Cobalt sulfides have been known for more than 30 years to be active toward oxygen reduction, and cobalt selenides have shown less activity. In this paper, a theoretical analysis is made of the four-electron reduction reaction of oxygen to water over the mixed anion and cation (202) surface of the pentlandite structure Co9Se8, one of several selenide phases. Reversible potentials for forming adsorbed reaction intermediates in acid are predicted using adsorption energies calculated with the Vienna ab initio simulation program (VASP) and the known bulk solution values together in a linear Gibbs energy relationship. Comparison with an earlier theoretical analysis of pentlandite structure Co9S8 shows that the overpotential is predicted to be larger for the selenide by around 0.22 V. Cobalt selenide electrodes of unspecified stoichiometry were prepared chemically on glassy carbon discs, and polarization curves were measured using rotating discs. When heat-treated at 900 °C, the onset potential for O2 reduction was found to be 0.5 V (normal hydrogen electrode, NHE), whereas electrodes not subject to heat-treatment were inactive. For Co3S4, onset potentials in the literature are ∼0.8 V (NHE), consistent with a ∼0.3 V higher measured overpotential for the selenide. The theoretical predictions for the pentlandite sulfide and selenide surfaces are in qualitative agreement.

Density functional theory and a linear Gibbs free energy relationship are employed in a theoretical investigation of catalytic properties of cobalt-graphite-nitride systems for O 2 reduction to hydrogen peroxide and water. Nitrided graphite edges, with N atoms substituting one or two CH groups, are modeled to establish some of the effects of N on edges with and without Co added. The calculations show that a bare graphite edge with one N atom, which the calculations indicate is not hydrogenated at potentials greater than 0.3 V, is not active for O 2 reduction because OOH bonds too weakly. At potentials lower than 0.3 V, for which N is hydrogenated, making it a radical center, the NH edge is not active for O 2 reduction because OOH bonds too strongly, resulting in a high overpotential for its reduction to H 2 O 2 on this site. Over a Co site bridging two N substituting for CH on an edge, the onset formation potential for OOH(ads) is about 0.4 V for Co 0 , 0.8 V for Co II in the form of Co(OH) 2 , 0.7 V for Co II in the form H 2 OsCo(OH) 2 , and 0.7 V for Co III as Co(OH) 3 . Later steps have higher predicted reversible potentials. A water molecule bonds to each of the Co centers but most weakly in the case of H 2 OCo(OH) 2 , which means this cobalt center is least likely to be blocked against O 2 interaction with it. All of the cobalt complexes are predicted to bond weakly, 1.5 eV and less, to the graphite edge N atoms, which means that the catalyst is not expected to be stable due to cobalt dissolution as soluble Co 2+ .

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