**1. Introduction**

In response to an increasingly carbon-constrained world, the adoption of policies aimed at developing new technologies has emerged in the face of cleaner energy production. In this context, direct alcohol fuel cells (DAFCs) have been recognized as promising systems to provide continuous and low-carbon power supply. Basically, a DAFC operates by electrochemically oxidizing an alcohol, such as methanol or ethanol, at the anode, to produce protons (H+ ions), and electrons. Protons are transferred to the cathode through the proton exchange

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

membrane and react with OH− ions, which are generated from the electrochemical reduction of oxygen, at the cathode, to produce water, heat, and electricity [1–4].

Since the above observations reinforce the potential of intentional rGO modification as a strategy for boosting the 2D mass and charge transfer, defect engineering in rGO, which refers to the introduction of controlled defects in the material structure, is focused on this mini-review. Although this chapter makes no attempt to be exhaustive, the present contribution describes new breakthroughs on defect engineering in rGO that have recently been published since 2017, including recent advances and trends on state-of-the-art synthesis and utilization of engineered rGO sheets as fuel-cell support materials for the methanol and ethanol oxidation

Defect Engineering in Reduced Graphene Oxide toward Advanced Energy Conversion

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reactions. Future perspectives for further development are also proposed.

**2. Overview of defect engineering in rGO for energy conversion**

proposed toward the development of the next-generation rGO support materials.

frameworks (MOFs) are proposed as future perspectives of further development.

**2.1. Current trends in 3D structure assembly for enhanced mass transfer**

Defect engineering in 2D semiconductor technology refers to the introduction of controlled defects at the atomic level, such as heteroatoms and size-controlled vacancies, for the modification of the two-dimensional structure and properties. In spite of these, other two strategies, a 3D structure assembly approach and a surface metal complexation methodology, have been included as part of a broadened view of defect engineering in reduced graphene oxide, as summarized in **Figure 1**. To produce tailor-made support materials with desirable characteristics for fuel-cell catalysis, the usage and/or combination of defect-induced procedures is

In the first subsection of this mini-review, recent progresses on the synthesis and electrocatalysis of 3D engineered rGO-based platinum catalysts toward methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) are discussed. Then, advances in heteroatom doping for designing highly conductive three-dimensional rGO-based platinum catalysts and the impacts on electrocatalysis are presented. In the last subsection, research directions on surface metal complexation and size-controlled defect formation through metal-organic

Crumpling the sheets into porous frameworks has been highlighted as an attractive methodology for enabling the interaction among nanoparticles and reactants. Kwok et al. [44] produced a high-quality platinum-decorated rGO aerogel with the aid of a solvothermal method. Their observations indicated that the rGO aerogel porous framework can be optimized by simply changing the GO concentration input for the gelation process. Tests on the supported ultrafine platinum nanoparticles (sizes ranging from 1.5 to 3 nm) showed that the electrochemically active surface area (ECSA) increased by about 8.92 times in comparison to a benchmark Pt/C catalyst, resulting in a 358% increment in specific power for a methanol-fed fuel cell.

In order to enhance catalyst utilization, Radhakrishnan et al. [45] fabricated a three-dimensional assembly of platinum nanostructures with dominant (100) plane on rGO by a co-electrodeposition method. They found that the morphology, active site, and the electrochemical activity of the catalyst were highly dependent on the number of electrochemical cycling used for the deposition. Their nanocomposite showed a high mass activity toward MOR, which

In spite of the attractiveness of these non-stop and low-carbon energy generation systems, important commercialization issues still need to be addressed. The kinetics of the alcohol oxidation reaction largely determines the overall efficiency of the fuel cell. In order to boost conversion efficiencies, highly active catalysts are required because of the low operating temperatures (60–120°C). Therefore, far, it is undisputed that platinum (Pt) provides the best correlation between energy adsorption and exchange current density [5]. With studies demonstrating the high instability of Pt catalysts [6–9] and the overall performance dependence on large Pt loadings [10–12], it has become imperative to design improved, durable, and highly efficient electrocatalysts.

Various attempts, such as the dispersion of Pt on high area conductive supports [13–19] and its combination with another metal [20–23] have been addressed for improving Pt utilization in fuelcell reactions. Regarding the former approach, it is well-known that a suitable fuel-cell support provides a high surface-to-volume ratio of metal particles, which, in turn, maximizes the available area for electrochemical reactions [24]. In comparison to state-of-the-art C black, reduced graphene oxide (rGO) sheets have been demonstrated as an advanced electrocatalyst support for DAFCs due to the unique characteristics of the two-dimensional (2D) structure [24, 25]. The high theoretical surface area (2.630 m2 ·g−1 for a single layer) and ultra-large surface-to-volume ratio, when combined with the fast heterogeneous electron transfer (HET) rate, high specific capacitance (550 F·g−1), and intrinsic redox activity, make rGO an ideal platform for homogeneous dispersion of Pt nanoparticles and faster charge and mass transport properties [26–28].

In spite of the appealing properties noted above, restacking of the sheets due to the strong van der Waals interaction greatly reduces the accessible Pt surface area, resulting in low catalyst utilization, and transport pathway for reaction species. Material processing techniques, broadly defined as the approaches for tailoring physicochemical properties, have been extensively applied to control the interactions between rGO sheets and make them aggregationresistant in both wet and solid state [29]. In this context, some solutions have been paving the way for further research and development on the assembly of two- or three-dimensional (3D) structures with desirable microstructural features for electrocatalysis. Positive progresses, such as the development of intercalation composites [30–34] and the usage of geometrical modification strategies [35–37], have greatly improved the utilization of supported Pt catalysts by increasing the density of exposed active sites.

Besides tailoring the physicochemical properties of the 2D structure, further advantageous characteristics for energy-conversion applications may be achieved by tuning the bandgap relative to the Dirac point in the C─C double bond network. Through electronic modulation of the support, high catalyst activity may be achieved by tuning the interaction between support and catalyst surfaces. In this sense, geometrically modified and/or heteroatom-doped rGO sheets, that is, can facilitate the property control of the Pt-support electronic effects. By enriching catalyst electronic structure due to catalyst/support synergism, novel characteristics, such as smaller catalyst particle size, increased catalyst particle dispersion, increased catalyst durability and stability, can effectively improve catalyst utilization [38–43].

Since the above observations reinforce the potential of intentional rGO modification as a strategy for boosting the 2D mass and charge transfer, defect engineering in rGO, which refers to the introduction of controlled defects in the material structure, is focused on this mini-review. Although this chapter makes no attempt to be exhaustive, the present contribution describes new breakthroughs on defect engineering in rGO that have recently been published since 2017, including recent advances and trends on state-of-the-art synthesis and utilization of engineered rGO sheets as fuel-cell support materials for the methanol and ethanol oxidation reactions. Future perspectives for further development are also proposed.
