Iron(tris pyridyl-imine) Complexes As Redox Couples for Non-Aqueous Redox Flow Battery Applications

Abstract

Grid-scale energy storage technologies are much needed for efficient integration of intermittent energy sources, such as wind and solar, into the electrical grid. Redox flow batteries (RFBs) have decoupled power and stored energy qualities, in contrast to traditional batteries, storing energy chemically in scalable solutions, and thus have the potential to solve the large-scale energy storage issue. Conventional redox flow chemistries are aqueous-based, such as Iron-chrome and all vanadium, where operating cell voltages are limited by water splitting reactions. Therefore, non-aqueous RFBs, which can operate at much higher voltages, are gaining attention and are typically only limited by the potential window of the non-aqueous solvent. Despite having enlarged potential windows that could provide higher energy density, most of the non-aqueous RFBs still operate at very low concentration, current densities and suffer from poor cycle life. In addition, the high cost of metals, such as vanadium, ruthenium, cobalt, etc., are an obstacle to achieving the DOE’s long term target of 150 USD per kWh for RFBs. In this presentation, therefore, we will discuss the synthesis and characterization of iron pyridine-imine complexes for redox couples. The performance of these redox couples in a bulk electrolysis cell and flow battery set-up will also be discussed. A mechanistic study to understand the fundamental degradation mechanisms of the redox couples under continuous charge/discharge operations will also be discussed. Our results indicate that iron complexes possess good reversibility and have the potential to compete, and potentially replace, other redox systems used in non-aqueous RFBs. Acknowledgement This work is supported by the Laboratory Directed Research & Development Program at Los Alamos National Laboratory.