RUDN University chemist created a flower-like carbon from flour for zinc-ion supercapacitors

RUDN University chemist has created an unusual carbon structure that can be used as a cathode in zinc-ion supercapacitors. The batteries made of it have a higher capacity and energy density than existing alternatives. The results are published in Carbon.

Supercapacitors can store 100 times more energy than usual batteries. They charge faster and can withstand more recharge cycles. One of the most promising supercapacitors is based on zinc—ion. However, its real capacities, which can be achieved experimentally, are significantly less than those calculated theoretically. This is due to the limitations of the characteristics of carbon compounds that are used as cathodes. In an attempt to find the best carbon structure, scientists are studying carbon nanotubes, chemically activated graphene, layered porous carbon and hollow carbon spheres. A RUDN University chemist proposed a new 3D structure that improves the properties of zinc-ion supercapacitors.

"Ionic based hybrid supercapacitors are promising platforms to optimize the properties of energy storage devices. However, restricted by insufficient adsorption sites of activated carbon cathodes, the energy capacities of ZSCs are inferior to the expectation, especially at high power output", said Prof. Rafael Luque from the Joint Institute for Chemical Research, RUDN University.

To get a new 3D structure, chemists mixed melamine, boric acid and mouldy flour in water. The mixture was placed in an autoclave at 180℃ for 15 hours. The result is structures to a carnation or hydrangea—uneven spheres with a large number of pores. This "bouquet" was then pyrolyzed for 2 hours, under gradually raising temperature up to 900℃. During pyrolysis, the auxiliary compounds in the "flowers" disintegrated, leaving carbon frame only. Chemists carried out similar procedures using flour and melamine as starting compounds, only flour. All the obtained structures were studied using a scanning electron microscope. Then, chemists made zinc-ion supercapacitors from the resulting flower-like carbon (BCF) and measured its characteristics.

RUDN University chemists compared the structure of the obtained compounds and concluded that boric acid did not affect the formation of the flower-like structure—in fact, melamine crystals and mouldy flour became the basis for it. It also turned out that the BCF consists of many "petals"—thin sheets connected to each other in a single ball structure. These connected nanopetals provided fast charge transfer inside the flower and low resistance. The battery capacity based on BCF turned out to be greater than that of other similar devices—133.5 mAh/gram. The energy density (that is, the amount of energy that can store 1 kg of battery) also exceeded the existing zinc-ion alternatives.

"The suitable mesopores and nanosheet structure were beneficial for electrolyte ion penetration and exchange. This work paves the way to the design of isolated carbon segments into assembled carbon superstructure for energy storage materials", said Prof. Rafael Luque from the Joint Institute for Chemical Research, RUDN University.

More information:
DOI: 10.1016/j.carbon.2021.04.093

Provided by RUDN University