Researchers at the Vellore Institute of Technology University, Chennai, have successfully converted leather solid waste (wet blue leather splits) containing chromium (III) into porous carbon matrix for use as electrodes in supercapacitors using a simple, sequential, two-step process. This approach not only yielded “excellent porous electrode material for supercapacitors”, but also effectively addressed the management of chromium-containing leather solid waste, which is considered to be the major issue of leather manufacturing industry. The results were published in the Journal of Hazardous Materials.
Chromium (Cr) is widely used in leather tanning as it imparts toughness to leather. Though Cr(III) present in leather waste is not toxic, it can undergo spontaneous oxidation and get oxidised into Cr(VI) which is toxic. The conventional disposal methods, such as land filling and incineration, cannot be considered as an ideal way of disposing the waste in an eco-friendly manner.
“The prime constituent of leather is collagen fibre. So we thought of converting the collagen fibre into carbon fibre without oxidising the Cr(III) to Cr(VI),” says Dr. L. John Kennedy from the Physics Division - Materials, School of Advanced Sciences, VIT University, Chennai, and the corresponding author of the paper.
As a first step, the leather waste was precarbonised by heating it for four hours at 400 degree C. The precarbonised material was soaked in potassium hydroxide overnight and then heated at higher temperature in an inert atmosphere to produce porous carbon that contains inter-connected nanopores of all three sizes — micropores (less than 2 nm), mesopores (2-50 nm) and macropores (over 50 nm). Since the carbon contains all the three types of pores, it is called hierarchical porous carbons (HPCs).
Role of pores
The pores of different sizes are formed by pore drilling and pore widening due to the combined effect of potassium hydroxide and temperature. In addition to pore formation, graphitic stable carbon structure is also formed. The chromium present in the leather induces graphitisation in the carbon material; the graphitic content present in the material improves its electrical conductivity property. “We are not only taking care of chromium disposal, the metal actually improves the property of the carbon,” he says.
Hierarchical porous carbon is considered as a promising material for making electrodes that can be used in supercapacitor devices.
The three types of interconnected pores have very different roles in rendering the carbon a good electrode material. (While the micropores enhance the electrical double layer formation, the mesopores provide ion-transport pathways with low resistance, and the macropores serve as ion-buffering reservoirs to reduce the diffusion distance.)
At 900 degree C, the specific capacitance value was 1960 Farad per gram using one molar of potassium chloride electrolyte; the specific capacitance value was less at lower temperatures. “The mesopore volume increases with increasing temperature and this leads to higher specific capacitance,” Dr. Kennedy says.
As the hierarchical porous carbon is highly porous and has higher surface area, adequate ions diffuse into the inner pores of the electrode material. Since more ions are adsorbed on these electrodes compared to normal electrodes, the charge storage capacity becomes higher.
“We have explored and proved the potential of converting hazardous leather waste to an excellent electrode material for energy storage device concept,” he says.