Theoretical Predictions Concerning Oxygen Reduction on Nitrided Graphite Edges and a Cobalt Center Bonded to Them

Abstract

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+ .

Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction

“…Experimental studies on Co-based pyrolyzed porphyrin electrocatalysts in acidic medium showed that Co-N 4 centres promote a complete 4e À reduction of O 2 to H 2 O and Co-N 2 centres are responsible for the formation of the H 2 O 2 intermediate. 57 This indicated that in the presence of Co the ORR activity was likely dominated by graphitic Co-N x defects. 55,56 However, the location of the Co-N 2 type site is ambiguous since it can be either graphitic or located at the edges of the carbon support.…”

Bifunctional Oxygen Reduction Reaction Mechanism on Non-Platinum Catalysts Derived from Pyrolyzed Porphyrins

“…28 The study found that a bare graphite edge with one N atom cannot reduce oxygen at potentials above 0.3 V vs reversible hydrogen electrode. Further, the analysis showed that oxygenated surface groups such as -OOH or -OH are absolutely necessary in the reduction of oxygen.…”

Molecular-Level Insights into Oxygen Reduction Catalysis by Graphite-Conjugated Active Sites

“…Nitrogen doped (N-doped) carbon materials have emerged as potentially viable replacements for platinum in low temperature fuel cells. 6,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] These materials are typically prepared via high temperature pyrolysis of carbon and nitrogen precursors, and therefore display a large distribution of nitrogen moieties. 21,32,33 The intrinsic heterogeneity of the active sites in these materials has impeded repeated attempts to uncover their mechanism of action and systematically improve their performance.…”