2.2 Reactive Molten Salt Synthesis of Carbon Nanostructures
9
of the initial carbon materials. It should be noticed that a considerable increase in the
graphitization degree can often be achieved by longtime heat treatment of carbon at
temperatures more than 2500 °C under protective atmospheres [38, 39]. The growth of
graphite crystallites, thus the progress of graphitization observed during the molten
salt heat treatment, can be attributed to the combination of various effects. These
effects include the reactive dissolution of impurities from carbonaceous materials into
molten salts [30], the reduction of the d-spacing fluctuation between graphite basal
planes and the closure of the Mrozowski cracks in graphite crystals. The latter was
suggested to be caused by compressive stresses induced by the salt network in the
graphite matrix [32] contributing to ordering of the carbon structure (Fig. 2.4).
Furthermore, Li et al. [40] reported that the treatment of Ni-encapsulated
MWCNTs in the LiCl–KCl eutectic molten salt promotes the uncapping of the carbon capsules and simplifying the purification of nanotubes by the subsequent acid
leaching. The influence of molten salts on carbonaceous materials can further be
promoted by applying electric potentials, which will be discussed later.
2.2.2 Capture and Conversion of CO 2 in Molten Salts
In the above-mentioned studies, carbon was sourced from carbonaceous solid materials exposed to the molten salts. The source of carbon used in molten salt processes can
also be carbon dioxide (CO 2 ) which is considered to be the main greenhouse gas and
probably the main cause of the accelerated global warming and ocean acidification
with catastrophic environmental consequences. Capture of CO 2 and its conversion
into carbon materials for various applications, therefore, is an interesting approach to
tackle CO 2 emission challenges. Carbon dioxide can be captured in a single molten
salt or often a mixture of molten salts to form amorphous carbons, CNTs and nanodiamonds. The molten salts used in this approach include molten carbonates, such
as Li 2 CO 3 , Na 2 CO 3 , K 2 CO 3 [33, 39–42], Cs 2 CO 3 [43] and CaCO 3 [44] as well as
chlorides such as LiCl, NaCl, KCl and CaCl 2 [45–48].
In molten carbonate salts, carbonate ions can be electrochemically reduced on
the cathode electrode to form amorphous solid carbon and oxygen ions O
2− , which
can subsequently absorb the CO 2 gas, promoting the continuous capture and electrochemical conversion of CO 2 to carbon [33]. In molten chloride, the addition of
alkali metal or alkaline earth metal oxides leads to the formation of oxygen ions with
a strong affinity for the absorption of CO 2 , resulting in the formation of carbonates
[47]. Ideally, using photovoltaic (PV) energy and an inert anode, CO 2 can be captured and electrochemically transferred into various carbon nanostructures (Fig. 2.5a
[49]).
Molten salts can also be used to promote the adsorption of CO 2 on porous solid
adsorbents. For instance, MgO is an attractive material for the CO 2 capture application [50, 51] since MgO particles easily react with the surrounding CO 2 molecules to
form MgCO 3 . However, MgCO 3 layer formed around MgO is impervious, sharply
9
of the initial carbon materials. It should be noticed that a considerable increase in the
graphitization degree can often be achieved by longtime heat treatment of carbon at
temperatures more than 2500 °C under protective atmospheres [38, 39]. The growth of
graphite crystallites, thus the progress of graphitization observed during the molten
salt heat treatment, can be attributed to the combination of various effects. These
effects include the reactive dissolution of impurities from carbonaceous materials into
molten salts [30], the reduction of the d-spacing fluctuation between graphite basal
planes and the closure of the Mrozowski cracks in graphite crystals. The latter was
suggested to be caused by compressive stresses induced by the salt network in the
graphite matrix [32] contributing to ordering of the carbon structure (Fig. 2.4).
Furthermore, Li et al. [40] reported that the treatment of Ni-encapsulated
MWCNTs in the LiCl–KCl eutectic molten salt promotes the uncapping of the carbon capsules and simplifying the purification of nanotubes by the subsequent acid
leaching. The influence of molten salts on carbonaceous materials can further be
promoted by applying electric potentials, which will be discussed later.
2.2.2 Capture and Conversion of CO 2 in Molten Salts
In the above-mentioned studies, carbon was sourced from carbonaceous solid materials exposed to the molten salts. The source of carbon used in molten salt processes can
also be carbon dioxide (CO 2 ) which is considered to be the main greenhouse gas and
probably the main cause of the accelerated global warming and ocean acidification
with catastrophic environmental consequences. Capture of CO 2 and its conversion
into carbon materials for various applications, therefore, is an interesting approach to
tackle CO 2 emission challenges. Carbon dioxide can be captured in a single molten
salt or often a mixture of molten salts to form amorphous carbons, CNTs and nanodiamonds. The molten salts used in this approach include molten carbonates, such
as Li 2 CO 3 , Na 2 CO 3 , K 2 CO 3 [33, 39–42], Cs 2 CO 3 [43] and CaCO 3 [44] as well as
chlorides such as LiCl, NaCl, KCl and CaCl 2 [45–48].
In molten carbonate salts, carbonate ions can be electrochemically reduced on
the cathode electrode to form amorphous solid carbon and oxygen ions O
2− , which
can subsequently absorb the CO 2 gas, promoting the continuous capture and electrochemical conversion of CO 2 to carbon [33]. In molten chloride, the addition of
alkali metal or alkaline earth metal oxides leads to the formation of oxygen ions with
a strong affinity for the absorption of CO 2 , resulting in the formation of carbonates
[47]. Ideally, using photovoltaic (PV) energy and an inert anode, CO 2 can be captured and electrochemically transferred into various carbon nanostructures (Fig. 2.5a
[49]).
Molten salts can also be used to promote the adsorption of CO 2 on porous solid
adsorbents. For instance, MgO is an attractive material for the CO 2 capture application [50, 51] since MgO particles easily react with the surrounding CO 2 molecules to
form MgCO 3 . However, MgCO 3 layer formed around MgO is impervious, sharply