Scalable and efficient solar-driven atmospheric water harvesting enabled by bidirectionally aligned and hierarchically structured nanocomposites

Water scarcity is a critical global challenge. Atmospheric water harvesting (SAWH), particularly solar-driven SAWH, offers a sustainable solution by extracting water from the abundant atmospheric moisture. While high-performance sorbents like metal-organic frameworks, covalent organic frameworks, and salt-based composites exist, slow sorption/desorption kinetics in packed sorbents limit efficiency. This slowness stems from poor heat and mass transport within the densely packed sorbent particles. The challenge lies in accelerating water sorption/desorption kinetics by understanding the structure-performance relationship of these packed sorbents and improving SAWH device design. This paper focuses on addressing this challenge by developing a novel material and device design that synergistically improves water uptake and transport within a SAWH system. The research hypothesizes that a bidirectionally aligned and hierarchically structured nanocomposite (BHNC) will significantly enhance water sorption and desorption kinetics, leading to improved SAWH efficiency and scalability. This hypothesis is based on the idea that strategically designed structures can optimize both water convection (vertical movement) and intrapore diffusion (radial movement), thus minimizing transport resistance.

Existing research on SAWH highlights the potential of various sorbents. Metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and salt-based composites have shown promise due to their high water uptake capacity. However, a common limitation is slow sorption/desorption kinetics, hindering practical applications. Studies have explored the use of modified structures such as vertically aligned nanocomposites and honeycomb structures to reduce diffusion resistance. However, these approaches often involve trade-offs between water uptake and kinetics. The literature lacks comprehensive innovations at both material and device levels to achieve truly scalable, energy-efficient, and high-yielding SAWH. This study aims to address this gap by designing a novel sorbent with an optimized structure and a device that enhances heat and mass transfer.

The researchers developed a facile synthesis strategy for BHNC blocks with ordered hierarchical structures. This involved creating a bidirectionally aligned porous graphene hydrogel matrix (BPGHM) using a bidirectional freezing assembly process with a specially designed copper template. The copper template introduces a radial temperature gradient alongside the typical vertical gradient, resulting in a highly ordered hierarchical structure with both vertically and radially oriented microchannels. After freeze-drying, a crosslinking process using calcium ions enhances the BPGHM's strength. Finally, lithium chloride (LiCl) is loaded onto the BPGHM to create the BHNC, where submicrometer LiCl crystals are evenly coated on the BPGHM surface, avoiding salt agglomeration. Characterization techniques included scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and mercury porosimetry. The water sorption performance of BHNC was evaluated using various methods, including isotherm measurements, thermogravimetric analysis (TGA), and a custom-built device to assess the sorption kinetics of the scalable BHNC blocks under controlled airflow, temperature and relative humidity. The device for testing the sorption kinetics contained temperature and humidity sensors, a wind velocity sensor, and an electronic balance to monitor changes in water uptake in real time. Finally, a prototype scalable solar-driven SAWH device was constructed, integrating multiple BHNC blocks, a solar air collector, a heat recovery exchanger, and an air-cooled condenser. This prototype was tested under both indoor and outdoor conditions to evaluate water production. The performance of the SAWH system was analyzed using theoretical calculations to optimize energy efficiency.

The BHNC exhibited superior water uptake capacity across a wide range of relative humidity (RH) conditions. It achieved 0.9 g_water g_sorbent^-1 at 15% RH, 1.36 g_water g_sorbent⁻¹ at 30% RH, 2.36 g_water g_sorbent⁻¹ at 60% RH, and an ultrahigh 6.61 g_water g_sorbent⁻¹ at 90% RH. This multi-step water sorption involved chemisorption, deliquescence, and absorption. The BHNC also showed remarkably fast sorption kinetics, surpassing those of state-of-the-art salt-based composites. The bidirectional structure was crucial in achieving this, as evidenced by comparative experiments with unordered and unidirectional structures. The scalable BHNC blocks, when assembled into arrays, maintained fast sorption/desorption kinetics. Even with three packed units in series, equilibrium was reached within 210 min for sorption and 80 min for desorption. The solar-driven SAWH prototype, employing 24 BHNC blocks and a heat recovery system, demonstrated rapid cycling and high water production. Indoor tests yielded 2,820 ml water kg_sorbent^-1 day^-1, while outdoor tests achieved 2,000 ml water kg_sorbent^-1 day^-1 even at low RH (below 30%). The collected water was analyzed and found to meet World Health Organization (WHO) drinking water quality standards, confirming the absence of LiCl leakage.

The results demonstrate the success of the hypothesized bidirectional structure in enhancing SAWH performance. The ultrahigh water uptake and fast sorption/desorption kinetics of BHNC directly address the limitations of previous SAWH technologies. The synergistic effects of the vertically oriented channels (for convection) and radially oriented channels (for diffusion) were essential in minimizing transport resistance. The scalable design of the BHNC blocks and the prototype SAWH device with heat recovery represent significant advancements toward practical, large-scale atmospheric water harvesting. The achievement of high water production under both indoor and outdoor conditions, including low-humidity environments, expands the applicability of this technology to a wider range of climates. The successful implementation of a heat recovery cycle further improves the energy efficiency of the system, making it more sustainable.

This research successfully synthesized BHNC with ultrahigh water uptake and ultrafast sorption kinetics. A scalable solar-driven SAWH prototype was engineered, achieving high water production. The bidirectional structure, along with the heat recovery system, resulted in a highly efficient and scalable SAWH technology. Future research could explore alternative hygroscopic salts, further optimize the BHNC structure, and improve the efficiency of the solar air collector to enhance the overall performance and cost-effectiveness of the SAWH system. Investigating the long-term durability and stability of the BHNC under various environmental conditions is also crucial for practical applications.

While the study demonstrates significant advancements in SAWH, some limitations exist. The long-term durability and stability of the BHNC under continuous operation require further investigation. The cost-effectiveness of large-scale production and implementation of the BHNC-based SAWH system needs to be assessed. The current prototype's performance might be sensitive to variations in solar irradiance and ambient conditions, requiring further optimization for consistent performance across diverse climates.