Conversion of spent activated carbon into a value-added product, MnO2/C nanocomposite, a bi-functional electrocatalyst for oxygen reaction and an electrode material for supercapacitors

Activated carbon (AC) is used for purification of water and air, deodorization, decolouration, decontamination, sewage treatment, as energy storage materials, catalysts, etc. AC loses its activity during usage due to the clogging up of pores by foreign media, resulting in spent activated carbon (SAC). SAC is primarily treated in three ways: regeneration, use as fuel in kilns and landfill disposal. However, all these processes cause severe harm to the environment. Regenerating SAC and using it again for a similar purpose is an expensive affair. In this work, the SAC from a water filter is converted into MnO₂/C nanocomposites by hydrothermal reaction with KMnO₄ of different concentrations and used as electrocatalysts for oxygen reduction and oxygen evolution reactions (ORR and OER) and as an electrode material in supercapacitors. The conversion of SAC to MnO₂/C nanocomposites is confirmed by XRD, vibrational and X-ray photoelectron spectroscopic studies. MnO₂/C nanocomposite synthesized using KMnO₄:SAC ratio of 2:1 demonstrates a good ORR (onset potential: 0.846 V and current density:7.59 mA cm^-2) and OER (overpotential of 445 mV to reach 10 mA cm^-2) activities. Also, it displays good capacitive performance (252 F g^-1 at 1 A g^-1, rate retention and stable cycle life over 10,000 cycles).