Neutron Radiography As a Powerful Method to Visualize Reactive Flows in Redox Flow Batteries
Authors: Jacquemond Rémy Richard , van der Heijden Maxime , Boz Emre Burak , Kowalski Jeffrey A , Greco Katharine , Nijmeijer Kitty , Brushett Fikile R. , Boillat Pierre , Forner-Cuenca Antoni 
Published: July 2022

Spatial and temporal gradients in reactant concentration, influenced by local microstructure and surface properties, govern the performance and durability of various advanced electrochemical systems. The cell and stack performance is typically assessed using traditional electrochemical diagnostics (e.g. polarization curves, electrochemical impedance spectroscopy) and the influence of materials is macroscopically evaluated based on empirical comparison of novel materials with the current state-of-the-art. While this is a valid approach to identify promising candidates, valuable information is lost due to the difficulty of identifying performance-limiting factors. Operando imaging of electrochemical systems, in tandem with complementary electrochemical diagnostics, has been instrumental in the development of advanced polymer electrolyte fuel cells1,2 and, more recently, lithium-ion batteries3,4. Over the past few years, several groups have developed novel imaging and spectroscopic techniques for operando characterization of redox flow batteries, which is the focus of this work. Wong et al. employed fluorescence microscopy and particle velocimetry to image concentration and velocity distributions near the electrode-flow field interface5. Tanaka et al. visualized flow distribution in redox flow batteries with infrared thermography6. Zhao et al. employed in-situ nuclear magnetic resonance to track reaction mechanisms occurring within the electrolyte7. Finally, several groups recently employed X-ray tomographic microscopy to visualize gas pockets within the liquid electrolyte imbibed porous electrode8–10. While these methods have provided important insights, an approach that enables quantitative mapping of species concentrations, in a non-invasive fashion and within an operating cell, has remained elusive.
In this presentation, I will discuss our recent efforts to develop neutron radiography as an operando characterization method for non-aqueous redox flow batteries. We leverage the high attenuation of organic materials (i.e., high hydrogen content) in solution and, combined with isotopic labelling, we perform subtractive neutron imaging to quantify the concentration of active species and supporting electrolytes. 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 non-aqueous electrolyte containing a TEMPO/TEMPO+ redox couple and study the influence of electrode microstructure, membrane type (e.g. porous or dense), and flow field design. To resolve the concentration profiles across the different layers, we employ the in-plane imaging configuration11 and correlate these concentration profiles to cell performance via polarization measurements under different operating conditions. In the final part of the talk, I will discuss our latest experimental campaign in which we investigated the use of energy-selective neutron radiography to deconvolute concentrations of active species and supporting electrodes during operation.

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