This image shows birnessite with water molecules (red oxygen, white hydrogen) and potassium cations (dark blue/lavender) at the outer surfaces and within the interlayer. Image: Karthik Ganeshan.The adsorption of electrolyte ions at the surface of an electrode is a ubiquitous process in both existing and emerging electrochemical energy technologies. To try to understand this adsorption process in more detail, researchers at North Carolina (NC) State University turned to a 'classic' material known as birnessite.
Birnessite is a hydrated layered form of manganese oxide that can quickly store and release a variety of positive ions, or cations, from electrolytes over many cycles. This makes it a promising material for use in high-power electrochemical energy storage, or in emerging electrochemical technologies such as desalination and recovering rare elements from water. What’s more, birnessite is abundant, easy to make and non-toxic.
The mechanism by which birnessite can take up and release cations has been described as both faradaic (involving charge transfer) and non-faradaic (involving only electrostatic ion adsorption). To clarify this apparent contradiction, the researchers utilized both experimental and computational approaches.
“In the energy storage community, we normally think of charge storage as being either faradaic or non-faradaic,” explains Shelby Boyd, a postdoctoral researcher at NC State and first author of a paper on this work in Nature Materials.
“At planar interfaces, faradaic refers to the specific adsorption of an ion to an electrode with corresponding charge transfer, as in a redox reaction. Non-faradaic refers to purely electrostatic adsorption without charge transfer. People have largely presented these mechanisms of charge storage as being mutually exclusive. But what we found with birnessite is that the nanoconfined interlayer structural water mitigates the interactions between intercalated cation and the birnessite. This results in an intermediate behavior from the two types of adsorption extremes at planar interfaces.”
The researchers were able to prove experimentally and theoretically that water in between the layers of birnessite can effectively serve as a buffer that makes capacitive behavior possible without causing significant structural change in the birnessite.
Ultimately, the researchers say these findings highlight two future directions for their work, both of which are promising for the broader field of electrochemistry.
“The field of electrochemistry is undergoing a renaissance,” says Veronica Augustyn, an assistant professor of materials science and engineering at NC State and corresponding author of the paper. “The ability to connect experimental results with atomistic-scale modeling of the electrochemical interface allows us to probe deeper than ever before and ask questions like: What roles are the solvent playing? What might happen when the reaction occurs under confinement? By understanding the capacitive mechanism of a material like birnessite, we set the stage for understanding more complex electrochemical reactions.”
This story is adapted from material from North Carolina State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.