Air-conditioners and dehumidifiers are essential equipment for making homes comfortable and keeping basements mold-free. But these machines consume immense amounts of energy. A new device that dries air while simultaneously generating energy could help.

The humidity digester reported in the journal Joule combines super moisture-absorbent hydrogels with a ferroelectric-semiconductor hybrid material that splits the absorbed water in the presence of light. Connected to a solar cell, the device can reduce humidity by 12% and generate a low current under ambient indoor light. This could lead to low cost, energy-efficient dehumidifying systems, and could help to reduce the cooling load on air-conditioners.

“High relative humidity affects how we perceive temperature and, more than heat, humidity causes thermal discomfort to people,” says Swee Ching Tan, professor of materials science and engineering at the National University of Singapore. The new device “maintains humidity at thermally comfortable levels at zero energy expense.”

There has been ample research in recent years on photoelectrochemical systems that, like plants, split water in the presence of sunlight to produce energy. But scaling these technologies up is a challenge because they require large amounts of water and extra electricity input.

So Tan and his colleagues decided to tap into atmospheric humidity as the water source. For this, they used zinc- or cobalt-based hydrogels developed in Tan’s laboratory. These gels can absorb more than four times their own weight of water from moist air.

The research team coated these hydrogels on an anode formed by embedding ferroelectric barium titanate (BTO) nanoparticles in the semiconductor bismuth vanadate (BVO). The BVO absorbs photons to generate electron-hole pairs that are then used to split the absorbed water, Tan says. “The role of BTO is to create a built-in electric field to enhance the separation of the photogenerated charges. This accelerates the water splitting reaction.”

When the anode is connected to a solar cell, the voltage bias further enhances water splitting and produces a current. The hybrid system continually absorbs humidity and generates a low current. The device that used the Co gel produced a current of 0.4 mA/cm², twice that of the Zn hydrogel device, because the Co gel absorbs twice as much water. After about 24 hours, the relative humidity decreased by about 7.5% and 12% for the Zn and Co hydrogel systems, respectively.

To demonstrate what a practical household device might look like, the researchers used two hydrogels to create a painting of a jockey riding a horse on the anode substrate. Such paintings, connected to solar cells that work with indoor ambient light could potentially be a decorative device that help dehumidify living spaces with zero energy input, they say.

“This strategy of combining atmospheric humidity harvesting with photoelectrochemical water splitting is novel,” says Ruzhu Wang, director of the Institute of Refrigeration and Cryogenics at Shanghai Jiao Tong University.

But the prototype will need much more work before it can be practical. Right now, the hydrogel absorbs water much faster than the photoanode can split it. “We are currently looking to develop novel materials that can be interfaced with the hydrogel to aid in faster water breakdown,” Tan says.

Read the abstract in Joule.