Operando Imaging Research Articles

Nanoscale Operando Imaging of Electrically Driven Charge-Density Wave Phase Transitions.

Structural transformations in strongly correlated materials promise efficient and fast control of materials' properties via electrical or optical stimulation. The desired functionality of devices operating based on phase transitions, however, will also be influenced by nanoscale heterogeneity. Experimentally characterizing the relationship between microstructure and phase switching remains challenging, as nanometer resolution and high sensitivity to subtle structural modifications are required. Here, we demonstrate nanoimaging of a current-induced phase transformation in the charge-density wave (CDW) material 1T-TaS2. Combining electrical characterizations with tailored contrast enhancement, we correlate macroscopic resistance changes with the nanoscale nucleation and growth of CDW phase domains. In particular, we locally determine the transformation barrier in the presence of dislocations and strain, underlining their non-negligible impact on future functional devices. Thereby, our results demonstrate the merit of tailored contrast enhancement and beam shaping for advanced operando microscopy of quantum materials and devices.

Probing Multiscale Dynamics of Energy-Dense Batteries by Operando Imaging

Probing Multiscale Dynamics of Energy-Dense Batteries by Operando Imaging

Operando imaging in electrocatalysis: insights into microstructural materials design.

Electrocatalysis plays a pivotal role in renewable energy conversion and associated chemical production, enabling a variety of emerging sustainability technologies with societal impacts. Achieving marked improvement in electrocatalytic performance relies on a deep understanding of catalyst microstructures and catalytic mechanisms, with a particular emphasis on the detailed, spatiotemporally resolved characterizations of the underlying fundamental electrocatalytic processes. This fundamental need drives the development of operando imaging techniques, which improve the ability to detect dynamic structural changes in electrocatalysts and establish clear structure-performance relationships for morphologically complex, hierarchically structured catalytic materials. This review aims to highlight significant advancements in the application of operando imaging techniques to develop a deeper understanding of important heterogeneous electrocatalytic reactions critical for emerging sustainability technologies. We summarize the up-to-date key mechanistic insights regarding these reactions achieved through a range of operando imaging techniques, including electron microscopies, X-ray imaging techniques, scanning probe microscopies, and optical microscopies. We conclude by pointing out emerging directions and future prospects within the field of operando imaging in electrocatalysis.

Healing of keyhole porosity by means of defocused laser beam remelting: Operando observation by X-ray imaging and acoustic emission-based detection

One of the remaining challenges in Laser Powder Bed Fusion (LPBF) of metals is the control of the formation of keyhole pores, resulting from a local excessive energy input during processing. Such defects can lead to degraded mechanical properties and are typically detected and/or removed after the process through non-destructive quality-inspection procedures and porosity-removal treatments. Monitoring and controlling the formation of defects during the LPBF process can allow circumventing such time-consuming and costly post-process stages. This paper develops a new approach to perform in-situ healing of deep keyhole pores, using a positively defocused laser beam with finely tuned laser remelting process parameters. Synchrotron radiographic images of the process zone are acquired during laser remelting. The use of operando imaging enables the visualization of pore removal during processing, and unveils the effect of various remelting conditions on the healing efficiency. The acoustic signals generated during laser remelting are recorded using a high-sensitivity optical microphone, and analyzed in parallel with the X-ray images, allowing the acoustic signature of defect healing to be identified. The present paper demonstrates for the first time that an airborne acoustic sensor can be used to monitor the healing of keyhole pores during LPBF.

Understanding Photo(electro)catalysts for Energy Conversion via Operando Functional Imaging.

Understanding the fundamental processes of charge carrier generation, separation, transport, and reactivity on the surface of semiconductors is crucial for the rational design of high-performance photo(electro)catalysts for various energy conversion applications. Nevertheless, the ubiquitous and intricate heterogeneity exhibited by individual catalysts poses a significant obstacle to achieving a comprehensive understanding of the structure-function relationships using bulk-level, ensemble-averaged characterizations. This review highlights the emerging operando imaging techniques capable of providing local functional information, such as surface photovoltage, photoluminescence, charge carrier reaction rate, and photocurrent, at the single particle to subparticle level. By establishing correlations between the acquired local functional information and the specific structural characteristics of the catalyst, a quantitative, holistic understanding of the structure-function relationship in photo(electro)catalysts can be achieved. This understanding can serve as a foundation for guiding the design of photocatalysts with enhanced energy conversion performance.

