Fe and Co adatoms on bilayer borophene as single-atom catalysts for the oxygen-reduction reaction: A theoretical study

This study employs first-principles density-functional-theory (DFT) calculations and ab initio molecular dynamic (AIMD) simulations to investigate the stability, electronic properties, and oxygen-reduction reaction (ORR) activity of $M$ adatoms ($M=\text{Fe},\text{Co}$) on free-standing bilayer borophene (BB) with different coverages. Our findings indicate that metals strongly bind to the BB surface, particularly at the hollow sites, inducing metallicity. We analyze the dissociation energy of ${\text{O}}_{2}$ and $\mathrm{OOH}$ after the adsorption on the metal center of BBM while ORR activity was assessed through the free-energy adsorption of their intermediates. The stability of the systems at electrochemical conditions was investigated by Pourbaix analysis as well as by AIMD simulations, which include explicit solvents. Our results suggest that $\mathrm{BBCo}$ in a low-coverage adatom configuration would exhibit competitive ORR activity, with a theoretical overpotential of around 1 V. However, this activity would only be feasible in alkaline environments where the stability $\mathrm{BBCo}$ is preserved. Hubbard-$U$ corrections and the hybrid functional approaches within DFT are taken into consideration, and subsequent results are compared.