Optimizing nanocasting techniques for stable bismuth-mesoporous silica composites in thermal energy storage application

Economically storing solar energy for use throughout the night is a major challenge facing the widespread transitions towards green energy generation and combating global warming. While most efforts are focused on electrochemical batteries, storing solar energy as heat is a viable alternative. Phase change materials (PCMs) utilize the solid – liquid transition to reversibly store heat at a constant temperature. The leakage of the molten phase limits the use of PCMs, but it can be alleviated by impregnation into porous matrices. Metals can be used for high temperature stationary heat storage, but are incompatible with oxide matrices due to the large difference in density and surface tension. The optimization of mesoporous silica – bismuth composites synthesis through nanocasting followed by reduction is reported. The resulting materials exhibit metallic bismuth both inside the mesopores and as an interparticle phase, leading to materials with stability towards oxidation, reversible heat storage, shape stability and reliability. A nanoconfined Bi phase could be obtained for low reaction times (4 h) and temperatures (125 °C) and it is correlated with increased stability towards oxidation in air. The samples with 50 % wt. metal retain their macroscopic shape above the metal melting point without leakage. All composites retain 50–96 % of their theoretical heat of fusion, which remains unchanged after 50 heating – cooling cycles. Nanocasting metal salts under hydrophobic solvents is a promising route for obtaining nanocomposites for thermal energy storage with both nanoconfined and interparticle metal phases.