Innovative Applications of Nanoporous Materials for the Environment: From Water Purification to Rechargeable Mechanical Batteries

A research Group, coordinated by Carlo Massimo Casciola of the Sapienza Department of Mechanical and Aerospace Engineering, has employed advanced molecular simulation techniques to design nano-structured materials with controllable wetting or drying behaviour. The study, published on ACS Nano, represents a significant step forward in the engineering of porous materials for energetic and environmental applications

Thanks to the presence of numerous miniscule pores (only a few nanometres in width), certain devices developed with nano-porous materials, the so-called HLS Systems (Heterogenous Lyophobic Systems) have an extraordinary capability of storing energy. These devices behave like a liquid battery. They are “charged” when the water pressure increases filling the pores and they “discharge” when, as the pressure diminishes, water is expelled, providing mechanical energy. However, to date, the “switch” allowing this mechanism to take place was not well understood and difficult to control.

Now, a new study conducted at the Sapienza Department of Mechanical and Aerospace Engineering has shed new light on this possibility by investigating the behaviour of various porous materials via high-pressure intrusion and extrusion experiments on water. The results of the study published on ACS Nano, point to an innovative strategy to control the movement of fluids in the materials under investigation.

“The microscopic mechanism underlying the expulsion of water,” explains Alberto Giacomello, one of the researchers on the team, “is related to the presence of nanometer-sized bubbles in the interconnections between the pores that reduce the contact between the walls of the material and the water, thereby developing true superhydrophobic pores.”

What seems to determine the capacity of nanoporous materials to spontaneously dewet appears to be the shape of the pores: the absence of an interconnection between the pores blocks the expulsion of absorbed water. However, if the pores are interconnected, the pores can “dry up” and expel the water, even at room temperature and at extremely high pressures, equivalent to those present hundreds of meters below sea level.

The researchers employed macroscopic models and atomic simulations to demonstrate that the phenomenon occurs when there are hydrophobic cavities equal to or less than one nanometre on the walls of the nanoporous material. Moreover, they also repeated the experiment, substituting the water with mercury and using other nanoporous materials, obtaining analogous results that confirmed the general validity of the mechanism.

“Our study provides theoretical and computational tools enabling scientists and engineers to design nano-structure materials capable of fully exploiting the characteristics of liquids in nanometer/sized confines,” points out Carlo Massimo Casciola, who coordinated the research project, “such as the ability to reversibly wet or dry a surface or increase their surface mobility. Numerous new applications are possible in this field, including mechanical energy accumulators for renewable sources and energy harvesting applications, vibration and shock absorbers, water purification techniques and surfaces capable of regenerating their superhydrophobicity.”

References:

Pore Morphology Determines Spontaneous Liquid Extrusion from Nanopores - Matteo Amabili, Yaroslav Grosu, Alberto Giacomello, Simone Meloni, Abdelali Zaki, Francisco Bonilla, Abdessamad Faik, and Carlo Massimo Casciola, ACS Nano 2019 DOI: 10.1021/acsnano.8b07818