**2.1. Introduction**

optical transparency, and tunable band gap [1, 2]. This material has a theoretical surface area

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

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

many other interdisciplinary fields such as nanotechnology and nanomedicine.

features, electrocatalytic applications, and future perspectives.

V-1 s-1 at a carrier density of ~1012 cm-2, and the highest

of 2630 m2

g-1, a mobility of 200,000 cm2

380 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

In terms of electrocatalysis, graphene-supported metal oxides nanohybrid materials have exhibited promising applications because of the following well-recognized advantages. *Firstly*, the large surface area and 2D flexibility of graphene nanosheets can offer sufficient space to accommodate different nanomaterials and also prevent their agglomeration. *Second‐ ly*, due to the good superficial characteristics of graphene, solid-air contact efficiency increases and simultaneously the amount of oxygen adsorption also increases. *Thirdly*, the electrical conductivity of graphene promotes the electron transfer rate on the surface. *Finally*, the structural defects of graphene also provide more active sites for further modification with different functional groups to promote the selective electrocatalysis.

Incorporation of inorganic nanomaterial onto the surface of graphene has attracted tremen‐ dous attention for the development of new-generation catalytic materials [13]. These novel nanostructures show superior electrocatalytic activity, selectivity, and long-term stability, which can serve as promising electrode material for different electrochemical reactions. Different metal oxides are promising in this regard for the replacement of noble-metal-based electrocatalysts. But normally metal oxides are poorly conductive and often suffer from dissolution and aggregation during the electrochemical reactions [14, 15]. It was proposed that anchoring metal oxide nanostructures onto graphene surface by suitable synthetic procedure could solve the problems. Indeed, some graphene-supported metal oxide nanocomposites have displayed remarkable improvements in electrocatalytic activity and stability toward some crucial electrochemical reactions in amperometric sensors and energy storage and conversion.
