Poly(norepinephrine) As a Functional Additive for Hybrid Cellulose/Agarose-Based Hydrogel Membranes: Application to Supercapacitors

Wisinska, Natalia

A rapidly growing interest in renewable energy sources requires not only developing efficient energy storage systems but also incorporating a greater number of eco-friendly components. Electrochemical double-layer capacitors (EDLCs) are a class of energy storage devices capable to store the electrical charge due to the separation of oppositely charged ions in the electrical field which results in the formation of an electrical double layer (EDL) at the electrode/electrolyte interface. EDLCs consist in general of two porous carbon-based electrodes pre-soaked with electrolyte and separated with a membrane (separator). The simple electrostatic mechanism of energy storage, coupled with a lack of chemical changes and faradaic transitions during operation, results in high electrical capacitance compared to classical capacitors, significantly higher power density in contrast to batteries, and practically unlimited life span. Currently, commercial EDLCs typically rely on organic solvents, such as acetonitrile or propylene carbonate, with the addition of ionically-conductive salts. However, there are several drawbacks when it comes to practical applications involving particular low conductivity, toxicity, flammability and high cost. This resulted in an increased interest in aqueous electrolytes such as KOH, H₂SO₄ or simple inorganic salts, which although they have a limited potential window, exhibit many positive features including higher ionic conductivity, lower viscosity, increased safety, lower cost and ease of assembly under ambient atmosphere.

Modern and technologically advanced charge storage devices often require high safety flexible and deformable devices for specific applications. However, at the current state-of-the-art, the EDLCs suffer from two prominent limitations (i) the possibility of electrolyte leakage and (ii) high standards of technology to safely encapsulate electrolytes in the device. Therefore, a lot of research is held to develop alternatives for currently used liquid (aqueous and organic) electrolytes. One of the solutions to overcome these limitations are solid-state EDLCs. Those systems use an ionically-conductive polymer or hydrogel membrane, which serves as both the separator and the electrolyte. Cellulose, built of β-(1→4)-linked D-glucose units, is one of the most prevalent and easily degradable biopolymers. Albeit, its 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. 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 regeneration of microcrystalline cellulose, i.e. its dissolution using an aqueous mixture of NaOH/urea, and further processing into a hydrogel membrane in the presence of cross-linking agent epichlorohydrin. To improve the mechanical strength and electrolyte uptake, in-situ polymerized norepinephrine and agarose were subsequently incorporated obtaining an interpenetrating polymer network (IPN). The structure and morphology of the membranes were characterized with SEM/EDX, CP/MAS 13C-NMR, AT-FTIR, TGA, contact angle, and elementary analysis. The ionic conductivity was determined using impedance spectroscopy over a wide range of temperatures (5-60°C). The relation between stress and strain in the materials was also determined to diagnose the mechanical properties. The cellulose-based hydrogel membranes were further used as a support for various aqueous electrolytes, including H₂SO₄, Na₂SO₄, i.e. most commonly used for aqueous EDLCs. Also, the alternative electrolyte was used, i.e. silicotungstic acid, H₄SiW₁₂O₄₀ which according to our recent results seems to be a promising candidate to replace conventional acidic electrolytes [1]. The designed systems were compared, in terms of energy, power and cycleability, with their analogues using conventional polypropylene separators and a liquid electrolyte.

[1] N.H. Wisinska, M. Skunik-Nuckowska, S. Dyjak, P.J. Kulesza, Factors affecting the 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

Acknowledgement

Financial support was provided by the National Science Center under Preludium 19 grant no. 2020/37//N/ST4/01679. This work was implemented as a part of Operational Project Knowledge Education Development 2014–2020 co-financed by the European Social Fund, Project No POWR.03.02.00-00-I007/16-00 (POWER 2014-2020)