## 1 Introduction  
In recent years, emerging technologies such as Internet of Things, 5G communication and satellite networks bring about kinds of electromagnetic waves in our environment, which are harmful to human health, electronic devices, aerospace, information security, etc. Therefore, in order to solve above problems, the development for high-performance electromagnetic absorbing materials with low reflection, wide absorption band and small density becomes an urgent need. Graphene aerogels have been widely studied in the electromagnetic absorption applications because of their excellent electrical properties, lightness and high specific surface area. However, their poor matching impedance and single attenuation mechanism lead to unsatisfactory absorption performance [1]. So various inorganic second phases, such as ferrite [2; 3; 4; 5], Ni [6; 7; 8], MXene [9; 10], SiC [11] and others [12; 13] are added in graphene aerogels to tune impedance matching properties for reaching good microwave absorption performance [6]. However, additional inorganic phases usually have a large density, which will increase the weight of aerogels and make the preparation process complex. Thus, the lightweight, pure graphene aerogel realizing efficient microwave absorption has been rarely reported to date [14].  
In order to meet the demands for light weight and high efficiency, researchers focus on microstructural adjustment of graphene aerogels. Various methods including mechanical compression, oriented structure and changing porosity have been applied to improve microwave absorption performance of aerogels [4; 15; 16; 17; 18]. Reflection loss (RL) is one of the important indexes to evaluate the microwave absorption performance. It refers to the ratio of the incident power of microwave to the reflected power and is usually expressed in dB. For a single-layer microwave absorber, the reflection loss can be expressed as a function of the normalized input impedance of a metal-backed absorber [19; 20]. From the relative permeability and dielectric constant in a given frequency range, the reflection loss of the material under different thickness can be calculated (the thickness of the absorber is matching thickness). Although RL\({}_{\text{min}}\) of graphene aerogels can be reduced to below \(-60\) dB by above methods, a large thickness (about 5 mm) or high fill rate (0.74 wt%) is necessary [15; 21]. This is mainly due to the single structure feature of normal graphene aerogels, which introduces the single microwave absorption way. Therefore, building multiple microstructures to bring about various absorption mechanisms will be useful for efficient microwave absorption.  
Herein, to obtain the strong microwave absorption performance, an open-cell structure was introduced into graphene aerogels via the self-assembly and freeze-drying process. The OCGA with multiple microstructures exhibited an enhanced RL\({}_{\text{min}}\) of \(-51.22\) dB, whose density was only 4.81 mg/cm\({}^{3}\). To improve the microwave absorption properties further, the OCGA with a gradient cell diameter structure (OCGA-G) was designed and prepared. This gradient aerogels without any additional phase showed a maximum effective absorption bandwidth (EAB, below \(-10\) dB) of 8.55 GHz and a RL\({}_{\text{min}}\) of \(-62.58\) dB at the thickness of 3.2 mm and ultra-low fill rate (0.53 wt%). In our opinion, the multiple, gradient structure of OCGA-G greatly increases the microwave transmission path, strengthens multiple reflection loss and provides an excellent impedance matching characteristic, so that OCGA-G achieves the ultra-light, broadband and strong microwave absorption.