Lignin gives supercapacitor 900x performance boost

Technology News | September 11, 2020

By Nick Flaherty

The “green” supercapacitor, say the researchers, is flexible, lightweight, and cost-effective, and in the near future could charge devices – even electric cars – within a few minutes.

“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,” said Prof Hong Liang, in the Department of Mechanical Engineering. “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.”

Supercapacitors store charge on metal plates or electrodes. However, unlike basic capacitors, they can be made in different sizes, shapes, and designs, depending on the intended application. Furthermore, supercapacitor electrodes can also be built with different materials.

For their work, the researchers were attracted to manganese dioxide nanoparticles for designing one of the two supercapacitor electrodes.

“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 Liang. “But a major drawback of manganese dioxide is that it suffers from lower electrical conductivity.”

Past research, say the researchers, has shown that lignin – a natural polymer that glues wood fibers together – used with metal oxides enhances the electrochemical properties of electrodes. However, there have been few studies looking into combining manganese dioxide and lignin to leverage both of their useful properties.

To create their electrode, the researchers treated purified lignin with a commonly available disinfectant called potassium permanganate. They then applied high heat and pressure to initiate an oxidation reaction that results in the breaking down of potassium permanganate and the deposition of manganese dioxide on lignin.

Next, they coated the lignin and manganese dioxide mixture on an aluminum plate to form the “green” electrode. Finally, the researchers assembled the supercapacitor by sandwiching a gel electrolyte between the lignin-manganese dioxide-aluminum electrode and another electrode made of aluminum and activated charcoal.

When testing their newly designed green electrode, say the researchers, they found that their supercapacitor had very stable electrochemical properties. In particular, the specific capacitance – or the ability of the device to store an electrical charge – changed little, even after thousands of cycles of charging and discharging.

Also, for an optimal lignin-manganese dioxide ratio, the specific capacitance was observed to be up to 900 times more than what has been reported for other supercapacitors. These supercapacitors are also very light and flexible, say the researchers – properties that extend their use as structural energy storage elements in vehicles, for example.

“In this study, we have been able to make a plant-based supercapacitor with excellent electrochemical performance using a low-cost, sustainable method,” says Liang. “In the near future, we’d like to make our supercapacitors 100% environmentally friendly by incorporating only green, sustainable ingredients.”

Due to the superior electrochemical performance, say the researchers, the supercapacitor shows exceptional potential for future sustainable and green electronics. For more, see “Design and synthesis of high performance flexible and green supercapacitors made of manganese-dioxide-decorated alkali lignin.”

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