In the present-day scenario, Energy and Environment together are the two most important
subject matter of discussion and concern. Increasing energy demands of the growing population
across the globe is leading fossil fuels exhaustion and thereby causing some series impacts on the
environment. Electrochemical energy storage devices such as batteries and supercapacitors, are
the alternative, environment-friendly and sustainable solution. Supercapacitors (SCs), also
known as electrochemical capacitors possess high power density and long cycle life in comparison
to batteries. However, the low energy density of SCs make them inferior to batteries and hinder
their implementation for real life applications. This brings up the concept of hybrid
supercapacitors (HSCs), involving the energy storage due to the combination of two different
charge storage mechanisms- non-faradaic and faradaic. In this thesis, two types of HSCs namely
composite-type HSCs and redox-electrolyte HSCs are explored.
In the first part, multi-component metal oxides (also known as high entropy oxides (HEOs)) are
prepared via sol-gel auto combustion technique. The as-prepared HEOs are utilized as a novel
catalyst to grow high yield multi-walled carbon nanotubes (MWCNTs) by chemical vapor
deposition. The obtained nanocomposite HEO-MWCNTs is implemented as an electrode material
for composite-type HSCs for the first time. Other metal oxides and carbon-based materials are
also prepared, and their performances are compared.
In the second part, single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes
(DWCNTs), and multi-walled carbon nanotubes (MWCNTs) are used as electrode materials for
redox-electrolyte HSCs in potassium ferricyanide K3[Fe(CN)6] based redox-additive electrolyte.
Their performances with and without redox additive in neutral electrolyte are compared. Further,
biowaste-derived activated carbon is synthesized and used for redox-electrolyte HSCs in iron
chloride FeCl3 based redox-additive electrolyte.
Water scarcity and pollution in many parts of the world is a series threat to mankind. Clean water
accessibility is becoming challenging with time, therefore, there is an urgent need to look for new
purification technologies. Desalination by several known methods like distillation, reverse
osmosis, electrodialysis, and ultrafiltration demands high energy consumption, heavy
equipment, and cost. Electrochemical desalination by capacitive deionization (CDI) is a simple,
economic, environment-friendly, and low-energy consumption technology. In the third part of
thesis, multi-component metal oxides incorporated surface modified biowaste-derived activated
carbon-based electrodes are prepared and a lab-scale CDI unit is fabricated. Also, sea water
desalination by CDI technology utilizing coconut-shell derived activated carbon is demonstrated.