Design of Mixed-Metal MOF-74-MgNi for Water Adsorption-Driven Solar Thermal Energy Storage and Heat Transformation Applications

Adsorption solar thermal energy storage and heat transformation are ecologically benign and energy-efficient technologies. Efficient adsorbents are the key to this technology. In this paper, two metal ions, Mg²⁺ and Ni²⁺, were introduced into the metal-organic framework MOF-74. The synthesis conditions were adjusted to obtain MOF-74 with a high water adsorption capacity and stability. The results show that MOF-74-MgNi-2 has the maximum water adsorption capacity. At a low moisture pressure P/P₀ = 0.1, its water adsorption capacity is 0.44 g/g, 0.12 g/g, and 0.10 g/g greater than that of MOF-74-Mg and MOF-74-Ni, respectively. Its saturated water adsorption capacity at P/P₀ = 0.9 is as high as 0.62 g/g, which is 1.3 times that of MOF-74-Ni and 1.1 times that of MOF-74-Mg, respectively. Its superior water adsorption capacity is explained by the combination of experiment and molecular simulation, which takes into account the pore structure and electrostatic potential energy distribution. Its thermal breakdown temperature is greater than 250 °C. Its water adsorption capacity decreases by only 9.0% in the 10th cycle. Under conventional refrigeration conditions, its refrigeration coefficient and working capacity are 0.75 and 0.43 cm³/cm³, respectively, which are greater than those of the majority of the regularly used adsorbents. In addition, it satisfies the primary technological goals of adsorption-based thermal batteries with a heat storage capacity of up to 1394 kJ/kg. The mixed-metal method is shown to be useful in the design of high-performance MOF-74 for solar thermal energy storage and heat transformation.