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Nanomaterials 2020, 10, 1407 7 of 26 catalyst in PEMFC and anion exchange membrane fuel cells (AEMFCs), with high power densities of 250 and 125 mW/cm2, respectively, demonstrating their use in real applications [79]. In seeking to prepare effective cathode electrocatalysts for ORR, diverse synthetic strategies involving various architectures on carbon supports were explored, engaging Pt bimetallic [81] or multi-metallic nanoparticles on grapheme [82], core-shell structures [50,54,83], etc. The electrical conductivity and stability of the decorated metallic nanoparticles could be effectively improved through hybridization with carbon nanostructures [50,54,84], while it could also address the lack of stability that metal catalysts face during the electrolysis process due to dissolution or agglomeration issues [85]. However, the smoothness of the carbon nanomaterial makes the interaction between the active site and the carrier very weak. In many cases, defects are held responsible for many electrochemical activities ascribed to graphitic materials. Indeed, the introduction of heteroatoms can induce defects and irregular structures of graphene. In this matter, the defected nature of CNHs due to their nanoscale pores and dahlia flower-like morphology could be more favorable for the dispersion of the nanoparticles and prevent their agglomeration compared to graphene, and thus, further promote ORR. Aiming at exploring this claim, Pd nanoparticles were uniformly loaded on N and B dual-doped CNHs, in the form of the hybrid Pd-N-B-CNHs, by a one-step reduction method and their ORR activities were studied in alkaline media [86]. Electrochemical studies showed that Pd-N-B-CNHs exhibited similar onset potential to that of the commercial Pt/C and a more positive half-wave potential by 0.167 V vs. SCE. The large pore size distribution, the presence of numerous pyrrolic-N and charged B+ sites, the defect carbon nanostructure, and the synergetic effect between metal and N-B-CNHs facilitated superior activity towards ORR. Additionally, the electrocatalyst showed long-term stability and resistance to methanol, underscoring the high potentiality of Pd-N-B-CNHs for fuel cells [86]. However, in some cases, the defected nature of CNHs has not worked for their benefit. Particularly, the corrosion resistance of CNH-based electrocatalysts was evaluated in strong conditions [87]. In this frame, CNHs presented a three times faster corrosion rate than that of carbon black (CB) and commercial Pt electrode. This observation is contradictory to the studies that want CNTs [88,89] to be more tolerant compared to CB due to their higher graphitic structure [90]. One would expect that CNHs will present a better oxidation profile than highly amorphous CB, however, their numerous defected sites are most likely prone to electro-oxidation. The aforementioned facts stress the importance of a good overall electrocatalyst for applying in real fuel cells. Furthermore, most metastable nanostructures of bimetallic or multi-metallic Pt-based electrocatalysts usually use synthetic procedures concerning complicated methods such as electrochemical etching or deposition processes, which limit the application of such catalysts at a large scale [91]. In addition, when these nanostructures involve noble metals, including Pt, Pd, and Au, purification becomes more complicated than that of monometallic catalysts. Consequently, the development of supported monometallic-Pt nanoparticle catalysts with high durability and electrocatalytic activity for ORR is of high significance for promoting the utilization of PEMFCs. As a matter of fact, Pt-loaded CNH-based catalysts have shown high performance in polymer electrolyte fuel cells [92–94]. Additionally, Pt nanoparticles supported on N-doped CNHs (Pt/N-CNHs) exhibited enhanced durability and catalytic activity for ORR compared to commercially available Pt/C catalysts in an acidic environment [95]. Specifically, Pt/N-CNHs showed 1.6 times higher activity in terms of onset potential than the commercial Pt/C, while its catalytic activity was higher by 75% overall. Transition metal chalcogenides (TMCs) is another category of electrocatalysts that has received attention due to their multifunctional behavior and ease of synthesis [96]. TMCs are highly tolerant towards fuels typically used in fuel cell anodes, such as methanol and formic acid. Transition metal chalcogenides (e.g., selenides and sulfides of Co, Mn, Ni, Ru, and Fe) are considered possible alternatives to Pt, since they perform well towards various electrode reactions such as oxygen reduction, oxygen evolution, and hydrogen evolution [96,97]. For example, Ru modified with Se electrocatalysts have showcased superior activity against ORR than pure metals [98] and have been combined with CB with very close activity to Pt/C [87]. Among them, CoSe2 and CoS2 have shown the most promisingPDF Image | Carbon Nanohorn-Based Electrocatalysts for Energy Conversion
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