Polyoxometalate/hydroquinone dual redox electrolyte for hybrid energy storage systems

Abstract

A dual redox-active (proton-conductive) electrolyte composed of the mixture of two different soluble redox species is considered here for application in hybrid (battery-type) energy storage devices operating like high-power supercapacitor-type systems with an enhanced specific energy. 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) 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, 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.