Electrocatalytic oxygen reduction by Co(III) porphyrins incorporated in aerogel carbon electrodes

Co(III) tetra(o-aminophenyl)porphyrin (Co(III)TAPP) and Co(III) tetra(p-sulfonatephenyl)porphyrin (Co(III)TPPS) were incorporated by adsorption or electropolymerization (Co(III)TAPP only) in aerogel carbon (AEC) electrodes. The adsorption rates as well as the maximum amounts of adsorbed species were similar for both metalloporphyrins (2.8 x 10(5) s(-1) and 3.1 +/- 0.1 mu mol/cm(2), respectively). Bare AEC showed a reversible couple with E-1/2 = +036 V, attributed to the reduction of surface quinone-related functionalities. These interact with Co(III)TAPP and Co(III)TPPS to form complexes on the surface with E-1/2 = +0.41 and +0.43 V, respectively. As a result, electroreduction of oxygen for the adsorbed as well as for the electropolymerized porphyrins occurs not only at the potentials dictated by the Co(III/Co(II) porphyrin redox couple (similar to+0.25 and similar to-0.1 V for Co(III)TAPP and Co(III)TPPS, respectively), but also at much more positive potential (similar to+0.4 V). Moreover, RRDE experiments show insignificant H2O2 production (similar to 0.3%) during O-2 reduction, compared to the high H2O2 yields reported for these metalloporphyrins when dissolved or immobilized on other electrode substrates. (C) 2008 Elsevier B.V. All rights reserved.