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Nanomaterials 2020, 10, 1407 8 of 26 activities against ORR [99,100], but still not as good as of Pt-based catalysts in both acidic and alkaline conditions. This is mostly due to the instability of the selenide and sulfide at higher positive potentials [101]. In order to overcome this obstacle, CoSe2 nanoparticles decorated on nitrogen-doped CNHs (CoSe2/N-CNHs) were tested against ORR and compared to CoSe2 nanoparticles supported on carbon. The CoSe2/N-CNHs hybrid showed lower onset overpotential by 50 mV compared to carbon supported CoSe2. More interestingly, CoSe2/N-CNHs showed a half-wave potential value comparable to that of Pt/C, underlying the increase in the density of active reaction centers of CoSe2 due to the presence of N-CNHs as well as the synergetic effect between those two [102]. In addition, CNHs were also employed as a support for a bimetallic metal–organic framework featuring different Zn:Co molar ratios [103]. The 3D conductive network of CNHs led to an ORR performance comparable to that of Pt/C as well as excellent durability and methanol tolerance. It is also worth mentioning that the optimum electrocatalyst was used in real Zn-air battery test and made a higher peak power density (185 mW/cm2) than that of commercial Pt/C catalyst (160 mW/cm2) [103]. Fe and Co are considered promising non-precious metal counterparts for heteroatom doping, since their mutual coordination and synergetic effect with commonly used dopants (e.g., N, S, B) have proven highly effective for ORR electrocatalysis [104,105]. Their activity can be enhanced by tuning the surface area and microporosity to increase the metal-nitrogen coordinated sites. Particular emphasis has been given in using carbon nanostructures to activate the reaction centers [106], however, it is a challenge to remain intact the physicochemical properties of the carbon matrix. In this regard, the unconventional morphology of CNHs fits perfect the abovementioned criteria. Specifically, oxidized CNHs were simultaneously doped with Fe and N at 900 ◦C [107]. The as-prepared electrocatalyst exhibited 40% greater activity than the commercial Pt/C. Specifically, it showed more negative onset potential and half-wave potential by 20 and 30 mV, respectively, against ORR compared to Pt/C. The high electrocatalytic activity was attributed to the synergetic effect of the coordinated N atoms at the edges of the micropores of CNHs with Fe. It is worth mentioning that the ORR activity of the tested electrocatalyst was improved after 1000 cycles, while it was used as a cathode in fuel cells with maximum power density of 35 mW/cm2 under alkaline conditions [107]. Moreover, graphitic carbon nitride (g-C3N4), a relatively new type of carbon-based material that possesses a graphene like sp2 bond structure, has attracted considerable attention as a non-precious metal catalyst for ORR due to its abundant nitrogen dopants and defects. Doping of g-C3N4 with transition metals is an efficient way to improve their catalytic activity, since interactions between different transition metals and heteroatom structures do control the inherent nature of the active sites. A way to overcome the poor electroconductivity and the limited surface area that g-C3N4 face is the introduction of CNHs. Within this scope, Co-doped g-C3N4 was combined with CNHs and evaluated as electrocatalysts for ORR (Co-g-C3N4/CNHs) [108]. The CNHs’ dahlia flower morphology with high surface area, defects, and porosity aided the formation of high Co-N active sites. Thus, Co-g-C3N4/CNHs showed improved catalytic activity compared to non-doped g-C3N4/CNHs and intact CNHs, emphasizing the need for further investigating of the role of Co in these hybrid nanostructures [108]. Hydrogen peroxide is produced mainly via the anthraquinone process, which is an energy-consuming procedure depending on palladium. Because of that, there is tremendous interest in seeking alternative synthetic methods of low cost and energy. Without any doubt, electrocatalytic processes for the reduction in molecular O2 are quite tempting, especially those that rely on metal-free based electrocatalysts. Carbon supports and N-doped porous carbon have previously been found to be active for H2O2 production through O2 reduction [109,110]. The ability to tune electrocatalysts’ selectivity toward the two- or four-electron pathway is of high concern, since promotion of the ORR often only happens at high overpotentials, where the production of H2O is favored. In this route, N-doped graphitized CNHs (g-N-CNHs) were prepared and tested as catalysts for the ORR to produce H2O2 by combining the properties of CNHs and a unique method of N-doping [111]. Briefly, N-doped CNHs were prepared via coating and annealing of polydopamine; a process that restricted them toPDF Image | Carbon Nanohorn-Based Electrocatalysts for Energy Conversion
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