Harvesting energy from wooden floors

Swiss researchers have created a nanogenerator that enables wooden flooring to generate energy from our footfalls — enough to power LED lightbulbs and small electronics. Their work has been published in the journal Matter.

The team began by transforming wood into a nanogenerator by sandwiching two pieces of functionalised wood between electrodes. Like a shirt-clinging sock fresh out of the dryer, the wood pieces become electrically charged through periodic contacts and separations when stepped on; a phenomenon called the triboelectric effect. The electrons can transfer from one object to another, generating electricity. However, there’s one problem with making a nanogenerator out of wood.

“Wood is basically triboneutral … [which] means that wood has no real tendency to acquire or to lose electrons,” said senior author Guido Panzarasa, group leader in the professorship of Wood Materials Science located at ETH Zürich and Empa. This limits the material’s ability to generate electricity, so the challenge is making wood that is able to attract and lose electrons.

To boost wood’s triboelectric properties, the scientists coated one piece of the wood with polydimethylsiloxane (PDMS), a silicone that gains electrons upon contact, while functionalising the other piece of wood with in-situ-grown nanocrystals called zeolitic imidazolate framework-8 (ZIF-8) — a hybrid network of metal ions and organic molecules that has a higher tendency to lose electrons. They also tested different types of wood to determine whether certain species or the direction in which wood is cut could influence its triboelectric properties by serving as a better scaffold for the coating.

The researchers found that a triboelectric nanogenerator made with radially cut spruce, a common wood for construction in Europe, performed the best. Together, the treatments boosted the triboelectric nanogenerator’s performance: it generated 80 times more electricity than natural wood. The device’s electricity output was also stable under steady forces for up to 1500 cycles.

“Our focus was to demonstrate the possibility of modifying wood with relatively environmentally friendly procedures to make it triboelectric,” Panzarasa said. “Spruce is cheap and available and has favourable mechanical properties. The functionalisation approach is quite simple, and it can be scalable on an industrial level. It’s only a matter of engineering.”

The researchers found that a wood floor prototype with a surface area slightly smaller than a piece of paper can produce enough energy to drive household LED lamps and small electronic devices such as calculators. They successfully lit up a lightbulb with the prototype when a human adult walked upon it, turning footsteps into electricity.

Besides being efficient, sustainable and scalable, the newly developed nanogenerator also preserves the features that make the wood useful for interior design, including its mechanical robustness and warm colours. The researchers say that these features might help promote the use of wood nanogenerators as green energy sources in smart buildings. They also say that wood construction could help mitigate climate change by sequestering CO₂ from the environment throughout the material’s lifespan.

The next step for the team is to further optimise the nanogenerator with chemical coatings that are more eco-friendly and easier to implement. “Eventually,” Panzarasa said, “the research that we do should lead to applications in the real world.

“The ultimate goal is to understand the potentialities of wood beyond those already known and to enable wood with new properties for future sustainable smart buildings.”

Image credit: ©stock.adobe.com/au/Leszek Czerwonka

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