Investigating Transport through Membranes in Non-Aqueous Redox Flow Batteries with Neutron Radiography

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.[1] P. Boillat, E. H. Lehmann, P. Trtik, M. Cochet, Current Opinion in Electrochemistry 2017, 5, 3.[2] J. Eller, T. Rosén, F. Marone, M. Stampanoni, A. Wokaun, F. N. Büchi, J. Electrochem. Soc. 2011, 158, B963.[3] B. Michalak, H. Sommer, D. Mannes, A. Kaestner, T. Brezesinski, J. Janek, Sci Rep 2015, 5, 15627.[4] D. P. Finegan, M. Scheel, J. B. Robinson, B. Tjaden, I. Hunt, T. J. Mason, J. Millichamp, M. Di Michiel, G. J. Offer, G. Hinds, D. J. L. Brett, P. R. Shearing, Nat Commun 2015, 6, 6924.[5] A. A. Wong, M. J. Aziz, S. Rubinstein, ECS Trans. 2017, 77, 153.[6] E. W. Zhao, T. Liu, E. Jónsson, J. Lee, I. Temprano, R. B. Jethwa, A. Wang, H. Smith, J. Carretero-González, Q. Song, C. P. Grey, Nature 2020, 579, 224.[7] E. W. Zhao, E. Jónsson, R. B. Jethwa, D. Hey, D. Lyu, A. Brookfield, P. A. A. Klusener, D. Collison, C. P. Grey, J. Am. Chem. Soc. 2021, 143, 1885.[8] H. Tanaka, Y. Miyafuji, J. Fukushima, T. Tayama, T. Sugita, M. Takezawa, T. Muta, Journal of Energy Storage 2018, 19, 67.[9] J. T. Clement, Investigation of Localized Performance and Gas Evolution in All-Vanadium Redox Flow Batteries via in-Situ Distributed Diagnostic Techniques, Ph.D. thesis, University of Tennessee, 2016.[10] P. Boillat, G. Frei, E. H. Lehmann, G. G. Scherer, A. Wokaun, Electrochem. Solid-State Lett. 2010, 13, B25. Figure 1

Operando Imaging of Over-Discharge-Induced Surface Morphology Evolutions of LiMn2O4 Submicron-Sized Particles by Electrochemical High-Speed Atomic Force Microscopy.

Spinel LiMn2O4 is a promising cathode material but suffers from severe capacity fading during battery operation. One of capacity fade mechanisms results from changes in its morphology and structure due to over-discharge. In this work, for the first time, we successfully tracked the morphologic evolution of LiMn2O4 submicron-sized particles during over-discharging by our home-made electrochemical high-speed atomic force microscopy (EC-HS-AFM). Seven hundred and sixty successive EC-HS-AFM images were stably captured at an imaging speed of ∼0.85 fps at corresponding potentials during over-discharging in ∼15 min, from which evolutions of nanoscale wrinkle-like and step-like structures on the particle surface were clearly observed. The phenomena could be resulted from the complex stresses due to structural distortion during the phase transformation from cubic (LiMn2O4) to tetragonal (Li2Mn2O4), and the formation of the Li2Mn2O4 phase was confirmed by ex situ XRD. Moreover, the particle surface area as a function of the potential was quantitatively extracted from the EC-HS-AFM images, revealing the irreversible expansion/contraction of the particles, and this finding obtained at the nanoscale was consistent with the macroscopic results tested by cyclic voltammetry and galvanostatic charge/discharge methods. These results demonstrate that the EC-HS-AFM is a powerful tool to establish the correlation between the over-discharge-induced surface morphology changes and irreversibility of the Li-ion insertion/extraction as well as capacity fading.

