A Model for the pH-Dependent Selectivity of the Oxygen Reduction Reaction Electrocatalyzed by N-Doped Graphitic Carbon

Abstract

Nitrogen-doped graphitic carbon materials have been extensively studied as potential replacements for Pt-based electrocatalysts for the oxygen reduction reaction (ORR). However, little is known about the catalytic mechanisms, including the parameters that determine the selectivity of the reaction. By comparing theoretical calculations of the ORR selectivity at a well-defined graphene nanostructure with experimental results, we propose a model based on interfacial solvation to explain the observed preference for the four-electron pathway in alkaline electrolytes. The hydrophobic environment around the active sites, as in enzymatic catalysis, restricts the access of water and destabilizes small ionic species such as peroxide, the product of the two-electron pathway. This model, when applied to acidic electrolytes, shows the ORR preferring the two-electron pathway, consistent with the well-known pH-dependent ORR selectivity catalyzed by graphitic carbon materials. Because of the similarity between more complex N-doped graphitic carbon materials and our model system, we can extend this model to the former and rationalize nearly all of the previously reported experimental results on the selectivity of ORR catalyzed by these materials.

Polycyclic aromatic hydrocarbons in the graphene era

“…Moreover, among the pyridinic-N-doped nanographenes, the larger ones displayed higher activities in the ORR, probably due to the higher HOMO levels, which facilitated the electron transfer to oxygen. Further theoretical studies on compound 252 suggested that the ORR preferred the four-electron pathway in alkaline electrolytes but the two-electron pathway in acidic conditions, consistent with the experimental pH-dependent selectivity observed in the ORR catalyzed by N-doped graphene [312]. This work confirmed the electroactivity of the pyridinic-N atoms, providing clear insights into the more complex N-doped graphene; thus, synthesizing nanographene molecules with different types of nitrogens would be desirable.…”

Modifying Commercial Carbon with Trace Amounts of ZIF to Prepare Derivatives with Superior ORR Activities

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controlled to obtain efficient activities; otherwise the porous structures might collapse, and the active centers might be hindered. [10][11][12][13][14] Such catalysts should be further explored to fulfill the requirements of cell optimizations and applications.Combining MOFs with carbon materials, such as graphene or carbon nanotubes to enhance performance has been widely studied in the fields of sensing, [15,16] separation, [17] catalysis, [18] supercapacitor, [19] etc. On the other hand, the mechanisms to provide high activities are convoluted due to the diversity of potential functional centers in such materials.

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A Facile and Versatile Electrochemical Tuning of Graphene for Oxygen Reduction Reaction in Acidic, Neutral and Alkali media

“…Based on the theoretical model for the pH‐dependent electrocatalytic activities of nitrogen incorporated graphenes, a similar mechanism is proposed to explain the observed ORR activity of N−Gr at different pH, as shown in Figure . The ORR and its mechanistic pathways can be affected by the pH, the interfacial solvation, the dielectric constants of the electrolyte (medium), the activation barrier, and the free energy change.…”

“…When the N−Gr is placed in an O 2 saturated electrolyte, it forms a peroxygraphene anion by the chemisorption of molecular oxygen. Breaking of either O−O or C−O bond in the peroxy graphene anion rules the mechanism of the ORR, i. e. the breaking of O−O leads to 4e − pathway and the breaking C−O leads to 2e − pathway . However, the breaking of these bonds depends on the factors mentioned above.…”