**2.2. Major synthetic methods**

The increasing interest in graphene-based metal oxides nanohybrid materials for different electrocatalytic applications has led to a variety of new processes proposed for the synthesis of nanocomposite materials. Some recently established methods are discussed in this section.

## *2.2.1. Solvothermal and hydrothermal synthetic approaches*

Solvothermal and hydrothermal synthetic approaches are one of the most common synthetic strategies for the development of different graphene-based nanohybrid electrocatalysts. Song et al. [16] reported a new hydrothermal approach for the preparation of CuO/GO nanocom‐ posite materials, which used cupric acetate and graphene oxide as precursors. The reaction was conducted for 10 h under different temperatures (120, 150, and 180°C). The final product was washed by deionized water and dried for the electrocatalytic applications. Dong et al. [17] introduced another hydrothermal approach for the preparation of Graphene/Co3O4 nanowire composite. They used presynthesized 3D graphene and CoCl2.6H2O as starting materials. This reaction was proceeded in autoclave at 120°C for 16 h. Dai et al. [18] also used a hydrothermal approach for the synthesis of covalent hybrid of spinel manganese−cobalt oxide and graphene. They employed a two-step procedure to synthesize MnCo2O4–graphene oxide nanohybrids. In the first nucleation step, Co(OAc)2 and Mn(OAc)2 were mixed at 80°C with mildly oxidized graphene oxide in an ethanol/water NH4OH solution. In the second step, hydrothermal treatment was done at 150°C to achieve the nitrogen-doped graphene. And the final material showed an excellent electrocatalytic activity for oxygen reduction reaction (ORR). Dai et al. [19] also used cobalt acetate and GO as a precursor for the hydrothermal synthesis of Co3O4/ graphene hybrid bifunctional catalyst for ORR and water oxidation or oxygen evolution reaction (OER).

**Figure 1.** Schematic illustration of the hydrothermal synthesis of 3D graphene/MnO2 (a), and schematic illustration of electrons transfer on the 3D hybrid material (b). (Reproduced with permission from ref. 44 Copyright 2014 American Chemical Society)

Wang et al. [20] reported a hydrothermal approach for the synthesis of CoO/rGO nanocom‐ posite using GO, Co (Ac)2.4H2O, Co(NH2)2 as a precursor (190°C, 2 h). Mullen et al. [21] successfully synthesized 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles by hydrothermal approach for the efficient electrocatalysis of ORR. Graphene oxide, iron acetate, and polypyrrole were hydrothermally assembled at 180°C for 12 h to form a 3D graphene-based hydrogel. The hydrogel was further dehydrated and annealed at 600°C for 3 h under nitrogen atmosphere.

Hydrothermal methods for the synthesis of graphene-based hybrid materials can be carried out at different temperature ranges, up to 190°C [22]. Figure 1 shows an example for the preparation of 3D graphene–MnO2 composites. There are several advantages of using hydrothermal process. *Firstly*, the strong electrolyte water possesses a high diffusion coeffi‐ cient and dielectric constant under hydrothermal reaction conditions, which helps to remove the oxygen-containing groups via dehydration and also accelerates heterolytic bond cleavage. *Secondly*, this is a green reduction approach and does not add any further impurities to the final product. *Thirdly*, the degree of reduction and the properties of the hybrid material can be tuned by adjusting temperature and pressure of the reaction. And *finally*, the process could be easily implemented in an industry scale with a relatively simple setup (an autoclave) and low cost. A possible disadvantage of the method could be the consumption of significant amount of energy.
