Carbon-based materials have been widely studied over the last few years, attracting the attention of scientists as environmentally friendly and economically viable alternatives to the already commercially established metal-based catalysts. Among the advantages of using carbon as a base material for catalysts, the following are worth mentioning: it can be produced from biomass and waste materials, it is cheaper and more abundant than metals, and carbon-based materials are also tunable for a wide variety of catalytic processes, since their structure can be functionalized with oxygenated functions, defects, vacancies, and heteroatom doping. We propose the investigation of carbon vacancy combined with cerium doping effects on graphene catalytic activity for the oxygen reduction reaction (ORR), with density functional theory (DFT) calculations. This reaction has recently gained attention as it can occur by a 4-electron mechanism for fuel cells applications, or by a 2-electron mechanism for hydrogen peroxide in situ electrogeneration and organic pollutants removal for water treatment. From a theoretical point of view, it is also viable to use graphene as a computational model to mimic catalytic sites on carbon-based materials. Summarily, we have employed a bi-dimensional graphene model, previously optimized, with 128 atoms, in which a double vacancy was created to accommodate the Ce atom. Plane-wave basis calculations were performed with the PWScf code available in the Quantum ESPRESSO suite, with GGA-PBE functionals to describe the exchange-correlation energy, and SSSP pseudopotentials to describe the core electrons. The associative mechanism of the ORR was considered in this study, since the first reaction intermediate species OOH can lead either to the H2O2 formation on the 2-electron pathway or the O-O bond break might occur over the next steps, following the 4-electron pathway. It was noted that the presence of a cerium atom can greatly enhance the catalytic activity of graphene for the ORR towards H2O generation, reducing the determinant intermediate step free energy from ΔGOOH,gr = 5.72 eV to ΔGOOH,gr+Ce = 3.98 eV at U = 0 V and from ΔGOOH,gr = 2.03 eV to ΔGOOH,gr+Ce = -0.11 eV at U = 1.23 V. In contrast, the reaction would not occur over pristine graphene, as indicated by the positive Gibbs free energy variation over its course. The cerium atom in this network also favors the 4-electron pathway, since it was observed the H2O2 molecule spontaneously breaks into two OH species rather than disorbing from the catalytic site. This is a novel work that hopefully can provide theoretical insight for the future development of carbon and graphene-based catalysts for the oxygen reduction reaction.

References Density functional theory studies of oxygen reduction reaction for hydrogen peroxide generation on graphene-gased catalysts J. Electroanal. Chem., v. 895, p. 115429, 2021.

Assessing the Oxygen Reduction Reaction by a 2-electron Mechanism on Ceria Surfaces Phys. Chem. Chem. Phys, v. 23, p. 18580, 2021.