CHENNAI: Researchers at the Indian Institute of Science Education and Research (IISER) in Kolkata have used human hair to produce cost-effective, metal-free cathodes for use in solar cells. This is the first instance where a bio-waste-derived electrode has been used as cathode in a quantum dot sensitised solar cell device.

While metal-free cathodes produced in the past have not performed as well as the traditional metal-based ones, the performance of graphitic porous carbon cathode produced by a team, led by Prof. Sayan Bhattacharyya from the Department of Chemical Sciences, has been at par with metal-based cathodes. The results have been published in the journal Carbon.

“The graphitic porous carbon cathode shows an impressive performance to help converting visible sunlight to electricity [power conversion] much higher than commercially available activated carbon cathodes and are comparable with commonly used cathodes made of platinum metal and metal sulfides,” said Prof. Bhattacharyya, who is the corresponding author of the paper. “Our ‘green’ cathodes have the potential to bring down the cost of solar cells.”

Besides its higher efficiency to convert visible sunlight to electricity, the cathode was found to generate high open-circuit voltage, which is at par with conventional platinum and activated carbon cathodes. Thereby, the power conversion efficiencies can also be enhanced.

Cheap and simple

Producing graphitic porous carbon cathode using human hair is also simple, quick and inexpensive. Unlike in the case of other synthetic porous carbons, no physical or chemical activation process or templates were required to produce the pores of 2-50 nm diameter. The porosity, along with high surface area to volume ratio, plays an important role in adsorption-desorption of electrolyte.

The cleaned and dry human hair was first treated with sulphuric acid at 165 degrees C for 25 minutes to achieve precarbonisation. It was then heated to different temperatures in the presence of an inert gas for six hours to carbonise and bring better electrical conductivity for efficient charge transfer.

A material with high electrical conductivity can be produced by carbonising at higher temperature, but it will not be highly porous.

“With increasing temperature, the pores collapse and the porosity starts reducing. Therefore, a trade-off is required between electrical conductivity and porosity,” said Prof. Bhattacharyya. “We found 850 degrees C was the optimum temperature to achieve highly catalytic graphitic porous carbon for fabricating efficient green cathodes.”

Though others had developed methods to produce porous carbon matrices using human hair for supercapacitor applications, large-scale production of porous carbon with graphitic edges and catalytically active sites by a facile synthesis procedure has been rare.