Polyoxometalate/Hydroquinone Hybrid Redox Electrolyte for Electrochemical Energy Storage

Abstract

There has been progress and growing interest in the development of novel and efficient energy storage systems meeting high power and energy demands of modern devices and advanced technologies. Among important requirements for such charge storage devices are high volumetric and gravimetric energy and power, large number of charging/discharging cycles, improved safety and reasonable cost. At the current stage of technology, common charge storage systems, such as batteries and electrochemical capacitors, rarely meet those expectations. Indeed the present systems offer either high energy density (batteries) or high peak power (electrochemical capacitors) but barely both characteristics. Lately, there have been attempts to consider aqueous electrolytes containing dissolved redox species capable of undergoing fast electrode reactions at the interfaces formed with highly porous carbon electrodes. Consequently, the energy density significantly increases while keeping the power performance of a typical double-layer capacitor. Since most of redox-electrolyte-based systems utilize aqueous solutions, such advantages as enhanced safety, environment-friendliness and reasonable cost could be retained. Such redox-active species as iodide, quinone, phenelenediamine, ferrocyanide, methylene blue, thiocyanide and cerium sulphate have been successfully applied as positive or negative electrode-supporting electrolytes. The redox processes (battery-type characteristics) are typically observable in the potential range of a single electrode only and, therefore, the opposite electrode operating according to the electrical double-layer mechanism (capacitive behavior) limits the overall cell performance in terms of capacity and energy. To overcome this drawback, application of dual redox-type electrolytes comprising two independent redox couples separated in the positive and negative electrode compartments has been recently proposed. From the mechanistic point of view, such devices resemble conventional redox-flow batteries but operating in static conditions, i.e. more favourable from the practical and economical point of view.By taking into account the above deliberations, a dual redox-active (proton-conductive) electrolyte composed of the mixture of two different soluble redox species is considered here for application in energy storage devices with an enhanced specific energy [1]. The Keggin-type polyoxometalate (phosphotungstic acid), undergoing fast (electrochemically reversible) one-electron redox processes has been selected to support the negative electrode. In order to develop the potential difference in the cell, the hydroquinone, capable of oxidation/reduction according to the fast two-electron mechanism has been adopted and its electroactivity was predominantly confined to the positive electrode. It enabled us to design a hybrid charge storage cell with the operating voltage of 0.8 V and the specific energy of 20 Wh kg-1 (per total mass of both electrodes). In the constant power discharge mode, 13 Wh kg-1 of the energy has been preserved at the power of 1 kW kg-1 during discharging down to ½ of the maximum voltage which is consistent with the fast charging/discharging dynamics of the proposed hybrid system. A mixed hierarchical micro-porous (predominantly mesoporous) porous carbon of high (>2 cm3g-1) total pore volume and the BET surface area approaching 1000 m2 g-1 has been selected as the electrode material permitting electrical double layer charging and supporting electroactivity of redox species and unimpeded mass transport. Also, the normalization of electrical parameters against the total mass of electrodes and electrolyte resulted in a ca. five-fold increase of discharge capacity and specific energy in comparison to the performance of a simple electrical double-layer capacitor. 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, K. Węgrzyn, S. Dyjak, N. H. Wisińska, P. J. Kulesza, Energy Storage Mater., 21 (2019) 427-438