Thermodynamic limits of atmospheric water harvesting with temperature-dependent adsorption

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Abstract

Adsorption-based atmospheric water harvesting (AWH) has vast potential for addressing global water shortage. Despite innovations in adsorbent materials, fundamental understanding of the physical processes involved in the AWH cycle and how material properties impact the theoretical limits of AWH is lacking. Here, we develop a generalized thermodynamic framework to elucidate the interplay between adsorbent properties and operating conditions for optimal AWH performance. Our analysis considers the temperature dependence of adsorption, which is critical but has largely been overlooked in past work. Using metal-organic framework (MOF) as an example, we show that the peak energy efficiencies of single-stage and dual-stage AWH devices, after considering temperature-dependent adsorption, increased by 30% and 100%, respectively, compared with previous studies. Moreover, in contrast to common understanding, we show that the adsorption enthalpy of MOFs can also be optimized to further improve the peak energy efficiency by 40%. This work bridges an important knowledge gap between adsorbent materials development and device design, providing insight toward high-performance adsorption-based AWH technologies.

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Water harvesting from air with metal-organic frameworks powered by natural sunlight.

Hyunho Kim, Sungwoo Yang, Sameer Rao … (2017)

Atmospheric water is a resource equivalent to ~10% of all fresh water in lakes on Earth. However, an efficient process for capturing and delivering water from air, especially at low humidity levels (down to 20%), has not been developed. We report the design and demonstration of a device based on a porous metal-organic framework {MOF-801, [Zr6O4(OH)4(fumarate)6]} that captures water from the atmosphere at ambient conditions by using low-grade heat from natural sunlight at a flux of less than 1 sun (1 kilowatt per square meter). This device is capable of harvesting 2.8 liters of water per kilogram of MOF daily at relative humidity levels as low as 20% and requires no additional input of energy.

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Reassessing the projections of the World Water Development Report

Alberto Boretti, Lorenzo Rosa (2020)

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Tuning the Topology and Functionality of Metal−Organic Frameworks by Ligand Design

Dan Zhao, Daren Timmons, Daqiang Yuan … (2011)

Metal-organic frameworks (MOFs)-highly crystalline hybrid materials that combine metal ions with rigid organic ligands-have emerged as an important class of porous materials. The organic ligands add flexibility and diversity to the chemical structures and functions of these materials. In this Account, we summarize our laboratory's experience in tuning the topology and functionality of MOFs by ligand design. These investigations have led to new materials with interesting properties. By using a ligand that can adopt different symmetry conformations through free internal bond rotation, we have obtained two MOFs that are supramolecular stereoisomers of each other at different reaction temperatures. In another case, where the dimerized ligands function as a D(3)-Piedfort unit spacer, we achieve chiral (10,3)-a networks. In the design of MOF-based materials for hydrogen and methane storage, we focused on increasing the gas affinity of frameworks by using ligands with different geometries to control the pore size and effectively introduce unsaturated metal centers (UMCs) into the framework. Framework interpenetration in PCN-6 (PCN stands for porous coordination network) can lead to higher hydrogen uptake. Because of the proper alignment of the UMCs, PCN-12 holds the record for uptake of hydrogen at 77 K/760 Torr. In the case of methane storage, PCN-14 with anthracene-derived ligand achieves breakthrough storage capacity, at a level 28% higher than the U.S. Department of Energy target. Selective gas adsorption requires a pore size comparable to that of the target gas molecules; therefore, we use bulky ligands and network interpenetration to reduce the pore size. In addition, with the help of an amphiphilic ligand, we were able to use temperature to continuously change pore size in a 2D layer MOF. Adding charge to an organic ligand can also stabilize frameworks. By ionizing the amine group within mesoMOF-1, the resulting electronic repulsion keeps the network from collapsing, giving rise to the first case of mesoporous MOF that demonstrates the type IV isotherm. We use dendritic hexacarboxylate ligands to synthesize an isoreticular series of MOFs with (3,24)-connected network topology. The cuboctahedral cages serve as building blocks that narrow the opening of the mesocavities into microwindows and stabilize these MOFs. The resulting materials have exceptionally high surface areas and hydrogen uptake capacities. Despite the many achievements in MOF development, there is still ample opportunity for further exploration. We will be continuing our efforts and look forward to contributing to this blossoming field in the next decade.