Relevance of the electronic structure of the substrate to O2 molecule adsorption on Fe-N-C single-atom catalysts under electrochemical potential

The local electronic structure of the metal active site often dominates the molecular adsorption on single-atom catalysts (SACs), a key process for understanding the catalytic mechanism. Herein, using constant-potential method, we delve into the role of the substrate's electronic structure in molecular adsorption on metal-embedded nitrogen-doped graphene (M-N-C) SACs under electrochemical potential. Using ${\mathrm{O}}_{2}$ as a model adsorbate and ${\mathrm{FeN}}_{4}$ and ${\mathrm{FeNC}}_{3}$ as model SACs, we observe a decrease in ${\mathrm{O}}_{2}$ adsorption strength with increasing electric potential on ${\mathrm{FeN}}_{4}$. This behavior is primarily due to changes in the electronic structure of the ${\mathrm{FeN}}_{4}$ substrate upon ${\mathrm{O}}_{2}$ adsorption, which causes a downward shift in the Fermi level and, consequently, an upward shift in the potential of zero charge. This shift leads to different responses in the total energy of ${\mathrm{FeN}}_{4}$ with and without ${\mathrm{O}}_{2}$ adsorption under applied potential. Further, we find that the ${\mathrm{O}}_{2}$ adsorption energy difference between ${\mathrm{FeN}}_{4}$ and ${\mathrm{FeNC}}_{3}$ follows a quadratic dependence on electric potential, with the parabola's opening controlled by the quantum capacitance difference between ${\mathrm{FeN}}_{4}$ and ${\mathrm{FeNC}}_{3}$ with adsorbed ${\mathrm{O}}_{2}$ . Notably, the electronic states responsible for this difference are distributed across the entire Fe-N-C SACs plane, highlighting again the critical role of the substrate's electronic structure in modulating ${\mathrm{O}}_{2}$ adsorption. This paper underscores the significant role of the substrate's electronic structure for molecular adsorption on the metal active center of SACs, enhancing the understanding of fundamental molecule-catalyst interactions under electrochemical potential.