Towards building a sustainable, high performance, and affordable redox flow battery

Abstract

As the number of generation sources from intermittent renewable technologies on the electric grid increases, the need for large-scale energy storage devices is becoming essential to ensure grid stability. Aqueous redox flow batteries (AORFB) are the most promising choice as they can improve the efficiency of the existing grid infrastructure by providing cost-effective storage together with a long life. Although the RFBs have been recognized as a viable technology for large-scale energy storage, the extensive utilization of the RFBs has been limited by the high cost. Several strategies have been published to reduce the cost of the RFB. One effective way is to reduce the cost of the flow battery components by reducing the chemical cost of the electrolyte, the cost of electrode treatments, and finally, the cost of the ion exchange membrane. The work presented in the first part of the thesis aims to reduce the cost of the electrolyte by developing an anolyte utilizing an organic crystal indigo carmine (5,5'-indigodisulfonic acid sodium salt) (IC-Na), a water-soluble derivative of the naturally occurring dye indigo. The 5,5'-indigodisulfonic acid (IC-H) is obtained through the substitution of sodium ions in IC-Na with protons (H+). The aqueous solubility of IC-H was increased dramatically from 0.035 M to 0.760 M (1.52 M electron concentration) in protic solvents. The diffusion coefficients (IC-Na: 1.06 ×10-5 and IC-H: 2.19 ×10-5 cm 2 s −1) and reaction rate constants (IC-Na: 1.93 × 10-4 and IC-H: 1.86 ×10-4 cm s-1) of IC-Na and IC-H indicate rapid reaction kinetics. The highly soluble and affordable IC-H with fast redox kinetics was used as a sustainable anolyte by pairing with different catholytes. Moreover, computational study was also conducted which signifies the prospect of further improvements in solubility and voltage window by tuning the structure. The second part demonstrates an energy-efficient, affordable, and scalable surface modification method of graphite felt (GF) electrode based on the electrochemical exfoliation to enhance the mass and charge transfer of the electrode. Exfoliation of the GF was conducted in ammonium sulfate ((NH4)2SO4) aqueous solution by breaking the weak van der Waals forces between the graphitic layers. The exfoliation incorporated sufficient oxygen functional groups that increase the active surface area, resulting in enhanced reaction kinetics at the electrode-electrolyte interface and improved hydrophilicity. Benefitting from the sufficient oxygen groups and superior wettability, the exfoliated GF (E-GF) shows brilliant electrocatalytic activity with minimized overpotential, higher volumetric capacity, and improved energy efficiency. The RFB assembled with the E-GF electrode delivered voltage and energy efficiencies of ~ 90 and 86 % at the current density of 100 mA cm-2, respectively. Finally, the last part of the thesis develops a stable and highly ion-selective membrane by utilizing proton conductive cellulose nanocrystals (CNCs) incorporated in semicrystalline hydrophobic poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) matrix. The high hydrophobicity of the PVDF-HFP matrix mitigates crossover of the electrolytes, whereas, the abundant and low-cost CNCs derived from wood provides high proton conductivity. The fundamental contributors for CNCs' excellent proton conductivity are the hydroxyl (-OH) functional groups, highly acidic sulfonate (-SO3H) functional groups, and the extensive intramolecular hydrogen bonding network. In addition, CNCs exhibit mechanically and chemically stable structure in the harsh acidic electrolyte. Therefore, because of the high proton conductivity, excellent ion selectivity, high chemical stability, and structural robustness, the vanadium redox flow battery (VRFB) assembled with homogeneous CNCs and PVDF-HFP (CNC/PVDF-HFP) membrane achieved a coulombic efficiency (CE) of 98.2 %, energy efficiency (EE) of 88.2 %, and a stable cycling performance for more than 650 cycles at a current density of 100 mA cm-2. The obtained membrane possess excellent flexibility, high mechanical tensile strength, and superior selectivity.--Author's abstract