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 × 10⁵ s^-1 and 3.1 ± 0.1 μmol/cm², respectively). Bare AEC showed a reversible couple with E_1/2 = +0.36 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 (∼+0.25 and ∼-0.1 V for Co(III)TAPP and Co(III)TPPS, respectively), but also at much more positive potential (∼+0.4 V). Moreover, RRDE experiments show insignificant H₂O₂ production (∼0.3%) during O₂ reduction, compared to the high H₂O₂ yields reported for these metalloporphyrins when dissolved or immobilized on other electrode substrates.