**1. Introduction**

Graphene, a single atomic thick layer of graphite with closely packed conjugated and hexag‐ onally connected carbon atoms, has attracted tremendous attention since its discovery in 2004 because of its large specific surface area, high-speed electron mobility, good mechanical strength, high electric and thermal conductivity, room temperature quantum hall effect, good

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optical transparency, and tunable band gap [1, 2]. This material has a theoretical surface area of 2630 m2 g-1, a mobility of 200,000 cm2 V-1 s-1 at a carrier density of ~1012 cm-2, and the highest electrical conductivity of 106 s cm-1 at room temperature [3, 4]. Also, graphene has a Young's modulus of ~1 TPa, breaking strength of 42 N m-1 [5], and thermal conductivity of 5000 W m-1 K-1 [6]. For all these versatile properties and obvious advantages, a rapid development of graphene-based materials has been witnessed in the fields of chemistry, physics, biology, and many other interdisciplinary fields such as nanotechnology and nanomedicine.

Electrocatalysis is a special type of catalysis that speeds up the rate of an electrochemical reaction occurring on electrode surfaces or at liquid/solid interfaces. Various kinds of electro‐ chemical reactions are involved for different electrocatalytic applications in energy and sensorrelated fields. Therefore, design and fabrication of advanced electrocatalysts with outstanding performance and low cost are of great significance for the commercialization of electrocata‐ lytic-system-based energy devices. Metal oxide nanostructures have been identified as one of the most important electrocatalytic materials due to their several advantages. *Firstly*, they have exceptional electrical, optical, and molecular properties. *Secondly*, there is further possibility to insert more functional groups on the surface for the immobilization of other biological catalysts. *Thirdly*, metal oxides have higher alkaline corrosion resistance compared to other materials in electrochemical environmental due to the stabilization of the higher oxidation state of the transition metals. *Finally*, their unique crystalline structures benefit in preventing the agglomeration of metal oxide nanostructures with their size retained [7]. Besides electro‐ catalytic activity and cost, the stability and durability of a catalyst are very critical issues for practical applications. In addition, a good catalyst support is needed, which should have a large surface area for catalyst dispersion, excellent electronic conductivity, and high electro‐ chemical stability in different electrolytes. In recent years, various catalyst support materials have been proposed and studied. In this regard, graphene nanosheets have shown promising characteristics for wide applications as 2D support materials for different electrocatalysts. In the last decade, intensive efforts have been devoted to functionalize graphene-based nano‐ materials and to explore their applications in sensors [8],[9], electrochemical energy storage [10, 11], electronics, optoelectronics [12], and others. Graphene-based nanocomposites provide a new option as electrode materials in the field of electrocatalytic applications due to their high electrical conductivity, high surface area, and richness of functional groups for further modification. The rapid development of low cost and facile preparation methods of graphenebased nanocomposites has promoted their practical industrial applications. The catalytic activity of the graphene-supported catalysts can be improved, due to enhanced electronic communication (e.g., charge transfer) between catalysts and support. Furthermore, arising from their synergistic effects of graphene sheets and functionalized components, the nano‐ composites can offer novel physicochemical properties and consequently improve electro‐ chemical performances. As a result, graphene-based nanocomposites have thus been regarded as one of the most promising hybrid materials that can drive the development of more efficient next-generation energy devices as well as applied in the fabrication of electrochemical and gas sensors. This chapter is focused on one of such nanocomposites, made of chemically exfoliated graphene and metal oxides. We overview and discuss their synthesis methods, structural features, electrocatalytic applications, and future perspectives.
