## 3 Results and Discussion
### OCGA with Gradient Structure for Efficient Microwave Absorption  
Gradient design is acknowledged as an effective strategy for improving the microwave absorption ability nowadays [40]. In order to reduce the thickness and improve the absorption performance of OCGAs further, the size distribution of cells is adjusted to obtain better impedance matching characteristics. By means of density difference between DE (1.08 g/cm\({}^{3}\)) and GO solution (1.01 g/cm\({}^{3}\)), the DE microspheres in Pickering emulsion can be accelerated to settle by the simple centrifugation treatment (Figure 8a). In addition, based on the equation of Stokes law (Equation (6)) [41], the larger the microspheres are, the greater the sedimentation rate is. \[v=\frac{2 gr^{2}(\rho-\rho^{\prime})}{9\eta} \tag{6}\]  
Here, \(g\) is the acceleration due to gravity, \(r\) is the droplet radius, \(\rho\) is the density of the dispersed phase (DE in this study), \(\rho^{\prime}\) is the density of the continuous phase (GO aqueous solution in this study), and \(\eta\) is the shear viscosity of the continuous phase. Based on this, the Pickering emulsions containing DE microspheres with different sizes were centrifuged to obtain a gradient distribution. The OCGA-G with gradient cell size cell was first prepared using this centrifugated emulsion through the process of freeze-drying, vapor ejection and chemical reduction in turn. Furthermore, as comparison groups, the OCGAs using the same Pickering emulsions after mixing (OCGA-m) and precipitating for 2 h (OCGA-P) were also prepared, respectively. The SEM of OCGA-G shows an obvious gradient structure along thickness direction (Figure 8b-d). At the top of OCGA-G, the cells are relatively integrated with an average diameter of 7 \(\upmu\)m and packed tightly. The middle position mainly contains the medium opening cells with a large average diameter of 12 \(\upmu\)m. The bottom region of OCGA-G is very loose and has the largest, completely opening cells with an average diameter of 28 \(\upmu\)m. These structure characteristics are due to that at the top regions, the small cells bear light  
Figure 7: Effect of cell structure on the compression performance. (**a**) Compressive strength of graphene aerogels. (**b**) Shapes of GA before, during compression and after compression. (**c**) Shapes of OCGA-20K before, during and after compression. centrifugal force, so that its structure is nearly unbroken. But the large microspheres at bottom bear strong centrifugal force, which causes emulsion breaking easily during the centrifugation. By comparison, OCGA-m does not show the gradient structure (Figure S5), and OCGA-P has an inconspicuous gradual morphology (Figure S6). The distribution state of the cells has a significant effect on the electromagnetic parameters (Figure S7) and microwave absorption performance of OCGAs. The RL\({}_{\text{min}}\) for OCGA-m (Figure 8e) is \(-35.44\) dB at a matching thickness of \(5\) mm. After precipitation treatment, the matched thickness of OCGA-P decreases to \(3.4\) mm, the RL\({}_{\text{min}}\) decreases slightly to \(-42.88\) dB. Furthermore, the peak shifts towards the high frequency region (Figure 8f). For OCGA-G, the RL\({}_{\text{min}}\) is declined dramatically to \(-62.58\) dB at \(14.73\) GHz, and the matching thickness is also reduced to \(3.2\) mm. Furthermore, the maximum value of EAB reaches \(8.55\) GHz in OCGA-G at a matching thickness of \(3.7\) mm (Figure 8g,h). It is noteworthy that the graphene filling rate in OCGA-G is only \(0.53\%\), which is apparently lower than that of graphene aerogel with similar microwave absorption performance (graphene filling rate is \(0.74\) wt%) in the reported literature [15]. We consider that these outstanding performances of OCGA-G root in its gradient structure, and the different positions in aerogels play different roles in the microwave absorption.