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

Water scarcity presents a major global challenge for sustainable development. Atmospheric moisture is an abundant potential water source, and sorption-based atmospheric water harvesting (SAWH) can capture water from air across climates using water-affinity sorbents and solar energy. Although various high-performance sorbents such as MOFs, COFs, and salt-based composites have been developed, practical SAWH devices often suffer from low water productivity due to slow sorption/desorption kinetics arising from poor heat transfer and mass transport in packed sorbent beds. Improving system-level performance requires innovations at both the material and device levels, targeting sorbents with high water uptake and climate adaptability, and device architectures that enhance heat and mass transport while improving energy efficiency. This work analyses the structure–performance relationship of packed sorbents and introduces bidirectionally aligned and hierarchically structured nanocomposites (BHNC) designed to reduce diffusion depth and tortuosity, thereby accelerating water transport and kinetics for efficient SAWH.

The paper surveys prior advances in sorbent materials for SAWH, including metal-organic frameworks, covalent organic frameworks, and salt-based composites, highlighting their benefits such as high water uptake and adaptability. Salt-based composites, formed by confining hygroscopic salts in porous matrices, are attractive for their low cost and capacity but are hindered by slow kinetics in packed configurations due to increased diffusion resistance and poor heat transfer. Prior device designs often rely on unordered sorbent packing, resulting in long diffusion paths and high tortuosity. Modified structures (e.g., vertically aligned composites with low tortuosity and honeycomb-structured hygroscopic polymers with reduced diffusion depth) have shown improvements, but comprehensive optimization of both diffusion depth and tortuosity is needed. The paper positions its contribution within this context by proposing and validating bidirectional, ordered hierarchical structures to overcome mass transport and heat transfer bottlenecks in scalable devices.

The study integrates theoretical modeling, materials synthesis, characterization, kinetics testing, and device prototyping. Theoretical models: Numerical models for unordered, unidirectional, and bidirectional packed structures were developed to quantify mass transport resistance (R_m,total) and heat transfer resistance (R_T,total), validated against experimental data. Models isolate the dominance of diffusion resistance and evaluate the impact of diffusion depth and tortuosity on sorption kinetics. Materials synthesis: A bidirectionally aligned porous graphene hydrogel matrix (BPGHM) was fabricated via bidirectional freeze-casting using a copper template with arrayed copper pillars to impose vertical (ΔT_v) and radial (ΔT_r) temperature gradients. Steps: prepare GO–sodium alginate (SA) solution; pour into mould; freeze with liquid nitrogen; freeze-dry; Ca2+ cross-linking (CaCl2, 2 wt%, 24 h) to strengthen; thermal reduction at 120 °C to form BPGHM; LiCl loading by immersion in 10 wt% solution and drying to yield BHNC. Control samples with unordered and unidirectional structures were prepared using low-conductivity moulds and non-directional freezing. Characterization: SEM and EDX mapping assessed morphology and LiCl distribution; mercury porosimetry measured pore size distributions; XPS determined surface chemistry (C/O≈3.0); XRD confirmed multi-step sorption states; contact angle measurements assessed wettability (BPGHM ~59°, BHNC ~6°). The BPGHM porosity is ~97%, with BHNC showing LiCl coating occupying 20–100 μm pores while leaving 100–1,000 μm channels for transport; LiCl loading ~69 wt%. Sorption measurements: Water sorption isotherms measured with ASAP 2020 at various RH and temperatures; TG-DSC and TGA (STA 449 C with MHG 32) quantified multi-step sorption at 30 °C (15%, 30%, 60% RH) and desorption at 90 °C, 4.2 kPa; high-RH capacity assessed at 30 °C, 90% RH; cyclic stability tested under low and high RH conditions. Kinetics testing: Custom rig controlled airflow velocity, temperature, and RH; sorption/desorption kinetics of single and multiple BHNC packed units (four blocks per unit) tested at 30 °C, 30% RH for sorption and 90 °C air for desorption; scalability evaluated by assembling up to three units in series; airflow variations examined. Device prototyping: A solar-driven SAWH prototype assembled with 24 BHNC blocks (six units) included a solar air collector (20 evacuated tubes), heat recovery air-to-air exchanger, and air-cooled condenser. An energy-saving closed-cycle with heat recovery was implemented, transferring heat from hot desorption air to preheat inlet air and pre-cooling condenser inlet air. Sensors logged temperatures and humidities; indoor tests used an electric heater; outdoor tests used solar heating. Water quality of collected condensate was analysed via ICP and ion chromatography.

