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MN4/C non-precious metal catalytic materials for the oxygen reduction reaction.
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José H. Zagal
Departamento de Química de los Materiales, Facultad de Química y Biología Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago.Chile
corresponding author: jose.zagal@usach.cl.
jose.zagal@usach.cl
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For many years, several transition M-N4 metal compounds have been proposed as catalytic materials for the oxygen reduction reaction (ORR). The most common materials investigated include macrocyclic metal complexes having phthalocyanines or porphyrins as ligands. This complexes show high catalytic activity but show low stability in the corrosive environment of fuel cells. However thermal treatment up to temperatures of 1000 ºC or more of mixtures of these complexes with carbon powders and other ingredients increases both activity and stability. After pyrolysis, the generation of a M-N4 type moiety has been identified as the active catalytic site for oxygen reduction. It has also been shown that this M-N4 moiety can be generated from inexpensive ingredients that do not necessarily require a M-4 complex. Some precursors that can generate a catalyst by pyrolysis are: (i) a transition metal salt, (ii) a source of nitrogen (acetonitrile, NH3, polyaniline, polypyrrol, etc..) and (iii) an oxygen-free atmosphere. The reason for the high catalytic activity of these heat-treated materials is not clear.
On the other hand, over the years in many communications, in our group we have demonstrated that the electrocatalytic activity of M-N4 complexes for ORR is directly related to the Eo’, the M(III)(II) formal potential of the catalyst. When comparing the activities of many complexes at constant electrode potential (with the complexes adsorbed on graphite) a plot of log I versus Eo’ gives a straight line of slope +2RT/F, similar to a Tafel plot but with positive sign. The activity increases as the formal potential of the M-N4 complex becomes more positive. Based on these correlations it is possible to explain the very high catalytic activity of some MN4 non-precious catalysts for the reduction of O2. In fact, we have showsn recently that many heat-treated catalysts show similar correlations versus M(III)(II) formal potential to those observed with intact MN4 macrocyclic complexes. These correlations of activity versus formal potential of the catalyst can serve as powerful guidelines to improve the activity of M-N4 non-precious metal catalysts by shifting the M(III)/(II) formal potential to extreme positive values. Recent work my Mukerjee at al. has shown that heat-treatment increases the basicity of the surrounding of the M-N4 moiety and this causes the M(III)/(II) formal potential of the metal center to shift in the positive direction, favorable for the ORR.
The fundamentals of these linear correlations of activity versus formal potentials of the catalysts will be discussed.
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