Redox flow battery (RFB) is a promising technology that can store intermittency sources such as solar and wind energy at a large scale and low cost. Energy in a RFB is stored in a pair of electrolytes, where soluble redox-active ions are key. While mature RFBs run on inorganic electrolytes, organic molecules are increasingly explored to address issues such as high material cost, crossover, and slow kinetics. However, both types of electrolytes face the issue of low solubility, which limits the volumetric specific capacity and the energy density.

In this thesis, we aim to understand how molecular structures and electrolyte compositions determine the solubility of redox-active ions and thereby design high-capacity electrolytes for aqueous RFBs. First, we use the thermodynamic cycle of sol...[ Read more ]

Redox flow battery (RFB) is a promising technology that can store intermittency sources such as solar and wind energy at a large scale and low cost. Energy in a RFB is stored in a pair of electrolytes, where soluble redox-active ions are key. While mature RFBs run on inorganic electrolytes, organic molecules are increasingly explored to address issues such as high material cost, crossover, and slow kinetics. However, both types of electrolytes face the issue of low solubility, which limits the volumetric specific capacity and the energy density.

In this thesis, we aim to understand how molecular structures and electrolyte compositions determine the solubility of redox-active ions and thereby design high-capacity electrolytes for aqueous RFBs. First, we use the thermodynamic cycle of solid dissolution to understand the solubility difference among redox-active anthraquinone sulfonate salts. Unlike typical theoretical approaches that consider the salts as neutral molecules, we consider the dissociation and show the importance of ion hydration energy and solid lattice energy in determining solubility. We further establish an empirical relationship between their solubility and the solubility of common sulfate salts, based on which we propose a concept of inorganic-organic hybrid electrolyte and demonstrate its application with a vanadium-anthraquinone hybrid RFB.

Secondly, we apply the knowledge of ternary phase diagram to understand the mixed ion effect on the solubility of redox-active ferrocyanide salts. By characterizing the properties of the solid salts and the binary solutions, we postulate that potassium ferrocyanide-sodium ferrocyanide-water is a ternary eutectic system, which is proven by building the phase diagram at room temperature with the solubilities at different component ratios. We then apply the mixed-ion effect to iodide salts to boost the maximum capacity to over 250 Ah/L and show how we may predict the effect based on binary phase diagrams. At last, we extend the effect to dihydroxylanthraquinone and pair it up with ferrocyanide in a mixed-ion RFB that delivers a 40% higher energy density than that of a single cation.