- BHNC structure and transport: Modeling and experiments show diffusion resistance dominates mass transport; ordering the packed structure reduces resistances. Unidirectional structures have ~1/10 the mass transport resistance of unordered, and bidirectional structures further reduce it by ~60% relative to unidirectional. Heat transfer resistance similarly decreases with ordering. Bidirectional structures shorten effective diffusion depth and reduce tortuosity, maintaining performance with increased thickness. - Sorption capacity: Multi-step sorption (chemisorption, deliquescence, absorption) yields high uptake across climates: 0.90 g_water g_sorbent^−1 at 15% RH, 1.36 g g^−1 at 30% RH, 2.36 g g^−1 at 60% RH, and up to 6.61 g g^−1 at 90% RH (30 °C). BHNC outperforms state-of-the-art sorbents across RH ranges. - Kinetics: BHNC exhibits faster sorption kinetics than reported salt-based composites under arid conditions (20–40% RH). In scalable packed units (four blocks/unit), equilibrium sorption and desorption times are ~180 min and ~60 min, respectively, at 30 °C/30% RH (sorption) and ~90 °C air (desorption). With three units in series (100×100×120 mm), sorption/desorption complete in <210 min/<80 min under the same conditions. Higher airflow further accelerates kinetics. - Materials parameters: BPGHM porosity ~97%; LiCl loading ~69 wt%; LiCl forms submicrometre coatings uniformly on pore surfaces; hydrophilicity improved (contact angle ~6° for BHNC). - Device performance and energy efficiency: The closed-cycle SAWH with heat recovery improves thermal efficiency by about 2–4× versus conventional systems, reducing condenser cooling load and solar heating demand. Indoor rapid-cycling tests at 60% RH delivered up to 2,820 ml_water kg_sorbent^−1 day^−1 over 10 h. Outdoor solar-driven tests achieved up to 2,000 ml_water kg_sorbent^−1 day^−1 even at <30% RH (30 °C). - Water quality: No lithium detected in collected water; ion concentrations meet WHO drinking-water standards. - Stability: BHNC maintains shape with no solution leakage after 12 h sorption at 90% RH; exhibits stable multi-cycle sorption/desorption behavior.

The work addresses the longstanding bottleneck of slow sorption/desorption kinetics in packed sorbents by elucidating the structure–performance relationship and demonstrating that bidirectionally ordered hierarchical architectures minimize diffusion depth and tortuosity while enhancing heat transfer. Combining vertically oriented millimetre-scale convective channels with radially oriented micrometre-scale diffusion pathways synergistically reduces mass and thermal resistances, resulting in ultrafast kinetics and high uptake across humidities. System-level integration with a heat recovery cycle further translates material advantages into energy-efficient, rapid-cycling water harvesting, achieving record water productivities over a wide RH range while maintaining potable water quality. These findings bridge materials design and device engineering, showing a viable pathway for scalable, all-weather, solar-powered atmospheric water harvesting.

The study introduces a facile, scalable strategy to fabricate bidirectionally aligned and hierarchically structured nanocomposites (BHNC) that deliver ultrahigh water uptake (up to 6.61 g g^−1 at 90% RH) and ultrafast sorption/desorption kinetics via synergistic enhancements in heat and mass transport. By assembling BHNC arrays into a closed-cycle SAWH device with heat recovery, the system realizes rapid multi-cycle operation and high water yields of 2,000–2,820 ml kg_sorbent^−1 day^−1 across diverse climates, with improved thermal efficiency and safe water quality. The approach provides a blueprint for next-generation SAWH materials, device architectures, and energy-efficient system designs for scalable atmospheric water harvesting.