A Solid/Liquid High-Energy-Density Storage Concept for Redox Flow Batteries and Its Demonstration in an H₂-V System

Abstract

Redox flow batteries (RFBs) are ideal for large-scale, long-term energy storage applications. However, the limited solubility of most ions and compounds in aqueous and non-aqueous solvents (1-1.5 M) restricts its use in the days-energy storage scenario, which necessitates a huge volume of solution in the numerous tanks and the vast floorspace for these tanks, making an RFB systems costly. To resolve the low energy storage density issue, this work presents a novel way in which the reactants and products are stored in both solid and soluble forms and only the soluble ions are circulated through the batteries. Storing the active ions in solid form can greatly increase the storage energy density of the system. With a solid to liquid storage ratio of 2:1, for example, the energy density of the electrolyte of vanadium sulfate (VOSO4), an active compound used in the all-vanadium RFB, can be increased from 40 Ah L-1 to 163 Ah L-1 (>4X), allowing an existing 6-hr RFB system to become a 24-hr system with minimal modifications. To show how the concept works, an H2-V flow battery with a solid/liquid storage system is used, and its successful demonstration validates the solid-liquid storage concept.

Modification of Nafion's nanostructure for the water management of PEM fuel cells

“…The reference exchange current density () and the catalyst loading density () used in the CL are unlikely to be affected by the hydrophobic treatment. Furthermore, our in‐situ ECSA measurement reveals that the hydrophobic treatment has minimal influence on the cathode's ECSA ( in Equation ()) 10 . Therefore, Equation () shows that the only possible reason for the fuel cell improvement in the kinetic‐controlled region is related to the oxygen concentration in the ionomer near the catalyst surface (), which is supposed to be determined by the oxygen solubility in the Nafion ionomer phase.…”

“…It provides more efficient pathways for the oxygen transport for the dissolved oxygen from the ionomer/air interface to the ionomer/catalyst interface. A more detailed explanation of the oxygen transport phenomenon along this direction is given in reference 10.…”

“…The lack of water molecules inside the ionomer results in the water‐saturated and the water‐unsaturated channels of stacked layers of the water‐saturated and water‐unsaturated domains. The details of the channel formation in the cross‐sectional view of the ionomer film can be found in the graphic abstract of this work and Figure 1 of reference 10. The oxygen transport from the gas pores to the catalyst agglomerate via the Nafion ionomer can be described by phase‐separated parallel‐diffusion mode 23,24 with two distinct sets of oxygen solubilities and diffusivities inside the ionomer layer.…”

“…Our group has developed several approaches to directly modify the cathode CL's structure. One method is to mix hydrophobic PTFE in the catalyst ink, of which details, advantages, and disadvantages have been discussed in the previous publications 9,10 . In this work, we would like to discuss another approach, the direct modification of Nafion ionomer’ structure 11,12 .…”

Vanadium Electrolyte Densification and Gel Formation Processes

“…12 Our previous research identified a type of pseudostable, precipitated VOSO 4 (V 4+ ) solid with a more amorphous, low-crystalline, gel-like structure. 14 This gel-like solid exhibits a high dissolution rate. Since it is easier to reconvert the solid electrolyte into its liquid form when sulfate anions remain dissociated from vanadium cations, it is necessary to maintain additional water molecules in the secondary solvation shell.…”