HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
of platinum-based catalysts at the PEFC cathode to accelerate the otherwise sluggish kinetics of the oxygen reduction reaction (ORR). Thus, the development of inexpensive noble metal-free catalysts for the ORR is one of the key research topics for future fuel cell designs. Among various types of non-noble metal catalysts reported so far, one promising class is metal-nitrogen-carbon (Me-N x -C y ) type materials in which transition metals, particularly Fe and Co, are bound to nitrogen doped carbon (NDC). The early reports on the application of such materials date back to the sixties of the last century when Jasinski demonstrated that N-coordinated transition metals can be active sites for the ORR. [1] Similarly, the ORR-activity of metal-free NDCs was shown based on experimental and theoretical results. [2] The moderate acidic performance of NDCs can be enhanced by introducing non-noble metal ions such as iron or cobalt. [3] The research on such non-noble metal catalysts was greatly promoted by the groundbreaking results of competitive activities compared to standard Pt-based catalysts obtained by Lefèvre et al. in 2009. [4] A porous activated carbon was used as a conductive backbone and mixed with phenanthroline as nitrogen and iron(II) acetate as iron source. After heat treatment, first in inert and second in NH 3 -atmosphere, a very active ORR-catalyst in acidic medium was obtained. Later, Wu et al. reported highly active and stable ORR electrocatalysts Iron-or cobalt-coordinated heteroatom doped carbons are promising alternatives for Pt-based cathode catalysts in polymer-electrolyte fuel cells. Currently, these catalysts are obtained at high temperatures. The reaction conditions complicate the selective and concentrated formation of metal-nitrogen active sites. Herein a mild procedure is introduced, which is conservative toward the carbon support and leads to active-site formation at low temperatures in a wet-chemical metal-coordination step. Active-site imprinted nitrogen doped carbons are synthesized via ionothermal carbonization employing Lewis-acidic Mg 2+ salt. The obtained carbons with large tubular porosity and imprinted N 4 sites lead to very active catalysts with a half-wave potential (E 1/2 ) of up to 0.76 V versus RHE in acidic electrolyte after coordination with iron. The catalyst shows 4e − selectivity and exceptional stability with a half-wave potential shift of only 5 mV after 1000 cycles. The X-ray absorption fine structure as well as the X-ray absorption near edge structure profiles of the most active catalyst closely match that of iron(II)phthalocyanine, proving the formation of active and stable FeN 4 sites at 80 °C. Metal-coordination with other transition metals reveals that Zn-N x sites are inactive, while cobalt gives rise to a strong performance increase even at very low concentrations.
Carbon nanotubes (CNTs) are known as nano-architectured allotropes of carbon, having graphene sheets that are wrapped forming a cylindrical shape. Rolling of graphene sheets in different ways makes CNTs either metals or narrow-band semiconductors. Over the years, researchers have devoted much attention to understanding the intriguing properties CNTs. They exhibit some unusual properties like a high degree of stiffness, a large length-to-diameter ratio, and exceptional resilience, and for this reason, they are used in a variety of applications. These properties can be manipulated by controlling the diameter, chirality, wall nature, and length of CNTs which are in turn, synthesis procedure-dependent. In this review article, various synthesis methods for the production of CNTs are thoroughly elaborated. Several characterization methods are also described in the paper. The applications of CNTs in various technologically important fields are discussed in detail. Finally, future prospects of CNTs are outlined in view of their commercial applications.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.