A Viewpoint on X-ray Tomography Imaging in Electrocatalysis

With the emerging demands for clean energy and an economywithnet-zero greenhouse gas emissions, electrocatalysis areas have attractedtremendous interest in recent years. The electrochemical devices thatuse electrocatalysis, such as fuel cells, electrolyzers, and flowbatteries, consist of hierarchical structures, requiring comprehensionand rational designs across scales from millimeter and micrometerall the way down to atomic scale. In past decades, electron microscopytechniques such as scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) have been extensively utilized for imagingdifferent scales of these devices in both two and three dimensions.However, electron-based techniques for high-resolution imaging requireuninterrupted maintenance of a high-vacuum environment, leading todifficulties of sample preparation and lack of integrated observationwithout intrusion/disassembly. To overcome these disadvantages, moreand more efforts have been dedicated to the development of X-ray imagingtechniques recently, specifically absorption-based two-dimensional(2D) transmission X-ray microscopy and three-dimensional (3D) X-raytomography, due to much better transmission behaviors of X-rays thanelectrons. X-ray tomography imaging mostly focuses on answering questionsrelated to morphology and morphological changes at the microscaleor near 1 & mu;m resolution and nanoscale of 30 nm resolution. Themethod is nondestructive and it allows for the visualization of operandoelectrochemical devices, such as fuel cells, electrolyzers, and redoxflow batteries. Operando X-ray microscopic tomography typically focuseson catalyst layers and morphology changes during degradation, as wellas mass transport. Nanoscale tomography still predominantly is usedfor ex situ studies, as multiple challenges exist for operando studiesimplementation, including X-ray beam damage, sample holder design,and beamline availability. Both microscale and nanoscale tomographybeamlines now couple various spectroscopic techniques, enabling electrocatalysisstudies for both morphology and chemical transformations. This viewpointhighlights the recent advances in X-ray tomography for electrocatalysis,compares it to other tomographic techniques, and outlines key complementarytechniques that can provide additional information during imaging.Lastly, it provides a perspective of what to anticipate in comingyears regarding the method use for electrocatalysis studies.