## 3 Results and Discussion
### Effect of Graphene Framework on the Microwave Absorption Performance  
It is well known that the density of graphene aerogels could affect the framework character, which is the usual way to adjust microwave absorption properties. The relationship between the microwave absorption performance of OCGA and its density was investigated at first. OCGAs with densities of 2.43 mg/cm\({}^{3}\), 4.81 mg/cm\({}^{3}\) and 7.22 mg/cm\({}^{3}\) were prepared using the GO solution with the concentration of 5 mg/mL, 10 mg/mL and 15 mg/mL, respectively. The volume ratio of GO solution to DE was 4:1, and the rotational speed was 15k rpm. The obtained OCGA with low, medium and high densities were named OCGA-L, OCGA-M and OCGA-H, respectively.  
The framework morphology of OCGAs with different density was observed using SEM. As shown in Figure 4a-c, with the increase of density, the OCGA becomes denser, and the opening rate of the cell decreases obviously. Moreover, there are more graphene lamellae connecting the individual cells together as increase of density. These changes are easy to understand because of the increased graphene content. The different microstructures resulted in different microwave absorption performance of OCGAs, which were measured using the coaxial wire method. The results indicate that all the OCGAs display a typical frequency dependence of permittivity that both \(\varepsilon^{\prime}\) and \(\varepsilon^{\prime\prime}\) decrease with the increasing frequency. As the graphene content increases, a significant enhancement is achieved in \(\varepsilon^{\prime}\) and \(\varepsilon^{\prime\prime}\) (Figure 4d,e), which indicates that the aerogels have a growing ability of electrons store and dielectric losses. The change of \(\varepsilon^{\prime}\) and \(\varepsilon^{\prime\prime}\) is explained as follow. According to Equation (3), \(\varepsilon^{\prime}\) is related to the amount of polarization, which represents the ability to store electrical energy from the electric field. Aerogels containing much graphene can produce more dipole-like orientational polarization and interfacial polarization. Therefore, the increase in density of OCGA is responsible for the enhancement of \(\varepsilon^{\prime}\). In addition, according to Equation (4), the increase in \(\varepsilon^{\prime\prime}\) is not only attributed to enhanced polarization loss, but also to conduction loss. As the content of graphene increases, the conduction of aerogel. This promotes more electron migration between graphene cells and leads to an increase of the conduction loss. Therefore, \(\varepsilon^{\prime}\) and \(\varepsilon^{\prime\prime}\) both significantly enhance with the increasing graphene content. Furthermore, because the OCGAs are composed of pure graphene without any magnetic component, all samples show \(\mu^{\prime}\) and \(\mu^{\prime\prime}\) at around 1 and 0, indicating a negligible magnetic loss for incident electromagnetic wave [38].  
The microwave absorption performance was calculated and shown in Figure 4f-h. The OCGAs with the lowest or highest density did not show the best microwave absorption ability. This is because that the dielectric loss of OCGA-L is too poor, so a larger thickness is required to reach the RLmin. For OCGA-H with higher dielectric loss, most microwaves cannot enter inside this dense aerogel due to the mismatched impedance. Proper dielectric loss and good impedance matching (Figure S1) are obtained in OCGA-M, which reaches a RLmin of \(-37.62\) dB at the matched thickness of 4.5 mm and the largest EAB of 7.07 GHz at the matching thickness of 4.0 mm. Thus, the OCGA with medium density was adopted in the follow-up study.  
Figure 4: Characterization of OCGAs with different graphene framework. SEM images of **(a)** OCGA-L, **(b)** OCGA-M and **(c)** OCGA-H. **(d)** Real part (\(\epsilon^{\prime}\)) and **(e)** imaginary part (\(\epsilon^{\prime\prime}\)) of permittivity, and RL of **(f)** OCGA-L, **(g)** OCGA-M and **(h)** OCGA-H.