Regulating Spin-Valence States of Vanadium-Based Single-Atom Oxygen Reduction Catalysts through Substrate and Axial Ligand Engineering

We study the effect of different carbon-based oxygen reduction reaction (ORR) catalysts on which atomically dispersed vanadium is anchored: graphene and graphitic carbon nitride (g-C3N4), by means of density functional theory. We found that different substrates and axial ligands modulate the spin and valence states of the vanadium metal center, thus modifying the catalytic activity. We observed that the magnetic moments of the metal center are related to the number of available electrons to be transferred from the catalyst to the adsorbate. The bare V-N4-graphene active site prefers a dissociative ORR mechanism due to strong binding between the vanadium metal center to oxygen-containing intermediates, and thus the OOH intermediate cannot be stably adsorbed. After the adsorption of the –OH ligand, the number of available electrons in the vanadium metal center is decreased, thus enhancing the ORR activity and shifting the ORR overpotential to 0.34 V, and the active site may prefer an associative mechanism. However, the magnetic moment of the catalysts is not the only factor affecting the adsorption behavior of the catalyst. Population analysis shows V-N4-graphene active site has the same number of available electrons in the d orbital of the vanadium metal center as the OH-V-g-C3N4. However, the V-N4-graphene active site exhibits stronger binding to oxygen-containing intermediates compared to OH-V-g-C3N4. We found that the valence states of the catalysts also affect the catalytic activity of the catalyst. The availability of an occupied dz2 orbital in the V-N4-graphene active site makes it have a stronger interaction with oxygen-containing intermediates compared to OH-V-g-C3N4. These combined effects lead to a spin-valence synergistic effect. These findings can pave the way to the design of better catalysts for efficient ORR in the future.