New research, led by Yingjie Zhang, assistant professor of Materials Science and Engineering at the University of Illinois at Urbana-Champaign, reports on direct imaging of the liquid solvation layers in an electrochemical supercapacitor device. The researchers identified key molecular reconfiguration effects during the capacitive charge/discharge processes. These findings are published in ACS Nano.
Liquid solvation layers, also called electric double layers (EDLs), occur ubiquitously at all types of solid-liquid interfaces and are crucial for a large range of natural and engineered processes, such as biomolecular signal transduction, corrosion control, and electrochemical energy conversion and storage. “In this study we were aiming to obtain structural information of these EDLs in order to understand the structure-property relationship for energy storage processes; but to date, it has been challenging to resolve the molecular structures of EDLs using other types of techniques such as X-ray spectroscopy and electron microscopy,” said Shan Zhou, a postdoctoral research associate and lead author on the paper.
To address this problem, the Zhang lab recently developed a new technique to image solid-liquid interfaces. Their invention, called "electrochemical three-dimensional atomic force microscopy (EC-3D-AFM)," enables atomic-scale 3D imaging of both the electrode surface and electric double layers within conditions in an electrochemical cell. “EDL structures are critical for batteries and supercapacitors," said Kaustubh Panse, a graduate student and co-lead author on the work. "Our EC-3D-AFM technique allows us to directly see the EDLs and understand how these energy storage systems work.”
Schematic showing the EC-3D-AFM setup and a 3D image of EDLs on a graphite surface.
Detailed features of the EDLs have been identified from the 3D molecular density maps using this technique. For example, the multilayer nature of the EDLs and zigzag-like patterns within each layer can be clearly resolved. The observed quasi-periodic zigzag features of EDLs are due to both the molecular tilt and the coexistence of cations and anions, which were proved by molecular dynamics (MD) simulations from the Narayana Aluru group here at the University of Illinois at Urbana-Champaign.
More interestingly, when applying different voltages to the graphite surface, pronounced structural reconfigurations were observed, especially at the first ionic layer. Combining experimental results with MD simulations, the authors identified that the molecular arrangement and orientation in the first EDL layer play a dominant role in capacitive charge storage. “We are excited about these results and are looking forward to see how this powerful imaging technique and these mechanistic understandings will benefit other researchers in areas of not only energy storage, but also molecular biology and others,” said Zhang.
The paper “Three-Dimensional Molecular Mapping of Ionic Liquids at Electrified Interfaces” can be found here: https://pubs.acs.org/doi/10.1021/acsnano.0c07957
This work was supported by the American Chemical Society Petroleum Research Fund and the National Science Foundation.