Investigating Transport through Membranes in Non-Aqueous Redox Flow Batteries with Neutron Radiography
Authors: Jacquemond Remy Richard , van der Heijden Maxime , Boz Emre Burak , Carreon-Ruiz E. Ricardo , Muñoz Perales Vanesa , Nijmeijer Kitty , Boillat Pierre , Forner-Cuenca Antoni 
Published: September 2023

Electrochemical technologies offer a strong platform to decarbonize and electrify our energy economy and thus are poised to play an increasingly important role. Nevertheless, all electrochemical technologies suffer from thermodynamic, kinetic, and other operational losses that can drastically limit the performance of the device. Current macroscopic characterization techniques used to assess those losses (i.e., polarization, electrochemical impedance spectroscopy) fail to assess local inefficiencies challenging complete understanding of the system being investigated. Operando imaging of electrochemical systems, in tandem with electrochemical diagnostics offers a viable option to resolve local information and correlate it with macroscopic electrochemical performance. This dual characterization strategy has been successfully applied for the development of other electrochemical technologies (i.e., polymer electrolyte fuel cells[1,2], lithium-ion batteries[3,4]). In the past years, several groups developed and investigated imaging and spectroscopic techniques for operando characterization of redox flow batteries (RFBs) such as fluorescence microscopy[5], nuclear magnetic resonance spectroscopy[6,7] and infrared thermography[8]. Only recently, Clement et al. reported for the first time the use of neutron radiography to perform through-plane imaging of the gas evolution during operation of an all-vanadium RFB[9]. However, an approach enabling non-invasive operando concentration mapping of the species of interest within the reactor area has remained elusive.

In this work, we develop two novel neutron radiography operando methodologies to quantify concentration distributions in RFBs. We first perform cuvette experiments to calibrate the neutron attenuation response of the redox active materials (i.e., TEMPO and TEMPO+) and supporting salts (i.e., BF4
-) used in this study. We find a linear response between the neutron absorption and the concentrations of the species of interest in the electrolyte in the studied range (0.0 to 0.5 M). Next, we performed experiments with redox flow cells under operation and use the ex-situ calibrations together with subtractive neutron imaging to quantify species concentration in the electrolyte. By employing an in-plane optical imaging setup[10], we can resolve species concentration across the porous electrodes (Figure 1). We observe strong variations in the concentration profiles within the reactor area upon changes in the operating conditions and link those differences to material properties (i.e., membrane electrostatic charge). Finally, I will discuss our latest results obtained using time-of-flight neutron imaging to deconvolute concentration maps for TEMPO/TEMPO+ and BF4
- species. Using this methodology, we study mass transport across a porous separator and a dense anion exchange membrane. We find that, while TEMPO/TEMPO+ and BF4
- show almost symmetrical concentration changes in the case of a porous separator, asymmetric transport occurs when a charge selective membrane is used where only BF4
- is allowed to crossover the anion exchange polymer. The developed methodology offers a non-invasive diagnostic tool to image concentration distributions in electrochemical cells and can be applied to support theoretical and experimental efforts aiming to understand the role of materials and reactor designs on complex mass transport phenomena.

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Figure 1