MECHANICAL AND STRUCTURAL PROPERTIES OF GEL POLYMER ELECTROLYTES FOR FLEXIBLE SUPERCAPASITORS

ABSTRACT

In this article, a unique self-healing, flexible double network gel polymer electrolyte derived from raw gellan gum and polyacrylamide is prepared and used as gel electrolyte in a supercapacitor.  Gellan gum and polyacrylamide that have been cross-linked using Na+ ions have good mechanical and self-healing characteristics.

АННОТАЦИЯ

В данной статье рассмотрен уникальный самовосстанавливающийся двумерно сшитый гибкий гель-полимерный электролит, состоящий из природной геллановой камеди и полиакриламида, который используется в качестве гелевого электролита в суперконденсаторе. Геллановая камедь и полиакриламид, сшитые ионами Na+, обладают хорошими механическими характеристиками самовосстановления.

Keywords: gellan gum, polyacrylamide, supercapacitor, electrolyte, stretchability, bending

Ключевые слова: геллановая камедь, полиакриламид, суперконденсатор, электролит, эластичность, изгиб.

Introduction

At present, storage batteries, capacitors, and supercapacitors (SC) are used as energy storage devices [1,2]. Among them, SC is now widely used by manufacturers because of its small size, large energy storage, fast charging, and safety. Such a demand leads not only to a change in the structure, appearance, and shape of electronic devices, but also in the devices for storage and transmission of energy used on their basis. The SC is the next generation of capacitors, which store electrical energy several times faster than conventional capacitors and have a large capacity. They differ from conventional capacitors with high capacitance density and high energy. In capacitors, an electric field is created between the electrodes at a certain distance from each other, and the energy of the electric field is accumulated in the volume between the electrodes. In SC, energy is accumulated due to the electric double layer, which appears in the liquid/solid electrolyte between 2 homogeneous electrodes and generates opposite charges at the electrode/electrolyte interface. The double layer at electrode 1 collects many electrons, which in turn attracts electrolyte cations. Electrode 2 collects anions on the surface of the double layer. The two electrical double layers are separated by a series of electrolytes that generate and store charge, energy, and voltage. In liquid electrolytes, the thickness of the electric double layer is 0.1 nm, and its electrical capacity is 10^–4 F/cm². In these studies, the mechanical and structural properties of polymer gel electrolytes, which can be used in SCs, were studied.

Experimental

Double network (DN) Gel polymer electrolyte (GPE) was synthesized by a one-pot method (Chen et al.), initially, Gelan Gum (GG) (respectively 0-0.5 gr), Acrylamide  (6g), Na₂SO₄ (0.2-1 moll), and deionised water (10 mL) were added into glasses beakers. The bubbling was mobility role and cross-linker of the sample. The results showed high tensile stress (2 MPa), elongation break (40 mm/mm), and good conductivity (0.29S/cm) is obtained simultaneously. Besides, the DN GPE can self-recover almost 55% at 60⁰ C temperature and for 8 hours. SCs can be facilely prepared by sandwiching the Polymer gel (PG)  DN GPE with two identical electrodes, which are fabricated by electrochemical depositing active carbon, without extra components. The objective of the current study was  achieve novel PG DN GPE with the combined advantages of excellent mechanical property and self-healing behaviors, which permits such flexible removed in the sample by ultrasonic vibrator and vacuum pumping, heated at 95⁰C water, and stirred for 2 hours under stirring speed at 10 m/r. After been a homogeneous solution, then Irgacure 2959 (0.085g) added, the solution was stirred again for 10 min. After all the reagents were dissolved, the resulting solution was rapidly injected into a glass mold (10cm × 60cm). As photo-polymerized under a UV Lamp with 365 nm wavelength and 8-watt power for 3 hours, finally the sample was kept at room temperature  for 2 hours to complete the polymerization process. By this method GG/PAM DN gels successfully prepared. PAM DN gels were also synthesized by the same process except for no GG added. The enhanced mechanical properties may result from the enhanced interaction between GG networks  Na⁺ serving as bridges.

