Electrochemical cells – or batteries, as a well-known example – are complex technologies that combine chemistry, physics, materials science and electronics. More than power sources for everything from smartphones to electric vehicles, they remain a strong motivation for scientific inquiry that seeks to fully understand their structure and evolution at the molecular level.
A team led by Yingjie Zhang, a professor of materials science and engineering in The Grainger College of Engineering at the University of Illinois Urbana-Champaign, has completed the first investigation into a widely acknowledged but often overlooked aspect of electrochemical cells: the nonuniformity of the liquid at the solid-liquid interfaces in the cells. As the researchers reported in the Proceedings of the National Academy of Sciences, microscopic imaging revealed that these interfacial structures, called electrical double layers (EDLs), tend to organize into specific configurations in response to chemical deposition on the surface of the solid.
"There's a tendency to think of electrochemical cells just for their technological utility as batteries, but there is still plenty of science to do on them that will inform the technological applications," said Qian Ai, a graduate student in Zhang's research group and the study's lead author. "In our work, we carefully examined EDLs with 3D atomic force microscopy, a technique designed to sense small forces. We observed the molecular structure of inhomogeneous EDLs surrounding surface clusters for the first time."
Electrochemical cells take advantage of mobile charges inside liquid electrolytes to maintain an electrical imbalance that gives rise to a voltage difference between two terminals. The earliest investigations of these systems over 100 years ago revealed the existence of EDLs at the interface between the liquid electrolyte and solid conductor mediating the voltage difference. They consist of electrolytes self-organized into nanometer-thick layers at the interface.
Past work has shown that solid-liquid interfaces in batteries are heterogeneous, exhibiting spatially varying chemical compositions and morphologies, sometimes forming surface clusters. However, these attempts to study and model electrochemical cells focused only on model systems with flat and uniform surfaces. The result is a knowledge gap that impedes our understanding of electrochemical cells and battery technology.
To investigate the heterogeneous interfaces, the team used 3D atomic force microscopy, a technique designed to sense small forces. This method allowed them to correlate the inhomogeneity in EDLs with the surface clusters, structures that nucleate at the initial stages of battery charging. Based on the data, the researchers proposed three primary responses in the EDLs: "bending," in which the layers appear to curve around the cluster; "breaking," in which parts of the layers detach to form new intermediate layers; and "reconnecting," in which the EDL layer above the cluster connects to a nearby layer with an offset in the layer number.
"These three patterns are quite universal," Ai said. "Those structures are mainly due to the finite size of the liquid molecules, not their specific chemistry. We should be able to predict the liquid structure based on the solid's surface morphology for other systems."
Going forward, the researchers look forward to expanding their findings.
"This is groundbreaking," Zhang said. "We have resolved the EDLs in realistic, heterogeneous electrochemical systems, which is a holy grain in electrochemistry. Besides the practical implications in technology, we are starting to develop new chapters in electrochemistry textbooks."
Lalith Bonagiri, Kaustubh Panse, Jaehyeon Kim and Shan Zhou also contributed to this work.
The study, "Nucleation at solid–liquid interfaces is accompanied by the reconfiguration of electrical double layers," is available online. DOI: 10.1073/pnas.2421635122
Support was provided by the Air Force Office of Scientific Research.
Yingjie Zhang is an Illinois Grainger Engineering assistant professor of materials science and engineering in the Department of Materials Science and Engineering . He is a faculty affiliate of the Materials Research Laboratory and the Beckman Institute for Advanced Science and Technology .
