## 3 Results and Discussion
### Effect of Open-Cell Structure on the Microwave Absorption Performance
#### 3.4.1 Effect of Cell Number on Microwave Absorption Performance  
The cell number was regulated by the ratio of DE to GO solution during emulsion preparation process. The OCGAs with a growing number of cells were fabricated using Pickering emulsion with increased DE/GO solution ratios (_v/v_), 0, 0.125, 0.250 and 0.375 at the same rotational speed. The obtained graphene aerogels were named GA, OCGA-DE1, OCGA-DE2 and OCGA-DE3, respectively. As shown in Figure 5a, the GA prepared without DE shows a smooth lamellar structure and has lots of large pores. When a little DE is added, the obtained aerogels still show a lamellar structure, but there are lots of hollow bulges in the lamellae, which are unformed cells with a diameter of about 7 \(\upmu\)m (Figure 5b). With the rising content of DE, the number of cells having approximately the same diameter increases further, which disperse uniformly in the aerogel (Figure 5c). However, the graphene lamellae between cells almost disappear when more cells are formed (Figure 5d), because most of the graphene have been assembled to form cells. The electromagnetic parameters of graphene aerogels with different number of cells were tested, and their RL of the aerogels at different matching thicknesses was calculated. The results indicate that the GA without cell structure shows a RL\({}_{\min}\) of \(-30.65\) dB (Figure 5e). When a few cells are formed, the RL\({}_{\min}\) of aerogel increase slightly to \(-23.34\) dB of OCGA-DE1 (Figure 5f). As the number of cells increases further, the RL\({}_{\min}\) decreases first and then increases (Figure 5g,h). Finally, the OCGA-DE2 obtains the smallest RL\({}_{\min}\) of \(-51.22\) dB. As we all know, a large number of defects and interfaces exist in reduced GO (Figure 5i), leading to various polarization, which can greatly consume the incident microwave. For the  
Figure 5: Effect of cell structure on the microwave absorption performance. SEM images of (**a**) GA, (**b**) OCGA-DE1, (**c**) OCGA-DE2, (**d**) OCGA-DE3. RL of (**e**) GA, (**f**) OCGA-DE1, (**g**) OCGA-DE2 and (**h**) OCGA-DE3. (**i**) Polarization from carbon. (**j**) Microwave absorption mechanism of GA (The black arrow indicates incident microwave, the red arrow indicates reflected microwave, and the green arrow indicates transmitted microwave). (**k**) Microwave absorption mechanism of OCGA. conventional graphene aerogels without cells, the graphene framework provides lamellae for electromagnetic wave reflection and conductive paths for electron migration. However, the graphene lamellae occupy less space, and there is little apparent obstacle for the transmission of microwave (Figure 5). Therefore, there is a little reflection loss in this aerogel. When a small amount of DE is added, the emerging hollow bulges affect the electron migration, which will reduce the conduction loss, so that microwave absorption property decreases firstly. Then with the increase of cell number, the cells are opened gradually, and the specific surface area of the OCGA increases obviously. The various loss mechanisms in this multiple structure are formed, as shown in Figure 5. On the one hand, beyond absorption of graphene framework, the open-cell structure results in frequent refraction and reflection of microwaves inside and outside the cells, which greatly increases the transmission path of microwave and improves the reflection loss. On the other hand, we consider that the cells construct a microscale spherical conductive structure, which can respond to incidence microwaves as huge resistance-inductance-capacitance coupled circuits [39]. When the incident microwaves reach this structure, the induced currents under time-varying electromagnetic fields are generated easily in the spherical circuit and converts the electromagnetic energy into heat. Based on above theory, the various loss mechanisms for microwaves exist in OCGA with modest cells, which will greatly improve the microwave absorption ability of aerogel.