This journal is © the Owner Societies 2017 Phys. Chem. Chem. Phys., 2017, 19, 20441--20450 | 20441 Cite this: Phys. Chem. Chem. Phys., 2017, 19, 20441 Comparison of the catalytic activity for O 2 reduction of Fe and Co MN4 adsorbed on graphite electrodes and on carbon nanotubes Ricardo Venegas, ab Francisco J. Recio,* ab Cesar Zun ˜ iga, ab Marco Viera, cd Marı ´ a-Paz Oyarzu ´ n, c Nataly Silva, c Karinna Neira, c Jose ´ F. Marco, e Jose ´ H. Zagal* c and Federico Tasca * c We have compared the electrocatalytic activity of several substituted and unsubstituted Co and Fe N4-macrocyclic complexes (MN4) for the electro-reduction of oxygen with the complexes directly adsorbed on the edge plane of pyrolytic graphite or adsorbed on carbon nanotubes (CNTs). In the presence of CNTs, one order of magnitude higher surface concentrations of MN4 catalysts per geometric area unit could be adsorbed leading to a lower overpotential for the oxygen electro-reduction and activities in the same order of magnitude as the commercially available Pt/C catalysts in basic pH. From Koutecky–Levich regression analysis, the total number of electrons transferred was approximately 2 for all the Co complexes and 4 for all the Fe ones, both in the presence and in the absence of the carbon nano- tubes. Furthermore, the Tafel slopes did not vary due to the presence of the CNTs and presented values in the range of 0.06 V decade 1 for the CoN4 compounds and in the range of 0.04 V decade 1 for FeN4. When plotting the log of kinetic current densities (i.e. log j k ) at a constant potential for each complex divided by the surface concentration G, and the number of electrons transferred n for the ORR for each catalyst, versus the difference between the redox potential of the metal active site of the Co(II)/(I) or Fe(III)/(II) catalyst and the reversible potential of the reaction they promote, the catalytic activity increases when the formal potential of the complex becomes more positive and the data obtained with complexes adsorbed on graphite are in agreement with the data obtained when using CNTs indicating that the increase in j k when CNTs are present is only due to an increase in the number of active sites per geometric area of the electrode. 1. Introduction The oxygen reduction reaction (ORR) is one of the most studied reactions in modern electrochemistry due to its importance in energy conversion. In fact, fuel cells based on the reduction of O 2 at the cathode are very good candidates among all forms of alternative energy sources for solving the dependence on hydrocarbons for locomotive and energy storing purposes. 1–5 However, the kinetics of the reduction of O 2 are slow at ambient temperatures and the reaction to proceed at rates required for the good performance of fuel cells needs the presence of catalysts. At present platinum and its alloys are the industrial standard catalysts for the ORR occurring at the cathode of fuel cells. 2 Developing inexpensive materials to substitute Pt is of extreme importance for fuel cell cost reduction and market expansion. 2 Metal N4-macrocyclic complexes are very versatile materials (MN4) that have been extensively studied as electrocatalysts not only for the ORR, but also for many other reactions which involve anodic reactions (e.g. oxidation of H 2 O 2 6–8 nitrite, 9 cysteine, 10,11 glucose 12 and hydrazine 13,14 among others 15–21 ). Through intensive research on MN4, it has been established that the activity and stability of those catalysts towards the ORR are affected by: (a) the nature of the central metal atom; (b) the formal redox potential of the metal centre involved in the catalytic process; (c) the surface concentration of the catalyst and the consequent formation of stacks, which in some cases lead to poor electron conductivity; 22–24 (d) the pH of the a Facultad de Quı ´mica, Departamento de Quı ´mica Inorga ´nica, Pontificia Universidad Cato´lica de Chile, Avda. Vicun ˜a Mackenna 4860, Macul, Santiago, Chile. E-mail: javier.recio@uc.cl b Centro de Nanotecnologı ´a y Materiales Avanzados, CIEN-UC, Pontificia Universidad Cato´lica de Chile, Santiago, Chile c Facultad de Quı ´mica y Biologı ´a, Departamento de Quı ´mica de los Materiales Universidad de Santiago de Chile, Casilla 40, Correo 33, Sucursal Matucana, Santiago 9170022, Chile. E-mail: Federico.Tasca@usach.cl, jose.zagal@usach.cl d Facultad de Ciencias Naturales, Matema ´ticas y Medioambiente, Universidad Tecnolo ´gica Metropolitana, Santiago, Chile e Instituto de Quı ´mica Fı ´sica ‘‘Rocasolano’’, CSIC, Madrid, Spain Received 12th May 2017, Accepted 10th July 2017 DOI: 10.1039/c7cp03172f rsc.li/pccp PCCP PAPER Published on 26 July 2017. Downloaded by Instituto de Quimica Fisica Rocasolano (IQFR) on 13/12/2017 15:36:48. View Article Online View Journal | View Issue