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G. Pandey et al.
cost (Miller and Simon 2008; Simon et al. 2014). Some of the commonly used supercapacitors are carbon-based Electrochemical Double Layer Capacitor (SC-EDLC).
However, a major limitation of such supercapacitors is that the charge is confined to
the surface thus leading to an overall low-energy density. HNT due to their nanotubular structure are capable of storing surplus charge within its lumen. This storage can
be enhanced by carefully selecting guest molecules that are capable of enhancing
energy storage. For instance, in a study, HNT had been used as support to first introduce sulphydryl groups on the surface through treatment by mercaptotriethoxysilane
(KH-90), followed by growing Nickel Sulphide (Ni 3 S 2 ) nanoparticles on the surface.
Further carbon nanoparticles were immobilized on the surface to ensure efficient
charge dispersion. The synthesized supercapacitor showed excellent energy storage
performance, by delivery a capacity of 450.4 C/g and a high charge retention of
82.6% even over 2000 cycles. This supercapacitor also showed a maximum energy
density at 79.6 Wh/kg, thereby suggesting the potential use of such eco-friendly
supercapacitors (Li et al. 2017a, b). Apart from this, HNT have also been used in
the development of Phase Change Material (PCM) for energy storage. In this study,
Graphene aerogels (GA) had been grafted on the surface of HNT, followed by their
implementation in the preparation of polyurethane-based solid–solid PCMs. This
structure showed enhanced light thermal and electrothermal energy conversion and
storage ability. In case of light-thermal, η was 75.6%, while in case of electrothermal,
η was 67.2%. The developed materials also showed enhanced thermal stability and
reliability and therefore can be efficiently used in energy storage (Zhou et al. 2019).
Another method of energy storage is through the use of Vanadium Flow Batteries
(VFB). Ion exchange membranes are highly crucial structures in such flow batteries,
since they act as an efficient physical separation between the oppositely charged
electrolytes. These structures also prevent the mixture of vanadium ions. Therefore,
a proper selection of the membrane is highly crucial such that it has excellent ion
selectivity, mechanical strength and chemical stability. Implementation of HNT with
guest molecules immobilized on it in such membranes can fulfil these factors. In a
study, PDA had been immobilized on HNT and incorporated in sulfonated poly(ether
ether ketone) membranes. The presence of dopamine on the surface enhanced the
proton conductivity, while the nanostructure of the tube acted as a crosslinker to form
a strong mesh like membrane structure. This membrane showed excellent durability
and high performance at 40–200 mA/cm
2 . It also showed high energy efficiency of
75% and a robust coulombic efficiency at 99% (Yu et al. 2018).
3.2.2 Fuel
Apart from energy storage, finding alternative fuel sources or implementation of
methods that can increase the yield of existing fuels is crucial. For instance, crude oil,
which is the most widely used fuel across the world is catalytically hydrocracked in
order to increase the yield of different products. Over the years, thermal and catalytic
cracking are the commonly used techniques for increasing the yields. However, they
G. Pandey et al.
cost (Miller and Simon 2008; Simon et al. 2014). Some of the commonly used supercapacitors are carbon-based Electrochemical Double Layer Capacitor (SC-EDLC).
However, a major limitation of such supercapacitors is that the charge is confined to
the surface thus leading to an overall low-energy density. HNT due to their nanotubular structure are capable of storing surplus charge within its lumen. This storage can
be enhanced by carefully selecting guest molecules that are capable of enhancing
energy storage. For instance, in a study, HNT had been used as support to first introduce sulphydryl groups on the surface through treatment by mercaptotriethoxysilane
(KH-90), followed by growing Nickel Sulphide (Ni 3 S 2 ) nanoparticles on the surface.
Further carbon nanoparticles were immobilized on the surface to ensure efficient
charge dispersion. The synthesized supercapacitor showed excellent energy storage
performance, by delivery a capacity of 450.4 C/g and a high charge retention of
82.6% even over 2000 cycles. This supercapacitor also showed a maximum energy
density at 79.6 Wh/kg, thereby suggesting the potential use of such eco-friendly
supercapacitors (Li et al. 2017a, b). Apart from this, HNT have also been used in
the development of Phase Change Material (PCM) for energy storage. In this study,
Graphene aerogels (GA) had been grafted on the surface of HNT, followed by their
implementation in the preparation of polyurethane-based solid–solid PCMs. This
structure showed enhanced light thermal and electrothermal energy conversion and
storage ability. In case of light-thermal, η was 75.6%, while in case of electrothermal,
η was 67.2%. The developed materials also showed enhanced thermal stability and
reliability and therefore can be efficiently used in energy storage (Zhou et al. 2019).
Another method of energy storage is through the use of Vanadium Flow Batteries
(VFB). Ion exchange membranes are highly crucial structures in such flow batteries,
since they act as an efficient physical separation between the oppositely charged
electrolytes. These structures also prevent the mixture of vanadium ions. Therefore,
a proper selection of the membrane is highly crucial such that it has excellent ion
selectivity, mechanical strength and chemical stability. Implementation of HNT with
guest molecules immobilized on it in such membranes can fulfil these factors. In a
study, PDA had been immobilized on HNT and incorporated in sulfonated poly(ether
ether ketone) membranes. The presence of dopamine on the surface enhanced the
proton conductivity, while the nanostructure of the tube acted as a crosslinker to form
a strong mesh like membrane structure. This membrane showed excellent durability
and high performance at 40–200 mA/cm
2 . It also showed high energy efficiency of
75% and a robust coulombic efficiency at 99% (Yu et al. 2018).
3.2.2 Fuel
Apart from energy storage, finding alternative fuel sources or implementation of
methods that can increase the yield of existing fuels is crucial. For instance, crude oil,
which is the most widely used fuel across the world is catalytically hydrocracked in
order to increase the yield of different products. Over the years, thermal and catalytic
cracking are the commonly used techniques for increasing the yields. However, they