The incorporation of water molecules in layered materials affects the ion storage capacity

The incorporation of water molecules in layered materials affects the ion storage capacity

Researchers have experimentally detected the structural change of water of hydration confined in the tiny pores at the nanometer scale of layered materials such as clays. Their findings potentially open the door to new options for ion separation and energy storage.

Investigating the interaction between the structure of water molecules that have been incorporated into layered materials, such as clays, and the configuration of ions in such materials has long been a major experimental challenge. But the researchers have now used a technique commonly used elsewhere to measure extremely small masses and nano-level molecular interactions to observe these interactions for the first time.

His research was published in nature communications on October 28, 2022.

Many materials take a layered form on the microscopic or nanometer scale. When dry, clays, for example, resemble a series of sheets stacked on top of each other. However, when such layered materials encounter water, that water can be confined and embedded in the spaces or holes, or, more accurately, ‘pores’, between the layers.

Such ‘hydration’ can also occur when water molecules or their constituent elements, in particular a hydroxide ion (a negatively charged ion combining a single oxygen atom and a single hydrogen atom) become integrated into the crystal structure of the material. This type of material, a ‘hydrate’, is not necessarily ‘moist’ although water is now part of it. Hydration can also substantially change the structure and properties of the original material.

In this ‘nanoconfinement’, the hydration structures (how the water molecules or their constituent elements are organized) determine the capacity of the original material to store ions (atoms or groups of atoms with a positive or negative charge).

This storage of water or charge means that such layered materials, from conventional clays to layered metal oxides and, crucially, their interactions with water, have widespread applications, from water purification to energy storage.

However, studying the interaction between this hydration structure and the ion configuration in the ion storage mechanism of such layered materials has turned out to be a great challenge. And efforts to analyze how these hydration structures change in the course of any movement of these ions (“ion transport”) are even more difficult.

Recent research has shown that such water structures and interactions with layered materials play an important role in giving the latter their high ion storage capacities, all of which in turn depend on how flexible the host layers are. Water. In the space between layers, the pores that are not filled with ions fill with water molecules, helping to stabilize the layered structure.

“Put another way, water structures are sensitive to how interlayer ions are structured,” said Katsuya Teshima, corresponding author of the study and a materials chemist at the Supra-Materials Research Initiative at Shinshu University. . “And although this configuration of ions in many different crystal structures controls how many ions can be stored, such configurations have until now rarely been systematically investigated.”

So Teshima’s group looked up the “quartz crystal microbalance with energy dissipation monitoring” (QCM-D) to help with their theoretical calculations. QCM-D is essentially an instrument that works like a balance that can measure extremely small masses and molecular interactions at the nano level. The technique can also measure small changes in power loss.

The researchers used QCM-D to demonstrate for the first time that the change in the structure of water molecules confined in the nanospace of layered materials can be observed experimentally.

They did this by measuring the “hardness” of materials. They investigated the layered double hydroxides (LDH) of a negatively charged class of clay. They found that hydration structures were associated with LDH hardening when any ion exchange reaction (an exchange of one type of ion with a different type of ion but with the same change) occurs.

“In other words, any change in the ionic interaction originates from the change in the hydration structure that occurs when ions are incorporated into the nanospace,” added Tomohito Sudare, a study collaborator now at the University of Tokyo.

In addition, the researchers found that the hydration structure is highly dependent on the charge density (the amount of charge per unit volume) of the layered material. This, in turn, is largely what governs the ion storage capacity.

The researchers now hope to apply these measurement methods together with knowledge of the ion hydration structure to devise new techniques to improve the ion storage capacity of layered materials, potentially opening new avenues for ion separation and analysis. sustainable energy storage.

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