Locally resolved diagnostics

In addition to cell-averaged characterization methods, locally resolved diagnostics provide information about the local properties within the cell, otherwise impossible to obtain or visualize by in-situ or ex-situ cell-averaged measurements. Properties ranging from local velocity and concentration profiles through the cell, to extracting the porous electrode three-dimensional structure, provide relevant information regarding transport processes within the electrochemical cell.

Several imaging techniques have been used to reconstruct digital 2D and 3D microstructures of RFB components at high spatial resolutions [1]. For example, X-ray tomographic microscopy (XTM) is frequently used in the battery community for obtaining the three-dimensional structure of porous electrodes (Figure 1) [2], both ex-situ [3] and operando [4] characterization of structural changes occurring in electrodes during operation, operando visualization of gas formation in the electrode [5–8], and investigating the electrolyte-gas diffusion layer interface [9, 10]. Additionally, XTM can be combined with microstructure-informed models to model transport processes within the electrochemical cell at the cell- or pore-level [11]. XTM can be used to obtain quantitative morphological information in the micrometer range; however, high X-ray doses are generally needed to obtain a high spatial and temporal resolution, resulting in a trade-off between high resolution and exposure time [12, 13].

Besides these microstructure-resolved imaging diagnostics, other techniques can be used to gain insight into the electrolyte distribution through the cell. Two operando examples are fluorescence microscopy and neutron imaging (Figure 1). Fluorescence imaging is a non-invasive imaging technique that can visualize velocity and concentration profiles within a transparent cell design with a high spatial resolution. Some examples include visualization of reactant distribution and mass transport in various flow field designs by injecting luminol solutions [14], and visualization of electrochemical reactions and heterogeneous transport of redox-active species in porous electrodes leveraging the natural fluorescence of quinones (Figure 1) [15]. Nonetheless, fluorescent species are required for operation, the technique is restricted to a limited penetration depth through porous electrodes, and it requires cell modifications. Alternatively, neutron imaging is a non-invasive imaging technique that has been deployed to visualize multiphase flows in polymer electrolyte fuel cells [16], bubble generation in vanadium RFBs [17], and most recently, to map concentration distributions in non-aqueous flow cells (Figure 1) [18]. Neutron imaging is a well-suited technique for steady-state imaging (relatively low temporal resolution) of electrochemical cells as it is non-invasive and minimal cell modifications are required [16]. The reason is that neutrons, as opposed to X-rays, feature high penetration depths even through high atomic weight elements which are used as engineering housing materials (e.g., aluminum, carbon, steel), and greatly attenuate with lighter elements such as hydrogen, lithium, and boron which are contained in redox active molecules, making neutron imaging an attractive method with unique properties. All things considered; operando imaging of electrochemical systems combined with complementary electrochemical diagnostics is particularly powerful to obtain microstructure-informed information through the battery.

Figure 1: Locally resolved diagnostic techniques (X-ray tomographic microscopy, fluorescence imaging, and neutron radiography) with their measured property, time resolution, spatial resolution, and required cell modifications. The fluorescence microscopy image was adapted from [104].

References:
[1] Z. Niu et al., Energy Environ. Sci. 14, 2549–2576 (2021).
[2] C. T. Wan et al., Adv. Mater. 33, 2006716 (2021).
[3] P. Trogadas et al., Electrochem. commun. 48, 155–159 (2014).
[4] X. Lu et al., Nat. Commun. 11, 1–13 (2020).
[5] R. Jervis et al., J. Phys. D. Appl. Phys. 49, 434002 (2016).
[6] F. Tariq et al., Sustain. Energy Fuels. 2, 2068–2080 (2018).
[7] K. Köble et al., J. Power Sources. 492, 229660 (2021).
[8] L. Eifert et al., ChemSusChem. 13, 3154–3165 (2020).
[9] J. Eller et al., J. Electrochem. Soc. 158, B963 (2011).
[10] A. Mularczyk et al., J. Electrochem. Soc. 167, 084506 (2020).
[11] A. G. Lombardo, B. A. Simon, O. Taiwo, S. J. Neethling, N. P. Brandon, J. Energy Storage. 24, 100736 (2019).
[12] J. Roth, J. Eller, F. N. Büchi, J. Electrochem. Soc. 159, F449–F455 (2012).
[13] Y. A. Gandomi et al., J. Electrochem. Soc. 165, A970–A1010 (2018).
[14] J. Rubio-Garcia, A. Kucernak, A. Charleson, Electrochem. commun. 93, 128–132 (2018).
[15] A. A. Wong, S. M. Rubinstein, M. J. Aziz, Cell Reports Phys. Sci. 2, 100388 (2021).
[16] P. Boillat, E. H. Lehmann, P. Trtik, M. Cochet, Curr. Opin. Electrochem. 5, 3–10 (2017).
[17] J. T. Clement, PhD thesis, University of Tennessee (2016).
[18] A. Forner-Cuenca, K. V. Greco, J. A. Kowalski, P. Boillat, F. R. Brushett, in ECS Meeting Abstracts (The Electrochemical Society, 2019).