Theoretical study of axial ligand modulation in BPN-supported M-N-C single-atom catalysts for oxygen reduction and...

The oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), as important reactions in fuel cells and metal-air batteries, necessitate the development of stable, efficient, and cost-effective non-precious metal-based materials as electrocatalysts. In this work, we conducted a theoretical study on eighteen M-N-C single-atom catalysts (SACs) supported on BPN and modified with axial ligands (TM-N-C@X catalysts, where the transition metals were V, Cr, Mn, Fe, Co, and Ni, and the axial ligand X = O, OH, OOH). The structures of all catalysts were optimized, and their thermodynamic and electrochemical stabilities were verified. By calculating the overpotentials, we found that the Cr-N-C@OOH catalyst and Mn-N-C@O catalyst exhibited overpotentials of 0.37 V and 0.57 V in the ORR and the OER, respectively, which were the lowest overpotentials and thus indicated the best performance in the ORR and the OER. In addition, based on the round-trip efficiency, Fe-N-C@OOH is identified as the best bifunctional ORR/OER catalyst, with the round-trip efficiency (RTE) of 41.1%. From the volcano plot, it can be seen that the Cr-N-C@OOH catalyst and Mn-N-C@O catalyst have moderate adsorption strengths for oxygen-containing reaction intermediates. By analyzing the COHP, ICOHP, and PDOS of the Cr-N-C@OOH catalyst, Mn-N-C@O catalyst, and the unmodified Cr-N-C and Mn-N-C catalysts without axial ligands, it was found that the Cr-N-C@OOH catalyst and Mn-N-C@O catalyst can adjust the adsorption strength of the transition metal active centers for *OH, which is more conducive to the desorption of *OH and thus improves the catalytic performance. This study provides a theoretical basis for enhancing the electrocatalytic performance of M-N-C SACs and offers design strategies for developing stable, cost-effective, and efficient non-precious metal-based materials.