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Nanosponges STORE and Recover Energy

06-12-2017

At first glance, nanoporous materials appear like grains of sand, but under an electronic microscope one can see the many tiny pores, measuring only a few nanometres, that characterize them. A single gram of material will contain over 1000 square meters in pores. Using this extraordinary property, engineers and materials scientists are now working to develop new technologies for energy storage in reliable, compact and economical devices.

A group from the Sapienza Department of Mechanical and Aerospace Engineering, coordinated by Carlo Massimo Casciola, has developed advanced molecular simulation techniques to design an optimal material for these devices. This innovative approach, which is described in the study published on the Proceedings of the National Academy of Sciences (PNAS), is a "virtual microscope" that enables researchers to investigate nanoscale phenomena with molecular resolution and long-time scales, which are inaccessible in normal simulations. At present, a wide variety of nanoporous materials, which differ both in terms of chemical composition and cavity geometry, can be used to develop HLS (Heterogeneous Lyophobic Systems): energy-storage devices based on the ability of the pores to behave as "molecular springs."

In fact, these devices are based on a mechanical form of energy storage. The nanoporous material is made hydrophobic and sealed in a container with water or other liquids with reduced wettability. By increasing the pressure of the liquid, it permeates the pores and energy is stored as surface tension. When the pressure is brought back to its original value, the formation of steam bubbles inside billions of pores causes the container to expand, making the stored energy available again.

By investigating rare nanoscale events, the study has created a bridge between macroscopic quantities, which are of interest in HLS studies (stored and dissipated energies; intrusion and extrusion pressures), and the microscopic properties of materials and liquids (chemistry and geometry of nanoporous materials and the formation of extremely asymmetric bubbles).

In particular, it was observed that nanosponges catalyse the formation of vapour bubbles, even in unexpected conditions: for example, water contained in hydrophobic nanopores can "boil" (form steam vapour) at room temperature and even at extremely positive pressures, equivalent to those recorded at depths of 1000 meters below sea level. These results deviate wildly from advanced hypotheses on macroscopic observations.

Simulations have also shown that, by increasing the nanopore size by a few nanometres, it is possible to change HLS behaviour from energy-storage devices with very high energy efficiencies into limited weight dampers that can disperse large quantities of mechanical energy.

"This study,” explains Casciola, “opens up new horizons for HLS design for a large number of applications ranging from renewables (such as solar) to energy recovery, as with new generation braking systems ... close collaboration with chemists and material scientists will optimize the compactness and efficiency of these devices."

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