Solar-Thermal Energy Storage and Conversion Capabilities of M-BTC MOFs Embedded with PEG-Based Phase Change Materials

Phase change materials (PCMs) are promising for solar-thermal energy storage, yet polymeric PCMs such as PEG suffer from supercooling, low thermal conductivity, and limited cycling stability. Here we investigate Co, Ni, and Co-Ni-doped 1,3,5-benzenetricarboxylate metal–organic frameworks (BTC MOFs) as porous matrices for PEG and systematically assess how metal substitution tunes the local coordination environment, melting behavior, and enthalpy of the resulting MOF–PCM composites. We find that disorder–order phase transitions are decisive in governing energy storage performance. Among the materials studied, Ni-BTC/PEG delivers the best results, enabled by its higher specific surface area, larger pore volume, and abundant surface hydroxyl groups that promote strong hydrogen bonding with PEG. Ni-BTC/PEG attains a high latent heat of 156 J/g, reduced supercooling (ΔT = 19.0 °C), and stable performance over 200 melt–freeze cycles, while its thermal conductivity increases by ∼38% relative to pure PEG. The composite also achieves a solar-thermal energy storage efficiency of 72.2%. These results identify the specific surface area and pore-size distribution as dominant, design-controllable parameters for MOF–PCM systems and position Ni-BTC/PEG as a strong candidate for practical solar-thermal energy storage applications.