Abstract
In situ transmission electron microscopy (TEM) can provide unparalleled structural and compositional information with high spatial and temporal resolution and is a standard tool for understanding and quantifying electrochemical nucleation and growth processes. Both solid state reactions and processes involving liquids can be examined. However, liquid phase reactions require particular care since the liquid needs to be confined in a thin layer and protected from the vacuum in the microscope. This problem is solved in the technique of electrochemical liquid cell TEM by confining the liquid between microfabricated electron transparent membranes on which the electrodes are patterned. Electrochemical liquid cell TEM has enabled quantification of nucleation and growth phenomena, dendrite formation, additive effects on morphology and SEI formation. Recent experimental developments have broadened the application space, but many more opportunities remain to be explored. Here we discuss how the existing strategies for electrochemical liquid cell TEM experiments can be adapted to allow quantitative data acquisition under an expanded range of electrochemical conditions. We first discuss the control of temperature, an important parameter in electrochemical systems such as batteries and fuel cells. Increasing the temperature while simultaneously applying a bias is achieved by customizing liquid cell chips to incorporate an electrically isolated heater strip beneath the electrodes. We find, as expected, that elevated temperature increases the rates of deposition and etching reactions and we discuss potential applications in electrocatalysis and codeposition. We then address the limitations of image resolution in liquid cell experiments. For nucleation and growth processes that take place on the surface of the electrode, the electrode materials such as Au or Pt often used in microfabricated liquid cells contribute strongly to electron scattering and thereby reduce the signal to noise ratio in the images. We show how few-layer graphene can be added to liquid cell chips to produce electrodes with minimal scattering, resulting in nucleation and growth movies with improved discrimination between nucleus shapes. We next consider the exciting consequences of instrumental improvements in microscopy - aberration correction, energy filtering and efficient electron detectors - in improving the resolution and lowering the dose required for imaging. Electron beam effects are unavoidable, however, and we discuss how the chemical changes caused by the beam can be modeled and perhaps mitigated for different electrolyte compositions and as a function of temperature. More speculative developments in liquid cell technique may allow us to measure stress, electrolyte composition and even the electrochemical double layer. Although a lot of work remains to be done, I hope to convince you that electrochemical TEM of liquids can address grand challenge questions in nucleation and growth through its unique view of electrochemical processes.