Wood That Drinks Air and Makes Electricity: A Tree-Inspired Sponge for Water–Energy Co-Generation

Original Authors: Haoyu Ma, Shengnan Li, Shaowei Wang, Weisheng Yang, Jingquan Han

Original paper is accessible at: https://doi.org/10.34133/research.1195

Two challenges in one device

Sorption-based atmospheric water harvesting (SAWH) has made rapid progress. MOFs, hygroscopic hydrogels, carbon sorbents, and salt-loaded porous matrices can now capture substantial amounts of water from air. Several practical challenges remain, however:

• Many high-performance sorbents are made from petrochemical feedstocks, which raises cost and disposal concerns.

• Many systems still depend on electric heating for desorption.

• Most devices do only one thing: capture water.

In parallel, a separate research field has grown around hydrovoltaic generation (HG), in which electricity is produced from water movement, evaporation, or humidity gradients through charged nanochannels.

This paper asks whether the two can be combined in a single, fully bio-based material:

Can one renewable material capture water from air, release it using sunlight, and generate electricity in the same cycle?

The authors answer with a double-layer wood sponge modeled on how a tree moves water.

The tree as a design template

A tree separates its water functions spatially. Roots take up water, vertically aligned vessels transport it, and leaves absorb sunlight and drive evaporation through transpiration.

The authors copy this layout in a block of balsa wood:

• The “root” layer is the lower part of the sponge, impregnated with lithium chloride (LiCl) to absorb moisture from air.

• The “vessels” are the wood’s natural vertically aligned microchannels, preserved through processing to guide water and ions in one direction.

• The “leaf” layer is a thin photothermal coating on top that absorbs sunlight and drives evaporation.

Because absorption sits at the bottom and evaporation at the top, a vertical humidity gradient forms through the thickness of the sponge. That gradient is what moves water during regeneration, and it is also what generates electricity.

Making the sponge, using the whole tree

The fabrication route is designed so that almost nothing from the wood is wasted.

1. Opening up the wood. Balsa blocks are treated with a deep eutectic solvent (choline chloride and oxalic acid) to remove lignin, then bleached under mild conditions. The vertical channel structure survives, while the surface area and mesoporosity increase markedly compared with natural wood.

2. Charging the channel walls. The delignified wood is carboxylated, which places negatively charged carboxyl groups along the channel walls. This makes the scaffold more hydrophilic and provides the surface charge needed for the hydrovoltaic effect.

3. Adding the sorbent. The sponge is soaked in LiCl solutions of different concentrations. A 15 wt% solution gave the best balance. Below that, uptake was limited. Above it, the salt aggregated inside the pores and blocked the channels, which slowed diffusion.

4. Recycling the lignin as a solar absorber. The lignin removed in step 1 is recovered from the solvent, chemically demethylated, and then coordinated with iron (Fe³⁺) to form a D-lignin–Fe complex. This complex is drop-cast onto the top surface as the photothermal layer.

The result is an all-biomass, two-layer device with asymmetric wettability: the top and bottom surfaces handle water differently, which reinforces directional transport.

How it performs as a water harvester

Under 60% RH and 25 °C, the sponge reached a water uptake of about 1.45 g/g. Across a 30–90% RH range, uptake rose to about 2.5 g/g at 90% RH. The sponge absorbed water faster and in larger amounts than commercial silica gel under the same test conditions.

A control experiment clarified the role of each component. A version of the sponge without LiCl showed a sharp drop in uptake. LiCl provides most of the sorption capacity, while the wood scaffold acts as a porous, hydrophilic host that holds the salt in place and keeps vapor pathways open.

The photothermal layer worked as intended. Under one-sun illumination, surface temperature reached about 68.5 °C with D-lignin–Fe, compared with about 53.5 °C for demethylated lignin alone and about 46.1 °C for raw lignin. Iron coordination broadened light absorption, especially in the near-infrared. A coating mass of about 60 mg gave the best desorption. Thicker coatings partially blocked the channels.

Because heating happens at the top surface rather than through the whole sponge, water evaporates near the surface and the gradient through the thickness is maintained. In cycling tests, the sponge absorbed between 0.75 and 1.86 g/g at 30–70% RH and released it rapidly under one sun, with stable performance and no visible damage over repeated cycles.

The authors are transparent about the comparison with other sorbents. Several LiCl composites and recent MOFs achieve higher peak uptake. They argue that this sponge was designed differently: sorption is used as a driver for transport and power generation, not as a standalone metric.

How it generates electricity

The electricity comes from a streaming potential. As water moves upward through the negatively charged channels, cations accumulate near the channel walls, the charges separate, and a voltage builds along the direction of flow.

LiCl plays a second role here. Dissolved Li⁺ ions act as a mobile charge reservoir that amplifies the ionic flux. Without LiCl, electrical output was negligible.

Key electrical results:

• Open-circuit voltage: about 430 mV at 60% RH under one sun

• Short-circuit current: about 6.3 µA

• Peak power density: about 2.9 mW/m² at an external load of about 10⁵ Ω

• Higher humidity and stronger sunlight both increased the voltage

• Reversing the electrodes reversed the polarity, which confirms that the output comes from directional ion migration

Connected in series, several sponge modules charged a capacitor to a few volts within minutes. The stored energy powered a thermohygrometer, a calculator, and an LED.

Outdoor demonstration

The authors ran a rooftop test in Nanjing with a day–night cycle: the sponges absorbed moisture at night, and sunlight drove desorption and condensation during the day. Over five consecutive cycles, water collection remained consistent from day to day, with no external electrical input. The output voltage tracked changes in temperature, humidity, and solar irradiance.

The collected water was analyzed for ions. Measured species were below WHO health-based guideline values where those exist. Lithium was also reported, although the WHO has no health-based guideline for it. For a LiCl-based device, that is a parameter worth monitoring closely in future work.

A sustainability check

A life cycle assessment compared the sponge with silica gel. The wood-based sponge showed lower impacts on global warming, fossil resource use, and freshwater consumption, partly because the biogenic carbon stored in wood was counted as a credit. Most of the remaining impact came from the hygroscopic substrate, meaning the LiCl loading and chemical modification, rather than from the lignin coating. That points to substrate preparation as the next target for improvement.

Open questions

The authors themselves identify several limits:

• Salt leakage: no brine seepage occurred under the standard cycling protocol, but leakage was detected after more than 24 hours of continuous adsorption at 70% RH.

• Salt crystallization: prolonged illumination caused LiCl to crystallize and the electrical output to decline. Under normal day–night cycles this effect is expected to be limited, but it defines an operating window.

• Durability: long-term performance beyond the tested cycling window still needs validation.

Scale is also worth keeping in perspective. A power density of about 2.9 mW/m² is suitable for low-power sensors and indicators, not for running pumps, fans, or treatment units. Water collection in the outdoor test was on the order of half a gram per gram of sponge per cycle, measured on small samples. The real value of this work is the concept: a single renewable scaffold in which water capture, solar regeneration, and power generation are designed together rather than assembled from separate parts.

Why this paper matters

The study shows how much functionality can be engineered into a common natural material. Wood already contains aligned transport channels, a modifiable cellulose surface, and lignin that can absorb sunlight. By restructuring these components instead of adding petrochemical ones, the authors produce a device that harvests water, regenerates with sunlight, and generates its own electricity.

For decentralized and off-grid AWH, that combination is interesting. A self-powered harvester that can also run its own humidity sensor or a status indicator is a small but meaningful step toward autonomous field units. The next steps are clear: keep the salt in place, extend lifetime, and increase both water output and power to levels that matter at device scale.