Laminated Structural Engineering Strategy toward Carbon Nanotube-Based Aerogel Films

Aerogel films with a low density are ideal candidates to meet lightweight application and have already been used in a myriad of fields; however, their structural design for performance enhancement remains elusive. Herein, we put forward a laminated structural engineering strategy to prepare a free-standing carbon nanotube (CNT)-based aerogel film with a densified laminated porous structure. By directional densification and carbonization, the three-dimensional network of one-dimensional nanostructures in the aramid nanofiber/carbon nanotube (ANF/CNT) hybrid aerogel film can be reconstructed to a laminated porous structure with preferential orientation and consecutively conductive pathways, resulting in a large specific surface area (341.9 m2/g) and high electrical conductivity (8540 S/m). Benefiting from the laminated porous structure and high electrical conductivity, the absolute specific shielding effectiveness (SSE/t) of a CNT-based aerogel film can reach 200647.9 dB cm2/g, which shows the highest value among the reported aerogel-based materials. The laminated CNT-based aerogel films with an adjustable wetting property also exhibit exceptional Joule heating performance. This work provides a structural engineering strategy for aerogel films with enhanced electric conductivity for lightweight applications, such as EMI shielding and wearable heating.


Electromagnetic Interference (EMI) Shielding Measurements
Electromagnetic interference shielding measurements of CNT-based aerogel films were carried out in a WR-90 rectangular waveguide using a 2-port network analyzer (N5227A) in the X-band frequency range (8. 2-12.4 GHz). The samples were cut into a rectangular shape in a dimension of 24 × 12 mm 2 , which is slightly larger than the sample holder dimension (22.84 × 10.14 mm 2 ). Electromagnetic interference shielding efficiency can be calculated according to the following formula: 1 = | 11 | 2 = | 22  where R, A, T are the reflection coefficient, absorption coefficient, and transmission coefficient, respectively; Sij represents the s-parameter obtained by the network analyzer; SER, SEA, SET, and SEMR are the reflection effectiveness, absorption effectiveness, total shielding effectiveness, and multiple internal reflection effectiveness, respectively.
Specific shielding effectiveness (SSE) is derived to compare the effectiveness of shielding materials by taking into account the density, which can be obtained by dividing the EMI SE by the density of materials as follows: 1 SSE=EMI SE/density =dB cm 3 g -1 .
However, SSE has a basic limitation, that is, it does not account for the thickness information.

Orientation measurements for aerogel film
Herman's orientation factor (f) was calculated to describe the degree of orientation of the nanofibers as well as CNTs relative to the film plane using Equation (3) where the mean-square cosine is calculated from the scattered intensity I(ø) by integrating over the azimuthal angle ø according to Equation (4): 2 where ø is the angle between the film plane and the nanofibers as well as CNTs. f = 1 when all nanofibers and CNTs are parallel to the film plane and f = 0 for random orientation of nanofibers and CNTs.

Input power density for electrothermal conversion
The voltage was input by Precision Measurement DC Supply and the corresponding current could be obtained simultaneously. The CNT-based aerogel film was cut into a certain size (1×2 cm 2 ). Assuming that electrical energy is completely converted into heat, the input power density (PA) can be calculated by Joule's law with the following formula: 2 Where U is the input voltage, I is the current through the aerogel film and A is the area of the aerogel film.    Without carbonization, the ANF/CNT aerogel film exhibit a smooth surface structure with some small pores ( Figure S4a). Following the carbonized temperature increase, the pores on the CANF/CNT films got bigger and bigger, due to the pyrolysis of ANF and the generated gases. For CANF/CNT-550 ( Figure S4b) and CANF/CNT-650 ( Figure S4c), a continuous smooth surface can be observed. When the carbonization temperature increases to 750 o C, a more porous structure can be observed on the surface ( Figure S4d). For CANF/CNT-850 ( Figure S4e) and CANF/CNT-950 ( Figure S4f), the smooth surface structures were destroyed and obvious nanofibers network structure can be observed.  In order to investigate the morphology of ANF after carbonization, we prepared a pure ANF aerogel film without CNTs and the ANF aerogel film was carbonized under 950 o C by the protection of Ar. From the SEM image, the pure ANF aerogel film shows a three-dimensional network structure of nanofibers ( Figure S6a), and the nanofiber morphology of ANF was maintained after carbonization treatment ( Figure S6b).