$\mathrm{Fe}$ and $\mathrm{Co}$ 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.

Figure 1

Top and side views of the bilayer borophene structure for a metal-adatom coverage of $\theta =1/4$. Gray and magenta balls represent boron and metal atoms. Dark-gray balls represent boron atoms forming interlayer $\mathrm{B}$—$\mathrm{B}$ bonds, and the shaded areas indicate the hollow hexagons.

Figure 2

Electronic band structures of free-standing bilayer borophene (BB) with $\mathrm{Fe}$ and $\mathrm{Co}$ adatoms at the hollow hexagon site. (a) Pristine BB, (b) ${\text{BBFe}}_{1/4}$, (c) ${\text{BBCo}}_{1/4}$, (d) ${\text{BBFe}}_1$, and (e) ${\text{BBCo}}_1$. The zero energy is set at the vacuum level. Red and blue curves indicate spin-up and spin-down bands. The dashed line indicates the Fermi energy.

Figure 3

Minimum energy path for the ${\text{OOH}}^{\ast }$ dissociation on (a) the $\mathrm{Fe}$ adatom of ${\text{BBFe}}_{1/4}$, (b) the $\mathrm{Co}$ adatom of ${\text{BBCo}}_{1/4}$, and (c) the $\mathrm{Pt}$(111) surface. Inset figures display the initial and final equilibrium geometries.

Figure 4

Reaction free energy diagrams for the ORR intermediates at different electrode potential versus reversible hydrogen electrode for (a) pristine BB, (b) ${\text{BBFe}}_{1/4}$, and (c) ${\text{BBCo}}_{1/4}$. The red lines represent the onset potential at which all reaction steps are downhill in free energy.

Figure 5

Surface Pourbaix diagram of the electrode potential versus the standard hydrogen electrode (SHE) as a function of pH for (a) pristine BB, (b) ${\text{BBFe}}_{1/4}$, and (c) ${\text{BBCo}}_{1/4}$. The area between the black lines represents the water-stability region (upper ${\text{H}}_2\text{O}/{\text{O}}_2$, lower ${\text{H}}^{+}/{\text{H}}_2$). The dashed line indicates the dissolution potential and the yellow area is the unstable region of metal dissolution.

Figure 6

AIMD simulation at 300 K of $\mathrm{BBFe}$-${\text{O}}_2$ in hydrochloric acid solution. (a) Initial geometry with ${\text{Cl}}^{-}$ surrounded by water plus the ${\text{OOH}}^{\ast }$ adsorbate. (b) Snapshot at 6 ps of simulation time. (c) Energy evolution during 6 ps of simulation time. The numbers in parentheses indicate key reaction steps described in the text.