toxicity, it is the time to develop non Li based cells. In this study, ionic conductivity of a gel polymer electrolyte (GPE) based on polymethylmethacrylate, ethylene carbonate, propylene carbonate and tetrapropylammonium iodide and its performance in an Mg / C: I2 cell was investigated. GPE was prepared by varying salt concentration using hot pressed method and the composition, 20 PMMA / 30 EC / 30 PC / 40 Pr4N I showed the highest conductivity of 5.02 x 10 Scm at 28C. The cell in the form Mg / GPE / C+I2 showed an average open circuit voltage of 1.9 V. The average short circuit current was 3.3 mA. It was possible to observe a good stability by the self discharge characteristics of the cell.


INTRODUCTION
Gel polymer electrolytes (GPE) have been found to be capable of playing a significant role in numerous electrochemical applications such as batteries, smart windows, super capacitors and photo electrochemical solar cells [1][2][3][4] . GPEs are basically considered to be consisting of a liquid electrolyte encapsulated within a polymer matrix. Use of such gel polymer electrolytes in devices has made the 'all solid state concept' a reality. At present, many electrochemical devices that are being used and being developed are based on lithium which is known as having some drawbacks such as safety issues, handling problems and toxicity 5,6 . Therefore, it would be of great interest to develop non Li based systems in order to avoid the drawbacks of Li based devices. At present, there is a particular interest on anodes like Mg, Zn and Na as they have some distinct advantages over Li such as low cost, natural abundance and environmental friendliness. Most of these metals are readily available in nature and also not hazardous. Also, several gel polymer electrolyte systems have been investigated with different cations other than Li and as such, studies are being carried out on non Li cells 7,8 . A special feature of such devices is that almost all of those GPEs are incorporating anions like CF3SO3 -, ClO4which are very common. However, there are some GPEs with Iodide based salts which are found to be iodide anion conductors. Such GPEs have been extensively studied for photo electrochemical solar cells where iodide ions are present as redox species 9,10 . In this work, the ionic conductivity of the gel polymer electrolyte based on polymethylmethacrylate (PMMA), ethylene carbonate (EC), propylene carbonate (PC) and tetrapropylammonium iodide (Pr4N + I -) and its performance in cells of the form, Magnesium (Mg) / PMMA: EC : PC: Pr4N + I -/ graphite(C) + I2 have been studied. The importance of this study is that properties and applications of only PMMA / EC / PC / Pr4N + Ihave not been reported before.

Preparation of GPE
For the purpose of reaching at an appropriate composition, several GPE films were prepared by varying amount of Pr4N + I with a constant quantity of PMMA + EC + PC. PMMA (Aldrich), EC (Aldrich), PC (Aldrich) and Pr4N + I (ABCR) were used as received. The composition that yields a good ionic conductivity at normal temperatures and good mechanical stability was selected for further studies. Suitable amounts of PMMA, EC, PC and Pr4N + I were mixed and the mixture was magnetically stirred further. It was heated at 80  C for 1 hr. Finally, the homogenous, hot mixture was pressed in between two well cleaned glass plates. Samples were prepared by varying the Pr4N + I concentration (by weight).

Characterization of GPE
A circular disc of 14 mm diameter was cut from a prepared GPE film and was sandwiched in between two stainless steel electrodes (SS) inside a spring loaded sample holder. A micrometer screw gauge was used to measure the thickness of the electrolyte film. Impedance measurements for different samples were taken in the frequency range, 0.01 Hz -0.1 MHz from room temperature to 55  C using Metroohm M101 impedance analyser. After determining the composition that results highest room temperature conductivity, it was used for further analysis.

DC polarization test
Disc shaped GPE sample was loaded in between two stainless steel electrodes in a sample holder and polarization measurements were done under an applied potential of 1 V. Current drop with time was observed for several hours.

Cell fabrication and characterization
For the cell fabrication, a cleaned Mg strip was used as one electrode. Graphite and I2 (Breckland Scientific Supplies) were mixed well in the ratio 4 : 1(by weight) and pellets were prepared. Several cells in the form, Mg / GPE / C : I2 were assembled inside spring loaded brass sample holders. Open circuit voltages and short circuit currents were measured using a digital multimeter. Discharge characteristics were observed under constant loads of 1 MΩ and 10 MΩ. Also, self discharge characteristics of some cells were monitored for several hours.

Ionic conductivity of GPE
Impedance data at different Pr4N + I concentrations was analysed using the Non Linear Least Square (NLLS) method developed by Boukamp (1989) and conductivity values were calculated 11 . Figure 1 shows the variation of room temperature conductivity with Pr4N + I concentration. At low salt concentrations, ionic conductivity was lower but with increasing concentration, ionic conductivity increased. After a certain salt concentration, further addition of salt reduced conductivity. Several other researchers have observed similar feature and have reported that the initial conductivity increase may be due to the building up of charge carriers with increasing salt content 12,13 . At high salt concentrations, build up of charge carriers is offset by the formation of neutral ion pairs as well as ion clouds made up of ion aggregates. The sample that had the optimized mechanical stability and highest ionic conductivity was of the composition, 20 PMMA / 30 EC / 30 PC / 40 Pr4N + I (in weight basis) and the conductivity was 5.02 × 10 -3 Scm -1 at 28  C. This value is rather higher than the values reported by Jeong et al., (2006) for a polyvinyl alcohol based MgTF system 12 . A comparable value has been observed by Lang et al., (2006) for a Sodium Iodide system with Polyacrylonitrile 14 . The electrolyte film made in our study had a satisfactory mechanical strength making it suitable for applications. Dependence of ionic conductivity on temperature of the sample which has the highest conductivity is illustrated in Figure 2. The curvature of the plot in Figure 2 is clearly implying that the conductivity can be described by the familiar Vogel -Tamman -Fulcher equation, where Ea is the pseudo activation energy, T0 is related to glass transition temperature, A is a pre exponential factor.
It can be seen from the plot that when the temperature increases, the conductivity also increases as expected. This is evidently due to decrease of viscosity with increasing temperature 13 .

DC polarization test
DC polarization curve taken with blocking electrodes is shown in Figure 3. The value of ionic transference number has been calculated and found to be 0.9. This value clearly shows that overall conductivity of GPE is predominantly ionic 15,16 .

Performance of cells
The cell voltage discharge profiles recorded as a function of time for two loads, 1 MΩ and 10 MΩ are shown in Figure 4. The possible electrochemical reactions can be stated as follows.
At the anode: At the cathode: Open circuit voltage has an average value of 1.9 V which is an appealing sufficient value for low power requirements. The average short circuit current was 3.3 mA. Under both loads, cells show more or less similar discharge characteristics. For a voltage reduction of 0.2 V, it has taken more than 10 hours. Average currents during discharge for 1 MΩ and 10 MΩ are 2 μA and 0.2 μA respectively. These two values are little lower but, make the cells suitable for low power applications.

CONCLUSIONS
The GPE system considered in this project is a good ionic conductor having negligible electronic conductivity. Therefore, it is quite suitable not only for electrochemical solar cells but also for primary cells. The cells having the electrodes other than Li are suitable for low power requirements such as illumination of LEDs. Those cells are having no environmental pollution issues as well as leaking, sealing problems. The results of this study may generate interest on cells based on GPEs with different types of anions instead of the commonly used ones.