Single vs Dual Shuttle Cycling of Polyferrocenyl Cathodes for Redox Targeting Flow Batteries

Abstract

Redox-targeting flow batteries (RTFBs) merge solution and solid-state redox chemistries to create scalable storage systems with the potential for high energy densities. This study systematically evaluates ferrocenyl derivatives as shuttles that charge/discharge an insoluble ferrocenyl polymer. The polymer utilization, voltaic efficiency, and discharge voltage of cells cycled by each shuttle alone (single system) or by combinations of shuttles (dual system) are directly compared. Here, the performance of a dual system exceeded that of any other system tested, and stable cycling was achieved with added polymer that quintupled the system’s capacity. Most importantly, experimental and computational mechanistic studies provide insights into the interplay between the shuttle’s redox potential (thermodynamic parameter) and the shuttle’s rate of redox targeting reaction (kinetic parameter) with the solid. These data reveal that the rate of redox targeting dictates a system’s performance and depth of charge once the potentials of shuttles and solids are comparable.

Organic Redox Targeting Flow Battery Utilizing a Hydrophilic Polymer and Its In‐Operando Characterization via State‐of‐Charge Monitoring of The Redox Mediator

“…Finally, Figure 7d displays the redox targetivity calculated by Equation (10) which further supports the previous interpretation. To recapitulate, the displayed percentages represent the percentage of the total applied current that is effectively charging/discharging the RT.…”

“…These findings are in line with the study about the important role of kinetic effects in redox targeting reactions reported by Sevov and co-workers recently. [10] The presented in-operando measurements of the RT's stateof-charge and the redox targetivity demonstrate the benefits and capabilities of this new measurement technique and the proposed figure-of-merit, respectively. Further methodological investigations are required, for example, to increase the accuracy of the method, to understand the impact of the various operational and physicochemical parameters of the flow battery, and to understand its yet unknown pitfalls.…”

“…The transfer of charge between the RT and RM per unit of time is -in general -a complex function of, for example, the equilibrium potential difference between RM and RT, [9,15] the redox targeting's reaction kinetics, [10,15] the mass transfer of the RM to the RT surface, and the inherent charge propagation within the RT. It is, thus, an implicit function of the charging current applied at the flow cell, the pumping speed, and electrolyte parameters like viscosity and temperature.…”

“…Currently, this approach is getting increasing attention by other researchers in the field. [10,11] In our former approach, N,N,N,2,2,6,6-heptamethylpiperidin-1-oxyl chloride (TMATEMPO) was utilized as RM and poly(TEMPO-methacrylate) (PTMA) as RT. However, PTMA can be considered as rather hydrophobic.…”

“…This approach excels in its design simplicity as well as its potential cost‐efficiency, reduces unwanted chemical interactions between the RT and RM, and naturally provides potential‐matched mediators and targets. Currently, this approach is getting increasing attention by other researchers in the field [10,11] …”