The potential of zero charge and solvation effects on single-atom M–N–C catalysts for oxygen electrocatalysis

Abstract

Metal–nitrogen–carbon (M–N–C) catalysts are a class of emerging materials for oxygen electrocatalysis. However, a precise understanding of the predominant factors that affect their electrocatalytic activities is still preliminary, significantly hampering the rational design of high-performance M–N–C electrocatalysts. Accurate structure–activity relationship modeling necessitates considering the potential of zero charge (PZC) and solvation effects, pivotal for pH-dependent activities on a reversible hydrogen electrode scale through direct impact on reaction energetics. These factors, however, have been largely omitted in theoretical and microkinetic models due to the computational intensity of explicit solvation models. Herein, we fill in this significant knowledge gap by employing large-scale sampling via ab initio molecular dynamics and structural relaxations based on density functional theory with van der Waals corrections, on twelve distinct M–N–C configurations (M₁-pyridine-N₄ and M₁-pyrrole-N₄; M = Cr, Mn, Fe, Co, Ni, and Cu) with explicit solvation models. Interestingly, our analysis reveals that the PZCs and solvation effects, particularly hydrogen bonding adjustments to crucial reaction intermediates (HO*, O*, and HOO*), vary substantially based on the M–N–C catalysts' structures, notably the metal type and nitrogen configuration (pyridine- or pyrrole-N). Besides, both the PZCs and solvation effects of M–N–C catalysts are found to be a function of HO* binding energy; however, the PZCs follow two distinct trends on the pyridine- and pyrrole-N structures, respectively. This study shows the intricate relationship between PZC/solvation effects and M–N–Cs, and emphasizes that these effects should be considered to further improve the accuracy of microkinetic modeling.