Nanoconfined Low-Temperature Supercritical Water for Thermal Energy Storage

This project, SuperLow, aims to elucidate the fundamental behavior of nanoconfined supercritical water (cSCWs), with a particular focus on investigating whether pseudo-boiling and pseudo-condensation phenomena can manifest under extreme nanoconfinement conditions. Such insights have the potential to unlock novel pathways for thermal energy storage (TES) technologies. Distinctively, SuperLow employs a pioneering approach that leverages highly non-wetting water–nanoporous material pairs during intrusion–extrusion processes, in contrast to the conventional use of wetting pairs in adsorption–desorption studies. Recent work by the principal investigators demonstrated that this methodology can induce an unprecedented reduction in the critical temperature of water by approximately 275 K, achieving supercritical behavior at near 100 °C—far below the bulk critical point. Despite considerable scientific interest, many fundamental properties of nanoconfined water remain poorly understood. In particular, the influence of nanopore size, topology, and surface hydrophobicity on phase behavior, critical temperature and pressure shifts, and the potential existence of the Widom line require systematic exploration. This project addresses these critical knowledge gaps by integrating high-pressure intrusion–extrusion calorimetry with operando structural characterization techniques and atomistic simulations. Confirming or refuting the presence of the Widom line and associated pseudo-phase transitions in cSCWs not only advances fundamental thermodynamics but also establishes a strong technological foundation for developing thermal energy storage materials optimized for low- to medium-temperature applications—an area currently lacking viable solutions. In the longer term, this innovative approach offers a promising route to enhancing energy efficiency by enabling the recovery of up to 468 TWh/year of waste heat dissipated below 100 °C in Europe alone. The use of inexpensive, abundant, and well-characterized nanoporous materials, e.g. silica, underscores the practical viability of this concept. The overarching goal of SuperLow is to stimulate a paradigm shift in supercritical water applications through the strategic lowering of critical temperatures via nanoconfinement. By intruding water in its liquid state under megapascals of pressure—rather than relying on vapor phase condensation at sub-saturation pressures—this work pioneers a novel experimental framework with broad implications for both fundamental science and sustainable energy technologies.

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