Redox flow batteries (RFBs) are promising rechargeable electrochemical devices for grid-scale energy storage, but further cost reductions are needed for ubiquitous adoption of this technology [1]. While recent research trends have focused on molecular discovery [2], advances in the science and engineering of electrochemical reactors can lead to cost reductions via increased power density and/or decreased component cost. Key to such efforts is deconvolution of coupled transport and kinetic processes within operating device and, to this end, imaging diagnostics, such as X-ray tomographic microscopy [3], infrared thermography [4], and electrochemiluminescence imaging [5], have been employed to visualize relevant properties (e.g., fluid distribution, temperature) in RFBs. However, obtaining quantitative property descriptions at high resolution and under representative operation conditions remains a challenge.

In this presentation, we introduce neutron radiography as an operando characterization method for RFBs which is particularly sensitivity to organic materials with high hydrogen content. To demonstrate the potential of this diagnostic tool, we characterize active species concentration distribution within a redox flow cell in a single electrolyte configuration with a nonaqueous electrolyte containing TEMPO/TEMPO⁺. We use subtractive neutron imaging [6] in combination with isotropic labelling to deconvolute the concentration of different electrolyte materials (i.e. supporting salts, active species). To resolve the concentration profiles across the different layers, we employ the in-plane imaging configuration [7] and correlate these profiles to cell performance via polarization and electrochemical impedance spectroscopy.

References

1. Darling et al., Energy & Environmental Science, 7, 3459 (2014).
2. A. Kowalski et al., Current Opinion in Chemical Engineering, 13, 45 (2016).
3. Tariq et al., Sustainable Energy & Fuels, 2, 2068 (2018).
4. Tanaka et al., Journal of Energy Storage, 19, 67 (2018).
5. Rubio-Garcia et al., Electrochemistry Communications, 93, 128 (2018).
6. Boillat et al., Journal of The Electrochemical Society, 162(6), F531 (2015).
7. Boillat et al., Electrochemistry Communications, 10(4), 546 (2008).

Acknowledgments

We gratefully acknowledge the financial support of the Swiss National Science Foundation (P2EZP2_172183) and the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy.