Effect of N and S co-doping of multiwalled carbon nanotubes for the oxygen reduction

The co-incorporation of S and N into multiwalled carbon nanotubes (CNTs) and the effects for the catalytic performance for the oxygen reduction reaction (ORR) in acidic and alkaline electrolytes has been studied. Subjecting CNTs to different ballmilling periods results in the formation of defects in their graphitic structure. Heteroatoms such as N and S can be actually incorporated into such defects leading to active catalysts for the ORR. In fact, the ORR activity in acid and alkaline media increases with the increasing amount of heteroatoms, especially N, actually incorporated in the catalysts. The use of precursors containing both N and S into their structure such as thiourea, results in a higher incorporation of surface N atoms than with similar N-containing precursors. As a consequence the ORR activity of the S/N/CNTs based catalysts is higher than that of N/CNT ones. This promotional effect of the presence of S is more significant when the ORR is measured in alkaline media suggesting that S-incorporation into the carbon matrix could actually play a direct role for the ORR.This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant number (D-006-432). The authors, therefore, acknowledge with thanks DSR technical and financial support. Economic support from project ENE2013-42322-R from the Spanish Ministry of Economy and Competitiveness is also acknowledged.Peer Reviewe

Influence of the electrolyte for the oxygen reduction reaction with Fe/N/C and Fe/N/CNT electrocatalysts

The behavior of Fe-based non-precious metal catalysts (NPMCs) in different electrolytes and the repercussion for the oxygen reduction reaction (ORR) has been studied. For this matter, a series of Fe-based NPMC electrocatalysts have been prepared from different carbon sources, carbon black and multiwalled carbon nanotubes. The catalysts have been subjected to chemical treatments in 0.5 M H2SO4 and thoroughly characterized. Their performance for the ORR in different electrolytes e.g. HClO4, H2SO4, CF3SO3H, KOH and NaOH has been studied. Higher ORR rates have been recorded in the alkaline electrolytes as compared to the acid ones. Remarkably, the effect of the electrolyte is almost negligible when measured at a given pH value; i.e., the ORR performance is not affected by the nature of the anion when measured in acid electrolytes or the cation when measured in alkaline electrolytes. On the other hand, the activity of NPMCs for the ORR decreases remarkably after treatment of the catalysts in 0.5 M H2SO4. This effect accounts to both the removal of active sites for the ORR during acid treatment and to the blockage of active sites due to the presence of adsorbed sulfates.This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant number (D- 006-432). The authors, therefore, acknowledge with thanks DSR technical and financial support. Economic support from projects ENE2010-15381 from the Spanish Ministry of Science and Innovation and Project 201080E116 from the CSIC is also acknowledged.Peer Reviewe

Effect of carbon nanotube diameter for the synthesis of Fe/N/multiwall carbon nanotubes and repercussions for the oxygen reduction reaction

The effect of the diameter of multiwalled carbon nanotubes for the incorporation of N and Fe and the consequences for the oxygen reduction reaction in acid medium has been studied. For this, a series of multiwalled carbon nanotubes with mean diameters of 10, 20 and 60 nm have been thermally treated in acid media and modified by addition of N- and Fe-groups by means of thermal treatments under inert atmosphere. The chemically treated nanotubes and the FeN-CNTs have been thoroughly characterized by N2 adsorption/desorption isotherms, Raman spectroscopy, X-ray photoelectron spectroscopy, elemental analysis, thermogravimetric analysis and X-ray diffraction. The performance of the FeN-CNTs for the oxygen reduction reaction (ORR) in acid medium has been evaluated by means of electrochemical techniques. We have found that the amount of nitrogen actually incorporated onto the multiwalled carbon nanotubes, which ranges between 2 and 3 wt.%, can be directly related with the number of defects of the chemically treated multiwalled nanotubes. On the other hand, the BET specific surface area of the FeN-CNTs increases with the decreasing diameter of the CNTs. A direct relationship between the nitrogen external surface area and the ORR performance has been observed.This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant number (D-006-432). The authors, therefore, acknowledge with thanks DSR technical and financial support. Economic support from projects ENE2010-15381 from the Spanish Ministry of Science and Innovation and Project 201080E116 from the CSIC is also acknowledged.Peer Reviewe

Effect of transition metal (M: Fe, Co or Mn) for the oxygen reduction reaction with non-precious metal catalysts in acid medium

The effect of the metal for the oxygen reduction reaction (ORR) in acid medium with non-precious metal catalysts has been investigated. A series of non-precious metal catalysts with typical formulation M/N/C with M being Mn, Co or Fe have been prepared by incorporating N onto an active carbon matrix by means of thermal treatments under inert atmospheres. The N-containing active carbons were further treated with the M-containing precursors based upon Mn, Co or Fe phthalocyanines and thermally treated under inert atmosphere. The performance for the ORR in acid medium of all of the catalysts has been evaluated by means of electrochemical techniques. The activity, both in terms of onset potential for the ORR and maximum current density at representative potentials between 900 and 700 mV follows the trend Fe > Co > Mn. In addition, the performance of the Fe-based catalysts obtained during the different stages of the catalyst preparation has been also evaluated. The catalysts obtained after the pyrolysis step are the only ones showing measurable rates for the ORR. Although the amount of N and Fe incorporated onto the carbon matrix decreases the pyrolysis treatment, this treatment leads to the formation of the real active sites for the ORR irrespectively of the nature of the transition metal.This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant number (D-006-432). The authors, therefore, acknowledge with thanks DSR technical and financial support. Economic support from projects ENE2010-15381 from the Spanish Ministry of Science and Innovation and Project 201080E116 from the CSIC is also acknowledged.Peer Reviewe

Oriented MFI membranes by gel-less secondary growth of sub-100 nm MFI-nanosheet seed layers

A zeolite membrane fabrication process combining 2D-zeolite nanosheet seeding and gel-free secondary growth is described. This process produces selective molecular sieve films that are as thin as 100 nm and exhibit record high permeances for xylene- and butane-isomers. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim