Cold-plasma modification of carbon black used as a catalyst for oxygen reduction in PEMFC's

Oxygen reduction in fuel cells can be catalyzed without using noble metals, substituting these by a Metal-Nitrogen catalytic position created over on the surface of a carbonaceous material (M-Nx/C). In this case, Fe or Co are the most commmonly used metal species as can be already described in literature (J. Phys. Chem. B. 104, 2000; 11238-11247). There is not need to say that the elimination of nobel metals in PEMFC's would represent an important developement due to the high price and limited ammount of these materials. Therefore the scope of the present work is to obtain chemically modified carbon supports that present activity towards the oxigen reduction. Special attention should be given to the plasma treatment that has been used in order to obtain the chemical modification. In the first step of the present work a cold plasma reactor has been used in order to create nitrogenated positions over on carbon black surface (CB). CB was thought to be a good carbonaceous support to modify due to the presence of different energetical domains on its surface, which could give a wider range of possibilities for the chemical reaction. To perform de chemical modification the reactor is fed with a mixture of ammonia and argon after vacuuming and reaching a low enough pressure (10-3mbar), then the gas is ionized by an elechtromagnetical source. Different power supplies and reaction times have been applied in order to study this N-enrichment reaction on both N-134 and Vulkan CB grades. Both CB's were treated in fluffy sate in order to ensure a complete and homogeneous modification. They have later been characterized by water dispersion and pH in water solution: the first assay indicates changes in the surface polarity, due to the increase in oxygenated and nitrogenated groups, whereas the second reflexes the nature of these surface groups, as oxygenated ones trend to decrease the pH value (these groups are frequently carboxylic acids), and the nitrogenated usually make it higher due to their ammoniacal origin. A raising trend has been observed for both variables as power and length of the plasma treatment were increased. Particular X-Ray Photoelectron Spectroscopy (XPS) measurements have also been done on modified samples. When analyzing the obtained data special attention has been paid to the presence of pyrrolic or pyridinic positions. The reason for this, is that these nitrogen positions have been described as the active ones for the the basis of the M-N catalyst, (poner aqui la referencia). Surface nitrogen percentages of up to a 2% have been observed, being more than a 60% of this N of either the pyridinic or the pyrrolic kind. In addition, the nitrilic nitrogen reaches a percentage of 25%. After creating obtaining these N-positions, the carbonaceous samples have been enriched with iron, much more usual as the catalytic metal in the existing bibliography due to the lower detrimental effects at the PEMFC's working conditions. The treated CB has been dispersed in a water solution of Fe(AcO)2 in a proportion of 2000ppm/g. CB. After this process about 2 grams of the resulting powdered material have been pyrolyzed in an Ar/H2 atmosphere, in order to create the Fe-N2/C and Fe-N4/C positions which will theorically catalyze the oxygen reduction reaction at the fuel cell?s cathode. Finally, the assumed catalytic capacity of this material has been demonstrated by RDE cyclic voltametries. After pyrolysis treatment, the powdered material has been dispersed in an ethanol-Nafion? solution, and pipetted onto a vitreous carbon RDE. Cyclic voltametries have been done, reaching a reduction potential of almost 0,4V vs. an Ag/AgCl reference electrode. The results show that the ammonia plasma treatment of CB is a potential important and competitive tool in oder to obtain active catalysts for the oxygen reduction when compared to the regular ammonia pyrolisis treatments, therefore, other studies such the maximization of the active sites by optimizing iron loading, pyrolisis temperature, as well as submitting the material to real PEMFC's (temperature, acidity, CO presence) are being planned as next steps to improve the knowledge and activity on this type of catalyst.