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Page 20 of 29 Teng et al. Microstructures 2023;3:2023019 https://dx.doi.org/10.20517/microstructures.2023.07

maintaining their structural stability, so the performance of filled carbon nanotubes is significantly better
than that of traditional anode materials.

Although filled carbon nanotubes show superior electrochemical performance, their performance slightly
decreases during cycling due to the instability of the electrode structure, including active and non-active
materials such as carbon nanotubes filled with active particles and binders. The significant expansion of
carbon nanotubes during lithiation can cause cracking, deformation, and partial peeling of the electrode
components during cycling, leading to the deterioration of cycling performance. To address these issues, it
is important to identify the optimal filling rate for different active materials to ensure the structural stability
of the expanded electrode. The use of multi-walled carbon nanotubes can further limit the range of volume
expansion. Designing the battery structure based on specific materials can also improve its overall
electrochemical performance.

The thermoelectric power generation
A major concern has been the potential effect of molecules encapsulated in CNTs on the thermal properties
of heterostructure systems. Kodama et al. developed a micro-nano processing method for determining the
[149]
effect of filling on thermal conductivity (κ) and thermoelectric potential (S), as shown in Figure 12 . The
results show that the filled CNTs reduce thermal conductivity by 35%-55% and increase thermoelectric
potential by about 40% compared to pristine CNTs at room temperature. Temperature-dependent
measurements from 40 to 320 K show that the peak of thermal conductivity changes as temperature
decreases.

[150]
Fukumaru et al. investigated the thermoelectric characteristics of CoCp @SWCNT heterostructures .
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Compared with original SWCNTs, the electrical conductivity of the heterostructures was significantly
improved by an order of magnitude. The negative Seebeck coefficient of -41.8 mV K at 320 K indicates that
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encapsulation of cobaltocene can convert the p-type pristine semiconducting SWCNT into an n-type.
Furthermore, the heterostructure has a high power factor and a low thermal conductivity (0.15 W m K ).
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Such a heterogeneous structure of conductivity, power factor, and thermal conductivity is very suitable for
the thermoelectric generation and is an attractive choice for the next generation of thermoelectric
appliances.

Catalyst
CNTs with a large internal surface area are very stable and good catalyst carriers. Their main function is to
immobilize and load nanoparticles and to provide an ideal local environment for certain chemical reactions.
Aygün et al. investigated the catalytic performance of Ru@SWCNTs vs. Ru coated on the surface of
SWCNTs . It has been demonstrated that the reason for improving the catalytic efficiency is not only the
[151]
stabilization of the catalytic particles but also the increase in the local concentration of the reactant
precursor, which is critical to the catalytic effect. Chamberlain et al. decompose synthetic catalytic
[152]
nanoparticles in SWCNTs . The size and morphology of the nanoclusters were controlled by the diameter
of SWCNTs, and efficient nanoparticle-filled SWCNTs provided a suitable environment for hydrogenation.
SWCNTs with different diameters can compare nanometers of different sizes, which is critical for catalytic
activity. The life of nanoparticles packed steadily into carbon nanotubes would be greatly extended.

Che et al. prepared highly aligned and monodisperse graphite-carbon nanoarrays using alumina films as
templates and filled carbon nanotubes with nanoparticles (Ru and Pt/Ru) . Supported catalysts are used in
[153]
electrocatalytic oxygen reduction and methanol oxidation of hydrocarbons. The catalytic activity was
significantly enhanced when CO and H O were converted to ethanol using Ru@CNTs.
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