Water scarcity is a pressing global challenge that threatens the sustainable development of human society. According to the United Nations, over 2 billion people lack access to safe drinking water, and this number is expected to rise due to population growth, urbanization, and climate change. To address this challenge, researchers have been exploring various approaches to harvest water from unconventional sources, such as air, fog, and dew. Among these approaches, sorbent-based atmospheric water harvesting (SAWH) has gained increasing attention due to its potential to provide a decentralized, renewable, and low-cost source of water.
However, SAWH faces several technical and practical challenges that limit its scalability and efficiency. One of the main challenges is the low water production rate, mainly due to the slow kinetics of water sorption and desorption in the sorbent material. While the past decade has seen substantial developments in new sorbents, the water productivity of reported SAWH devices remains low due to poor heat transfer and mass transport in packed sorbents, limiting their practical applications. Addressing these issues requires comprehensive innovations at both the material and device levels, aiming for superior SAWH sorbents, scalable devices with excellent heat and mass transfer, and energy-efficient systems.
To overcome these challenges, we developed a novel strategy to synthesize bidirectional-aligned and hierarchical-structured nanocomposites (BHNC) for efficient water production from air (Fig. 1a). Unordered structures, common in SAWH devices, exhibit high diffusion resistance, while modified unidirectional structures, such as vertically-aligned nanocomposites and honeycomb-structured hygroscopic polymer, aim to decrease diffusion resistance. The key lies in regulating diffusion depth and tortuosity factor.
Theoretical models and experimental results indicate that diffusion resistance plays a dominant role in water transport, and improving the orderliness of packed structures significantly reduces mass transport and heat transfer resistances (Fig. 1b-c). The unidirectional structure has a mass transport resistance of only 1/10 of the unordered structure, and this can be further lowered by 60% with a bidirectional structure (Fig. 1d). Additionally, the transition from unordered to unidirectional or bidirectional structures significantly reduces the overall heat transfer resistance (Fig. 1e). Combined with hygroscopic salt, this structure results in high water uptake of 1.36 gwater·gsorbent-1 (30%RH) and 6.61 gwater·gsorbent-1(90%RH), indicating the climate adaptability of BHNC for all-weather water harvesting from air. In addition, the ultrafast water sorption kinetics of BHNC surpasses the capabilities of reported salt-based sorbents, due to its short diffusion depth and low tortuosity factor.
Fig. 1 Design of packed sorbents with superior water transport kinetics. a, Schematic of water sorption from air and water desorption from sorbent during water harvesting. b, Schematic of multi-step moisture transport during water sorption process. Water molecules transport from the ambient air to the external surface of sorbents by undergoing surface resistance (Rsurf), which can be tuned by the external airflow rate. Subsequently, the water molecules further diffuse along the connected intra pores by overcoming diffusion resistance (Rsorb), which is mainly affected by the tortuosity factor (τ), the diffusion depth (δsorb), and packed porosity (e). Finally, the water molecules are captured by the sorption sites of sorbents, mainly determined by the intrinsic reaction resistance (Rreact). c, Morphological evolution from unordered structures to ordered structures of packed sorbents with enhanced heat transfer and mass transport. d, Numerical results of the total mass transport resistances of different packed structures. e, Numerical results of the total heat transfer resistances of different packed structures.
We further engineered a scalable and efficient solar-driven SAWH prototype with scalable BHNC blocks. The BHNC block facilitates water movement through various channels and captures water molecules at sorption sites, offering the fastest sorption kinetics and lowest mass transport resistance. Theoretical models were employed to analyze the influential mechanism of heat-mass transfer resistance, revealing that diffusion resistance and thermal resistance play dominant roles in the multi-step mass transport and heat transfer processes, respectively. This understanding aids in the timely removal of sorption heat and quick moisture supply, accelerating water sorption/desorption kinetics. The sorption and desorption processes of a single BHNC-packed unit reach equilibrium in 180 and 60 minutes, respectively. When three units are assembled in series, these processes are completed in under 210 and 80 minutes, respectively, maintaining efficiency even at a larger scale.
The integration of twenty-four BHNC blocks into a solar-driven SAWH prototype, assisted by the energy-saving design in heat-recovery device units, demonstrated rapid-cycling and high-yielding water production up to 2820 mLwater·kgsorbent-1·day-1. This work presents a scalable material and device system in the field and represents a significant step forward for advancing sustainable water harvesting technologies.