To better understand how ions penetrate very small gaps during the adsorption process from an electrolyte to the electrode, researchers turned to the 'classic' material, birnessite. 

The process of ion adsorption is a particularly ubiquitous process, which drives potential electrochemistry and energy applications. Researchers from North Carolina State University examined birnessite to gain new insights on the ion transfer process, with their findings offering new insights for both electrochemistry and energy studies in the future.

Researchers presented their findings in the paper "Effects of Interlayer Confinement and Hydration on Capacitive Charge Storage in Birnessite," appearing in the August issue of the journal Nature Materials.

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The Advantage and Potential of Birnessite

Birnessite is an oxide mineral of manganese that also contains traces of calcium, sodium, and potassium. The hydrated layered form of manganese oxide, also known as a phyllomanganate, has been found to quickly store and release a host of positive ions from electrolytes and maintain this for many cycles.

This makes the mineral a promising material for high-power electrochemical energy storage or emerging technologies like desalination and rare element filtering from water. A 2009 study from the University of Wisconsin-Madison even noted that the material could degrade the prion, a pathogenic protein form that causes a host of dangerous diseases.

Additionally, birnessite is an abundant mineral, being reported in various nodules around the world beyond Birness, Scotland, where the material got its name. It is also non-toxic and easy to process.

The mechanism by which birnessite stores and releases cations (positive ions) has been previously described as both faradaic, or involving charge transfer between ions, and non-faradaic, or the process that only involves electrostatic ion adsorption.

Observing Ion Response and Movement in Birnessite Samples

To better understand how the material functions, the NCSU researchers used a mix of computational and experimental approaches.

"In the energy storage community, we normally think of charge storage as being either faradaic or non-faradaic," explains Shelby Boyd, the first author of the paper of the new study and a postdoctoral researcher at NCSU, said in a university press release.

She additionally explains that for a planar interface, faradaic refers to the specific adsorption of a charged particle toward an electrode accompanied by a charge transfer, which is the same thing that happens in reduction-oxidation or redox reactions.

On the other hand, non-faradaic mechanisms are limited to electrostatic adsorption without the transfer of charges. While the two mechanisms are usually interpreted as being mutually exclusive, this is not the case with birnessite.

The interlayer structural water in the mineral mitigates the interaction between the birnessite and the cation, resulting in behavior partially characteristic of both faradaic and non-faradaic mechanisms.

Also, NCSU researchers showed through both theoretical and experimental approaches that water between the birnessite layers serves as a buffer. This allows the material to exhibit capacitive behavior without causing damage or structural change in the birnessite.

With these new findings, researchers identified two future directions for their work, both of them promising for the fields of electrochemistry and energy.

 

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