Operando spectral imaging of the lithium ion battery’s solid-electrolyte interphase

The lithium-ion battery is currently the preferred power source for applications ranging from smart phones to electric vehicles. Imaging the chemical reactions governing its function as they happen, with nanoscale spatial resolution and chemical specificity, is a long-standing open problem. Here, we demonstrate operando spectrum imaging of a Li-ion battery anode over multiple charge-discharge cycles using electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Using ultrathin Li-ion cells, we acquire reference EELS spectra for the various constituents of the solid-electrolyte interphase (SEI) layer and then apply these “chemical fingerprints” to high-resolution, real-space mapping of the corresponding physical structures. We observe the growth of Li and LiH dendrites in the SEI and fingerprint the SEI itself. High spatial- and spectral-resolution operando imaging of the air-sensitive liquid chemistries of the Li-ion cell opens a direct route to understanding the complex, dynamic mechanisms that affect battery safety, capacity, and lifetime.

Magnesium Anode Protection by an Organic Artificial Solid Electrolyte Interphase for Magnesium-Sulfur Batteries.

In the search for post-lithium battery systems, magnesium-sulfur batteries have attracted research attention in recent years due to their high potential energy density, raw material abundance, and low cost. Despite significant progress, the system still lacks cycling stability mainly associated with the ongoing parasitic reduction of sulfur at the anode surface, resulting in the loss of active materials and passivating surface layer formation on the anode. In addition to sulfur retention approaches on the cathode side, the protection of the reductive anode surface by an artificial solid electrolyte interphase (SEI) represents a promising approach, which contrarily does not impede the sulfur cathode kinetics. In this study, an organic coating approach based on ionomers and polymers is pursued to combine the desired properties of mechanical flexibility and high ionic conductivity while enabling a facile and energy-efficient preparation. Despite exhibiting higher polarization overpotentials in Mg-Mg cells, the charge overpotential in Mg-S cells was decreased by the coated anodes with the initial Coulombic efficiency being significantly increased. Consequently, the discharge capacity after 300 cycles applying an Aquivion/PVDF-coated Mg anode was twice that of a pristine Mg anode, indicating effective polysulfide repulsion from the Mg surface by the artificial SEI. This was backed by operando imaging during long-term OCV revealing a non-colored separator, i.e. mitigated self-discharge. While SEM, AFM, IR and XPS were applied to gain further insights into the surface morphology and composition, scalable coating techniques were investigated in addition to ensure practical relevance. Remarkably therein, the Mg anode preparation and all surface coatings were prepared under ambient conditions, which facilitates future electrode and cell assembly. Overall, this study highlights the important role of Mg anode coatings to improve the electrochemical performance of magnesium-sulfur batteries.

Neutron imaging for magnetization inside an operating inductor

Magnetic components are key parts of energy conversion systems, such as electric generators, motors, power electric devices, and magnetic refrigerators. Toroidal inductors with magnetic ring cores can be found inside such electric devices that are used daily. For such inductors, magnetization vector M is believed to circulate with/without distribution inside magnetic cores as electric power was used in the late nineteenth century. Nevertheless, notably, the distribution of M has never been directly verified. Herein, we measured a map of polarized neutron transmission spectra for a ferrite ring core assembled on a familiar inductor device. The results showed that M circulates inside the ring core with a ferrimagnetic spin order when power is supplied to the coil. In other words, this method enables the multiscale operando imaging of magnetic states, allowing us to evaluate the novel architectures of high-performance energy conversion systems using magnetic components with complex magnetic states.

Progressive and instantaneous nature of lithium nucleation discovered by dynamic and operando imaging.

The understanding of lithium (Li) nucleation and growth is important to design better electrodes for high-performance batteries. However, the study of Li nucleation process is still limited because of the lack of imaging tools that can provide information of the entire dynamic process. We developed and used an operando reflection interference microscope (RIM) that enables real-time imaging and tracking the Li nucleation dynamics at a single nanoparticle level. This dynamic and operando imaging platform provides us with critical capabilities to continuously monitor and study the Li nucleation process. We find that the formation of initial Li nuclei is not at the exact same time point, and Li nucleation process shows the properties of both progressive and instantaneous nucleation. In addition, the RIM allows us to track the individual Li nucleus's growth and extract spatially resolved overpotential map. The nonuniform overpotential map indicates that the localized electrochemical environments substantially influence the Li nucleation.

