Keggin Type Silicotungstic Acid As Electrolyte for Liquid and Solid-State Electrochemical Capacitors Utilizing Biopolymer Membranes
Authors: Skunik-Nuckowska Magdalena , Wisińska Natalia H , Dyjak Sławomir , Kulesza Pawel J. 
Published: May 2020

A dynamic progress in technologically advanced electronics and automotive industry, as well as a growing interest in renewable energy resources, requires efficient energy storage systems supporting conventional batteries and fuel cells in terms of power, durability, wide temperature performance and overall maintenance cost. In this respect the electrochemical capacitors (ECs) perfectly match these demands allowing for a large amount of energy to be stored and released rapidly, i.e. able to deliver high power pulses preventing the main energy storage unit (eg. battery) from fast performance failure. Additionally, due to the operation mechanism involving purely electrostatic charge storage in the electrical double layer, their life-span is almost unlimited while the application of highly porous carbon electrodes, with surface area often exceeding 2000 m2 g-1, ensures the energy density incomparably higher when compared with traditional tantalum capacitors.
Among important issue, ensuring high reliability of ECs is a proper selection of electrolytic solution serving as a source of ions supporting double-layer formation. Most of ECs, especially those commercially available, comprise of organic solvents offering a wide operating voltage which results in a high energy density, by its definition equal to . However, such drawbacks of organic electrolytes as low conductivity, toxicity, flammability and high cost cannot be neglected. For this reason, some of the recent attempts are focused on new concepts of aqueous electrolytes, replacing standard acid and alkali solutions by paying attention to the safety issues, price and environmental friendliness.
Recently, we reported on application of Keggin-type polyoxometalate (POM), namely phosphotungstic acid, H3PW12O40 (PW12) as electrolyte for aqueous ECs [1, 2]. Such features as its low toxicity, high ionic conductivity, chemical/electrochemical stability and possibility of using low cost stainless-steel current collectors, hindering in case of standard acidic electrolytes, have been emphasized. In the present study we extend this concept on application of Keggin-type silicotungstic acid, H4SiW12O40 (SiW12) as electrolyte operating down to -30°C. Five different activated carbon materials have been selected for electrochemical testing in order to match the porous structure of the electrode to a fairly large SiW12O40
4- anion (1-1.2 nm). It has been shown that for carbon materials with adequately wide pore size distribution in the microporous range, the electrical parameters of a capacitor, such as specific capacitance and specific energy, can be as high as for the standard acidic H2SO4 electrolyte but with better charging/discharging dynamics and consequently with the higher specific power (as derived from the constant power Ragone’plots). That can be attributed to the high mobility of protons and anions in the largely hydrated Keggin structure as well as to a great ionic and molar conductivity of SiW12. The capacitive properties of SiW12 have been compared with its P-containing counterpart, PW12 in order to discuss the influence of POMs redox chemistry on the performance of ECs.
We have also developed a strategy allowing for the immobilization of SiW12 within a porous biopolymer membrane. For this purpose, a microcrystalline cellulose was used due to its well-abundance, low cost and environmental aspects. Dissolution process, i.e. one of the major problem related with the cellulose-based membrane processing has been conducted using the NaOH/urea mixture by freezing followed the gelation at ambient temperature. The structure and stability of the gel was investigated by various physicochemical techniques including XRD, FT IR, TGA, SEM and AFM. H4SiW12O40-impregnated membrane applied as both separator and a polymer electrolyte in the symmetric solid-state capacitor utilizing activated carbon electrodes allowed to minimize the electrolyte content providing excellent capacitance characteristics and charge transport dynamics when compared to the liquid-type cells.

Acknowledgement

Financial support was provided by the National Centre for Research and Development (NCBR, Poland) under Techmatstrateg Grant no. 347431/14/NCBR/2018 as well as by the NCBR in the framework of European Union POWER 3.2 Project no. POWR.03.02.00-00-I007/16-00.
1. M. Skunik-Nuckowska, S. Dyjak, K. Grzejszczyk, N.H. Wisińska, F.Beguin, P. J .Kulesza, Electrochim. Acta 282 (2018) 533-543
2. M. Skunik-Nuckowska, K. Węgrzyn, S. Dyjak, N. H. Wisińska, P. J. Kulesza, Energy Storage Mater. 21 (2019) 427-438