(617c) Coupling Potential Dependency and Spin Effects in Graphene-Based Single Atom Catalysts for Oxygen Evolution and Reduction Reactions

Single-atom catalysts (SACs) are considered a new frontier in heterogeneous catalysis due to their superior transition-metal (TM) utility, activity, selectivity, and sufficient stability toward various electrochemical reactions. Traditionally, density functional theory (DFT) uses fixed number of electrons to identify the active sites for catalysis. However, under real experimental conditions, the electrode potential remains fixed via continuous charge transfer between intermediate and electrode surface. Additionally, the spin crossover in TM of SACs is often ignored along the reaction pathway in theoretical calculations.

Here, we computationally show that Ni-SAC prefers to be high spin (HS) states (S=1, triplet) when it breaks planar symmetry to tetrahedrally distorted NiN₄ structure (D2d symmetry). Later, we apply constant potential approach using VASPsol implicit solvation for OER and ORR thermodynamics calculations by exploring both charge and spin degrees of freedom for first-row transition metal-based N-doped graphene embedded as M-N₄C₄ moiety. When adding or removing electrons from system, we show that extrapolation energy to infinite vacuum is necessary. In Figure 1 below, we highlight the spin crossover between low spin (LS) to high spin (HS) in cobalt site as a function of applied potential. The charge density difference plots show that adding an electron (reduction) leads homogeneous distributed throughout the whole system while removing an electron (oxidation) results charge localization on cobalt and neighboring N-ligands. We will explore spin crossover effects for other 3d-TM based SACs for OER and ORR electrocatalysis. This electrochemistry research is supported by the U.S. Department of Energy, Catalysis Science Program to the SUNCAT Center for Interface Science and Catalysis.

Figure 1. Spin crossover between high spin (HS) and low spin (LS) state of Co-SAC as a function applied potential. Adding or removing electron shows different degree of charge localization (yellow and blue surfaces represent electron accumulation and depletion, respectively).