Resolving mass transport losses at the catalyst layer of polymer electrolyte membrane fuel cells through semi-empirical modelling

In this study, the sources of oxygen transport resistance in a polymer electrolyte membrane (PEM) fuel cell are resolved using two different approaches: pore network modelling and empirical modelling. This is achieved by applying operando imaging and electrochemical performance testing results to the pore network model and empirical model, respectively. In the literature, the impact of liquid water on oxygen transport resistance in a 5-layer membrane electrode assembly is typically attributed to the accumulation of liquid water within the gas diffusion layer. A comparison of the oxygen transport resistance values predicted by pore network modelling to the values predicted by empirical modelling shows that empirical models significantly overpredict the contribution of the substrate within gas diffusion layers to oxygen transport resistance at limiting current conditions (up to 440%). Statistical analysis of the discrepancy (residual) between the values of the oxygen transport resistance of the substrate predicted by the empirical and pore network models indicates that the residual can be attributed to oxygen transport resistance in locations that are related to water production but independent of the relative humidity of the inlet: the catalyst layer (CL) and/or CL-microporous layer interface.

Cinematographic Recording of a Metastable Floating Island in Two- and Three-Dimensional Crystal Growth.

Many chemical reactions go through a cascade of events in which a series of metastable intermediates appear, and crystal nucleation is no exception. Although the consensus on the energetics of nucleation suggests the formation of metastable states preceding the crystal growth, little experimental evidence has been reported for their dynamics at an atomistic level. Operando imaging of two-dimensional nucleation on a defect-free NaCl nanocrystal in carbon nanotubes using a millisecond angstrom-resolution transmission electron microscope revealed the formation of a metastable "floating island" (FI) that migrates thermally on the (100) facet of NaCl as the first intermediate of epitaxy. The speed of the migration at 298 K is estimated to be larger than 0.3 nm ms-1. When a crystal tumbles in a container, a space repeatedly forms between the crystal and the container wall that hosts the FI. Tumbling changes the surface energy repeatedly and promotes the conversion of the FI into a new epitaxial layer. We anticipate that this surface catalysis mechanism found on the nanoscale also operates in bulk heterogeneous nucleation where agitation and attrition accelerate crystallization.

Visualizing 4D Pore-Scale Gas Transport in Operating PEM Electrolyzers Using X-Ray Computed Tomography

Mass transport losses in polymer electrolyte membrane (PEM) electrolyzers are predominantly caused by the accumulation of oxygen gas within the pores of the porous transport layer (PTL), hindering reactant water delivery to reaction sites (1). To enhance mass transport and optimize electrolyzer performance, a comprehensive understanding of the mechanisms driving oxygen gas transport in the PTL is required. Numerous studies have utilized various imaging techniques - including optical, neutron, and X-ray radiography to elucidate the evolution and transport of oxygen gas in the multiphase flow regime during PEM electrolyzer operation (2). However, these techniques only capture two-dimensional (2D) images, resolving average gas distributions rather than a holistic pore-scale quantification. Few studies have used three-dimensional (3D) imaging to resolve the dynamic microscale distributions of oxygen gas in the PTL. Understanding the impact of key parameters affecting 3D oxygen gas transport, such as PTL morphology, will allow for tailored pore-scale material optimization for enhanced gas transport and improved performance of PEM electrolyzers.This study presents a non-destructive operando imaging approach using X-ray computed tomography (CT), enabling the visualization of transient 3D oxygen gas evolution and transport in the PTL of a PEM electrolyzer. Using state-of-the-art synchrotron X-rays, high temporal and spatial resolutions were achieved to capture the pore-scale formation and growth of oxygen gas pathways within the PTL at various operating conditions. Electrochemical performance of an in-house designed electrolyzer cell was also characterized while acquiring tomographic images to investigate the impact of observed transport mechanisms on performance. The methodology presented in this work showcases the capability of using X-ray CT to visualize the complex interfacial multiphase flow in PTLs. Findings from this study will provide valuable insight towards pore-scale material optimization of clean energy porous materials aimed at curtailing the costs of green hydrogen production.

