Surface Confinement of Oxygen in Carbon Supported Oxygen Reduction Catalysts

In 2011, we reported an oxygen adsorption phenomenon for heat treated, Fe/polyaniline based oxygen reduction catalysts.¹ We found that the extent of oxygen adsorption correlated with the surface density of the active sites (Fe‑N_{4 (2+2)}) and determined the thermodynamic as well as kinetic parameters associated with the adsorption process.¹

         In our recent studies on oxygen reduction catalysis by macrocyclic complexes of transition metals such as porphyrins, phthalocyanines and corroles, we found the occurrence of similar phenomena. The surface confined oxygen can be easily detected by voltammetry, as its reduction produces a high reduction peak bearing typical “surface properties”, such as, e.g., the reduction current proportionality to the scan rate or the (approximate) axial symmetry (Fig. 1).

         In this work, we compare the surface confinement of oxygen in presence of numerous carbon supported catalysts and their mixtures and conclude that an essential condition for the surface confinement of oxygen is, among others, a graphitic (π‑electronic) structure of the catalyst support. This conclusion explains the proportionality between the magnitude of the phenomenon and the density of the active sites in heat treated ORR catalysts,¹ where the active sites are believed to be embedded in graphene planes.^2,3

Acknowledgement

The financial support from the University of California (grant 12-LR-237440) is gratefully acknowledged.

References

1)       J. Chlistunoff, J. Phys. Chem. C, 115, 6496-6507 (2011).

2)       M. Lefèvre, J.-P. Dodelet, P. Bertrand, J. Phys. Chem. B, 106, 8705-8713 (2002).

3)       M. Lefèvre, E. Proietti, F. Jaouen, J.-P. Dodelet, Science, 324, 71-74 (2009).

Fig. 1. Cyclic voltammetry (100 mV s^‑1) of oxygen reduction catalyzed by carbon (XC72) supported cobalt phtalocyanine showing the predominantly surface character of the oxygen reduction peak. Catalyst ink composition: 46.5 mg XC72; 0.8 mg CoPC; 8 cm³ isopropanol; 55.9 μl 5% Nafion. Catalyst loading 0.1 mg (for XC72).