At a first look nanoporous materials may have the appearance of grains of sand, but their minuscule and countless pores account for surface areas of the order of 1000 m2 or more in a single gram of material. Leveraging this extraordinary surface concentration, engineers and materials scientists are working towards developing new ways of storing energy in compact, reliable, and inexpensive devices.

A promising route to these “molecular springs” is to render nanopores hydrophobic (“water-fearing”) and then seal them in a container with water or other non-wetting liquids: when the water pressure is increased, e.g., by an external energy source, water enters the pores and energy is stored in the form of surface energy. When the pressure is restored to the original value, bubbles form in the billions of pores and the container expands making the energy available again. This mechanical form of storage (Heterogeneous Lyophobic Systems – HLS) is promising in a number of applications, including renewable energy sources (e.g., solar) and energy recovery (e.g., from brakes).

A variety of nanoporous materials, with different chemical compositions and pore geometries, and liquids can be used to realize HLS: engineering the optimal one requires knowledge of the behavior of liquids in the narrow confines of the nanopores – of the order of 1 or few nanometers (1 billionth of a meter), where macroscopic theories are challenged. For energy storage, it is particularly important to reduce the difference between the pressure at which the liquid intrudes and that at which bubbles form (hysteresis), which is connected to the dissipation of energy and thus to lower efficiencies.

On the other hand, HLS with high hysteresis can be used as energy dampers, capable of dissipating, e.g., vibrations over an unprecedentedly broad range of frequencies. Summarizing, the working principle of HLS for energy storage or dissipation is rooted in the liquid intrusion and extrusion processes in nanopores; designing HLS requires microscopic insights into these nanoconfined phenomena, which are difficult to observe directly in experiments.
The team lead by Prof. Casciola at the Department of Mechanical and Aerospace Engineering has used state-of-the-art molecular dynamics simulations to reveal how water intrudes and how nanobubbles nucleate in the narrow confines of hydrophobic nanopores; this research has been published in the Proceedings of the National Academy of Sciences.

These “rare events” simulations are a sort of virtual microscope which allows the researchers to investigate nanoscale phenomena with an unprecedented resolution of a single water molecule and over long timescales which are out of reach of standard simulations. This innovative bottom-up approach allowed the researchers to bridge the macroscopic quantities of engineering interest for a HLS (e.g., the stored and dissipated energies; the intrusion and extrusion pressures) to the microscopic characteristics of the material and of the liquid: the chemistry and geometry of the nanoporous material and the formation of a highly asymmetric bubble.

In particular, results disclosed substantial deviations from the predictions of macroscopic theories on the intrusion pressure and on the time for forming a bubble. For instance, water in nanopores boils (i.e., forms bubbles) at ambient temperature and at extremely large pressures, equivalent to 1000m underwater. Simulations also showed that by increasing the size of the nanopore by few nanometers it is possible to tune the behavior of HLS from energy-storage devices, with very high storage efficiency, to dampers, which are compact means of dissipating energy.

This research is part of a broader project funded by the Advanced ERC Grant “BIC – Cavitation across scales: following Bubbles from Inception to Collapse” awarded to Prof. Casciola. The aim of the project is to understand the fundamental aspects of bubble formation, transport, and collapse, which gives rise to engineering cavitation. The present results are an example of how hydrophobic surfaces with nanoscale cavities catalyze the formation of bubbles, even in conditions at which one  would not expect them. These bubbles can then grow, be transported, and cause cavitation damage.

The research paved the way for understanding how the microscopic characteristics of a nanoporous material are connected to the engineering performance of HLS. Advanced molecular dynamics simulations show promise tool to design HLS for specific applications in the field of energy storage: the researchers of Sapienza University of Rome are collaborating with chemists and material scientists (the third author Y. Grosu who works at a Spanish research center devoted to energy research), who are capable of synthetizing a variety of nanoporous materials, in order to optimize the compactness and efficiency of energy storage devices based on nanoporous materials.
 

References:

Intrusion and extrusion of water in hydrophobic nanopores - Antonio Tinti, Alberto Giacomello, Yaroslav Grosu, and Carlo Massimo Casciola - Proceedings of the National Academy of Sciences of the United States of America. doi: 10.1073/pnas.1714796114

Further Information:

Carlo Massimo Casciola
Department of Mechanic and Aereospacial Engineering, Sapienza University of Rome
T (+39) 06 4458 5201
Email: carlomassimo.casciola@uniroma1.it