Characterizing Liquid Water Distribution in Polymer Electrolyte Fuel Cells Using Operando 2D and 3D X-Ray Imaging

The potential to reduce the cost of Polymer Electrolyte Fuel Cells (PEFCs) through high power density operation is limited by the attending water management challenges at such operating conditions [1]. Addressing PEFC water management requires detailed understanding of liquid water distribution characteristics of an operating fuel cell, to refine the design of membrane electrode assemblies (MEAs) for improved performance.Recent advances in X-ray operando imaging [2, 3] and custom hardware development [4], have enabled the visualization of liquid water in an operating PEFC, through the analysis of water phase segmented 3-dimensional (3D) images [2, 3] and 2-dimensional (2D) radiographs [5]. 2D image datasets may be acquired in a shorter time than 3D tomographic datasets but only provide grayscale information averaged through the thickness of the sample. 3D image datasets obtained from a reconstruction of many angular radiographs yield a 3D representation of the sample in addition to the grayscale information but take a long collection time. However, time dependent liquid water distribution characteristics such as channel water breakthrough, observable in the 2D image datasets, tend to be elusive to 3D imaging methods. Although efforts have been made to reduce the image acquisition time for 3D X-ray imaging by using Synchrotron sources and improved image processing techniques [2], these efforts tend to compromise image quality and are restricted to a brief temporal snapshot of liquid water distribution. Also, the brightness of the Synchrotron sources necessitates short exposure duration tests due to the likelihood of cell degradation caused by the sample’s exposure to radiation [4]. Given the unique capabilities of 2D and 3D imaging modes, we therefore focus on combining information accessible in both modes to further the understanding of PEFC liquid water distribution.In this work, an improved understanding of liquid water distribution in the PEFC is presented by integrating operando 2D and 3D image datasets acquired under the same cell operating conditions and using laboratory scale X-ray computed tomography (XCT) equipment which was interfaced with fuel cell testing and diagnostic tools. Analysis of 3D image datasets is extended beyond binary information to include the grayscale value (GSV) of liquid water to reveal a change in the through-plane values. These results, when combined with observed 2D liquid water breakthrough characteristics suggest that the change in GSV is indicative of the differences in the intermittency of liquid water presence in the different regions of the GDL; see Figure 1. These results therefore present a more complete interpretation of PEFC operando liquid water distribution, highlighting distinct characteristics from the microporous (MPL) region to the GDL-land interface. Unique findings and insight obtained using this methodology will be discussed. Acknowledgments Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Ballard Power Systems, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, and Canada Research Chairs. References Jiao and X. Li, Progress in energy and combustion Science, vol. 37, no. 3, pp. 221–291, 2011.Xu, S. Nagashima, H. P. Nguyen, K. Kishita, F. Marone, F. N. Büchi, and J. Eller, Journal of Power Sources, vol. 490, p. 229492, 2021.Nagai, J. Eller, T. Hatanaka, S. Yamaguchi, S. Kato, A. Kato, F. Marone, H. Xu, and F. N. Büchi, Journal of Power Sources, vol. 435, p. 226809, 2019.T. White, F. P. Orfino, M. El Hannach, O. Luo, M. Dutta, A. P. Young, and E. Kjeang, Journal of The Electrochemical Society, vol. 163, no. 13, pp. F1337–F1343, 2016.R. Banerjee, N. Ge, J. Lee, M. G. George, S. Chevalier, H. Liu, P. Shrestha, D. Muirhead, and A. Bazylak, Journal of The Electrochemical Society, vol. 164, no. 2, p. F154, 2017. Figure 1

