Plant-based supercapacitor keeps costs low and energy storage high

Supercapacitors have the potential to pave the way for electric vehicles that charge in minutes rather than hours, overcoming one of the barriers to widespread adoption and being good for drivers and the environment. In a step towards such a reality, scientists at Texas A&M University have demonstrated a plant-based supercapacitor with excellent energy storage potential.

With an ability to charge almost in an instant and discharge huge amounts of power as its needed, supercapacitors are an energy storage technology with massive potential. And we have seen a number of interesting advances made in building the devices out of sustainable materials, including upcycled plastic bottles, hemp and and even discarded cigarette butts.

The team at Texas A&M University looked to make use of a natural polymer that gives plants and trees their rigidity called lignin. This is produced in huge amounts as a waste product by the paper manufacturing industry, and we have actually seen some interesting breakthroughs in efforts to recycle the polymer into other products, such as stronger concrete and biopastes for 3D printing.

The authors of the new study, however, hope to use it to supercharge the performance of a material used in supercapacitor electrodes called manganese dioxide. Nanoparticles of this compound offer a number of benefits over other solutions, but the electrochemical performance is where they tend to fall down.

“Manganese dioxide is cheaper, available in abundance and is safer compared to other transition metal oxides, like ruthenium or zinc oxide, that are popularly used for making electrodes,” says study author Hong Liang. “But a major drawback of manganese dioxide is that it suffers from lower electrical conductivity.”

Previous research had indicated that lignin combined with metal oxides could boost the electrical performance of supercapacitor electrodes, but the team wanted to investigate how it could enhance the function of manganese oxide specifically. So they designed a supercapacitor in which these two components formed the key building blocks.

The team started by purifying the lignin in a common disinfectant and then applied heat and pressure, which caused the liquid to break down and for manganese dioxide to be deposited onto the lignin. This mixture was then used to coat an aluminum plate to form the electrode, which was paired with another electrode made of aluminum and activated charcoal to form the supercapacitor, with a gel electrolyte sandwiched in between.

The researchers describe the new device as light, flexible and cost effective, increasing its potential to be used as structural energy storage elements in vehicles. They also report that it stood up extremely well in testing, finding that it had “very stable electrochemical properties,” and that it maintained its ability to store an electrical charge over thousands of cycles.

The performance was compared to other cutting-edge supercapacitor designs through existing literature, including ones with electrodes made wholly from activated carbon, or graphene combined with other materials. It outperformed them all in terms of specific capacitance, a measure of the device's ability to store a charge. When compared to one supercapacitor with an electrode made from tin diselenide, the new device offered a specific capacitance that was 900 times greater.

“Integrating biomaterials into energy storage devices has been tricky because it is difficult to control their resulting electrical properties, which then gravely affects the devices’ life cycle and performance” says Liang. “Also, the process of making biomaterials generally includes chemical treatments that are hazardous. We have designed an environmentally friendly energy storage device that has superior electrical performance and can be manufactured easily, safely and at much lower cost.”

The research was published in the journal Energy Storage.

Source: Texas A&M University