Efficient water capture under low humidity using Ni-modified MOF-5: scalable atmospheric water harvesting systems

Atmospheric Water Harvesting (AWH) has advanced as a sustainable, energy-efficient, and cost-effective strategy to address global water scarcity. Among AWH techniques, sorption-based systems (SBAWH) offer significant advantages; however, their practical deployment is limited by the lack of efficient and scalable sorbent materials capable of high-water uptake under low to moderate relative humidity (RH) conditions. This study investigates the synthesis and performance of pristine MOF-5 and its nickel-doped derivatives (20% and 40% Ni-MOF-5), fabricated <i>via</i> a direct mixing approach, for application in SBAWH systems. The novelty of this work lies in the systematic comparison of pristine and Ni-substituted MOF-5 sorbents for enhanced water adsorption under low RH environments, an area that remains underexplored. Comprehensive material characterization was conducted using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) to clarify the structural and compositional attributes. Water sorption isotherms were obtained using a custom-built AWH prototype operating under controlled RH conditions (35-75%). At RH levels of 65%, 35%, and 55%, the water uptake capacities of MOF-5, 20% Ni-MOF-5, and 40% Ni-MOF-5 were 151, 162, and 142 mg g^-1, respectively. Thermodynamic analyses indicated that the adsorption process is spontaneous and exothermic, while isotherm and kinetic modeling confirmed a chemisorption dominated mechanism. The superior performance of 20% Ni-MOF-5, particularly under low humidity conditions, highlights its promise as a scalable and effective sorbent for next generation SBAWH systems.