Investigation of electrolytes and separators for redox flow batteries

Abstract

Batteries as energy storage and conversion technologies, especially redox flow batteries (RFBs), have gained momentum in electric grids. The state-of-the-art RFB uses vanadium-based raw materials for their electrolytes; hence, they are called vanadium redox flow batteries, VRFB. While VRFB performance is well studied, their sluggish kinetics and precipitation issues still need to be addressed. In chapter 4 of this thesis, a raw material for VRFB, vanadium pentoxide V2O5, is studied. The studies involve long-term solubility monitoring, electrochemical screening, and spectral studies. V2O5 long-term solubility increased by adding a 5% volume of hydrochloric acid or methanesulfonic acid. Its electrochemistry performance also improved. A three-fold increase was observed in the diffusion coefficient of the ion VO2+ (formed from dissolving V2O5 in acid) and of the kinetic rate constant of the electron transfer process. Electrochemistry and spectral data proved the mechanism of the improvement. The additives provide extra protons for the dissolution of V2O5. The -OH groups or -CH3SO3- anions in methanesulfonic acid increase the solution’s wettability on the carbon electrode surface. The Cl- anions in hydrochloric acid form coordinated complexes with the dissolved vanadium ions, reducing the chance for clustering and precipitating. In chapter 5 of this thesis, RFB performance is studied. The V2O5-based solution with HCl additive is chosen as the positive electrolyte. An organic quinone solution, made from anthraquinone-2,6-disulfonate sodium (AQDS 2,6) dissolved in sulfuric acid, is chosen as the negative electrolyte. To facilitate the use of a hybrid inorganic – organic ‘vanadium || organic quinone’ cell, a silica gel separator is employed. The silica gel separator is made by dispersing silica into an acidic solution, which is identical to the solutions used to dissolve V2O5 or AQDS-2,6. The full cell has a capacity of 4 AhL-1, energy efficiency of 81.4%, and capacity retention of 97% after 500 cycles. Its voltage losses were predicted from the previous electrochemistry screening studies and analyzed from the full cell data. The reactions were confirmed using spectral data. The results demonstrate the potential for this system to be used in next-generation RFBs for energy storage applications.