These researchers in Switzerland can obtain electricity from wood –

These researchers in Switzerland can obtain electricity from wood

Researchers from ETH Zurich and Empa have chemically modified the wood and made it more compressible, turning it into a mini-generator. When compressed, it generates an electrical voltage. Such wood could serve as a biosensor or as a building material that harvests energy.

Ingo Burgert and his team at the public research university ETH Zurich and the Swiss federal laboratory Empa have shown that wood is much more than just a building material. His research improves the properties of wood for use in new applications. For example, they have already developed high-strength, water-repellent and magnetizable wood.

Now, together with the Empa research group led by Francis Schwarze, the team has used a chemical and a biological process to generate electrical voltage from a type of wooden sponge. In doing so, they amplify what is known as the “piezoelectric effect” of wood.

Compression creates voltage

When a piezoelectric material is elastically deformed, it generates an electrical voltage. Measurement technology, in particular, exploits this phenomenon by using sensors that generate a load signal when subjected to mechanical stress. However, many of the materials that are often used for these sensors are not suitable for biomedical applications. Lead zirconate titanate (PZT), for example, cannot be used on the skin due to its toxic lead and must be specially disposed of.

Wood also has a natural piezoelectric effect, but it only generates a very low electrical voltage. If you want to increase the voltage, you have to change the chemical composition of the wood, and this also makes it more compressible.

From the wooden block to the sponge

To convert wood into an easily formable material, a component of the cell walls must be dissolved. The cell walls of wood consist of three basic substances: lignin, hemicellulose and cellulose. “Lignin is the stabilizing substance that trees need to grow. Without lignin, which connects cells and prevents rigid cellulose fibrils from bending, this would not be possible, ”says Burgert.

A few months ago, Jianguo Sun, a PhD student from Burgert’s team, along with colleagues from ETH and Empa, published a study in ACS Nano which explains how wood can deform if lignin is removed chemically. As a result, its piezoelectric effect is enhanced.

The researchers achieved this “delignification” by placing wood in a mixture of hydrogen peroxide and acetic acid. The acid dissolves the lignin, leaving a layer of cellulose. “The process preserves the hierarchical structure of the wood and prevents the disassembly of individual fibers,” explains Burgert.

Even a little pressure can generate usable energy in the wood sponge. Photo: ACS Nano / Empa

In this way, a piece of balsa wood becomes a white wood sponge, formed by layer upon layer of fine cellulose. The sponge can simply be compressed and then returned to its original shape. “The wooden sponge generates an electrical voltage 85 times higher than that of the native sponge. [untreated] wood, ”says Sun.

A mini generator on the wooden floor.

The team subjected a test cube with a side length of approximately 1.5 cm to about 600 load cycles. The wooden sponge proved to be surprisingly stable: For each load, the researchers measured a voltage of about 0.63 volts, which would be appropriate for a sensor. In additional experiments, the team tested the scalability of this mini generator. If 30 such wooden blocks are connected and loaded evenly with the body weight of an adult, enough electricity is generated to power a simple LCD screen.

Treatment with fungi instead of chemicals.

In a follow-up study just published in Progress of science, the ETH-Empa research team went one step further, seeking to produce the wood sponge without using chemicals. Researchers found the solution in nature: the fungus Ganoderma applanatum causes white rot in wood and gently degrades lignin and hemicellulose. “Although the electrical voltage generated was lower in the initial tests than with chemically treated wood, the fungal process is more environmentally friendly,” says Burgert.

There are clear advantages to such a simple renewable piezoelectric system. Researchers see several potential applications for wood sponges, for example as sustainable building materials that harvest energy in the use phase or skin-friendly pressure sensors for medical purposes.

However, there are still several steps to go before piezoelectric wood can be used as a biosensor, or even as an electricity collection parquet floor. Burgert and his colleagues are now exploring with various partners how to adapt the technology for industrial applications.

News source: ETH Zurich editorial team

Main photo: Joel and Jasmin Førestbird / Unsplash

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