Operando Optical Tracking of Single-Particle Ion Dynamics in Batteries

In order to advance lithium-ion battery technologies, particularly fast charging abilities, it is important to understand the non-equilibrium processes occurring in functioning materials under realistic conditions, in real time, and on the nano- to meso-scale. Currently, operando imaging of lithium-ion dynamics requires sophisticated synchrotron X-ray or electron microscopy methods. These techniques do not lend themselves to high-throughput material screening, and often fail to capture the behaviour of individual particles in batteries charging faster than 2C. In this work, we introduce a laboratory-based optical interferometric scattering microscope to resolve lithium-ion transport in battery materials at the single-particle level, and apply it to follow rapid cycling of the archetypical cathode material Li x CoO2. We directly visualise the insulator-metal, solid solution and lithium ordering phase transitions and determine rates of lithium insertion and removal from individual particles, identifying different mechanisms on charge vs. discharge. Finally, we capture the real-time formation of domain boundaries between different crystal orientations associated with the monoclinic lattice distortion at Li0.5CoO2. The high throughput nature of our methodology allows many particles to be sampled across the electrode and enables exploration of the role of defects, morphologies and cycling rate on battery degradation. The imaging concept is general and may be applied to study any active electrode material, and more broadly, any system where the transport of ions is associated with electronic or structural changes. Due to its straightforward lab-based implementation, we hope that this methodology will become an indispensable tool for high-throughput material discovery and mechanistic studies, to complement existing synchrotron-based methodologies. References Merryweather, A.J., Schnedermann, C., Jacquet, Q. et al. Operando optical tracking of single-particle ion dynamics in batteries. Nature 594, 522–528 (2021).

Neutron Radiography As a Powerful Method to Visualize Reactive Flows in Redox Flow Batteries

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. References 1 P. Boillat, E. H. Lehmann, P. Trtik and M. Cochet, Curr. Opin. Electrochem., , DOI:https://doi.org/10.1016/j.coelec.2017.07.012.2 J. Eller, T. Rosén, F. Marone, M. Stampanoni, A. Wokaun and F. N. Büchi, J. Electrochem. Soc., 2011, 158, B963.3 B. Michalak, H. Sommer, D. Mannes, A. Kaestner, T. Brezesinski and J. Janek, Sci. Rep., 2015, 5, 15627.4 D. P. Finegan, M. Scheel, J. B. Robinson, B. Tjaden, I. Hunt, T. J. Mason, J. Millichamp, M. Di Michiel, G. J. Offer, G. Hinds, D. J. L. Brett and P. R. Shearing, Nat. Commun., 2015, 6, 6924.5 A. A. Wong, M. J. Aziz and S. Rubinstein, ECS Trans. , 2017, 77, 153–161.6 H. Tanaka, Y. Miyafuji, J. Fukushima, T. Tayama, T. Sugita, M. Takezawa and T. Muta, J. Energy Storage, 2018, 19, 67–72.7 E. W. Zhao, T. Liu, E. Jónsson, J. Lee, I. Temprano, R. B. Jethwa, A. Wang, H. Smith, J. Carretero-González, Q. Song and C. P. Grey, Nature, 2020, 579, 224–228.8 R. Jervis, L. D. Brown, T. P. Neville, J. Millichamp, D. P. Finegan, T. M. M. Heenan, D. J. L. Brett and P. R. Shearing, J. Phys. D. Appl. Phys., , DOI:10.1088/0022-3727/49/43/434002.9 F. Tariq, J. Rubio-Garcia, V. Yufit, A. Bertei, B. K. Chakrabarti, A. Kucernak and N. Brandon, Sustain. Energy Fuels, 2018, 2, 2068–2080.10 K. Köble, L. Eifert, N. Bevilacqua, K. F. Fahy, A. Bazylak and R. Zeis, J. Power Sources, , DOI:10.1016/j.jpowsour.2021.229660.11 P. Boillat, D. Kramer, B. C. Seyfang, G. Frei, E. Lehmann, G. G. Scherer, A. Wokaun, Y. Ichikawa, Y. Tasaki and K. Shinohara, Electrochem. commun., 2008, 10, 546–550.

