Owing to their low specific weight, high stiffness, superior temperature and corrosion resistance, and ease of processability, carbon fiber-reinforced polymeric (CFRP) composites are used in broad range of applications from aerospace to marine industries [1]. CFRPs display excellent tensile strength; however, the brittleness, crack-prone matrix and the fiber–matrix interface with a weak adhesion usually lead to a failure of the composite structures in terms of delamination and catastrophic failure [2,3]. In this direction, carbon nanostructured forms (CNFs) can help to overcome these criticalities. The use of CNFs in bulk epoxy resins and their composites has been pursued by many authors, not only to increase the mechanical performance of the resulting composites but also to confer them functional properties, such as de-icing, self-sensing, and self-healing, or to save energy during the manufacturing process [4,5,6,7]. Multi-wall carbon nanotubes (MWCNs) have been successfully incorporated in CFRCs, imparting them electrical conductivity, whose value depends on the process adopted for the manufacturing [8,9]. Graphene-based nanoparticles are expected to further improve, with respect to CNTs, some functional properties depending on the thermal conductivity of the composite (thermal management), and to reduce the humidity content in composites based on epoxy resins [10]. For these reasons, many authors have made tremendous efforts to obtain graphene in the form of single layers, for example, training to completely exfoliate graphite particles (to reach almost 100% of exfoliation). Despite the large efforts made to obtain and employ perfect and almost single graphene layers, no high-performance materials, in terms of electrical properties and related functional properties, have been obtained. This is because defect-free single graphene layers tend to reassemble during the steps of nanocomposite manufacturing. To avoid reassembling arrangements, functionalization procedures are needed. This allows the attaching of chemical groups to the graphene layers able to prevent the re-assembling because of steric and energy factors. If the functionalization is performed through functional groups covalently bonded to the single graphene layers, the reassembling phenomena are prevented, but the change in the hybridization state of the carbon atoms, from sp² to sp³, in single layers results in the partial loss of the phenomenon of electron delocalization of carbon nanoparticles and, as a consequence, in the impoverishment of their electronic properties [11]. To meet this scientific and technical challenge, it is possible to use graphene oxide nanosheets, which are constituted of the stacking of different graphene layers (able to manifest the desired phenomenon of electron delocalization) and polar functional groups on the edges of these thin graphene-based blocks. GO nanoparticles dispersed in the polymeric matrix are expected to also improve the interfacial adhesion between woven carbon fibers and the epoxy matrix during the impregnation process of the woven fibers. In fact, GO nanoparticles are composed of polar groups, especially on the edges of the nanosheets, able to provide strong interaction with the hosting cured epoxy matrix (composed of a large number –OH polar groups) and the “core” part, similar to the structure of CFs, therefore being able to establish attractive interactions with the reinforcing CFs of the woven fibers. This is a hot topic worth investigating for the obvious applicative implications. There are many relevant papers in the literature that have revealed that the presence of GO nanosheets in the epoxy resin should allow the conferring of functional properties to the resin, whereas, in other papers, GO should promote better adhesion between carbon fibers and the polymeric matrix. This review aims to investigate recent advances achieved in this last direction. To improve the interfacial properties of the composites, a resilient interfacial adhesion between the fiber and matrix plays a significant role. In recent years, a series of methods have been introduced to improve the interfacial adhesion between the fiber and matrix such as the coating of fibers, sizing, 3D weaving, and tufting [12]. Unfortunately, these approaches are only applicable to textile laminates made with resin transfer molding (RTM) procedures, and are thus not applicable on prepreg laminates [13,14]. The poor defective surface of carbon fiber lacks the functional groups on its surface and thus has insufficient wettability and interaction with the polymer matrix. Functional groups such as hydroxyl –OH, carboxyl –COOH, and carbonyl –CO are added on the surface of CFs or in the epoxy matrix to boost their functionality as a CFRP reinforcement [15]. The effect of the functionalization of carbon fibers or epoxy with nanoparticles such as carbon nanotubes, nano clay, and graphene nanoplatelets has been investigated in detail [16,17,18,19,20,21,22,23,24,25,26]. However, control of the carbon nanotubes (CNTs) and nano clay dispersion is difficult and not cost-effective. In addition, their agglomeration onto a fiber surface leads to reductions in the glass transition temperature (T_g) and mechanical properties of the developed composites [27]. Recently, to attain good interfacial adhesion, the modified graphene was significantly incorporated in CFRP composites [28]. The improvement in the thermal conductivity of the CFRP by grafting carbon fibers with a 3D graphene network was examined, where an increase in thermal conductivity of about 165% in comparison with that of pure CFRP was observed for the graphene-incorporated composites [29]. The fatigue behavior and mode I fracture toughness of CFRP functionalized by graphene nanoplatelets (GNPs) was investigated, and it was observed that with 0.1% of GNPs, the mean fatigue life and mode I interlaminar fracture toughness increased to 155% and 40%, respectively [30]. A strong interaction was observed in the case of GO-incorporated composites as compared to unfunctionalized graphene sheets-incorporated composites due to the abundance of various functional groups [31]. The use of graphene oxide functionalization with polymers has gained momentum after this development. The functionalization of graphene oxide resulted in an improvement in interfacial adhesion in terms of preventing delamination and crack propagation. 

2. Applications of Carbon Fiber-Reinforced Polymer (CFRP) Composites