Hydroxyl functional ionic liquid-erythritol binary eutectic phase change materials for long-term thermal energy storage

Long-term phase change thermal storage is critical for large-scale renewable energy application, as it uses phase change materials' (PCMs) high latent heat to resolve spatiotemporal energy supply-demand imbalances. However, the practical application of erythritol (ERY) and similar PCMs is hindered by their crystallization kinetics, which cause prolonged supercooling, spontaneous crystallization, and an inability to release stored heat on demand. Herein, we propose a novel strategy, constructing a deep eutectic system via strong hydrogen bonds between hydroxy-functionalized ionic liquids (OH-ILs) and ERY to tailor its crystallization kinetics and phase behavior. Among composites, 1-(2-hydroxyethyl)-3-hydroxy pyridinium bromide (IL2)/ERY70 performs best, with melting enthalpy of 256.4 J/g and crystallization enthalpy of −159.8 J/g. Differential scanning calorimetry, in situ confocal micro-Raman spectroscopy demonstrate that the strong hydrogen-bonding interactions formed within the IL2/ERY system effectively reduce its melting point, and restructures its phase-change behavior and crystallization kinetics. The observed shift from cooling-induced to heating-induced cold crystallization, coupled with a transition from growth-dominated to nucleation-dominated kinetics, results in a more controllable, albeit slower, crystallization process. These synergistic modifications collectively enhance the supercooling stability of the material and significantly improve the controllability of energy release. In addition, IL2/ERY70 demonstrates exceptional cyclic stability, with its enthalpy varying only 5.1% over 50 cycles. Infrared analysis confirms its structural integrity, making it a promising high-performance material for long term thermal energy storage. This study provides an effective eutectic engineering strategy for advanced PCMs with high stability, controllable heat release, and long lifespan.