In-Situ Visualization of the Transition Metal Dissolution in Layered Cathodes

Abstract Transition metal dissolution in layered cathodes is one of the most intractable issues that deteriorate the battery performance and lifetime. It not only aggravates the structure degradation in cathode but also damages the solid electrolyte interphase in anode and even induces the formation of lithium dendrites. In this work, we investigate the dissolution behaviors of polycrystalline and single-crystalline layered cathode via operando X-ray imaging techniques. The cathode particle morphology appears to have a significant impact on the evolution of the dissolution dynamics. As a mitigation strategy, we reveal that doping with a trace amount of Zr in the layered cathode could improve its robustness against the transition metal dissolution. Our finding provides valuable insights for designing the next-generation highly stable layered battery cathodes.

Direct observation of crack formation mechanisms with operando Laser Powder Bed Fusion X-ray imaging

Laser powder bed fusion ( L -PBF) is a versatile additive manufacturing process that can print geometrically complex metal parts for a variety of applications. However, poor control of defect formation during processing hampers its widespread industrial adoption. Many materials suffer from a high crack susceptibility during L -PBF, which results in degraded mechanical properties, and is an obstacle to the certification of critical parts. In order to unveil the mechanisms of crack formation in a prone-to-cracking nickel-based superalloy, we employ high-speed synchrotron X-ray imaging in combination with a miniaturized L -PBF set-up that reproduces real processing conditions. This unique set-up provides operando imaging of crack formation during L -PBF. Complementary post-mortem inspection of crack morphology and thermal simulations supported by operando X-ray diffraction-based measurements of the temperature evolution allow to identify the cracking mechanism and to differentiate solidification cracking from liquation.

Operando Imaging of Ce Radical Scavengers in a Practical Polymer Electrolyte Fuel Cell by 3D Fluorescence CT-XAFS and Depth-Profiling Nano-XAFS-SEM/EDS Techniques.

There is little information on the spatial distribution, migration, and valence of Ce species doped as an efficient radical scavenger in a practical polymer electrolyte fuel cell (PEFC) for commercial fuel cell vehicles (FCVs) closely related to a severe reliability issue for long-term PEFC operation. An in situ three-dimensional fluorescence computed tomography-X-ray absorption fine structure (CT-XAFS) imaging technique and an in situ same-view nano-XAFS-scanning electron microscopy (SEM)/energy-dispersive spectrometry (EDS) combination technique were applied for the first time to perform operando spatial visualization and depth-profiling analysis of Ce radical scavengers in a practical PEFC of Toyota MIRAI FCV under PEFC operating conditions. Using these in situ techniques, we successfully visualized and analyzed the domain, density, valence, and migration of Ce scavengers that were heterogeneously distributed in the components of PEFC, such as anode microporous layer, anode catalyst layer, polymer electrolyte membrane (PEM), cathode catalyst layer, and cathode microporous layer. The average Ce valence states in the whole PEFC and PEM were 3.9+ and 3.4+, respectively, and the Ce3+/Ce4+ ratios in the PEM under H2 (anode)-N2 (cathode) at an open-circuit voltage (OCV), H2-air at 0.2 A cm-2, and H2-air at 0.0 A cm-2 were 70 ± 5:30 ± 5%, as estimated by both in situ fluorescence CT-X-ray absorption near-edge spectroscopy (XANES) and nano-XANES-SEM/EDS techniques. The Ce3+ migration rates in the electrolyte membrane toward the anode and cathode electrodes ranged from 0.3 to 3.8 μm h-1, depending on the PEFC operating conditions. Faster Ce3+ migration was not observed with voltage transient response processes by highly time-resolved (100 ms) and spatially resolved (200 nm) nano-XANES imaging. Ce3+ ions were suggested to be coordinated with both Nafion sulfonate (Nfsul) groups and water to form [Ce(Nfsul)x(H2O)y]3+. The Ce migration behavior may also be affected by the spatial density of Ce, interactions of Ce with Nafion, thickness and states of the PEM, and H2O convection, in addition to the PEFC operating conditions. The unprecedented operando imaging of Ce radical scavengers in the practical PEFCs by both in situ three-dimensional (3D) fluorescence CT-XAFS imaging and in situ depth-profiling nano-XAFS-SEM/EDS techniques yields intriguing insights into the spatial distribution, chemical states, and behavior of Ce scavengers under the working conditions for the development of next-generation PEFCs with high long-term reliability and durability.