Advanced Liquid Phase TEM for Nanoscale Investigation of Electrochemical Interfaces

Solid-solid and solid-liquid interfaces are key components for diverse systems of electrocatalytic chemical conversion and rechargeable battery. Critical electrochemical reactions at the diverse interfaces in those systems occur at the nanometer length scale. Despite the importance of the interfacial reactions, a lack of analytical tools that can characterize structural changes at the nanoscale level has hindered fundamental mechanistic understating of them. The recent development of liquid phase transmission electron microscopy (LPTEM) is gaining attention as a new tool to directly observe electrochemical reactions that occur in solid-solid and solid-liquid interfaces with high resolution. Here, we introduce a few examples where liquid phase TEM, in conjunction with other in situ characterization techniques, directly reveals important reactions at the interfaces of electrocatalysts and the electrolyte. Special efforts have been made to improve LPTEM as a reliable method for monitoring electrochemical process of supported metal catalysts in a condition of minimized electron beam perturbation. With this approach, LPTEM can successfully reveal how supported catalysts respond to electrochemical cycles in nanoscale spatial resolution. In situ observation based on LPTEM is also extended to investigation of important processes occurring at the electrode surfaces of diverse battery systems. Direct in situ TEM observation discloses the gradual growth of toroidal Li2O2 discharge product in the electrolyte with the redox mediator upon discharge in Li-air battery. The liquid phase TEM with electrochemical biasing capability is also applied to the studies of direct Li deposition on the metal electrode in Li metal battery system. Combined with cryo-TEM, it is found that the kinetics of Li deposition is determined by the mechanical and chemical properties of SEI, formed differently depending on the electrolyte.