**5. Conclusions and outlook**

The rapid development of graphene-based nanohybrid electrocatalysts for energy conversion, storage, and various electrochemical sensors has been driven by their unique structural features, novel physicochemical properties, high stability, and low cost. The immobilization of transition metal oxides on graphene by various methods via the use of interactions between the structural defects and functional groups on graphene's surface, contributed to the im‐ proved catalytic activity and stability of graphene-based transition metal oxide nanohybrid catalysts. In this chapter, we have discussed the recent development of graphene-supported transition metal oxide nanohybrid materials for their sensor and energy applications, includ‐ ing their synthesis, structural characterizations, key properties, and major applications. The control in the morphology and dimension of the graphene-based nanocomposite is of crucial importance for their electrocatalytic activities. As a consequence, multivalent transition metal oxides with special structural properties exhibit more efficiency for the electrocatalytic reactions than amorphous nanocomposites under similar experimental conditions. Electroca‐ talytic performance of graphene-based transition metal nanohybrid materials can be charac‐ terized by different electrochemical techniques in detail. Stability and durability are among the most important factors for promising electrocatalytic applications. The flexibility and large surface area of graphene sheets could prevent particles from agglomerationand facilitate accommodation of large amount of particles. Monotransition metal oxides anchored on graphene have already shown good electrocatalytic performance with long-term stability. However, the strategy of synthesizing bimetallic transition metal oxides is very promising for further elevating the electrocatalytic activity. Also, covalently bonded bimetallic oxides on graphene can offer better activity and longer durability than the physical mixture of two types of metallic nanoparticles. Therefore, the rational design of cationic substitution and covalent coupling with graphene supports can instruct the construction of advanced electrocatalysts for sensor- and energy-related applications. Thus, the advancement of sophisticated structurecontrolled methods and processes for the *in situ* synthesis of graphene-based transition metal oxide nanocomposites is crucial for the development of next-generation electrocatalysts. It should be noted, however, that the fundamental mechanisms behind the electrocatalytic performance of graphene-based nanocomposites are far from being fully understood. More‐ over, the electronic interactions between graphene and nanoparticles and the synergistic effects are needed to be explored. These remained challenges will motivate strongly many ongoing research and further development of this field.