Figure 1. The digital photographs of the PG3 DN GPE stretchability, knot-stretch ability, weight-loading (1200gr), bending, knotting and twisting respectively

As shown in Figure 1, PG DN GPE exhibit pure white, non-tacky, self-standing, extremely flexible and easy to manipulate. Besides, it can withstand mechanical deformation under different conditions without breakage. For this the as-prepared tough PG  DN GPE  electrolyte was a transparent elastomeric thin with superior stretchability and mechanical strength, allowing uniaxially and biaxially stretching, bending, loading, and knotting (Fig.1a-f). An intact PG  DN GPE was easily stretched up to more than 6 times its original length (Fig. 1a), while a knotted gel also bore the large strain of 600% without breaking (Fig.1b) and the sample did not disconnect even when loaded with 1200 g (Fig. 1c). PG DN GPE also can be bent easily and even rolled up without cracking, which makes it possible to be used in flexible energy storage systems. We used several measurement methods, such as Fourier Transform Infrared (FTIR) and X-ray photoelectron spectroscopy (XPS), to characterize PG  DN GPE. Self-healing PG  DN GPE polymers were confirmed by comparing their FTIR spectra with pure Polyacrylamide (PAM), GG and by the DN cross-linked PG  GPE.  Initially, Potassium bromide (KBr) is used with the sample to ensure the accuracy of the sample spectra. Cause, KBr is one of the compounds which does not absorb the provided radiations of "IR" and does not react with the sample, hence does not exhibit its absorption pattern in the graph.

Figure 2.   FTIR (a) and XPS (b) characterisation  relationships of  PG  DN GPE

Figure 2(a) shown PAM peaks at 3420 and 3194 cm⁻¹ for N-H stretching in primary amide, peaks at 1653 and 1616 cm⁻¹ for C=O and NH₂stretching, and a peek at 1420 cm⁻¹ for C-N stretching [3] In the spectrum of the GG, the broad peak observed between 3500 and 3200 cm⁻¹ is assigned to O-H stretching, whereas the peaks at 1715 and 1614 cm⁻¹ are attributed to C=O in carboxylic acid and asymmetric COO− stretching, respectively [3,4,5].  The GG gel shows a broad  peak at 3414 cm⁻¹ for the -OH groups. The peaks in 2923 cm⁻¹ and 2853 cm⁻¹ are for the asymmetric stretching vibration and symmetric stretching vibration of the methylene groups, respectively. The C=O stretching vibration of the -COOH group is presented in 1708 cm⁻¹[4]. The peak intensity of C-O stretching in the C-OH of PG DN GPE decreased compared to that of GG and PAM, while the peak intensity of symmetric C-O in the C-O-C structure increased  when the GG is with PAM. This indicates that the hydroxyl groups of GG react with the C-H groups of the  monomers, forming hydrogen bonds between them, providing further polymerization of PAM as a branch of the GG backbone. As a result of the addition of Na₂SO₄, the intensities of PAM and GG peaks were sharply reduced. From this, we can conclude that the Na ion can also simultaneously act as a bridge between GG and PAM while cross-linking the GG network. To improve the mechanical properties of GPE in further investigations we are planning to use the method of High-Performance Size Exclusion Chromatography [6,7] to establish the quantitative relationship between molar mass and tensile strain-stress, elongation degree of polymers in GPE. The given method allows expressing the determination of molar masses and polydispersity degrees of GPE components and gives reliable data for the optimal choice of polymer type in GPE to reach good performance exploitation characteristics of SC product. 

Conclusions

According to the above experiments a new GPE exhibits outstanding mechanical properties. Additionally, GPE has also shown promising self-healing ability under room conditions without external situations. This kind of ability gives us GPE use for a long time under mechanical deformation conditions. So, we offer novel GPE for flexible and wearable energy storage SC devices.

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