Lead-Free Perovskite Promises New Era of Piezoelectric Energy Harvesting

A team of researchers has created a polymer film filled with chalcogenide perovskite compound that generates electricity when stressed.

Researchers have created a polymer film filled with a chalcogenide perovskite compound that generates electricity when stressed. This phenomenon is known as the piezoelectric effect, which is simply the ability of certain materials to generate an electric charge when mechanical stress is applied.

The piezoelectric effect occurs in materials that lack crystal structural symmetry. Crystals, ceramics, polymers, and biological matter such as bone, DNA, and various proteins are different kinds of piezoelectric materials.

Such materials have the potential to collect the energy related to mechanical vibrations. The best thing about this form of energy is that it is present all around us in abundant supply and is renewable in nature.

However, as the latest research notes, piezoelectric materials that are best performing tend to have the chemical element lead (Pb), which can cause cancer, increase the risk of brain tumors, and hinder DNA repair.

Materials that contain lead are hazardous, and regulators have curtailed their use to protect the environment.

Given the toxicity of lead, which is a heavy, malleable, naturally occurring metal with a relatively low melting point, it is being increasingly phased out of materials and devices.

Hence, the team's goal was to create a material that was lead-free and able to be made inexpensively using elements that are commonly found in nature.

So, the team from the Rensselaer Polytechnic Institute (RPI) made use of a material that not only does not contain lead but is also one of the few high-performing ones. Hence, it is a great candidate for use in biomedical applications, machines, and infrastructure.

The lead-free material that the team used belongs to the chalcogenide perovskite family exhibiting piezoelectricity. BaZrS3 was the composition used in the study, which is reported to have a pronounced piezoelectric response.

Chalcogenide perovskites have been gaining a lot of attention and advances lately. This family of compounds is related to perovskite structures, which have many favorable properties such as low toxicity, high stability, direct band gaps, good carrier transport abilities, and strong light absorption.

These properties make perovskites really stand out in applications like photovoltaics, photodetectors, light-emitting devices, and photocatalysts.

Interestingly, most high-performing piezoelectric materials are non-centrosymmetric and hence display intrinsically high polarizability. However, many oxide perovskites, including the one used in the study, exhibit a centrosymmetric crystal structure, which is weakly piezoelectric in its pristine form. These compounds are actually non-polar because they inherently lack a net dipole moment.

The dipole moment is the scientific name for the way piezoelectric materials perform when under stress, which is deformation in a way that causes positive ions and negative ions in the material to separate. This dipole moment can be harnessed and turned into an electric current.

But with no net dipole moment, how did the team achieve piezoelectricity? Well, they leverage the loose packing within the chalcogenide perovskite structure to overcome the problem.

Scaling the Technology for Green Energy Applications

The latest study details that despite being centrosymmetric, lead-free chalcogenide perovskite materials become polarizable very quickly when it is deformed. This is due to a loosely packed unit cell, which has a lot of vacant space.

This significant volume of empty space allows extended displacement of ions, which, in turn, allows for the reduction of symmetry and results in an amplified displacement-mediated dipole moment.

The team performed a piezoresponse force microscopy (PFM) on BaZrS3 to confirm the piezoelectricity of the material.

PFM is a functional atomic force microscopy (AFM) model that has been recognized for the unique information it offers on the electromechanical properties of various materials on the nanometer scale.

Structural symmetry in the chalcogenide perovskite material, as per the team, can be easily broken under stress, which leads to an enhanced piezoelectric response. So, once confirmed, the team developed composites of BaZrS3 particles dispersed in polycaprolactone.

The new material synthesized contains barium, zirconium, and sulfur, which were then used to harvest energy from human body motion and power electrochemical and electronic devices.

The team tested the material's ability to generate electricity by subjecting it to bodily movements like running, walking, tapping fingers, and clapping. The electricity produced during the experiment was found to be enough to power LED banks, spelling out RPI.

“We are excited and encouraged by our findings and their potential to support the transition to green energy.”

– Nikhil Koratkar, Study co-author

The material, according to him, converts mechanical energy into electrical energy. According to Koratkar:

“The greater the applied pressure load and the greater the surface area over which the pressure is applied, the greater the effect.”

The energy harvesting film created by the team is just 0.3 millimeters thick and can be integrated into various machines, devices, and structures like buildings and highways to