Advanced imaging techniques prospects, locally resolved diagnostics

Throughout my work I emplyed advanced imaging techniques, including X-ray tomographic microscopy (XTM) and neutron imaging. Locally resolved diagnostics enable a deeper understanding of local properties within the reactor, providing key information regarding the operando transport processes in the electrochemical cell. Advanced imaging is thus crucial in flow battery research and should be developed further. Possible research directions include:

(1) Further investigation of redox flow batteries with the neutron radiography approach. First and foremost, full cell performance should be probed with energy-selective neutron imaging. By engineering an anolyte and a catholyte with distinct neutron attenuations, the concentration distribution during operation in both half-cells can be resolved. Moreover, neutron radiography can be used to gain a deeper understanding of the reactive transport processes in the porous electrode, which is of special interest for novel electrode structures to investigate their potential, design flaws, and points of improvement. Furthermore, the electrode-flow field interplay could be studied further, including studies on the electrolyte transport from the flow field channel to the porous electrode, flow channel utilization, and novel flow channel configurations. In addition, neutron radiography could be used to investigate electrolyte crossover with neutron-attenuating active species to study the impact of distinct membrane types, cell configurations, and electrode structures. Jacquemond et al. [1] recently studied electrolyte crossover through different membranes utilizing symmetric cell configurations. Their research could be extended to full cell designs and more membrane types (various charge densities and thicknesses), and can be combined with diverse reactor configurations to study the magnitude and effect of electrolyte crossover on the cell performance. Finally, cell malfunction studies could be performed including the investigation of channel utilization and salt precipitation, but also on hydrogen evolution [2] and electrode wetting. Such studies are not only interesting from a fundamental point of view but additionally for industrial use to optimize the stack configuration and design, and operating conditions.

(2) Utilizing operando and ex-situ XTM for porous electrode characterization. Operando X-ray computed tomography can be used to study gas formation through the reactor with high spatial resolution [3–6]. Whereas ex-situ XTM can be used to investigate the electrode structure and structural integrity, for example upon compression [7]. Particularly the structural comparison of novel electrodes, e.g., 3D printed electrodes or NIPS electrodes, before and after extensive electrochemical cycling with XTM is expected to be powerful and can gain insight into the local mechanical stability of novel electrodes.

(3) Developing additional operando locally resolved imaging techniques. Promising approaches [8] include nuclear magnetic resonance to track the state-of-charge [9], magnetic resonance imaging to analyze the electrolyte and temperature distributions [10, 11], CCD camera studies with colorimetric analysis to probe electrochemical reactions [12], and fluorescence/confocal imaging to visualize electrolyte reactions and transport behavior [13]. The latter, confocal imaging, is currently being explored by us. Inspired by the work of Wong et al. [13] we aim to study mass and reactive transport in model grid structures by combining confocal microscopy with 3D printing. Using our transparent flow cell (Figure 1a) we elect to study the reduction reaction of anthraquinone-2,6-disulfonate (AQDS) to H2AQDS (Figure 1b) [14] by making use of the change in fluorescence signal upon AQDS reduction. Combined with model grid structures with varying pillar designs, orientations, and locations, and distinct flow field designs, the mass transport through the reactor area can be studied. Figure 1: (a) In-house designed confocal imaging cell, and (b) a test run providing the cycling ability of the chemistry of interest for the confocal measurements: 100% state-of-charge 0.1 M AQDS in 1 M KCl and 100% state-of-charge 0.3 M ferrocyanide in 1 M KCl (with argon purging of the tanks). The cycling performance of the full-cel was obtained with the regular RFB cell configuration with a 2.55 cm2 geometrical area, with two stacked thermally treated SGL 39AA electrodes, an interdigitated flow field, a Nafion 212 membrane, and evaluated at 5 cm s-1 and 100 mA applied current.

References:
[1] R. R. Jacquemond, PhD thesis, Eindhoven University of Technology (2023).
[2] J. T. Clement, PhD thesis, University of Tennessee (2016).
[3] R. Jervis et al., J. Phys. D. Appl. Phys. 49, 434002 (2016).
[4] F. Tariq et al., Sustain. Energy Fuels. 2, 2068–2080 (2018).
[5] K. Köble et al., J. Power Sources. 492, 229660 (2021).
[6] L. Eifert et al., ChemSusChem. 13, 3154–3165 (2020).
[7] K. M. Tenny et al., Energy Technol. 10, 2101162 (2022).
[8] Y. A. Gandomi et al., J. Electrochem. Soc. 165, A970–A1010 (2018).
[9] E. W. Zhao et al., Nature. 579, 224–228 (2020).
[10] Z. Dunbar, R. I. Masel, J. Power Sources. 171, 678–687 (2007).
[11] I. E. Gunathilaka, J. M. Pringle, L. A. O’Dell, Nat. Commun. 12, 1–9 (2021).
[12] H. Park et al., Proc. Natl. Acad. Sci. U. S. A. 119, 1–9 (2022).
[13] A. A. Wong, S. M. Rubinstein, M. J. Aziz, Cell Reports Phys. Sci. 2, 100388 (2021).
[14] W. Lee, A. Permatasari, Y. Kwon, J. Mater. Chem. C. 8, 5727–5731 (2020).