Water scarcity is a pressing global issue, affecting approximately 2.2 billion people. This crisis is exacerbated by population growth and environmental pollution, with projections indicating that over half the world's population will face clean water shortages by 2050. Traditional water collection methods like rainwater harvesting, artificial precipitation, and desalination are often impractical in water-stressed regions, particularly in arid and landlocked areas. Atmospheric water harvesting (AWH) presents a viable alternative, given the vast amount of atmospheric moisture—12,900 cubic kilometers, six times the volume of all rivers globally. Several AWH technologies exist, including chiller-driven dew collection, radiative cooling-driven condensation, fog harvesting, and sorption-based AWH (SAWH). SAWH is particularly efficient in arid climates with low relative humidity (RH) due to the high water vapor affinity of sorbents. While solar-driven SAWH is a promising approach, its water productivity lags behind other solar-driven technologies like interfacial evaporation or desalination. This low efficiency stems from low water sorption capacity at low RH and the low thermal efficiency of current SAWH devices. Metal-organic frameworks (MOFs) offer superior water sorption capacity and lower regeneration temperatures compared to traditional sorbents like silica gel and zeolites. Despite advancements in sorbent materials such as MOFs, hydrogels, ion solutions, and salt-based composites, the inherent endothermic/exothermic nature of sorption/desorption processes demands significant energy input, hindering SAWH device efficiency. Integrating power generation into SAWH systems addresses this, enabling efficient utilization of the sorption/desorption heat. In arid regions, electricity scarcity compounds the water shortage. Solar-driven hybrid systems combining water production (via thermal evaporation or distillation) and power generation (using photovoltaics or salinity gradients) have been explored, but these are often infeasible due to persistent water scarcity. A promising approach is coupling solar-driven SAWH with thermoelectric generators (TEGs) for simultaneous water and power production. However, the intermittent nature of sunlight restricts nighttime power generation in such systems. This study aims to address these challenges by introducing a moisture-induced energy harvesting strategy capable of delivering both water and electricity around the clock.

The literature extensively documents the challenges of water scarcity and the potential of atmospheric water harvesting. Various technologies have been explored, including those based on radiative cooling and hydrophilic surfaces, but sorption-based AWH (SAWH) using materials like metal-organic frameworks (MOFs) has shown particular promise for arid climates due to its ability to extract water vapor even at low relative humidity. However, the efficiency of SAWH is often limited by the energy required for the sorption/desorption cycles. Several studies have explored the use of solar energy to drive SAWH, but the overall efficiency has remained relatively low. The integration of thermoelectric generators (TEGs) with SAWH has been proposed as a way to improve efficiency by utilizing the heat generated or absorbed during the sorption/desorption process. Research on TEGs, particularly those utilizing radiative cooling for nighttime operation, has also advanced. However, the relatively small temperature differences achievable with radiative cooling alone limit the power output of these devices. This paper builds upon the existing literature by proposing a synergistic approach that combines SAWH, TEGs, and both solar heating and radiative cooling to achieve continuous water production and power generation.

This research introduces a novel moisture-induced energy harvesting strategy, combining sorption-based atmospheric water harvesting (SAWH) and 24-hour thermoelectric power generation (TEPG). The core of the system is a hybrid SAWH-TEPG device, carefully designed to synergistically leverage the thermal effects of moisture sorption/desorption, solar energy, and radiative cooling. The device comprises several key components: a dual-functional coating layer (acting as both a solar absorber and infrared emitter), a thermoelectric generator (TEG) module, a SAWH module containing the sorbent material, and an air-cooling condenser. The SAWH module is thermally coupled to the TEG module through an aluminum block to facilitate efficient heat transfer. The entire assembly is insulated and covered with a polyethylene membrane to minimize energy losses. MIL-101(Cr), a metal-organic framework known for its high water uptake, fast sorption kinetics, and low desorption temperature, was selected as the sorbent. To enhance heat and mass transfer, MIL-101(Cr) powder was coated onto a copper foam substrate (MIL-101(Cr)@CF composite) using an in-situ impregnation method. The morphology, sorption performance, thermal stability, and thermal conductivity of the MIL-101(Cr)@CF composite were thoroughly characterized using various techniques including Scanning Electron Microscopy (SEM), Nitrogen adsorption isotherms, Thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC), and thermal conductivity measurements. A dual-functional coating layer, exhibiting high solar absorbance (~95%) and high emissivity (~90% in the 8-13 µm range), was designed for efficient solar thermal collection during the day and radiative cooling at night. The research involved both indoor and outdoor experiments. Indoor experiments used simulated conditions to evaluate performance under controlled humidity, temperature, and solar intensity. Outdoor experiments were conducted on a rooftop to assess real-world performance. Temperature profiles, open-circuit voltage (Voc), short-circuit current (Isc), water production, and power output were monitored and analyzed to validate the synergistic effects of moisture-induced energy harvesting. Specific equations were used to calculate thermoelectric power density and solar-driven water harvesting efficiency. The experimental design included a control device without the MIL-101(Cr)@CF sorbent to highlight the contributions of the moisture-induced energy harvesting strategy.

