Nitrogen-Treated Graphite and Oxygen Electroreduction on Pyridinic Edge Sites

On the basis of literature reports, some nitrogen-treated graphite electrodes have been found to catalyze the four-electron electroreduction of O2 to water in acid. In this study the linear Gibbs energy relationship is used to predict the reversible potentials for forming intermediates during O2 reduction in acid over graphene doped with two N atoms substituting for adjacent edge CH groups. This procedure, generally accurate within ∼0.2 V, is useful for estimating overpotentials for electrode surface catalyzed reactions. Using bond strengths from VASP slab-band density functional calculations, it is predicted that one of the edge N has H bonded to it at potentials of ∼1.70 V and below. In the first reduction step, the OOH that forms then dissociates on the edge into O that bonds strongly to N with OH weakly associated with it. The calculated reversible potential is ∼0.89 V. The OH is proposed to abstract H from an edge NH, forming H2O. The H is then replaced in a reduction reaction. The reversible potential for reducing the O(ads) to OH(ads) on the edge with is ∼−0.60 V, well negative of the potential range of interest for oxygen reduction, which means this edge structure will be stable at the potentials of interest. The edge has an unpaired electron and OOH bonds to the C atom bridging ON···NH with a strength corresponding to a reversible potential of ∼0.73 V. This means that the two-electron reduction product, H2O2, can form at a potential close to the ∼0.695 V standard reversible potential. The absence of any apparent pathway for the direct four-electron reduction suggests that (i) some other catalytic site structure involving substituent N is involved or (ii) the peroxide pathway is being followed with O2 and H2O generation when peroxy intermediates disproportionate or (iii) impurity transition metals are contributing to direct four-electron reduction.