Development of New Cellulose-Based Hydrogel Membranes for Aqueous Electrochemical Capacitors
Authors: Wisinska Natalia H. , Skunik-Nuckowska Magdalena , Mroziewicz Aleksandra , Kulesza Pawel J. 
Published: July 2021

A rapidly growing interest in renewable energy resources requires efficient energy storage systems as well as a need to elaborate a greater number of eco-friendly components. Electrochemical double-layer capacitors (EDLCs), also known as supercapacitors are a class of energy storage devices that store the energy due to the separation of oppositely charged ions at in the electrical field resulting the formation of electrical double layer (EDL) at the porous carbon electrode/electrolyte interface. EDLCs consist of two porous carbonaceous electrodes pre-soaked with electrolyte and separated with a membrane (separator). The simple electrostatic mechanism of energy storage, lack of chemical changes and faradaic transitions during operation results in high electrical capacitance with respect to classical capacitors, significantly higher power density in comparison to batteries, and practically unlimited life span. Currently, the commercially produced EDLCs typically rely on organic solvents such as acetonitrile or propylene carbonate with the addition of conductive salts. However, such drawbacks as their low conductivity, toxicity flammability and high cost, led to the growth of interest in aqueous electrolytes such as KOH, H2SO4 or simple inorganic salts. Among main advantages of aqueous solvents such features as their higher ionic conductivity, lower viscosity, increased safety, lower cost and ease of assembly under ambient atmosphere, should be underlined.
Modern and technologically advanced charge storage devices often require flexible and deformable devices for specific applications. Therefore, a lot of research is held into the development of alternatives for currently used liquid (aqueous and organic) electrolytes, which suffer from two prominent drawbacks — (i) the possibility of electrolyte leakage and (ii) high standards of technology to safely encapsulate electrolyte in the device. To overcome these limitations, the solid-state EDLCs using an ionically-conductive polymer or hydrogel membrane, serving as both the separator and the electrolyte, are under the spotlight. In this respect, a cellulose, built of β-(1→4)-linked D-glucose units, is one of the most prevalent and easily degradable biopolymer. Albeit the wide availability, biodegradability and low cost, the usage of cellulose is limited due to insolubility in most common solvents. The recent alternative, to toxic and flammable organic compounds, such as N,N- Dimethylformamide/N2O4, N-methylmorpholine oxide (NMMO), are ionic liquids (ILs), that have been gaining lately a lot of attention in energy storage systems. Various ILs based on imidazolium, pyridinium and ammonium cation paired with strongly basic anion (e.g., OAc-, HCOO-) were also recently used to dissolve cellulose [1]. However, the requirements of high-purity syntheses and the cost of some of the cations/anions may affect a large scale application.
Therefore, our research refers to an alternative route of chemical processing of microcrystalline cellulose, i.e. its dissolution using various wt% ratio of aqueous NaOH/urea mixtures, and further processing into a hydrogel membrane in the presence of cross-linking agents, such as glutaraldehyde and citric acid. The cellulose-based hydrogel membranes will be used as a support for various aqueous electrolytes, including H2SO4, KOH, K2SO4, i.e. most commonly used for aqueous EDLCs. Also, the alternative solutions will be used, i.e. based on polyoxometalates, or more precisely Keggin-type heteropolyacids (such as H4SiW12O40) which according to our recent results seem to be promising candidates to replace conventional acidic/neutral electrolytes [2]. The thickness of the membranes will be adjusted to introduce the minimum volume of electrolyte, i.e. necessary to fill the porosity and all void spaces in the electrode material and to avoid the electrolyte excess. A structure and morphology will be characterized with SEM/EDX, AFM, AT-FTIR, XRD and TGA. The ionic conductivity will be determined using impedance spectroscopy over wide range of temperatures. The designed systems will be compared, in terms of energy, power and cycleability, with their analogues using conventional polypropylene separator in the presence of excess of liquid electrolyte.

Acknowledgement

Financial support was provided by the National Centre for Research and Development (NCBR, Poland) under Techmatstrateg Grant no. 347431/14/NCBR/2018.
[1] D. Kasprzak, I. Stepniak, M. Galinski, Electrodes and hydrogel electrolytes based on cellulose: fabrication and characterization as EDLC components, J. Solid State Electrochem. 22 (2018) 3035–3047, https://doi.org/10.1007/s10008-018-4015-y
[2] N.H. Wisinska, M. Skunik-Nuckowska, S. Dyjak, P.J. Kulesza, Factors affecting performance of electrochemical capacitors operating in Keggin-type silicotungstic acid electrolyte, Appl. Surf. Sci. 530 (2020) 147273, https://doi.org/10.1016/j.apsusc.2020.147273.