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Go with the Flow: Redox Material Investigated for Non-Aqueous Flow Batteries

Research team also develops promising spectroscopy approach to address battery chemistry and design challenges

December 2017
Go with the Flow: Redox Material Investigated for Non-Aqueous Flow Batteries

The PTIO flow cell shows promise for overcoming challenges in today’s non-aqueous flow batteries.

Redox flow batteries are extremely promising solutions for energy storage, providing an attractive, moderate-cost option for improving power grid reliability and integrating low-cost renewable energy technologies.

Of special interest to the energy storage community are non-aqueous flow batteries. Due to their broad voltage window, these batteries have the potential to achieve high energy density—in other words, store a lot of energy. But a handful of hurdles, including materials, battery chemistry design, and architecture, have prevented non-aqueous batteries from reaching their full potential.

Another concern is battery status monitoring. Issues such as overcharging and degrading material can damage battery performance as well as lead to battery failure, serious safety challenges, and loss of investment. State of charge—or SOC—indicates the depth of flow battery charge or discharge, and timely monitoring of SOC can help detect potential risks before they reach threatening levels. However, SOC has not yet been fully investigated for non-aqueous flow cell batteries.

Leading the Charge for Improved Material

PNNL is addressing these battery design and chemistry challenges by developing a novel non-aqueous flow battery design based on a new redox material, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. Otherwise known as PTIO, this material exhibits an ambipolar electrochemical property—meaning it can serve as both anolyte (negative electrolyte) and catholyte (positive electrolyte) redox materials. This symmetry within the electrochemical property is found to solve challenges associated with crossover-related material degradation towards a durable, reliable energy storage system.

A “Spectra-tacular” Protocol Addressing the SOC Challenge

Fourier-transform infrared spectroscopy is a long-standing technique that allows researchers to “see”—via spectra—the absorption or emission of solids, liquids, or gasses. It also enables investigation of changes in molecules during flow battery operation. The technique is easily accessible, quick, low cost, and requires little space.

The research team developed a protocol for using this non-invasive technique, which accurately detected SOC properties within the PTIO flow battery. As part of the protocol, spectra from Fourier-transform infrared spectroscopy is paired with measurements from electron spin resonance spectroscopy—which detects and characterizes chemical systems with one or more unpaired electrons—to cross-validate, or “double-check,” SOC measurements. The protocol may lead to additional advances for maintaining safety and reliability during long-term flow battery operations.

The protocol is outlined in the paper and video, “A Protocol for Electrochemical Evaluations and State of Charge Diagnostics off a Symmetric Organic Redox Flow Battery,” published in the Journal of Visualized Experiments.

This research was funded by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the DOE Office of Science's Basic Energy Science program.

PNNL Research Team: Wentao Duan, Rama S. Vemuri, Dehong Hu, Zheng Yang, and Xiaoliang Wei

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