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Nanomaterials 2020, 10, 1407 5 of 26 applications in the pulp-and-paper, textile, synthetic chemical, and waste-water-treatment industries. Thus, recent trends in the electrochemical reduction of O2 require hydrogen peroxide as the desirable product, since selective synthesis of hydrogen peroxide through ORR provides a cheap, ecological, and safe way for its production [49]. Without a doubt, the investigation and exploration of novel, less expensive, and abundant electrocatalysts for the ORR is of paramount importance. Recent research has focused on highly active catalysts driving the 4e− ORR, including Pt alloys [48], core-shell nanostructures [50], transition metal oxides [51], and chalcogenides [52], and carbon-based non-noble catalysts [53,54]. Carbon-based materials, such as graphene, doped graphene, and CNTs have been studied as conductive supports in ORR due to their excellent conductivity and high surface area [52–55]. Although CNTs have been tested in electrocatalysis [53], they lack certain essential characteristics for an ideal electrocatalyst, such as low preparation cost, high porosity, and high surface area. In an attempt to overcome these limitations, CNHs were used as starting material to prepare CNT-encapsulated with iron oxide nanoparticles (Fe-CNH/CNT), by annealing a mixture of CNHs, melamine, and iron salt [56]. In that way, the as-prepared nanotubes featured a tubular morphology with loose graphene layers, while they maintained the high surface area of CNHs, which was indeed the highest reported for metal encapsulated nanotubes (750 m2/g). The extremely stable electrocatalyst was tested towards ORR and showed lower overpotentials of 0.16 and 0.10 V vs. RHE in alkaline and acidic solutions, respectively, than reference electrocatalyst Pt/C. The Fe-CNT was also tested as cathode catalyst in PEMFCs with maximum power density of 200 mW/cm2, showing the prospect of this type of encapsulating carbon morphologies as Pt-free catalysts for fuel cells [56]. There is no question that doping nanocarbon materials with heteroatoms can increase their catalytic activity and selectivity towards specific reactions. Particularly, the incorporation of heteroatoms such as N, B, S, and P within the graphene skeleton induces changes in the electron density distribution, due to the electronegativity difference of the foreign element with carbon atoms, that effectively tunes the electronic states of the material [57]. This effect, along with charge density, results in improvement in the electrical conductivity of the material. Heteroatom-doped carbon-based nanomaterials have been ascribed as promising cathodes for fuel cells. Nevertheless, their catalytic ability towards ORR is still insufficient compared to Pt-based catalysts and specifically, in terms of the onset potential. The creation of adequate active sites on the electrocatalysts is imperative in order to reduce the required energy for the reduction of O2 and achieve high-density chemisorption of dioxygen. It is widely accepted that the edges of carbon nanostructures are more active towards doping. Therefore, incorporation of more edges and/or improvement of the porosity of the surface are two effective ways to modulate the active sites [58]. Much effort has been given into enhancing the surface area of carbon nanomaterials, including hydrothermal methods, hard- and soft-template assisted syntheses, and annealing high surface area materials, such as metal–organic frameworks (MOFs) [59,60]. However, their electronic conductivity can be affected by the synthetic procedure [61]; thereby, it is extremely important to adopt preparation routes that can ensure electrocatalysts with high surface area that maintain their electronic conductivity. Normally, CNHs possess a surface area of around 300–400 m2/g, while it was observed that their surface area decreases by increasing treatment temperature [62]. Furthermore, when the surface of porous carbon nanomaterials increases, the electrical conductivity is reduced, implying strong relations between electrical conductivity and porosity, both of them being needful for materials aiming at energy-device applications. Modification of CNHs can further enhance their surface from 350 to 1700 m2/g [62]. A simple method for tuning CNHs’ surface area is oxidation [63–65]. In this manner, oxidation was used to open up the tips of CNHs and achieve high electrochemical surface area [65]. The oxidized CNHs were used as support for the immobilization of Pt nanoparticles and employed as anode electrocatalysts in PEMFC, with an overpotential of ~0.63 V [66]. Heteroatom doping is definitely an efficient strategy to enhance conductivity as it favorably modulates the electron density of the carbon nanomaterial [67]. Apart from that, in situ doping of heteroatoms can increase the number of defects of CNHs without using metal catalysts [57].PDF Image | Carbon Nanohorn-Based Electrocatalysts for Energy Conversion
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