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Nanomaterials 2020, 10, 1407 6 of 26 In this scenario, nitrogen-doped CNHs (N-CNHs) were prepared by thermally treating hydrogen peroxide functionalized CNHs with urea in order to obtain better ORR activity [68]. The so-produced electrocatalyst possessed high surface area of >1800 m2/g with well-defined microporosity and electrical conductivity. More interestingly, the CNH-based electrocatalyst showed enhanced ORR activity compared to intact CNHs, originating from the pyridinic form of the doped nitrogen. N-doped CNHs showed higher durability and methanol fuel tolerance compared to Pt/C, whereas they were used as a cathode catalyst in fuel cells and provided a maximum power density of 30 mW/cm2. In seeking of increasing the N-doping efficiency, CNHs with controlled defects on the conical edges and sidewalls were synthesized under mild oxidation and afterwards, were treated with nitrogen plasma [69]. The N-doped CNHs demonstrated significantly improved electrocatalytic activity for ORR compared to intact CNHs, resulting from the enriched edges with pyridinic-N atoms and the good electrical conductivity and excellent mass transport guaranteed from the inherent spherical three-dimensional quality of CNHs. In this context, the influence of Fe concentration was evaluated in the preparation of sulfur-doped CNHs regarding their morphology, chemical, and electrochemical properties [70]. The modified chemical vapor deposition method with ferrocene gave the most well-defined tubular nanostructures, with conical and cone-like shapes at lower Fe amounts. Sulfur-doped CNHs exhibited fair electrocatalytic activity against ORR but still far from the commercial Pt/C. Moreover, dual-doped carbon nanomaterials can further enhance the catalytic activity by disrupting the electroneutrality of graphitic π-system due to the synergistic effect between the doped atoms [71,72]. For instance, nitrogen- and boron-doped (N-B-doped) graphene exhibited enhanced ORR compared to that owed to N-doped graphene, justified by the high density of catalytic sites and the synergistic effect between N and B atoms [73,74]. In a similar manner, co-doped vertically aligned carbon nanotubes with N and P showed enhanced ORR activity, which was mainly attributed to the morphological modification, surface area expansion induced by the doping, and the increased doping concentration [75]. Co-doped nanocarbon materials often involve tedious and expensive preparation techniques that render their mass production unrealistic. On top of that, the presence of unnecessary by-products and difficult removable metal impurities make the identification of the nature of proposed synergistic effects hard to tell. Meanwhile, the determination of the dopant source may play a crucial role in the electrocatalytic performance of the catalyst. As a proof of concept, nitrogen-boron and nitrogen-phosphorous co-doped CNHs (N-B-CNH and N-P-CNH, respectively) were prepared by using two different N-sources for each co-doped material, namely nitrogen and melamine, and boron carbide and triphenylphosphine as B and P sources by the one-step arc discharge method [76]. It was found that the co-doping of N and P synthesized by nitrogen resulted in higher N-amounts in CNHs, causing high catalytic activity towards ORR. On the other hand, N-B dual-doping of CNHs by nitrogen favored the formation of hexagonal boron nitride, which is inert in ORR, resulting in reduced electrocatalytic activity. However, this was not noticed during the incorporation of N-B atoms in CNHs with melamine as N-source, where the synergetic effect among N-C-B due to polarization of adjacent atoms helped in marking the highest electrocatalytic activity overall [76]. Undoubtedly, increasing the accessibility of available active sites is critical for the enhancement in ORR performance. It is proven that the micropores are regarded as the hosts of the majority of active sites and they are consequential for increasing the number of active sites [77]. Partially unzipped CNTs, possessing active centers with improved characteristics to reduce dioxygen, were employed [78]. Taking into account their high porosity and surface area, CNHs were used in an electrospinning process to increase the specific surface area and pore volume of carbon fibers [79]. Markedly, CNHs increased the electrical conductivity and also helped create mesopores and macropores, which are extremely effective in mass transfer processes [80]. After doping with Fe and N, a series of dual-doped carbon fibers were prepared and tested in ORR. Actually, optimal catalyst showed the best electrocatalytic activity for the reduction in O2 recorded so far, with a half-wave potential value of 60 mV higher than that of the commercial Pt/C, in alkaline solution. Moreover, the electrocatalyst was tested as a cathodePDF Image | Carbon Nanohorn-Based Electrocatalysts for Energy Conversion
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