Researchers from the Swiss Federal Laboratories for Materials Science and Technology (Empa) have developed a new process to generate electricity from wood.

Reported recently in the Science Advances journal, the researchers comprising Ingo Burgert and his team at Empa and ETH Zurich, together with the Empa research group of Francis Schwarze and Javier Ribera, have developed a simple, environment-friendly process for generating electricity from a type of wood sponge, expanding the scope of wood beyond its use as a building material.

The research team explains that the so-called piezoelectric effect comes into play when you generate electricity from wood.

Piezoelectricity is an electric voltage created by the elastic deformation of solids. This phenomenon is mainly exploited by metrology, which uses sensors that generate a charge signal, for instance, when a mechanical load is applied.

However, such sensors often use materials that are unsuitable for use in biomedical applications, such as lead zirconate titanate (PZT), which cannot be used on human skin due to the lead content, and also makes ecological disposal difficult.

Therefore, using the natural piezoelectric effect of wood offers a number of advantages and can be developed further for use in sustainable energy production. However, wood is not flexible enough – when subjected to mechanical stress, only a very low electrical voltage is generated in the deformation process. Therefore, wood must be given the appropriate properties through special treatment, the researchers explained.

From wood to wood sponge

Wood cell walls consist of three basic materials: lignin, hemicelluloses and cellulose. Jianguo Sun, a Ph.D. student in Burgert's team, used a chemical process called delignification to create wood sponge.

"Lignin is what a tree needs primarily in order to grow to great heights. This would not be possible without lignin as a stabilising substance that connects the cells and prevents the rigid cellulose fibrils from buckling," explains Burgert.

Lignin needs to be extracted to transform wood into a material that can easily be deformed. This is achieved by placing wood in a bath of hydrogen peroxide and acetic acid, which dissolves the lignin, leaving a framework of cellulose layers.

The resulting white wood sponge consists of superimposed thin layers of cellulose that can easily be squeezed together and then expanded back into their original form, making wood elastic.

piezoelectric nanogenerator

This is how a piezoelectric nanogenerator works: After the rigid wooden structure has been dissolved, a flexible cellulose network remains. When this is squeezed, charges are separated, generating an electric voltage. Credit: ACS Nano / Empa

Electricity from wooden floors

Burgert's team subjected the test cube with a side length of about 1.5cm to about 600 load cycles. At each compression, the researchers measured a voltage of around 0.63V – enough for an application as a sensor. Scaling up their wooden nanogenerators, they were able to show that 30 such wooden blocks, when loaded in parallel with the body weight of an adult, can light up a simple LCD display. It would, therefore, be conceivable to develop a wooden floor that is capable of converting the energy of people walking on it into electricity. The researchers also tested the suitability as a pressure sensor on human skin and showed that it could be used in biomedical applications.

Natural delignification

Looking to modify the process so that it no longer required the use of aggressive chemicals, the researchers found a suitable candidate that could carry out the delignification naturally in the form of a biological process – the fungus Ganoderma applanatum and the cause of white rot in wood.

"The fungus breaks down lignin and hemicellulose in the wood particularly gently," says Empa researcher Ribera. This process could be easily controlled in the lab, he added.

Following their initial success with electricity generation from wood sponge, the researchers continue their work so that the 'piezo' wood can be used as a sensor or as an electricity-generating wood floor. In order to adapt the technology for industrial applications, the researchers are in talks with potential partners.

Image: Scanning electron microscopy (SEM) images of balsa wood (left) and delignified wood illustrate the structural changes. Credit: ACS Nano / Empa