Optimizing Sorbent Coatings for Compact Atmospheric Water Harvesting

Original Authors: Nathan Ortiz and Sameer Rao

Original paper is accessible at: https://doi.org/10.3389/frwa.2025.1606252

Atmospheric Water Harvesting (AWH) remains a promising technology for providing safe drinking water in regions with limited freshwater availability. Sorption-based AWH (SAWH) systems use materials such as zeolites and metal-organic frameworks (MOFs) to capture water vapor from the air. While research has advanced in developing high-capacity sorbents, system-level packaging and integration strategies often lag behind. Thin sorbent coatings, with inherently fast adsorption kinetics, offer a practical solution but typically suffer from low coating weights. This study introduces a silane-based coating method to optimize sorbent packaging and performance for compact AWH devices.

Methodological Framework

Researchers developed a two-step silane coating process, using zeolite AQSOA Z02 as the sorbent material, deposited on thin aluminum substrates. Coating thickness was controlled by sequential dipping and curing steps, producing single-, double-, and triple-layer samples.

• Thickness control: 229.4 μm (1 coat), 558.3 μm (2 coats), and 837.1 μm (3 coats).

• Characterization: Coatings were tested with dynamic vapor sorption (DVS) to measure water uptake and kinetic response at 25°C and 40% relative humidity.

• System projection: The coatings were modeled in a finned aluminum heat exchanger to estimate volumetric productivity and overall system performance.

Results and Implications

• Sorption Performance:Thinner coatings exhibited faster sorption kinetics, cycling more frequently per day. A single coat achieved a productivity of 6.68 kg water/kg coating/day, while thicker coatings slowed kinetics but increased coating weight.

• Optimal Thickness:Two coatings struck the best balance, achieving a maximum volumetric productivity of 458 kg water/m³ AHX/day, outperforming both thinner and thicker samples.

• Stability and Cycling:Multi-cycle testing confirmed consistent sorption capacity (~0.29 kg water/kg coating) with minimal variability, demonstrating strong mechanical and chemical stability.

• System Projections:A heat exchanger design with two coatings could produce the required 15 L/day for household-scale needs with the smallest system mass (21.4 kg) and volume, highlighting practical feasibility.

• Trade-offs:

  • Thinner coatings enable higher cycling frequency but require larger system footprints.

  • Thicker coatings provide more sorbent mass but slow down adsorption-desorption rates.

  • Two coatings provide the optimal compromise between performance and compactness.

Conclusion

This work demonstrates that silane-based sorbent coatings provide an effective method for compactly packaging sorbent powders while maintaining rapid sorption kinetics. By adjusting coating thickness, AWH devices can be tailored for efficiency, system mass, and footprint. The optimal two-layer design balances sorption kinetics and coating weight, enabling high volumetric productivity and practical system integration. Future development may include higher-performance sorbents and structural reinforcements for multi-layer coatings.

Overall, this study provides a blueprint for next-generation compact AWH devices, where material science and system engineering converge to enhance water productivity and sustainability.