Metal Complexes With Redox-Active Ligands As High Energy Density Nonaqueous Redox Flow Battery Electrolytes

Metal Complexes with Redox-active Ligands as High Energy Density Nonaqueous Redox Flow Battery Electrolytes

by Patrick J. Cappillino, Harry D. Pratt III, Nicholas S. Hudak, Neil C. Tomson, Travis M. Anderson, Mitchell R. Anstey

Renewable energy sources that do not contribute to accumulation of greenhouse gases, such as wind and solar, must become an increasing portion of the global energy portfolio.  While abundant, these sources are intermittent due to factors such as day-night cycles and weather.  Furthermore, their generation capabilities are poorly matched to peak and off-peak use of electricity. Grid-scale strategies to mitigate these challenges, such as load-shifting, require energy storage components to supplement our current infrastructure in which electricity is instantaneously generated and distributed.

Redox flow batteries (RFB) have attractive features for grid-scale energy storage, including separation of energy and power capacities and modular scalability.  Over the last several decades, a number of technologies based on vanadium and iron-chromium electrolytes were developed but have so far met limited commercial success. Further advancement requires fundamentally different redox chemistry with stable redox couples, large potential differences, and fast redox kinetics.

Herein we report new, non-aqueous RFB electrolytes that meet these criteria based on metal complexes containing “redox non-innocent” ligands (M-NIL). The abundance of well-characterized, reversible charge transfer events exhibited by these compounds has the potential to greatly improve their energy density over state-of-the-art, transition-metal based RFB electrolytes.  Details on their preparation, characterization and electrochemistry will be presented and their performance as RFB electrolytes will be discussed.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.