Concentration-dependent Cycling of Phenothiazine-based Electrolytes in Nonaqueous Redox Flow Cells

Increasing redox-active species concentrations can improve viability for organic redox flow batteries by enabling higher energy densities, but the required concentrated solutions can become viscous and less conductive, leading to inefficient electrochemical cycling and low material utilization at higher current densities. To better understand these tradeoffs in a model system, we study a highly soluble and stable redox-active couple, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT), and its bis(trifluoromethanesulfonyl)imide radical cation salt (MEEPT-TFSI). We measure the physicochemical properties of electrolytes containing 0.2-1 M active species and connect these to symmetric cell cycling behavior, achieving robust cycling performance. Specifically, for a 1 M electrolyte concentration, we demonstrate 94% materials utilization, 89% capacity retention, and 99.8% average coulombic efficiency over 435 h (100 full cycles). This demonstration helps to establish potential for high-performing, concentrated nonaqueous electrolytes and highlights possible failure modes in such systems.

Automated summary

Increasing concentrations of redox-active species in organic redox flow batteries can improve viability but can also lead to inefficient electrochemical cycling and low material utilization at higher current densities. We studied a highly soluble and stable redox-active couple, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT), and its bis(trifluoromethanesulfonyl)imide radical cation salt (MEEPT-TFSI), and demonstrated robust cycling performance with 1 M electrolyte concentration, achieving 94% materials utilization, 89% capacity retention, and 99.8% average coulombic efficiency over 435 h (100 full cycles). This study establishes the potential for high-performing, concentrated nonaqueous electrolytes while highlighting possible failure modes in such systems.