Toward Low-Energy Atmospheric Water Harvesting Through Vertical Pore Engineering

Original Authors: Beibei Yang, Zhenrong Tan, Conghua Yi, Siyu Yang, Xueqing Qiu, Dafeng Zheng

Original paper is accessible at: https://doi.org/10.1002/adfm.202514160

The hidden challenge in sorption-based AWH

Sorption-based atmospheric water harvesting (SAWH) is increasingly recognized as one of the most promising technologies for freshwater production in arid and semi-arid regions. Unlike condensation-based systems, sorption-driven AWH can operate under low humidity conditions with lower infrastructure requirements.

However, one major limitation continues to slow practical deployment:

👉 High desorption energy consumption

Many hygroscopic materials can adsorb large amounts of water, but releasing that water requires substantial thermal input. In addition, traditional sorbents often suffer from:

• Slow adsorption/desorption kinetics

• Salt leakage and agglomeration

• Poor structural stability

• High diffusion resistance inside porous networks

This study addresses these challenges through a bioinspired vertically layered composite architecture that simultaneously improves:

⚡ Adsorption kinetics🌡️ Desorption efficiency💧 Water productivity🔋 Energy recovery

A vertically layered biomass composite design

The researchers developed a new hygroscopic composite based on:

• Sodium alginate (SA)

• Calcium lignosulfonate (CLS)

• Polypyrrole (PPy) for photothermal conversion

• LiCl as the hygroscopic salt

The material was fabricated using:

❄️ Directional freeze-drying➡️ Vertically aligned pore formation🧱 In situ crosslinking

This process created a hierarchical vertically stratified pore structure, allowing rapid water-vapor transport while stabilizing LiCl inside the porous network.

Unlike random porous systems, the vertically aligned channels significantly reduced internal vapor diffusion resistance during both adsorption and desorption.

Why the vertically layered structure matters

The study demonstrates that pore architecture—not only chemistry—is a dominant factor controlling SAWH performance.

During adsorption:

• Vertically aligned pores accelerate vapor diffusion

• Superhydrophilic surfaces rapidly capture water molecules

• Capillary transport enhances water uptake

During desorption:

• Continuous channels provide low-resistance escape pathways

• Reduced hydrogen-bond confinement lowers desorption energy

• Faster vapor release improves cycle efficiency

This structural optimization enabled the material to achieve both:

✅ High adsorption capacity✅ Fast adsorption–desorption kinetics

—a combination rarely achieved simultaneously in SAWH systems.

Exceptional adsorption performance across wide humidity ranges

One of the strongest contributions of the work is the material’s performance under extremely broad humidity conditions.

The optimized composite (SP0.5@LiCl-10%) achieved:

• 💧 0.63 g/g at 7% RH

• 💧 0.76 g/g at 11% RH

• 💧 1.59 g/g at 30% RH

• 💧 3.51 g/g at 60% RH

• 💧 5.75 g/g at 95% RH 

Importantly, even under nearly desert-like conditions (7% RH), the material still efficiently captured atmospheric moisture.

The adsorption equilibrium was reached within approximately 5 hours, demonstrating rapid adsorption kinetics compared to many conventional sorbents.

Hydrogen-bond engineering reduces desorption energy

A particularly interesting aspect of this study is the investigation of water states inside the gel network.

Using:

• Raman spectroscopy

• Differential scanning calorimetry (DSC)

• Molecular dynamics simulations

the authors showed that the vertically layered porous structure promotes formation of:

💧 Intermediate water (IW)rather than strongly bound water.

This is critical because weakly hydrogen-bonded water requires significantly less energy for evaporation.

As a result:

• Desorption heat decreased substantially compared with pure LiCl systems

• Water molecules evaporated more rapidly

• Overall regeneration energy demand was reduced

The study directly links pore architecture → hydrogen bonding → desorption energy, providing an important mechanistic insight for future SAWH material design.

Photothermal desorption performance

The incorporation of polypyrrole (PPy) and lignosulfonate produced very strong solar absorption.

The material exhibited:

☀️ >96% light absorption (300–2500 nm)🌡️ Surface temperature of 93.5°C within 5 minutes under 1 sun irradiation

After adsorption at 30% RH:

• 💨 82% of absorbed water released within 30 minutes

• 💨 ~93% released within 1 hour

The desorption rate reached:

⚡ 0.02469 g·g⁻¹·min⁻¹ 

This rapid desorption was enabled by:

• efficient photothermal conversion

• low diffusion resistance

• reduced water-binding energy inside the porous network

Rapid cycling enables higher water productivity

Because both adsorption and desorption are fast, the system can operate in multiple daily cycles.

The authors optimized:

🔁 80 min adsorption + 60 min desorption

allowing:

✅ 6 water-harvesting cycles per day

This represents a major advantage over many SAWH systems limited to slow single-cycle operation.

Outdoor atmospheric water harvesting performance

Outdoor tests demonstrated strong real-world performance.

Under average outdoor conditions of:

• 🌡️ ~27.6°C

• 💧 ~70% RH

the material collected:

💧 2.42 L·m⁻²·day⁻¹or💧 6.95 kg_water·kg_sorbent⁻¹·day⁻¹ 

The system also maintained:

• stable performance under UV aging

• tolerance to saline marine fog environments

• adsorption stability over dozens of cycles

Heat recovery: turning water harvesting into energy generation

One of the most innovative aspects of the work is the integration of a:

🔋 thermoelectric generator (TEG)

to recover heat released during adsorption/desorption.

The system generated:

• ⚡ 612 mW·m⁻² during adsorption

• ⚡ 3.742 W·m⁻² during desorption 

This transforms the SAWH system into a:

💧 + 🔋 dual-function water-energy harvesting platform

The vertically layered structure also improved thermoelectric performance substantially compared with disordered pore systems.

Water quality and sustainability

Water-quality analysis showed that ion concentrations in collected water remained below:

✅ WHO drinking water standards

including:

• Li⁺

• Na⁺

• Ca²⁺

• Cl⁻

• SO₄²⁻

The estimated water production cost was as low as:

💲 0.1432 USD per kg of water

highlighting the economic potential of biomass-based SAWH systems.

Key insights (Important 🔥)

1. Vertically aligned pore structures dramatically improve vapor transport

2. Weak hydrogen-bond engineering lowers desorption energy

3. Fast adsorption + fast desorption enable multicycle operation

4. Photothermal conversion efficiency reached ~90.9%

5. Integrated thermoelectric recovery converts waste heat into electricity

6. Excellent performance across ultra-low to high humidity ranges (7–95% RH)

7. Biomass-derived materials can rival advanced MOF-based systems

Takeaway

This work demonstrates that the future of atmospheric water harvesting may depend less on exotic materials alone and more on:

👉 integrating pore architecture, photothermal engineering, and energy recovery into a unified system

By combining:

• biomass-derived materials

• vertically stratified transport pathways

• solar-driven desorption

• thermoelectric heat recovery

the study presents a practical pathway toward:

💧 low-energy, multicycle, and energy-recovering atmospheric water harvesting systems