Challenges and opportunities in operando electrochemical liquid cell TEM

The unparalleled spatial resolution of transmission electron microscopy (TEM) had historically been limited to solid samples, or low-vapor-pressure liquids, due to the difficulty of implementing a sufficiently pressurized environment inside the microscope column. But the fabrication, and now widespread commercial availability, of hermetically sealed liquid cells has enabled in situ imaging of dynamic physical and chemical processes. Operando electrochemical TEM (ec-TEM) is of particular interest, but challenging, due to the complexity of integrating electrodes and controllable microfluidic flow into ultrathin fluid cells. However, thanks to advances in cell fabrication, ec-TEM has been used to image a range of electrochemical phenomena, including nanoparticle synthesis, electrodeposition, and solid electrolyte interface formation, with micro- and nanoscale spatial resolution. Meanwhile, the use of thin films and two-dimensional (2D) materials to fabricate viewing windows has made atomic-scale imaging in liquids a reality. This perspective will discuss the current state of operando ec-TEM, showcasing the applications of the technique, its limitations, and recent advances, including the use of 2D materials and thin films to construct ultrathin liquid cells for atomic-scale electrochemical imaging.