In recent years, with the demand for clean energy and the development of a net zero greenhouse gas emission economy, the field of electrocatalysis has attracted great interest. Electrochemical devices that use electrocatalysis, such as fuel cells, electrolytic cells, and flow cells, are composed of hierarchical structures that require understanding and reasonable design, from millimeter and micrometer to atomic scales. In the past few decades, electron microscopy techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been widely used for two-dimensional and three-dimensional imaging of different scales of these devices. However, electronic based high-resolution imaging technology requires uninterrupted maintenance of a high vacuum environment, which makes sample preparation difficult and lacks comprehensive observation without invasion/disassembly. In order to overcome these shortcomings, more and more people have been devoted to the development of X-ray imaging technology in recent years, especially the absorption based two-dimensional (2D) transmission X-ray microscope and three-dimensional (3D) X-ray tomography technology, because the transmission behavior of X-rays is much better than that of electrons. X-ray tomography mainly focuses on answering questions related to morphology and morphological changes. This method is non-destructive and allows for visual operation of electrochemical devices such as fuel cells, electrolytic cells, and redox flow cells. In situ X-ray micro tomography usually focuses on the morphological changes and mass transfer of the catalyst layer and degradation process. Nanotomography is still mainly used for non in-situ research, as there are various challenges in the implementation of operational research, including X-ray beam damage, sample holder design, and beam line availability. Microscale and nanoscale chromatographic beams now combine various spectral techniques, making it possible to study the morphology and chemical transformation of electrocatalysis. This viewpoint emphasizes the latest progress in electrocatalytic X-ray tomography, compares it with other tomography techniques, and outlines key complementary technologies that can provide additional information during the imaging process. Finally, it provides a prospect for the next few years regarding the methods used in electrocatalytic research.

 

Background of electrocatalysis
 
Electrochemical technology has significant prospects in decarbonization of the energy sector and the transition of the economy to net zero. Hydrogen technology, such as fuel cells and electrolytic cells, is limited by cost and durability. It is necessary to improve the utilization, activity, and durability of electrocatalysts in order to make them widely used. Similarly, for the utilization and conversion of carbon, carbon dioxide reduction reaction (CO2RR) electrolytic cells require selective and durable electrocatalysts. The redox flow battery used for long-term power grid energy storage relies on electrocatalysts for oxidation/reduction reactions, requiring rich and durable catalysts. All of these technologies rely on nanoscale electrocatalysts and porous electrodes to increase the surface area and utilization rate of the catalyst. X-ray computed tomography (CT) may unravel the correlation between complex reaction processes and the porous morphology of electrocatalysts. Overall, Figure 1 summarizes the X-ray CT technology and how it can be applied to study nanoscale and microscale phenomena related to electrocatalysis in electrochemical devices.

Figure 1 Imaging Resolution as a Function of Field of View (FOV) for Planar Photography, Nano X-ray CT, and Micro X-ray CT
 

Why does electrocatalysis require X-CT?
 
Physical and chemical characterization methods such as electron microscopy and X-ray spectroscopy have had a significant impact on the development of electrocatalysis, and the improvement of these technological capabilities has led to a better understanding of the synthesis, reaction pathways, and degradation mechanisms of catalysts. Electron microscopy techniques, such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as well as related energy dispersive X-ray spectroscopy (EDS), can also provide structural and elemental information on the distribution of catalysts within the catalyst layer. Although these technologies provide valuable nanoscale structure and elemental information about catalysts and catalyst layer distribution, they are usually limited to two-dimensional (2D) analysis. In addition, based on electronic characterization, in order to avoid signal loss during the propagation process from the source to the sample to the detector, it is necessary to continuously maintain a high vacuum environment. In contrast, X-ray sources, especially hard X-rays (with energies greater than 5 keV), have weaker interactions with gas molecules and have milder requirements for sample preparation. Therefore, the electrocatalytic community interested in characterizing electrochemical devices tends to use X-ray techniques, such as X-ray diffraction (XRD), X-ray tomography (XRD), and X-ray fluorescence (XRF). Micro scale X-ray tomography (micro CT) is very beneficial for characterizing and understanding electrochemical systems at the micro scale. The use of this technology makes an important contribution to understanding the morphology of catalyst layers and their impact on mass transfer. Due to the spatial resolution limitations of micro CT, it is usually not used for studying nano particle catalysts alone or integrating them into the catalyst layer. In contrast, nanoscale X-ray tomography (X-ray NanoCT) is more conducive to non-invasive detection of the structure of aggregated electrocatalyst nanoparticles, as in high-resolution imaging, it better reflects the actual situation in the catalyst layer compared to well dispersed/prepared TEM samples. In 2018, Normile designed a special snake shaped PEMFC that utilizes synchrotron phase contrast X-ray micro CT under advanced light source (ALS) to perform high-resolution segmentation of platinum, carbon, film, and water, observing the formation of liquid water and film expansion. As shown in Figure 3, in the 3D reconstruction and segmentation results, the formation of liquid water in the platinum anode, platinum free cathode, cathode gap, and the detachment between the platinum catalyst and the expansion film can be clearly observed.

Figure 2 shows the volume rendering representation of PEFC, where the top is a cathode with a metal nitrogen carbon catalyst and the bottom is an anode with a platinum/carbon catalyst. Left: Gray level tomogram. Middle: 3D visualization of water and electrodes. Right image: 3D visualization of moisture region
 

X-ray CT in fuel cells/electrolyzers
 
X-ray CT has been widely used in the field of PEMFC, mainly for studying mass transfer (water management) and catalyst degradation after long-term operation. Some research groups have developed operable PEMFC sample holders with an active area of 1 square centimeter, which can visually observe the transformation of the fuel cell catalyst layer during operation. Figure 3 shows an example of catalyst layer crack formation during accelerated stress testing using in-situ X-ray CT. In terms of electrolytic cells, there have been many reports in recent years using X-ray tomography technology, especially in the field of polymer electrolyte membrane electrolysis (PEMWE). In proton exchange membrane fuel cells, a typical imaging research focus is on manufacturing differences, which may also be encountered in proton exchange membrane fuel cells. In 2020, Leonard compared the morphology and performance differences of porous transport electrodes (PTE), catalyst deposition on porous transport layers and catalyst coated films (CCM) surfaces, and catalyst deposition using X-ray micro CT and nano CT. It has been revealed that low ion connectivity in the PTE structure can lead to bottlenecks in proton transport and lead to high performance losses. Another imaging focus is the mass transfer of oxygen in PEMWEs. In 2022, Kulkarni et al. used X-ray micro CT to study the distribution of oxygen content under different catalyst loads. De Angelis revealed two-phase (water/oxygen) transport through X-ray micro CT in 2021, identifying four different types of oxygen cluster connections and how isotropic/anisotropic microstructures affect electrolytic performance. Another imaging research interest is distinguishing electrode components within components, which may require a combination of X-ray CT and other imaging characterization techniques such as planar photography and fluorescence.

Figure 3 Time resolved in situ visualization of electrode morphology changes during accelerated degradation of platinum cathode catalyst layer