Thermodynamics, Charge Transfer and Practical Considerations of Solid Boosters in Redox Flow Batteries

Abstract

Solid boosters are an emerging concept for improving the performance and especially the energy storage density of the redox flow batteries, but thermodynamical and practical considerations of these systems are missing, scarce or scattered in the literature. In this paper we will formulate how these systems work from the point of view of thermodynamics. We describe possible pathways for charge transfer, estimate the overpotentials required for these reactions in realistic conditions, and illustrate the range of energy storage densities achievable considering different redox electrolyte concentrations, solid volume fractions and solid charge storage densities. Approximately 80% of charge storage capacity of the solid can be accessed if redox electrolyte and redox solid have matching redox potentials. 100 times higher active areas are required from the solid boosters in the tank to reach overpotentials of <10 mV.

High Voltage Redox-Meditated Flow Batteries with Prussian Blue Solid Booster

“…Still, 110 mAh g −1 capacity only amounts to 68% utilization of the booster, and thus it seems that a redox mediator with more potent oxidizing abilities or a higher cut-off potential is called for to fully utilize rhombohedral Prussian blue. That being said, the potential matching between mediator and booster is complex and depends on concentration and state of charge for both mediator and booster [22].…”

“…Choosing redox mediators often means balancing energy efficiency vs. solid booster utilization. While a single potential redox mediator perfectly matched to a flat voltage plateau solid booster could reach as high as 80% utilization combined with good energy efficiency [22], it would not be optimal for this system. The sodium iodide allows for high booster utilization and is expected to handle the potential difference between high and low spin iron in rhombohedral Prussian blue, since the use of I 2 as an oxidizer and NaI as a reducing agent is equivalent to using two different redox mediators.…”

Redox Flow Batteries: Materials, Design and Prospects

“…The major drawbacks of this approach is that since there are two kinetic processes involved (in cell and in tanks), the device electrochemical performance tends to decrease (coulombic and voltage efficiency, current/power density), despite there still being some divergence about this topic [53,277]. Additionally, this double kinetics system leads to a dependence between power and capacity [272,278] and the screening process of choosing mediator and active species gets even harder when compared to other electrolytes [272].…”

“…Furthermore, the charge storage capacity of the solid particles can be restrained in the tanks and ca. 80% of this capacity can be reached depending on the compatibility established between the redox electrolyte and redox solid [278]. Furthermore, it is also possible to combine the slurry redox flow battery with other configurations, e.g., slurry-air [279].…”

Synergistic Catalysis of SnO2/Reduced Graphene Oxide for VO2+/VO2+ and V2+/V3+ Redox Reactions

“…However, this type of energy is not always available at all times, and in most cases, due to the oscillation of energy availability in its cycle and peak demand, supply and demand are not synchronized [ 10 , 11 , 12 , 13 , 14 , 15 ]. Energy storage equipment is needed to store and transform the electric energy generated by renewable energy sources to realize its large-scale application [ 16 , 17 , 18 , 19 , 20 ]. Among many energy storage devices, a vanadium redox flow battery (VRFB) has attracted much attention because of its unique performance [ 21 , 22 , 23 ].…”