(561b) Effect of Geometry, Spin, and Applied Potential in Oxygen Evolution and Reduction Reactions for Atomically Disperse Carbon-Based Electrocatalysts

Atomically dispersed carbon-based materials such as single atom catalysts (SACs) and metal-organic frameworks (MOFs) are promising electrocatalysts toward various electrochemical reactions. However, their activity, selectivity, and stability are largely dependent on external factors. Here we found that the square planar geometry of NiN₄ strongly prefer (~ -1 eV) a low spin (LS) state (S=0, singlet) than any other spin state. However, NiN₄ prefers to be high spin (HS) states (S=1, triplet) when it breaks planar symmetry to tetrahedrally distorted structure (D2d symmetry). Our calculated Aiso value for non-planar tetrahedrally distorted structure with dihedral angle between planes 153° to 123° very close to experimentally measured value of 20±5 MHz. Later, we applied constant potential approach using VASPsol implicit solvation for OER and ORR thermodynamics calculations on M-N₄C₄ (M = Co, Fe, Mn) moiety. We found that Fe-SAC OER activity largely depends on pH (Higher pH better activity) where Mn, Co-SAC shows insignificant effects. Co-SAC shows highest activity (overpotential 1.76 V) at acidic condition while Mn-SAC in basic conditions (1.75 V). Another interesting carbon-based material is MOF, consists of two distinct metal sites M1 and M2 where M1 is a phthalocyanine moiety and M2 is a node. Herein, we report a family of nine 2D-phthalocyanine MOF-based catalysts for alkaline ORR, where M = Co, Ni, Cu. Among all combinations, we found that Ni (M1) – Co (M2) combination has the best overall activity and delivers 5 mA cm^-2 at 0.7 V vs. RHE. Our calculation predicts that Ni as M1 is highly selective for H₂O₂ formation whereas Co as M₂ shows excellent activity for ORR. This electrochemistry research is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science Program to the SUNCAT Center for Interface Science and Catalysis.