Design of non-faradaic EDLC from plasticized MC based polymer electrolyte with an energy density close to lead-acid batteries
Graphical abstract
Introduction
Today, the polymer industry has rapidly developed, more than any other industry, due to the wide applications in the field of polymers compared to any other classes of materials [1], [2]. Heteroatoms with lone pair electrons in polar polymers, such as oxygen and nitrogen, allowed interacting directly with the cation of inorganic and transition metal salts, resulting in the dissolution. There have been extensive researches regarding solid polymer electrolytes (SPEs) based on natural polymers [3], [4], [5]. SPEs are essential for electrochemical devices owing to the benefits of solvent free, leakage free, light weight, elasticity, transparency, easy thin-film forming, good conductivity, easy handlingand wide electrochemical windows in comparison with commercial liquid electrolytes [6], [7], [8].
In order to minimize the impact of fossil foil resources on environment, different routes have been taken on both academic and industrial level to advance the usage ofrenewable and sustainable energy sources. Furthermore, the last two decades witnessed a significant advancement in energy production from renewable energy sources, which demands more efficient and powerful energy storage devices thatare also environmentally safe [9], [10], [11]. In this regard biodegradable materials, which are environmentally friendly, are among the most preferable choices to be employed in either energy conversion and energy storage devices such as solar cells, supercapacitors and batteries [12], [13]. Recent researches have revealed the possibility of employing biodegradable polymer electrolytes in energy conversion devices like dye sensitized solar cell (DSSC) [13], [14], energy storage devices such as batteries [15], [16], [17], [18], and supercapacitors [12], [19]. The overall conductivity of the studied biodegradable polymer electrolytes is ranging from (10−3-10−5 Scm−1), which is suitable for energy device applications [12], [15], [19]. However, further enchantment in ionic conductivity is necessary for better device performance, which can be achieved through careful selection of host polymer, dopant salt, plasticizer and filler [12].
Commonly used host polymers in polymer electrolytes for energy storage devices are of synthetic or natural polymer type [20]. Most of the synthetic polymers including polyethylene, polyvinyl chloride, polymethyl metacrylate, polycarbonate and polyacrylonitrile, which are often derived from petroleum resources, are considered as non-biodegradable, causing environmental pollution [21], [22]. Therefore, biopolymers can offer a great promise to be used as the host polymer in energy storage devices. Biopolymers made from renewable resources possess unique properties like affordability, solvent compatibility, abundancy and good film forming ability [23], [24], [25], [26]; additionally, most studied examples in polymer electrolyte are cellulose, starch, chitosan, dextran and carrageenan [27], [28], [29].
Cellulose, as the most abundant organic polymer from natural origin, is renewable and can be used as an alternative to petroleum-based polymers [30], [31].One of the most common types of cellulose derivative is methyl cellulose (MC), which is produced by partially substituting hydroxyl groups on the cellulose backbone with methoxy group as a result of reacting methyl chloride or dimethyl sulfate with alkali cellulose [32], [33]. Films formed from MC are transparent and possess good mechanical, thermal and chemical stabilities, as well as the excellent solubility [34], [35]. Free ions from the salt form interactions with the functional groups of the polymer host via a dative bond. All the functional groups of –OH, –O-CH3 and –C1-O-C4– with lone pair electrons in MC are involved in the ionic conduction [36]. To date, polymer electrolytes have been playing a major role in modern energy storage devices, including batteries and supercapacitors (SCs). SCs are of three main types, namely electric double-layer capacitors (EDLCs), pseudocapacitors (PDCs) and hybrid SCs. In EDLCs, charge is stored as Helmholtz’s double-layers formed from ion motion inside the electrolyte without any interaction between electrodes and electrolyte, which involves a non-Faradaic reaction (NFR) [37]. The adsorption of ions on the electrode surface provides charging-discharging cycles. In PDCs, energy is stored through Faradaic redox reaction between the electrode and electrolyte. The combination of both Faradaic reaction and NFR is the mechanism of charge storage in hybrid capacitors. Both high energy and high power densities can be attained using super EDLC [38], [39]. Here, the energy storage is achieved by the charge accumulation on the surface of carbon electrode in a form of potential energy [40]. These SCs are recognized by long cycle life, simple fabrication and fast charge–discharge rate [41]. Among carbon materials, activated carbons (ACs) are the most often used ideal active materials of EDLC electrodes, owing to their good electric conductivity, high surface area and low cost [42]. Thus, two porous electrodes and an electrolyte film are incorporated in the EDLC device.
Overall, polymer based EDLC exhibits low energy density compared to the batteries. However, this shortcoming can be minimized through improving ionic conductivity and electrochemical properties of the polymer-based electrolyte. The intensive literature survey revealed that the effect of glycerol on MC-NaI system has not been explored yet. Hence, in this research, glycerol as the appropriate plasticizer was used to enhance conductivity of the MC host polymer, since it contains three –OH groups. This study includes exploring the effect of glycerol content on the conductivity and electrochemical properties of the MC polymer doped with sodium iodide (NaI). Then, the EDLC device will be fabricated and its performance using the optimum plasticized concentration will be thoroughly investigated. The results presented in this research can be regarded as an innovation in EDLC devices. The outcomes highlight the possibility of using such polymer electrolytes as mediator between the electrodes in energy device applications. Extra research work is required to satisfy the suitability of plasticized electrolytes for electrochemical device application and extending the cyclability of the devices.
Section snippets
Materials and samplespreparation
The host polymer methylcellulose (MC) (4000 cP) and sodium iodide (NaI) (99.5%) were purchased from Sigma Aldrich and used without further purification. 1 g of MC was dissolved in 90 ml of 1% acetic acid solution at ambient temperature and stirred consciously for 2 h. Then, a fixed amount of NaI salt with 50 wt.% was added to the solution in order to prepare the polymer electrolyte and the mixture was stirred for 3 h to get a homogenous solution. The prepared polymer electrolyte then was
Conductivity and transport study
The electric charge displacement within the bulk material occurs via two interrelated physical phenomena, namely polarization and diffusion. In case of polarization, the charge mobility is tightly localized in a restricted volume of matter; whereas, diffusion of charge occurs through an extended distance of matter due the existing charge concentration gradient, which gives rise to a DC conductivity, σdc [43]. Generally, different kinds of polarization such as atomic, dipolar, electronic and
Conclusion
One of the main challenges facing polymer based electrolytes to be employed in energy device applications is their low ionic conductivity. This study highlighted the role of plasticizer on enhancing the DC conductivity of the MC:NaI:glycerol polymer electrolyte system prepared by solution casting technique. The impact of glycerol content on improving the DC conductivities of the samples was thoroughly explored using impedance analysis. It was found that the plasticizer can significantly improve
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to acknowledge all support for this work by the University of Sulaimani, King Saud University and Komar University of Science and Technology. The authors (S. M. Alshehri, T. Ahmed) are grateful to the researchers supporting project number (RSP-2021/29), King Saud University, Riyadh, Saudi Arabia for funding.
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