Iron Strapped Porphyrins on Carbon Nanotubes As Hybrid Materials for the ORR Reaction

One of the most important challenge of nanosciences is the fabrication of functional hybrid materials, preserving and combining the properties of their building blocks. Among the different classes of nanomaterials, carbon nanotubes are promising for electronics, opto-electronics, catalysis and composite applications. Within the context of sustainable development and renewable energy, we and others have been envisioning the use of functionalized or doped carbon nanotubes in electrocatalytic systems.1,2 In such systems, catalytic sites need to be supported on conducting materials. Carbon nanotubes, thanks to their electrical conductivity and their high surface, area appear to be the ideal material for that purpose. In actual Proton Exchange Membrane Fuel Cell Devices (PEMFC), the reactions at the electrodes and in particular the reduction of oxygen are performed by platinum nanoparticles. The cost of platinum is an incentive to developing new catalysts-based on non-noble metals. Thus, whereas in nature oxygen reduction is performed by iron porphyrins in the active center of enzymes, bio-inspired catalysts based on cobalt or iron macrocycles have been extensively studied for Oxygen Reduction Reaction (ORR).3,4 Here, we describe the synergic effect on catalytic activity of carbon nanotubes and strapped iron porphyrin hybrids for ORR. In particular, we demonstrate that combining both MWNTs and porphyrins leads to better catalytic activity compared to isolated nanotubes or porphyrins on their own. This study hence highlights the importance of carbon support for the catalysis: nanotubes ensure the availability of electrons to porphyrin catalysts and allow ORR to occur via the 4-electron pathway, avoiding the production of hydrogen peroxide. For this study, two series of iron-strapped porphyrins have been explored.5-6 In the first one, the porphyrins were adsorbed at the surface on nanotubes whereas, for the second one, porphyrins were synthesized with propargylic ether groups at their periphery, allowing covalent linkage by Hay coupling at the surface of the nanotubes. Each of the nanotube hybrids was characterized and tested for ORR in a series of electrochemical measurements. References [1] Zhang, W.; Sherrell, P.; Minett, A. I.; Razal, J. M.; Chen, J. Energy Environ. Sci. 2010, 3, 1286-1293. [2] Morozan, A.; Jousselme, B.; Palacin, S. Energy Environ. Sci. 2011, 4, 1238-1254. [3] Zagal, J. H.; Griveau, S.; Silva, J. F.; Nyokong, T.; Bedioui, F. Coord. Chem. Rev. 2010, 254, 2755-2791. [4] Morozan, A.; Campidelli, S.; Filoramo, A.; Jousselme, B.; Palacin, S. Carbon 2011, 49, 4839-4847. [5] Hanana, M.; Arcostanzo H.; Das P.K; Bouget M.; Le Gac S. Okuno H.; Cornut R.; Jousselme B.; Dorcet V.; Boitrel, B.; Campidelli S., New J.Chem.,2018,42, 19749--19754. [6] Boitrel, B; Bouget, M.; Das, P. K.; Le Gac, S.; Roisnel, T.; Hanana, M.; Arcostanzo, H.; Cornut, R.; Jousselme B.; Campidelli S., J. Porphyr. Phthalocyanines, 2019, doi: 10.1142/S1088424619501232 Figure 1