The study demonstrated that the synergistic integration of SAWH and TEPG significantly enhances both water production and power generation. During daytime operation, the endothermic nature of moisture desorption from the MIL-101(Cr)@CF sorbent cools the cold side of the TEG, increasing the temperature difference and boosting power output. The solar heating further enhances this effect, resulting in impressive thermoelectric power densities. At night, the exothermic process of moisture sorption provides heat to the TEG's hot side, while radiative cooling lowers the cold side. The combination of sorption heating and radiative cooling creates a substantial temperature difference, enabling nighttime power generation. The heat consumed by the TEG module during nighttime operation accelerates moisture sorption. Indoor experiments under simulated conditions confirmed these synergistic effects. The hybrid SAWH-TEPG device demonstrated a 346% improvement in thermoelectric power density compared to a conventional radiative cooling-based TEG design during nighttime. During daytime operation under one standard sun (1000 W m⁻²), the average open-circuit voltage (Voc) increased from 492.3 mV to 571.9 mV due to the cooling effect of moisture desorption. The water production rate reached 150 g m⁻² h⁻¹ and further increased with higher solar intensities. Outdoor experiments confirmed the continuous operation and high performance of the hybrid system. The device showed consistent water collection and power generation over a week, achieving a maximum Voc of 505 mV and an average water uptake of 800 g m⁻². The daytime water harvesting efficiency (η) was calculated as 21.7%, and annual water production was estimated to range from 295.7 to 612.6 L m⁻² depending on location climate. The maximum daytime power density reached 685 mW m⁻², while the nighttime maximum reached 21 mW m⁻².

The findings demonstrate the feasibility and efficacy of the proposed moisture-induced energy harvesting strategy for simultaneous water production and 24-hour power generation. The synergistic effects observed significantly outperform conventional systems, offering a sustainable and potentially transformative solution for water and electricity scarcity in remote areas. The high water production and power densities achieved are competitive with other state-of-the-art technologies. The ability to operate continuously without energy storage is a significant advancement. The results are highly relevant to the fields of water resource management, renewable energy, and sustainable development. This work provides a compelling example of how leveraging natural resources and thermodynamic principles can address critical global challenges. The device's modular design suggests scalability and adaptability to various environmental conditions. Future studies could focus on optimizing sorbent materials, improving device design for enhanced heat and mass transfer, and exploring the integration of advanced energy storage solutions to further enhance performance.

This research successfully demonstrated a novel moisture-induced energy harvesting strategy for simultaneous atmospheric water production and 24-hour power generation. A hybrid SAWH-TEPG device, leveraging the synergistic effects of solar heating, radiative cooling, and moisture sorption/desorption, achieved high water production (750 g m⁻²) and impressive power densities (685 mW m⁻² daytime, 21 mW m⁻² nighttime). This sustainable and continuous system offers a significant advancement towards addressing water and electricity shortages in arid and remote regions. Future research could focus on material optimization, device design refinements, and system scalability for wider implementation.

While the study demonstrates significant advancements, some limitations exist. The long equilibration time between sorption and desorption processes suggests room for improvement in sorbent and device design. The outdoor experiments were conducted in a specific location (Shanghai, China), and the generalizability of the results to other climatic conditions requires further investigation. The long-term durability and stability of the materials and the device under various environmental conditions still need further evaluation.