Atomistic Electrocatalysts for Modulating the Oxygen Reduction Reaction Selectivity in Carbon-Based Materials: Active-Site Engineering, Local Environment, and Magnetism.

The oxygen reduction reaction (ORR) is an electrochemical process that is key to tackling global concerns regarding the conversion and storage of clean energy as well as the development of sustainable water treatment. We mainly focus on nonprecious metal catalysts, specifically harnessing Carbon-based electrocatalysts. In the current invited perspective, we highlight three main ways to control the ORR selectivity, which is still a challenge under development: (i) engineering the active sites where the use of single-atom, double-atom, or small clusters of atoms of transition metals in the carbon matrix allow including more active sites for the reaction, (ii) using coordination shells and modifying the local environment of the active-sites with more electronegative elements generates a strong positive electrostatic potential in the active site thus improving the metal-O₂ interaction, and (iii) using spin-selection with magnetic single atoms where the magnetic moment strength of the single-atom and the triplet-to-singlet transition in the O₂ after adsorption. More attention should be paid to this effect since the magnetic properties are directly correlated with the O₂ adsorption strength, and at the same time, the selectivity of the O₂ adsorption is directly related to the two- or four-electron pathway. Selectivity is commonly discussed in carbon-based catalysts but is not always linked to atomistic effects. Therefore, it is necessary to understand and rationally design alternative electrocatalysts that can synergistically combine active transition metal centers, different local environments in their coordination shells, and magnetic control.