Improving the Catalytic Activity of Metal Complexes and Pyrolyzed Non-Precious Metal- N4 Catalysts for the Reduction of O2

Thursday, May 15, 2014: 16:00
Floridian Ballroom F, Lobby Level (Hilton Orlando Bonnet Creek)


The massive applications of fuel-cells in daily life has been hampered in part by the high cost of catalytic materials for the O2 cathode that involve noble metals like Pt. MN4 metal macrocyclics [1] and several CuN4 and CuN2 complexes [2] are active for the reduction of O2 but they lack long-term stability in fuel conditions. However, they could useful in disposable air-batteries. Many authors have shown (3-4) that heating MN4 metal macrocyclics together with carbonaceous materials and    conducting polymers    at temp -eratures as high as 1000 oC produce materials of ill-defined structure but more active and stable. The MNx inner structure is probably retained. It has remained unclear why the heat-treated materials exhibit such high catalytic activity. In many papers (1) we have demonstrated that the catalytic activity of intact MN4 metal complexes is directly linked to the M(III)/(II) formal potential, the more positive the highest the activity and this is also valid for Cu N4 and CuN2 complexes [2]   For example for Fe-and Co-containing MN4 catalysts a plot of (log i)E versus the formal potential Eo M(III)/(II) of the catalyst gives a linear correlation with a slope of +0.160 V/decade, i.e. a value that is close to a Tafel slope of -0.120 V/decade but with opposite sign. The redox potential of the catalyst can be tuned by preparing MN4 chelates with appropriate groups located on the ligand or by axial ligation. [1] We have proposed that the very high activities of the heat-treated CNxM materials (x=2,4) can be explained by the shift of the formal potential of the MN4 moiety to more positive values [5] and this has been supported by recent findings of Mukerjee et al. that correlate the activity with the Lewis basicity of the graphitic support, accessed via C 1s photoemission spectroscopy [6].  All this suggests that preparing heat-treated materials that would have redox couples more shifted to more extreme positive potentials (ca 0.8 V or more) by creating an electron-acceptor environment around the metal that could survive the high pyrolyzing temperatures. Many catalysts heat-treated at 1000oC do not exhibit clear redox couples but this is the right direction to go and hypothetically it is possible to overpass the catalytic activity of Pt. A volcano correlation has not yet been found for intact and pyrolyzed MN4 systems so to prepare catalysts that have redox potentials beyond 0.8 V or closer to the thermodynamic potential of the H2O/O2couple (1.22 V vs NHE) should provide the highest possible activity. In this paper we show several correlations using many complexes and pyrolyzed MN4 catalysts which show that there is a clear hope of obtaining catalysts better than Pt and having cheaper fuel-cells for broad applications. Some other explanations of these trends will be discussed that propose a new semi-empirical model of electrocatalysis.

 Acknowledgements: This work has been funded by Fondecyt 1100773, Dicyt-USACH and Núcleo Milenio Project. P07-006


1. J. H. Zagal, S. Griveau, J.. Silva, T. Nyokong, F. Bedioui, Coord.Chem. Revs 257 (2010) 2755 and references there in.

 2. A.Gewirth Coord.Chem. Revs 254 (2013) 130     and references there in.

3.J.P .Dodelet      in      N4-Macrocyclic  Metal Complexes, J.H.Zagal, F.Bedioui, J-P Dodelet eds. Springer New York (2006) 83.

4. R.Bashuam, P.Zelenay Nature 443 (2006) 63. 5. J. H. Zagal, I. Ponce, D. Báez, R. Venegas, J. Pavez, M. Páez, M. Gulppi Electrochem. Solid State Letters 15 (2012) B90

6. N.Ramaswamy, U. Tylus, Q.Jia, S. Mukerjee, J. Am. Chem. Soc. 135 (2013) 15449